Scheelite-type Structure Research Articles

Ab-Initio study of water molecule adsorption on monoclinic Scheelite-Type BiVO4 surfaces

Herein, we present a study on the adsorption of one water molecule (bonded to either Bi or V sites) and two water molecules (bonded to both Bi and V sites) onto seven low-index surfaces (001), (010), (011), (100), (101), (110), and (111) as well as one high-index surface (211) of a monoclinic scheelite-type BiVO4 crystal structure using ab initio calculations. By predicting the adsorption energies for different facets, we find that water adsorption is more likely to occur on Bi sites. However, for the (001) and (211) surfaces, water adsorption is more likely on the V sites. Furthermore, we find that the studied low-index facets can be grouped into four distinct categories. Facets within the same group exhibit similar water adsorption energies. These groups are ((001)), ((010), (100)), ((110)), and ((011), (101), (111)). For low-index surfaces, favorable adsorption occurs on the (001) surface on the V sites.

A comprehensive study on the structural, radioluminescence and thermoluminescence properties of scheelite and wolframite type tungstates

In this study, the structural and spectroscopic properties of AWO4 (A = Ca, Sr, Ba, Mg, Zn and Cu) ceramics synthesized by conventional solid-state reaction technique were investigated. The local structure around the W and accessible various metal cations have been investigated using X-ray absorption spectroscopy (XAS). Local structural changes have been observed in scheelite-type structures and wolframite type structures. β-irradiated thermoluminescence (TL) glow curve for all samples were recorded and analyzed to obtain the trapping parameters, such as activation energy (E), order of kinetics (b) and frequency factor (s). The scheelite-type tungstates exhibited a distinct TL peak of second order kinetics indicating the presence of deeper traps, whereas wolframite-type tungstates did not exhibit such peak. Apart from BaWO4, and CuWO4, all the other samples demonstrated broad radioluminescence (RL) in the range 400 nm–700 nm when subjected to X-ray excitation. The RL and TL properties of the AWO4 ceramics are correlated with their respective crystallographic structure.

Tetrahedra Rotational and Displacive Disorder in the Scheelite-Type Oxide CsReO4.

Scheelite-type metal oxides are a notable class of functional materials, with applications including ionic conductivity, photocatalysis, and the safe storage of radioactive waste. To further engineer these materials for specific applications, a detailed understanding of how their properties can change under different conditions is required─not just in the long-range average structure but also in the short-range local structure. This paper outlines a detailed investigation of the metal oxide CsReO4, which exhibits an uncommon orthorhombic Pnma pseudo-scheelite-type structure at room temperature. Using synchrotron X-ray diffraction, the average structure of CsReO4 is found to undergo a transformation from the orthorhombic Pnma pseudo-scheelite-type structure to the tetragonal I41/a scheelite-type structure at ∼440 K. In the X-ray pair distribution function analysis, lattice strain and rotations of the ReO4 tetrahedra are apparent above 440 K despite the increase in long-range average symmetry, revealing a disconnect between the structural models at different length scales. This study demonstrates how the bonding requirements and ionic radii of the A-site cation can induce disorder that is detectable at different length scales, affecting the physical properties of the material.

New sustainable yellow pigments in scheelite-type solid solutions: (LiLaCa)1/3MoO4−BiVO4 with high NIR reflectance as cool colorants

Abstract New toxic-metal-free sustainable yellow pigments have been developed in scheelite-type solid solutions: [(Li0.33Ca0.33La0.33)xBi1−x][MoxV1−x]O4. The developed pigments show a structural transition from monoclinic to tetragonal scheelite-type structures upon the substitution of LiCaLaMoO4 into the BiVO4 system. The absorption edge is gently blue-shifted from the parent BiVO4 by widening the bandgap upon the incorporation of various metal ions such as Li, La, Ca, and Mo into the lattice, allowing different intense yellow hues to be produced. The typical pigment [(LiLaCa)0.198Bi0.8][Mo0.2V0.8]O4 showed the most vivid yellow hue with a b* value of 78.32, which is comparable to the commercial Sicopal yellow (b* = 76.9). Further, the developed toxic-metal-free pigments have high near-infrared reflectance, making them sustainable as eco-friendly cool colorants for paint and plastic formulations.

