Preparation, structural study from neutron diffraction data and magnetism of the disordered perovskite Ca(Cr0.5Mo0.5)O3

We report on the preparation and characterization of the Ca(Cr_0.5Mo_0.5)O₃ perovskite, obtained in the search of the hypothetical double perovskite Ca₂CrMoO₆. This material was prepared in polycrystalline form by solid state reaction in H₂/Ar flow. It has been studied by X-ray and neutron powder diffraction (NPD) and magnetic measurements. Ca(Cr_0.5Mo_0.5)O₃ crystallizes in the orthorhombic Pbnm (No. 62) space group, with the unit-cell parameters a=5.4110 (4) Å, b=5.4795 (5) Å, c=7.6938 (6) Å. There is a complete disordering of Cr³⁺ and Mo⁵⁺ over the B-site of the perovskite, and the (Cr,Mo)O₆ octahedra are tilted by 12.4° in order to optimize the Ca-O bond lengths. The magnetic susceptibility is characteristic of a ferrimagnetic behavior, with T_C=125 K, and a small saturation magnetization at T=5 K, of 0.05 μ_B/f.u.     

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.     

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.     

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.     

Structures, phase transitions and microwave dielectric properties of the 6H perovskites Ba3BSb2O9, B=Mg, Ca, Sr, Ba

We present a complete temperature-composition phase diagram for Ba₃BSb₂O₉, B=Mg, Ca, Sr, Ba, along with their electrical behavior as a function of B. These compounds have long been recognized as 6H-type perovskites, but (with the exception of B=Mg) their exact structures and properties were unknown due to their low symmetries, temperature-dependent phase transitions, and difficulties in synthesizing pure samples. The full range of possible space group symmetries is observed, from ideal hexagonal P6₃/mmc to monoclinic C2/c to triclinic . Direct second-order transitions between these phases are plausible according to group theory, and no evidence was seen for any further intermediate phases. The phase diagram with respect to temperature and the effective ionic radius of B is remarkably symmetrical for B=Mg, Ca, and Sr. For B=Ba, a first-order phase transition to a locally distorted phase allows a metastable hexagonal phase to persist to lower temperatures than expected before decomposing around 600 K. Electrical measurements revealed that dielectric permittivity corrected for porosity does not change significantly as a function of B and is in a good agreement with the values predicted by the Clausius-Mossotti equation.     

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.     

Preparation, crystal structure and magnetic behavior of new double perovskites Sr2B′UO6 with B′=Mn, Fe, Ni, Zn

Sr₂B′UO₆ double perovskites with B′=Mn, Fe, Ni, Zn have been prepared in polycrystalline form by solid-state reaction, in air or reducing conditions. These new materials have been studied by X-ray diffraction (XRD), magnetic susceptibility and magnetization measurements. The room-temperature crystal structure is monoclinic (space group P2₁/n), and contains alternating B′O₆ and UO₆ octahedra sharing corners, tilted along the three pseudocubic axes according to the Glazer notation a⁻a⁻b⁺. The magnetic measurements show a spontaneous magnetic ordering below T_N=21 K for B′=Mn, Ni, and T_C=150 K for B′=Fe. From a Curie-Weiss fit, the effective paramagnetic moment for B′=Mn (5.74 μ_B/f.u.) and B′=Ni(3.51 μ_B/f.u.) are significantly different from the corresponding spin-only moments for the divalent cations, suggesting the possibility of a partial charge disproportionation B′²⁺+U⁶⁺⇔B′³⁺+U⁵⁺, also accounting for plausible ferrimagnetic interactions between B′ and U sublattices. The strong curvature of the reciprocal susceptibility for B′=Fe precludes a Curie-Weiss fit but also suggests the presence of ferrimagnetic interactions in this compound. This charge disproportionation effect is also supported by the observed B′O distances, which are closer to the expected values for high-spin, trivalent Mn, Fe and Ni cations.     

