Compression mechanisms of symmetric and Jahn-Teller distorted octahedra in double perovskites: A2CuWO6 (A=Sr, Ba), Sr2CoMoO6, and La2LiRuO6

Synchrotron X-ray powder diffraction was used to study the high pressure dependence of the lattice parameter and structural evolution of the monoclinic La₂LiRuO₆ and tetragonal Sr₂CoMoO₆, Sr₂CuWO₆, and Ba₂CuWO₆ double perovskite phases. The c lattice parameters of Sr₂CuWO₆ and Ba₂CuWO₆ decreased more rapidly than the a lattice parameters and Ba₂CuWO₆ exhibited a more anisotropic compression compared to Sr₂CuWO₆. Based on lower pressure refinements of Ba₂CuWO₆, the anisotropic compression is proposed to be due to the preferential compression of the Cu-O bonds containing cooperative Jahn-Teller distortions aligned parallel to the c-axis, which is in contrast to Sr₂CoMoO₆, where the change in the octahedral tilting and symmetric bond compression is the prevailing compression mechanism. The bulk moduli were obtained from a fit of the volume-pressure data using the second-order Murnaghan equation of state.     

The crystal and magnetic structures of Sr2YRuO6

Time-of-flight powder neutron diffraction data have been used to refine the crystal structure of the ordered, distorted perovskite Sr₂YRuO₆. Yttrium and ruthenium are octahedrally coordinated in this material with average MO bond lengths of 2.202 and 1.955 Å, respectively. Constant wavelength neutron diffraction data show that Sr₂YRuO₆ is a Type I antiferromagnet at 4.2 K with an ordered magnetic moment of 1.85 μ_B per Ru⁵⁺ ion. The Néel temperature of Sr₂YRuO₆ was determined to be 26 K. The data suggest that the 4d³ electrons in this material are localized rather than itinerant.     

Structural studies of the disorder and phase transitions in the double perovskite Sr2YTaO6

The evolution of the crystal structure of the double perovskite Sr₂YTaO₆ from room temperature to 1250 °C has been studied using powder neutron and synchrotron X-ray diffraction. At room temperature Sr₂YTaO₆ crystallises in a monoclinic superstructure with the space group P2₁/n. The tilting of the octahedra evident in the room temperature structure is progressively lost on heating, resulting in a sequence of phase transitions that ultimately yields the cubic structure in space group Fm3?m. The high temperature tetragonal and cubic phases are characterised by strongly anisotropic displacements of the anions. The amount of defects in the crystal structure of Sr₂YTaO₆ is found to be sensitive to the preparative method.     

Temperature and composition dependent structural investigation of the defect perovskite series Sr1−xTi1−2xNb2xO3, 0≤x≤0.2

The crystal structure of the defect perovskite series Sr_1−xTi_1−2xNb_2xO₃ 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 Sr_1−xTi_1−2xNb_2xO₃ with minor SrTiO₃ intergrowth, 0.125<x≤0.2 was a pure Sr_1−xTi_1−2xNb_2xO₃ solid solution adopting the cubic perovskite type structure (Pm3¯m) and for x>0.2 Sr_0.8Ti_0.6Nb_0.4O₃ and Sr₃TiNb₄O₁₅ formed a two phase region. The cubic structure for Sr_0.8Ti_0.6Nb_0.4O₃ 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.     

The Jahn-Teller distortion and cation ordering in the perovskite Sr2MnSbO6

The perovskite Sr₂MnSbO₆ has been synthesized using conventional ceramic techniques and structurally characterized using high-resolution powder X-ray and neutron diffraction. The structure is tetragonal in space group I4/m. The octahedra were found to feature Jahn-Teller (JT) distortion due to the presence of Mn³⁺, and this is identified as strongly contributing to the octahedral tilting. Evidence for B-site cation ordering is presented however there is extensive anti-site disorder. The disordering of the Mn³⁺ and Sb⁵⁺ cations is believed to be a result of the similar size of these two cations and the polarizability of the Sb⁵⁺ cation. The structure was found to undergo a transition to cubic symmetry at 521 °C with removal of the octahedral tilting leading to the quenching of the JT distortion. This phase transition was found to be continuous and tricritical in nature.     

