Single crystal growth, optical absorption and luminescence properties under VUV-UV synchrotron excitation of type III Pr³⁺:KGd(PO₃)₄

Abstract

Scintillator materials are widely used for a variety of applications such as high energy physics, astrophysics and medical imaging. Since the ideal scintillator does not exist, the search for scintillators with suitable properties for each application is of great interest. Here, Pr³⁺-doped KGd(PO₃)₄ bulk single crystals with monoclinic structure (space group: P2₁) are grown from high temperature solutions and their structural, thermal and optical properties are studied as possible candidates for scintillation material. The change in the unit cell parameters as a function of the Pr³⁺ level of doping and temperature is studied. Differential thermal analysis reveals that KGd_0.942Pr_0.058(PO₃)₄ is stable until 1140 K. The 5d₃, 5d₂ and 5d₁ levels of Pr³⁺ with respect to the ³H₄ ground state are centred at 166, 196 and 218 nm, respectively, in this host. The luminescence of KGd_0.990Pr_0.010(PO₃)₄, by exciting these 5d levels, shows intense emissions centred at 256 and 265 nm from the 5d₁ to ³F_3,4 and ¹G₄ levels of Pr³⁺ with a short decay time of 6 ns. The ⁶P_3/2,5/2,7/2 → ⁸S_7/2 transitions of Gd³⁺ appear after exciting the 5d levels of Pr³⁺ and the 4 f levels of Gd³⁺, showing an energy transfer between Pr³⁺ and Gd³⁺.

Introduction

Inorganic scintillation materials are widely used in a variety of applications in the field of particles and ionizing radiation detection such us medical imaging, dosimetry, nuclear physics and astrophysics¹. Current research is focused on the search for new materials with improved scintillation properties².

Ray imaging techniques for medical imaging include planar X-ray photography, computed tomography (CT) and positron emission tomography (PET). In the first of these, the number of UV-vis photons emitted by the scintillator material per energy unit of the incoming X-ray photons (light yield) should be high in order to decrease the X-ray dose to the patient. In CT, the stability of the light output should be as high as possible to achieve reliable images and therefore better diagnostics. In PET, a fast decay time of the UV-vis photons emitted by the scintillator is required for any improvement in spatial resolution and sensitivity, since this technique is based on a precise temporal measurement of two simultaneously emitted gamma photons at nearly 180° during a positron-electron annihilation process^3,4.

Ce³⁺ and Pr³⁺ have been used as doping ions in the vast majority of the new single crystal scintillators reported over the last approximately 20 years because of the fast decay time of the 5d → 4 f radiative transitions (usually from 10 to 60 ns), together with the high quantum efficiency of these transitions at room temperature². The scintillation properties of Ce³⁺- and Pr³⁺-doped garnets have been optimized by the growth of multicomponent doped hosts like (Gd,Lu)₃Ga₃Al₂O₁₂^5,6,7,8. As regards aluminium perovskite crystals, fast lifetimes corresponding to the 5d → 4 f transitions of Ce³⁺ and Pr³⁺ ions doped in YAlO₃ host have been obtained at around 18 and 8 ns, respectively². Oxyorthosilicates have also been investigated because they have good scintillation properties, especially (Lu/Y)₂SiO₅ (LYSO) doped with Ce³⁺, which is used in PET imaging². The Ce³⁺-doped LYSO compound was introduced in 2000 with the composition Ce:Lu_1.8Y_0.2SiO₅ by Cooke et al.⁹. Lu₂SiO₅ (LSO) doped with Pr³⁺ has also been studied and the photoluminescence of the 5d → 4 f electronic transition at 273 nm shows a fast decay time of 6–7 ns. However, since the 5d₁ level of Pr³⁺ in LSO is close to the conduction band, there is a degradation of the light yield of this scintillator at room temperature¹⁰.

In polyphosphate compounds, the photoluminescence of rare earth ions in LiLaP₄O₁₂ was studied by Blasse and Dirken¹¹, with a Ce³⁺ concentration quenching being observed. The luminescence of LiY_0.9Ce_0.1P₄O₁₂ as a function of temperature was reported by Shalapska et al.¹² with a decay time for the Ce³⁺ 5d₁ → 4 f transition of 18.6 ns at room temperature. In the research carried out by Zhong et al. into Ce³⁺-doped MGdP₄O₁₂ (M = Li, Na, K, Cs)¹³, the energy transfer between Gd³⁺ and Ce³⁺ ions and its luminescence was studied and the energy level diagrams of the Gd³⁺ and Ce³⁺ in these compounds were put forward. Ce³⁺- and Pr³⁺-doped NaLa(PO₃)₄ were studied under VUV-UV excitation by Kang et al.¹⁴, showing decay times for the 5d → 4 f transitions of Ce³⁺ of 22.7–23.8 ns and of Pr³⁺ of 9.9–12.9 ns.

Type III KGd(PO₃)₄ is a monoclinic crystal with a non-centrosymmetric crystalline structure (space group: P2₁) that has nonlinear optical properties similar to KH₂PO₄ (KDP)¹⁵ and a deep UV cut-off of its transparency window at 160 nm¹⁶. It is non-hygroscopic and chemically stable and its high hardness (close to quartz in the Mohs scale) means the surfaces can be polished to a good optical quality¹⁷. Considering the literature mentioned above, the 5d → 4 f transitions of Pr³⁺ in KGd(PO₃)₄ host is expected to have decay times faster than Ce³⁺ in the same host. Ce³⁺-doped KGd(PO₃)₄ single crystals have already been studied and show interesting luminescence characteristics for scintillator applications¹⁸. The aim of this paper is therefore to grow Pr:KGd(PO₃)₄ single crystals from high temperature solutions with different Pr³⁺ concentrations in order to characterize their thermal stability, study their optical absorption and luminescence properties under UV-VUV synchrotron excitation, and discuss their usefulness as a new scintillator material. However, it should first be mentioned that, a priori, the functionality of Pr:KGd(PO₃)₄ single crystals for some scintillator applications could be limited due to the 0.0117% natural abundance of the ⁴⁰K radioisotope, which means an increase in the background counts is expected. Nevertheless, the ⁴⁰K radioactive isotope together with others such as ¹⁷⁶Lu, ⁸⁷Rb, and ¹³⁸La is present in some scintillator materials that are promising or already in use¹⁹.