Crystal structures of disordered A2Mn3+M5+O6 (A=Sr,Ca; M=Sb,Nb, Ru) perovskites

Polycrystalline samples of A₂MnMO₆ (A=Sr, Ca; M=Nb, Sb, Ru) were prepared by conventional solid state synthesis and their crystal structures were determined using neutron powder diffraction data. All six compounds can be classified as distorted, disordered perovskites. The Mn³⁺/M⁵⁺ distribution is disordered in all six compounds. The strontium containing compounds, Sr₂MnMO₆ (M=Nb, Sb, Ru), undergo out of phase rotations of the octahedra about the c-axis (tilt system a⁰a⁰c⁻) leading to tetragonal I4/mcm space group symmetry. The calcium containing compounds, Ca₂MnMO₆ (M=Nb, Ru, Sb), have orthorhombic Pnma space group symmetry, as a result of a GdFeO₃-type octahedral tilting distortion (tilt system a⁻b⁺a⁻). A cooperative Jahn–Teller distortion is observed in Sr₂MnSbO₆ and Sr₂MnRuO₆, but it is much smaller than the distortion observed in LnMnO₃ (Ln=lanthanide ion) perovskites. It is possible that Jahn–Teller distortions of the MnO₆ octahedra take place on a short-range length scale in the other four compounds, but there is little or no evidence for cooperative ordering of the local distortions. These findings demonstrate a link between orbital ordering, cation ordering and octahedral tilting.     

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.     

Synthesis, structures, and phase transitions of barium bismuth iridium oxide perovskites Ba2BiIrO6 and Ba3BiIr2O9

The Ba-Bi-Ir-O system is found to contain two distinct perovskite-type phases: a rock-salt ordered double perovskite Ba₂BiIrO₆; and a 6H-type hexagonal perovskite Ba₃BiIr₂O₉. Ba₂BiIrO₆ 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 Ba₂BiIrO₆ had not previously been identified and reported. A solid solution Ba₂Bi_1+xIr_1−xO₆, analogous to Ba₂Bi_1+xRu_1−xO₆, 0≤x≤2/3, was not observed. The 6H-type phase Ba₃BiIr₂O₉ undergoes a clean second-order phase transition P6₃/mmc→C2/c at 750 K, unlike 6H-type Ba₃LaIr₂O₉, the P6₃/mmc structure of which is highly strained below ∼750 K but fails to distort coherently to the monoclinic phase.     

Ln0.5A0.5MnO3 (Ln=Lanthanide,A= Ca,Sr) Perovskites Exhibiting Remarkable Performance in the Thermochemical Generation of CO and H2 from CO2 and H2O

Perovskite oxides of the Ln_0.5A_0.5MnO₃ (Ln=lanthanide, A=Sr, Ca) family have been investigated for the thermochemical splitting of H₂O and CO₂ to produce H₂ and CO respectively. The amounts of O₂ and CO produced strongly depend on the size of the rare earth ions and alkaline earth ions. The manganite with the smallest rare earth possessing the highest distortion and size disorder as well as the smallest tolerance factor, gives out the maximum amount of O₂, and, hence, the maximum amount of CO. Thus, the best results are found with Y_0.5Sr_0.5MnO₃, which possesses the highest distortion and size disorder. Y_0.5Sr_0.5MnO₃ shows remarkable fuel production activity even at the reduction and oxidation temperatures as low as 1200 °C and 900 °C, respectively.     

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.     

Mechano-chemical synthesis K2MF6 (M = Mn,Ni) by cation-exchange reaction at room temperature

In order to establish the power of mechanochemistry to produce industrially important phosphors, synthesis of K₂MnF₆ has been attempted by the successive grinding reactions of manganese (II) acetate with ammonium fluoride and potassium fluoride. The progress of reaction was followed by ex-situ characterization after periodic intervals of time. Cubic symmetry of K₂MnF₆ was evident from its powder X-ray diffraction pattern which was refined successfully in cubic space group (Fm-3m) with a = 8.4658 (20) Å. Stretching and bending vibration modes of MnF₆²⁻ octahedral units appeared at 740 and 482 cm⁻¹ in the fourier transformed infrared spectrum. Bands at 405 and 652 cm⁻¹ appeared in the Raman spectrum and they were finger-print positions of cubic K₂MnF₆. Other than the ligand to metal charge transfer transition at 242 nm, transitions from ⁴A_2g to ⁴T_1g, ⁴T_2g and ²T_2g of Mn⁴⁺-ion appeared at 352, 429, 474 and 569 nm in the UV–visible diffuse reflectance spectrum of the sample. Red emission due to Mn⁴⁺ was observed in the photoluminescence spectrum with a decay time of 0.22 ms. Following the success in forming cubic K₂MnF₆, this approach has been extended to synthesize cubic K₂NiF₆ at room temperature. All these results confirmed the susceptibility of acetate salts of transition metals belonging to first-row of the periodic table to facile fluorination at room temperature aided by mechanical forces.     

