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CsNdNb2O7is a hybrid improper ferroelectric (HIF) where oxygen octahedral rotation modes induce spontaneous polarization. While CsLaNb2O7is also believed to be an HIF, its polar structure and ferroelectric properties remain poorly understood. This study investigates their solid solution, CsLa1–xNdxNb2O7, by constructing a temperature–composition phase diagram using synchrotron X-ray diffractometry, first-principles calculations, and dielectric measurements. Upon cooling, the Nd-rich compositions (x= 0.5, 0.75, and 1) undergo phase transitions from a P4/mmmto a C2/mand then to a polar P21amphase. Both transition temperatures decrease as x decreases. The polar P21amphase, accompanied by ferroelectric polarization switching, is stabilized at room temperature only when x= 0.75 and 1. For La-rich compositions, the nonpolar C2/mphase remains down to room temperature without exhibiting any indication of transformation into the polar P21amphase. Theoretical calculations suggest that a symmetry trapping effect hinders the nonpolar (C2/m)-to-polar (P21am) phase transition.

Tailoring size, shape, and connectivity of oxygen coordination octahedra in perovskite-related oxides is known to play a key role in engineering their material properties, and furthermore, the interplay among the different types of oxygen octahedral distortions can open up new strategies for structure-driven control of functionalities. Here, we report that in layered perovskites AgRTiO4(R: rare earth), the biaxial negative thermal expansion (NTE) arises from the interplay between the structural distortions that alter the octahedral connectivity and shape, which is a fundamentally different mechanism from those in the conventional NTE materials. AgRTiO4was previously identified as having an orthorhombic (Pbcm) structure, but our experimental and theoretical study reveals that this compound adopts an acentric tetragonal (P4̅21m) structure due to (Φ00)(0Φ0)-type TiO6octahedral rotations, as in the previously reported Na and K analogs, NaRTiO4and KRTiO4. Thorough structural analysis reveals that the competition of the octahedral rotations with octahedral deformations drives the biaxial NTE; the decrease in the rotation amplitude caused by heating results in octahedral deformations, i.e., an out-of-plane elongation and an in-plane compression of TiO6octahedra, leading to shrinkage of the lattice parameters aand b(a= b). The Ag+-R3+layered ordering produces a built-in electric field compelling Ti4+to off center, which is the source for the otherwise unfavorable coexistence of octahedral rotations and deformations. Despite the competition between the two octahedral distortions being predicted to be active in other members of the ARTiO4series (A= Na, K, and Rb) as well, we do not observe experimentally the biaxial NTE for the Na members. Detailed analysis of calculated electronic structures highlights the essential role played by Ag–O–Ti covalent bonding in enhancing the octahedral deformation of AgRTiO4, which is directly responsible for the biaxial NTE. The present study provides an important example of functional properties that emerge from the coupling among distinct distortions of octahedral frameworks.

The ability to reproducibly synthesize highly conductive solid electrolytes (SEs) is a prerequisite for the widespread usage of solid-state batteries. However, reported ionic conductivities of SEs exhibit significant variation even in materials with the same nominal composition. In this study, the thermodynamic origin of such sample-dependent variations is discussed using sodium-ion conducting Na3SbS4as a model SE. The impact of uncontrolled variations in elemental chemical potentials on the ionic conductivity is investigated with theory and experiments. The elemental chemical potentials are uniquely defined when the system is constrained to have zero thermodynamic degrees of freedom. First, we establish the relationship between the chemical potentials and sodium-ion conductivity in Na3SbS4by computing the phase diagram and native defect formation energies. From these calculations, we identify two distinct three-phase equilibrium regions (zero degrees of freedom) with the highest ratio of sodium-ion conductivity, which are then experimentally probed. Transport measurements reveal an abrupt change in the bulk ion transport of the phase-pure samples, with a room-temperature ionic conductivity of 0.16–1.2 mS cm–1with a standard deviation of 50% when the elemental chemical potentials are not controlled, that is, not uniquely defined. In contrast, we show that by controlling the chemical potentials and therefore, the defect formation energies through the experimental concept of phase boundary mapping, the sample-dependent variation is reduced to 15% with a high average ionic conductivity of 0.94 mS cm–1. This study highlights the existence of “hidden” thermodynamic states defined by their chemical potentials and the need to precisely control these states to achieve reproducibly high ionic conductivity.

The utilization of earth-abundant and high-capacity sulfur in solid-state batteries presents a promising strategy to circumvent the use of rare transition metals and enhance achievable specific energy. However, numerous challenges remain. The transport limitation within the cathode composite, particularly with sulfide electrolytes during charging, has been identified as a major degradation mechanism in solid-state Li–S batteries. This degradation is linked to electrolyte oxidation and a concomitant reduction in the effective ionic conductivity of the cathode composite. Inspired by the sufficiently high oxidation stability of halide-based electrolytes, we investigated their compatibility with solid-state Li–S batteries in this work. The electrochemical stability of halides in contact with conductive additives, the stability window of fast ion transport in the composite electrodes, and chemical compatibility with sulfur-active materials (e.g., S and Li2S), in addition to the cyclability of the halide-based composite electrodes, are explored. Three halides were employed as model electrolytes: Li3InCl6, Li3YCl6, and Li3YBr6. Despite its high oxidation stability, Li3InCl6exhibited rapid degradation due to electrolyte reduction. The composite with Li3YCl6lost its capacity because of chemical incompatibility, especially with Li2S, resulting in the formation of LiYS2at the interface. In contrast, Li3YBr6demonstrated superior performance, maintaining a capacity of 1100 mAh gS–1for 20 cycles (normalized to the sulfur content in the cathode material). This study elucidates the degradation mechanisms of halide-based solid-state Li–S batteries and proposes potential design strategies to mitigate chemical incompatibility issues.

A novel perovskite fluoride, LixCoF3, which has an exceptionally low tolerance factor (0.81), has been synthesized via low-temperature lithium intercalation into a distorted ReO3-type fluoride CoF3using organolithium reagents. Interestingly, this reaction is completed within 15 min at room temperature. Synchrotron X-ray diffractometry and optical second harmonic generation at room temperature have revealed that this compound shows a high-temperature LiNbO3-type structure (space group: R3̅c) involving Li–Co antisite defects and A-site splitting along the cdirection. A-site splitting is consistent with the prediction based on hybrid Hartree–Fock density functional theory calculations. Co-L2,3edge X-ray absorption spectroscopy, as well as bond valence sum analysis, has verified the divalent oxidation state of Co ions in the lithiated phase, suggesting that its composition is close to LiCoF3(x≈ 1). This compound exhibits a paramagnetic-to-antiferromagnetic transition at 36 K on cooling, accompanied by weak ferromagnetic ordering. The synthetic route based on low-temperature lithiation of metal fluorides host paves the way for obtaining a new LiNbO3-type fluoride family.