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1.
The high pressure behavior of aluminum tungstate [Al2(WO4)3] has been investigated up to ∼18 GPa with the help of Raman scattering studies. Our results confirm the recent observations of two reversible phase transitions below 3 GPa. In addition, we find that this compound undergoes two more phase transitions at ∼5.3 and ∼6 GPa before transforming irreversibly to an amorphous phase at ∼14 GPa.  相似文献   

2.
Phase transitions in MgAl2O4 were examined at 21-27 GPa and 1400-2500 °C using a multianvil apparatus. A mixture of MgO and Al2O3 corundum that are high-pressure dissociation products of MgAl2O4 spinel combines into calcium-ferrite type MgAl2O4 at 26-27 GPa and 1400-2000 °C. At temperature above 2000 °C at pressure below 25.5 GPa, a mixture of Al2O3 corundum and a new phase with Mg2Al2O5 composition is stable. The transition boundary between the two fields has a strongly negative pressure-temperature slope. Structure analysis and Rietveld refinement on the basis of the powder X-ray diffraction profile of the Mg2Al2O5 phase indicated that the phase represented a new structure type with orthorhombic symmetry (Pbam), and the lattice parameters were determined as a=9.3710(6) Å, b=12.1952(6) Å, c=2.7916(2) Å, V=319.03(3) Å3, Z=4. The structure consists of edge-sharing and corner-sharing (Mg, Al)O6 octahedra, and contains chains of edge-sharing octahedra running along the c-axis. A part of Mg atoms are accommodated in six-coordinated trigonal prism sites in tunnels surrounded by the chains of edge-sharing (Mg, Al)O6 octahedra. The structure is related with that of ludwigite (Mg, Fe2+)2(Fe3+, Al)(BO3)O2. The molar volume of the Mg2Al2O5 phase is smaller by 0.18% than sum of molar volumes of 2MgO and Al2O3 corundum. High-pressure dissociation to the mixture of corundum-type phase and the phase with ludwigite-related structure has been found only in MgAl2O4 among various A2+B3+2O4 compounds.  相似文献   

3.
The reported pressure-induced amorphization in α-NaVO3 has been re-investigated using Raman spectroscopy. Discontinuous changes are noted in the Raman spectrum above 5.6 GPa implying large structural changes across the transition. The decrease in frequency of the V-O stretching mode across the transition suggests that the vanadium atom may be in octahedral coordination in the high pressure phase. Excessive broadening of the internal modes is observed above 6 GPa. New peaks characteristic of a crystalline phase gain in intensity at higher pressures in the bending modes region; however, the transformation is not complete even at 13 GPa. Co-existence of phases is noted over a significant pressure range above the onset of transition. Pressure released spectrum is found to be a mixture of crystalline α-phase, traces of crystalline β-phase and highly disordered phase consisting of V-O units in five- and six-fold coordination.  相似文献   

4.
The high-pressure behavior of Y2(WO4)3 has been investigated at room temperature by in situ X-ray diffraction and Raman scattering measurements. Both the studies show that beyond ∼3 GPa, this compound smoothly transforms from the ambient orthorhombic phase to a disordered phase. The structural modifications are found to be reversible up to ∼4 GPa but become irreversible at higher pressures. Low pressures of transformation imply that these changes are intrinsic and not due to non-hydrostatic stresses. In addition, the correlation between the stability range of orthorhombic phase and counter cation size supports that this compound has a large field of negative thermal expansion in this family of compounds.  相似文献   

5.
The evolution of the unit-cell parameters of CaZrO3 perovskite, an orthorhombic perovskite belonging to space group Pbnm, have been determined to a pressure of 8.7 GPa at room temperature using single-crystal X-ray diffraction measurements. A fit of a third-order Birch-Murnaghan equation of state to the pressure-volume data yields values of V0=258.04(2) Å3, KT0=154(1) GPa and K0′=5.9(3). Although CaZrO3 perovskite does not exhibit any phase transitions in this pressure range, the compression of the structure is anisotropic with [010] approximately 20% less compressible than either [100] or [001]. Compressional moduli for the unit cell parameters are: Ka0=142(1) GPa and Ka0′=4.4(2), Kb0=177(2) GPa and Kb0′=9.4(5), Kc0=146(2) GPa and Kc0′=5.4(4). Comparison with other orthorhombic Ca-oxide perovskites shows that there is systematic increase in compressional anisotropy with increasing distortion from cubic symmetry.  相似文献   

