High-Pressure Low-Temperature Optical Studies of BaWO4:Ce,Na

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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High-Pressure Low-Temperature Optical Studies of BaWO4:Ce,Na Crystals Damian Wlodarczyk,*,† Lev-Ivan Bulyk,† Marek Berkowski,† Michal Glowacki,† Katarzyna M. Kosyl,† Slawomir M. Kaczmarek,‡ Zbigniew Kowalski,‡ Aleksander Wittlin,† Hanka Przybylinska,† Yaroslav Zhydachevskyy,† and Andrzej Suchocki*,†,§ †

Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland Institute of Physics, West Pomeranian University of Technology in Szczecin, Al. Piastow 17, 70-310 Szczecin, Poland § Institute of Physics, Kazimierz Wielki University, Weyssenhoffa 11, 85-072 Bydgoszcz, Poland

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‡

ABSTRACT: We report detailed optical studies of BaWO4:Ce and BaWO4:Ce,Na single crystals. The material does not emit any luminescence at ambient pressure under near-UV (325 nm) excitation. Efficient green light is emitted only at high pressure (HP) and low temperature (LT). The luminescence is of excitonic character, since the lowest Ce3+ 5d level is degenerate with the conduction band also under hydrostatic pressures. To explain these phenomena, absorption measurements were made together with powder X-ray diffraction (XRD) and confocal micro-Raman and Fourier transform infrared (FTIR) spectroscopy. Raman experiments confirm the existence of a metastable phase, induced by certain nonhydrostatic conditions, before the reversible transition at a high-pressure range above 9 GPa, where efficient photoluminescence (PL) occurs. Although the phase transition is reversible, it proceeds with a prominent hysteresis observed in luminescence and Raman experiments. FTIR focuses on the existence of Ce3+ multisites observed during LT measurements.

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INTRODUCTION In most recent years BaWO4 has gained much more attention in comparison to any other alkaline-earth tungstate reported in the literature.1 This is due to its unique, versatile properties applicable directly to developed technologies. This material is a promising scintillating medium2−4 which can also be used in optoelectronics5−7 (i.e., solid state lasers, optical fibers,4−6 filters8) or as a detector of weak massive particle interactions,9 neutrinoless double-β decays, and radioactive decays of isotopes.10,11 Recent reports have even stated that, in addition to light-emitting diodes,12 this compound could be applied for humidity sensors,13 microwave dielectrics,14 and photocatalyst development.15 Nowadays there is special interest in codoped BaWO4, since its emission in the visible spectral range can be tuned to blue, green, and red light depending on the excitation wavelength.16 Typically, excitation with short-wavelength UV light (210−280 nm) gives blue emission, while longer wavelengths (e.g., 325 nm) induce an additional green component, which is the object of our study. These phenomena are also discussed in many other papers such as those, for example, by Lou17 and Ma et al.18 Since numerous origins of both PL types in BaWO4 have been proposed, ranging from defect centers associated with interstitial oxygen atoms to specific structural disorder, additional crystal lattice studies have been carried out.19−22 Some of them concern not only XRD but also lattice dynamics studied via Raman © XXXX American Chemical Society

spectroscopy, since barium tungstate is a good, scatteringactive crystal (Raman shifter), considered for pump pulse lasers in order to acquire stimulated Raman spectra.15,23−28 BaWO4 crystals belong to the I41/a space group (SG) under ambient conditions.29−32 The W6+ ions are hosted within tetrahedral O2− cells having C4h symmetry. The tetrahedra are separated by compressible, pseudocubic BaO8 dodecahedra with S4 point symmetry.33−35 The complex polymorphism of tungstates under compression has been extensively studied and is still under discussion.36−43 Because of large kinetic barriers associated with nonhydrostatic stress, tungstates can transform from scheelites to the denser monoclinic BaWO4-II HP phase (SG P21/n) through the metastable fergusonite (SG I2/ a).30,31,36−39 Schematic crystallographic structures of those three BaWO4 phases are shown in Figure 1. During these welldocumented transitions the coordination number of tungsten atoms increases from 4 to 6 and that of barium from 8 to 12, thus creating WO6 octahedra connected at the edges and corners with irregular BaO12 polyhedra.44,45 These features greatly influence BaWO4 PL properties. Since barium has one of the largest ionic radii among all A-site cations in tungstates, BaWO4 has the largest energy gap (Eg). This is related to the band structure of these materials, in which the bottom of the Received: December 26, 2018

A

DOI: 10.1021/acs.inorgchem.8b03606 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Instruments and Methods. Luminescence data were gathered using a near-UV He-Cd 325 nm laser and Triax 320 monochromator from ISA Yobin Yvon-Spex equipped with a Spectrum One CCD detector. UV−vis absorption spectra were examined using a Cary 5000 spectrophotometer. Raman spectra were taken on a Monovista CRS+ setup (S&I Ltd.) working with Trivista Software, equipped with a 0.75 m length monochromator and 532 nm laser set to a power density of 7.5 mW/ μm2. The resolution was roughly 1 cm−1. Line positions were established by fitting the observed spectra with Gaussian curves. Research was conducted in two configurations: under HP at room temperature and at low temperatures under ambient pressure (AP). To achieve HP, samples were polished to about 35 × 35 × 25 μm3 cuboids and placed in a Diacell CryoDAC-LT Almax easyLab diamond anvil cell equipped with anvils having 450 μm culets. Argon was chosen as the pressure-transmitting medium (PTM), and ruby was placed inside the cell as a pressure gauge. Infrared absorption spectra were measured on a BOMEM DA3 FTIR spectrometer equipped with a globar source, KBr beam splitter, and MCT detector. The resolution was set to 1 cm−1. For LT measurements samples were placed on a cold finger inside a CF-102 Oxford Instruments cryostat with KRS-5 windows.

Figure 1. Three crystallographic structures of BaWO4: ambient- and low-pressure scheelite (a); intermediate pressure (7−9 GPa) fergusonite (b); high-pressure monoclinic BaWO4-II (c).

conduction band is made of 5d states of W6+. All of these complex relations were briefly summarized by LacombaPerales et al.46 While the source of blue emission has been widely accepted as a charge-transfer transition within (WO4)2− groups,17,47−54 the origin of green luminescence has not yet been firmly assigned. On the basis of experimental and theoretical studies several authors suggested that the origin of green emission is extrinsicrelated either to (WO3 + F) centers52−55 or oxygendeficient complexes (WO3 + VO).56 Others considered defect centers containing interstitial oxygen (WO4 + Oi)57−59 or proposed slight intrinsic distortions of the (WO4) tetrahedra as a source of green light.60 Theoretical studies performed by Campos50 and Longo et al.61 attributed it to both oxygenvacancy complexes and slight intrinsic distortions. Our samples were additionally doped with Ce3+ ions with the expectation of efficient spin- and parity-allowed 5d → 4f optical transitions, exhibited in many hosts.62−65 It was reported that Ce can form covalent bonds, which may influence the energy levels of the host, thus decreasing the energies of blue and green emission.65 However, opposite effects were also reported.66 In this paper we present the results of RT (roomtemperature), LT (low-temperature), and HP (high-pressure) photoluminescence (PL) investigations of BaWO4:Ce. Additionally, to support our conclusions, absorption, FTIR, and Raman experiments were carried out. We also studied BaWO4 crystals codoped with sodium ions in order to efficiently compensate the charge deficiency emerging from Ce3+/Ba2+ substitution.

