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1.
High-resolution neutron and synchrotron X-ray powder diffraction have been used to examine various compositions across the system (1− x)CaTiO 3− xLa 2/3TiO 3. 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.
The crystal structure of the A-site deficient perovskite Ln1/3NbO 3 ( Ln=Nd, Pr) at room temperature has been determined, for the first time, as orthorhombic in space group Cmmm using high-resolution neutron powder diffraction. Pertinent features are the alternation of unoccupied layers of A-sites and layers partly occupied by Ln cations, as well as out-of-phase tilting of the NbO 6 octahedra around an axis perpendicular to the direction of the cation/vacancy ordering. The phase transition behaviour of Nd 1/3NbO 3 has also been studied in situ. This compound undergoes a continuous phase transition at around 650 °C to a tetragonal structure in space group P4/ mmm due to the disappearance of the octahedral tilting. The analysis of spontaneous strains shows that this phase transition is tricritical in nature. 相似文献
3.
La 3NbO 7 and Nd 3NbO 7 are insulating compounds that have an orthorhombic weberite-type crystal structure and undergo a phase transition at about 360 and 450 K, respectively. The nature of the phase transitions was investigated via heat capacity measurements, synchrotron X-ray and neutron diffraction experiments. It is here shown that above the phase transition temperature, the compounds possess a weberite-type structure described by space group Cmcm (No. 63). Below the phase transition, the high temperature phase transforms into a weberite-type structure with space group Pmcn (No. 62). The phase transformation primarily involves the off-center shifting of Nb 5+ ions inside the NbO 6 octahedra, combined with shifts of one third of the Ln3+ ( Ln3+=La 3+ and Nd 3+) ions at the center of the LnO 8 polyhedra towards off-center positions. The phase transition was also proven to have great impacts on the dielectric properties. 相似文献
4.
Li 2CoTi 3O 8 has an ordered Li 2BB′ 3O 8 spinel structure, space group P4 332, at room temperature with 3:1 ordering of Ti and Li on the octahedral sites, and Li, Co disordered over the tetrahedral site. Rietveld refinement of variable temperature neutron powder diffraction data has shown an order-disorder phase transition in Li 2CoTi 3O 8 which commences at ∼500 °C with Li and Co mixing on the tetrahedral and 4-fold octahedral sites and is complete at a first order structural discontinuity at ∼915 °C. The fraction of Ti on the 12-fold octahedral site exhibits a small decrease with increasing temperature, which may suggest that the disordering involves all three cations. Above 930 °C, the structure, space group Fd3¯ m, has Li, Co and Ti sharing a single-octahedral site and Li, Co sharing a tetrahedral site, although Co still exhibits a preference for tetrahedral coordination. A labelling scheme for ordered and partially ordered 3:1 spinels is devised which focuses on the occupancy of the Li, B cations. 相似文献
5.
Structures of the double perovskites Ba 2Sr 1−xCa xWO 6 have been studied by the profile analysis of X-ray diffraction data. The end members, Ba 2SrWO 6 and Ba 2CaWO 6, have the space group I2/ m (tilt system a0b−b−) and Fm3¯ m (tilt system a0a0a0), respectively. By increasing the Ca concentration, the monoclinic structure transforms to the cubic one via the rhombohedral R3¯ phase (tilt system a−a−a−) instead of the tetragonal I4/ m phase (tilt system a0a0c−). This observation supports the idea that the rhombohedral structure is favoured by increasing the covalency of the octahedral cations in Ba 2MM′O 6-type double perovskites, and disagrees with a recent proposal that the formation of the π-bonding, e.g., d0-ion, determines the tetragonal symmetry in preference to the rhombohedral one. 相似文献
6.
The structure of 14 compounds in the series Ba 2LnTaO 6 have been examined using synchrotron X-ray diffraction and found to undergo a sequence of phase transitions from I2/ m monoclinic to I4/ m tetragonal to cubic symmetry with decreasing ionic radii of the lanthanides. Ba 2LaTaO 6 is an exception to this with variable temperature neutron diffraction being used to establish that the full series of phases adopted over the range of 15-500 K is P2 1/ n monoclinic to I2/ m monoclinic to rhombohedral. The chemical environments of these compounds have also been investigated and the overbonding to the lanthanide cations is due to the unusually large size for the B-site in these perovskites. 相似文献
7.
