Structural phase transitions and photoluminescence properties of oxonitridosilicate phosphors under high hydrostatic pressure

Spectroscopic properties of a series of (Sr0.98-xBaxEu0.02)Si2O2N2 (0 ≤ x ≤ 0.98) compounds has been studied under high hydrostatic pressure applied in a diamond anvil cell up to 200 kbar. At ambient pressure the crystal structures of (Sr0.98-xBaxEu0.02)Si2O2N2 (0 ≤ x ≤ 0.98) are related to the ratio of strontium to barium and three different phases exists: orthorhombic Pbcn(0.78 ≤ x ≤ 0.98), triclinic P1 (0 < x ≤ 0.65) and triclinic P1 (0.65 < x < 0.78). It was found that Eu2+ luminescence reveals abrupt changes under pressure (decay time, energy and shape) which indicate the variation of the local symmetry and crystal field strength in Eu2+ sites. These changes are attributed to the reversible pressure-induced structural phase transitions of triclinic (Sr0.98-xBaxEu0.02)Si2O2N2 into orthorhombic structure. Pressure in which phase transition occurs decreases linearly with increasing of Ba composition in (Sr0.98-xBaxEu0.02)Si2O2N2 series. Additionally, very different pressure shifts of the Eu2+ luminescence in different phases of (Sr0.98-xBaxEu0.02)Si2O2N2:Eu from −40 cm−1/kbar to 0 cm−1/kbar have been observed. This effect is explained by different interaction of the Eu2+ 5d electron with the second coordination sphere around the impurity cations.

Rare earth activated oxonitridosilicate compounds have been widely explored as advanced phosphors in white emitting diodes (WLEDs) due to very efficient and thermally stable luminescence that covers a broad spectral range, under excitation of blue and/or UV light [1][2][3][4][5] . Moreover, since these materials have been synthesized under ambient pressure they have a lower production cost than high-pressure synthesized nitridosilicates materials such as M 2 Si 5 N 8 :Eu (M = Ca, Sr, Ba), MAlSiN 3 :Eu (M = Ca, Sr) [5][6][7][8][9][10] . Among oxonitridosilicate phosphors, there is an increasing interest in layered MSi 2 O 2 N 2 :Eu (M = Ca, Sr, Ba) which show efficient yellow, green and cyan emission, respectively [11][12][13][14][15][16][17] . The present research in rare-earth ions doped MSi 2 O 2 N 2 (M = Ca, Sr, Ba) mainly focuses on the development of an appropriate host composition by mixing alkaline earth metal cations to adjust their emission properties. For example, structure variations in MSi 2 O 2 N 2 :Eu (M = Ba, Sr) with gradually substitution of Ba with Sr leads to continous or abrupt changes in luminescence properties of these compounds 14,15,18,19 .
Based on high resolution synchrotron XRD measurements, it has been shown that crystal structures of the (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 (0 ≤ x ≤ 0.98) are related to the ratio of strontium to barium. Depending on barium content three different phases exist: Phase A (0.78 ≤ x ≤ 0.98), Phase B (0 < x ≤ 0.65) and Phase C (0.65 < x < 0.78) 18 . Phase A and Phase B are isostructural to well-known crystal structures of BaSi 2 O 2 N 2 and SrSi 2 O 2 N 2 , respectively, whereas Phase C adopts a distorted variation of the BaSi 2 O 2 N 2 type structure 14 . The crystal structure of Phase A with orthorhombic unit cell is described by Pbcn space group, while Phase B with a triclinic unit cell is described by P1 space group. Phase C is a new crystal structure that is different from Phase A and Phase B and can be regarded as a "transition state" between them, since it combines the unit cell-metrics of the Phase A type with corrugated metal-ion layers found in the Phase B. Nominally Phase C with triclinic unit cell is attributed to P1 space group. Detailed studies on the high resolution transmission electron microscopy and scanning electron diffraction analysis of selected areas, have shown that Phase C usually contains a small amount of nanocrystalline domains of Phase B with a triclinic unit cell 18,19 . This could be due to inhomogenous distribution of Ba and Sr components in the mixed (Sr 0.98-x Ba x )Si 2 O 2 N 2 crystals. In (Sr0.98-xBaxEu0.02)Si 2 O 2 N 2 , the barium, strontium and europium ions are in the channels, which are formed by [SiON 3 ] tetrahedra and forms linear chains in the structure. In the case of Phase A and Phase C structures, cation chains are located in a small distance from each other, which is almost equal to the intra-chain distance forming a plane in the structure, while in the case of Phase B cation chains are separated from each other much more than the intra-chain distance. Thus the second coordination sphere around Eu 2+ ions in Phase A and Phase C consist of Sr 2+ /Ba 2+ cations forming rectangle while in the case of Phase B the Sr 2+ /Ba 2+ cations form a line. Cation distribution in the second coordination sphere can results in a very different spectroscopic properties of Eu 2+ luminescence. Poort et al. 20 have suggested that electron-lattice interaction energy, manifested by large Stokes shift between emission and absorption spectra is large when Eu 2+ ions occupy the lattice sites that belongs to the linear cationic chain.
