Structures and crystal chemistry of the double perovskites Ba2LnB′O6 (Ln=lanthanide and B′=Nb, Ta):: Part II—Temperature dependence of the structures of Ba2LnB′O6

The structures of eight members of the series of double perovskites of the type Ba₂LnB′O₆ (Ln=La³⁺-Sm³⁺ and Y³⁺ and B′=Nb⁵⁺ and Ta⁵⁺) were examined both above and below room temperature using synchrotron X-ray powder diffraction. The La³⁺ and Pr³⁺ 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 La³⁺ and Pr³⁺ 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.     

Crystal structures and chemistry of double perovskites Ba2M(II)M′(VI)O6 (M=Ca, Sr, M′=Te, W, U)

Structures of the double perovskites Ba₂M(II)M ′(VI)O₆ (M=Ca, Sr, M′=Te, W, U) at room temperature have been investigated by the Rietveld method using X-ray and neutron powder diffraction data. For double perovskites with M=Sr, the observed space groups are I2/m (M′ =W) and (M′=Te), respectively. In the case of M=Ca, the space groups are either monoclinic P2₁/n (M′=U) or cubic (M′=W and Te). The tetragonal and orthorhombic symmetry reported earlier for Ba₂SrTeO₆ and Ba₂CaUO₆, respectively, were not observed. In addition, non-ambient X-ray diffraction data were collected and analyzed for Ba₂SrWO₆ and Ba₂CaWO₆ in the temperature range between 80 and 723 K. It was found that the rhombohedral structure exists in Ba₂SrWO₆ above room temperature between the monoclinic and the cubic structure, whereas the cubic Ba₂CaWO₆ undergoes a structural phase transition at low temperature to the tetragonal I4/m structure.     

Synthesis, structure, and magnetic properties of SrMn1−xGaxO3−δ (x=0-0.5) perovskites

We report the synthesis of SrMn_1−xGa_xO_3−δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques derived from thermogravimetric measurements, we have extended the solubility limit of random substitution of Ga³⁺ for Mn in the cubic perovskite phase to x=0.5. In the cubic perovskite phase the maximum oxygen content is close to 3−x/2, which corresponds to 100% Mn⁴⁺. Maximally oxygenated solid solution compounds are found to order antiferromagnetically for x=0-0.4, with the transition temperature linearly decreasing as Ga content increases. Increasing the Ga content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.5Ga_0.5O_2.67(3) below 12 K. These properties are markedly different from the long-range antiferromagnetic order below 180 K observed for the layer-ordered compound Sr₂MnGaO_5.50 with nominally identical chemical composition.     

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.     

Orthorhombic superstructures within the rare earth strontium-doped cobaltate perovskites: Ln1−xSrxCoO3−δ (Ln=Y, Dy-Yb; 0.750?x?0.875)

A combination of electron, synchrotron X-ray and neutron powder diffraction reveals a new orthorhombic structure type within the Sr-doped rare earth perovskite cobaltates Ln_1−xSr_xCoO_3−δ (Ln=Y³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺and Yb³⁺). Electron diffraction shows a C-centred cell based on a 2√2a_p×4a_p×4√2a_p superstructure of the basic perovskite unit. Not all of these very weak satellite reflections are evident in the synchrotron X-ray and neutron powder diffraction data and the average structure of each member of this series could only be refined based on Cmma symmetry and a 2√2a_p×4a_p×2√2a_p cell. The nature of structural and magnetic ordering in these phases relies on both oxygen vacancy and cation distribution. A small range of solid solution exists where this orthorhombic structure type is observed, centred roughly around the compositions Ln_0.2Sr_0.8CoO_3−δ. In the case of Yb³⁺ the pure orthorhombic phase was only observed for 0.850?x?0.875. Tetragonal (I4/mmm; 2a_p×2a_p×4a_p) superstructures were observed for compositions having higher or lower Sr-doping levels, or for compounds with rare earth ions larger than Dy³⁺. These orthorhombic phases show mixed valence (3+/4+) cobalt oxidation states between 3.2+ and 3.3+. DC magnetic susceptibility measurements show an additional magnetic transition for these orthorhombic phases compared to the associated tetragonal compounds with critical temperatures > 330 K.     

