Chiolite-like Ca5Te3O14: An X-ray and neutron diffraction study

The crystal structure of Ca₅Te₃O₁₄ 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 Ca₅Te₃O₁₄ is chiolite-like and consists of a framework of corner-linked TeO₆ octahedral layers in which a linear TeO₂ group of every fourth octahedron is substituted by a Ca atom. This type of structure was previously observed in BaSr₄U₃O₁₄. The relationship between the chiolite-like structure and the fluorite structure is discussed.     

Structural investigation of Ca2MnO4 by neutron powder diffraction and electron microscopy

The first member of the Ruddlesden-Popper family, Ca₂MnO₄, has been revisited. Coexistence of two structures has been shown from electron microscopy at room temperature and neutron diffraction data have evidenced two antiferromagnetic structures at low temperature. Two forms, with an orthorhombic Aba2 ( and c≈12 Å) and a tetragonal I4₁/cad ( and c≈24 Å) symmetries, were found to coexist coherently within the same matrix.     

Synchrotron and neutron powder diffraction study of phase transition in weberite-type Nd3NbO7 and La3NbO7

La₃NbO₇ and Nd₃NbO₇ 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 Nb⁵⁺ ions inside the NbO₆ octahedra, combined with shifts of one third of the Ln³⁺ (Ln³⁺=La³⁺ and Nd³⁺) ions at the center of the LnO₈ polyhedra towards off-center positions. The phase transition was also proven to have great impacts on the dielectric properties.     

X-ray and neutron powder diffraction study of the double perovskites Ba2LnSbO6 (Ln=La, Pr, Nd and Sm)

The crystal structures of Ba₂LnSbO₆ (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 Ba₂LnSbO₆ 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 LnO₆/SbO₆ octahedra about the primitive cubic [111]_p-axis. On the other hand, the structure of Ba₂SmSbO₆ is found to be cubic. All compounds contain an ordered arrangement of LnO₆ and SbO₆ octahedra.     

Hydrothermal synthesis of the perovskite manganites Pr0.5Sr0.5MnO3 and Nd0.5Sr0.5MnO3 and alkali-earth manganese oxides CaMn2O4, 4H-SrMnO3, and 2H-BaMnO3

We report the synthesis of the perovskite manganites Pr_0.5Sr_0.5MnO₃ and Nd_0.5Sr_0.5MnO₃ using mild hydrothermal conditions. Both are formed as polycrystalline powders from solutions of metal salts in aqueous potassium hydroxide at 240 °C, and crystallise as a tetragonal polymorph (space group I4/mcm). Scanning electron microscopy shows both materials to contain cuboid-shaped crystallites several microns in dimension, and the average particle size is verified by light scattering measurements. We also report the first hydrothermal synthesis of 2H-BaMnO₃ and 4H-SrMnO₃, and the first subcritical hydrothermal synthesis of CaMn₂O₄ (marokite). Despite the formation of these alkali-earth manganese oxides at 240 °C, we have been unable to isolate rare-earth manganese oxides LnMnO₃ using similar conditions. We discuss the formation of perovskite manganites in hydrothermal reactions by relating our new results to those manganites already reported to form under hydrothermal conditions, and rationalise the trends seen by considering tolerance factor of the perovskite and the variance of the A-site metal radius.     

Strained Structure in Ho0.5Sr0.5MnO3

The Ho_0.5Sr_0.5MnO₃ perovskite, synthesized in air, has been studied by combining neutron powder and electron diffraction techniques. The Pnma-type structure exhibits a strong tilting of the MnO₆ octahedra. This octahedra tilting and microtwinning involve a complex strained structure. No structural transition is observed down to 1.4 K, but short-range A-type antiferromagnetism running over only a few perovskite subcells is evidenced below ≈90 K. The different behavior of this perovskite compared to other Ln_0.5Sr_0.5MnO₃ perovskites is discussed in terms of A-site cationic mismatch.     