Results and discussion

Bulk single crystal growth

Table 1 shows the crystal growth experiments carried out for different levels of Pr³⁺ doping and the crystals obtained.

Table 1 Crystal growth experiments for different Pr³⁺ doping levels and the crystals obtained.

The saturation temperature of all solutions was around 993 K and no significant changes were observed with Pr³⁺ doping at the levels studied in this work. The saturation temperature is slightly higher than that previously reported for the crystal growth of Ce:KGd(PO₃)₄ crystals¹⁸ grown in similar experimental conditions. The crystals obtained were generally transparent, free of inclusions and cracks, and slightly greenish due to Pr³⁺ doping. The sizes obtained were a* × b × c* = 7.6–13.6 mm × 17.1–24.1 mm × 10.7–13.6 mm and the weights ranged from 2.7 to 6.9 g. As can be seen in Table 1, the crystal dimension in b direction was always higher than in a* and c* directions. This faster growth in the b crystallographic direction has already been reported by us for different doping KGd(PO₃)₄ crystals^16,17,18 and is in agreement with the non-presence of the (010) crystalline face. The growth rate of the {hkl} form is inversely proportional to interplanar spacing d_hkl and the sequence in this crystal is d₀₀₁ > d₁₀₀ > d₀₂₀. The chosen orientation of the seed also reinforces this behaviour, together with the thermal gradients in the solution. The crystal growth rate varies from 6.1 × 10⁻³ to 17.4 × 10⁻³ g·h⁻¹.

As an example, Fig. 1 shows an as-grown Pr:KGd(PO₃)₄ single crystal and a scheme of the crystal morphology in which the main faces of the crystal can be observed. These are generally {001}, {100}, {011}, {0–11}, {110}, {1–10}, {10-1}, {101} and {-1-11}.

Figure 1

(a) As-grown single crystal of Pr:KGd(PO₃)₄, and (b) crystal scheme with the faces observed.

Using the atomic percentage of each element present in the chemical compound as obtained from the EPMA (electron probe microanalysis) results, the chemical formula of each crystal was determined and the distribution coefficient of Pr³⁺ in KGd(PO₃)₄ calculated according to the formula K_Pr = ([Pr]/([Pr] + [Gd]))_crystal/([Pr]/([Pr] + [Gd]))_solution, where [Pr] and [Gd] are the Pr³⁺ and Gd³⁺ concentrations, respectively, in atomic % in the crystal and the solution. Table 2 shows, for the five doping levels studied, the Pr³⁺ atomic ratio with respect to Gd³⁺ in the crystal, the chemical formula, the number of Pr³⁺ ions per unit cell volume in the crystal and the distribution coefficient of Pr³⁺ (K_Pr). For the crystals obtained from solutions with 2.00 and 5.00 atomic % of Pr³⁺ in the solution, the atomic percentage of each element was measured at several points along the a* and c crystallographic directions in a plate perpendicular to b crystallographic axis, obtaining up to about 85 results per sample. These results (see Fig. S1 in Supporting Information) showed that the variation of the measured values of Pr³⁺ atomic concentration along the crystallographic directions is of the same order as the error in the measurements. Therefore, the results indicate the uniformity of Pr³⁺ atomic concentration of the growing crystals along the a* and c crystallographic directions, up to 5.8 atomic % of Pr³⁺ in the crystal. In the samples obtained from solutions with 0.25, 0.50 and 1.00 atomic % of Pr³⁺ in the solution, the EPMA measurements were carried out far from the undoped KGd(PO₃)₄ seed, i.e. in the last stages of the crystal growth. Taking into account the values of the Pr³⁺ distribution coefficient in KGd(PO₃)₄ and their error, it can be observed that the Pr³⁺ distribution coefficient is not far from the unit in any of the concentrations studied. Besides, the results do not show any significant tendency for its value (K_Pr) to decrease or increase as the level of Pr³⁺ doping in the solution increases.

Table 2 Compositional results for Pr:KGd(PO₃)₄. K_Pr denotes the distribution coefficient of the Pr³⁺ in the crystal.

Structural characterization

X-ray powder diffraction analysis was carried out to study the evolution of the unit cell parameters of KGd_1-xPr_x(PO₃)₄ depending on the Pr³⁺ doping concentration. The refinement of the unit cell parameters was carried out using the TOPAS program²⁰. Table 3 shows the unit cell parameters of the crystals studied, while Fig. 2 shows the evolution of these parameters as a function of the praseodymium content in KGd(PO₃)₄. It can be seen that there is an ascending linear behaviour of the unit cell parameters on increasing the Pr³⁺ content in the crystals.

Table 3 The unit cell parameters and unit cell volume of Pr:KGd(PO₃)₄ single crystals at different Pr³⁺ doping concentrations.

Figure 2

Evolution of the a, b, c and β unit cell parameters and unit cell volume of Pr:KGd(PO₃)₄ single crystals as a function of the praseodymium content in KGd(PO₃)₄.

As Table 3 and Fig. 2 show, the a and β parameters remain practically the same, the b and c parameters increase slightly and the unit cell volume clearly increases when the Pr³⁺ concentration in the crystal increases. This behaviour is expected because the ionic radius of Pr³⁺ with coordination VIII is higher than the ionic radius of Gd³⁺ with the same coordination (1.126 Å and 1.053 Å, respectively²¹).

Thermal stability

Figure 3 shows the thermogram obtained for KGd_0.942Pr_0.058(PO₃)₄ in both the heating and cooling processes in the range 500–1273 K. The weight change during the experiment was not significant.

Figure 3

Thermogram of KGd_0.942Pr_0.058(PO₃)₄ in heating and cooling processes in the range 500–1273 K.

An endothermic peak beginning at 1140 K can be observed in the heating process, which is attributed to the incongruent melting process of KGd_0.942Pr_0.058(PO₃)₄. This temperature is so similar to the 1142 K obtained for undoped KGd(PO₃)₄²² that it can be said that there is no appreciable difference in the incongruent melting temperature of KGd(PO₃)₄ with the Pr³⁺ doping, at least up to 5.8 atomic % of Pr³⁺ in the crystal. Meanwhile, no heat exchange in the cooling process of the sample was observed, which means that no crystalline phase transitions were produced during the process.