Crystal structures of the double perovskites Ba2Sr1−xCaxWO6

Structures of the double perovskites Ba₂Sr_1−xCa_xWO₆ have been studied by the profile analysis of X-ray diffraction data. The end members, Ba₂SrWO₆ and Ba₂CaWO₆, 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 Ba₂MM′O₆-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.     

Crystal structures and phase transition in the system SrTiO3-La2/3TiO3

The distribution of A-site cations in the perovskite system La_xSr_1−3x/2TiO₃ depends on the concentration of La³⁺ ions and associated vacancies. For small x (x?0.2), the substitutions are expected to be random. For x?0.55, the cations are ordered in such a way that successive layers of A-sites are occupied to greater and lesser degree, and this ordering drives a tetragonal distortion. For x from about 0.3 to 0.5, the X-ray patterns show diffuse peaks indicative of similar ordering, but this is not long-range order and no tetragonal distortion results. The lower temperature structures also exhibit out-of-phase tilting of the TiO₆ octahedra, setting in at temperatures varying linearly with composition from 105 K for x=0, to about 650 K at x=2/3.     

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.     

Phase equilibria in systems Ce-M-Sb (M=Si, Ge, Sn) and superstructure Ce12Ge9−xSb23+x (x=3.8±0.1)

Phase relations in the ternary systems Ce-M-Sb (M=Si, Ge, Sn) in composition regions CeSb₂-Sb-M were studied by optical and electron microscopy, X-ray diffraction, and electron probe microanalysis on arc-melted alloys and specimens annealed in the temperature region from 850 to 200 °C. The results, in combination with an assessment of all literature data available, were used to construct solidus surfaces and a series of isothermal sections. No ternary compounds were found to form in the Ce-Si-Sb system whilst Ce₁₂Ge_9−xSb_23+x (3.3<x<4.2) and CeSn_xSb₂ (0.1<x<0.8) participate in phase equilibria in the composition region investigated. Crystallographic parameters for the ternary compound Ce₁₂Ge_9−xSb_23+x (x=3.8±0.1) were determined from X-ray single crystal and powder diffraction. For the binary system Ge-Sb a eutectic was defined L⇔(Ge)+(Sb) at 591.6 °C and 22.5 at%. Ge EPMA revealed a maximal solubility of 6.3 at% Ge in (Sb) at the eutectic temperature.     

Order-disorder in In perovskites: The example of A(In2/3B″1/3)O3 (A=Ba, Sr; B″=W, U)

We describe the preparation and structural characterization of four In-containing perovskites from neutron powder diffraction (NPD) and X-ray powder diffraction (XRPD) data. Sr₃In₂B″O₉ and Ba(In_2/3B″_1/3)O₃ (B″=W, U) were synthesized by standard ceramic procedures. The crystal structure of the W-containing perovskites and Ba(In_2/3U_1/3)O₃ have been revisited based on our high-resolution NPD and XRPD data, while for the new U-containing perovskite Sr₃In₂UO₉ the structural refinement was carried out from high-resolution XRPD data. At room temperature, the crystal structure for the two Sr phases is monoclinic, space group P2₁/n, where the In atoms occupy two different sites Sr₂[In]_2d[In_1/3B″_2/3]_2cO₆, with a=5.7548(2) Å, b=5.7706(2) Å, c=8.1432(3) Å, β=90.01(1)° for B″=W and a=5.861(1) Å, b=5.908(1) Å, c=8.315(2) Å, β=89.98(1)° for B″=U. The two phases with A=Ba should be described in a simple cubic perovskite unit cell (S.G. Pm3¯m) with In and B″ distributed at random at the octahedral sites, with a=4.16111(1) Å and 4.24941(1) Å for W and U compounds, respectively.     