6.
Mn/Fe mixed oxide solids doped with Al2O3 (0.32-1.27 wt.%) were prepared by impregnation of manganese nitrate with finely powdered ferric oxide, then treated with different amounts of aluminum nitrate. The obtained samples were calcined in air at 700-1000 °C for 6 h. The specific surface area (SBET) and the catalytic activity of pure and doped precalcined at 700-1000 °C have been measured by using N2 adsorption isotherms and CO oxidation by O2. The structure and the phase changes were characterized by DTA and XRD techniques. The obtained results revealed that Mn2O3 interacted readily with Fe2O3 to produce well-crystallized manganese ferrite (MnFe2O4) at temperatures of 800 °C and above. The degree of propagation of this reaction increased by Al2O3-doping and also by increasing the heating temperature. The treatment with 1.27 wt.% Al2O3 followed by heating at 1000 °C resulted in complete conversion of Mn/Fe oxides into the corresponding ferrite phase. The catalytic activity and SBET of pure and doped solids were found to decrease, by increasing both the calcination temperature and the amount of Al2O3 added, due to the enhanced formation of MnFe2O4 phase which is less reactive than the free oxides (Mn2O3 and Fe2O3). The activation energy of formation (ΔE) of MnFe2O4 was determined for pure and doped solids. The promotion effect of aluminum in formation of MnFe2O4 was attributed to an effective increase in the mobility of reacting cations.  相似文献   

7.
High-pressure X-ray diffraction measurements were carried out for polymeric CdI2-type compounds MTe2 (M=Pt, Pd) to investigate if they undergo a structural phase transition under pressure as does IrTe2. Up to 27 GPa at room temperature PtTe2 does not undergo any structural phase transition. In contrast, however, an abrupt change in the inter-atomic distances occurs in PdTe2 above 15.7 GPa at room temperature, and above 5 GPa at 300 °C, but the volume vs. pressure curve exhibits no discontinuity. To account for the differences between the isostructural compounds PtTe2, PdTe2 and IrTe2, their electronic structures and bonding were analyzed on the basis of first principles electronic band structure calculations.  相似文献   

8.
The high pressure behavior of U2O(PO4)2 has been investigated with the help of Raman scattering and X-ray diffraction measurements up to ∼14 and 6.5 GPa, respectively. The observed changes in the Raman spectra as well as the X-ray diffraction patterns suggest that U2O(PO4)2 undergoes a phase transition at ∼6 GPa to a mixture of a disordered ambient pressure phase and a new high pressure phase. The new phase resembles the triclinic mixed-valence phase of uranium orthophosphate (U(UO2)(PO4)2). On release of pressure the initial phase is not retrieved.  相似文献   

9.
The orthorhombic perovskites, GdAlO3 and GdFeO3, have been studied using single-crystal X-ray diffraction up to 8.52 and 8.13 GPa, respectively, in a diamond anvil cell at 298 K. The evolution of the structures of GdAlO3 and GdFeO3 involves compression of both the GdO12 and the octahedral (AlO6 and FeO6) sites. The compression of the GdO12 site is anisotropic in both perovskites, with the four longest Gd-O distances compressing more than the eight shorter Gd-O bond lengths, resulting in a decrease in the distortion of GdO12 with pressure. In GdAlO3, the GdO12 site is less compressible than the AlO6 site, resulting in an increase of both the interoctahedral Al-O1-Al and Al-O2-Al angles with increasing pressure. Thus GdAlO3 perovskite becomes less distorted with increasing pressure. In GdFeO3, the GdO12 site displays a similar compressibility as the FeO6 site, with little change in the Fe-O2-Fe angle with pressure but an increase of the Fe-O1-Fe tilting angle. Thus GdFeO3 perovskite becomes less distorted with increasing pressure, but the change is not as pronounced as GdAlO3. The high-pressure behavior of GdAlO3 and GdFeO3 is similar to orthorhombic YAlO3 perovskite but contrasts with orthorhombic CaSnO3, which becomes more distorted with increasing pressure.  相似文献   