Neutron diffraction study of the crystal structure and structural phase transition of La0.7Ca0.3−xSrxCrO3 (0≤x≤0.3): The relationship between thermodynamic behavior and crystal structure changes at the phase transition

The crystal structure of the La_0.7Ca_0.3−xSr_xCrO₃ series, including the compositional and temperature dependence of the structural parameters, has been studied by variable temperature neutron diffraction measurements. The extent of the distortions from the ideal cubic perovskite structure has been evaluated quantitatively using the average bond lengths and the mean volumes of the [CrO₆] octahedron and [(La/Ca/Sr)O₁₂] polyhedron, and has been shown to decrease with increase of Sr content or temperature. At the structural phase transition from the orthorhombic (Pnma) structure to the rhombohedral one, the volume of the [CrO₆] octahedron decreases whereas that of the [(La/Ca/Sr)O₁₂] polyhedron shows little difference, resulting in an overall decrease in the level of distortion. The change in the degree of distortion at the phase transition decreases with increase of Sr content, in agreement with the smaller variation of the enthalpy and volume for the specimens with higher Sr content.     

Raman scattering study and electrical properties characterization of elpasolite perovskites Ln2(BB′)O6 (Ln=La, Sm…Gd and B,B′=Ni, Co, Mn)

Two series of elpasolite perovskites Ln₂CoMnO₆ and Ln₂NiMnO₆ (Ln=La, Pr, Nd, Sm, Gd) have been prepared. The electronic band gap and magnetic Curie temperature vary systematically as a function of the rare earth cation size within both series. Here we used Raman scattering spectroscopy along with the results of previous structural studies to show that there is little change in octahedral distortion but significant changes in the octahedral tilting angle upon decreasing lanthanide ionic radius. The data indicate differences in the orbital overlap and bond strengths between the two series of materials that allow us to understand variations in the magnetic and electrical properties within and between the two perovskite series.     

Structural and magnetic properties of Ba3La3Mn2W3O18

A quaternary phase, Ba₃La₃Mn₂W₃O₁₈, was synthesized in reduced atmosphere (5% H₂/Ar) at 1200 °C and characterized by using powder X-ray diffraction, electron diffraction and high resolution TEM. Ba₃La₃Mn₂W₃O₁₈ crystallizes in rhombohedral space group with the cell parameters, and , and can be attributed to the n=6 member in the B-site deficient perovskite family, A_nB_n−1O_3n. The structure can be described as close-packed [La/BaO₃] arrays in the sequence of (hcccch)₃, wherein the B-site cations, W and Mn, occupy five octahedral layers in every six octahedral layers, which leave a vacant octahedral layers separating the 5-layer perovskite blocks. The B-cation layers in the perovskite block alternate along the c-axis in a sequence of W⁶⁺-Mn²⁺-W⁵⁺-Mn²⁺-W⁶⁺. The bond valence calculation and optical reflection spectrum confirm the presence of W⁵⁺. This compound behaves paramagnetically in wide temperature range and weak antiferromagnetic interaction only occurs at low temperatures.     

The nature of the orthorhombic to tetragonal phase transition in Sr1−xCaxMnO3

The orthorhombic-tetragonal phase transition in the perovskite series Sr_1−xCa_xMnO₃ 0.4?x?0.6 has been studied by synchrotron X-ray powder diffraction. At room temperature the Ca rich oxides x?0.45 have the orthorhombic Pbnm superstructure whereas Sr_0.6Ca_0.4MnO₃ is two phases with both tetragonal I4/mcm and orthorhombic Pbnm. Analysis of the octahedral tilts suggest the co-existence of these two phases is a consequence of a first-order I4/mcm to Pbnm transition. The evolution of the structure of Sr_0.5Ca_0.5MnO₃ with temperature is also described and this is found to evolve from orthorhombic to tetragonal and ultimately cubic.     