Crystal structure, microstructure and reducibility of LaNixCo1−xO3 and LaFexCo1−xO3 Perovskites (0<x≤0.5)

Nickel and iron substituted LaCoO₃ with rhombohedrally distorted perovskite structure were obtained in the temperature range of 600-900 °C by thermal decomposition of freeze-dried citrates and by the Pechini method. The crystal structure, morphology and defective structure of LaCo_1−xNi_xO₃ and LaCo_1−xFe_xO₃ were characterized by X-ray diffraction and neutron powder diffraction, TEM and SEM analyses and electron paramagnetic resonance spectroscopy. The reducibility was tested by temperature programmed reduction with hydrogen. The products of the partial and complete reduction were determined by ex-situ XRD experiments. The replacement of Co by Ni and Fe led to lattice expansion of the perovskite structure. For perovskites annealed at 900 °C, there was a random Ni, Fe and Co distribution. The morphology of the perovskites does not depend on the Ni and Fe content, nor does it depend on the type of the precursor used. LaCo_1−xNi_xO₃ perovskites (x>0.1) annealed at 900 °C are reduced to Co/Ni transition metal and La₂O₃ via the formation of oxygen deficient Brownmillerite-type compositions. For LaCo_1−xNi_xO₃ annealed at 600 °C, Co/Ni metal, in addition to oxygen-deficient perovskites, was formed as an intermediate product at the initial stage of the reduction. The interaction of LaCo_1−xFe_xO₃ with H₂ occurs by reduction of Co³⁺ to Co²⁺ prior to the Fe³⁺ ions. The reducibility of Fe-substituted perovskites is less sensitive towards the synthesis procedure in comparison with that of Ni substituted perovskites.     

Crystal structure investigations of ZrAsxSey (x>y, x+y≤2) by single crystal neutron diffraction at 300 K, 25 K and 2.3 K

Large single crystals of ZrAs_xSe_y (x>y, x+y≤2, PbFCl type of structure, space group P4/nmm) were grown by Chemical Transport. Structural details were studied by single crystal neutron diffraction techniques at various temperatures. One single crystal specimen with chemical composition ZrAs_1.595(3)Se_0.393(1) was studied at ambient temperature (R1=5.10 %, wR2=13.18 %), and a second crystal with composition ZrAs_1.420(3)Se_0.560(1) was investigated at 25 K (R1=2.70%, wR2=5.70 %) and 2.3 K (R1=2.30 %, wR2=4.70 %), respectively. The chemical compositions of the crystals under investigation were determined by wavelength dispersive X-ray spectroscopy. The quantification of trace elements was carried out by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry. According to the crystal structure refinements the crystallographic 2a site is occupied by As, together with a significant amount of vacancies. One of the 2c sites is fully occupied by As and Se (random distribution). With respect to the fractional coordinates of the atoms, the crystal structure determinations based on the data obtained at 25.0 K and 2.3 K did not show significant deviations from ambient temperature results. The temperature dependence of the displacement parameters indicates a static displacement of As on the 2a sites (located on the (0 0 1) planes) for all temperatures. No indications for any occupation of interstitial sites or the presence of vacancies on the Zr (2a) site were found.     

Crystal structure of Ca2Ln3Sb3O14 (Ln=La, Pr, Nd and Y): A novel variant of weberite

The crystal structures of Ca₂Ln₃Sb₃O₁₄ (Ln=La, Pr, Nd and Y) and Ca₂Sb₂O₇ at room temperature were refined by the Rietveld method using combined X-ray and neutron powder diffraction data. Ca₂Sb₂O₇ adopts the weberite structure having the space group Imma. The structures of Ca₂Ln₃Sb₃O₁₄ 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.     