Comparative electrochemical investigation for scheelite structured metals tungstate (MWO4 (M = Ni, Cu and Co)) nanocubes for high dense supercapacitors application

Scheelite structured metal tungstate MWO4 (M = Ni, Cu and Co) nanocubes were synthesized through the chemical reflux for supercapacitors application and ceyltrimethylammonium bromide (C-TAB) as surfactant. In X-ray diffraction (XRD) result are fit with relevant JCPDS cards, synthesized materials are closely matched with monoclinic and triclinic crystal phase corresponding to NiWO4, CoWO4 and CuWO4 with Scheelite type structure. To resist the growth of the particles and succeeding nanocubes morphology were achieving by using PEG-400 and C-TAB act as a surfactant. The prepared modified electrodes were examined electrochemical analysis after successive coating of working material in empty Ni foil. From the galvanostatic charge-discharge (GCD) comparative analysis, fast ions movements are interacts through the aqueous electrolyte medium with nanocubes NiWO4 electrode are achieving specific capacitance of 1185 Fg−1 at 0.5 Ag−1 and cyclic stability 93.084 % (retentivity) formerly compare to CuWO4 and CoWO4 electrodes.

Comparative characteristic of Bi- and La- doped (Ca/Sr)MoO4 -based materials with a defect scheelite-type structure

CaMoO4- and SrMoO4-based scheelite-type phases are noteworthy functional materials, whose properties strongly correlate with their structure. This work is devoted to La- or Bi-doped scheelite-type molybdates. The purpose of the present study is to quantify the effect of isolated electron pairs of bismuth on the distortion of the structure and related properties. Conventional solid-state technology was used for the synthesis of (Ca/Sr)1–3xLa2xФxMoO4 and Sr1–3xBi2xФxMoO4, (0.025≤ x ≤ 0.275). The structure was investigated by X-ray powder diffraction and Raman spectroscopy. Rates of structure distortion were characterised by the analysis of the autocorrelation function (AAF) of Raman spectra. Energy gaps were calculated by the Kubelka-Munk method. The conductivity was studied with a.c. impedance spectroscopy. For (Ca/Sr)1−3x(Bi/La)2xФxMoO4 series 0.025 ≤ x ≤ 0.15 compositions show a basic defect scheelite structure, while 0.15 x ≤ 0.225 compositions of Bi-doped samples exhibit tetragonal supercells. The chemical compression of unit cell is more evident in the case of Bi-doping, indicating the preferred orientation of the isolated electron pairs. The distortion of MoO4 polyhedra showed by AAF was more significant for Sr1−3xBi2xФxMoO4 than for Sr1−3xLa2xФxMoO4, the Δcorr parameters for Bi-doped compositions were almost double in comparison with La-doped one in the range of 50–600 cm–1 of the Raman shift. The «critical» x = 0.15 point was also clearly indicated by Δcorr parameter. The AAF of the Raman spectra of solid oxides was shown to be a good tool for prediction of properties and points of phase transitions in solid oxides.

Fast microwave synthesis and white luminescent emission from Pb1–2xCaxSrxMoO4 (x=0; 0.3; and 0.5) particles

Scheelite with ABO4 structure has been increasingly highlighted in the optoelectronic field. In this study, PbMoO4, Pb0.4Ca0.3Sr0.3MoO4, and Ca0.5Sr0.5MoO4 particles were fast synthesized via microwave-assisted hydrothermal method for 1, 2, 4, and 8 min. The particles were characterized by X-ray diffraction (XRD) and Rietveld refinement, ultraviolet-visible (UV–vis) absorption spectroscopy, photoluminescence (PL), and field emission scanning electron microscopy (FE-SEM). The diffractogram showed a scheelite-type tetragonal structure and a unit cell with space group I41/a, as well as the presence of defects with an increase in the synthesis time in PbMoO4 and Pb0.4Ca0.3Sr0.3MoO4 samples. Rietveld refinement data possibilities the evaluation of distortions in the tetrahedral [MoO4] clusters and FE-SEM images showed octahedral, spherical particles, and flower-like morphology due to the additions of calcium and strontium in the structure. The bandgap of the materials varied between 3.32 and 4.18 eV, which was affected by the increase in the presence of dopants in the materials. The photoluminescence presented by the PL emission spectral showed a typical emission peak in green with the PL signal intensity decreasing with the synthesis time increase for the PCSMO samples, and the longer the synthesis time, the higher the PL signal for CSMO samples. CCT coordinates have been calculated for all samples and its value exhibited that, overall emission is near white light. This parameter infers that Pb0.4Ca0.3Sr0.3MoO4 sample, synthesized in 1 min, may have potential application in commercial LEDs, due to near white emission and fast microwave synthesis.