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RESULTS AND DISCUSSION UV and Visible Absorption. UV and visible absorption was measured for three samples: pure BaWO4, BaWO4:Ce3+ oriented crystals, and BaWO4:Ce3+,Na+ codoped samples. The results presented in Figure 2 show that after Ce3+ doping an

Figure 2. Absorption spectra of pure BaWO4, doped with 0.5 atom % Ce3+ in [100] (a) and [001] (b) orientationd and additionally codoped with 1 atom % Na+ (c).

additional absorption peak appears with a maximum at 325 nm. Another broad band around 275 nm can be assigned to the host (either excitonic or charge-transfer transitions) or to some contamination of the starting materials: e.g., with strontium. It is also possible that the absorption is due to some tungsten oxide (WO3 or W3O8) inclusions, but that is much less probable. As will be discussed in the next section, some small inclusions were detected, mainly in the pure BaWO4 crystal, while the 275 nm band is more intense in doped crystals. The band gap of BaWO4 was investigated as a function of temperature. The results of absorption measurements in the band gap region are presented in Figure 3a. Band-to-band absorption is observed at photon energies above 5 eV. The measured dependences were fitted assuming either a direct or indirect band gap. Better results were obtained for a direct band gap. In this case, the absorption coefficient in the region of the band edge should be proportional to the square root of the photon energy ℏω: α ≈ (ℏω − Eg)1/2, where Eg is the band

EXPERIMENTAL SECTION

Synthetic Procedures and Crystallography. Single-crystal samples of undoped BaWO4, BaWO4:0.5 atom % Ce, BaWO4:1 atom % Ce, and BaWO4:0.5 atom % Ce, 1 atom % Na were obtained by the Czochralski method using a Malvern MSR4 puller. The starting materials BaCO3 (5 N), WO3 (4 N), CeO2 (4 N), and Na2CO3 (5 N) were dried at 300 °C for 12 h before weighting. Reagents were then mixed together in stoichiometric molar ratios and pressed into cylindrical pellets before melting in the inductively heated iridium crucible. All of the crystals were grown under an ambient N2 atmosphere on an iridium rod used as a seed, with pulling rate of 3 mm/h and 20 rpm rotation speed. Cubic samples with (100) and (001) orientation were cut afterward from single-crystal boules and polished. The crystal structure was examined with a Siemens D5000 diffractometer using Ni-filtered Cu Kα radiation in Debye−Scherrer geometry. Le Bail refinement was performed to determine lattice constants from collected diffractograms using FullProf Suite software. Single-crystal orientation was determined with use of the Laue method. B


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Figure 3. Temperature dependence of BaWO4 absorption with thickness of about 100 μm in the band gap region (a). Band-gap dependence as a function of temperature with fit for modified Varshni formula shown as a solid line (b).

Figure 4. Temperature dependent PL spectra at ambient pressure of both pure (a) and Ce3+ (0.5 atom %), Na+ (1 atom %) doped (b) BaWO4. Excitation wavelength: 275 nm.

et al.30 This effect is compensated most probably by electron− phonon interaction. Luminescence. None of the studied single crystals show any emission at ambient pressure and low temperatures, when excited with the 325 nm laser line, i.e., within the Ce3+ absorption band. This is in contrast to previous reports, where red luminescence of BaWO4 under long-wavelength UV excitation was observed,17,71−74 however, only in powdered samples. On the other hand, under excitation with a shorter wavelength, 275 nm, all our samples emit at low temperatures in the blue-green region. Representative PL spectra for undoped and Ce,Na codoped BaWO4 are presented in Figure 4a and 4b, respectively. The spectra in Figure 4b can be deconvoluted into two bands peaking approximately at 470 and 575 nm. Small differences in peak positions and shape of the dominating band are observed depending on the doping level. The peak positions red-shift with increasing temperature. Both peaks were previously assigned to the BaWO4 matrix and are in good agreement with other reports.15,16,20,67,75 In the undoped BaWO4 crystal, in addition to the host-related PL bands, a luminescence peak at 430 nm can be observed (see Figure 4a). We relate it to the presence of inclusions, most probably tungsten oxide particles, which were not properly dissolved in the melt. Such inclusions are visible under the microscope, as shown in the inset in Figure 5. Under UV excitation these parts give intense, blue-shifted emission, while the spectrum

gap energy. On this basis, the band gap can be evaluated as the intersection of the straight lines, fitted to the square of the absorption coefficient, with the energy axis at α = 0. The temperature dependence of the band gap energy is plotted in Figure 3b. Although there are different data on the band gap energy of BaWO4 available in the literature,15,20,46,67 our results are in good agreement with the recent estimation of this value.30 The temperature dependence of Eg was fitted using the Varshni equation,68 modified by O’Donnell and Chen69 to account for electron−phonon coupling: ÉÑ ÄÅ ÑÑ ÅÅ i ⟨ℏΩ⟩ zy zz − 1ÑÑÑ Eg (T ) = Eg (0) − S ·⟨ℏΩ⟩·ÅÅÅÅcothjjj ÑÑ ÅÅÇ (1) k 2kT { ÑÖ where Eg(0) is the band gap at zero temperature, S is a dimensionless coupling constant, ⟨ℏΩ⟩ is the effective phonon energy, and k is the Boltzmann constant. In Figure 3b the solid line represents the best fit to eq 1. The best fit parameters are Eg(0) = 5.22 ± 0.02 eV, S = 4.2 ± 0.2, and ⟨ℏΩ⟩ = 0.032 ± 0.002 eV. As can be seen, the band gap energy remains approximately constant up to about 70 K. With rising temperature it decreases and is reduced by about 140 meV at 310 K. It is a moderate change in comparison to other oxide compounds: e.g., the yttrium aluminum oxide family.70 The decrease of the band gap energy with temperature is in apparent contradiction to the expected energy increase due to increasing lattice parameter, pointed out by Lacomba-Perales C


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refinement is shown in Figure 7b. The unit cell parameters of undoped BaWO4 correspond very well to those of the reference material in the database. Crystals which contain only Ce3+ reveal slightly larger unit cells in comparison to those codoped with Na+. On the other hand, sodium-compensated samples do not differ significantly from the pure BaWO4. However, in the latter we detect a few trace-intensity peaks from an unidentified minor phase of tungsten oxidesmost likely W3O8 or WO3. FTIR Spectra. Temperature-dependent infrared absorption spectra of pure BaWO4 are presented in Figure 8a, in comparison to the crystal sample doped with 1 atom % Ce3+ and 2 atom % Na+ in Figure 8b. At temperatures below 150 K sharp, Ce-related absorption lines are visible between 2100 and 2350 cm−1 and around 3000 cm−1. They are attributed to transitions between different 4f−4f Ce3+ energy sublevels 2 F5/2 (#1−#3) ↔ 2F7/2 (#4−#7), as indicated in the figure.76,77 Figure 9 shows low-temperature absorption spectra in the region of intracenter Ce3+ transitions from the lowest level of the ground 2F5/2 multiplet (#1) to the two lowest levels of the excited 2F7/2 multiplet (#4 and #5) depending on Ce: concentration (Figure 9a) and crystal orientation (Figure 9b). Instead of one absorption line for each transition, expected for isolated Ce3+ ions substituting Ba2+, at least six are detected. This points out the existence of at least six Ce3+related centers with different local surroundings, influenced by either the charge-compensating Na+ ions or subsequent defects in the near vicinity. The line positions are independent of Ce concentration and crystal orientation. They are presented in Table 2. In all infrared spectra the #1 → #6 transition is missing, which is probably associated with its lower oscillator strength.78 The presence of Ce3+ multicenters was confirmed by LT EPR measurements by Leniec et al.79 performed on samples cut from the same bulk crystals investigated in the present study. They have found that Ce3+ ions in BaWO4 occupy mainly two sites with S4 and C2v symmetry, with the former being the most abundant. In addition, several sites with lower symmetry were also detected in EPR experiments. The strongest FTIR absorption lines can be unambiguously assigned to the substitutional Ce center with tetragonal symmetry. Scheme 1 shows the determined multiplet level positions for this center (solid lines) together with the detected transitions (arrows). Unknown level positions are indicated by dashed lines.

