The possibility of tuning the optical band gap, crystal structure and persistent luminescence performance of a Cr3+-doped LiGa5O8 spinel by partially replacing Ga with Al and/or In has been studied extensively. For this purpose, a series of Cr3+-doped Li(Ga1-x-yAlxIny)5O8 (x = 0…0.5; y = 0…0.1) microcrystalline phosphors were synthesised using a conventional solid-state reaction method and characterised using powder X-ray diffraction, SEM-EDX and luminescence techniques. DFT-based electronic structure calculations were carried out for the same Li(Ga1-x-yAlxIny)5O8 compositions, and the results were compared with the experimental ones. Based on the studies performed, the mechanism of Al and In incorporation into the LiGa5O8 spinel structure as well as the tuning of the crystal lattice parameters, the local structure of M3+ (M = Ga, Al, and In) cations and the optical band gap of the material have been established. The multicentre structure and the broadening of the local structural disorder of the octahedrally coordinated Cr3+ centres observed in this case have been confirmed by high-resolution, low-temperature photoluminescence measurements. Band gap engineering through alterations in the chemical composition of the LiGa5O8 spinel, as well as the depth of the native point defects responsible for charge trapping, allows for the efficient tuning of the thermoluminescence and persistent luminescence properties of Li(Ga1-x-yAlxIny)5O8:Cr3+ phosphors. Thus, the room-temperature persistent luminescence performance of the phosphors modified by the addition of Al and annealing under an oxygen-free atmosphere was increased threefold compared to the pristine LiGa5O8:Cr3+ phosphor synthesised under the same conditions.

The work introduces a new type of UV-emitting high-Z OSL detectors based on YAlO3:Bi3+ (YAP: Bi) perovskite. Two kinds of YAP: Bi solid state detectors have been fabricated and compared to BeO Thermalox®995 chips. The first ones are single-crystalline detectors cut from a Czochralski grown crystal. The second are detectors cut from the high-density bulk ceramics prepared by the high-pressure high-temperature (HPHT) pressing technique from nanocrystalline powder derived from sol-gel synthesis. The YAP: Bi ceramic detectors studied suffer from strong thermal fading, which makes their use impractical, at least until technological methods to modify this property of ceramics are found. The single crystalline YAP: Bi detectors show very low thermal fading, a wide linearity range of dose response and a sensitivity comparable to BeO, which together with the optical registration in the same UV range, compatible optical stimulation by blue light and similar registration times in CW-OSL mode make them ideal high-Z detectors that can be used alone or in tandem with BeO detectors.

Crystals of Ca10.5-xTMx(VO4)7 (TM = Co, Cu), belonging to the whitlockite family, were synthesized by solid-state reaction and studied as a function of the TM content (x) for the first time. The structure was refined at ambient conditions and at high temperatures up to 1200 K using the Rietveld method. The unit cell size significantly decreases with increasing TM content up to the solubility limit, xlim, which is 0.78(3) for TM = Co and 0.75(4) for TM = Cu. Occupancy factors show a preference for the M5 site by Co/Cu. The unit cell size varies smoothly with temperature, while the axial ratio exhibits nonlinear behaviour above approximately 800 K. The thermal expansion coefficient was determined from 300-1100 K. Atomic arrangement modifications at higher temperatures are indicated by changes in the axial ratio, the thermal expansion coefficient, and the reduction of fractional TM occupancy at the M5 site at specific temperatures.

Optical spectra of transitions between the 3H6 and 3F4 manifolds of Tm3+ ions in disordered crystal lattices of Gd3Ga5O12–Gd3Al5O12, Lu2SiO5–Gd2SiO5 solid solutions and in LiNbO3 crystals were recorded at several temperatures between 80 K and 300 K. Obtained experimental data were analyzed to examine the spectroscopic peculiarities of Tm3+ transitions that governs the temperature dependence of absorption and emission properties. The impact of structural disorder on the lasing ability at cryogenic temperatures upon resonant (in-band) optical pumping was assessed. In particular, it was found that peak stimulated emission cross section values at respective free generation wavelengths increase by a factor of two when samples examined are cooled down from 300 K to 80 K. At 80 K the laser operation wavelength can be tuned in regions 1940–2063 nm for GAGG: Tm3+, 1764–1853 nm for LNO: Tm3+ and 1778–2074 nm for LGSO: Tm3+.