Magnetic and Electrical Characteristics of Nd3+ and Mn2+-Co-doped Calcium Molybdato-Tungstates Single Crystals.

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.

Structural and Dielectric Characterization of Nanocrystalline BaWO<sub>4</sub>

BaWO4 samples prepared by the chemical precipitation method were structurally characterized by X-ray diffraction and scanning electron microscopic measurements. The results confirm the tetragonal scheelite-type structure of the BaWO4 sample. The dependence of εꞌ, tanδ, and ac conductivity with frequency and temperature was studied for the nanocrystalline BaWO4 calcined samples. It is found that all these factors hinge on temperature and frequency. The values of εꞌ and tanδ are high at low frequencies and their values decrease quickly with the increase in frequency. Both εꞌ and tanδ attain steady values at high frequency regime. The space charge polarization is the cause of exponential decrease of εꞌ at the low frequency. It is also found that the calcination temperature is a key factor in determining the dielectric properties of a material. The low and steady value of the dissipation factor and an almost steady value of the εꞌ suggest the suitability of BaWO4 materials in low-temperature co-fired ceramic applications. The deviation of ac conductivity against frequency comply with Jonscher’s power law, however, a small deviation in the low-frequency region is due to the Maxwell-Wagner-type effect.

Luminescence properties of Ln3+ doped BaMoO4 (Ln3+ = Sm3+ and Dy3+) phosphors under UV excitation

Ln3+ doped BaMoO4 (Ln 3+ = Sm3+ and Dy3+) nanoparticles have been synthesized successfully using ethylene glycol as a solvent. All the prepared samples are well indexed into the pure scheelite-type tetragonal structure of BaMoO4, the doping of trivalent ions (Ln3+) in to the bivalent host ions (Ba2+) sites of the BaMoO4 do not change the pure tetragonal structure of BaMoO4 although there is a charge difference between Ba2+ and Ln3+. The SEM image of the prepared BaMoO4 samples shows a number of uniform shuttle-like nanocrystalline with protrusion in the middle. The doping of Ln3+ ions do not change the morphology of the BaMoO4. The photoluminescence study has been carried in detail by measuring the excitation and emission spectra of the prepared samples. The excitation spectrum consists of a broad band with a maximum at about 268 nm thereby demonstrating the transfer of energy from MoO2-4 groups to the doped Ln3+ ions. Yellow emission is dominated over blue in case of Dy3+ doped BaMoO4 and in Sm3+ doped BaMoO4 red emission dominated over other lights under UV excitation. The luminescence intensity is highest at 10 at.% Ln3+ under the UV excitation. Under this UV excitation BaMoO4:Sm3+ exhibits strong orange-red and BaMoO4:Dy3+ greenish-yellow emissions. Excitation of Dy3+ in thin film of BaMoO4 emits an intense yellow color which could be applicable in the field of biological assays and biological fluorescence labeling.

Electrochemical properties of double molybdate LiSm(MoO4)2 ceramics with ultra-low sintering temperature

A novel double molybdate LiSm(MoO4)2 composition was prepared through a conventional solid-state ceramic route for its potential application as a solid-state Li+-ion conductor for low-loss dielectric ceramics. Ceramic disks were prepared by uniaxial pressing and sintered at the low temperature of 620 °C. Phase formation was confirmed through powder X-ray diffraction (XRD), which was indexed to the tetragonal crystal system with a Scheelite-type structure. Laser Raman spectroscopy showed that the MoO42− tetrahedra in the LiSm(MoO4)2 material are more regular, suggesting high symmetry for the compound. Scanning Electron Microscopy (SEM) revealed the formation of polygonal grains with an average grain size of less than 1 μm. The electrochemical properties of the ceramic system were investigated by Electrochemical Impedance Spectroscopy (EIS) in O2, air, and Ar (250 °C – 500 °C). Data were systematically analyzed using an Equivalent Circuit Model (EQM) combined with a Distribution Function of Relaxation Times (DFRT) analysis. The results revealed the coexistence of both a delocalized (long-range conductivity) and a localized (dipole relaxation) phenomenon, mainly distinguishable at lower temperatures (i.e., for T ≤ 350 °C) and in a pure O2 atmosphere. This localized phenomenon was tentatively attributed to the presence of trapped hole-vacancy pairs with reduced mobility. Conversely, at higher temperatures in O2 (i.e., for T ≥ 350 °C) and in the whole temperature range in less oxidizing conditions (air and Ar), such a localized phenomenon was no longer observed, as a likely result of a decrease in hole concentration with decreasing oxygen partial pressure (pO2). The current work, thus, underscores a different perspective for the development and characterization of ceramic-based solid-state Li+-ion conductors for their application as low-loss dielectric ceramics.

