Perovskite PbCoO synthesized at 12 GPa was found to have an unusual charge distribution of PbPbCoCoO with charge orderings in both the A and B sites of perovskite ABO. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to PbPbCoCoO quadruple perovskite structure. It is shown that the average valence distribution of PbCoO between PbCrO and PbNiO can be stabilized by tuning the energy levels of Pb 6s and transition metal 3d orbitals.

The discovery of unusual negative thermal expansion (NTE) provides the opportunity to control the common but much desired property of thermal expansion, which is valuable not only in scientific interests but also in practical applications. However, most of the available NTE materials are limited to a narrow temperature range, and the NTE effect is generally weakened by means of various modifications. Here, we report an enhanced NTE effect that occurs over a wide temperature range ( V  = -5.24 × 10 -5 °C -1 , 25-575 C), and this NTE effect is accompanied by an abnormal enhanced tetragonality, a large spontaneous polarization, and a G-type antiferromagnetic ordering in the present perovskite-type ferroelectric of (1-x)PbTiO3-xBiCoO3. Specifically, for the composition of 0.5PbTiO3-0.5BiCoO3, an extensive volumetric contraction of ~4.8 % has been observed near the Curie temperature of 700 °C, which represents the highest level in PbTiO3-based ferroelectrics.According to our experimental and theoretical results, the giant NTE originates from a synergistic effect of the ferroelectrostriction and spin-crossover of cobalt on the crystal lattice. The actual NTE mechanism is contrasted with previous functional NTE materials, in which the NTE is simply coupled with one ordering such as electronic, magnetic, or ferroelectric ordering. The present study sheds light on the understanding of NTE mechanisms and it attests that NTE could be simultaneously coupled with different orderings, which will pave a new way toward the design of large NTE materials.

PbMO3 (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO3 remain unclear. Herein, we reveal that PbFeO3 crystallizes into an unusual 2ap × 6ap × 2ap orthorhombic perovskite super unit cell with space group Cmcm. The distinctive crystal construction and valence distribution of Pb2+0.5Pb4+0.5FeO3 lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb2+ and Pb4+) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the a-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the b-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe3+ magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature.

The valence distribution and local structure of Bi 1−x Pb x NiO 3 (x ≤ 0.25) were investigated by comprehensive studies of Rietveld analysis of synchrotron X-ray diffraction (SXRD) data, X-ray absorption spectroscopy (XAS), hard X-ray photoemission spectroscopy (HAXPES), and pair distribution function (PDF) analysis of synchrotron X-ray total scattering data. Disproportionation of Bi ions into Bi 3+ and Bi 5+ was observed for all the samples, but it was a longranged one with distinct crystallographic sites in the P1̅ triclinic structure for x ≤ 0.15, while the ordering was short-ranged for x = 0.20 and 0.25. An intermetallic charge transfer between Bi 5+ and Ni 2+ , leading to large volume shrinkage, was observed for all the samples upon heating at ∼500 K.

Spin state transition and intermetallic charge transfer can essentially change material structural and physical properties with a fashion of excluding external chemical doping. However, these two effects have rarely been found to occur sequentially in a specific material. In this article, we show the realization of these two phenomena in a perovskite oxide PbCoO3 with a simple ABO3 composition under high pressure. PbCoO3 possesses a peculiar A-and B-site ordered charge distribution Pb 2+ Pb 4+ 3Co 2+ 2Co 3+ 2O12 with insulating behavior at ambient conditions. The high spin Co 2+ gradually changes to low spin with increasing pressure up to about 15 GPa, leading to the anomalous increase of resistance magnitude. Between 15 GPa and 30 GPa, the intermetallic charge transfer occurs between Pb 4+ and Co 2+ cations. The accumulated charge-transfer effect triggers a metal-insulator transition as well as a first-order structural phase transition toward a Tetra.-I phase at the onset of ~20 GPa near room temperature. On further compression over 30 GPa, the charge transfer completes, giving rise to another first-order structural transformation toward a Tetra.-II phase and the reentrant electrical insulating behavior.