Figure 4 shows the X-ray powder diffractogram of KGd_0.942Pr_0.058(PO₃)₄ at room temperature, its evolution with the temperature up to 1273 K and the cooling process up to room temperature. The temperatures written to the right of the graph are used as labels, since it is expected that the temperature distribution in the sample support during these measurements was not homogeneous. This could lead to a partial incongruent melting when the thermocouple of the diffractometer chamber indicated 1093 K.

Figure 4

X-ray powder experimental diffractogram of KGd_0.942Pr_0.058(PO₃)₄ at room temperature, those selected at several temperatures describing its evolution with temperature in both the heating and cooling processes and three X-ray powder diffraction standard patterns. The labelling of the patterns is related to the temperature read in the central part of the sample support by the thermocouple.

The diffraction standard patterns of KTb(PO₃)₄ (89–1424 ICDD database²³ and GdPO₄ (83–0657 ICDD database²⁴, together with a Pt diffraction peak (the sample holder was of Pt), are also shown in Fig. 4. The diffraction standard pattern of type III KTb(PO₃)₄ (space group: P2₁) was used due to the non-existence of the type III KGd(PO₃)₄ powder diffraction standard pattern in the version of the ICDD database used. Hence at room temperature all diffraction peaks correspond to the monoclinic crystalline phase of type III KGd(PO₃)₄ together with a diffraction peak belonging to the Pt crystalline phase of the sample holder. Till 1078 K, there are no extra peaks of any other crystalline phase, and the diffraction peaks belonging to KGd(PO₃)₄ present a decrease in their sharpness and intensity, related to the loss of crystallinity. At 1093 K a small peak appeared at 29.2° and was identified as GdPO₄. At 1108 K, the two crystalline phases (KGd(PO₃)₄ and GdPO₄) are coexistent, and at 1123 K the KGd(PO₃)₄ crystalline phase has totally decomposed. In Ponceblanc et al.²⁵, differences in the phase transition temperature of the same compound were also observed depending on both the heating rates and technique used. From 1093 to 1108 K, as the intensity of the type III KGd(PO₃)₄ peaks decreased, the intensity of the GdPO₄ peaks increased. From 1123 K to 1273 K only the diffraction peaks of GdPO₄ can be observed, meaning that this crystalline compound is stable at this range of temperatures. Throughout the cooling process from 1273 K until room temperature (see the last four diffractograms) there are no significant changes, so the GdPO₄ remains stable. This means that the phase transition is not reversible, as expected for an incongruent melting, and in our case the solidification of the liquid phase leads to an amorphous phase.

Therefore, according to the differential thermal analysis and X-ray powder diffraction results, KGd_0.942Pr_0.058(PO₃)₄ decomposes at 1140 K into GdPO₄ and liquid phase, which probably consisted of a mixture of phosphorus and potassium oxides, since the sample weight remained practically constant.

The studies on KGd(PO₃)₄²² and KYb_0.029Gd_0.971(PO₃)₄¹⁷ are comparable to that presented in our work. The results for the first compound show that KGd(PO₃)₄ decomposes irreversibly at 1142 K into Gd(PO₃)₃, GdPO₄, Gd₂P₄O₁₃ and an amorphous phase, and that at room temperature after the cooling process only GdPO₄ remains. Regarding the second case, KYb_0.029Gd_0.971(PO₃)₄ decomposes irreversibly at 1130 K into Gd(PO₃)₃, at 1223 K Gd(PO₃)₃, GdPO₄, Gd₂P₄O₁₃, GdP₅O₁₄ and an amorphous phase coexist, and at room temperature after the cooling process the GdPO₄ and Gd₂P₄O₁₃ crystalline phases remain. Thus the difference in the thermal evolution observed for the KGd_0.942Pr_0.058(PO₃)₄ is that this crystal is decomposed into a unique crystalline compound, GdPO₄, and a liquid phase. The intermediate Gd(PO₃)₃ crystalline compound observed in the previous studies is not observed in our case, and neither are the crystalline phases Gd₂P₄O₁₃ and GdP₅O₁₄ from the previous works present in our case. Only the GdPO₄ crystalline phase is observed to be stable till room temperature in all three studies^17,22.

Another compound whose thermal decomposition has been studied in the literature is KLa(PO₃)₄²⁶, which decomposes into La(PO₃)₃, LaPO₄ and an amorphous phase containing phosphorus and potassium oxides. Thus, the thermal decomposition products of KLa(PO₃)₄ correlate with those of KGd(PO₃)₄ except for the fact that La₂P₄O₁₃ is not present.

Linear thermal expansion tensor

Bearing in mind the X-ray powder diffractograms measured in the range from room temperature up to 773 K and using the Le Bail method²⁷, the unit cell parameters at different temperatures in the P2₁ space group were refined. The parameters relating to goodness of fit are R_wp and R_exp, whose values must fulfil the expression R_wp ≤ 2·R_exp for it to be considered that a good fit is obtained. In all cases these parameters are around R_wp = 21.06 and R_exp = 18.08. Table 4 shows the unit cell parameters of KGd_0.942Pr_0.058(PO₃)₄ at different temperatures, while Fig. 5 shows the relative thermal evolution of these parameters with respect to those at room temperature as a function of temperature. It can be seen that the unit cell parameters follow a linear trend. The a and b parameters clearly increase with temperature, as does the c parameter but in minor proportion, while the β parameter decreases slightly. From these results, the linear thermal expansion coefficients in each crystallographic direction can be calculated using the expression α = (ΔL/ΔT)/L_RT, where ΔL/ΔT is the slope of the linear fit of the change of each unit cell parameter with the temperature, and L_RT is the unit cell parameter at room temperature, 303 K. The linear thermal expansion coefficients in the crystallophysical system X₁∣∣a, X₂∣∣b and X₃∣∣c* are α₁₁ = 12.00 × 10^-6 K⁻¹, α₂₂ = 12.40 × 10⁻⁶ K⁻¹, α₃₃ = 7.46 × 10⁻⁶ K⁻¹ and α₁₃ = 1.45 × 10⁻⁶ K⁻¹.

Table 4 Unit cell parameters and unit cell volume of KGd_0.942Pr_0.058 (PO₃)₄ at different temperatures.

Figure 5

Evolution of the unit cell parameters of KGd_0.942Pr_0.058(PO₃)₄ from room temperature up to 773 K.