Enhanced emission intensity in (Li+/Ca2+/Bi3+) ions co-doped NaLa(MoO4)2: Dy3+phosphors and their Judd-Ofelt analysis for WLEDs applications

In the present study, we have synthesized a series of Dy3+ ion doped NaLa(MoO4)2phosphors by the conventional solid-state method at 750 °C for 4h. All the compounds were crystallized in the tetragonal scheelite type structure with space group (I41/a, No.88). The morphology and functional group were confirmed by the field emission scanning electron microscopy (FE-SEM) and Fourier transform infrared(FTIR)spectroscopy. Upon near-Ultraviolet (n-UV) excitation, the PL spectra exhibit the two characteristic emissions of Dy3+ ions, blue (4F9/2 → 6H15/2) at 487 nm and yellow (4F9/2 → 6H13/2) at 574nm respectively. The optimum concentration of Dy3+ionis 3 mol% and then quenching occurred due to multipolar interaction. Further, enhanced the emission intensity by co-doping with monovalent (Li+), divalent (Ca2+) and trivalent (Bi3+) ions. Among them, Li+ ion co-doped samples are shown maximum intensity (50 times) more than Dy3+ doped phosphors as relaxation of parity restriction of electric dipole transition because of local distortion of crystal field around the Dy3+ ions. In addition, by incorporation of Eu3+ ions into NaLa(MoO4)2:Dy3+system, tuned the emission color from white to red, owing to energy transfer from Dy3+ to Eu3+ ions. The intensity parameters (Ω2, Ω4) and radiative properties such as transition probabilities (A T), radiative lifetime (τ rad), and branching ratio were calculated using the Judd-Ofelt theory. CIE color coordinates, CCT values indicates that these phosphors exhibit an excellent white emission. The determined radiative properties, CIE and CCT results revealed that the Dy3+-activated NaLa(MoO4)2phosphors are potential materials for developing white LEDs, and optoelectronic device fabrications.

Structural and Spectroscopic Effects of Li+ Substitution for Na+ in LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 Upconversion Scheelite-Type Phosphors

New triple molybdates LixNa1−xCaLa0.5(MoO4)3:Er3+0.05/Yb3+0.45 (x = 0, 0.05, 0.1, 0.2, 0.3) were manufactured successfully using the microwave-assisted sol-gel-based technique (MAS). Their room-temperature crystal structures were determined in space group I41/a by Rietveld analysis. The compounds were found to have a scheelite-type structure. In Li-substituted samples, the sites of big cations were occupied by a mixture of (Li, Na, La, Er, Yb) ions, which provided a linear cell volume decrease with the Li content increase. The increased upconversion (UC) efficiency and Raman spectroscopic properties of the phosphors were discussed in detail. The mechanism of optimization of upconversion luminescence upon Li content variation was shown to be due to the control of excitation/energy transfer channel, while the control of luminescence channels played a minor role. The UC luminescence maximized at lithium content x = 0.05. The mechanism of UC optimization was shown to be due to the control of excitation/energy transfer channel, while the control of luminescence channels played a minor role. Over the whole spectral range, the Raman spectra of LixNa1−xCaLa0.5(MoO4)3 doped with Er3+ and Yb3+ ions were totally superimposed with the luminescence signal of Er3+ ions, and increasing the Li+ content resulted in the difference of Er3+ multiple intensity. The density functional theory calculations with the account for the structural disorder in the system of Li, Na, Ca, La, Er and Yb ions revealed the bandgap variation from 3.99 to 4.137 eV due to the changing of Li content. It was found that the direct electronic transition energy was close to the indirect one for all compounds. The determined chromaticity points (ICP) of the LiNaCaLa(MoO4)3:Er3+,Yb3+ phosphors were in good relation to the equal-energy point in the standard CIE (Commission Internationale de L’Eclairage) coordinates.

