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.     

Temperature and pressure dependent structural studies of the ordered double perovskites Sr2TbRu1−xIrxO6

High resolution powder diffraction studies are reported for the series of mixed RuIr perovskites Sr₂TbRu_1−xIr_xO₆. Using a combination of synchrotron X-ray and neutron powder diffraction precise structures are established for the two end-member oxides, where the Tb oxidation state changes from +3 in the Ru oxide to +4 in the Ir containing oxide. The structures of both oxides are monoclinic. Composition dependent studies show that this valence transition is first order. Variable temperature diffraction show no evidence for any structural or valence state transitions. However, upon application of pressure Sr₂TbRu_0.3Ir_0.7O₆ undergoes a valence state transition at low pressures.     

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.     

Phase segregation in mixed Nb-Sb double perovskites Ba2LnNb1−xSbxO6

The phase composition of two series of mixed Nb⁵⁺-Sb⁵⁺ double perovskites formed between the pairs Ba₂EuNbO₆-Ba₂PrSbO₆ and Ba₂NdSbO₆-Ba₂NdNbO₆ have been studied using synchrotron X-ray powder diffraction methods. In both series extensive phase segregation is observed demonstrating limited solubility of Sb⁵⁺ in these Nb⁵⁺ perovskites, irrespective of the precise structures of the double perovskite. Evidence for a monoclinic I2/m phase in the series formed between tetragonal I4/m Ba₂EuNbO₆ and rhombohedral Ba₂EuNbO₆ is presented. It is postulated that this phase segregation is a consequence of competing bonding requirements of the Nb⁵⁺ and Sb⁵⁺ cations associated with their electronic configurations.     

Synthesis, structures and phase transitions in the double perovskites Sr2−xCaxCrNbO6

The synthesis and crystal structures of nine members of the rock-salt ordered double perovskites Sr_2−xCa_xCrNbO₆ is presented. The crystal structures of the end members of the series Sr₂CrNbO₆ and Ca₂CrNbO₆ were refined using powder neutron diffraction data and are cubic in and monoclinic in P2₁/n, respectively, in both cases there being considerable anti-site Cr-Nb mixing. Variable temperature and/or composition studies suggest a direct first-order P2₁/n to transition, a suggestion supported by selected area electron diffraction studies.     

Electron doping effect on structural and magnetic phase transitions in Sr2−xNdxFeMoO6 double perovskites

Polycrystalline Sr_2−xNd_xFeMoO₆ (x=0.0, 0.1, 0.2, 0.4) materials have been synthesized by a citrate co-precipitation method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature-dependent NPD data shows that the compounds (x=0.0, 0.1, 0.2) crystallize in the tetragonal symmetry in the range 10-400 K and converts to cubic symmetry above 450 K. The unit cell volume increases with increasing Nd³⁺ concentration, which is an electronic effect in order to change the valence state of the B-site cations. Antisite defects at the Fe-Mo sublattice increases with the Nd³⁺ doping. The Curie temperature was increased from 430 K for x=0 to 443 K for x=0.4. The magnetic moment of the Fe-site decreases while the Mo-site moment increases with electron doping. The antiferromagnetic arrangement causes the system to show a net ferrimagnetic moment.     

Structural and electronic phase transitions in Sr1−xCexMnO3 perovskites

The structures of eight members of the series Sr_1−xCe_xMnO₃ with 0.075?x?0.4 have been established using synchrotron X-ray powder diffraction. These exhibit the sequence of structures

     

Composition- and temperature-dependent phase transitions in 1:3 ordered perovskites Ba4−xSrxNaSb3O12

A series of 25 members of the 1:3 ordered perovskite family of the type Ba_4−xSr_xNaSb₃O₁₂ has been synthesized and their structures determined using synchrotron X-ray and neutron powder diffraction techniques. At room temperature the sample Ba₄NaSb₃O₁₂ has a cubic structure in space group with a=8.2821(1) Å, where the Na and Sb cations are ordered in the octahedral sites but there is no tilting of the (Na/Sb)O₆ octahedra. As the average size of the A-site cation decreases, through the progressive replacement of Ba by Sr, tilting of the octahedra is introduced firstly lowering the symmetry to tetragonal in P4/mnc then to orthorhombic in Cmca and ultimately a monoclinic structure in P2₁/n as seen for Sr₄NaSb₃O₁₂ with a=8.0960(2) Å, b=8.0926(2) Å, c=8.1003(1) Å and β=90.016(2)°. The powder neutron diffraction studies show that the orthorhombic and tetragonal phases in Cmca and P4/mnc co-exist at room temperature for samples with x between 1.5 and 2.     

