共查询到20条相似文献,搜索用时 687 毫秒
1.
Zhaoming Zhang Gregory R. Lumpkin Christopher J. Howard Karl R. Whittle Keiichi Osaka 《Journal of solid state chemistry》2007,180(3):1083-1092
High-resolution neutron and synchrotron X-ray powder diffraction have been used to examine various compositions across the system (1−x)CaTiO3−xLa2/3TiO3. The structures at room temperature were determined according to composition: in Pbnm for 0?x?0.5, Ibmm for 0.5<x<0.7, then I4/mcm for 0.7?x<0.9, and finally in Cmmm for x?0.9. Although the four structures are the same as those proposed previously in an X-ray diffraction study, the phase boundaries are somewhat different, in particular the Pbnm↔Ibmm phase boundary has been extended from x?0.4 to x>0.5 in the current study based on our high-resolution neutron diffraction data. From in situ measurements to identify structures above room temperature, an approximate composition-temperature phase diagram has been constructed, involving four temperature-induced phase transitions: Pbnm→I4/mcm, Ibmm→I4/mcm, I4/mcm→Pm3¯m and Cmmm→P4/mmm. 相似文献
2.
A.I. Becerro S.A.T. RedfernM.A. Carpenter K.S. KnightF. Seifert 《Journal of solid state chemistry》2002,167(2):459-471
The influence of small amounts of Fe3+ on the phase transitions of CaTiO3 perovskite has been studied by means of in situ high-temperature neutron diffraction. The same sequence of phase transitions as observed in CaTiO3 is shown by both CaTi0.9Fe0.1O2.95 and CaTi0.8Fe0.2O2.90 perovskites: from orthorhombic Pnma symmetry at room temperature (RT) to cubic Pm3m at high temperature, with an intermediate I4/mcm tetragonal phase which exists over a temperature range of about 100°C. The two phase boundaries in the temperature vs composition phase diagram of the system CaFexTi1−xO3−x/2 (0≤x≤0.4) decrease in a quasi-linear manner with increasing Fe content up to x=0.2 and then they both drop abruptly to RT. The existence of a second orthorhombic phase (Cmcm), which has been postulated for CaTiO3, is ruled out in the Fe-doped CaTiO3 perovskites in view of the behavior of specific diffraction peaks. Strain analysis shows first-order thermodynamic character for the Pnma→I4/mcm transition, while the character of the Pm3m→I4/mcm transition could be second order or tricritical. Shear strains behave more or less classically, as described by order parameter coupling and shear strain/order parameter coupling models. The volume strain has an anomalous coupling with the order parameter components, which appears to be temperature-dependent. 相似文献
3.
Phase transitions that occurred in perovskite BaPbO3 have been investigated using high-resolution time-of-flight neutron powder diffraction. The structure at room temperature is orthorhombic (space group Imma), which is derived from the cubic aristotype by tilting the PbO6 octahedra around the two-fold axis (tilt system a0b−b−). The orthorhombic structure shows anisotropic line broadening attributed to the presence of micro twins. At above about 573 K, BaPbO3 undergoes a discontinuous phase transition to a tetragonal structure (space group I4/mcm) with the tilting of the PbO6 octahedra being about the four-fold axis of the cubic aristotype (tilt system a0a0c−). With further increasing the temperature, BaPbO3 experiences a continuous phase transition to a simple cubic structure (space group Pm3¯m) at above about 673 K. The later phase transition is characterised by a critical exponent of β=0.36, depicted by the three-dimensional Heisenberg universality class. The earlier reported Imma→I2/m phase transition above room temperature has not been observed. 相似文献
4.
L. Vasylechko V. Vashook A. Senyshyn M. Knapp M. Berkowski U. Bismayer 《Journal of solid state chemistry》2003,172(2):396-411
Crystal structure and anisotropy of the thermal expansion of single crystals of La1−xSrxGa1−2xMg2xO3−y (x=0.05 and 0.1) were measured in the temperature range 300-1270 K. High-resolution X-ray powder diffraction data obtained by synchrotron experiments have been used to determine the crystal structure and thermal expansion. The room temperature structure of the crystal with x=0.05 was found to be orthorhombic (Imma, Z=4, a=7.79423(3) Å, b=5.49896(2) Å, c=5.53806(2) Å), whereas the symmetry of the x=0.1 crystal is monoclinic (I2/a, Z=4, a=7.82129(5) Å, b=5.54361(3) Å, c=5.51654(4) Å, β=90.040(1)°). The conductivity in two orthogonal directions of the crystals has been studied. Both, the conductivity and the structural data indicate three phase transitions in La0.95Sr0.05Ga0.9Mg0.1O2.92 at 520-570 K (Imma-I2/a), 770 K (I2/a-R3c) and at 870 K (R3c-R-3c), respectively. Two transitions at 770 K (I2/a-R3c) and in the range 870-970 K (R3c-R-3c) occur in La0.9Sr0.1Ga0.8Mg0.2O2.85. 相似文献
5.