Optical absorption and luminescence spectra were recorded at different temperatures for the Czochralski grown crystals of Lu2SiO5-Gd2SiO5 solid solution (LGSO) single doped with Er3+ and double-doped with Er3+ and Yb3+. Optical anisotropy and intensities of Er3+ transitions in LGSO were determined from analysis of room temperature polarized absorption spectra. The crystal field splitting of multiplets and linewidths of Er3+ transitions were derived from low-temperature 6 K ab-sorption and luminescence spectra. Spectra of the Er3+ luminescence excited at 380 nm and of up-converted luminescence excited at 803 nm and 975 nm were recorded for samples with different Yb3+ concentration as a function of temperature between 300 K and 775 K. Thermographic parameters of LGSO:0.5%Er,5%Yb sample derived for FIR (H-2(11/2)/S-4(3/2)) are comparable to those observed in other hosts co-doped with Er and Yb ions, corroborating suitability of this system for the purpose of optical thermometry. For all samples studied the relative thermographic sensitivity Sr(max) values for the S-4(3/2)/F-4(9/2), F-4(7/2)/S-4(3/2) and F-4(7/2)/F-4(9/2) pairs are smaller as compared to H-2(11/2)/S-4(3/2) pair but they occur at higher temperatures and therefore may be of interest for specific applications.

Single crystals of Ca1-3x-yMny□xNd2x(MoO4)1-3x(WO4)3x molybdato-tungstates (□ denotes vacant sites) with a scheelite-type structure have been successfully grown by the Czochralski technique in an inert atmosphere. This paper presents the results of structural, optical, magnetic, and electrical properties, as well as the broadband dielectric spectroscopy measurements of single crystals with different Nd3+ ion concentrations, i.e., when x = 0.0050 or x = 0.0098, and with constant content of Mn2+ ions, i.e., when y = 0.0050. Our magnetic studies have shown that substitution of diamagnetic Ca2+ ions in the CaMoO4 matrix with paramagnetic Nd3+ ones with a content not exceeding 0.02 and having a screened 4f-shell revealed a significant effect of orbital diamagnetism and Van Vleck's paramagnetism. Both single crystals have revealed residual electrical conductivity without an intrinsic region and a change of sign of the Seebeck coefficient at ca. 230 K. Dielectric spectroscopy measurements have shown constant values of relative permittivity (εr ∼ 8) and loss tangent (tan δ ∼ 0.01) both up to 400 K and up to 1 MHz, as well as the Fermi energy (∼0.04 eV) and the Fermi temperature (∼500 K) determined for both crystals from the diffusion component of thermopower. These results suggest the presence of shallow acceptor and donor levels in the studied crystals.

Single crystals of Pb1−3x▯xNd2x(MoO4)1−3x(WO4)3x (PNMWO) with scheelite-type structure, where ▯ denotes cationic vacancies, have been successfully grown by the Czochralski method in air and under 1 MPa. This paper presents the results of structural, optical, magnetic and electrical, as well as the broadband dielectric spectroscopy measurements of PNMWO single crystals. Research has shown that replacing diamagnetic Pb2+ ions with paramagnetic Nd3+ ones, with a content not exceeding 0.01 and possessing a screened 4f-shell, revealed a significant effect of orbital diamagnetism and Van Vleck’s paramagnetism, n-type electrical conductivity with an activation energy of 0.7 eV in the intrinsic area, a strong increase of the power factor above room temperature for a crystal with x = 0.005, constant dielectric value (~30) and loss tangent (~0.01) up to room temperature. The Fermi energy (~0.04 eV) and the Fermi temperature (~500 K) determined from the diffusion component of thermopower showed shallow donor levels.

The independently measured pressure and temperature evolution of Ca 3 V 2 O 8 is characterized by X-ray diffraction and DFT calculations. A pressure-induced phase transition is discovered and the crystal structure for the HP phase reported.

