Enhanced Spontaneous Polarization by V4+ Substitution in a Lead-Free Perovskite CaMnTi2O6

Anisotropic Thermal Expansion and a Second-Order Charge Order Transition in the Ferrimagnetic Dy2CuZnMn4O12 Perovskite with Triple A-Site Cation Ordering

Dy2CuZnMn4O12 perovskite, belonging to the A-site columnar-ordered quadruple perovskite family with the general composition of A2A'A″B4O12, was prepared by a high-pressure, high-temperature method at 6 GPa and 1500 K. Its crystal structure was studied by synchrotron powder X-ray diffraction between 100 and 800 K. The ideal cation distribution (without antisite disorder) was found to be realized within the sensitivity of the synchrotron X-ray diffraction method. Between 100 and 400 K, it crystallizes in space group Pmmn (no. 59) and has layered charge ordering of Mn3+ and Mn4+ at the B sites. Above 425 K, it crystallizes in space group P42/nmc (no. 137) with one crystallographic B site and an average Mn3.5+ oxidation state. The charge ordering transition (at TCO = 425 K) appears to be of the second order as no anomalies were found on differential scanning calorimetry curves and temperature dependence of the unit cell volume, and the orthorhombic a and b lattice parameters merge gradually. The compound demonstrates anisotropic thermal expansion with the c lattice parameter decreasing with increasing temperature above 280 K. A ferrimagnetic transition occurs at TC = 116 K with an additional, gradual rise of magnetic susceptibilities below 45 K, probably due to increases of the ordered moments of the Dy sublattices.

A-Site Columnar-Ordered Perovskite CaZnV2O6 as a Pauli-Paramagnetic Metal

In this study, we successfully synthesized a novel A-site columnar-ordered perovskite CaZnV₂O₆. This compound features a square-planar-coordinated Zn²⁺ disorder, which is the same characteristic as the centrosymmetric paraelectric CaMnTi₂O₆. Unlike CaMnTi₂O₆, which shows a centrosymmetric paraelectric-noncentrosymmetric ferroelectric transition, CaZnV₂O₆ retains Pauli-paramagnetic metallicity arising from itinerant V⁴⁺ d¹ electrons and centrosymmetry down to 5 K. Based on analogous compounds, we expect CaZnV₂O₆ to provide a new playground for the electronic and magnetic states of V⁴⁺.

CaFeFeNbO6 - an iron-based double double perovskite

Ordering of cations is important for controlling properties of ABO₃ perovskites, and CaFeFeNbO₆ is the first example of an Fe-based AA'BB'O₆ double double perovskite, with Ca²⁺/Fe²⁺ ordered on A-site columns, and Fe³⁺/Nb⁵⁺ at the octahedral B-sites. Substantial (37%) antisite disorder of the latter cations leads to spin glassy magnetism below a freezing transition at 12 K. The CaMnFeNbO₆ analogue also shows substantial cation disorder and spin glassy behaviour. Comparison of synthesis pressures for ordered materials based on different A-site transition metals, suggests that pressures of at least 14-18 GPa will be required to discover the expected plethora of double double perovskites based on A' cations smaller than Mn²⁺.