Figure 5. Luminescence of a BaWO4 single crystal at 70 K under 275 nm excitation. The red PL spectrum stems from an inclusion-free region, while the black spectrum is from a region containing inclusions. Pictures of the crystal are shown in the insets.

collected from inclusion-free regions is identical to that of Figure 4b at the same temperature. In contrast to the inclusion-related luminescence, the intensity of the host PL decreases monotonically with rising temperature. In both pure and doped samples it disappears above 140 K. In Figure 6 we show the temperature dependence of the PL intensity for the stronger band. It can be approximated by the Arrhenius equation I0 I (T ) = ΔE 1 + A exp − kT (2)

(

)

where I0 is the intensity at 0 K, ΔE is the activation energy, and k is the Boltzmann constant. Calculated activation energies from the fits of eq 2 to the experimental data are equal to about 9 and 14 meV for undoped and Ce,Na-doped BaWO4 crystals, respectively. Powder X-ray Diffraction. The obtained materials were examined by powder X-ray diffraction. The diffractograms are presented in Figure 7a and are in a good agreement with the JCPDS standard. The collected diffractograms were also refined with the Le Bail method, and the determined lattice constants are presented in Table 1. An exemplary Le Bail

Figure 6. Temperature dependence of the 575 nm PL intensity in BaWO4 (a) and BaWO4:Ce3+ (0.5 atom %), Na+ (1 atom %) (b). D


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Figure 7. (a) XRD patterns of the examined BaWO4:Ce and BaWO4:Ce,Na powders together with the JCPDS Card Standard No. 43-0646 for the pure SG I41/a BaWO4 phase. (b) Exemplary Le Bail fit for BaWO4:Ce3+ (1 atom %). Red circles show experimental data, the black line shows the calculated profile, the green vertical bars show Bragg reflections, and the blue solid line shows the difference between experimental and calculated data.

Table 1. Lattice Parameters and Unit Cell Volumes of Studied BaWO4 Samples Compared to PDF-2 43-0646 Data a (Å) c (Å) V (Å3)

43-0646 PDF-2

undoped

0.5 atom % Ce

1 atom % Ce

0.5 atom % Ce, 1 atom % Na

1 atom % Ce, 2 atom % Na

5.6123 12.7059 400.21

5.6113(6) 12.706(2) 400.07(8)

5.61533(7) 12.7206(2) 401.11(1)

5.61524(6) 12.7210(2) 401.105(7)

5.6105(5) 12.710(1) 400.08(6)

5.6113(9) 12.708(2) 400.1(1)

Figure 8. Temperature-dependent FTIR absorption spectra of BaWO4 single crystals: (a) pure and (b) doped with 1 atom % Ce3+, 2 atom % Na+.

The weak absorption lines at 2140 and 2170 cm−1 are related to an unknown Ce3+-related defect. They are found only in some crystals. On the other hand, the broad bands visible between 2400 and 2900 cm−1 are present in every examined sample, also undoped. The structure of these additional lines is very weakly temperature dependent, except for small broadening visible only at higher temperatures. In contrast, the lines associated with Ce3+ exhibit a quite different temperature behavior: i.e., due to strong electron−lattice coupling they broaden strongly at temperatures around 50−80 K and at higher temperatures become indiscernible. The origin of the spectral lines in the 2400−2900 cm−1 region is not known. They may be associated with vibrational modes of CO2 molecules80 or bicarbonate-like anions81−83 left after the Czochralski synthesissimilar spectra were also reported for BaWO4 by Ge et al.84

High-Pressure Luminescence. Pressure-dependent luminescence spectra of undoped and Ce-doped BaWO4 single crystals, excited with the 325 nm laser line at 10 K, are presented in Figure 10. Under increasing pressure no PL is detected up to about 9 GPa. At this pressure, abruptly, bright luminescence appears in both crystals. The luminescence is composed of a more intense green band centered at 550 nm and a weaker band at approximately 610 nm. While the positions of the peaks do not change with further increase of pressure, the emission intensity gradually decreases. This behavior is only partially reversible. During release of pressure the PL intensity initially increases (again without a change in the peak position) up to 9 and 7.5 GPa in pure and Ce-doped BaWO4 crystals, respectively, and then decreases with further pressure release down to about 4 GPa, where the PL disappears (see Figure 10c,d). The observed hysteresis in the pressure dependence of the PL intensity is shown in Figure 11. E


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Figure 9. LT Ce3+ absorption spectra in BaWO4 single crystals (a) depending on Ce/Na doping level (0.5 atom % Ce3+, 1 atom % Na+, and 1 atom % Ce3+, 2 atom % Na+) maintaining the same [100] crystal orientation and (b) at different axes ([100] or [001]) for the same Ce/Na concentration (1 atom % Ce3+, 2 atom % Na+).

Scheme 1. Energy Structure of Ce3+ (4f1) in BaWO4a

Table 2. FTIR Absorption Lines of Ce3+ Multicenters at 5 K Observed in Differently Doped (0.5 atom % Ce3+, 1 atom % Na+; 1 atom % Ce3+, 2 atom % Na+) BaWO4 Single Crystals Oriented along the [100] Axisa strength and width wavenumber (cm−1)

0.5 atom %

2140 2170 2185 2192 2198 2208 2288 2295 2304 2311 2321 2328 2339 2983

mw, sh mw, sh w, sh mw, sh mw, sh m, sh w, br w, br w, sh mw, sh w, br w, br mw, sh w, sh

1 atom %

Ce3+ 2F5/2 →2F7/2 transition assignment #1 → #4

mw, sh m, sh m, sh s, sh mw, br mw, br m, sh m, sh mw, br mw, br mw, sh mw, sh

#1 → #5

#1 → #7

a

Energies of specific 4f−4f transitions of Ce are given with accuracy better than 1 cm−1. Abbreviations: w, weak; mw, medium-weak; m, medium; s, strong; br, broad; sh, sharp.

The level positions determined for the dominant center with tetragonal symmetry are drawn with solid lines, while unknown level positions are drawn with dashed lines. Arrows show detected transitions, associated with different Ce3+ centers.

Moreover, we note that at 6 GPa in the Ce-doped sample and below 5 GPa in pure BaWO4 the luminescence peaks shift abruptly by 50 nm toward higher wavelengths. We relate the appearance of host PL at 9 GPa to the phase transition of barium tungstate to the monoclinic BaWO4-II crystallographic structure. While under increasing pressure the transition proceeds abruptly, the reversal to the ambient scheelite phase is more gradual, as indicated by the observed hysteresis. Moreover, the sudden shift of the PL wavelength at about 6 GPa points to the involvement of an intermediate, metastable crystallographic phase. The exact correspondence of the PL spectra in pure and Cedoped BaWO4 under pressure means that the Ce3+ 5d1 excited states remain degenerate with the conduction band at least up to 14 GPa. Still, the enhancement of the excitonic PL intensity in Ce-/Na-codoped crystals (by 1 order of magnitude in comparison to pure BaWO4) may be related to the presence of

one of the dopants. In another case, luminescence intensity, excited by the 275 nm laser line at 6.8 GPa, is strongly temperature-dependent, which is shown in Figure 12. The fit with eq 2 shows that the luminescence is quenched at higher temperatures with an activation energy of ΔE = 39 meV. Larger value of activation energy in comparison to that observed at ambient pressure is related to the transition from a tetragonal to a fergusonite phase accompanied by an abrupt change of the energy structure of the system. Raman Spectra. Micro-Raman spectra of different BaWO4 crystals were measured in order to investigate the differences between undoped and Ce-/Na-doped samples. The undoped sample was not oriented in any specific direction, contrary to the doped specimen cut along [100] or [001] axes. The Raman spectra under ambient conditions are presented in Figure 13. As can be seen, the intensity ratio of all 13 registered Raman modes changes with doping and crystal orientation.

a

3+

F


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Inorganic Chemistry

Figure 10. Luminescence of BaWO4 and BaWO4:Ce3+ (0.5 atom %), Na+ (1 atom %) as a function of pressure increase (a, b) and release (c, d) at 10 K. Excitation wavelength: 325 nm.

Figure 11. Intensity of 610 nm (black symbols) and weak (red symbols) PL bands vs pressure in BaWO4:Ce3+ (0.5 atom %), Na+ (1 atom %) (a) and pure BaWO4 (b) single crystals measured at 10 K under 325 nm excitation. The direction of pressure changes is indicated with arrows.

Figure 12. Temperature dependence of BaWO4:Ce3+ (0.5 atom %), Na+ (1 atom %) luminescence: spectra (a) and intensity (b) at a pressure of 6.8 GPa. Excitation wavelength: 275 nm.