Magnetic and Electrical Characteristics of Nd3+-Doped Lead Molybdato-Tungstate Single 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.

Effect of Ca2+ site substitution on structural, optical, electrical and magnetic properties in Nd3+ and Mn2+-co-doped calcium molybdato-tungstates

Ca 1-3x-y Mn y [] x Nd 2x (MoO 4 ) 1-3x (WO 4 ) 3x molybdato-tungstates (▯ denotes vacant sites) were successfully synthesized by high-temperature solid-state reaction. New materials crystallize in scheelite-type structure within whole homogeneity range of solid solution ( x ≤ 0.2000 and y = 0.0200). Morphological features and particle size distribution were investigated by SEM and laser diffraction methods, respectively. Spectroscopic measurements in the UV–vis range was carried out to determine optical direct band gap ( E g ), Urbach energy ( E U ) and confirmation of structural disorder. Refractive index ( n ) was calculated using four different models. Magnetic studies revealed paramagnetic behavior with long-range ferrimagnetic and short-range antiferromagnetic interactions. New materials showed weak n -type electrical conductivity and thermoelectric power factor (S 2 σ) that strongly depends on Nd 3+ ions content. Dielectric parameters, i.e. relative permittivity (ε r ) and energy loss ( tanδ ) are insignificantly dependent on Nd 3+ ions concentration. These effects were considered in terms of structural defects, thermal activation of charge carriers, and the Maxwell–Wagner polarization.

Luminescence properties of BaMO4:Eu3+ (M: Mo or W) phosphors derived from co-precipitation reaction

Luminescent BaMO4:xmol%Eu3+ materials (M: Mo or W, and x: 0, 2, 4, 6, 8, and 10 mol%) were successfully obtained by a coprecipitation method at room temperature without additional thermal treatment, leading to highly crystalline materials with reduced reaction times and low manufacturing cost. Structural analyses by powder X-ray diffraction and vibrational Raman techniques of the [WO4]2- and [MoO4]2- groups confirm a characteristic scheelite-type structure. The results indicate an average crystallite size at around 30 nm, and a highly pure phase has been supported by Rietveld refinement. SEM-EDS data of BaMO4:xmol%Eu3+ materials identified polycrystalline particles with bipyramidal-like morphology and homogeneous europium ion distribution. Additionally, the band gap energy (Eg) of barium molybdate and tungstate materials were calculated from reflectance data by the single-constant Kubelka-Munk function. Furthermore, the emission intensity, lifetime, and intrinsic emission quantum yield (QEu3+Eu3+) of the materials have been determined and discussed. The luminescent properties of these materials are significantly influenced by the LMCT excitation bands (O2- → Mo6+, W6+, and Eu3+) as well as their intense red emission bands assigned to Eu3+ transitions. The experimental intensity parameter values Ω2 and Ω4 were evaluated from the emission spectra, using the magnetic dipole 5D0 → 7F1 transition as the standard reference. It was observed that the Ω2 values are much higher than the Ω4 values. This result is related to the fact that the 5D0 → 7F2 transition presents a much higher intensity than 5D0 → 7F1 one suggesting a low local symmetry around the Eu3+ ion, which might be due to angular distortions in the local coordination geometry. The high QEu3+Eu3+ values (60–79%) indicated an overall high emission intensity for the prepared phosphors. These are special photonic features of the Eu3+-doped molybdate and tungstate, suggesting they could be suitable for luminescent materials applications.

New Gd3+ and Mn2+-Co-Doped Scheelite-Type Ceramics-Their Structural, Optical and Magnetic Properties.

New Gd3+- and Mn2+-co-doped calcium molybdato-tungstates with the chemical formula of Ca1−3x−yMny▯xGd2x(MoO4)1−3x(WO4)3x (labeled later as CaMnGdMoWO), where ▯ denotes vacant sites in the crystal lattice, 0 < x ≤ 0.2500 and y = 0.0200 as well as 0 < y ≤ 0.0667 and x = 0.1667 were successfully synthesized by high-temperature solid-state reaction method and combustion route. Obtained ceramic materials crystallize in scheelite-type structure with space group I41/a. Morphological features and grain sizes of powders under study were investigated by SEM technique. Spectroscopic studies within the UV-vis spectral range were carried out to estimate the direct band gap (Eg) and Urbach energy (EU) of obtained powders. EPR studies confirmed the existence of two types of magnetic objects, i.e., Mn2+ and Gd3+ ions, and significant antiferromagnetic (AFM) interactions among them.

