Negative thermal expansion (NTE) materials, which can compensate for the thermal expansion of structural materials, have attracted much attention in the field of nanoscale electronics and optical devices requiring precise positioning. The present paper demonstrates a reversible colossal NTE in the perovskite-type oxide lanthanoid-substituted BiCoO3. The 6.1% volume shrinkage in Bi0.82Nd0.18CoO3 is the largest ever observed in Pb-free NTE materials. Synchrotron X-ray diffraction and Co L-edge soft X-ray absorption spectroscopy measurements and machine learning force field molecular dynamics simulations confirm that the origin of the NTE is the coordination change from the CoO5 pyramid with high-spin Co3+ to the CoO6 octahedron with the LaCoO3-type Co3+ spin state (intermediate-spin state or mixture of high-spin and low-spin states) due to melting of the dxy orbital ordering in the d6 electron configuration. The NTE properties as functions of ionic radius and concentration of substituting Ln ions are well explained by the nucleation mechanism of the reconstructive martensitic phase transition. The present results offer a new strategy for developing large-NTE materials.

Negative Thermal Expansion (NTE) is a key property for achieving dimensional stability in advanced technologies, yet understanding its underlying mechanisms, particularly the dynamic processes of phase transitions, remains a formidable challenge. In this study, we applied large-scale molecular dynamics simulations using the Crystal Hamilton Graph Network (CHGNet), a pretrained universal machine learning force field (MLFF), to elucidate the atomic-level mechanisms of the NTE phenomenon in the perovskite oxide system Bi1−xLaxCoO3. CHGNet achieved a computational speed approximately 20,000 times faster than first-principles calculations and qualitatively reproduced the chemical trend of the NTE transition temperature systematically decreasing as La substitution increased. Leveraging this computational efficiency, we successfully visualized the dynamic process of the NTE phase transition at the atomic level, which had previously been unobservable. The results revealed a nucleation and propagation mechanism wherein the phase transition initiates heterogeneously in La-rich regions and propagates to Bi-rich regions. These findings demonstrate that universal MLFFs are powerful tools for more quickly elucidating complex phase transition phenomena and open up new possibilities for computational science-driven exploration of new materials.

BiFeO3 is the most intensively investigated multiferroic material. It has a cycloidal spin modulation superimposed on G-type antiferromagnetic ordering of Fe3+ (S = 5/2), which prohibits the appearance of a net magnetization. We found that the simultaneous substitution of Ru4+ or Ir4+ for Fe3+ and Ca2+ for Bi3+ suppressed the cycloidal modulation and induced canted weak ferromagnetism at room temperature, while preserving the polar rhombohedral crystal structure. Moreover, the A- and B-site substitutions substantially lowered the ferroelectric transition temperature and resulted in a 1.77% volumetric negative thermal expansion around room temperature. These findings open an avenue for designing new BiFeO3-based functional materials.

Two negative thermal expansions (NTEs) with different mechanisms were observed in solid solutions of perovskite-type oxides PbCrO3 and PbTiO3. PbCr1-xTixO3 was found to adopt a cubic structure the same as that of PbCrO3 for x <= 0.6 and a PbTiO3-type tetragonal structure for x >= 0.7. The NTE observed at x <= 0.6 was accompanied by a cubic-to-cubic phase transition originating from the rearrangement of Pb2+/Pb4+ in a complex local structure called a charge glass. The volume shrinkage of -2.5% observed in PbCrO3 is sufficiently large despite the absence of intermetallic charge transfer, which is the origin of pressure-induced cubic-to-cubic phase transition and 9.8% volume collapse. The NTE in the tetragonal phase was caused by the ferroelectric-to-paraelectric phase transition, the same as in PbTiO3. We succeeded in significantly lowering the working temperature of PbTiO3 as an NTE material by Cr substitution while retaining a large volume shrinkage of 0.6%.

Bi0.5Pb0.5FeO3 with 1:1 mixture of Bi and Pb having charge degrees of freedom at the A-site of perovskite oxide ABO3 is obtained for the first time by high-pressure synthesis. Comprehensive synchrotron X-ray powder diffraction, optical second harmonic generation, Mössbauer spectroscopy, and hard X-ray photoemission spectroscopy measurements revealed that Bi0.5Pb0.5FeO3 is a canted antiferromagnetic insulator crystalizing in a nonpolar tetragonal I4/mcm structure with √2a × √2a × 2a unit cell and has unusually Pb charge disproportionated Bi3+0.5Pb2+0.25Pb4+0.25Fe3+O3 charge distribution. The valence of transition metal M in Bi0.5Pb0.5MO3 changes from 3.5+ to 3+ and finally to 2+ from Mn to Fe and to Ni, from left to right in the periodic table as the 3d-level becomes deeper. The valences of Bi and Pb increase to compensate for the decrease in the M's valence, and Pb changes from 6s2 (2+) to 6s0 (4+) before Bi changes.

An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Li4OBr2, an n = 1 member of the inverse Ruddlesden-Popper (iRP) phase Li3n+1OnBrn+1, was prepared for the first time by high-pressure (HP) synthesis. The changes in the stability and the ionic conductivity induced by F or I substitution for Br were also investigated by density functional theory (DFT) calculations and were experimentally attempted as well. The experimentally observed activation energy of Li4OBr2 was 0.63 eV, which was further lowered by F or I substitution for Br or the introduction of Li-Br defects.

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.

Mn perovskite oxides with half-integer valence (3.5+) exhibit charge ordering (CO) that is the origin of intriguing properties such as the colossal magnetoresistance effect. Although CO melts below the room temperature in most of the compounds, Bi0.5Sr0.5MnO3 preserves the CO state up to 475 K. It is such explained that 6s2 lone pair of Bi stabilizes the charge ordering. 6s2 lone pair has a steric activity and disturbs the electron transfer through the Mn-O-Mn bond. In this study, we replaced Sr by Pb with 6s2 lone pair, and investigated the properties of Bi0.5Pb0.5MnO3 in order to clarify the contribution of 6s2 lone pair to the CO. Synchrotron XRD measurement clarified the presence of superlattice structure originating from CO and orbital ordering (OO) up to 500 K. The CO state was preserved up to 550 K, much higher than Bi0.5Sr0.5MnO3. It is confirmed that increase in the fraction of ions with lone pair at the A site of Mn perovskite makes the CO state stable.

Bi and Pb ions with charge degree of freedom depending on 6s2 and 6s0 electronic configurations were combined with the Mn ion in a perovskite oxide. Comprehensive theoretical and experimental investigations revealed the Bi3+0.5Pb2+0.5Mn3+0.5Mn4+0.5O3 charge ordered state with CE-type spin and dz2 orbital orderings as observed in La0.5Ca0.5MnO3, Nd0.5Sr0.5MnO3, and Bi0.5Sr0.5MnO3. The charge and orbital orderings were preserved above 500 K owing to the stereochemical activity of Bi3+ and Pb2+ ions which stabilized the structural distortion.

The pyrochlore-type Ca2Bi2O7 and Sr2Bi2O7 have been synthesized from a low-temperature hydrothermal route using NaBiO3·nH2O as a starting material. The crystal structures of these compounds were refined using synchrotron powder X-ray diffraction data. The cell parameters were found to be a = 10.75021 (5) Å and 10.94132 (6) Å for Ca2Bi2O7 and Sr2Bi2O7, respectively. Density functional theory calculations showed the metallic band structure, but the negligible mixing of O2 2p bands with the A-site alkaline-earth-metal states and weak overlap with the conduction bands result in the semiconducting behavior.