7.1 GPa and above 9.0 GPa, respectively. The determined pressure coefficients and calculated Grüneisen parameters are given in Table 3. Note that we do not characterize the metastable fergusonite phase, since in the narrow pressure range of its existence the observed signals behave incoherentlysome exhibit a blue shift and some a rapid red shift. Among the low-energy signals in scheelite, a soft T (Bg) mode associated with ferroelastic properties is found at 64 cm−1. The characteristic blue shift of the line position and decrease in intensity under pressure is associated with gradually decreasing ferroelectricity. This phenomenon is taking place due to the translation of WO4 tetrahedra along the c axis during the long-range, secondorder phase transition to the fergusonite structure. The vanishing of the T (Bg) mode together with the emergence of a new, red-shifting, pressure-hardening T (Ag) mode at a

Pressure-dependent Raman spectra, shown in Figure 14, support the luminescence data presented earlier. The scheelite (SG I41/a) phase of barium tungstate persists up to 7.1 GPa. At 9.0 GPa it transforms into monoclinic BaWO4-II (SG P21/ n), through the intermediate, metastable fergusonite (SG I2/a) phase, observed between 7.1 and 9.0 GPa. These changes diminish simultaneously the band gap of BaWO4, since W6+ 5d states form the bottom of the conduction band in this material.46 The number of registered Raman lines increases from 13 in scheelite up to 37 in BaWO4-II HP structures, respectively. Their energies and group theory mode assignments with literature references can be found in Table 3. Some of the observed lines are phase-specific. Most of them have moderate, monotonic pressure dependences, as shown in Figure 15. A linear red shift with pressure is mostly observed in the scheelite and monoclinic BaWO4-II HP phases: i.e., below G


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symmetrydopants are known to change the valence and coordination number of the W ion in tungsten oxide clusters this can lead to some unusual mode splitting or even the emergence of new signals during complex phase transitions taking place in this material. This theory is supported by Hardcastle and Wachs,86 who give theoretical equations compensating, to some extent, the changes in bond distance and Pauling’s bond strength. Therefore, the sodium codopant can be responsible for the new signals presented in this work. Codoping may also explain changes in the observed “phonon gap” (lack of phonons with energies between 400 and 800 cm−1) which is specific for scheelitesit closes up to 450−600 cm−1 for fergusonite and BaWO4-II HP phases. Raman spectra measured during pressure release are presented in Figure 16. The observed strong hysteresis indicates the existence of large kinetic barriers with possible, weak shape memory properties.87 It seems that BaWO4:Ce,Na prefers to remain in the PL-active, nonferroelastic, monoclinic BaWO4-II phase down to 4.9−4.1 GPa, before transforming directly to tetragonal scheelite. Temperature-dependent Raman scattering experiments performed at ambient pressure confirm the thermal stability of the scheelite phase down to 4.2 K. The only signal that varies with decreasing temperature is the soft mode at 64 cm−1its energy increases at the rate of 0.00208 cm−1/K. The obtained Raman spectroscopy results are consistent with previous reports. Manjon et al.40,88 have encountered the same set of phase transformations measuring BaWO4 and the scheelite-like PbWO4 up to 16−17 GPa. Both studies seem to resolve, at least partially, some controversy regarding previous structural research performed by Jayaraman et al.43 The authors of refs.40,88 support their claims by providing precise lattice dynamics calculations with concise experimental data showing abrupt kinetic barriers that scheelite and stolzite undergo throughout excessive compression - mainly similar sluggish, second-order phase transitions to fergusonite, between 7.0 and 9.0 GPa, and to monoclinic phases respectively, at higher pressures. The kinetic barriers also influence the structure recovery process during careful and

Figure 13. Raman scattering spectra at ambient pressure of undoped, unoriented BaWO4 and two specimens doped with 0.5 atom % Ce3+ and 1 atom % Na+ oriented along [100] and [001] axes. The spectra were normalized to the intensity of the 927 cm−1 band (Ag mode).

similar energy (just above 63 cm−1) was already observed after the phase transition by Errandonea and Manjon in PbWO4.85 The assignment of other modes, at 114, 148, 167, 644, 650, and 705 cm−1, to fergusonite in Figure 15 is ambiguous, since they are observed in the literature only at pressures higher than those in our casesome sources ascribe them directly to the BaWO4-II HP phase. The discrepancies in line positions between our work and literature data disappear after the second phase transition (above 9.0 GPa) to monoclinic BaWO4-II HP. The measured energies are identical with those reported in the references given in Table 3. However, there are still some modes, e.g., at 85, 112, 148, 307, 670, 965, and 976 cm−1, that have been rarely or never observedwe relate them to the incorporation of charge-compensating sodium dopant. Some of these signals can be assigned on the basis of ab initio, total-energy calculations made by Lacomba-Perales et al.,30 but the origins of a few are still unknown. Manjon et al.40 suggest that the main difference in bending (energies below 400 cm−1) and stretching (900−950 cm−1) modes in tungstates during pressure application is related to changes of local atomic

Figure 14. Pressure-dependent Raman spectra at room temperature of BaWO4 doped with 0.5 atom % Ce3+ and 1 atom % Na, oriented along [100]. Phase transitions are noticeable at 7.1 GPa (to the metastable fergusonite phase) and at 9.0 GPa to monoclinic HP BaWO4-II. H


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Table 3. Initial Energies (E0) (i.e. Energies at Ambient Pressure, Estimated from Linear Pressure Dependence), Pressure Coefficients (dE/dP), and R2 Parameters of Various Raman Modes Determined from RT-HP Experiments on Doped BaWO4 Presented in Figure 14 for both Scheelite and Monoclinic BaWO4 II HP Phasesa phase 1: scheelite E0 (cm−1) 63 64 79 85 112 114 117 148 152 162 167 192 198 260 285 307 334 342 347 355 365 414 434 448 615 644 650 670 705 721 761 792 797 827 832 833 841 905 920 927 934 965 976

phase 2, 1, 1, 1, 2, 1, 3 2 1, 2 2 1, 2, 3 3 2, 1, 2, 1, 1, 2, 2, 3 3 3 3 3 3 3 3 3 2, 1, 2, 1, 3 2, 2, 1, 1, 3 3 3

3 2 2, 3 2 3 2, 3

2, 3

2 3

3 2, 3 3 2, 3 2, 3 3 3

3 2, 3 3 2, 3 3 3 2, 3 2, 3

characteristics s, sh w, sh m, sh w, sh w, sh w, sh w, sh w, left shoulder w, br w, left shoulder w, sh w, br w, br w, left shoulder w, left shoulder mw, left shoulder s, sh m, sh mw, right shoulder mw, sh s, right shoulder mw, right shoulder w, right shoulder w, right shoulder w, left shoulder m, left shoulder m, br m, right shoulder m,br m, br mw, left shoulder m, sh m, sh m, sh m, sh m, right shoulder m, sh w, sh w, left shoulder vs, sh w, right shoulder w, right shoulder w, right shoulder

dE/dP (cm−1/GPa)

R2

phase 3: BaWO4-II HP γi (B0 = 47 GPa)6

dE/dP (cm−1/GPa)

R2

0.035

0.866

−0.818 0.822 −0.171

0.961 0.946 0.781

−0.601 0.489 −0.095

−0.009

0.937

0.766

0.989

0.316

0.303 −0.085 −0.281

0.786 0.913 0.719

4.551

0.992

1.407

1.387

0.818

6.686

0.999

1.637 2.171 2.321 2.293 2.489 0.586 2.419 1.835 0.922 2.358 2.989 2.408 1.417 1.895 1.342 0.108 0.913 1.632 −0.184 2.031 −0.328 3.417 3.241 2.746 3.257 2.472 3.152 1.932 2.367 2.628 2.661 1.906

0.899 0.918 0.886 0.927 0.756 0.953 0.955 0.991 0.937 0.917 0.921 0.771 0.969 0.992 0.898 0.819 0.917 0.862 0.831 0.703 0.957 0.954 0.919 0.986 0.989 0.991 0.989 0.997 0.992 0.993 0.989

3.108

0.998

0.437

2.695 3.479

0.978 0.996

0.365 0.461

3.489

0.998

0.206

2.278

0.991

0.129

3.285 3.094

0.999 0.997

0.168 0.157

mode assignment T (Ag) soft, T (Bg) T (Eg) T (Eg) Ag T (Eg) Bg Bg R (Ag) Ag Bg R (Eg) Bg Ag Bg Ag ν2 (Ag) Ag ν4 (Bg) ν4 (Eg) Bg Ag Bg Ag Bg Ag Bg Bg Ag Bg Ag Bg ν3 (Eg) Bg ν3 (Bg) Bg Ag Bg ν1 (Bg) ν1 (Ag) Ag unknown unknown

ref 30, 31, 30, 31, 30, 31 30 30 30, 31, 30 ,40 30 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 40 30, 37, 31, 37, 31, 40 30, 31, 30, 31, 30, 31 30, 31, 30, 31, 30, 31, 31 31, 37, 31, 37, 30, 31, 30, 31, 31, 37, 31, 40