Thermoelectric properties of polycrystalline La1−xSrxCoO3

Thermoelectric properties of polycrystalline La_1−xSr_xCoO₃, where Sr²⁺ is substituted in La³⁺ site in perovskite-type LaCoO₃, have been investigated. Sr-doping increases the electrical conductivity (σ) of La_1−xSr_xCoO₃, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS²) is 3.7×10⁻⁴ W m⁻¹ K⁻² at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m⁻¹ K⁻¹ at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La_1−xSr_xCoO₃ are improved by Sr-doping, and the figure of merit (Z=σS² κ⁻¹) reaches 1.6×10⁻⁴ K⁻¹ for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.     

Studies on magnetic and calorimetric properties of double perovskites Ba2HoRuO6 and Ba2HoIrO6

Magnetic properties of double perovskite compounds Ba₂HoRuO₆ and Ba₂HoIrO₆ have been reported. Powder X-ray and neutron diffraction measurements show that these compounds have a cubic perovskite-type structure with the space group and the 1:1 ordered arrangement of Ho³⁺ and Ru⁵⁺ (or Ir⁵⁺) over the 6-coordinate B sites. Results of the magnetic susceptibility and specific heat measurements show that Ba₂HoRuO₆ exhibits two magnetic anomalies at 22 and 50 K. Analysis of the temperature dependence of magnetic specific heat indicates that the anomaly at 50 K is due to the antiferromagnetic ordering of Ru⁵⁺ ions and that the anomaly at 22 K is ascribable to the magnetic interaction between Ho³⁺ ions. Neutron diffraction data collected at 10 and 35 K show that the Ba₂HoRuO₆ has a long range antiferromagnetic ordering involving both Ho³⁺ and Ru⁵⁺ ions. Each of their magnetic moments orders in a Type I arrangement and these magnetic moments are anti-parallel in the ab-plane with each other. The magnetic moments are aligned along the c-direction. On the other hand, Ba₂HoIrO₆ is paramagnetic down to 1.8 K.     

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 growth, structures, and optical properties of the cubic double perovskites Ba2MgWO6 and Ba2ZnWO6

Single crystals of the tungstates Ba₂MgWO₆ and Ba₂ZnWO₆ have been grown for the first time. The crystals were prepared with molten potassium carbonate acting as a flux. According to the single-crystal X-ray diffraction structure determination, the compounds crystallize in space group Fm3¯m of the cubic system with a double perovskite structure, A₂BB′O₆. These structural findings were confirmed with neutron diffraction on polycrystalline samples synthesized by a high-temperature solid-state route. Both sets of diffraction data reveal that the M²⁺ and W⁶⁺ cations are fully ordered on the B and B′ sites. Ba₂MgWO₆ and Ba₂ZnWO₆ exhibit room-temperature luminescence with green and yellow emissions, respectively.     

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.     

A structural study of the perovskite series Ca1−xNaxTi1−xTaxO3

Compounds in the solid solution series Ca_1−xNa_xTi_1−xTa_xO₃ were synthesized at 1300 °C, followed by annealing at 850 °C or 800 °C with quenching and/or slow cooling to room temperature. Rietveld refinement of their powder X-ray diffraction patterns show that all compounds are single-phase ternary perovskites which adopt the space group Pbnm (a≈b≈√2a_p; c≈2a_p; Z=4) at ambient conditions. The unit cell parameters and cell volumes of the compounds increase regularly with increasing values of x. The coordination of the A-site cations changes throughout the series from eight for CaTiO₃ to nine for NaTaO₃. Compounds with 0?x ?0.4 have A-site cations in eight fold coordination, whereas the coordination of those with 0.4<x<0.9 is ambiguous. Analysis of the crystal chemistry of the compounds shows that the change in coordination at x=0.4 is related to the departure of the B-site cations from the second coordination sphere of the A-site cations, as in compounds with x>0.4 the A-^IIO distances become less than the A-B intercation distances. Contemporaneous with these coordination changes, the tilt angles of the BO₆ polyhedra decrease with increasing values of x. This solid solution series is unusual in that these structural and coordination changes occur regardless that Goldschmidt tolerance factors remain essentially constant at approximately 0.89, and observed tolerance factors, assuming eight fold coordination of the A-site cations, range only from 0.91 to 0.93 (0?x?0.8).     

The Al stabilized phase Li3−3xAlxBO3

The structure of an Al³⁺ stabilized phase Li_3−3xAl_xBO₃ (x≈0.18) was determined by means of single crystal X-ray diffraction. This phase crystallizes in space group P6₁22 or P6₅22, with lattice constants , and Z=6. The unit cell consists of six layers of BO₃ groups with Li⁺ cations distributing statistically on five crystallographic sites, none of which is fully occupied. The Li sites are close to each other and a three-dimensional network results when Li sites only within 1.65 Å are connected. Significant ionic conductivity was observed for this phase.     