Crystal structure of the monoclinic and cubic polymorphs of BiMn7O12

The complex perovskite BiMn₇O₁₂ occurs with two polymorphic structures, cubic and monoclinic. Currently their crystal structures are investigated with high-resolution synchrotron powder X-ray diffraction at room temperature. Rietveld analysis reveals unusual behavior for, respectively, the oxygen and bismuth atoms in the monoclinic and cubic phases. Bond valence calculations indicate that all the Mn atoms in both the phases are in trivalent state. Possible roles of the 6s² lone-pair electrons of Bi³⁺ in BiMn₇O₁₂ are discussed in comparison with the LaMn₇O₁₂ phase that is isomorphic to monoclinic BiMn₇O₁₂. Multiple roles of the lone-pair electrons are revealed, causing (i) A-site cation deficiency, (ii) octahedral tilting, (iii) A-site cation displacement, and (iv) Mn³⁺ Jahn-Teller (JT) distortion. Relationships between the monoclinic and cubic phases are discussed with emphasis on the MnO₂ and MnO₆ local structural aspects. All Mn atoms in the monoclinic polymorph have distorted coordination consistent with JT-active Mn(III) high spin, whereas for the cubic polymorph, the B-site Mn atoms show regular octahedral coordination.     

Structural characterisation of the perovskite series SrxCa1−x−yNdyMnO3: Influence of the Jahn-Teller effect

The crystal structures of the perovskite manganites Sr_xCa_1−x−yNd_yMnO₃ with y=0.1 or 0.2 have been investigated using synchrotron X-ray powder diffraction. At room temperature the structures change from depending on the cation distribution, the different structures exhibiting different tilts of the MnO₆ octahedra. High temperature diffraction measurements demonstrate the presence of, an apparently continuous, isosymmetric I4/mcm to I4/mcm phase transition associated with the removal of long range orbital ordering. Heating the manganites to still higher temperatures results in a continuous transition to the cubic structure. A feature of such transitions is the continuous evolution of the octahedral tilt angle through the I4/mcm to I4/mcm phase transition. The orthorhombic structures do not exhibit orbital ordering and although a first order transition to the tetragonal structure is observed in Sr_0.4Ca_0.5Nd_0.1MnO₃, this high temperature tetragonal structure does not exhibit orbital ordering.     

Crystal structure, oxidation state and magnetism of La1.2Sr2.4RuO7: a new member of the [A2O][AnBn−1O3n] series of hexagonal perovskites

Single crystals of the new compound La_1.2Sr_2.4RuO₇, an oxide related to the hexagonal perovskites, were grown from a BaCl₂ flux. The structure was solved by single crystal X-ray diffraction. La_1.2Sr_2.4RuO₇ crystallizes in the space group with a=5.760(1) Å and c=18.273(3) Å. It is one of the rare examples of oxides with isolated RuO₆-octahedra. The structure consists of alternating layers of RuO₆-octahedra and trigonal (La,Sr)O₆-prisms. These prisms are capped by one additional oxygen ion, which occupies a distorted position within the (La,Sr)O₆-layers. La_1.2Sr_2.4RuO₇ is the second member of the general [A₂O][A_nB_n−1O_3n] family of hexagonal perovskites with n=2 and the first ruthenate possessing this structure. XANES investigations, bond valence sum calculations and magnetic measurements show that ruthenium takes the oxidation state +5. Although the ruthenium ions have quite long distances, a medium strong antiferromagnetic interaction between these paramagnetic centers was observed.     

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.     

Synthesis and crystal structure of the Sr2MnGa(O,F)6 oxyfluorides

The fluorine-containing derivatives of Sr₂MnGaO_5.5 were prepared by treatment with XeF₂ at temperatures ranging from 300°C to 600°C. The compounds crystallize in a tetragonal unit cell with a_t≈a_p, c_t≈2a_p (a_p—the parameter of the perovskite subcell). An increase in fluorine content is accompanied by a reduction of the Mn oxidation state due to a partial replacement of oxygen by fluorine. The crystal structure of Sr₂MnGaO_4.78F_1.22 was determined by electron diffraction and X-ray powder diffraction (a=3.85559(2) Å, c=7.78289(6) Å, S.G. P4/mmm, R_I=0.012, R_P=0.019). The structure consists of alternating (MnO₂), (SrO) and (GaO_0.78F_1.22) layers. The Ga atoms are situated in slightly elongated octahedra, the MnO₆ octahedra are characterized by two short apical Mn-O distances of 1.876(8) Å and four long equatorial ones of 1.9278(1) Å. This is interpreted as an “apically compressed” type of Jahn-Teller distortion, in contrast to the “apically elongated” one in the Sr₂MnGaO_5+δ brownmillerites with different oxygen content. Possible structural reasons for the reversed Jahn-Teller effect are discussed.     