Sanjay Kumar Mishra Dhananjai Pandey Pierre Ranson Jean-Paul Pinan-Lucarre 《Journal of solid state chemistry》2005,178(9):2846-2857
Results of powder X-ray diffraction and Raman scattering studies on the phase transitions in Sr1−xCaxTiO3 (SCT) are presented for x=0.04, 0.06 and 0.12 in the temperature range 8-473 K. It is proposed that the space group of SCT in the composition range 0.06?x?0.35 is Imma with a0b-b- tilt system and not I4/mcm with a0a0c- tilt system, as assumed by earlier workers. The lowering of the crystal symmetry from I4/mcm to Imma is supported by the observation of additional Raman lines, in agreement with the factor group analysis for the Imma space group. The structural Eg mode, characteristics of the non-cubic phase, is shown to be present even in the cubic phases of x=0.06 and x=0.12 but not of x=0.04 indicating the change in the local structure of the cubic phase of SCT for x?0.06. The presence of symmetry forbidden TO2 mode in the Raman spectra of the cubic phase of SCT for x<0.06 and its absence for x?0.06 provides yet another characteristic feature distinguishing the I4/mcm and Imma space groups. The implications of the change in the tilt system from a0a0c- to a0b-b- on the development of the polar order is also discussed. 相似文献
6.
A new complete solid solution of NASICON-type compounds between LiZr2(PO4)3 and La1/3Zr2(PO4)3 was evidenced with the general formula Li1−xLax/3Zr2(PO4)3 (0?x?1). These phases were synthesized by a complex polymerizable method and structurally characterized from Rietveld treatment of their X-ray and neutron powder diffraction data. This solid solution results from the substitution mechanism Li+→1/3La3++2/3□ leading to an increase of the vacancies number correlated to an increase of the La content. According to this substitution mechanism, the general formula can then be written Li1−xLax/3□2x/3Zr2(PO4)3 (0?x?1) in order to underline the correlation between the La content and the vacancies rate. For all the compounds, the structure is clearly related to that of the NASICON family with three crystallographic domains evidenced. For 0?x?0.5, all the members adopt at high temperature the typical NASICON-type structure (s.g. R3¯c), while at lower temperature, their structure distorts to a triclinic form (s.g. C 1¯), as observed for LiZr2(PO4)3 prepared above 1100 °C. Moreover, in this domain, the reversible transition is clearly soft and the transition temperature strongly depends of the x value. For 0.6?x?0.9, the compounds crystallize in a rhombohedral cell (s.g. R3¯), while for x=1, the phase La1/3Zr2(PO4)3 is obtained (s.g. P3¯, Z=6, a=8.7378(2) Å, c=23.2156(7) Å).This paper is devoted to the structure analysis of the series Li1−xLax/3Zr2(PO4)3 (0?x?1), from X-ray and neutron powder thermo diffraction and transmission electron microscopy (TEM) studies. 相似文献
7.
The room temperature structure of perovskite CeAlO3 has been reinvestigated by X-ray powder diffraction. The Rietveld refinement has confirmed the tetragonal symmetry; but revealed a super cell, a=5.32489(6) Å and c=7.58976(10) Å, with the space group I4/mcm. In CeAlO3, the distortion from the ideal cubic perovskite is caused by the cooperative tilting of the AlO6 octahedra around the primitive cubic [001]p-axis. 相似文献
8.
The structural properties of the system La1−xCexY2Ni9 with xCe=0, 0.5 and 1 have been investigated by electron probe microanalysis, powder X-ray diffraction and absorption spectroscopy. The compound LaY2Ni9 adopts a rhombohedral structure of PuNi3-type (R-3m space group, Z=3). It can be described as an intergrowth between RM5 (Haücke phase) and RM2 (Laves phase) type structures. Among the two available crystallographic sites for R atoms, lanthanum occupies preferentially the site 3a leading to a partially ordered ternary compound. Substitution by cerium involves anisotropic variations of the cell parameter with a decrease of a and an increase of c leading to an overall cell volume reduction. Increasing cerium content does not induce any symmetry change but leads to a statistical distribution of the rare earths over the two sites 3a and 6c involving an evolution toward a pseudo-binary compound. This behavior is related to the intermediate valence state of cerium observed by X-ray absorption spectroscopy. The hydriding properties of the two compounds LaY2Ni9 and CeY2Ni9 are described in relation with their crystallographic structure. 相似文献
9.