The crystal structures of Ba 2LnSbO 6 ( Ln=La, Pr, Nd and Sm) at room temperature have been investigated by profile analysis of the Rietveld method using either combined X-ray and neutron powder diffraction data or X-ray powder diffraction data. It has been shown that the structure of Ba 2LnSbO 6 with Ln =La, Pr and Nd are neither monoclinic nor cubic as were previously reported. They are rhombohedral with the space group . The distortion from cubic symmetry is due to the rotation of the LnO 6/SbO 6 octahedra about the primitive cubic [111] p-axis. On the other hand, the structure of Ba 2SmSbO 6 is found to be cubic. All compounds contain an ordered arrangement of LnO 6 and SbO 6 octahedra. 相似文献
8.
High-resolution time-of-flight neutron powder diffraction was carried out to investigate the crystal structures of CeAlO 3 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 CeAlO 3 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 CeAlO 3 is a rare one in oxide perovskites. 相似文献
9.
Unlike ordering of the octahedral B-site cations, ordering of the larger A-site cations in stoichiometric perovskites is rare. Herein the A- and B-site ordering characteristics of several double perovskites with AA′ BB′O 6 stoichiometry have been investigated. The compounds investigated include NaLaMgWO 6, NaLaMgTeO 6, NaLaScNbO 6, NaLaScSbO 6, NaLaTi 2O 6, and NaLaZr 2O 6. Group theoretical methods are used to enumerate the possible structures of AA′ BB′ X6 double perovskites that result from the combination of rock salt ordering of the B-site cations, layered ordering of the A-site cations, and octahedral tilting distortions. This combination results in 12 possible structures in addition to the aristotype. Among the compounds investigated only NaLaMgWO 6 and NaLaScNbO 6 show significant long-range ordering of the A-site cations, Na + and La 3+. A complete structural characterization is presented for NaLaMgWO 6. This compound possesses monoclinic C2/ m (#12) space group symmetry, with unit cell dimensions of , , , β=90.136(1)° at room temperature. The results presented here show that in AA′ BB′O 6 perovskites layered ordering of A-site cations creates a bonding instability that is compensated for by a second-order Jahn-Teller distortion of the B′ cation. These two distortions are synergistic and the removal of one leads to the disappearance of the other. 相似文献
10.
The structures of eight members of the series of double perovskites of the type Ba 2LnB′O 6 ( Ln=La 3+-Sm 3+ and Y 3+ and B′=Nb 5+ and Ta 5+) were examined both above and below room temperature using synchrotron X-ray powder diffraction. The La 3+ and Pr 3+ containing compounds had an intermediate rhombohedral phase whereas the other tantalates and niobates studied have a tetragonal intermediate. This difference in symmetry appears to be a consequence of the larger size of the La 3+ and Pr 3+ cations compared to the other lanthanides. The temperature range over which the intermediate symmetry is stable is reduced in those compounds near the point where the preferred intermediate symmetry changes from tetragonal to rhombohedral. In such compounds the transition to the cubic phase involves higher order terms in the Landau expression. This suggests that in this region the stability of the two intermediate phases is similar. 相似文献
11.
The REMn 0.5Ni 0.5O 3 (RE = La, Nd) and YMn 0.5Co 0.5O 3 compounds, prepared by solid-state reaction under air, were investigated by X-ray diffraction (XRD) and neutron powder diffraction (NPD). The La-compound contains some lanthanum vacancies decreasing the lattice parameters and increasing the crystal distortion. Nearly stoichiometric compositions were found for the other compounds prepared under similar conditions. The NPD data of all compounds indicate some ordering of the mixed transition ions over the octahedral site, since the crystal structure is better described in monoclinic space group P21/n that accounts for two distinguishable octahedral sites, rather than in orthorhombic Pbnm. Such results contrast with those obtained from X-ray diffraction where the mixed ions appear to be randomly distributed over the only octahedral site of the Pbnm space group. The low-temperature NPD patterns of YMn0.5Co0.5O3 exhibit some magnetic peaks, below 70 K. The magnetic structure at 1.4 K consists of two collinear ferromagnetic sublayers with saturated magnetic moments of 3.26(2) μB and 2.14(2) μB per ion, at the 2c- and 2d-site, respectively. The low-temperature NPD patterns of LaMn0.5Ni0.5O3 and NdMn0.5Ni0.5O3 exhibit an obvious increase in intensity over the only nuclear peak, at 2θ 36.89° and 36.40°, respectively. Although such behavior indicates a ferromagnetic ordering, the magnetic structure could not be refined because of the faint magnetic properties, more likely due to the incomplete atomic ordering. 相似文献
12.