Crystals of Gd3Al2.5Ga2.5O12:Er3+, (Lu0.3Gd0.7)2SiO5:Er3+ and LiNbO3:Er3+ compounds differing in origin and the nature of their inherent structural disorder were crystalized. Optical absorption and luminescence spectra for transitions between the 4I15/2 and the 4I13/2 multiplets of Er3+ ions for the crystal samples were recorded versus temperatures in the region of 80–300 K. Gathered data were analyzed thoroughly providing the in-depth knowledge of the effects of temperature on intensities, wavelengths and bandwidths of Er3+ transitions. The information acquired together with the knowledge of significant structural dissimilarities of the host crystals chosen made it possible to propose an interpretation of the impact of a structural disorder in Er3+-doped crystals on their spectroscopic properties, and to determine their lasing ability at cryogenic temperatures upon resonant (in-band) optical pumping.

High resolution spectra of optical absorption and luminescence were recorded at temperatures below 6 K for Pr3+, Sm3+, Er3+, and Yb3+ ions embedded in Gd3Ga5O12–Gd3Al5O12 and Lu2SiO5–Gd2SiO5 solid solution crystals fabricated by the Czochralski method. Detailed analysis of acquired spectral bands related to transitions between the lowest energy components (0–0) lines of initial and terminal multiplets provided quantitative information regarding bandwidths, peak transition energies and number of band components. Combination of these data with structural peculiarities of the hosts made it possible to propose the interpretation of optical interaction in systems under study.

The effect of application of filters, made of different materials and various thickness, is studied by Monte Carlo calculations using MCNP6.2 code. The calculated data were validated by experimental studies (benchmark tests). Experimental results obtained for YAlO3:Mn high-Z TL detectors irradiated to different standard ISO radiation qualities (X-ray series N-40, N-60, N-80, N-100, N-120, N-150 and N-200 as well as isotopic series S-Cs) modified by various metal (copper and aluminum) filters of thickness of 0.5, 0.8 and 1 mm. The experimental results are compared with results of Monte Carlo simulations done for the same 'radiation-attenuator-detector' combinations and geometry. Obtained results show good consistence between the experimental and calculated data that testifies adequacy of the used calculations and their applicability to modeling of modification of an output from the high-Z detectors exposed to photons of various energies.

Optical absorption and luminescence spectra of Yb3+ ions in Gd3(Al0.5Ga0.5)5O12, (Lu0.3Gd0.7)2SiO5 and YAl3(BO3)4 hosts crystals were examined as a function of temperature between 80 and 300 K. The analysis of obtained data provided detailed information on the thermally induced change of intensities, wavelengths and bandwidths of Yb3+ transitions that governs the temperature dependence of absorption and emission cross section spectra. This information, combined with structural peculiarities of the systems studied made it possible to propose generalizations regarding the importance of the structural disorder in the Yb3+-doped laser crystals operating at cryogenic temperatures.

The work presents an experimental study of (Y, Gd)AlO3:Mn2+ crystals grown by the Czochralski method under inert gas atmosphere and by the floating-zone method in the ambient air atmosphere. The Mn2+/Mn4+ ratio, the energy depth of the main dosimetric and shallow traps are analyzed in comparison with the YAlO3:Mn2+ and (Y, Lu)AlO3:Mn2+ crystals studied before.

A detailed electron paramagnetic resonance EPR), optical absorption, luminescence, and thermoluminescence (TL) study of Mn-doped YAlO3 (YAP) single crystals was performed. The crystals were grown by the Czochralski method from stoichiometric (Y/Al = 1) and yttrium-rich (Y/Al = 1.04) melts and codoped with either Si or Hf ions. The EPR measurements revealed the presence of only one type of Mn2+ center, that is, isolated Mn ions occupying Y sites (Mn-Y(2+)). It was found that only in yttrium-rich crystals, the Mn-Y(2+) ions undergo recharging to Mn-Y(2+) under ionizing irradiation, indicating that this process requires the availability of sufficiently deep electron traps. The initial charge state is fully restored only after subsequent warming above 600 K. The presented results demonstrate, moreover, that Mn-Y(3+) + e -> Mn-Y(2+) recombination is not the most efficient excitation channel of the green T-4(1) -> (6)A(1) emission of Mn-Y(2+), possibly because of the huge energy difference between the recombination (>5.39 eV) and excitation (3 eV) energies. In contrast, energy transfer to Mn-Y(2+) proves to be dominant. A general model of trapping and recombination mechanisms responsible for TL of YAP:Mn crystals above room temperature is proposed. Besides Mn(Y)(2+ )ions and the defect-related electron and hole traps intrinsic to the YAP lattice, the model includes also unintentional dopants such as FeAl acting as deep hole traps, as well as Mn-Al(4+) and Cr-Al(3+) ions acting both as deep hole and electron traps.