10.
The tetragonal compound Bi2CuO4 was investigated at high pressures by using in situ Raman scattering and X-ray diffraction (XRD) methods. A pressure-induced structural transition started at 20 GPa and completed at ∼37 GPa was found. The high pressure phase is in orthorhombic symmetry. Raman and XRD measurements revealed that the above phase transition is reversible.  相似文献   

11.
Three novel metal polyphosphides, α-SrP3, BaP8, and LaP5, were prepared in BN crucibles by the reaction of the respective stoichiometric mixtures under a high pressure of 3 GPa at 950-1000°C. Their crystal structures were determined from single-crystal X-ray data (α-SrP3: space group C2/m, a=9.199(6) Å, b=7.288(3) Å, c=5.690(3) Å, β=113.45(4)°, Z=4, R1/wR2=0.0684/0.1180 for 471 observed reflections and 22 variables; BaP8: space group P−1, a=6.762(2) Å, b=7.233(2) Å, c=8.567(2) Å, α=86.32(2)°, β=84.31(2)°, γ=70.40(2)°, Z=2, R1/wR2=0.0476/0.1255 for 2702 observed reflections and 82 variables; LaP5: space group P21/m, a=4.885(1) Å, b=9.673(3) Å, c=5.577(2) Å, β=105.32(2)°, Z=2, R1/wR2=0.0391/0.1034 for 1272 observed reflections and 31 variables). α-SrP3 is isostructural with SrAs3 and the crystal structure consists of two-dimensional puckered polyanionic layers 2[P3]2− that stack along the c-axis yielding channels occupied by Sr2+ counterions. BaP8 crystallizes in a new structure type which contains a three-dimensional infinite polyanionic framework 3[P3]2−, with large channels hosting the barium cations. LaP5 is a layered compound containing 2[P5]3− polyanionic layers separated by La3+ ions. All three compounds exhibit expected diamagnetic behaviors.  相似文献   

12.
Changes in structure and dielectric properties at elevated temperatures have been investigated on single-crystals of sodium potassium niobate, Na0.5K0.5NbO3, grown by the flux method. Single-crystal X-ray diffraction studies revealed that the crystals underwent orthorhombic-tetragonal and tetragonal-cubic phase transitions at 465 and 671 K during heating and 446 and 666 K during cooling, respectively. Both transitions were accompanied by volumetric discontinuities of collapse upon heating and expansion upon cooling, suggesting that the transitions were of the first order. The coordination numbers of an Nb showed a decreasing tendency with decreasing temperature, i.e., 6 in cubic, 5+1 in tetragonal and 4+2 in orthorhombic. An Na atom occupied a slightly different position from the K atom in 12-fold coordination, resulting in fewer coordination numbers of 8+4 in cubic and tetragonal and 7+5 in orthorhombic. The spontaneous polarisation (Ps) estimated from the atom positions and formal charges were approximately 0.29 C m−2 in orthorhombic and 0.18 C m−2 in tetragonal. The contribution of the alkaline oxide components to Ps was estimated to be approximately 15% in both ferroelectric forms. The temperature-induced transitions were also confirmed through the dielectric constant and dielectric loss at various frequencies and the differential scanning calorimetry.  相似文献   

13.
A high-pressure Raman scattering study of the tungstate Al2(WO4)3 is presented. This study showed the onset of two reversible phase transitions at 0.28±0.07 and 2.8±0.1 GPa. The pressure evolution of Raman bands provides strong evidences that both the transitions involve rotations/tilts of nearly rigid tungstate tetrahedra and that the structure of the stable phase in the 0.28-2.8 GPa range may be the same as the structure of the ambient pressure, low-temperature monoclinic (C2h5) ferroelastic phase of Al2(WO4)3.  相似文献   