Selective substitution of vanadium for molybdenum in Sr2(Fe1−xVx)MoO6 double perovskites

The crystal and magnetic structures of Sr₂(Fe_1−xV_x)MoO₆ (0.03?x?0.1) compounds are refined by alternately using X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) data collected at room temperature. The refinement results reveal that the V atoms selectively occupy the Mo sites instead of the Fe sites for x?0.1. The 3d/4d cation ordering decreases with the increase of the V content. Slight distortions in the lattice and metal octahedra are shown at 300 K, and the distortions increase at 4 K. The magnetic structure at 4 K can be modeled equally well with the moments aligning along [001], [110] or [111] directions. The total moments derived from the NPD data for the [110] and [111] direction models agree well with the magnetic measurements, whereas the [001] model leads to a smaller total moment. Bond valence analysis indicates that Sr ions are properly located in the structure and Mo ions are compatible with both the Fe sites and the Mo sites. The electronic effects are suggested to be responsible for the selective occupation of the V on the Mo sites due to the different distortions of the FeO₆ and MoO₆ octahedra.     

Neutron powder diffraction and first-principles computational studies of CuLixMg2−x (x≅0.08), CuMg2, and Cu2Mg

A small addition of Li changes the orthorhombic structure of CuMg₂ to hexagonal CuLi_xMg_2−x (x=0.08). Determining the Li content of the ternary phase and Li atomic positions was our main objective for this work. For this reason we performed neutron diffraction at several different temperatures below and above room temperature. The results obtained on two neutron powder diffractometers were compared with X-ray diffraction (XRD) data, and with first-principles calculations. The first-principles calculations are in good agreement with Rietveld-refined data from neutron diffraction, but do not show a marked preference for one of several possible Li sites. The pair distribution function (PDF) fitting is consistent with Li substituting only Mg1 (1/2, 0, z). Interstitial spaces in the structure of CuMg₂ and of CuLi_xMg_2−x were also considered, but are unlikely to be occupied by Li. Neutron diffraction data for binary CuMg₂ and Cu₂Mg were also obtained.     

Structural phase transition and magnetic properties of the Sr2FeRe1−xBx′O6 (B′=Nb, Ta; x=0, 0.1) double perovskites

The double perovskites, Sr₂FeReO₆ and Sr₂FeRe_0.9M_0.1O₆ (M=Nb, Ta) have been obtained by soft synthesis methods which yield homogeneous particles of micrometric grain size. The materials have been studied by X-ray and neutron powder diffraction, scanning electron microscopy and magnetic measurements. Rietveld refinements show that the compounds adopt a tetragonal I4/mmm structure at high temperatures and monoclinic P2₁/n below the transition temperature. The magnetic structures were determined by neutron powder diffraction at 100 and 300 K for the Sr₂FeReO₆, Sr₂FeRe_0.9Nb_0.1O₆ and Sr₂FeRe_0.9Ta_0.1O₆ phases, respectively. Evidence for a ferrimagnetic coupling between the Fe³⁺ and Re⁵⁺ sublattices has been observed. Magnetic measurements yield magnetic moments lower than the theoretical ones being in accord with the antisite disorder of 25% in the B-B′ positions.     

Synthesis and characterization of Sr0.75Y0.25Co1−xMxO2.625+δ (M=Ga, 0.125?x?0.500 and M=Fe, 0.125?x?0.875)