The possibility of band gap engineering (BGE) in RAlO3 (R = Y, La, Gd, Yb, Lu) perovskites in the context of trap depths of intrinsic point defects was investigated comprehensively using experimental and theoretical approaches. The optical band gap of the materials, Eg, was determined via both the absorption measurements in the VUV spectral range and the spectra of recombination luminescence excitation by synchrotron radiation. The experimentally observed effect of Eg reduction from ∼8.5 to ∼5.5 eV in RAlO3 perovskites with increasing R3+ ionic radius was confirmed by the DFT electronic structure calculations performed for RMIIIO3 crystals (R = Lu, Y, La; MIII = Al, Ga, In). The possibility of BGE was also proved by the analysis of thermally stimulated luminescence (TSL) measured above room temperature for the far-red emitting (Y/Gd/La)AlO3:Mn4+ phosphors, which confirmed decreasing of the trap depths in the cation sequence Y → Gd → La. Calculations of the trap depths performed within the super cell approach for a number of intrinsic point defects and their complexes allowed recognizing specific trapping centers that can be responsible for the observed TSL. In particular, the electron traps of 1.33 and 1.43 eV (in YAlO3) were considered to be formed by the energy level of oxygen vacancy (VO) with different arrangement of neighboring YAl and VY, while shallower electron traps of 0.9-1.0 eV were related to the energy level of YAl antisite complexes with neighboring VO or (VO + VY). The effect of the lowering of electron trap depths in RAlO3 was demonstrated for the VO-related level of the (YAl + VO + VY) complex defect for the particular case of La substituting Y.

The whitlockite-related materials have attracted researchers' attention because of their potential application in various fields, especially in optoelectronics. In the present work, the structure of novel whitlockite-related oxides Ca10TM0.5(VO4)7 (TM = Co, Cu) is studied at room and high temperatures, using X-ray powder diffraction. These compounds form by fractional substitution of divalent transition metal atoms into the Ca3(VO4)2 lattice. Rietveld refinements provided the structural details. The lattice parameters are a = 10.78074(6) Å, c = 37.8196(2) Å, and V = 3806.67(4) Å3 for Ca10Co0.5(VO4)7 and a = 10.78710(7) Å, c = 37.8997(3) Å, and V = 3819.23(4) Å3 for Ca10Cu0.5(VO4)7. Structure refinement results show that among the five available sites (M1-M5), the M2+ ions select the M5 site. This finding is confirmed by analysis of interatomic distances: due to the difference in size between TM and Ca ions sharing the M5 site, the M5-O distance shortens by about 5.0% for Ca10Co0.5(VO4)7 and 2.7% for Ca10Cu0.5(VO4)7 with respect to the unsubstituted parent compound, Ca3(VO4)2. The observed trends in the crystallographic properties of the studied crystals are in line with those of previously reported structurally related phosphates, Ca10.5-xMx(PO4)7 (M = Mg or divalent transition metal). Moreover, the observed tendency for occupation of M5 by small divalent ions follows the earlier theoretical results. For cobalt and copper substituted orthovanadate and orthophosphate whitlockite related materials, a linear variation in the unit cell size is demonstrated. The common equation for evaluation of volume is applicable to the substitution of the two transition metals in orthovanadate and orthophosphate whitlockite related materials. Thermal expansion is investigated for both compounds. The variations of the lattice parameters and the thermal expansion coefficient with temperature are determined in the 300-810 K range. The lattice parameter, a, expands by 0.80% for Ca10Co0.5(VO4)7 and 0.74% for Ca10Cu0.5(VO4)7 in this range. The lattice parameter, c, enlarges by about 0.70% for both samples. In the studied temperature range, the volume thermal expansion coefficient of Ca10Co0.5(VO4)7 increases from 37.2 to 44.8 MK-1 and for Ca10Cu0.5(VO4)7, it increases from 35.1 to 45.2 MK-1; the observed expansion anisotropy is smaller than those of other related compounds.