14.
Thin crystals of La2O3, LaAlO3, La2/3TiO3, La2TiO5, and La2Ti2O7 have been irradiated in situ using 1 MeV Kr2+ ions at the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), Argonne National Laboratory (ANL). We observed that La2O3 remained crystalline to a fluence greater than 3.1×1016 ions cm−2 at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline-amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (Tc) of 647 K for LaAlO3, 840 K for La2Ti2O7, 865 K for La2/3TiO3, and 1027 K for La2TiO5. The Tc values observed in this study, together with previous data for Al2O3 and TiO2, are discussed with reference to the melting points for the La2O3-Al2O3 and La2O3-TiO2 systems and the different local environments within the four crystal structures. Results suggest that there is an observable inverse correlation between Tc and melting temperature (Tm) in the two systems. More complex relationships exist between Tc and crystal structure, with the stoichiometric perovskite LaAlO3 being the most resistant to amorphisation.  相似文献   

15.
Subsolidus phase relationships in the In2O3-WO3 system at 800-1400°C were investigated using X-ray diffraction. Two binary-oxide phases—In6WO12 and In2(WO4)3—were found to be stable over the range 800-1200°C. Heating the binary-oxide phases above 1200°C resulted in the preferential volatilization of WO3. Rietveld refinement was performed on three structures using X-ray diffraction data from nominally phase-pure In6WO12 at room temperature and from nominally phase-pure In2(WO4)3 at 225°C and 310°C. The indium-rich phase, In6WO12, is rhombohedral, space group (rhombohedral), with Z=1, a=6.22390(4) Å, α=99.0338(2)° [hexagonal axes: aH=9.48298(6) Å, c=8.94276(6) Å, aH/c=0.9430(9)]. In6WO12 can be viewed as an anion-deficient fluorite structure in which 1/7 of the fluorite anion sites are vacant. Indium tungstate, In2(WO4)3, undergoes a monoclinic-orthorhombic transition around 250°C. The high-temperature polymorph is orthorhombic, space group Pnca, with a=9.7126(5) Å, b=13.3824(7) Å, c=9.6141(5) Å, and Z=4. The low-temperature polymorph is monoclinic, space group P21/a, with a=16.406(2) Å, b=9.9663(1) Å, c=19.099(2) Å, β=125.411(2)°, and Z=8. The structures of the two In2(WO4)3 polymorphs are similar, consisting of a network of corner sharing InO6 octahedra and WO4 tetrahedra.  相似文献   

16.
The La(Mn0.5Co0.5)1−xCuxO3−δ series with x=0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8 and 1 was synthesized by the Pechini method to obtain insight into the phase formation in the quasi-ternary LaMnO3-LaCoO3-“LaCuO3” system caused by the instability of LaCuO3 under ambient conditions. After sintering at 1100°C some remarkable results were obtained: LaMn0.3Co0.3Cu0.4O3−δ crystallized as a single phase in the orthorhombic perovskite structure typical of LaCuO3. Among the synthesized compositions this compound showed the highest electrical conductivity in air at 800°C (155 S cm−1) and also the highest thermal expansion coefficient (α30−800°C=15.4×10−6 K−1). The LaCuO3−δ composition also crystallized as a single phase but in a monoclinic structure although previous investigations have shown that other phases are preferably formed after sintering at 1100°C. The electrical conductivity and thermal expansion coefficient were the lowest within the series of compositions, i.e. 9.4 S cm−1 and 11.9×10−6 K−1, respectively.  相似文献   

17.
The phase system Al2O3-TiO2 was investigated in the compositional range from 48:52 to 62:38 mol% Al2O3:TiO2. The samples were prepared by melting the binary oxides in an arc-imaging furnace and the obtained samples were examined by powder X-ray diffraction. The recorded powder patterns could be interpreted in terms of intergrowth structures consisting of two basic building blocks, which were deduced from the known crystal structures of β-Al2TiO5 and Al6Ti2O13. The structure of a new ordered compound with the formula Al16Ti5O34 is proposed. The thermal stability was estimated from DTA and tempering experiments and showed that all prepared samples decompose at temperatures around 800 °C into the binary oxides corundum and titania.  相似文献   