Neutron diffraction study and magnetotransport properties of stoichiometric CaMoO3 perovskite prepared by a soft-chemistry route

Polycrystalline CaMoO₃ perovskite has been prepared by soft-chemistry procedures, followed by controlled annealing under reducing conditions (H₂/N₂ flow). The crystal structure, studied from neutron powder diffraction data, can be described in an orthorhombic unit cell, space group Pbnm (No. 62). The lattice parameters were a=5.4510(1) Å, b=5.5821(1) Å and c=7.7803(2) Å. In the perovskite network the MoO₆ octahedra are tilted by 13.5° in order to optimize the Ca-O bond lengths; the tilting scheme corresponds to a GdFeO₃-like superstructure. The perovskite is fully oxygen stoichiometric, as demonstrated from the refinement of the oxygen occupancy factors. Resistivity and transport measurements indicated that CaMoO₃ behaves as a metal; at low temperatures (5 K) a small positive magnetoresistance is observed, reaching a maximum value of 1.4% at 9 T. The magnetic susceptibility is predominantly Pauli paramagnetic-like, although a non-negligible temperature-dependent component due to isolated Mo⁴⁺ spins is patent at low temperatures.     

Lattice instabilities in Cs2MFe(CN)6 (M = Mg2+, Ca2+, and Sr2+): The crystal structure of Cs2BaFe(CN)6·2H2O

The room temperature Raman spectra of Cs₂MFe(CN)₆ (M = Mg²⁺, Ca²⁺, and Sr²⁺) suggest that these salts undergo phase transformations similar to those found in Cs₂LiCr(CN)₆ where the distortion from the high-symmetry phase proceeds primarily along two modes of vibration. The distortion involves an antiferroelectric rotation of the hexacyanide moiety and a cesium translation. On the basis of the spectra a correlation has been made between the size of M and the apparent transition temperature. In going down the alkaline earths, the apparent transition temperature increases. The structure of the barium salt determined at room temperature shows the crystal latitce contains two waters of hydration. Many similarities have been found between Cs₂BaFe(CN)₆·2H₂O and the low-symmetry phase structure of Cs₂LiCr(CN)₆.     

The Crystal Structures, Microstructure and Ionic Conductivity of Ba2In2O5 and Ba(InxZr1−x)O3−x/2

This paper describes the results of electron microscopy, high-temperature powder neutron diffraction, and impedance spectroscopy studies of brownmillerite-structured Ba₂In₂O₅ and perovskite structured Ba(In_xZr_1−x)O_3−x/2. The ambient temperature structure of Ba₂In₂O₅ is found to adopt Icmm symmetry, with disorder of the tetrahedrally coordinated (In³⁺) ions of the type observed previously in Sr₂Fe₂O₅. Ba₂In₂O₅ undergoes a ∼6-fold increase in its ionic conductivity over the narrow temperature range from ∼1140 K to ∼1230 K, in broad agreement with previous studies. This transition corresponds to a change from the brownmillerite structure to a cubic perovskite arrangement with disordered anions. Electron microscopy investigations showed the presence of extended defects in all the crystals analyzed. Ba(In_xZr_1−x)O_3−x/2 samples with x=0.1 to 0.9 adopt the cubic perovskite structure, with the lattice parameter increasing with x.     

Structural and magnetic study of the cation-ordered perovskites Ba2−xSrxErMoO6

A series of perovskite phases have been prepared from the appropriate carbonates and oxides by heating under reducing conditions at temperatures up to 1300 °C. Complete ordering between ErO₆ and MoO₆ octahedra and a disordered distribution of Sr²⁺ and Ba²⁺ occur in all compounds. Neutron powder diffraction experiments show that the substitution of Sr²⁺ into Ba₂ErMoO₆ introduces a progressive reduction in symmetry from Fm3¯m (x=0) to I4/m (x=0.5, 0.8) to P2₁/n (x=1.25, 1.75, 2.0). Magnetic susceptibility measurements indicate that all of these compounds show Curie-Weiss paramagnetism and that for x<1.25 this behaviour persists down to 2 K. The monoclinically distorted compounds show magnetic transitions at low temperature and neutron diffraction has confirmed the presence of long-range antiferromagnetic order below 2.5 and 4 K in Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆, respectively. Ba_0.75Sr_1.25ErMoO₆, Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆ do not undergo structural distortion on cooling from room temperature.