40, 43, 85 85

40

40, 37, 37, 40 40, 40 37, 40 37, 37, 40, 40 37, 40 37, 37, 40

43 40 40 43 40 40 40 43 40 40 40

40 40 37 37, 40 40, 43 37, 40 40, 43 40 40 40 40, 43 40

a Grüneisen parameters (γi) were calculated for the first phase using the bulk modulus (B0) established by Gomis et al.31 Abbreviations: 1, scheelite; 2, fergusonite; 3, monoclinic BaWO4-II HP; w, weak; mw, medium-weak; m, medium; s, strong; vs, very strong; br, broad; sh, sharp.

in all spectra and predominated by the ν1 (Ag) mode. However, there are some other lines (mainly at low energies) associated with external (transitional and rotational) modes, which occur at slightly different positions than predicted by Manjon et al.40 Different energies of these modes are possibly related to doping of the crystals. Grüneisen parameters calculated in this work oscillate between theoretical and experimental values given by Manjon et al.40 For external modes, below 300 cm−1, they are closer to the given experimental lines, while those of the internal,

slow pressure release−back-transition is hindered down to 5 GPa for PbWO4 and to 3 GPa, for BaWO4. The pronounced hysteresis essentially proves the authors’ statement, that fergusonite and high-pressure phases are preferred and predominant in the spectra after the energy barrier is crossed. From all observed 13 Raman modes for scheelite, we also registered an elusive ν4 (Eg) band which has been obtained after deconvolution of nearby, overlapping, internal signals from WO4 tetrahedra - just around 330−350 cm−1. The second group of signals, just above 900 cm−1, is more intense I


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Inorganic Chemistry

Figure 15. Pressure dependence of Raman modes emerging in scheelite (solid symbols), fergusonite (open symbols), and the BaWO4-II HP phase (semisolid symbols) up to 14.1 GPa. Dotted lines above 7 GPa show the onset of a scheelite to fergusonite transition and dashed lines below 9 GPa that of a fergusonite to monoclinic BaWO4-II transition.

the conclusion that there is a narrow pressure range in which both HP phases can really coexist with each other at the same time. The main change in the spectra, observed during the transition through the metastable phase, occurs between 400 and 800 cm−1a previously empty range. Sudden broadening of internal modes together with splitting of ν3 bands below 800 cm−1 is also noticeable. The most exceptional, however, is the slow emergence of several modes above 350 and 950 cm−1 that might suggest some changes in BaWO4 symmetry. Such modifications are mostly noticeable in Ca2+, Sr2+, and Ba2+ tungstates and less so in Pb2+.40,89,90

■

CONCLUSIONS The luminescence properties of BaWO4:Ce single crystals can be interpreted as follows. The observed luminescence is of excitonic origin, independent of applied pressure. In order to observe it at AP it is necessary to use near band gap excitation: i.e., an excitation wavelength of 275 nm or shorter. The luminescence is strongly temperature quenched, with a relatively low activation energy of a few meV. Under 325 nm excitation, resonant with the strong absorption associated with the Ce3+ dopant, no luminescence is observed. This is related to the location of the Ce3+ 5d levels in the conduction band of BaWO4.91 Pressure application has a strong effect on the luminescence properties of BaWO4 crystals. First of all, the band gap energy decreases with increasing pressure due to its band structure. The increased crystal field induces a larger splitting of the tungsten 5d states, which form the bottom of the conduction band, thus reducing the band gap energy.38 Therefore, the 5d levels of Ce3+ remain degenerate with the conduction band

Figure 16. RT Raman spectra of [100] BaWO4 doped with 0.5 atom % Ce3+ and 1 atom % of Na+ collected during pressure release. The transition from the BaWO4-II HP to the scheelite phase occurs between 4.9 and 4.1 GPa.

stretching bands (above 300 and 900 cm−1) are closer to the Grüneisen values from ab initio calculations of the Manjon group. These discrepancies might partially result from applying different values of the bulk modulus (we used 47 GPa instead of 52 GPa due to the different PTM used in our studies).39,40 Finally, we also agree upon the division of registered Raman signals into two groups above the second-order phase transition: (1) bands that appear at 7.0 and disappear at 9.0 GPa and (2) modes that can be followed up to a certain pressure threshold. Most of the latter slowly lose intensity above 9.0 GPa.40,88 This different pressure behavior leads to J


Article

Inorganic Chemistry under pressure and the Ce3+ 5d−4f luminescence is suppressed. A further decrease of the band gap energy, related to a phase transition from scheelite to the HP crystal structure, allows us to observe excitonic luminescence under longer wavelength excitation (325 nm). The efficiency of this luminescence is higher in Ce-doped crystals due to the resonance of the 325 nm excitation with the Ce-related absorption band. These processes are illustrated schematically in Scheme 2.

fwhm, full width at half-maximum; PTM, pressure-transmitting medium

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(1) Cerny, P.; Jelinkova, H.; Basiev, T. T.; Zverev, P. G. Highly Efficient Picosecond Raman Generators Based on the BaWO4 Crystal in the near Infrared, Visible, and Ultraviolet. IEEE J. Quantum Electron. 2002, 38 (11), 1471−1478. (2) Annenkov, A. A.; Korzhik, M. V.; Lecoq, P. Lead Tungstate Scintillation Material. Nucl. Instrum. Methods Phys. Res., Sect. A 2002, 490 (1−2), 30−50. (3) CERN. LHC Experiments Committee. The Compact Muon Solenoid: Technical Proposal, CERN-LHCC-9438; CERN: Geneva, Switzerland, 1994. (4) Kobayashi, M.; Ishii, M.; Usuki, Y.; Yahagi, H. Scintillation Characteristics of PbWO4 Single Crystals at Room Temperature. Nucl. Instrum. Methods Phys. Res., Sect. A 1993, 333 (2−3), 429−433. (5) Nikl, M.; Bohacek, P.; Mihokova, E.; Kobayashi, M.; Ishii, M.; Usuki, Y.; Babin, V.; Stolovich, A.; Zazubovich, S.; Bacci, M. Excitonic Emission of Scheelite Tungstates AWO4 (A = Pb, Ca, Ba, Sr). J. Lumin. 2000, 87−89, 1136−1139. (6) Nikl, M.; Bohacek, P.; Mihokova, E.; Solovieva, N.; Vedda, A.; Martini, M.; Pazzi, G. P.; Fabeni, P.; Kobayashi, M.; Ishii, M. Enhanced Efficiency of PbWO4:Mo,Nb Scintillator. J. Appl. Phys. 2002, 91 (8), 5041−5044. (7) Brenier, A.; Jia, G.; Tu, C. Raman Lasers at 1.171 and 1.517 μm with Self-Frequency Conversion in SrWO4:Nd3+ Crystal. J. Phys.: Condens. Matter 2004, 16 (49), 9103−9108. (8) Balakshy, V. I.; Asratyan, K. R.; Molchanov, V. Y. Acousto-Optic Collinear Diffraction of a Strongly Divergent Optical Beam. J. Opt. A: Pure Appl. Opt. 2001, 3 (4), S87−S92. (9) Angloher, G.; Bucci, C.; Cozzini, C.; von Feilitzsch, F.; Frank, T.; Hauff, D.; Henry, S.; Jagemann, T.; Jochum, J.; Kraus, H. Cresst-II: Dark Matter Search with Scintillating Absorbers. Nucl. Instrum. Methods Phys. Res., Sect. A 2004, 520 (1−3), 108−111. (10) Alessandrello, A.; Bashkirov, V.; Brofferio, C.; Bucci, C.; Camin, D.; Cremonesi, O.; Fiorini, E.; Gervasio, G.; Giuliani, A.; Nucciotti, A.; et al. A Scintillating Bolometer for Experiments on Double Beta Decay. Phys. Lett. B 1998, 420 (1−2), 109−113. (11) Cozzini, C.; Angloher, G.; Bucci, C.; von Feilitzsch, F.; Hauff, D.; Henry, S.; Jagemann, T.; Jochum, J.; Kraus, H.; Majorovits, B.; et al. Detection of the Natural α Decay of Tungsten. Phys. Rev. C: Nucl. Phys. 2004, 70 (6), 064606. (12) Yang, X.; Liu, J.; Yang, H.; Yu, X.; Guo, Y.; Zhou, Y.; Liu, J. Synthesis and Characterization of New Red Phosphors for White LED Applications. J. Mater. Chem. 2009, 19 (22), 3771−3774. (13) Tamaki, J.; Fujii, T.; Fujimori, K.; Miura, N.; Yamazoe, N. Application of Metal Tungstate-Carbonate Composite to Nitrogen Oxides Sensor Operative at Elevated Temperature. Sens. Actuators, B 1995, 25 (1−3), 396−399. (14) Choi, G.-K.; Kim, J.-R.; Yoon, S. H.; Hong, K. S. Microwave Dielectric Properties of Scheelite (A = Ca, Sr, Ba) and Wolframite (A = Mg, Zn, Mn) AMoO4 Compounds. J. Eur. Ceram. Soc. 2007, 27 (8− 9), 3063−3067. (15) Vidya, S.; Solomon, S.; Thomas, J. K. Synthesis, Characterization, and Low Temperature Sintering of Nanostructured BaWO4 for Optical and LTCC Applications. Adv. Condens. Matter Phys. 2013, 2013, 1−11. (16) Dabre, K. V.; Dhoble, S. J.; Lochab, J. Synthesis and Luminescence Properties of Ce3+ Doped MWO4 (M = Ca, Sr and Ba) Microcrystalline Phosphors. J. Lumin. 2014, 149, 348−352. (17) Lou, Z.; Cocivera, M. Cathodoluminescence of CaWO4 and SrWO4 Thin Films Prepared by Spray Pyrolysis. Mater. Res. Bull. 2002, 37 (9), 1573−1582. (18) Ma, L.; Sun, Y.; Gao, P.; Yin, Y.; Qin, Z.; Zhou, B. Controlled Synthesis and Photoluminescence Properties of Hierarchical BaXO4 (X = Mo, W) Nanostructures at Room Temperature. Mater. Lett. 2010, 64 (10), 1235−1237.