The high pressure study of the (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 doped with Eu 2+ presented in this contribution is continuation and extension of the research work presented in ref. 18. where the relations between the luminescence properties and the content of Ba and Sr have been discussed.
It is well known that pressure is an important physical parameter that can induce the structural phase transitions 21,22 . X-ray diffraction study of (Sr 0.98 Eu 0.02 )Si 2 O 2 N 2 under hydrostatic pressure has shown that in pressure range up to 96.5 kbar triclinic structure was conserved 23 . The main motivation on performing high hydrostatic pressure was to investigate whether high pressure influences the structural stability of (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 (0 < x ≤ 0.98) to induce phase transitions. Phase transitions were identified trough a comparison of the luminescence and Raman properties under pressure. High pressure, which permits us to continuously vary structural parameters, allow to answer question whether phase transitions are related to structural parameters like bond length and lattice constants. To achieve that, complementary studies of chemical pressure (through replacement of Sr 2+ by larger Ba 2+ ) and external high hydrostatic pressure up to 200 kbar (20 GPa) applied in diamond anvil cell (DAC) have been carried out. Finally, we have developed the phase diagram of (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 which shows the relation between chemical and external pressure compression effects.
Properties of complex consisting of Eu 2+ ions and its negatively charged coordination sphere can be described using crystal field approximation in such case high hydrostatic pressure causes the increase of the energies of the 4f 7 , 4f 6 5d 1 electronic configuration with respect to the host bands energies and the increase of the 4f 6 5d 1 levels splitting 24,25 . The superposition of these effects are seen as a pressure-induced diminishing of the energy of luminescence related to the 4f 6 5d 1 → 4f 7 transition. The influence of the second coordination sphere may bring about drastic changes in this effect, since it can magnify or cancel the influence of the first coordination sphere. As a result, different pressure behavior of the luminescence band related to the 4f 6 5d → 4f 7 transition of Eu 2+ ions in different phases is expected.
The study of luminescence under high hydrostatic pressure results in better understand the relationship between pressure and crystal phases of oxyntridosilicate phosphors, and further clarify the influences of these phase transitions on photoluminescence properties, which might be general to oxynitride materials and will be useful in tuning optical and other properties that are sensitive to local coordination environments.

Results
For spectroscopic studies the representatives of three different phases were selected: ( Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 system for x = 0.98 (Phase A) x = 0.49 (Phase B) and x = 0.75 (Phase C) are presented in Fig. 1a-c, respectively. To interpret the spectra the first and the second coordination sphere in the vicinity of Eu 2+ ions in different crystal phases were considered. The first and the second coordination of Eu 2+ in Sr 2+ /Ba 2+ sites in Phase A, B and C are presented in Fig. 2a-c, respectively. PLE spectra consist of several bands related to the parity allowed transitions from the ground state 4f 7 ( 8 S 7/2 ) to excited states of the 4f 6 5d 1 electronic configuration of Eu 2+ ions. The PL spectra consist of broad band(s) which is (are) related to the 4f 6 5d 1 → 4f 7 (d-f) transition of Eu 2+ ions. The maximum of luminescence band varies from blue to green spectral region, depending on the ratio of Ba 2+ to Sr 2+ . The PL spectrum of the (Ba 0.98 Eu 0.02 ) Si 2 O 2 N 2 consists of a single band peaking at 495 nm with a full width at half maximum (FWHM) equal to 32 nm (see Fig. 1a). In the orthorhombic phase (space group Pbcn, Phase A) 18 of (Ba 0.98 Eu 0.02 )Si 2 O 2 N 2 , Eu 2+ ions can occupy Ba 2+ site which is coordinated by eight oxygen ions creating cuboid and two long ranged nitrogen ions (see Fig. 2a) 13,14 . The Ba-O bond lengths range from 2.74 Å to 3.10 Å and the average distance is equal to 2.89 Å whilst Ba-N distances are equal for booth nitrogen and the distance is equal to 3.31 Å.