High-resolution time-of-flight neutron powder diffraction was carried out to investigate the crystal structures of CeAlO3 over a wide temperature range between 4.2 and 1423 K. Confirming the recent result of X-ray powder diffraction, the room temperature structure is tetragonal with the space group I4/mcm (tilt system (a0a0c−)). The tetragonal structure persists down to 4.2 K. However, above room temperature CeAlO3 undergoes three phase transitions: first to the orthorhombic Imma structure (tilt system (a0b−b−)) at, e.g., 373 K, then to the rhombohedral structure (tilt system (a−a−a−)) at, e.g., 473 K, and finally, to the primitive cubic structure which exists above 1373 K. The sequence of phases, , which occurs in CeAlO3 is a rare one in oxide perovskites. 相似文献
10.
Michael A. Carpenter Susana Rios Wilson Crichton 《Journal of solid state chemistry》2007,180(1):360-369
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
11.
Phase equilibria, crystal structure, and transport properties in the (100−x) La0.95Ni0.6Fe0.4O3-xCeO2 (LNFCx) system (x=2-75 mol%) were studied in air. Evolution of phase compositions and crystal structure of components was observed. The LNFCx (2≤x≤10) are three-phase and comprise the perovskite phase with rhombohedral symmetry (R3?c), the modified ceria with fluorite structure (Fm3?m), and NiO as a secondary phase. These multiphase compositions exhibit metallic-like conductivity above 300 °C. Their conductivity gradually decreases from 395.6 to 260.6 S/cm, whereas the activation energy remains the same (Ea=0.04-0.05 eV), implying the decrease in the concentration of charge carriers. Phase compositions in the LNFCx (25≤x≤75) are more complicated. A change from semiconducting to metallic-like conductivity behavior was observed in LNFC25 at about 550 °C. The conductivity of LNFCx (25≤x≤75) could be explained in terms of a modified simple mixture model. 相似文献
12.
The crystal structure of the defect perovskite series Sr1−xTi1−2xNb2xO3 has been investigated over a range of temperatures using high-resolution synchrotron X-ray diffraction, neutron diffraction and electron diffraction. Three distinct regions were observed: 0<x≤0.125 was a solid solution of Sr1−xTi1−2xNb2xO3 with minor SrTiO3 intergrowth, 0.125<x≤0.2 was a pure Sr1−xTi1−2xNb2xO3 solid solution adopting the cubic perovskite type structure (Pm3¯m) and for x>0.2 Sr0.8Ti0.6Nb0.4O3 and Sr3TiNb4O15 formed a two phase region. The cubic structure for Sr0.8Ti0.6Nb0.4O3 was stable over the temperature range 90-1248 K and the thermal expansion co-efficient was determined to be 8.72(9)×10−6 K−1. Electron diffraction studies revealed diffuse scattering due to local scale Ti/Nb displacements and slightly enhanced octahedral rotations that did not lead to long range order. The octahedral rotations were observed to ‘lock-in’ at temperatures below ∼75 K resulting in a tetragonal structure (I4/mcm) with anti-phase octahedral tilting about the c-axis. 相似文献
13.
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
14.
The Sr deficient perovskite Sr0.92NbO3 was synthesized from Sr5Nb4O15 and Nb and its crystal structure was determined using powder neutron diffraction. At room temperature the structure is orthorhombic in space group Pnma with both in-phase and out-of-phase tilting of the NbO6 octahedra. High temperature measurements have shown that the oxide undergoes a sequence of phase transitions with increasing temperature: Pnma→P4/mbm→Pm3¯m. The intermediate tetragonal phase has only in-phase tilts of the NbO6 octahedra, rather than the out-of-phase tilts present in the more commonly observed I4/mcm structure, due to initial softening at the M point rather than R point. The tetragonal phase exists only over a very narrow temperature range. The importance of M-M and M-O bonding in controlling the transition temperatures in SrMO3 perovskites is discussed. 相似文献
15.
Anita Fossdal 《Journal of solid state chemistry》2004,177(8):2933-2942
The phase relations in the pseudo-binary system SrO-Fe2O3 have been investigated in air up to 1150°C by means of powder X-ray diffraction and thermal analysis. Sr3Fe2O7−δ, SrFeO3−δ and SrFe12O19 are stable phases in the entire investigated temperature region, whereas Sr2FeO4−δ and Sr4Fe3O10−δ decompose above 930±10°C and 850±25°C, respectively. Sr4Fe6O13±δ is entropy-stabilized relative to SrFeO3−δ and SrFe12O19 above 775±25°C. Extended solid-solution Sr1±xFeO3−δ was demonstrated. On the Fe-deficient side, the extent of solid solubility appeared to decrease gradually with temperature, whereas an abrupt decrease due to formation of Sr4Fe6O13±δ was observed above 775°C on the Sr-deficient side. 相似文献
16.