Scheme 2. Pressure Influence on Luminescence Properties of BaWO4 Crystals

Although doping of BaWO4 with Ce ions does not activate Ce3+ luminescence at any pressure and any temperature, their presence may contribute to the efficiency of exciton formation. We observe that in crystals doped with Ce3+ the intensity of the excitonic emission is significantly higher than that in pure crystals. The presence of Ce3+ in the studied crystals has been confirmed by UV absorption, mid-IR absorption, and EPR measurements.79 Ce3+ ions form multicenters in BaWO4; however, these are apparently nonluminescent.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail for D.W.: [email protected]. *E-mail for A.S.: [email protected]. ORCID

Damian Wlodarczyk: 0000-0002-5923-7569 Katarzyna M. Kosyl: 0000-0001-6876-1381 Andrzej Suchocki: 0000-0001-7126-1951 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partially supported by Project DEC-2015/17/ B/ST5/01658 of the National Science Center in Poland and by the EU within the European Regional Development Fund through an Innovative Economy grant (POIG.01.01.02-00108/09).

■

ABBREVIATIONS AP, ambient pressure; UV, ultraviolet; HP, high pressure; LT, low temperature; XRD, X-ray diffraction; FTIR, Fourier transform infrared spectroscopy; PL, photoluminescence; SG, space group; Eg, energy gap; RT, room temperature; UV−vis, ultraviolet−visible range; JCPDS, Joint Committee on Powder Diffraction Standards; EPR, electron paramagnetic resonance; K


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Inorganic Chemistry (19) Basiev, T. T.; Danileiko, Y. K.; Doroshenko, M. E.; Fedin, A. V.; Gavrilov, A. V.; Osiko, V. V.; Smetanin, S. N. High-Energy BaWO4 Raman Laser Pumped by a Self-Phase-Conjugated Nd:GGG Laser. Proc. SPIE 2012, 14 (7), 917−921. (20) Cavalcante, L. S.; Sczancoski, J. C.; Espinosa, J. W. M.; Varela, J. A.; Pizani, P. S.; Longo, E. Photoluminescent Behavior of BaWO4 Powders Processed in Microwave-Hydrothermal. J. Alloys Compd. 2009, 474 (1−2), 195−200. (21) Yin, Y.; Gan, Z.; Sun, Y.; Zhou, B.; Zhang, X.; Zhang, D.; Gao, P. Controlled Synthesis and Photoluminescence Properties of BaXO4 (X = W, Mo) Hierarchical Nanostructures via a Facile Solution Route. Mater. Lett. 2010, 64 (6), 789−792. (22) Lima, R. C.; Anicete-Santos, M.; Orhan, E.; Maurera, M. A.; Souza, A. G.; Pizani, P. S.; Leite, E. R.; Varela, J. A.; Longo, E. Photoluminescent Property of Mechanically Milled BaWO4 Powder. J. Lumin. 2007, 126 (2), 741−746. (23) Du, S.; Shi, Y.; Zhang, D.; Li, Q.; Feng, B.; Zhang, J.-Y.; Zang, J.-C. High-Peak Power Multi-Wavelength Picosecond Pulses Generated from a BaWO4 Raman-Seeded Optical Parametric Amplifier. Opt. Commun. 2009, 282 (14), 2960−2963. (24) Č erný, P.; Zverev, P. G.; Jelínková, H.; Basiev, T. T. Efficient Raman Shifting of Picosecond Pulses Using BaWO4 Crystal. Opt. Commun. 2000, 177 (1−6), 397−404. (25) Š ulc, J.; Jelínková, H.; Basiev, T. T.; Doroschenko, M. E.; Ivleva, L. I.; Osiko, V. V.; Zverev, P. G. Nd:SrWO4 and Nd:BaWO4 Raman Lasers. Opt. Mater. (Amsterdam, Neth.) 2007, 30 (1), 195− 197. (26) Li, L.; Zhang, X.; Liu, Z.; Wang, Q.; Cong, Z.; Zhang, Y.; Wang, W.; Wu, Z.; Zhang, H. A High Power Diode-Side-Pumped Nd:YAG/ BaWO4 Raman Laser at 1103 nm. Laser Phys. 2013, 23 (4), 045402. (27) Zhao, J.; Zhang, X.; Guo, X.; Bao, X.; Li, L.; Cui, J. DiodePumped Actively Q-Switched Tm, Ho:GdVO4/BaWO4 Intracavity Raman Laser at 2533 nm. Opt. Lett. 2013, 38 (8), 1206−1208. (28) Gao, L.; Wang, Q. P.; Zhang, X. Y.; Liu, Z. J.; Bai, F.; Chen, X. H.; Shen, H. B.; Lan, W. X. High-Power Nd:YVO4 /BaWO4 Intracavity Raman Laser Emitting at 1,103 nm. Appl. Phys. B: Lasers Opt. 2012, 109 (1), 9−13. (29) Sleight, A. W. Accurate Cell Dimensions for ABO4 Molybdates and Tungstates. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1972, 28 (10), 2899−2902. (30) Lacomba-Perales, R.; Martinez-García, D.; Errandonea, D.; Le Godec, Y.; Philippe, J.; Le Marchand, G.; Chervin, J. C.; Polian, A.; Múñoz, A.; López-Solano, J. Experimental and Theoretical Investigation of the Stability of the Monoclinic BaWO4-II Phase at High Pressure and High Temperature. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81 (14), 144117. (31) Gomis, O.; Sans, J. A.; Lacomba-Perales, R.; Errandonea, D.; Meng, Y.; Chervin, J. C.; Polian, A. Complex High-Pressure Polymorphism of Barium Tungstate. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86 (5), 054121. (32) Gürmen, E.; Daniels, E.; King, J. S. Crystal Structure Refinement of SrMoO4, SrWO4, CaMoO4, and BaWO4 by Neutron Diffraction. J. Chem. Phys. 1971, 55 (3), 1093−1097. (33) Mürk, V.; Nikl, M.; Mihoková, E.; Nitsch, K. A Study of Electron Excitations in CaWO4 and PbWO4 Single Crystals. J. Phys.: Condens. Matter 1997, 9 (1), 249−256. (34) Martini, M.; Meinardi, F.; Spinolo, G.; Vedda, A.; Nikl, M.; Usuki, Y. Shallow Traps in PbWO4 Studied by Wavelength-Resolved Thermally Stimulated Luminescence. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60 (7), 4653−4658. (35) Itoh, M.; Fujita, M. Optical Properties of Scheelite and Raspite PbWO4 Crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62 (19), 12825−12830. (36) Tan, D.; Xiao, W.; Zhou, W.; Chen, M.; Zhou, W.; Li, X.; Li, Y.; Liu, J. High Pressure X-Ray Diffraction Study on BaWO4-II. High Pressure Res. 2012, 32 (2), 262−269. (37) Tan, D.-Y.; Xiao, W.-S.; Zhou, W.-G.; Song, M.-S.; Xiong, X.-L.; Chen, M. Raman Investigation of BaWO4-II Phase under Hydrostatic Pressures up to 14.8 GPa. Chin. Phys. Lett. 2009, 26 (4), 046301.