We consider this to be the first coordination sphere. The Ba 2+ ions form linear chains in the lattice. The distance between two separate Ba 2+ chain is almost equal to distance between Ba 2+ ions in a given chain, thus the second coordination sphere consist of an Ba 2+ cations forming rectangle around Eu 2+ central ion (see Fig. 2a).
The PL spectrum of the (Sr 0.49 Ba 0.49 Eu 0.02 )Si 2 N 2 O 2 is presented in Fig. 1b. The spectrum consists of a broad band with a maximum at 560 nm and FWHM equal to 80 nm. In the triclinic phase (space group P1, Phase B) 12,14,16 of the (Sr 0.49 Ba 0.49 Eu 0.02 )Si 2 N 2 O 2 there are four different Sr 2+ /Ba 2+ crystallographic sites, which can be occupy by Eu 2+ ions. These four sites are very similar, and have nearly the same surrounding. Each of them can be depicted as a distorted trigonal prism of six oxygen ions around Sr 2+ /Ba 2+ which is capped by one N ion (see Fig. 2b) 12,16 . The Sr/Ba-O bond lengths range from 2.50 Å to 3.05 Å (the average distance is equal to 2.67 Å) and Sr/Ba-N distance is equal to 2.58 Å.  The Sr 2+ /Ba 2+ ions in this case are in channels, which forms almost linear chains and the distance between two separate Ba 2+ /Sr 2+ chains are longer than between Ba 2+ /Sr 2+ ions in a given chain. Thus the second coordination sphere around central ions consist of two Sr 2+ /Ba 2+ cations forming the line (see Fig. 2b). When Eu 2+ ion is built into this chain the attractive potential of the close positive charges orients and elongates the 5d orbital of Eu 2+ into the chain direction partially out of the first coordination sphere 20 . As the result, the 5d electron is more delocalized than in the case of Phase A, where no preferential orientation and elongation of 5d orbital occurs. Such delocalization of the 5d 1 electron in Eu 2+ is responsible for the fact that the energy of electron-lattice interaction in the excited state of Eu 2+ is larger than in Phase A and results in a lower energy of the luminescence band and larger FWHM.
The PL spectra of the (Sr 0.23 Ba 0.75 Eu 0.02 )Si 2 O 2 N 2 obtained with different excitation wavelength are presented in Fig. 1c. Under UV to blue excitation, the emission spectrum consists of two broad bands (solid curve). The first band peaking at 470 nm with FWHM equal to 41 nm is attributed to the d-f emission of Eu 2+ occupying Sr 2+ /Ba 2+ positions in Phase C (triclinic P1). In this phase there is one Ba 2+ /Sr 2+ site, which can be depicted as a distorted cuboid of eight O 2− ions (mean distance between oxygen ions and central ion is equal to 2.92 Å) and two long-ranged N 3− ions 3.14 Å and 3.57 Å.
It is worth noting that among these three environments of Eu 2+ ions (Phase A, B, C) the average distance between the Eu 2+ ion and nearest O/N ions (first coordination sphere) is the largest in Phase C and smallest in Phase B. Similar but smaller is in the phase A. The differences in the average distances result in the differences in the crystal field splitting of the 4f 6 5d electronic manifold of Eu 2+ ion. The smallest crystal field splitting hence the maximum of Eu 2+ luminescence has the highest energy in Phase C. The strong shift of Eu 2+ luminescence into red in Phase B is the result of the largest crystal field splitting.
The second coordination sphere around central ions consists of the Ba 2+ cations forming rectangle around central ion (see Fig. 2c). This situation is similar to the case of Phase A, and there is no preferential orientation in the space and elongation of d orbitals, thus the Eu 2+ luminescence is observed at high energy with a small FWHM. The second band in the yellow range of the spectrum is attributed to the occurrence of a small amount of nanocrystalline domains that contains less Ba than x = 0.75 (domains of Phase B). This could be due to inhomogenous distribution of a Ba and Sr components in the mixed (Sr 0.98-x Ba x )Si 2 O 2 N 2 crystals.
Emission spectra related to Eu 2+ in domains of Phase B were obtained when the (Sr 0.23 Ba 0.75 Eu 0.02 )Si 2 O 2 N 2 was excited with wavelength 488 nm (dotted curve in Fig. 1c). This emission consists of a broad band peaked at 575 nm with FWHM equal to 83 nm. Position of the maximum of the luminescence band allows to relate this emission to the Phase B domains of (Sr0.98-xBaxEu0.02)Si 2 O 2 N 2 with concentration x = 0.63 18 .