Structural and electrical properties evolution in Ba1−xSrxRuO3 synthesized under high pressure

The 6H and 6M Ba_1−xSr_xRuO₃ at x?0.6 with the normal and distorted hexagonal BaTiO₃ structures were synthesized by using high-pressure and high-temperature method. It is found that the unit cell volume deviates from Vegard's law between 0.3 and 0.4 for the solid solutions due to the increasing distortion degree of crystal structure. With the increasing x, the electrical resistivity at the same temperature is increasing. With the substitution of Sr for Ba ion, the 6H BaRuO₃ transforms to a Fermi-liquid metal at x=0.25 from the primal non-Fermi-liquid metal, and then becomes a semiconductor at low temperature when x is larger than 0.4.     

Order-disorder transition and valence state of ytterbium in YbAuxGa2−x (0.26≤x≤1.31)

The intermetallic compounds YbAu_xGa_2−x (0.26≤x≤1.31) were synthesized by melting of elemental components and subsequent annealing. The crystal structure of YbAu_1.04Ga_0.96 was investigated using single-crystal X-ray diffraction data: structure type TiNiSi, space group Pnma, a=7.1167(3) Å, b=4.5019(3) Å, c=7.7083(3) Å, R_F=0.028 for 27 variables and 441 reflections. At 600 °C this compound is described as partially substituted TiNiSi type and shows a homogeneity range around the equiatomic composition YbAu_xGa_2−x (0.94≤x≤1.19). For the gallium- (0.26≤x≤0.83) and gold-rich (1.21≤x≤1.31) regions, the KHg₂ type of crystal structure (space group Imma) with mixed Au/Ga occupation is found. A temperature-driven phase transition for the composition YbAuGa from ordered TiNiSi to disordered KHg₂ structure type is observed at 629 °C. Yb L_III X-ray absorption spectra indicate an intermediate valence of +2.5 for Yb atoms in YbAuGa. For samples deviating from this composition a further reduced valence of Yb is observed. Magnetic susceptibility studies show a non-magnetic 4f¹⁴ ground state of Yb atoms with thermal fluctuations towards the 4f¹³ state.     

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.     

Structural phase transitions and crystal chemistry of the series Ba2LnB′O6 (Ln=lanthanide and B′=Nb or Sb)

The structures of 28 compounds in the two series Ba₂LnSbO₆ and Ba₂LnNbO₆ have been examined using synchrotron X-ray and in selected cases neutron powder diffraction at, below and above ambient temperature. The antimonate series is found to undergo a sequence of phase transitions from monoclinic to rhombohedral to cubic symmetry with both decreasing ionic radii of the lanthanides and increasing temperature. Compounds in the series Ba₂LnNbO₆, on the other hand, feature an intermediate tetragonal structure instead of the rhombohedral structure exhibited by the antimonates. This difference in symmetry is thought to be caused by π-bonding in the niobates that is absent in the antimonates. The bonding environments of the cations in these compounds have also been examined with overbonding of the lanthanide and niobium cations being caused by the unusually large B-site cations.     

Synthesis, crystal structure and magnetic properties of the spin ladder compounds La2(Cu1−xZnx)2O5 and La8(Cu1−xZnx)7O19

The influence of Zn-doping on the crystal structure and magnetic properties of the spin ladder compounds La₂Cu₂O₅ (4-leg) and La₈Cu₇O₁₉ (5-leg) have been investigated. The La₂(Cu_1−xZn_x)₂O₅ and La₈(Cu_1−xZn_x)₇O₁₉ solid solutions were obtained as single phases with x=0-0.1 via the solid-state reaction method in the temperature range between 1005-1010 °C and 1015-1030 °C in oxygen and air atmospheres, respectively. The lattice parameters a and c of the monoclinic crystal structures as well as the unit cell volume V increase with increasing x, while b and β decrease for both series. The magnetic susceptibilities χ of both series show a very similar behavior on temperature as well as on Zn-doping, which is supposed to be due to the similar Cu-O coordination in both La₂Cu₂O₅ and La₈Cu₇O₁₉. For low Zn-doping (x?0.04), a spin-chain like behavior is found. This quasi-one-dimensional behavior is strongly suppressed in both series for x?0.04. Here, the maximum (characteristic for spin chains) in χ(T) disappears and χ(T) decreases monotonically with increasing temperature.