Manganite charge and orbitally ordered and disordered states probed by Fe substitution into Mn site in , and (, Gd, Sm, Nd, Pr, La)

The layered manganese oxides LnBaMn_1.96Fe_0.04O_y (Ln=Y, Gd, Sm, Nd, Pr, La) have been synthesized for y=5,5.5 and 6. In the oxygen-saturated state (y=6) they exhibit the charge and orbital order at ambient temperature for Ln=Y, Gd, Sm, but unordered e_g-electronic system for Ln=La, Pr, Nd. Fourfold increase of quadrupole splitting was observed owing to the charge and orbital ordering. This is in agreement with the jumplike increase in distortion of the reduced perovskite-like cell for the charge and orbitally ordered manganites compared to the unordered ones. Substitution of 2% of Mn by Fe suppresses the temperatures of structural and magnetic transitions by 20–50 K. Parameters of the crystal lattices and the room-temperature Mössbauer spectra were studied on 40 samples whose structures were refined within five symmetry groups: P4/mmm, P4/nmm, Pm-3m, Icma and P2/m. Overwhelming majority of the Fe species are undifferentiated in the Mössbauer spectra for most of the samples. Such the single-component spectra in the two-site structures are explained by the preference of Fe towards the site of Mn(III) and by the segmentation of the charge and orbitally ordered domains.     

Long-range magnetic ordering in Ba2CoS3: A neutron diffraction study

Neutron powder diffraction has been used to determine the magnetic structure of the quasi-one-dimensional compound Ba₂CoS₃, which contains linear [001] chains of vertex-sharing CoS₄ tetrahedra, spaced apart by Ba²⁺ cations. At 1.5 K the Co²⁺ cations in the chains are antiferromagnetically ordered with an ordered magnetic moment of 1.97(4) μ_B per cation aligned along [100]. Each Co²⁺ cation is ferromagnetically aligned with four cation in neighbouring chains and antiferromagnetically aligned with two others.     

The ferroelectric phase of CdTiO3: A powder neutron diffraction study

The synthesis of bulk samples of polycrystalline CdTiO₃ in both the rhombohedral ilmenite and orthorhombic perovskite forms is described and the structures of these have been refined using powder neutron diffraction data. This involved the preparation of samples enriched in ¹¹⁴Cd. Cooling perovskite-type CdTiO₃ to 4 K induces a ferroelectric phase transition, with the neutron data suggesting the low temperature structure is in Pna2₁. Mode analysis shows the polar mode to be dominant at low temperatures. The ilmenite-structure of CdTiO₃ is compared with that of ZnTiO₃. The refined scattering length of the ¹¹⁴Cd is estimated to be 5.56 fm. Attempts to dope CdTiO₃ with Ca and Sr are described.     

Neutron powder diffraction study of tetragonal Li7La3Hf2O12 with the garnet-related type structure

We have successfully synthesized a polycrystalline sample of tetragonal garnet-related Li-ion conductor Li₇La₃Hf₂O₁₂ by solid state reaction. The crystal structure is analyzed by the Rietveld method using neutron powder diffraction data. The structure analysis identifies that tetragonal Li₇La₃Hf₂O₁₂ has the garnet-related type structure with a space group of I4₁/acd (no. 142). The lattice constants are a=13.106(2) Å and c=12.630(2) Å with a cell ratio of c/a=0.9637. The crystal structure of tetragonal Li₇La₃Hf₂O₁₂ has the garnet-type framework structure composed of dodecahedral La(1)O₈, La(2)O₈ and octahedral HfO₆. Li atoms occupy three types of crystallographic site in the interstices of this framework structure, where Li(1) atom is located at the tetrahedral 8a site, and Li(2) and Li(3) atoms are located at the distorted octahedral 16f and 32g sites, respectively. These Li sites are filled with the Li atom. The present tetragonal Li₇La₃Hf₂O₁₂ sample exhibits bulk Li-ion conductivity of σ_b=9.85×10⁻⁷ S cm⁻¹ and grain-boundary Li-ion conductivity of σ_gb=4.45×10⁻⁷ S cm⁻¹ at 300 K. The activation energy is estimated to be E_a=0.53 eV in the temperature range of 300-580 K.     