Photoluminescence spectra of the (Sr0.98-xBaxEu0.02)Si 2 O 2 N 2 (x = 0.98, 0.49, 0.75) measured at different high hydrostatic pressures are presented in Fig. 3(a-d). Energies of the (Sr0.98-xBaxEu0.02)Si 2 O 2 N 2 (x = 0.98, 0.49 and 0.75) emission maxima versus pressure are presented in Fig. 4. In Fig. 4  In the case of the (Ba 0.98 Eu 0.02 )Si 2 O 2 N 2 (Phase A) a red shift of the 4f 6 5d 1 − 4f 7 emission band is observed (see Fig. 3a). The energy of the emission maximum shifts linearly with increasing pressure with a rate of about-20 cm −1 /kbar (see Fig. 4, green circles).  Fig. 3(c,d). In Fig. 3(c) the 4f 65d1 − 4f 7Eu 2+ emission, obtained under excitation with wavelength 442 nm is presented. When pressure increases up to 20 kbar, energy of the maximum of emission band decreases (the red shift is approximately equal to − 40 cm −1 /kbar). Above 20 kbar, the red shift rate diminishes and is approximately equal to − 20 cm −1 /kbar (see solid blue triangles in Fig. 4) which is similar to that observed for orthorhombic (Ba 0.98 Eu 0.02 ) Si 2 O 2 N 2 and high pressure orthorhombic (Sr 0.49 Ba 0.49 Eu 0.02 )Si 2 O 2 N 2 structures. We propose that this change in the rate of the PL pressure shift, is the result of phase transition from triclinic (Phase C) to orthorhombic (Phase A) phase of the (Sr 0.23 Ba 0.75 Eu 0.02 )Si 2 O 2 N 2 system. Room temperature luminescence decays of (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 for x = 0.98 (Phase A), x = 0.49 (Phase B) and x = 0.75 (Phase C and domains of Phase B) recorded at different pressures were monitored at the maximum of the 5d 1 4f 6 → 4f 7 transition. The example of the PL decays for the (Sr 0.49 Ba 0.49 Eu 0.02 )Si 2 O 2 N 2 are shown in Fig. 5. All decays related to the Eu 2+ emission in compounds under study were close to single exponential. The decay times versus pressure for all considered samples are collected in Fig. 6. The PL decay time obtained for the (Ba 0.98 Eu 0.02 ) Si 2 O 2 N 2 (green circles), at ambient pressure is equal to 0.38 μ s and increase gradually to 0.54 μ s at 200 kbar. In the absence of nonradiative processes this behavior can be expected because the luminescence lifetime elongates as the emission is red shifted with increasing pressure 26 .
The PL decay time obtained for the (Sr 0.49 Ba 0.49 Eu 0.02 )Si 2 O 2 N 2 (orange squares) at ambient pressure is equal to 1.13 μ s. The decay time slightly decrease with increasing pressure to 0.89 μ s at 50 kbar which is accompanied by the decrease of emission intensity and at 50 kbar luminescence with maximum at 560 nm vanishes completely.   One should discuss the phenomenon of the Eu 2+ luminescence in Phase B. In contrast to Phases A and C the Eu 2+ emission in Phase B consists of broad band characterized by FWHM equal to 83 nm, which decays with the decay time 1.2 μ s, whereas the Eu 2+ emission in A and C phases is represented by much sharper band, characterized by FWHM equal to 41 nm decaying with the lifetime ~0.36 μ s. Additionally very different pressure shifts of the 4f 6 5d → 4f 7 luminescence of Eu 2+ in the Phases A and C (where it is equal to − 20 cm −1 /kbar and − 40 cm −1 / kbar, respectively) and in Phase B (where it is equal zero) have been observed. This different pressure dependences of the emission can be attributed to different interaction of the Eu 2+ system in the 4f 6 5d 1 state with the second coordination sphere of positive ions. In the case of Eu 2+ in (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 in Phase B, the 5d electron of Eu 2+ is attracted by the positively charged chain and therefore is delocalized and can be partly located out of the first coordination sphere of negative ions. This results in large Stokes shift and FWHM of the luminescence, actually much larger than in the case of the 4f 6 5d → 4f 7 luminescence of Eu 2+ in (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 in Phase A and Phase C.
