New Insight into Phase Formation of MxMg2Al4+xSi5−xO18:Eu2+ Solid Solution Phosphors and Its Luminescence Properties

Here we reported the phase formation of MxMg2Al4+xSi5−xO18:Eu2+ (M = K, Rb) solid solution phosphors, where M+ ions were introduced into the void channels of Mg2Al4Si5O18 via Al3+/Si4+ substitution to keep the charge balance. XRD results revealed that the as-prepared phosphors with different M+ contents were iso-structural with Mg2Al4Si5O18 phase. The combined analysis of the Rietveld refinement and high resolution transmission electron microscopy (HRTEM) results proved that M+ ions were surely introduced into the intrinsic channels in Mg2Al4Si5O18. The emission peaks of MxMg2Al4+xSi5−xO18:Eu2+ (M = K, Rb) phosphors with various x values performed a systematic red-shift tendency, which was ascribed to the elongation of [MgO6] octahedra. The temperature stable photoluminescence and internal quantum efficiency (QE) of MxMg2Al4+xSi5−xO18:Eu2+ (M = K, Rb) phosphors were enhanced owing to the filling of M+ in the void channels suggesting a new insight to design the solid solution phosphors with improved photoluminescence properties.

Silicates have attracted great attention in recent years due to their high chemical stability, heat stability, low cost, excellent weather resistance and variety of crystal structures 1 . Amongst them, cordierite silicates have been widely used as a high quality refractory materials, integrated circuit board, catalyst carrier, ceramic foam and aviation materials etc., which are attributed to its easy preparation, fire resistance, good thermal shock resistance, and mechanical properties resistant to corrosion at higher temperature 2,3 . The cordierite compound is represented by a magnesium/aluminum aluminosilicate with the crystallo-chemical formula Mg 2 [6] Al 3 [4] (Si 5 Al [4] O 18 ). Herein, Mg 2 Al 4 Si 5 O 18 has a complex structure with six tetrahedral units [(Si/Al)O 4 ], forming Si 6 O 18 -type 6-membered rings with one Al substituted for one Si in the ring [4][5][6][7][8][9]

Results
Phase and crystal structure analysis. Figure 1    viewpoints, one is that the Eu 2+ can enter the void channels, and the other viewpoint think that Eu 2+ will replace the cations. Piriou et al. demonstrated that the Eu ion cannot enter the void channels based on the site-selective spectroscopy 10 . In the present case, it is assumed that Eu 2+ (r = 1.17 Å when coordinate number (CN) = 6) ions will occupy the Mg 2+ (r = 0. 72 Å when CN = 6) sites, because both the Al 3+ (r = 0.39 Å when CN = 4) and Si 4+ (r = 0.26 Å when CN = 4) sites are too small to accommodate the Eu 2+ ions. In order to further analyze the crystal structure of the as-prepared samples, the Rietveld structural refinement for these samples were performed using TOPAS 4.2 (Bruker AXS TOPAS V4: General profile and structure analysis software for powder diffraction data. -User's Manual. Bruker AXS, Karlsruhe, Germany. 2008.). Figure S1 (electronic supporting information) demonstrates the observed, calculated, and difference patterns. Based on the Rietveld refinement results, negligible amounts of impurity phases were identified in the samples, and all of these samples exhibit the same crystalline hexagonal crystal system with a space group P6/mcc. The final weighted R factors (R wp ) of the samples were successfully converged at a satisfactory level, and the refined structural parameters of these samples are listed in Table  S1. The unit cell parameters and Al 3+ /Si 4+ ratio in tetrahedra become larger with increasing M + content, which is ascribed to the fact that the M + were introduced into the void channels.  (Fig. 3c,f,i) confirm the presence of all the elements (including the C and Cu from the sample holder), which were detected from one complete microcrystal (red square region) (Fig. 3c,f,i). The Eu element can't be detected clearly due to its low concentration, but it can be confirmed by the emission spectra in the following section.  Fig. 4a,b. Therefore Rb + leads to bigger red-shift amount than that of K + , which could be due to the difference of the ion radii of Rb + and K + , so that the larger structural distortion can be expected. Furthermore, Fig. 5 presents the room temperature decay curves of Eu 2+ luminescence in K x Mg 2 Al 4+x Si 5−x O 18 :0.03Eu 2+ (a) and Rb x Mg 2 Al 4+x Si 5−x O 18 :0.03Eu 2+ (b) series, respectively. All the decay curves can be well fitted with a second order exponential equation: where I is the luminescence intensity, A 1 and A 2 are constants, τ is the time, τ 1 and τ 2 are rapid and slow lifetimes for exponential components, respectively. Furthermore, the effective lifetime constant (τ * ) can be calculated as: Scientific RepoRts | 5:12149 | DOi: 10.1038/srep12149 Based on the decay curves in      inferior to that of BAM, this phosphor has comparatively good temperature quenching effect and the thermal stability can be further enhanced via the optimization of composition and preparation experiment. Generally, the thermal quenching of emission intensity can be explained by the temperature dependence of the electron-phonon interactions in the luminescence center and thermally activated photo-ionization of lanthanide. These two mechanisms are strongly related to the crystal structure of host lattices and crystallinity of the phosphors, and are based on the observed thermal quenching rates 13,14 .
In order to give a quantitative analysis of the thermally stable luminescence behaviors, the Arrhenius equation was employed to calculate the respective activation energy as follows 15,16 : where I 0 is the initial PL intensity of the phosphor at room temperature, I T is the PL intensity at different temperatures, c is a constant, ΔE is the activation energy for thermal quenching, and k is Boltzmann constant (8.62 × 10 −5 eV). According to the equation, the activation energy ΔE can be calculated from a plotting of ln[(I 0 /I)−1] against 1/kT, where a straight slope equals -ΔE. As shown in Fig. 6b