(38) Lacomba-Perales, R.; Errandonea, D.; Segura, A.; Ruiz-Fuertes, J.; Rodríguez-Hernández, P.; Radescu, S.; López-Solano, J.; Mujica, A.; Muñ oz, A. A Combined High-Pressure Experimental and Theoretical Study of the Electronic Band-Structure of ScheeliteType AWO4 (A = Ca, Sr, Ba, Pb) Compounds. J. Appl. Phys. 2011, 110 (4), 043703. (39) Errandonea, D.; Pellicer-Porres, J.; Manjón, F. J.; Segura, A.; Ferrer-Roca, C.; Kumar, R. S.; Tschauner, O.; López-Solano, J.; Rodríguez-Hernández, P.; Radescu, S.; et al. Determination of the High-Pressure Crystal Structure of BaWO4 and PbWO4. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73 (22), 224103. (40) Manjón, F. J.; Errandonea, D.; Garro, N.; Pellicer-Porres, J.; Rodríguez-Hernández, P.; Radescu, S.; López-Solano, J.; Mujica, A.; Muñoz, A. Lattice Dynamics Study of Scheelite Tungstates under High Pressure BaWO4. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74 (14), 144111. (41) Grzechnik, A.; Crichton, W. A.; Marshall, W. G.; Friese, K. High-Pressure X-Ray and Neutron Powder Diffraction Study of PbWO4 and BaWO4 Scheelites. J. Phys.: Condens. Matter 2006, 18 (11), 3017−3029. (42) Panchal, V.; Garg, N.; Chauhan, A. K.; Sangeeta; Sharma, S. M. High Pressure Phase Transitions in BaWO4. Solid State Commun. 2004, 130 (3−4), 203−208. (43) Jayaraman, A.; Batlogg, B.; VanUitert, L. G. High-Pressure Raman Study of Alkaline-Earth Tungstates and a New PressureInduced Phase Transition in BaWO4. Phys. Rev. B: Condens. Matter Mater. Phys. 1983, 28 (8), 4774−4777. (44) Fujita, T.; Yamaoka, S.; Fukunaga, O. Pressure Induced Phase Transformation in BaWO4. Mater. Res. Bull. 1974, 9 (2), 141−146. (45) Kawada, I.; Kato, K.; Fujita, T. IUCr. BaWO4-II (a HighPressure Form). Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1974, 30 (8), 2069−2071. (46) Lacomba-Perales, R.; Ruiz-Fuertes, J.; Errandonea, D.; Martínez-García, D.; Segura, A. Optical Absorption of Divalent Metal Tungstates: Correlation between the Band-Gap Energy and the Cation Ionic Radius. Europhys. Lett. 2008, 83 (3), 37002. (47) Chen, D.; Shen, G.; Tang, K.; Zheng, H.; Qian, Y. LowTemperature Synthesis of Metal Tungstates Nanocrystallites in Ethylene Glycol. Mater. Res. Bull. 2003, 38 (14), 1783−1789. (48) Chen, S.-J.; Li, J.; Chen, X.-T.; Hong, J.-M.; Xue, Z.; You, X.-Z. Solvothermal Synthesis and Characterization of Crystalline CaWO4 Nanoparticles. J. Cryst. Growth 2003, 253 (1−4), 361−365. (49) Zhai, R.; Wang, H.; Yan, H.; Yoshimura, M. Preparation of Crystalline CaWO4 Thin Films by Chemical Bath Deposition. J. Cryst. Growth 2006, 289 (2), 647−651. (50) Campos, A. B.; Simões, A. Z.; Longo, E.; Varela, J. A.; Longo, V. M.; de Figueiredo, A. T.; De Vicente, F. S.; Hernandes, A. C. Mechanisms behind Blue, Green, and Red Photoluminescence Emissions in CaWO4 and CaMoO4 Powders. Appl. Phys. Lett. 2007, 91 (5), 051923. (51) Xu, L.; Ping, Y.; Yunfei, T.; Yang, L.; Dingquan, X. Synthesis and Characterization of Nano-Calcium Tungstate Thin Film Via Chemical Solution Processing. Ferroelectrics 2009, 383 (1), 27−32. (52) Groenink, J. A.; Blasse, G. Some New Observations on the Luminescence of PbMoO4 and PbWO4. J. Solid State Chem. 1980, 32 (1), 9−20. (53) Grasser, R.; Scharmann, A.; Strack, K.-R. On the Intrinsic Nature of the Blue Luminescence in CaWO4. J. Lumin. 1982, 27 (3), 263−272. (54) Korzhik, M. V.; Pavlenko, V. B.; Timoschenko, T. N.; Katchanov, V. A.; Singovskii, A. V.; Annenkov, A. N.; Ligun, V. A.; Solskii, I. M.; Peigneux, J.-P. Spectroscopy and Origin of Radiation Centers and Scintillation in PbWO4 Single Crystals. Phys. Status Solidi 1996, 154 (2), 779−788. (55) Baccaro, S.; Borgia, B.; Cecilia, A.; Dafinei, I.; Diemoz, M.; Fabeni, P.; Nikl, M.; Martini, M.; Montecchi, M.; Pazzi, G.; et al. Investigation of Lead Tungstate (PbWO4) Crystal Properties. Nucl. Phys. B, Proc. Suppl. 1998, 61 (3), 66−70. L


