Fergusonite-type Structure Research Articles

Exploring Phase Transition and Structural Complexity in the Mixed Cation Uranium Oxide CaUNb2O8.

The structures and high-temperature phase transition of CaUNb2O8 were studied in situ using synchrotron X-ray and neutron powder diffraction. Rietveld refinements provided an accurate description of the crystal structures of both the monoclinic fergusonite-type I2/b structure observed at room temperature and the tetragonal scheelite-type I41/a structure found at high temperatures. Bond valence sum analysis showed Nb5+ to be octahedrally coordinated in the monoclinic fergusonite-type structure, akin to other ANbO4 materials. Rietveld analysis of the variable temperature data allowed for the determination of accurate unit cell parameters and atomic coordinates, as well as revealing a reversible phase transition around ∼750 °C. The Nb-O bond distances display anomalous behavior, with a discontinuity in the longer Nb-O(1') distance coinciding with the phase transition suggestive of a reconstructive phase transition. Mode analysis identified the Γ2+ mode as the primary mode that drives the phase transition; this is linearly coupled to the induced spontaneous strain within the monoclinic fergusonite-type structure. Analysis of the temperature dependence of the Nb(z) positional parameter, as well as of the ϵ1-ϵ2 and ϵ6 strain parameters, showed that the phase transition is not strictly second order, with the critical exponent β ≠ 1/2. This study demonstrates the complex structural features of mixed cation metal oxides at elevated temperatures.

Structural instability in LaNbO4 under compression

In this work, we report a high-pressure study on fergusonite-type LaNbO4. Powder x-ray diffraction and Raman spectroscopic experiments support the occurrence of a phase transition between 11 and 14 GPa. The transition takes place from a monoclinic fergusonite-type structure (space group I2/a) to another monoclinic structure (space group P21/c). The phase transition is reversible, and the high-pressure phase is isomorphic to the high-pressure phase of HoNbO4. The high-pressure phase remains stable up to 33.3 GPa, the highest pressure reached in the present measurements. Density-functional theory calculations found that in the pressure range of the studies; the high-pressure phase has a higher enthalpy than the low-pressure fergusonite phase. We propose that the high-pressure phase is metastable and it is observed because of non-hydrostatic conditions in the experiments. The pressure dependence of unit-cell parameters of the low-pressure phase and the room-temperature equation of state are reported. The pressure dependence of various Raman and IR frequencies as obtained from experiment and theory is also reported. For the fergusonite phase, we have also obtained the isothermal compressibility tensor, elastics constants, and elastic moduli.

High-pressure monoclinic–monoclinic transition in fergusonite-type HoNbO4

In this paper we perform a high-pressure (HP) study of fergusonite-type HoNbO4. Powder x-ray diffraction experiments and ab initio density-functional theory (DFT) simulations provide evidence of a phase transition at 18.9(1.1) GPa from the monoclinic fergusonite-type structure (space group I2/a) to another monoclinic polymorph described by space group P21/c. The phase transition is reversible and the HP structural behavior is different than the one previously observed in related niobates. The HP phase remains stable up to 29 GPa. The observed transition involves a change in the Nb coordination number from 4 to 6, and it is driven by mechanical instabilities. We have determined the pressure dependence of unit-cell parameters of both phases and calculated their room-temperature equation of state. For the fergusonite-phase we have also obtained the isothermal compressibility tensor. In addition to the HP studies, we report ambient-pressure Raman and infrared (IR) spectroscopy measurements. We have been able to identify all the active modes of fergusonite-type HoNbO4, which have been assigned based upon DFT calculations. These simulations also provide the elastic constants of the different structures and the pressure dependence of the Raman and IR modes of the two phases of HoNbO4. According to ab initio calculations, the reported phase transition is related to a mechanical instability and a phonon softening.