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.     

A structure and phase analysis investigation of the “1:1” ordered A2InNbO6 perovskites (A=Ca, Sr, Ba)

The structures of the Ba₂InNbO₆, Sr₂InNbO₆ and Ca₂InNbO₆ “1:1” complex perovskites have been refined from neutron powder diffraction data. Both the A=Ca and Sr compounds occur at room temperature in P12₁/n1 (a=a_p+b_p, b=-a_p+b_p, c=2c_p) perovskite-related superstructures while the A=Ba compound occurs in the , a=2a_p, elpasolite structure type. In the case of the A=Ca compound, an extensive Ca₂[(Ca_2x/3In_1−xNb_x/3)Nb]O₆ ‘solid solution’ field spanning compositions between Ca₄Nb₂O₉ and Ca₂InNbO₆ in the CaO-InO_3/2-NbO_5/2 ternary phase diagram is shown to exist. Under the conditions of synthesis used, the ‘solid solution’ field stops just short of the ideal 1:1 Ca₂InNbO₆ composition.     

Electronic, magnetic and structural properties of A2VMoO6 perovskites (A=Ca, Sr)

The perovskites Sr₂VMoO₆ and Ca₂VMoO₆ have been synthesized by liquid-mix technique in citrate melts, and their electronic, magnetic and structural properties have been investigated. No signs of V/Mo ordering are seen by synchrotron X-ray powder diffraction, but despite the chemical disorder both oxides are highly conductive and Pauli paramagnetic. Electrical conductivities of these solid solutions are comparable or higher than those reported for polycrystalline AMoO₃ end members. It is suggested that the delocalized metallic conductivity of these compounds with two different transition-metal atoms implies valence equilibrium between the degenerate oxidation-state couples V⁴⁺Mo⁴⁺ and V³⁺Mo⁵⁺.     

K_2NiF_4型Dy_(0.5)Sr_(1.5)MO_4(M=Mn,Fe,Co,Ni)稀土复合氧化物的合成和催化性能考察

用重稀土镝的氧化物与锶、锰、铁、钴、镍的硝酸盐为原料,制备了A_2BO_4型Dy_(0.5)Sr_(1.5)MO_4(M=Mn,Fe,Co,Ni)稀土复合氧化物,用XRD技术考察了物相,合成了K_2NiF_4型四方结构的Dy_(0.5)Sr_(1.5)MO_4,并研究了其催化性能。对结构容纳因子的适用性作了讨论。     

Preparation and structural study from neutron diffraction data of R2MoO6 (R=Dy, Ho, Er, Tm, Yb, Y)

The title compounds have been prepared as polycrystalline powders by thermal treatments of stoichiometric mixtures of R₂O₃ and MoO₃ in air. The room-temperature crystal structure for all the series has been refined from high-resolution neutron powder diffraction data. All the phases are isostructural (space group C2/c, Z=8) with the polymorph α-R₂MoO₆, typified by Sm₂MoO₆. The structure contains four zigzag, one-dimensional MoO₅ polyhedral rows per unit cell, running through the RO₈ polyhedral framework along the [001] direction. MoO₅ form discrete units (i.e. do not share common oxygen), with Mo-O distances ranging from 1.77 to 2.24 Å, although the oxygen coordination can be extended to distances of about 3.1 Å, giving rise to strongly distorted MoO₈ scalenohedra. Thus, MoO₈ and RO₈ polyhedra are fully ordered in R₂MoO₆ compounds, which in fact can be considered as superstructures of fluorite (M₃O₆), containing 24 MO₂ fluorite units per unit cell, with unit-cell parameters related to that of cubic fluorite ( Å). A bond valence study demonstrates that the present crystal structure is especially stable for small rare-earth cations, and becomes more unstable when the R³⁺ size increases, thus explaining the observed preference of the large rare-earth molybdates for polymorphs β and γ with the same stoichiometry.