Perovskite-structure TlMnO₃: a new manganite with new properties

Perovskite-structure TlBO3 (B = Cr, Mn) for thermomechanical and optoelectronic applications: an investigation via a DFT scheme

Here, we have employed the density functional theory on TlBO3 (B = Cr, Mn) to study the structural, mechanical, electronic, optical, and thermal properties for the first time. Spin polarization causes a metallic-to-semiconducting transition.

Tl2Ir2O7: A Pauli Paramagnetic Metal, Proximal to a Metal Insulator Transition

A polycrystalline sample of Tl₂Ir₂O₇ was synthesized by high-pressure and high-temperature methods. Tl₂Ir₂O₇ crystallizes in the cubic pyrochlore structure with space group Fd3̅m (No. 227). The Ir⁴⁺ oxidation state is confirmed by Ir-L₃ X-ray absorption near-edge spectroscopy. Combined temperature-dependent magnetic susceptibility, resistivity, specific heat, and DFT+DMFT calculation data show that Tl₂Ir₂O₇ is a Pauli paramagnetic metal, but it is close to a metal-insulator transition. The effective ionic size of Tl³⁺ is much smaller than that of Pr³⁺ in metallic Pr₂Ir₂O₇; hence, Tl₂Ir₂O₇ would be expected to be insulating according to the established phase diagram of the pyrochlore iridate compounds, A³⁺₂Ir⁴⁺₂O₇. Our experimental and theoretical studies indicate that Tl₂Ir₂O₇ is uniquely different from the current A³⁺₂Ir⁴⁺₂O₇ phase diagram. This uniqueness is attributed primarily to the electronic configuration difference between Tl³⁺ and rare-earth ions, which plays a substantial role in determining the Ir-O-Ir bond angle, and the corresponding electrical and magnetic properties.

Hybridization-Switching Induced Mott Transition in ABO_{3} Perovskites

We propose the concept of a "hybridization-switching induced Mott transition" which is relevant to a broad class of ABO_{3} perovskite materials including BiNiO_{3} and PbCrO_{3} that feature extended 6s orbitals on the A-site cation (Bi or Pb), and a strong A-O covalency induced ligand hole. Using ab initio electronic structure and slave rotor theory calculations, we show that such systems exhibit a breathing phonon driven A-site to oxygen hybridization-wave instability which conspires with strong correlations on the B-site transition metal ion (Ni or Cr) to trigger a Mott insulating state. This class of systems is shown to undergo a pressure induced insulator to metal transition accompanied by a colossal volume collapse due to ligand hybridization switching.

Perovskite-Type InCoO3 with Low-Spin Co3+: Effect of In-O Covalency on Structural Stabilization in Comparison with Rare-Earth Series

Perovskite rare-earth cobaltites ACoO₃ (A = Sc, Y, La-Lu) have been of enduring interest for decades due to their unusual structural and physical properties associated with the spin-state transitions of low-spin Co³⁺ ions. Herein, we have synthesized a non-rare-earth perovskite cobaltite, InCoO₃, at 15 GPa and 1400 °C and investigated its crystal structure and magnetic ground state. Under the same high-pressure and high-temperature conditions, we also prepared a perovskite-type ScCoO₃ with an improved cation stoichiometry in comparison to that in a previous study, where synthesis at 6 GPa and 1297 °C yielded a perovskite cobaltite with cation mixing on the A-site, (Sc_0.95Co_0.05)CoO₃. The two perovskite phases have nearly stoichiometric cation compositions, crystallizing in the orthorhombic Pnma space group. In the present investigation, comprehensive studies on newly developed and well-known Pnma ACoO₃ perovskites (A = In, Sc, Y, Pr-Lu) show that InCoO₃ does not fulfill the general evolution of crystal metrics with A-site cation size, indicating that InCoO₃ and rare-earth counterparts have different chemistry for stabilizing the Pnma structures. Detailed structural analyses combined with first-principles calculations reveal that the origin of the anomaly for InCoO₃ is ascribed to the A-site cation displacements that accompany octahedral tilts; despite the highly tilted CoO₆ network, the In-O covalency makes In³⁺ ions reluctant to move from their ideal cubic-symmetry position, leading to less orthorhombic distortion than would be expected from electrostatic/ionic size mismatch effects. Magnetic studies demonstrate that InCoO₃ and ScCoO₃ are diamagnetic with a low-spin state of Co³⁺ below 300 K, in contrast to the case of (Sc_0.95Co_0.05)CoO₃, where the high-spin Co³⁺ ions on the A-site generate a large paramagnetic moment. The present work extends the accessible composition range of the low-spin orthocobaltite series and thus should help to establish a more comprehensive understanding of the structure-property relation.

High-pressure synthesis, crystal structure and magnetic properties of TlCrO3 perovskite

TlMO(3) perovskites (M(3+) = transition metals) are exceptional members of trivalent perovskite families because of the strong covalency of Tl(3+)-O bonds. Here we report on the synthesis, crystal structure and properties of TlCrO(3) investigated by Mössbauer spectroscopy, specific heat, dc/ac magnetization and dielectric measurements. TlCrO(3) perovskite is prepared under high pressure (6 GPa) and high temperature (1500 K) conditions. The crystal structure of TlCrO(3) is refined using synchrotron X-ray powder diffraction data: space group Pnma (no. 62), Z = 4 and lattice parameters a = 5.40318(1) Å, b = 7.64699(1) Å and c = 5.30196(1) Å at 293 K. No structural phase transitions are found between 5 and 300 K. TlCrO(3) crystallizes in the GdFeO(3)-type structure similar to other members of the perovskite chromite family, ACrO(3) (A(3+) = Sc, In, Y and La-Lu). The unit cell volume and Cr-O-Cr bond angles of TlCrO(3) are close to those of DyCrO(3); however, the Néel temperature of TlCrO(3) (TN≈ 89 K) is much smaller than that of DyCrO(3) and close to that of InCrO(3). Isothermal magnetization studies show that TlCrO(3) is a fully compensated antiferromagnet similar to ScCrO(3) and InCrO(3), but different from RCrO(3) (R(3+) = Y and La-Lu). Ac and dc magnetization measurements with a fine step of 0.2 K reveal the existence of two Néel temperatures with very close values at T(N2) = 87.0 K and T(N1) = 89.3 K. Magnetic anomalies near T(N2 )are suppressed by static magnetic fields and by 5% iron doping.