Strong Narrow Red Emission in a Perturbed Fergusonite System: Y3Mg2Nb3O14:Eu3+ for White LED Applications

A series of intense red-emission phosphors, Y3Mg2Nb3O14:Eu3+, with higher color purity were prepared via the conventional high-temperature ceramic route. The influence of Mg2+ ion incorporation in the highly symmetrical YNbO4 lattice on the luminescence properties was studied using powder x-ray diffraction, scanning electron microscopy with energy dispersive spectrometry, UV–visible absorption spectroscopy, and photoluminescence measurements. The developed phosphors crystallize into a monoclinic fergusonite-type structure with a C2/c space group even with the addition of Mg into the system. These phosphors display intense, sharp, red emission with a maximum at 613 nm under near-UV excitation. The photoluminescence studies suggest that Mg addition perturbs the A site network forming cation pairs of Eu3+-Y3+, Eu3+-Mg2+, and Eu3+-Eu3+ connected with O2− ions. The preferential occupation of cations in the A site network by the incorporation of Mg2+ ions in the YNbO4 host lattice induces Eu3+-Eu3+ separation and results in intense, sharp, red emission due to increased radiative probabilities. Moreover, the color purity of the Y3−xMg2Nb3O14:xEu3+ phosphor could achieve 92.4% that is attributed to the narrow full width at half maximum (FWHM ∼ 4 nm) of the red emission peaking at 613 nm. More importantly, the luminescence intensity of the developed phosphor is 9.4 times greater than that of the Philip’s red phosphor under near-UV excitation. Because of the intriguing red luminescence, phosphors exhibit Commission Internationale de l’Elcairage color coordinates (0.63, 0.35) close to the National Television System Committee standards, low correlated color temperature values for warm white lighting, and greater color purity (92.4%) for improving the color rendering index, which makes them a potential candidate for use in the fabrication of white light-emitting diodes.

Monoclinic-tetragonal-monoclinic phase transitions in Eu0.1Bi0.9VO4 under pressure

The promising technological material Eu0.1Bi0.9VO4, has been studied for the first time at room-temperature under high-pressure, up to 24.9 GPa, by means of in situ angle dispersive powder x-ray diffraction (XRD). The compound undergoes two phase transitions at 1.9 and 16.1 GPa. The first transition is from the monoclinic fergusonite-type structure (space group I2/a) to a tetragonal scheelite-type structure (space group I41/a), being a ferroelastic-paraelastic transformation similar to that previously reported for isomorphic pristine BiVO4. The second phase transition is first-order in nature. The scheelite-type and the second high-pressure phase coexist in a wide pressure range. A monoclinic structure (space group P21/n) is proposed for the second high-pressure phase. Both transitions are reversible upon decompression. Details of the different crystal structures are reported. All the three observed structures are composed of network of VO4 tetrahedra and BiO8 (or EuO8 due to the substitution of Bi by Eu) dodecahedra. The room-temperature P–V equation of state and axial anisotropic compressibilities of the fergusonite and scheelite polymorphs are also given. In particular, the isothermal compressibility tensor for the monoclinic fergusonite phase has been calculated.

Thermophysical properties of M′-LuTaO4: Structural and calorimetric studies

Lutetium orthotantalate with M′-fergusonite type structure was synthesized using a reverse coprecipitation method. Phase and chemical composition, as well as microstructure of the synthesized sample, were characterized by X-ray diffraction (XRD), μ-X-ray fluorescence and Fourier-transform infrared spectroscopies, and scanning electron microscopy. Heat capacity of M′-LuTaO4 was first studied by adiabatic and differential scanning calorimetry (DSC) in the temperature range from 10 to 1300 K. Using a temperature dependence of heat capacity, the standard thermodynamic functions (entropy Smo(T), enthalpy change Hmo(T)–Hmo(0) and derived Gibbs energy Фmo(T)) were calculated in the range of T→0–1300 K. The standard molar entropy of M′-LuTaO4 at 298.15 K is 123.12 ± 0.50 J K−1 mol−1. A comparison of the experimental heat capacity values, obtained by DSC, with those calculated using the empirical Neumann-Kopp rule showed a reasonable agreement between the two sets of data only up to ≈1000 K. The high-temperature evolution of the lattice parameters for M′-LuTaO4 was studied by high-temperature XRD (HTXRD). According to the high-temperature heat capacity study and the HTXRD measurements, there were no phase transformations up to 1300 K. Based on the HTXRD data, the linear and volume thermal expansion coefficients were obtained for the first time.