Since pressure decrease distance between Eu 2+ ion and ligand ions, it is expected that the pressure induces the red shift of the d-f luminescence due to the increase of crystal field splitting of 5d electronic manifold. This effect has been previously observed in many different compounds 27 . The first coordination sphere of the Eu 2+ in (Sr 0.98-x Ba x Eu 0.02 ) Si 2 O 2 N 2 is build with negative ions having approximately cubic symmetry. These causes the splitting of the 5d orbital into doubly degenerated the lower state (e) and triply degenerated the higher state (t) 28 . In the (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 having Phase A the second coordination sphere is built with four positive ions (approximately C 4v symmetry) which can be considered as an octahedron without vertical ions. This configuration magnifies the splitting caused by first configurational sphere and additionally splits the double degenerated the lower state (e) into the non-degenerated a 1 and b 1 states. As the result large pressure shift of the luminescence related to the 4f 6 5d → 4f 7 luminescence Eu 2+ emission in Phases A and C is observed. Due to the delocalization of the 5d electron in (Sr 1-x Ba x Eu 0.02 )Si 2 O 2 N 2 in Phase B the energy of the lowest state of the 5d 1 4f 6 electronic manifold can be not sensitive to actual positions of negative ions forming the first coordination sphere and finally does not dependent on pressure.
Raman spectroscopy has been applied to confirm structure changes of (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 for x = 0.1, 0.3, 0.49, 0.75 and 0.98 and pressure-induced phase transition of the (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 . The ambient pressure Raman spectra of (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 for different Ba concentrations are presented in Fig. 7. The crystal structure of oxynitridosilicates (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 are built of SiON 3 tetrahedral layers with the same general topology for all samples. It is expected that internal stretching and bending vibrations of the tetrahedral SiON 3 groups contribute to Raman peaks in the range 400-1200 cm −1 for all samples 29,30 . In this spectral region, the Raman spectrum of (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 for x = 0.98 is dominated by an intense lines with energies equal to 415 cm −1 (labeled ν a ) and 1020 cm −1 and for x ≤ 0.75, 401~405 cm −1 (labeled ν c and ν b ) and 1020 cm −1 . Difference between position of ν a with respect to ν b and ν c peaks are due to different degree of distortion of SiON 3 groups. The external vibrations involving the entire SiON 3 as well as their neighboring Sr 2+ /Ba 2+ ions To verify our hypothesis about pressure-induced phase transformations from triclinics (Phase B and C) to orthorhombic phase (Phase A) Raman spectra of the (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 for x = 0.49, 0.75 and 0.98 have been measured at different pressures up to ~80 kbar and the results are presented in Fig. 8a-c. It should be noted here that in our experiments the Eu 2+ luminescence with laser beam excitation of 632 nm contributes greatly to the background level causing the Raman spectra were very difficult or even impossible to measure under pressure greater than 80 kbar. In the case of (Ba 0.98 Eu 0.02 )Si 2 O 2 N 2 (see Fig. 8a). It is seen that due to the decrease of bonding length, energy of all Raman peaks increases with pressure. This increase is observed with a slightly different pressure rate for all lines causing that some of unresolved peaks at ambient pressure (see for example ν a ) start to be resolved at higher pressure, but the main Raman pattern and number of peaks does not change with pressure. These result confirms findings from luminescence studies, that the orthorhombic phase of (Ba 0.89 Eu 0.02 )Si 2 O 2 N 2 is preserved in measured pressure range. Figure 8b shows high-pressure Raman spectra of (Sr 0.49 Ba 0.49 Eu 0.02 ) Si 2 O 2 N 2 . The most interesting effect occurs above 32 kbar where a significant change in the Raman lines structure is observed. Let us focus on the most intensive vibration peak ν b related to the internal stretching and bending vibrations of the SiON 3 groups. It is seen that due to the increase of Si-O/N bonding strength energy of ν b increases when pressure increases. In the range of pressure between 40 kbar and 50 kbar additional peak labeled ν a appears and simultaneously intensity of ν b decreases. Above 50 kbar peak ν a remains exclusively. The ν a Raman line can be attributed to an orthorhombic structure of the (Ba 0.98 Eu 0.02 )Si 2 O 2 N 2 . The appearance of ν a peak at ~42 kbar confirms the phase transition of the (Sr 0.49 Ba 0.49 Eu 0.02 )Si 2 O 2 N 2 from triclinic to orthorhombic phase. Figure 8c shows high-pressure Raman spectra of (Sr 0.23 Ba 0.75 Eu 0.02 )Si 2 O 2 N 2 which is representative of Phase C. Also in this case we have observed significant changes in Raman spectra but at lower pressure of about 26 kbar suggesting a structural transition occurs at this point. At this pressure additional peak labeled ν a appears and simultaneously intensity of ν c decreases. Also obvious changes in Raman spectra below 400 cm −1 are observed. Upon decompression, the luminescence and structural properties of the (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 (x = 0.98, 0.49 and 0.75) of these crystals returns to ambient state. It means that the high-pressure induced phase transition of the (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 (0.49 and 0.75) is reversible.