The crystal structures of Ca 2Ln3Sb 3O 14 ( Ln=La, Pr, Nd and Y) and Ca 2Sb 2O 7 at room temperature were refined by the Rietveld method using combined X-ray and neutron powder diffraction data. Ca 2Sb 2O 7 adopts the weberite structure having the space group Imma. The structures of Ca 2Ln3Sb 3O 14 are, however, neither the orthorhombic nor the tetragonal chiolite as has been suggested previously. They crystallize in the monoclinic space group I2/ m11 belonging to a hitherto unknown type of deformation of the parent (orthorhombic) weberite structure. 相似文献
13.
The Ba-Bi-Ir-O system is found to contain two distinct perovskite-type phases: a rock-salt ordered double perovskite Ba 2BiIrO 6; and a 6H-type hexagonal perovskite Ba 3BiIr 2O 9. Ba 2BiIrO 6 undergoes a series of symmetry-lowering phase transitions on cooling , all of which are second order except the rhombohedral→monoclinic one, which is first order. The monoclinic phase is only observed in a 2-phase rhombohedral+monoclinic regime. The transition and 2-phase region lie very close to 300 K, making the room-temperature X-ray diffraction patterns extremely complex and potentially explaining why Ba 2BiIrO 6 had not previously been identified and reported. A solid solution Ba 2Bi 1+xIr 1−xO 6, analogous to Ba 2Bi 1+xRu 1−xO 6, 0≤ x≤2/3, was not observed. The 6H-type phase Ba 3BiIr 2O 9 undergoes a clean second-order phase transition P6 3/ mmc→ C2/ c at 750 K, unlike 6H-type Ba 3LaIr 2O 9, the P6 3/ mmc structure of which is highly strained below ∼750 K but fails to distort coherently to the monoclinic phase. 相似文献
14.
Polycrystalline samples of La 18Li 8Rh 4MO 39 ( M=Ti, Mn, Ru) have been prepared by a solid-state method and studied by neutron powder diffraction. They are isostructural with La 18Li 8Rh 5O 39 and adopt the cubic space group with a ∼12.22 Å. Their structure consists of a La-O framework containing intersecting channels that run along 〈111〉. These channels are occupied by chains made up of alternating, face-sharing trigonal-prismatic and octahedral coordination polyhedra; there are two crystallographically distinct types of octahedral site. The prisms are occupied by Li and the transition metals are disordered over the two octahedral sites. 相似文献
15.
The structure of the double perovskite Ba 2PrIrO 6 has been re-examined by profile analysis of X-ray diffraction data. It has been shown that neither the monoclinic P2 1/n nor the tetragonal P4 /mnc space group correctly describes its structure. Ba 2PrIrO 6 has cubic symmetry, space group , cell parameter . In agreement with earlier observations, Ba 2PrIrO 6 is an ordered perovskite containing the valence pair Pr(IV)/Ir(IV). 相似文献
16.
Bulk SrMnO x samples with oxygen contents 2.5? x<2.7 have been studied using a combination of neutron time-of-flight and high-energy high-resolution synchrotron X-ray diffraction measurements along with thermogravimetric analysis. We report the identification and characterization of two new oxygen-vacancy ordered phases, Sr 5Mn 5O 13 (SrMnO 2.6-tetragonal P4/ m a=8.6127(3) Å, c=3.8102(2) Å) and Sr 7Mn 7O 19 (SrMnO 2.714-monoclinic P2/ m a=8.6076(4) Å, b=12.1284(4) Å, c=3.8076(2) Å, γ=98.203(2)°). The nuclear and magnetic structures of Sr 2Mn 2O 5 are also reported (SrMnO 2.5 nuclear: orthorhombic Pbam, magnetic: Orthorhombic Ay type Pcbam with cM=2c). In the three phases, oxygen-vacancies are ordered in lines running along one of the (100) directions of the parent cubic perovskite system. Oxygen-vacancy ordering allows the charge and orbital ordering of the Mn 3+ and Mn 4+ cations in the new phases. 相似文献
17.