The effect of replacing Co³⁺ by Ga³⁺ and Fe³⁺ in the perovskite-related tetragonal phase Sr_0.75Y_0.25CoO_2.625 with unit cell parameters: a=2a_p, and c=4a_p (314 phase) has been investigated. The 314 phase is formed by Sr_0.75Y_0.25Co_1−xM_xO_2.625+δ, with x?0.375 for M=Ga and x?0.625 for M=Fe. High-resolution transmission electron microscopy and electron diffraction revealed frequent microtwinning in the iron-containing compounds, in contrast to the Ga-substituted 314 phases. Diffraction experiments and electron microscope images indicated that at higher Fe contents, 0.75?x?0.875, a disordered cubic perovskite structure forms. The crystal structures of Sr_0.75Y_0.25Co_0.75Ga_0.25O_2.625 and Sr_0.75Y_0.25Co_0.5Fe_0.5O_2.625+δ were refined using neutron powder diffraction data. It was found that the oxygen content of Sr_0.75Y_0.25Co_0.5Fe_0.5O_2.625+δ is higher than in Fe-free 314 phase, so that δ corresponds to 0.076, whereas δ=0 in Sr_0.75Y_0.25Co_0.75Ga_0.25O_2.625+δ. Magnetization measurements on the unsubstituted Sr_0.7Y_0.3CoO_2.62 and Ga-substituted Sr_0.75Y_0.25Co_0.75Ga_0.25O_2.625 compounds indicate the presence of a ferromagnetic-like contribution to the measured magnetization at 320 and 225 K, respectively, while replacing Co by Fe leads to the suppression of this contribution. A neutron diffraction study shows that the Sr_0.75Y_0.25Co_0.5Fe_0.5O_2.625+δ compound is G-type antiferromagnetic at room temperature, whereas Sr_0.75Y_0.25Co_0.75Ga_0.25O_2.625 does not exhibit magnetic ordering at room temperature.     

Synthesis and single-crystal X-ray diffraction studies of new framework substituted type II clathrates, Cs8Na16AgxGe136−x (x<7)

New inorganic type II clathrates with Ag atoms substituting for framework Ge atoms, Cs₈Na₁₆Ag_xGe_136−x (x=0, 5.9, and 6.7), have been synthesized by reaction of the pure elements at high temperature. Structural refinements have been performed using single crystal X-ray diffraction. The materials crystallize with the cubic type II clathrate crystal structure (space group ) with a=15.49262(9) Å, 15.51605(6) Å, and 15.51618(9) for x=0, 5.9, and 6.7, respectively, and Z=1. The structure is formed by a covalently bonded Ag-Ge framework, in which the Cs and Na atoms are found inside two types of polyhedral cages. Ag substitutes for Ge in the tetrahedrally bonded framework positions, and was found to preferentially occupy the most asymmetric 96g site. The proven ability to substitute atoms for the germanium framework should offer a route to the synthesis of new compositions of type II clathrates, materials that are of interest for potential thermoelectrics applications.     

The crystal structure of the new ternary antimonide Dy3Cu20+xSb11−x (x≈2)

New ternary antimonide Dy₃Cu_20+xSb_11−x (x≈2) was synthesized and its crystal structure was determined by direct methods from X-ray powder diffraction data (diffractometer DRON-3M, CuKα-radiation, R_I=6.99%,R_p=12.27%,R_wp=11.55%). The compound crystallizes with the own cubic structure type: space group , Pearson code cF272, . The structure of the Dy₃Cu₂₀Sb_11−x (x≈2) can be obtained from the structure type BaHg₁₁ by doubling of the lattice parameter and subtraction of 16 atoms. The studied structure was compared with the structures of known compounds, which crystallize in the same space group with similar cell parameters.     

B-site disordering in Ba3Ln2MoO9 (Ln=Ho, Er) perovskites: A neutron diffraction study

We describe the preparation, structure determination and magnetic properties of two Ba perovskites containing rare-earth cations at the B-sublattice. Ba₃Ln₂MoO₉ (Ln=Ho³⁺ and Er³⁺) were synthesized by ceramic procedures. Joint X-ray (XRPD) and neutron (NPD) powder diffraction refinements were carried out to analyse the crystal structure. At room temperature, both phases are tetragonal, space group I4/mcm, Z=4. Ln and Mo atoms are found to be distributed at random over the octahedral sites of the perovskites. Magnetic measurements at 0.1 T show that both samples are paramagnetic between 3 and 300 K, following a Curie-Weiss law. M vs. H curves show a region of paramagnetic behaviour and above 2.5 T a magnetic saturated system is observed. Finally, the temperature evolution of the NPD patterns of Ba₃Ho₂MoO₉ reveals the absence of long-range magnetic ordering down to 2 K.