The binary phase diagram of KNO3-KClO3 is studied by means of differential scanning calorimetry (DSC) and high-temperature X-ray diffraction. The limited solid solutions, K(NO3)1−x(ClO3)x (0<x<0.20) and K(NO3)1−x(ClO3)x (0.90<x<1.0), were formed in the KNO3-based solid solutions and KClO3-based solid solutions phase, respectively. For KNO3-based solid solutions, KNO3 ferroelectric phase can be stable from 423 to 223 K as a result of substituting of NO3 by ClO3-radicals. The temperatures for solidus and liquidus have been determined based on limited solid solutions. Two models, Henrian solution and regular solution theory for KNO3-based (α) phase and KClO3-based (β) phase, respectively, are employed to reproduce solidus and liquidus of the phase diagram. The results are in good agreement with the DSC data. The thermodynamic properties for α and β solid solutions have been derived from an optimization procedure using the experimental data. The calculated phase diagram and optimized thermodynamic parameters are thermodynamically self-consistent. 相似文献
17.
Brendan J. Kennedy Jimmy Ting Zhaoming Zhang Michihiro Miyake 《Journal of solid state chemistry》2009,182(4):954-90
Fifteen perovskite-type compounds Sr0.9−xCaxCe0.1MnO3, x=0-0.9 in steps as fine as 0.05, have been synthesised by solid state methods, and the room temperature structures characterised using X-ray synchrotron powder diffraction. At low Ca contents (x?0.45) the structures are tetragonal in space group I4/mcm and at high Ca contents (x?0.55) the compounds are orthorhombic in space group Pbnm. At room temperature these two phases co-exist in the compound with x=0.5. XANES measurements show the Ce to be present as Ce4+ in all the oxides. High temperature structures are reported for selected members. 相似文献
18.
Christophe T.G. Petit 《Journal of solid state chemistry》2010,183(6):1231-6194
A-site substituted cerium orthovanadates, Ce1−xSrxVO4, were synthesised by solid-state reactions. It was found that the solid solution limit in Ce1−xSrxVO4 is at x=0.175. The crystal structure was analysed by X-ray diffraction and it exhibits a tetragonal zircon structure of space group I41/amd (1 4 1) with a=7.3670 (3) and c=6.4894 (1) Å for Ce0.825Sr0.175VO4. The UV-vis absorption spectra indicated that the compounds have band gaps at room temperature in the range 4.5-4.6 eV. Conductivity measurements were performed for the first time up to the strontium solid solution limit in air and in dry 5% H2/Ar with conductivity values at 600 °C ranging from 0.3 to 30 mS cm−1 in air to 30-45 mS cm−1 in reduced atmosphere. Sample Ce0.825Sr0.175VO4 is redox stable at a temperature below 600 °C although the conductivity is not high enough to be used as an electrode for solid oxide fuel cells. 相似文献
19.
The (Ca1-2xNaxLax)TiO3 (0?x?0.5) A-site substituted perovskite compounds have been synthesized and characterized by XRD and Raman spectroscopy at room temperature. The XRD powder diffraction study suggests that the end-member Na1/2La1/2TiO3 crystallizes in the tetragonal space group I4/mcm. The phase transition from Pbnm to I4/mcm is located between x=0.34 and 0.39 and is driven by the variation of ionic radii at the A-site. The observed Raman modes are in agreement with group theory analysis, and the relationships between the behavior of structural parameters (e.g. Ti-O-Ti bond angle), indicated by long-range order, and the corresponding Raman frequency shifts and intensity evolution, indicated by short-range order, are established and discussed in terms of the radius effect and the mass effect. 相似文献
20.
Here we employ high-quality samples of (Sr1−xLax)CuO2 and (Nd2−xCex)CuO4 and XANES spectroscopy at O-K, Cu-L2,3 and Ce-M4,5 edges to gain comprehensive understanding of the electronic structure and doping in n-type high-Tc superconductors. Not only common but also slightly different features are revealed for the two systems. From O-K-edge spectra, the UHB is found essentially independent of the electron-doping level for both the systems, in line with our understanding that the doped electrons do not go to the O site in n-type copper-oxide superconductors. Another common observation is that the main CuII peak at the Cu-L3 edge (due to transitions to the CuII-3d orbitals) systematically decreases in intensity upon electron doping, hence verifying the fact that the doped electrons go to the Cu site. The difference then between the two systems is that in (Sr1−xLax)CuO2 the weaker CuII peak due to transitions to the CuII-4s orbital depends on the degree of doping. Moreover, it was found that with increasing x, electron density increases much faster in (Sr1−xLax)CuO2 than in (Nd2−xCex)CuO4. This is a consequence of two phenomena: a tiny increase in oxygen content concomitant to the CeIV-for-NdIII substitution and the somewhat lower Ce-valence value of +3.8 compared to the nominal tetravalent state. 相似文献