In summary we developed the phase diagram of (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 which has been shown on Fig. 9. The dotted lines represent the phase boundaries between different phases in the pressure and Ba content space. Two points corresponding to pressures of phase transitions for (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 x = 0.3 and x = 0.1 (black squares in Fig. 9) were obtained at high pressure luminescence experiments, however the respective luminescence and Raman spectra are not presented in this contribution. The phase diagram shows that pressure in which phase transition between Phase B and A occurs decreases linearly with increasing Ba concentration in (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 . The Boundary between Phase B and A is indicated by dotted red line. For concentration rage 0.65 < x < 0.78 and pressure lower than 20 kbar the (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 system exists in Phase C. The boundary between Phase B and C is indicated by dotted black line. SrCO     initial chemicals were used without further purification. Aluminum oxide crucibles and cylindrical molybdenum crucibles (20 mm × 50 mm) were used in the sintering process of the samples.

Synthesis.
A series of oxonitridosilicate compounds, (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 (0.1 ≤ x ≤ 0.98), was prepared using two-step solid-state reaction processes. Nonstoichiometric amounts of SrCO 3 , BaCO 3 , SiO 2 , and Eu 2 O 3 powders, in which the molar ratio of (Sr, Ba)/Si = 0.8 were ground in an agate mortar for 30 min to form a homogeneous mixture. The mixture was then placed in aluminum oxide crucible and fired at 1250 to 1350 °C for 6 h under flowing 95%N 2 − 5% H 2 atmosphere in a horizontal tube furnace. The sintered products were ground, yielding crystalline powder (Sr 0.98-x Ba x Eu 0.02 )Si 2 O 2 N 2 (0.1 ≤ x ≤ 0.98). The crystalline powder was mixed with stoichiometric amounts of Si 3 N 4 and reground for 30 min in an agate mortar. After forming a homogeneous mixture, the mixture was loaded into a cylindrical molybdenum crucible with a screw-cap and fired again at 1500 to 1550 °C for 6 h under flowing 95% N 2 − 5%H 2 atmosphere in the horizontal tube furnace. The second sintered products were ground again, yielding the resulting phosphor powder. The details of samples preparation and ambient pressure structural characterization were presented and discussed in paper 18 . Spectroscopic characterization. Raman spectra were recorded at room temperature and ambient pressure on a Horiba JobinYvon Lab Ram Aramis spectrometer with a He-Ne laser providing excitation light at 633 nm with the 1200 l/mm grating.
Photoluminescence excitation spectra were acquired using a FluoroMax-4P TCSPC spectrofluorometer produced by Horiba, containing Czerny-Turner monochromators for excitation and emission. An excitation source in this system was a 150-W ozone-free Xenon lamp. Fluorescence intensity was measured using a R928 Side-on photomultiplier. Steady state luminescence spectra were excited with He-Cd laser with the wavelength of 442 nm. The photoluminescence spectra were recorded on a SR-750-D1 luminescence spectrometer with an Andor CCD camera DU420A-OE type, which can detect signal on large wavelength range 250-1000 nm with an accuracy of 0.5 nm. The spectra were corrected for instrumental spectral response using a standard lamp.
The experimental setup for luminescence kinetics consists of a PL 2143 A/SS laser as the excitation source and a PG 401/SH parametric optical generator. This system can generate 30 ps laser pulses, with the frequency of 10 Hz with wavelengths ranging from 220 to 2200 nm. The emission signal was analyzed by a Bruker Optics 2501S spectrometer and the Hamamatsu Streak Camera model C4334-01 with a final spectral resolution 0.47 nm. Luminescence decays were obtained by the integration of streak camera images over the wavelength intervals. Details of the experimental setup are described in the paper 31 . High hydrostatic pressure was applied in a Merrill Bassett type DAC 32 . Polydimethylsiloxane oil was used as the pressure-transmitting medium, and pressure was measured by the shift of the R 1 luminescence line of ruby.