High-pressure phase transformations in NdVO4 under hydrostatic, conditions: a structural powder x-ray diffraction study

Room temperature angle dispersive powder x-ray diffraction experiments on zircon-type NdVO4 were performed for the first time under quasi-hydrostatic conditions up to 24.5 GPa. The sample undergoes two phase transitions at 6.4 and 19.9 GPa. Our results show that the first transition is a zircon-to-scheelite-type phase transition, which has not been reported before, and contradicts previous non-hydrostatic experiments. In the second transition, NdVO4 transforms into a fergusonite-type structure, which is a monoclinic distortion of scheelite-type. The compressibility and axial anisotropy of the different polymorphs of NdVO4 are reported. A direct comparison of our results with former experimental and theoretical studies on other rare-earth orthovanadates found in literature highlights the importance of the role played by non-hydrostatic stresses in their high-pressure structural behavior.

High-pressure behavior ofCaMoO4

We report a high-pressure study of tetragonal scheelite-type CaMoO4 up to 29 GPa. In order to characterize its high-pressure behavior, we have combined Raman and optical-absorption measurements with density-functional theory calculations. We have found evidence of a pressure-induced phase transition near 15 GPa. Experiments and calculations agree in assigning the high-pressure phase to a monoclinic fergusonite-type structure. The reported results are consistent with previous powder x-ray-diffraction experiments, but are in contradiction with the conclusions obtained from earlier Raman measurements, which support the existence of more than one phase transition in the pressure range covered by our studies. The observed scheelite-fergusonite transition induces significant changes in the electronic band gap and phonon spectrum of CaMoO4. We have determined the pressure evolution of the band gap for the low- and high-pressure phases as well as the frequencies and pressure dependences of the Raman-active and infrared-active modes. In addition, based upon calculations of the phonon dispersion of the scheelite phase, carried out at a pressure higher than the transition pressure, we propose a possible mechanism for the reported phase transition. Furthermore, from the calculations we determined the pressure dependence of the unit-cell parameters and atomic positions of the different phases and their room-temperature equations of state. These results are compared with previous experiments showing a very good agreement. Finally, information on bond compressibility is reported and correlated with the macroscopic compressibility of CaMoO4. The reported results are of interest for the many technological applications of this oxide.

ScVO4 under non-hydrostatic compression: a new metastable polymorph

The high-pressure (HP) behaviour of scandium vanadate (ScVO4) is investigated under non-hydrostatic compression. The compound is studied by means of synchrotron-based powder x-ray diffraction (XRD) and optical-absorption techniques. The occurrence of a non-reversible phase transition is detected. The transition is from the zircon structure to the fergusonite-type structure and takes place around 6 GPa with nearly 10% volume discontinuity. XRD measurements on the pressure cycled sample confirm for the first time that the fergusonite-type ScVO4 can be recovered as the metastable phase at ambient conditions. Raman spectroscopic measurements verify the metastable phase to be of a fergusonite-type phase. Theoretical calculations also corroborate the experimental findings. The fergusonite phase is found to be stiffer than the ambient-pressure zircon phase, as indicated by the observed experimental and theoretical bulk moduli. The optical properties and lattice-dynamics calculation of the fergusonite ScVO4 are discussed. At ambient pressure the band gap of the zircon (fergusonite)-type ScVO4 is 2.75 eV (2.3 eV). This fact suggests that the novel metastable polymorph of ScVO4 can have applications in green technologies; for instance, it can be used as photocatalytic material for hydrogen production by water splitting.

Polymorphism in Strontium Tungstate SrWO4 under Quasi-Hydrostatic Compression.