By diagonalizing the tensor, the thermal expansion values in the principal axes of the tensor X₁’, X₂’∣∣b and X₃’ are obtained, these being α’₁₁ = 12.43 × 10⁻⁶ K⁻¹, α’₂₂ = 12.40 × 10⁻⁶ K⁻¹ and α’₃₃ = 7.03 × 10⁻⁶ K⁻¹. The X₁’ axis is at 16.31° clockwise from the a axis, while X₃’ is at 14.44° from the c axis with the positive b axis pointing toward the observer (see Fig. 6).

Figure 6

Linear thermal expansion ellipsoid of KGd_0.942Pr_0.058(PO₃)₄ at 303 K.

Pr³⁺ spectroscopy in KGd(PO₃)₄ single crystals

Optical absorption. To identify the Pr³⁺ absorption bands and distinguish them from the Gd³⁺ bands in Pr:KGd(PO₃)₄, the optical absorption of an undoped sample of KGd(PO₃)₄ was measured (see Fig. S2 in Supporting Information). Figure 7 shows the unpolarized optical absorption cross sections of the ³H₄ → 5d₁ electronic transition and the 4 f → 4 f electronic transitions of Pr³⁺ ions in KGd(PO₃)₄ in the range 205 to 2475 nm at room temperature.

Figure 7

Unpolarized optical absorption cross sections of Pr:KGd(PO₃)₄ at room temperature. Light propagation direction is along the b axis in figure (a) and along the a* axis in figures (b–f). All labels indicate the electronic transitions of Pr³⁺.

In Fig. 7a, a broad band centred at 218 nm (45872 cm⁻¹, 5.69 eV) is observed, which corresponds to the electronic transition from the ³H₄ ground state of Pr³⁺ to its lowest 5d level (5d₁) in KGd(PO₃)₄. From the crystallographic point of view, there is only one site expected, with C₁ point symmetry, for the Pr³⁺ and Gd³⁺ ions in the KGd(PO₃)₄ crystal²⁸. Therefore all Pr³⁺ ions have the same crystal field, and consequently only one band for the ³H₄ → 5d₁ transition is expected. This transition of Pr³⁺ has been systematically studied in many hosts and it can be predicted by considering the study carried out by Dorenbos²⁹. In this work, the value of the ³H₄ → 5d₁ transition of Pr³⁺ in phosphate hosts varies from 212 nm for LaP₃O₉³⁰, through 222 nm for YPO₄^30,31, to 224 nm for YP₃O₉³⁰. In addition, since the average energy difference of the first spin-allowed 4 f → 5d transition of Pr³⁺ (³H₄ → 5d₁) with respect to the transition of Ce³⁺ (²F_5/2 → 5d₁) in the same host²⁹ is 12240 ± 750 cm⁻¹ and because the ²F_5/2 → 5d₁ transition of Ce³⁺ in KGd(PO₃)₄ is centred at 302.5 nm (33058 cm⁻¹, 4.10 eV)¹⁸, the expected position of the ³H₄ → 5d₁ transition of Pr³⁺ in KGd(PO₃)₄ is in the range 217.2–224.5 nm (46048–44548 cm⁻¹, 5.71–5.52 eV). Therefore the experimental position of the ³H₄ → 5d₁ transition of Pr³⁺ in KGd(PO₃)₄ (218 nm) is consistent with the values found in other phosphates and within the calculated range.

The value of the optical absorption cross section of Pr³⁺ in KGd(PO₃)₄ for the ³H₄ → 5d₁ transition (at 218 nm) is about 780 × 10⁻²⁰ cm². This high value is expected since 4 f → 5d transitions are parity-allowed transitions.

It should be noted that, of the optical absorption measurements, only the 5d₁ absorption band was identified out of all the 5d_x levels of Pr³⁺ in KGd(PO₃)₄. The reason for this is that, although the UV limit of the equipment used is 175 nm, the measurements were carried out in air atmosphere, and below 190 nm the air absorption hid the other 5d_x levels. The 5d₂ and 5d₃ energy levels, together with the 5d₁ energy level already determined, of Pr³⁺ in KGd(PO₃)₄ were quantified in the UV-VUV synchrotron measurements (see next section) by studying the excitation spectra for several emission wavelengths.

Figure 7b–f show the optical absorption cross section of the Pr³⁺ 4 f → 4 f transitions in KGd(PO₃)₄ in the energy ranges 430–490, 570–620, 925–1125, 1375–1720 and 1850–2475 nm, in which the ³H₄ → ¹I₆ + ³P_J, ³H₄ → ¹D₂, ³H₄ → ¹G₄, ³H₄ → ³F₄ + ³F₃ and ³H₄ → ³F₂ + ³H₆ electronic transitions of Pr³⁺, respectively, were observed. Although the ³H₄ → ³P₂ pseudo-hypersensitive transition does not have the highest optical absorption cross section, its value of 4.4 × 10⁻²⁰ cm² is in the suitable range to be used for exciting 4 f → 4 f transitions for optical amplification applications.

Optical emission

Figure 8 shows the emission spectra of Pr:KGd(PO₃)₄ under λ_exc = 218 nm (45872 cm⁻¹, 5.69 eV), 196 nm (51020 cm⁻¹, 6.33 eV) and 166 nm (60241 cm⁻¹, 7.47 eV) at room temperature.

Figure 8

Optical emission spectra of KGd_0.990Pr_0.010(PO₃)₄ under (a) 218 nm, (b) 196 nm, and (c) 166 nm excitation. Labels in purple and italics indicate the electronic transitions of Gd³⁺ and labels in green and bold those of Pr³⁺.

In Fig. 8a, the most intense emissions centred at 305, 312 and 323 nm correspond to the ⁶P_3/2 → ⁸S_7/2, ⁶P_5/2 → ⁸S_7/2 and ⁶P_7/2 → ⁸S_7/2 transitions of Gd³⁺, respectively, obtained by exciting the 5d₁ level of Pr³⁺ in KGd(PO₃)₄. This means that an energy transfer (ET) between Pr³⁺ and Gd³⁺ occurs. In other hosts such as (Gd,Lu)₃Ga₃Al₂O₁₂:Pr, energy transfers from 5d levels of Pr³⁺ to 4 f levels of Gd³⁺ were also observed⁵.