Discussion
It is well-known that red-shift behavior of Eu 2+ emission is supposed to appear in M x Mg 2 Al 4+x Si 5−x O 18 : 0.03Eu 2+ with increasing M + concentration. One explanation for this phenomenon is that addition of M + dopants leads to the expansion of the tetrahedra [Al/SiO 4 ] and then the values of d(Mg/Eu-O) decrease (Fig. 7a), so that red-shift in PL spectra can be observed. However, as can be seen in Fig. S3  which are consistent with the variation of the PL emission intensities. As a reference, we have also measured the QE value of BAM, and the value is 48.3% under 365 nm excitation. These results indicated that the QE can be greatly enhanced by introducing M + ions into the channels of Mg 2 Al 4 Si 5 O 18 :Eu 2+ , and these new solid solution phosphors can be potential in the practical use.
In  3 were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China, and the stoichiometric amounts of them were dissolved in ethanol under stirring. After this, the designed amounts of Si(OC 2 H 5 ) 4 (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) were added successfully in the above solution. The resultant mixtures continued to be stirred at 80 °C for 30 min, and then heated at 110 °C for 10 h in an oven until homogeneous gels were formed. After being dried, the gels were ground and treated at 900 °C for 10 h in the air, and then fully ground and sintered at 1330 °C for 4 h under a 10%H 2 -90%N 2 gas mixture. Finally, they were furnace-cooled to room temperature, and ground again into powder for the following measurement.
Structure and optical measurements. The powder X-ray diffraction (XRD) analysis were conducted on a D8 Advance diffractometer (Bruker Corporation, Germany) operating at 40 kV and 40 mA with Cu Kα radiation (λ = 0.15406 nm), and the scanning rate was fixed at 4 o /min. The powder diffraction pattern for Rietveld analysis was collected with the same diffractometer. The step size of 2θ was 0.016°, and the counting time was 1 s per step. Rietveld refinement was performed by using TOPAS 4.2 software. High resolution transmission electron microscopic (HRTEM) images were characterized by a JEOL JEM-2010 microscope with an accelerated voltage of 200 kV. Room temperature excitation and emission spectra were measured on a fluorescence spectrophotometer (F-4600, HITACHI, Japan) with a photomultiplier tube operating at 400 V, and a 150 W Xe lamp used as the excitation lamp. The decay curves were recorded on an Edinburgh instrument (FLSP920) with a nF900 flash lamp was used as the excitation resource. The temperature-dependence luminescence properties were measured on the same F-4600 spectrophotometer, which was combined with a self-made heating attachment and a computer-controlled electric furnace (Tianjin Orient KOJI Co., Ltd, TAP-02). Quantum efficiency was measured using the integrating sphere on the FLSP920 fluorescence spectrophotometer (Edinburgh Instruments Ltd., UK).