Structural investigation and sonophotocatalytic properties of the solid solutions Sr(Mo1−xWx)O4 crystals synthesized by the sonochemical method

In this paper, the solid solutions of strontium molybdate-tungstate [Sr(Mo1−xWx)O4] crystals with (x = 0, 0.25, 0.50, 0.75, and 1) were synthesized by the sonochemical method. Their structure, morphology, optical, and sonophotocatalytic properties were performed in function of the replacement of Mo6+ by W6+ cations into the lattice. Their structure and elemental composition were characterized using X-ray diffraction, Rietveld refinement, micro-Raman, energy-dispersive X-ray, and Fourier-transform infrared spectroscopies proving that all samples are monophasic, crystalline, and exhibit a scheelite-type tetragonal structure. Field-emission scanning electron microscopy images revealed the octahedral and dumbbell-like morphologies for SrMoO4 and SrWO4 crystals. Moreover, it is noted to pass through spindle-like morphology for the microcrystals containing both Mo6+ and W6+ cations (x = 0.25, 0.50, and 0.75). Ultraviolet-visible diffuse reflectance spectroscopy showed a directly proportional increase in the optical band gap (Egap) values from 4.27 to 5.01 eV. These data indicate an increase in the intermediary electronic levels between valance and conduction bands with the increase in the concentration of W6+ cations in the lattice. Finally, we have obtained good sonophotocatalytic performances for SrMoO4 crystals (90%), and mainly to Sr(Mo0.25W0.75)O4 crystals (98%) in the degradation of Rhodamine B dye until 240 min under UV-C light.

Tetrahedral Displacive Disorder in the Scheelite-Type Oxide RbReO4.

Oxides exhibiting the scheelite-type structure are an important class of functional materials with notable applications in photocatalysis, luminescence, and ionic conductivity. Like all materials, understanding their atomic structure is fundamental to engineering their physical properties. This study outlines a detailed structural investigation of the scheelite-type oxide RbReO4, which exhibits a rare long-range phase transition from I41/a to I41/amd upon heating. Additionally, in the long-range I41/a model, the Re-O tetrahedral distance undergoes significant contraction upon warming. Recent studies of other scheelite oxides have attributed this apparent contraction to incoherent local-scale tetrahedral rotations. In this study, we use X-ray pair distribution function analysis to show that RbReO4 undergoes a unique symmetry-lowering process on the local scale, which involves incoherent tetrahedral displacements. The rare I41/a to I41/amd long-range phase transition was found to occur via a change from static to dynamic disorder on the local scale, which is due to the combination of the size of the A-site cation and lattice expansion. This demonstrates how careful manipulation of the ionic radius of the A-site in the scheelite structure can be used to induce local-scale disorder, which has valuable implications for tailoring the physical properties of related materials.

Effect of ligand environment of rare-earth ions on temperature measurement performance of SrAO4:xEu3+ (A = Mo and W) phosphors

Molybdate and tungstate with scheelite-type structure are excellent self-luminescent materials, which can be used as ideal hosts for the doping of rare-earth ions. In this study, a series of Eu3+-activated SrAO4 (A = Mo and W) phosphors were successfully synthesized, and their crystal structures, photoluminescence properties, and temperature measurement performance were analyzed in detail. These phosphors were excited by UV light (291 nm and 247 nm, respectively), with clear energy transfer (ET) (MoO42−→Eu3+ or WO42−→Eu3+). According to fluorescence intensity ratio (FIR) and Judd–Ofelt (J–O) theory, compared to SrWO4:0.01Eu3+ phosphor, SrMoO4:0.01Eu3+ phosphor exhibited better thermal stability, with relatively low Sa value (maximum values were 5.082 %K−1 and 20.74 %K−1, respectively), and their Sr values were not significantly different (maximum values were 0.864 %K−1 and 0.83 %K−1, respectively). Sa value was negatively correlated to central asymmetry of Eu3+, but the optimal Sr value tended to be more suitable for central asymmetry of Eu3+. In addition, Eu3+ exhibited stronger central asymmetry as well as covalency of Eu–O bond in SrMoO4. Results reveal that SrMoO4:xEu3+ and SrWO4:xEu3+ can be used for luminescent thermometers.