Also in Fig. 8a, parity-allowed transitions from the d to f levels of Pr³⁺, which are of great interest for scintillation applications, are also observed. These broad, intense bands correspond to the 5d₁ → ³H₅, 5d₁ → ³H₆, 5d₁ → ³F_3,4 and 5d₁ → ¹G₄ electronic transitions of Pr³⁺, centred at 229, 239, 256 and 265 nm, respectively. The emission band corresponding to the electronic transition from the 5d₁ level to the ³H₄ ground state of Pr³⁺ does not appear, probably due to the self-absorption effect, as occurs in AREP₂O₇ hosts (A = Na, K, Rb, Cs; RE = Y, Lu)³². We should also note that the significantly weaker broad bands centred at 358 and 435 nm correspond to the 5d₁ → ¹D₂ and 5d₁ → ³P₂ transitions. The most intense band originating in a 5d level is the broad band located around 256–265 nm, which corresponds to the overlapping of three electronic transitions, 5d₁ → ³F_3,4 and 5d₁ → ¹G₄. By exciting the 5d₁ level of Pr³⁺ at 210 nm in LiYF₄ at 10 K, it can be observed how 5d₁ → ³H₄, 5d₁ → ³H₅, 5d₁ → ³H₆, 5d₁ → ³F_3,4 and 5d₁ → ¹G₄ transitions appear at 220, 230, 245, 255 and 272 nm, respectively, while 5d₁ → ¹D₂ and 5d₁ → ³P₂ transitions do not appear. It should also be noted that the band that corresponds to the 5d₁ → ³H₅ transition is the most intense, while that corresponding to the 5d₁ → ¹G₄ transition is the least³³. Under direct 4 f → 5d₁ excitation (280 nm) of Pr³⁺ in Lu₃Al₅O₁₂, Lu₃Al₄GaO₁₂ and Lu₃Al₃Ga₂O₁₂ hosts, the emission bands corresponding to the 5d₁ → ³H_J and 5d₁ → ³F_J electronic transitions appear centred at 310 and 360 nm, respectively, with the first band being the most intense. The bands corresponding to the 5d₁ → ¹G₄, 5d₁ → ¹D₂ and 5d₁ → ³P_J transitions do not appear. The emission peaks corresponding to some of the 4 f → 4 f electronic transitions of Pr³⁺ are insinuated in the visible range from 480 to 760 nm³⁴. The emission spectrum of La_0.999Pr_0.001PO₄ at 300 K under the direct 4 f → 5d₁ excitation (193 nm) of Pr³⁺ shows intense emission bands centred at about 230, 240 and 255 nm corresponding to the 5d₁ → ³H₄, 5d₁ → ³H₅ and 5d₁ → ³H₆, ³F₂ electronic transitions, respectively. Two very weak, broad emission bands appear centred around 375 and 440 nm corresponding to the 5d₁ → ¹D₂ and 5d₁ → ¹I₆,³P_J transitions. It should be noted that an emission peak centred at approximately 610 nm, with an intensity similar to the two previous bands, corresponds to the ¹D₂ → ³H₄ electronic transition. The most intense emission bands correspond to the 5d₁ → ³H₄ and 5d₁ → ³H₅ transitions, and the least intense to the 5d₁ → ¹D₂ transition³⁵.

The weaker bands observed in the visible region of Fig. 8a are 4 f → 4 f transitions corresponding to Gd³⁺, which belong to the ⁶G_7/2 → ⁶P_7/2, ⁶G_11/2,9/2,5/2 → ⁶P_3/2, ⁶G_13/2 → ⁶I_7/2 and ⁶G_3/2 → ⁶I_7/2 transitions.

Information on the energy value of the 5d₂ and 5d₃ levels of Pr³⁺ in this host was obtained by studying the excitation spectra of Pr:KGd(PO₃)₄ for different emission wavelengths (see below in Fig. 9). In Fig. 8b, it can be seen that the emission spectrum of Pr:KGd(PO₃)₄ under λ_exc = 196 nm (5d₂ level of Pr³⁺ and ⁶G_13/2 level of Gd³⁺ excitation) is very similar to that obtained by exciting the 5d₁ level as regards the intensity ratio of the emission bands in the range from 200 to 500 nm. The main difference obtained by exciting at 196 nm is in the 500–825 nm visible range, where the ⁶G_7/2 → ⁶P_7/2, ⁶G_11/2,9/2,5/2 → ⁶P_3/2, ⁶G_13/2 → ⁶I_7/2 and ⁶G_3/2 → ⁶I_7/2 transitions of Gd³⁺ are significantly intensified. Other new peaks appeared at 570–578, 632, 662, 689, 751, 759, 777 and 800 nm and correspond to the ⁶G_11/2,9/2,5/2 → ⁶P_7/2, ⁶G_7/2 → ⁶P_3/2, ⁶G_13/2 → ⁶I_9/2,17/2, ⁶G_13/2 → ⁶I_{11/2,15/2,13/2}, ⁶G_11/2,9/2,5/2 → ⁶I_9/2,17/2, ⁶G_7/2 → ⁶I_7/2, ⁶G_11/2,9/2,5/2 → ⁶I_{11/2,15/2,13/2} and ⁶G_7/2 → ⁶I_9/2,17/2 transitions of Gd³⁺. Under excitation at 196 nm, the visible bands corresponding to the 4 f → 4 f electronic transitions of Gd³⁺ are more intense than the UV band corresponding to the 5d → 4 f electronic transitions of Pr³⁺. This could be related to a simultaneous energy transfer process from Pr³⁺ to Gd³⁺ and also direct Gd³⁺ excitation leading to a larger electronic population in the Gd³⁺ emitting levels. As in our work, photon cascade emissions of Gd³⁺ in the UV-Visible-near IR range were observed in GdBaB₉O₁₆ under ⁸S_7/2 → ⁶G_J excitation (202 nm)³⁶, and also under 195 nm excitation (⁸S_7/2 → ⁶G_J) in NaY_0.8Gd_0.2FPO₄ and NaGdFPO₄³⁷.

Figure 9

Excitation spectra of Pr:KGd(PO₃)₄ crystal under VUV radiation for (a) λ_emi = 265 nm and (b) λ_emi = 592 nm. Labels in purple and italics indicate the electronic transitions of Gd³⁺ and labels in green and bold those of Pr³⁺.