We have successfully synthesized a high-purity polycrystalline sample of tetragonal Li 7La 3Zr 2O 12. Single crystals have been also grown by a flux method. The single-crystal X-ray diffraction analysis verifies that tetragonal Li 7La 3Zr 2O 12 has the garnet-related type structure with a space group of I4 1/ acd (no. 142). The lattice constants are a=13.134(4) Å and c=12.663(8) Å. The garnet-type framework structure is composed of two types of dodecahedral LaO 8 and octahedral ZrO 6. Li atoms occupy three crystallographic sites in the interstices of this framework structure, where Li(1), Li(2), and Li(3) atoms are located at the tetrahedral 8 a site and the distorted octahedral 16 f and 32 g sites, respectively. The structure is also investigated by the Rietveld method with X-ray and neutron powder diffraction data. These diffraction patterns are identified as the tetragonal Li 7La 3Zr 2O 12 structure determined from the single-crystal data. The present tetragonal Li 7La 3Zr 2O 12 sample exhibits a bulk Li-ion conductivity of σb=1.63×10 −6 S cm −1 and grain-boundary Li-ion conductivity of σgb=5.59×10 −7 S cm −1 at 300 K. The activation energy is estimated to be Ea=0.54 eV in the temperature range of 300–560 K. 相似文献
18.
B-site disordered RFe 0.5V 0.5O 3 compounds, with R=La, Nd, Eu and Y, have been prepared by solid-state reaction technique and their structures and magnetic properties have been investigated through X-ray powder diffraction, time-of-flight neutron powder diffraction and magnetization measurements at temperatures ranging from 5 to 700 K. The four compounds can be described as distorted perovskites with space group symmetry Pbnm and a+b−b− tilt system. The studied compounds also show antiferromagnetic ordering with Neel temperatures of 299, 304, 304, and 335 K respectively. The magnetic structures of R=La, Nd and Y compounds were determined from the neutron powder diffraction as Gz with observed magnetic moments of 2.55, 2.54 and 2.69 μB at 30, 40 and 40 K, respectively. 相似文献
19.
The crystal structure of Ca 5Te 3O 14 at room temperature was studied by the Rietveld method using combined X-ray and neutron powder diffraction data. The compound crystallizes in the space group Cmca with the lattice parameters a=10.4268(2) Å, b=10.3908(2) Å and c=10.4702(2) Å. The structure of Ca 5Te 3O 14 is chiolite-like and consists of a framework of corner-linked TeO 6 octahedral layers in which a linear TeO 2 group of every fourth octahedron is substituted by a Ca atom. This type of structure was previously observed in BaSr 4U 3O 14. The relationship between the chiolite-like structure and the fluorite structure is discussed. 相似文献
20.
A new vanado-molybdate LiMg 3VMo 2O 12 has been synthesized, the crystal structure determined an ionic conductivity measured. The solid solution Li 2−zMg 2+zV zMo 3−zO 12 was investigated and the structures of the z=0.5 and 1.0 compositions were refined by Rietveld analysis of powder X-ray (XRD) and powder neutron diffraction (ND) data. The structures were refined in the orthorhombic space group Pnma with a∼5.10, b∼10.4 and c∼17.6 Å, and are isostructural with the previously reported double molybdates Li 2M2(MoO 4) 3 ( M=M 2+, z=0). The structures comprise of two unique (Li/Mg)O 6 octahedra, (Li/Mg)O 6 trigonal prisms and two unique (Mo/V)O 4 tetrahedra. A well-defined 1:3 ratio of Li +:Mg 2+ is observed in octahedral chains for LiMg 3VMo 2O 12. Li + preferentially occupies trigonal prisms and Mg 2+ favours octahedral sheets. Excess V 5+ adjacent to the octahedral sheets may indicate short-range order. Ionic conductivity measured by impedance spectroscopy (IS) and differential scanning calorimetry (DSC) measurements show the presence of a phase transition, at 500-600 °C, depending on x. A decrease in activation energy for Li + ion conductivity occurs at the phase transition and the high temperature structure is a good Li + ion conductor, with σ=1×10 −3-4×10 −2 S cm −1 and Ea=0.6 to 0.8 eV. 相似文献
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