The structural and vibrational properties of SrWO4 have been studied experimentally up to 27 and 46 GPa, respectively, by angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy measurements as well as using ab initio calculations. The existence of four polymorphs upon quasi-hydrostatic compression is reported. The three phase transitions were found at 11.5, 19.0, and 39.5 GPa. The ambient-pressure SrWO4 tetragonal scheelite-type structure (S.G. I41/a) undergoes a transition to a monoclinic fergusonite-type structure (S.G. I2/a) at 11.5 GPa with a 1.5% volume decrease. Subsequently, at 19.0 GPa, another structural transformation takes place. Our calculations indicate two possible post-fergusonite phases, one monoclinic and the other orthorhombic. In the diffraction experiments, we observed the theoretically predicted monoclinic LaTaO4-type phase coexisting with the fergusonite-type phase up to 27 GPa. The coexistence of the two phases and the large volume collapse at the transition confirm a kinetic hindrance typical of first-order phase transitions. Significant changes in Raman spectra suggest a third pressure-induced transition at 39.5 GPa. The conclusions extracted from the experiments are complemented and supported by ab initio calculations. Our data provides insight into the structural mechanism of the first transition, with the formation of two additional W-O contacts. The fergusonite-type phase can be therefore considered as a structural bridge between the scheelite structure, composed of [WO4] tetrahedra, and the new higher pressure phases, which contain [WO6] octahedra. All the observed phases are compatible with the high-pressure structural systematics predicted for ABO4 compounds using crystal-chemistry arguments such as the diagram proposed by Bastide.

A new red-emitting material K0.5Na0.5NbO3:Eu3+ for white LEDs

K0.5Na0.5NbO3 (KNN):xEu3+ materials were fabricated via the conventional solid-state reaction method. The microstructure and photoluminescence property of KNN:xEu3+ were investigated. XRD and SEM analyses evidenced that the fergusonite-type structure EuNbO4 were homogeneously precipitated among KNN host. Photoluminescent properties of KNN:xEu3+ were significantly influenced by the variations between KNN and EuNbO4 phase. Within the quenching concentrations, the KNN:xEu3+ exhibited intense narrow band red emissions centered at 615nm from 5D0→7F2 transition with good thermal stability (20–200°C), under the excitation wavelength at a near-ultraviolet (393nm) or blue light (464nm). The red emission intensity reached a maximum at x=0.20 due to the concentration quenching effect. Nevertheless further increasing the Eu3+-doped concentration over x=0.25 did not decrease the red emission intensity, but significantly enhanced the red emission due to the intrinsic luminescence of EuNbO4. These results suggest the KNN:xEu3+ could be potentially applicable in W-LEDs under near-ultraviolet or blue chip excitation.

The high-pressure monazite-to-scheelite transformation in CaSeO4

AbstractThe high-pressure monazite – scheelite structure transition has been observed at P >4.57 GPa in CaSeO4 by synchrotron X-ray powder diffraction. It is a first-order transition with a 4.5% volume change and is severely hindered kinetically. Scheelite-type CaSeO4 remains to a maximum experimental pressure of 42.2 GPa and no (002) reflection, specifically indicative of a subgroup transition to a fergusonite-type structure, is observed. Scheelite-type CaSeO4 remains at ambient conditions, where the tetragonal unit cell has parameters of a = 5.04801(11) c = 11.6644(5) Å and V = 297.21(3) Å3 with Dcalc = 4.090 g cm–3. The diffraction pattern of the recovered material was refined in space group I41/a to Rp = 0.98%, wRp = 1.91%, GoF = 0.59, RFobs = 5.04%, wRFobs = 4.27%. The oxygen is located on the general 16f site at (0.2578(8) 0.3699(14) 0.5755(4)) and shares four identical bonds with Se (4a: ½ ½ ½) at 1.644(5) Å. The Ca (4b: 0, 0, ½) is eight-coordinated via O at 4 × 2.440(6) Å and 4 × 2.504(5) Å. This is further evidence of the dissimilarity of sulfate and selenate at high pressure and temperature conditions and the closer resemblance of the selenates to the orthophosphates, arsenates and vanadates, where this type of transition sequence has been described.

In situhigh-pressure synchrotron x-ray diffraction study of CeVO4and TbVO4up to 50 GPa

Room-temperature angle-dispersive x-ray diffraction measurements on zircon-type TbVO${}_{4}$ and CeVO${}_{4}$ were performed in a diamond-anvil cell up to 50 GPa using neon as a pressure-transmitting medium. In TbVO${}_{4}$, we found at 6.4 GPa evidence of a nonreversible pressure-induced structural phase transition from zircon to a scheelite-type structure. A second transition to an $M$-fergusonite-type structure was found at 33.9 GPa, which is reversible. Zircon-type CeVO${}_{4}$ exhibits two pressure-induced transitions: first, an irreversible transition to a monazite-type structure at 5.6 GPa and, second, at 14.7 GPa, a reversible transition to an orthorhombic structure. No additional phase transitions or evidences of chemical decomposition are found in the experiments. The equations of state and axial compressibility for the different phases are also determined. Finally, the sequence of structural transitions and the compressibilities are discussed in comparison with other orhtovanadates and the influence of nonhydrostaticity commented.