Figure 8c shows the emission spectrum of Pr:KGd(PO₃)₄ under λ_exc = 166 nm (5d₃ level of Pr³⁺ and ⁶H_13/2 level of Gd³⁺ excitation). In this spectrum it is important to note that the main ⁶P_3/2,5/2,7/2 → ⁸S_7/2 electronic transitions of Gd³⁺ and the 5d₁ → ³F_3,4 and 5d₁ → ¹G₄ emission bands of Pr³⁺ have a similar intensity. In the 500–825 nm range, the Gd³⁺ emissions located at 617 nm (⁶G_11/2,9/2,5/2 → ⁶P_3/2) and 700 nm (⁶G_3/2 → ⁶I_7/2) greatly decreased in intensity compared to the emission spectrum obtained by exciting at 196 nm and slightly decreased compared to the emission spectrum obtained by exciting at 218 nm. It should also be noted that three new emission peaks belonging to Gd³⁺ appear. These are centred at 561 nm (⁶G_3/2 → ⁶P_5/2), 595 nm (⁶G_11/2,9/2,5/2 → ⁶P_5/2) and 645 nm (⁶G_7/2* → ⁶I_{11/2,15/2,13/2}).

Finally, it is important to note that in Pr:KGd(PO₃)₄, no 4 f → 4 f transitions of Pr³⁺ were observed under excitations in the 120–248 nm range. The assignation of the Gd³⁺ and Pr³⁺ transitions was checked by comparing the Pr:KGd(PO₃)₄ emission spectra with those of the undoped KGd(PO₃)₄ excited at λ_exc = 218, 196 and 166 nm (see Fig. S3 in Supporting Information) and by consulting the Dieke’s diagram³⁸, the extended Dieke’s diagram³⁹, and the work carried out by Wegh et al.⁴⁰ and by Yang et al.³⁶.

In the excitation spectrum of Pr:KGd(PO₃)₄ crystal under VUV-UV radiation from 120 to 248 nm for λ_emi = 265 nm corresponding to the 5d₁ → ¹G₄ electronic transition (see Fig. 9a), the excitation of the 5d_3, 5d₂ and 5d₁ levels of Pr³⁺ in KGd(PO₃)₄ is produced at wavelengths around 166, 196 and 218 nm, respectively. Some 4 f levels of Gd³⁺ were also excited giving rise to this Pr³⁺ emission, which could be explained by an energy transfer from Gd³⁺ to Pr³⁺.

As for the excitation spectrum for the Gd³⁺ emission at λ_emi = 592 nm (see Fig. 9b), although this emission could also be observed by excitation of the 5d energy levels of Pr³⁺ and the consequent energy transfer to Gd³⁺, it seems that it is more favoured when the ⁶G_J levels of Gd³⁺ are excited.

Given the calculations explained in our previous work on type III Ce:KGd(PO₃)₄ single crystals¹⁸, the energy of the exciton creation (E^ex) and the approximate energy difference from the bottom of the conduction band to the top of the valence band (E_VC) of the type III KGd(PO₃)₄ host were predicted at 7.57 eV (164 nm) and at 8.17 eV (152 nm), respectively. Therefore, as already mentioned in the previous work, the E^ex band could appear in the excitation spectra (Fig. 9), but it would not be appreciated due to an overlapping with the ⁸S_7/2 → ⁴F_9/2 transition of Gd³⁺.

Figure 10 shows the energy levels scheme for Pr³⁺ and Gd³⁺ and the electronic transitions assigned to the observed emissions by excitation at 218, 196 and 166 nm in a KGd_0.990Pr_0.010(PO₃)₄ single crystal.

Figure 10

Energy level diagram of Pr³⁺ and Gd³⁺ in Pr:KGd(PO₃)₄ and emissions observed under excitation at (a) 218 nm, (b) 196 nm, and (c) 166 nm. The thickness of the arrows is related to the intensity of the emissions represented.

Decay time measurements

Time profiles were recorded in two time regimes to measure the fast and slow components of the decay curves of the emission at 256 nm and 312 nm, respectively. These curves are shown in Fig. 11. Pr:KGd(PO₃)₄ crystals were excited at 218 nm (³H₄ → 5d₁ of Pr³⁺) and 166 nm (³H₄ → 5d₃ of Pr³⁺ and ⁸S_7/2 → ⁶H_13/2 of Gd³⁺) under pulsed synchrotron radiation. In order to improve the photon flux reaching the sample, the excitation radiation was not exactly monochromatic but had a bandwidth of around 7%.

Figure 11

(a) Fast component of the decay curves of Pr:KGd(PO₃)₄ at different concentrations of praseodymium under excitations at 166 and 218 nm for λ_emi = 256 nm. (b) Slow component of the decay curve of KGd_0.942Pr_0.058(PO₃)₄ under excitation at 300 nm for λ_emi = 312 nm.

As can be seen in Fig. 11a, for the fast component all the decay curves can be fitted to single exponential decays with a time constant of around 6 ns. This value can be attributed to the lifetime of the 5d₁ level of Pr³⁺ in KGd(PO₃)₄, which is significantly shorter than the lifetimes obtained in Ce:KGd(PO₃)₄¹⁸. The lifetimes obtained for the 5d₁ emitting electronic state of Pr³⁺ in different hosts are usually longer than 6 ns, as observed in Table 5. The shortening of this lifetime in the KGd(PO₃)₄ host could be related to the energy transfer from Pr³⁺ to Gd³⁺, this being an additional depopulation channel through a non-radiative decay of the 5d₁ emitting state of praseodymium.

Table 5 Values for the lifetime of the 5d₁ level of Pr³⁺ in several hosts.

It can also be seen that no significant changes in lifetime with the Pr³⁺ doping content of KGd(PO₃)₄ came about, at least up to 5.8 atomic % Pr:KGd(PO₃)₄. And no important quenching of the emission is expected due to the concentration effect, so no shortening of the lifetime. This can be seen in the long lifetime of the LiPrP₄O₁₂ phosphate (Table 5), where Pr³⁺ is not a doping element but a host element.