Optical absorption spectra and luminescence spectra were recorded as a function of temperature between 295 K and 800 K for single crystal samples of Gd2SiO5:Dy3+, Lu2SiO5:Dy3+, LiNbO3:Dy3+, and Gd3Ga3Al2O12:Dy3+ fabricated by the Czochralski method and of YAl3(BO3)4:Dy3+ fabricated by the top-seeded high temperature solution method. A thermally induced change of fluorescence intensity ratio (FIR) between the 4I15/2→ 6H15/2 and 4F9/2 → 6H15/2 emission bands of Dy3+ was inferred from experimental data. It was found that relative thermal sensitivities SR at 350 K are higher for YAl3(BO3)4:Dy3+ and Lu2SiO5:Dy3+than those for the remaining systems studied. Based on detailed examination of the structural peculiarities of the crystals it was ascertained that the observed difference between thermosensitive features cannot be attributed directly to the dissimilarity of structural factors consisting of the geometry and symmetry of Dy3+ sites, the number of non-equivalent Dy3+ sites, and the host anisotropy. Instead, it was found that a meaningful correlation between relative thermal sensitivity SR and rates of radiative transitions of Dy3+ inferred from the Judd–Ofelt treatment exists. It was concluded that generalization based on the Judd–Ofelt parameters and luminescence branching ratio analysis may be useful during a preliminary assessment of thermosensitive properties of new phosphor materials.

Single crystal of Eu3+-doped cadmium molybdate (Cd0.9268▯0.0244Eu0.0488MoO4, where ▯ denotes cationic vacancies) has been successfully grown by the Czochralski method in air and under 1 MPa. X-ray diffraction analysis indicates that as-grown single crystal exhibits tetragonal scheelite-type structure (a = b = 5.16188(14) Å; c = 11.2080(5) Å; space group I41/a). Eu3+ ions do not show long-range order and they are randomly distributed in CdMoO4 framework substituting Cd2+ ones. UV–vis diffuse reflectance measurements revealed very close optical band gap (Eg) values, i.e. ∼1.74 eV along [100] and [001] crystallographic directions that are twice smaller than Eg of microcrystalline pure CdMoO4 as well as powder Eu3+-doped single crystal. Magnetic and electrical studies of Eu3+-doped cadmium molybdate single crystal showed a paramagnetic and n-type semiconducting behaviour with the metal-insulator transition above 350 K along both crystallographic directions. Dielectric results analysis using the Cole-Cole fit function revealed that the dipole relaxation process has different time scale depending on the crystallographic direction and exhibits Arrhenius temperature dependence for both studied directions. This fact is accompanied by the colossal dielectric permittivity with εr > 8⋅103. The above results are considered in the framework of narrow europium multiplets of energy comparable to thermal energy.

We use micro-beam synchrotron x-ray diffraction to study the pressure-induced amorphization of nano sized and single crystals of Y3Ga6012 up to pressures exceeding 1 Mbar in static compression. The abrupt pressure-induced amorphization found for both 56 nm and bulk micrometric crystals at around 76 GPa independently of the pressure transmitting medium employed demonstrates its intrinsic nature, previously predicted at 79 GPa by ab initio calculations. The single crystal structural solution at 50 GPa shows that the contraction of the unit-cell, mostly accommodated by the compressible YO8 dodecahedra, gives rise to a regularization and tilting increase of the Ga06 polyhedra with the Y O-Ga angle changing from 104.84 to 102.34 in 50 GPa. We obtain a bulk modulus of 178(3) GPa for the single crystal and 172(3) GPa for the nanocrystals in excellent agreement with previous calculations. (C) 2020 Elsevier B.V. All rights reserved.