Article

Inorganic Chemistry

(76) Syvorotka, I. I.; Sugak, D.; Wierzbicka, A.; Wittlin, A.; Przybylińs ka, H.; Barzowska, J.; Barcz, A.; Berkowski, M.; Domagała, J.; Mahlik, S.; et al. Optical Properties of Pure and Ce3+ Doped Gadolinium Gallium Garnet Crystals and Epitaxial Layers. J. Lumin. 2015, 164 (1), 31−37. (77) Przybylińska, H.; Ma, C.-G.; Brik, M. G.; Kamińska, A.; Szczepkowski, J.; Sybilski, P.; Wittlin, A.; Berkowski, M.; Jastrzębski, W.; Suchocki, A. Evidence of Multicenter Structure of Cerium Ions in Gadolinium Gallium Garnet Crystals Studied by Infrared Absorption Spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87 (4), 045114. (78) Wittlin, A.; Przybylińska, H.; Berkowski, M.; Kamińska, A.; Nowakowski, P.; Sybilski, P.; Ma, C.-G.; Brik, M. G.; Suchocki, A. Ambient and High Pressure Spectroscopy of Ce3+ Doped Yttrium Gallium Garnet. Opt. Mater. Express 2015, 5 (8), 1868−1880. (79) Leniec, G.; Kaczmarek, S. M.; Bodzionny, T.; Fuks, H.; Kowalski, Z.; Berkowski, M.; Glowacki, M. Site Symmetries of Cerium Ions in BaWO4 Single Crystals Codoped with Sodium Ions. Appl. Magn. Reson. 2018, DOI: 10.1007/s00723-018-1095-x. (80) Schädle, T.; Pejcic, B.; Mizaikoff, B. Monitoring Dissolved Carbon Dioxide and Methane in Brine Environments at High Pressure Using IR-ATR Spectroscopy. Anal. Methods 2016, 8 (4), 756−762. (81) Bhattacharyya, K.; Wu, W.; Weitz, E.; Vijayan, B. K.; Gray, K. A. Probing Water and CO2 Interactions at the Surface of Collapsed Titania Nanotubes Using IR Spectroscopy. Molecules 2015, 20 (9), 15469−15487. (82) Alavi, M. A.; Morsali, A. Syntheses of BaCO3 Nanostructures by Ultrasonic Method. Ultrason. Sonochem. 2008, 15 (5), 833−838. (83) Ma, X.; Su, C.; Yang, L.; Li, L.; Wang, K.; Zhou, J.; Yuan, S. Size- and Morphology-Controlled Biomimetic Synthesis of Hierarchical Hollow BaCO3. CrystEngComm 2012, 14 (24), 8554−8561. (84) Ge, W.; Zhang, H.; Wang, J.; Liu, J.; Li, H.; Cheng, X.; Xu, H.; Xu, X.; Hu, X.; Jiang, M. The Thermal and Optical Properties of BaWO4 Single Crystal. J. Cryst. Growth 2005, 276 (1−2), 208−214. (85) Errandonea, D.; Manjón, F. J. On the Ferroelastic Nature of the Scheelite-to-Fergusonite Phase Transition in Orthotungstates and Orthomolybdates. Mater. Res. Bull. 2009, 44 (4), 807−811. (86) Hardcastle, F. D.; Wachs, I. E. Determination of the Molecular Structures of Tungstates by Raman Spectroscopy. J. Raman Spectrosc. 1995, 26 (6), 397−405. (87) Wei, Z. G.; Sandstroröm, R.; Miyazaki, S. Shape-Memory Materials and Hybrid Composites for Smart Systems: Part I ShapeMemory Materials. J. Mater. Sci. 1998, 33 (15), 3743−3762. (88) Manjon, F. J.; Errandonea, D.; Garro, N.; Pellicer-Porres, J.; López-Solano, J.; Rodríguez-Hernández, P.; Radescu, S.; Mujica, A.; Muñoz, A. Lattice Dynamics Study of Scheelite Tungstates under High Pressure PbWO4-II. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74 (14), 144112. (89) Christofilos, D.; Ves, S.; Kourouklis, G. A. Pressure Induced Phase Transitions in Alkaline Earth Tungstates. Phys. Status Solidi B 1996, 198 (1), 539−544. (90) Christofilos, D.; Papagelis, K.; Ves, S.; Kourouklis, G. A.; Raptis, C. High-Pressure Raman Study and Lattice Dynamical Calculations for SrWO4. J. Phys.: Condens. Matter 2002, 14 (47), 12641−12650. (91) Dorenbos, P.; Rogers, E. G. Vacuum Referred Binding Energies of the Lanthanides in Transition Metal Oxide Compounds. ECS J. Solid State Sci. Technol. 2014, 3 (8), R150−R158.

(56) Sokolenko, E. V.; Zhukovskii, V. M.; Buyanova, E. S.; Krasnobaev, Y. A. Luminescent Properties of Oxygen-Disordered Scheelite-Structure Tungstates: Steady-State Luminescence. Inorg. Mater. 1998, 34 (5), 611−615. (57) Shi, C.; Wei, Y.; Yang, X.; Zhou, D.; Guo, C.; Liao, J.; Tang, H. Spectral Properties and Thermoluminescence of PbWO4 Crystals Annealed in Different Atmospheres. Chem. Phys. Lett. 2000, 328 (1− 2), 1−4. (58) Chen, Y.; Shi, C.; Hu, G. Influence of Sb Doping on the Luminescent Properties of PbWO4 Single Crystals. J. Appl. Phys. 2000, 87 (3), 1503−1506. (59) Qi, Z.; Shi, C.; Zhou, D.; Tang, H.; Liu, T.; Hu, T. The Green Emission and Local Structure of the Scintillator PbWO4. Phys. B 2001, 307 (1−4), 45−50. (60) Sinelnikov, B. M.; Sokolenko, E. V.; Zvekova, E. G. Effect of Nonstoichiometry on the 420-nm Luminescence of Calcium Tungstate. Inorg. Mater. 1996, 32 (9), 999−1001. (61) Longo, V. M.; Figueiredo, A. T. de; Campos, A. B.; Espinosa, J. W. M.; Hernandes, A. C.; Taft, C. A.; Sambrano, J. R.; Varela, J. A.; Longo, E. Different Origins of Green-Light Photoluminescence Emission in Structurally Ordered and Disordered Powders of Calcium Molybdate. J. Phys. Chem. A 2008, 112 (38), 8920−8928. (62) Luminescent Materials and Applications; Kitai, A., Ed.; Wiley: 2008. (63) Luminescence: From Theory to Applications; Ronda, C. R., Ed.; Wiley-VCH: Weinheim, Germany, 2007. DOI: 10.1002/ 9783527621064. (64) Energy Transfer. In Luminescent Materials; Blasse, G., Grabmaier, B. C., Eds.; Springer-Verlag: Berlin, Heidelberg, 1994; pp 91−107. DOI: 10.1007/978-3-642-79017-1_5. (65) Fang, Y.; Xiong, Y.; Zhou, Y.; Chen, J.; Song, K.; Fang, Y.; Zhen, X. Structure and Optical Properties of Cerium(III)-Doped PbWO4 Micro-Crystals Synthesized by a Facile Wet Chemical Method. Solid State Sci. 2009, 11 (6), 1131−1136. (66) Naik, S. J.; Subramanian, U.; Tangsali, R. B.; Salker, A. V. Optical Absorption and Photoluminescent Studies of Cerium-Doped Cobalt Tungstate Nanomaterials. J. Phys. D: Appl. Phys. 2011, 44 (11), 115404. (67) Mohamed Jaffer Sadiq, M.; Samson Nesaraj, A. Soft Chemical Synthesis and Characterization of BaWO4 Nanoparticles for Photocatalytic Removal of Rhodamine B Present in Water Sample. J. Nanostructure Chem. 2015, 5 (1), 45−54. (68) Varshni, Y. P. Temperature Dependence of the Energy Gap in Semiconductors. Physica 1967, 34 (1), 149−154. (69) O’Donnell, K. P.; Chen, X. Temperature Dependence of Semiconductor Band Gaps. Appl. Phys. Lett. 1991, 58 (25), 2924− 2926. (70) Wang, Y.; Hrubiak, R.; Turczyński, S.; Pawlak, D. A.; Malinowski, M.; Włodarczyk, D.; Kosyl, K. M.; Paszkowicz, W.; Przybylińska, H.; Wittlin, A.; et al. Spectroscopic Properties and Martensitic Phase Transition of Y4Al2O9:Ce Single Crystals under High Pressure. Acta Mater. 2019, 165, 346−361. (71) Ryu, J. H.; Park, G. S.; Kim, K. M.; Lim, C. S.; Yoon, J.-W.; Shim, K. B. Synthesis of CaWO4 Nanocolloidal Suspension via Pulsed Laser Ablation and Its Optical Properties. Appl. Phys. A: Mater. Sci. Process. 2007, 88 (4), 731−736. (72) Feng, W.-L.; Zhao, M.-F.; Xue, J.-Y.; Tian, X.-J. Photoluminescence Properties of (Ba1−xEux)WO4 Red Synthesized by the Coprecipitation/Calcination Method. J. Alloys Compd. 2012, 521, 146−149. (73) Liao, J.; You, H.; Zhang, S.; Jiang, J.; Qiu, B.; Huang, H.; Wen, H. Synthesis and Luminescence Properties of BaWO4:Pr3+ Microcrystal. J. Rare Earths 2011, 29 (7), 623−627. (74) Cao, R.; Xu, H.; Peng, D.; Gou, Q.; Zhou, S.; Liu, P.; Yu, X. Synthesis and Luminescence Properties of BaWO4:Sm3+, Mo6+/K+ Red Phosphor. J. Mater. Sci.: Mater. Electron. 2015, 26 (9), 6776− 6780. (75) Tyagi, M.; Sangeeta; Sabharwal, S. C. Luminescence Properties of BaWO4 Single Crystal. J. Lumin. 2008, 128 (9), 1528−1532. M


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