As previously stated, the emission at around 312 nm observed in Fig. 8 corresponds to some of the 4 f → 4 f electronic transitions of Gd³⁺. The decay time of these electronic transitions (lifetime could be in μs or ms) is significantly slower than those from the 5d energy levels and could not be measured at the DESIRS beamline (DESIRS-6.65 m Monochromator) due to the interpulse duration of 1118 ns of the synchrotron radiation. Figure 11b shows its slow decay time. By fitting to a single exponential decay, the lifetime obtained is 9 ms. As reported previously in the literature, the origin of this slow component could be attributed to the Gd³⁺ emission corresponding to the electronic transition ⁶P_J → ⁸S_7/2 along with some contribution from the trapping effect, since the lifetime of the emitting ⁶P_7/2 level of Gd³⁺ is of the same order (4.9 ms in NaY_0.80Gd_0.20PO₄ and 6.36 ms in NaGd(PO₃)₄)^41,42 as the 9 ms.

Conclusions

Type III Pr³⁺-doped KGd(PO₃)₄ single crystals of up to 5.8 atomic % of Pr³⁺ substituting Gd³⁺ with high crystalline quality have been grown by the top seeded solution growth-slow cooling technique from self-flux solutions. It has been demonstrated that KGd_0.942Pr_0.058(PO₃)₄ is thermally stable up to 1140 K, where it suffers an irreversible decomposition into a unique crystalline compound, GdPO₄, and a liquid phase. The X₁’ principal axis of the thermal tensor of this crystal is at 16.31° clockwise from the a crystallographic direction when the positive b axis (parallel to the X₂’ principal axis) is pointing toward the observer and the X₃’ principal axis is at 14.44° clockwise from the c axis. The absorption bands corresponding to the ³H₄ → 5d₁, ³H₄ → 5d₂ and ³H₄ → 5d₃ electronic transitions of Pr³⁺ in KGd(PO₃)₄ are centred at 218, 196 and 166 nm, respectively. The ¹S₀ energy level of Pr³⁺ overlaps with the 5d₁ level of the same ion in this host, preventing the non-radiative relaxation from the 5d₁ level to the ¹S₀ energy level and the radiative relaxation between 4 f levels that would deteriorate the scintillation efficiency of these crystals. Under ultraviolet excitation, an intense, broad emission band located around 256–265 nm was observed in all grown crystals, corresponding to the 5d₁ → ³F_3,4 and 5d₁ → ¹G₄ electronic transitions of Pr³⁺. The lifetime of the 5d₁ level of the Pr³⁺ in type III KGd(PO₃)₄ was measured for the emission band centred at 256 nm and a lifetime of around 6 ns was obtained, which is of great interest for scintillator applications. In most cases this lifetime is shorter than the lifetime obtained for the same level of Pr³⁺ in other hosts. Moreover, the emission spectra show a broad band in the visible range corresponding to the 5d₁ → ³P₂ transition of Pr³⁺ with enough intensity under 218 nm excitation, which could be an appropriate transition for use in scintillator applications. It should also be noted that the ⁶P_3/2,5/2,7/2 → ⁸S_7/2 electronic transitions of Gd³⁺ were observed centred at 305, 312 and 323 nm by exciting the 5d levels of Pr³⁺, although it would be interesting to study whether the same behaviour occurs under X-ray excitation.

Experimental

Single crystal growth

Type III praseodymium-doped KGd(PO₃)₄ single crystals, with doping levels ranging up to 5 atomic % of Pr³⁺ substituting Gd³⁺ in the solution, were grown from self-flux solutions using the top seeded solution growth-slow cooling (TSSG-SC) technique. The growth solutions, with a weight of around 130 g, were placed in a platinum cylindrical crucible 50 mm in diameter and 50 mm in height. The initial reagents used were K₂CO₃ (99%), Gd₂O₃ (99.9%), Pr₂O₃ (99.9%) and NH₄H₂PO₄ (≥99.0%). The compositions of the solutions, chosen according to the KGd(PO₃)₄ primary crystallization region in the K₂O – Gd₂O₃ – P₂O₅ ternary system²², were K₂O: ((1−x) Gd₂O₃ + x Pr₂O₃): P₂O₅ = 36: 4: 60, mol %, with x ranging from 0 to 0.05.

A platinum stirrer with a diameter of 18 mm, located at 12–14 mm below the solution surface and rotating at 55 rpm with a change of rotation direction every 50 s, was used to mix the solution. This stirring was needed because of the high level of dynamic viscosity of the growth solution, around 19 Pa·s⁴³. An a* oriented KGd(PO₃)₄ seed was placed in contact with the surface of the solution at 12 mm from the solution centre, rotating with the same angular velocity as the stirrer. The use of this crystallographic direction in the KGd(PO₃)₄ seeds leads to the growth of high crystalline quality crystals^16,22. The crystallographic c direction of the seed was oriented in radial direction, while its b direction was tangential to the rotation movement in order to achieve a good aerodynamic orientation of the crystal during its movement. Note that the morphology of this crystal usually presents an edge perpendicular to the b crystallographic direction, while it has natural {001} faces. With the aim of further improving the mixing of the solution, high thermal gradients (around 11 K·cm⁻¹ in depth, with the hottest point at the bottom) were applied.

Once the solution was homogeneous, its saturation temperature was determined by accurately measuring the growth/dissolution rate of the KGd(PO₃)₄ seed depending on the temperature, which will then be used to start the growth of the single crystal. Beginning at the saturation temperature of the solution, cooling rates of 0.1 K·h⁻¹ for the first 15 K and 0.05 K·h⁻¹ for the next 10–15 K were applied to create supersaturation and grow the single crystal. At the beginning of the crystal growth experiments, the cooling rate was higher in order to initiate growth and not lose contact between the crystal seed and the solution. During this initial cooling rate regime, the supersaturation of the solution increases gradually because of the difficulty in mixing the solution due to its high viscosity. After decreasing the temperature of the furnace 15 K, a second cooling rate was applied that was slower than the first to avoid an additional increase in the supersaturation of the solution, since this could induce nucleation in different points of the solution and also inclusions of solution inside the crystals. The growth rate can be maintained even with a slower cooling rate due to the accumulated supersaturation of the solution and the larger crystal surface.

After finishing the thermal cooling ramps, the crystal was removed from the solution and maintained at a few mm above the surface of the solution while the furnace was cooled to room temperature at a rate of 20–25 K·h⁻¹.

