Symmetry-mode analysis of the phase transitions in SrLaZnRuO6and SrLaMgRuO6ordered double perovskites

High-pressure synthesis, crystal structures, and magnetic properties of 5d double-perovskite oxides Ca2MgOsO6 and Sr2MgOsO6

Double-perovskite oxides Ca2MgOsO6 and Sr2MgOsO6 have been synthesized under high-pressure and high-temperature conditions (6 GPa and 1500 °C). Their crystal structures and magnetic properties were studied by a synchrotron X-ray diffraction experiment and by magnetic susceptibility, specific heat, isothermal magnetization, and electrical resistivity measurements. Ca2MgOsO6 and Sr2MgOsO6 crystallized in monoclinic (P21/n) and tetragonal (I4/m) double-perovskite structures, respectively; the degree of order of the Os and Mg arrangement was 96% or higher. Although Ca2MgOsO6 and Sr2MgOsO6 are isoelectric, a magnetic-glass transition was observed for Ca2MgOsO6 at 19 K, while Sr2MgOsO6 showed an antiferromagnetic transition at 110 K. The antiferromagnetic-transition temperature is the highest in the family. A first-principles density functional approach revealed that Ca2MgOsO6 and Sr2MgOsO6 are likely to be antiferromagnetic Mott insulators in which the band gaps open, with Coulomb correlations of ∼1.8-3.0 eV. These compounds offer a better opportunity for the clarification of the basis of 5d magnetic sublattices, with regard to the possible use of perovskite-related oxides in multifunctional devices. The double-perovskite oxides Ca2MgOsO6 and Sr2MgOsO6 are likely to be Mott insulators with a magnetic-glass (MG) transition at ∼19 K and an antiferromagnetic (AFM) transition at ∼110 K, respectively. This AFM transition temperature is the highest among double-perovskite oxides containing single magnetic sublattices. Thus, these compounds offer valuable opportunities for studying the magnetic nature of 5d perovskite-related oxides, with regard to their possible use in multifunctional devices.

Mode-crystallography analysis of the crystal structures and the low- and high-temperature phase transitions in Na0.5K0.5NbO3

Na0.5K0.5NbO3has been synthesized by the conventional solid-state reaction process. The crystal structures and phase transitions, at low and high temperature, determined from the Rietveld refinements of X-ray and neutron powder diffraction data are reported. The structure evolution of Na0.5K0.5NbO3in the temperature range from 2 to 875 K shows the presence of three phase transitions. The first one, at ∼135 K, is discontinuous from the rhombohedralR3c(No. 161) space group to the room-temperature orthorhombicAmm2 (No. 38) space group; the second is discontinuous from the orthorhombic to the tetragonalP4mmspace group (No. 99) at ∼465 K, and the third is continuous from the tetragonal to the cubic Pm\overline{3}m space group (No. 221) at ∼700 K. The obtained phase-transition sequence isR3c→Amm2 →P4mm→Pm\overline{3}m. No previous studies at low temperature have been carried out on the material with composition Na0.5K0.5NbO3. In the course of the determination of the three experimentally found phases, a novel method of refinement is presented. This is a step forward in the use of the symmetry-adapted modes as degrees of freedom in the refinement process: the parameterization of a direction in the internal space of the, in this case, sole irreducible representation, GM4−, responsible for the symmetry breaking from the parent cubic space group to the polar distorted low-symmetry phases. Eventually, this procedure enables the calculation of the spontaneous polarization.

Synthesis, structural, magnetic and phase-transition studies of the ferromagnetic La2CoMnO6 double perovskite by symmetry-adapted modes

A powdered La₂CoMnO₆ double perovskite was synthesized by the solid-state reaction method, and its crystal structure and phase-transitions were investigated by (Mode Crystallography) Rietveld analysis using X-ray and neutron diffraction data. Three indistinguishable ferromagnetic models with the space groups P2₁/n and P2′1/n′ are proposed.