Structural properties of two deuterides LaY2Ni9D12.8 and CeY2Ni9D7.7 determined by neutron powder diffraction and X-ray absorption spectroscopy

The crystal structure of two deuterides RY₂Ni₉D_x (R=La; x=12.8 and R=Ce; x=7.7) have been investigated by means of neutron powder diffraction and X-ray absorption spectroscopy. The structures are best described in the space group . The deuterium location has been determined for both compounds. The nature and the occupancy factors of the different D sites are presented. Comparisons are made between the La-based deuteride and the Ce-one in relation with the crystal structure of the intermetallic compounds. Differences in site occupancies within the RM₂ and RM₅ building units of the PuNi₃-type structure are discussed and heterogeneous mixed valence state is reported for the cerium compound.     

Transitions between P21, , and P6322 modifications of SrAl2O4 by in situ high-temperature X-ray and neutron diffraction

The results of in situ high-temperature X-ray and neutron powder diffraction experiments reconcile inconsistencies in previous reports on the symmetry of high-temperature phases of SrAl₂O₄. The material undergoes two reversible phase transitions and at 680 and 860 °C, respectively, and the latter one is experimentally observed and characterized for the first time. The higher symmetry above the transition is gained by disordering off-center split site of oxygen atoms around trigonal axis rather than by unbending Al–O–Al angle to the ideal value 180°. The analysis of the literature suggests that it is a common feature of the P6₃22 phases of stuffed tridymites.     

Structure, magnetism and transport of the perovskite manganites Ln0.5Ca0.5MnO3 (Ln=Ho, Er, Tm, Yb and Lu)

It was found that the manganese perovskite oxides Ln_0.5Ca_0.5MnO₃ (Ln=Ho, Er, Tm, Yb and Lu) have an orthorhombic structure (space group Pnma). The Mn-O-Mn angles were calculated to be ∼148-150°, revealing an existence of a large crystallographic distortion in these oxides. Electrical resistivity measurements indicated both an insulating nature and a small magnetoresistance effect, both of which are owing to narrow bandwidths of the Mn-3d electrons arising from the crystallographic distortion. DC magnetization measurements showed the three characteristic temperatures, which could be assigned to charge-order, antiferromagnetism of Mn moments, and possible glassy states. All of these temperatures were decreased for the heavier Ln ions, which is explained in connection with both a difference of ionic radii of Ln³⁺ and Ca²⁺, and a lowering of electron transfer. The charge-ordering transition was not clearly observed only for Lu_0.5Ca_0.5MnO₃ containing the smallest lanthanide ion, plausibly due to a large randomness of magnetic interactions arising from the ionic radii difference of Lu³⁺ and Ca²⁺. In addition, preliminary measurements of AC dielectric response suggested that these manganites belong to a so-called multiferroic system.     

Structures and crystal chemistry of the double perovskites Ba2LnB′O6 (Ln=lanthanide B′=Nb and Ta): Part I. Investigation of Ba2LnTaO6 using synchrotron X-ray and neutron powder diffraction

The structure of 14 compounds in the series Ba₂LnTaO₆ 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. Ba₂LaTaO₆ 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 P2₁/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.     

The crystal structure of a new bismuth tellurium oxychloride Bi0.87Te2O4.9Cl0.87 from neutron powder diffraction data

A new bismuth tellurium oxychloride was obtained by reaction of BiOCl and TeO₂ in air. According to energy dispersive X-ray spectroscopy and neutron powder diffraction refinement the composition of the substance was determined as Bi_0.87Te₂O_4.9Cl_0.87. The new compound crystallizes in the trigonal system space group R 3¯ (#148), Z=6, a=4.10793(4), c=31.1273(4) Å, χ²=3.20, wR_p=0.0369. Bi_0.87Te₂O_4.9Cl_0.87 has a new type of layered structure constructed by Bi-Te-O layers separated by chloride ions. The Te atoms in Bi_0.87Te₂O_4.9Cl_0.87 show an unusual umbrella-like environment. A comparison with known related structures has been made.     