18.
Lattice parameters of a synthetic powder sample of Ca0.35Sr0.65TiO3 perovskite have been determined by the method of Le Bail refinement, using synchrotron X-ray diffraction patterns collected at pressures up to 15.5 GPa with a membrane-driven diamond anvil cell. At ambient conditions, diffraction data were consistent with the I4/mcm structure reported previously in the literature for the same composition. Diffraction data collected at high pressures were consistent with tetragonal (or, at least, pseudo-tetragonal) lattice geometry, and no evidence was found for the development of any of the orthorhombic structures identified in other studies of (Ca, Sr)TiO3 perovskites. Additional weak reflections, which could not be accounted for by the normal I4/mcm perovskite structure, were detected in diffraction patterns collected at pressures of 0.9-2.5 GPa, and above ∼13.5 GPa, however. Small anomalies in the evolution of unit cell volume and tetragonal strain were observed near 3 GPa, coinciding approximately with breaks in slope with increasing pressure of bulk and shear moduli for a sample with the same composition which had previously been reported. The anomalies could be due either to new tetragonal↔tetragonal/pseudo-tetragonal phase transitions or to subtle changes in compression mechanism of the tetragonal perovskite structure.  相似文献   

19.
Two new compounds, La3Ru8B6 and Y3Os8B6, were synthesized by arc melting the elements. Their structural characterization was carried out at room temperature on as-cast samples by using X-ray diffractometry. According to X-ray single-crystal diffraction results these borides crystallize in Fmmm space group (no. 69), Z=4, a=5.5607(1) Å, b=9.8035(3) Å, c=17.5524(4) Å, ρ=8.956 Mg/m3, μ=25.23 mm−1 for La3Ru8B6 and a=5.4792(2) Å, b=9.5139(4) Å, c=17.6972(8) Å, ρ=13.343 Mg/m3, μ=128.23 mm−1 for Y3Os8B6. The crystal structure of La3Ru8B6 was confirmed from Rietveld refinement of X-ray powder diffraction data. Both La3Ru8B6 and Y3Os8B6 compounds are isotypic with the Ca3Rh8B6 compound and their structures are built up from CeCo3B2-type and CeAl2Ga2-type structural fragments taken in ratio 2:1. They are the members of structural series R(A)nM3n−1B2n with n=3 (R is the rare earth metal, A the alkaline earth metal, and M the transition metal). Structural and atomic parameters were also obtained for La0.94Ru3B2 compound from Rietveld refinement (CeCo3B2-type structure, P6/mmm space group (no. 191), a=5.5835(9) Å, c=3.0278(6) Å).  相似文献   

20.
Singlet-triplet energy gaps in cyclopenta-2,4-dienylidene, as well as its 2- or 3-halogenated derivatives, are compared and contrasted with their sila, germa, stana, and plumba analogues; at HF/6-31G* and B3LYP/ 6-311++G(3df, 2p) levels of theory. Energy gaps (ΔGt-s), between triplet (t) and singlet (s) states, appear linearly proportional to: (i) the size of the group 14 divalent element (M = C, Si, Ge, Sn and Pb), (ii) the angle ∠C-M-C, and (iii) the ΔG(LUMO-HOMO) of the singlet state involved. The magnitude of ΔGt-s, for each 2- and/or 3-substituted species studied, increases with an order of: carbenes < silylenes < germylenes < stanylenes < plumbylenes. This order reverses for the barriers of the ring puckering. The puckering occurs with more ease for every singlet, compared to its corresponding triplet form.Regardless of the group 14 element (M) employed, every 3-halo-substituted species is more stable than the corresponding 2-halo-substituted isomer. For M = Pb, Sn and/or Ge; 3-halo-substituted species have higher ΔGt-s than their corresponding 2-halo-substituted analogues. For M = Si, similar ΔGt-s are found for 2- and 3-halogenated isomers. For M = C, 3-halo-substituted species have lower ΔGt-s than their corresponding 2-halo-substituted analogues.Every cyclic singlet has a larger ∠C-M-C angle, than its corresponding cyclic triplet state, except for 3-halosilacyclopenta-2,4-dienylidenes where triplet has a larger ∠C-M-C angle than its corresponding singlet state.  相似文献   

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