Electron probe microanalysis (EPMA) with wavelength dispersive spectrometry (WDS) was used to determine the Pr content of the crystals. In this non-destructive technique, an electron beam is focused on the sample and the characteristic X-rays emitted (specific to each element of the sample) are dispersed by crystals (WDS) before being recorded and compared with the emission of standard compounds containing the elements to be analysed. The X-rays of the sample and the standards are obtained under the same measurement conditions. The equipment used was a JEOL JXA-8230. The standards used were an undoped KGd(PO₃)₄ single crystal for K, Gd, P and O measurements and an REE-1 for determining the Pr concentration. An accelerating voltage of 20 kV and a current of 20 nA were applied, with measuring time of 10 s for K, P, Gd and O and 100 s for Pr peaks and 5 s and 50 s for background measuring, respectively. Kα lines of K, P and O and Lα lines of Gd and Pr were used. The dispersive crystals were PETJ for K, PETH for P, LDE1 for O, and LIFL for Gd and Pr measurements. The detection limit of Pr³⁺ was around 105 ppm.

Structural characterization and thermal stability

The evolution of the unit cell parameters of Pr:KGd(PO₃)₄ with the Pr³⁺ content was studied by X-ray powder diffraction measurements, using a D5000 Siemens X-ray powder diffractometer in vertical θ-θ configuration with the Bragg-Brentano geometry. The X-ray diffraction patterns of undoped KGd(PO₃)₄ and 1, 2 and 5 atomic % Pr:KGd(PO₃)₄ in solution were obtained using Cu Kα radiation and recorded in the 2θ range from 10 to 70°. The measurements were made with a step size of 0.03° and a step time of 7 s. The unit cell parameters were refined using the TOPAS program²⁰, the Le Bail method²⁷ and the crystal data for undoped type III KGd(PO₃)₄ studied by Parreu et al.²⁸ (171710 ICSD database).

The thermal stability of the KGd(PO₃)₄ doped with praseodymium was studied by X-ray powder diffraction. The equipment used was a Bruker-AXS D8-Discover diffractometer equipped with a Cu source, a parallel incident beam (Göbel mirror), a HI-STAR GADDS (general area detector diffraction system) and a MRI BTS-Solid temperature chamber with a platinum ribbon heating stage. The powder samples were placed in the centre (occupying an area of ~1 × 1 mm²) on the platinum ribbon. This stage was covered with a beryllium dome to maintain temperature. The sample was heated and cooled at a rate of 10 K·min⁻¹. Diffraction patterns in the heating and cooling cycles were recorded every 15 K between 1048 and 1273 K and twice at room temperature, one diffraction pattern before the heating process and the other after the cooling process. The measurements were made in the 2θ range from 18 to 52° (one frame) with a detector-sample distance of 15 cm, an exposition time of 300 s per frame and a delay time of 60 s before each frame.

To complement the study of the thermal stability of the KGd(PO₃)₄ doped with praseodymium, differential thermal and thermogravimetric analyses (DTA-TGA) were performed using a TA Instruments SDT 2960 Simultaneous DSC-TGA. Al₂O₃ was used as the reference material, and the heating and cooling rates were at 10 K·min⁻¹ with an air flux of 90 cm³·min⁻¹.

The evolution of the unit cell parameters of the crystals grown from a 5 atomic % Pr:KGd(PO₃)₄ in solution with temperature was also studied by X-ray powder diffraction. The equipment was the same D5000 Siemens X-ray powder diffractometer previously used to study the Pr:KGd(PO₃)₄ unit cell parameters, but with an Anton-Paar HTK10 temperature chamber with a platinum ribbon heating stage. The sample was placed in the centre (occupying an area of ~9 × 5 mm²) on the platinum ribbon. The diffraction patterns were recorded at temperatures of 298, 323, 373, 473, 573, 673 and 773 K (in which the monoclinic P2₁ crystalline phase of KGd(PO₃)₄ is stable), in the 2θ range from 10 to 70° with a step size of 0.03°, a step time of 5 s and a delay time of 300 s before each measurement. As before, the unit cell parameters were refined using the TOPAS program²⁰, the Le Bail method²⁷ and the crystal data for undoped type III KGd(PO₃)₄ studied by Parreu et al.²⁸ (171710 ICSD database).

Optical characterization

The bulk single crystals obtained were cut in plates perpendicular to the crystallographic a*, b and c* directions with a diamond saw. The plates were initially lapped and then polished with Al₂O₃ particle solutions to a size of 0.1 μm using a Logitech polishing machine. These plates were used for the optical absorption and emission studies. The unpolarized optical absorption of Pr³⁺ in KGd(PO₃)₄ was studied using a CARY 5000 UV-Vis-NIR spectrophotometer at room temperature in the wavelength range from 205 to 2475 nm, while the unpolarized optical absorption of undoped KGd(PO₃)₄ was studied in the wavelength range from 190 to 315 nm.

The emission spectroscopy was studied under vacuum ultraviolet-ultraviolet (VUV-UV) excitation in the wavelength range from 120 to 248 nm (10–5 eV). Experiments were performed in the DESIRS beamline at SOLEIL Synchrotron, France (proposal number 20151215, standard). The samples were placed in a vacuum chamber which can be evacuated to a pressure below 2 × 10⁻⁵ bars. A lithium fluoride window at the entrance of the vacuum chamber separates it from the synchrotron line. The monochromatized synchrotron light reached the sample at an angle of 90°. The emitted light from the sample was collected at 45°, focused with a silica lens and analysed with an Ocean Optics Jaz spectrometer (minimum spectral resolution 0.3 nm). The emission spectra were recorded in the range from 192 to 886 nm. To obtain the excitation spectra, the intensity obtained for a particular emission wavelength was plotted in front of the excitation wavelength in the excitation wavelength range from 120 to 248 nm.

Lifetime measurements were also carried out in the DESIRS beamline of SOLEIL Synchrotron, France (proposal number 20161324, standard) in a single bunch mode operation for pulsed radiation. The same configuration in the vacuum chamber as in previous measurements was used. The light was guided with an optical fibre to an ANDOR spectrograph (Shamrock 193i) with a grating of 150 lines·mm⁻¹, coupled to an iStar Intensified Charge Coupled Device (ICCD) (model DH734-18F-03). When the level of vacuum in the chamber was lower than 2 × 10⁻⁵ bars, the window between this chamber and the synchrotron was removed in order to increase the photon flux reaching the sample.

The decay time of the 4 f → 4 f electronic transitions, which was significantly slower than those of the 5d energy levels, was measured with a Cary Eclipse fluorescence spectrophotometer.