The structure of CeAlO3 by Rietveld refinement of X-ray powder diffraction data

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.     

Neutron powder diffraction study of the crystal and magnetic structures of BiNiO3 at low temperature

The crystal and magnetic structures of the charge ordered perovskite BiNiO₃ have been studied at temperatures from 5 to 300 K using neutron diffraction. Rietveld analysis of the data shows that the structure remains triclinic (space group ) throughout the whole temperature range. Bond-valence sum calculations based on the Bi-O and Ni-O bond distances confirm that the charge distribution is Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃ down to 5 K. The magnetic cell is identical to that of the triclinic superstructure and a G-type antiferromagnetic model gives a good fit to the magnetic intensities, with an ordered Ni²⁺ moment of 1.76(3) μ_B at 5 K. However, BiNiO₃ is ferrimagnetic due to the inexact cancellation of opposing, inequivalent moments in the low symmetry cell.     

Ab initio structure determination and Rietveld refinement of Bi10Mo3O24 the member n=3 of the Bi2n+4MonO6(n+1) series

Bi₂O₃-MoO₃ system shows a large panoply of phases depending on Bi/Mo ratio, among them, the low temperature phases of the homologous series Bi_2(n+2)Mo_nO_6(n+1) with n=3, 4, 5 and 6. They exhibit, alike most of the phases of this system, strong fluorite sub-network. Nevertheless, a multitechnique approach has been followed in order to solve the crystal structure of the n=3 member, i.e. Bi₁₀Mo₃O₂₄. From ab initio indexing X-ray powder pattern cell parameters were derived. It belongs to the monoclinic system, space group C2, with cell parameters: a=23.7282(2) Å, b=5.64906(6) Å, c=8.68173(9) Å, β=95.8668(7)° with Z=2. The matrix relating this cell with the fluorite one is 4 0 1/0 1 0/ 0  and a cationic localization was derived. HRTEM allowed the cationic Bi and Mo order to be modified and specified, as well as to build up a full structural ab initio model on the basis of crystal chemistry considerations. Simultaneous Rietveld refinement of multipattern X-ray and neutron powder diffraction data taking advantage of the neutron scattering length for O location have been performed. The goodness of the model was ascertained by low reliability factors, weighted R_b=4.97% and R_f=3.21%. This complex Bi₁₀Mo₃O₂₄ structure, with 5Bi, 2Mo and 13O in different crystallographic positions of the asymmetric unit, shows good agreement between observed and calculated patterns within the data resolution. Moreover, the determination of this structure sets the basis for the crystallographic characterization of the complete family Bi_2(n+2)Mo_nO_6(n+1), whose guidelines are also evidenced in this paper.     

Composition and temperature dependent phase transitions in Co-W double perovskites, a synchrotron X-ray and neutron powder diffraction study

High-resolution synchrotron and neutron powder diffraction techniques were used to determine precise structures for the series of perovskite oxides A_2−xSr_xCoWO₆ (ACa, or Ba, 0?x?2). The studies demonstrated that the symmetry decreases as the average size of the A-site cation decreases with a sequential introduction of in-phase and out-of-phase tilting of the BO₆ octahedra. A cubic structure in Fm3¯m with rock-salt like ordering of the Co and W cations was formed for Ba_2−xSr_xCoWO₆ with x∼<1.4. As the Sr content was increased, the materials became tetragonal in I4/m and ultimately monoclinic in P2₁/n. A mixture of monoclinic and tetragonal phases occurs in Sr₂CoWO₆ at room temperature but this was purely monoclinic at 20 K.     

High-resolution neutron powder diffraction study on the phase transitions in BaPbO3

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.