Microwave Dielectric Properties of RENbO<sub>4</sub> (RE=Y, Yb,Ce) Ceramics

In the search of more microwave dielectric materials with high-quality factor (Qf ) and high relative dielectric constant (r), RENbO4 (where RE = Y, Ce, Yb) niobate ceramics were synthesized from a solid-state ceramic route, their sintering behavior and microwave dielectric properties were investigated. All of the RENbO4 ceramics formed a single-phase fergusonite-type structure (monoclinic, with space group I2/c), except CeNbO4. They all have an excellent quality factor and dielectric constant value in range of 20 – 28, and the inherent reasons were analyzed.

Theoretical study of the scheelite-to-fergusonite phase transition in YLiF4 under pressure

Abstract Recent experimental works on high-pressure X-ray diffraction have shown that YLiF 4 undergoes a reversible phase transition around 10 GPa from the ambient pressure scheelite-type to a fergusonite-type structure. Different theoretical works have proposed either the M-fergusonite or M′-fergusonite as this second stable structure. Also, small changes in photoluminescence and Raman signals have been observed around 6 GPa, and have not been completely explained yet. In this work we present a first-principles Density Functional Theory structural study of YLiF 4 . We found that the anomalies at 6 GPa can be related to the structural changes observed in a scheelite to M′-fergusonite transition. We also have studied briefly different possibilities for an experimentally observed but not completly characterised third stable structure above 17 GPa.

Crystal structure of the high-pressure modification of NdTaO 4

The crystal structure of the metastable NdTaO 4 high-pressure modification (HPM) has been studied by X-ray powder diffraction. It was found that the crystal structure of HPM NdTaO 4 belongs to the monoclinic LaTaO 4-type structure (space group P2 1/c , a=7.5920(7) Å, b=5.4673(5) Å, c=7.7022(7) Å, β=100.032(6)°, Z=4). The difference in structure between LnTaO 4 with Ln=Nd–Lu (fergusonite-type structure) and that with Ln=La–Pr (monoclinic LaTaO 4-type structure) is most likely due to the difference in size of the rare-earth cations.

Spectroscopic study of Bi xEu 1−xVO 4 and Bi yGd 1−yVO 4 mixed oxides

The mixed oxides Bi x Eu 1− x VO 4 and Bi y Gd 1− y VO 4 crystallize in a zircon-type structure, for 0 < x < 0.6 and 0 < y < 0.64, and in a fergusonite-type structure, for 0.94 < x < 1 and 0.93 < y < 1. A process of competition between the dominant and the constrained effects of the lone-pair 6 s 2 of Bi 3+ is discussed. The diffuse reflectance spectroscopic studies of these mixed oxides are presented. The observed broad bands are attributed to charge transfer processes and the sharp peaks in the Bi x Eu 1− x VO 4 spectra are ascribed to intra-configurational 4 f – 4 ftransitions of the Eu 3+ ion. The broad absorption shift in BiLnVO 4 (Ln : Eu and Gd) compounds to the longer wavelengths range, when Bi is introduced in the LnVO 4 lattice, is ascribed to charge transfer processes in a Bi-VO 4 center and are interpreted assuming a Jahn-Teller effect in the excited state of Bi 3+. The concept of an internal pressure of Bi 3+ ions is also used to explain the broad A-band shifts.

Ferroelastics with the fergusonite type structure

Abstract Ferroelastic phase transitions in RNbO4 compounds (R = La-Lu) with fergusonite-type structure have been studied. Spontaneous shear strain, thermal expansion and dielectric properties were examined. The phenomenological analysis of the ferroelastic phase transitions in RNbO4 has been carried out. The data obtained indicate that all RNbO4 compounds are pure proper ferroelastics.