In oxides, the substitution of non-oxide anions (F(-),S(2-),N(3-) and so on) for oxide introduces many properties, but the least commonly encountered substitution is where the hydride anion (H(-)) replaces oxygen to form an oxyhydride. Only a handful of oxyhydrides have been reported, mainly with electropositive main group elements or as layered cobalt oxides with unusually low oxidation states. Here, we present an oxyhydride of the perhaps most well-known perovskite, BaTiO(3), as an O(2-)/H(-) solid solution with hydride concentrations up to 20% of the anion sites. BaTiO(3-x)H(x) is electronically conducting, and stable in air and water at ambient conditions. Furthermore, the hydride species is exchangeable with hydrogen gas at 400 °C. Such an exchange implies diffusion of hydride, and interesting diffusion mechanisms specific to hydrogen may be at play. Moreover, such a labile anion in an oxide framework should be useful in further expanding the mixed-anion chemistry of the solid state.

SrFeO(2.5) and SrCoO(2.5) are able to intercalate oxygen in a reversible topotactic redox reaction already at room temperature to form the cubic perovskites Sr(Fe,Co)O(3), while CaFeO(2.5) can only be oxidized under extreme conditions. To explain this significant difference in low temperature oxygen mobility, we investigated the homologous SrFeO(2.5) and CaFeO(2.5) by temperature dependent oxygen isotope exchange as well as by inelastic neutron scattering (INS) studies, combined with ab initio (DFT) molecular dynamical calculations. From (18)O/(16)O isotope exchange experiments we proved free oxygen mobility to be realized in SrFeO(x) already below 600 K. We have also evidence that low temperature oxygen mobility relies on the existence of specific, low energy lattice modes, which trigger and amplify oxygen mobility in solids. We interpret the INS data together with the DFT-based molecular dynamical simulation results on SrFeO(2.5) and CaFeO(2.5) in terms of an enhanced, phonon-assisted, low temperature oxygen diffusion for SrFeO(3-x) as a result of the strongly reduced Fe-O-Fe bond strength of the apical oxygen atoms in the FeO(6) octahedra along the stacking axis. This dynamically triggered phenomenon leads to an easy migration of the oxide ions into the open vacancy channels and vice versa. The decisive impact of lattice dynamics, giving rise to structural instabilities in oxygen deficient perovskites, especially with brownmillerite-type structure, is demonstrated, opening new concepts for the design and tailoring of low temperature oxygen ion conductors.

In contrast to polar cation displacements driving oxides into noncentrosymmetric and ferroelectric states, inversion-preserving anion displacements, such as rotations or tilts of oxygen octahedra about cation coordination centers, are exceedingly common. More than one nonpolar rotational mode in layered perovskites can lift inversion symmetry and combine to induce an electric polarization through a hybrid improper ferroelectric (HIF) mechanism. This form of ferroelectricity expands the compositional palette to new ferroelectric oxides because its activity derives from geometric rather than electronic origins. Here, the new Ruddlesden-Popper HIF Sr 3 Zr 2 O 7 , which is the first ternary lead-free zirconate ferroelectric, is reported and room-temperature polarization switching is demonstrated. This compound undergoes a first-order ferroelectric-to-paraelectric transition, involving an unusual change in the "sense" of octahedral rotation while the octahedral tilt remains unchanged. Our experimental and first-principles study shows that the paraelectric polymorph competes with the polar phase and emerges from a trilinear coupling of rotation and tilt modes interacting with an antipolar mode. This form of hybrid improper "antiferroelectricity" is recently predicted theoretically but has remained undetected. This work establishes the importance of understanding anharmonic interactions among lattice degrees of freedom, which is important for the discovery of new ferroelectrics and likely to influence the design of next-generation thermoelectrics.

In synthesizing mixed anion oxides, direct syntheses have often been employed, usually involving high temperature and occasionally high pressure. Compared with these methods, here we show how the use of a titanium perovskite oxyhydride (BaTiO2.5H0.5) as a starting material enables new multistep low temperature topochemical routes to access mixed anion compounds. Similar to labile ligands in inorganic complexes, the lability of H(-) provides the necessary reactivity for syntheses, leading to reactions and products previously difficult to obtain. For example, BaTiO2.5N0.2 can be prepared with the otherwise inert N2 gas at 400-600 °C, in marked contrast with currently available oxynitride synthetic routes. F(-)/H(-) exchange can also be accomplished at 150 °C, yielding the oxyhydride-fluoride BaTi(O, H, F)3. For BaTiO2.4D0.3F0.3, we find evidence that further anionic exchange with OD(-) yields BaTiO2.4(D(-))0.26(OD(-))0.34, which implies stable coexistence of H(+) and H(-) at ambient conditions. Such an arrangement is thermodynamically unstable and would be difficult to realize otherwise. These results show that the labile nature of hydride imparts reactivity to oxide hosts, enabling it to participate in new multistep reactions and form new materials.

We present an x-ray-diffraction study performed in the different states of the molecular glass-forming liquid triphenyl phosphite ͑TPP͒. Investigations carried out in the apparently amorphous state, the so-called ''glacial'' state, formed from isothermal aging at different temperatures in the range ͓210 K, 222 K͔, confirm previous findings about the structural description of the glacial state in terms of nanocrystallized domains. The very close relationship between the diffraction patterns of the glacial and crystalline states reveals that the glaciation corresponds to an aborted crystallization process. By fitting the whole diffraction pattern of the crystalline phase, all the Bragg peaks can be indexed in a monoclinic unit cell. From this indexation, a structural organization of glacial TPP is given and the domain size can be evaluated for different aging temperatures.

Hybrid improper ferroelectricity, which utilizes nonpolar but ubiquitous rotational/tilting distortions to create polarization, offers an attractive route to the discovery of new ferroelectric and multiferroic materials because its activity derives from geometric rather than electronic origins. Design approaches based on group theory and first principles can be utilized to explore the crystal symmetries of ferroelectric ground states, but in general do not make accurate predictions for some important parameters of ferroelectrics, such as Curie temperature (T C). Here, we establish a predictive and quantitative relationship between T C and the Goldschmidt tolerance factor, t, by employing n = 2 Ruddlesden-Popper (RP) A 3 B 2 O 7 as a prototypical example of hybrid improper ferroelectrics. The focus is placed on an RP system, (Sr 1−x Ca x) 3 Sn 2 O 7 (x = 0, 0.1, and 0.2), which allows for the investigation of the purely geometric (ionic-size) effect on ferroelectric transitions, due to the absence of the second-order Jahn-Teller active (d 0 and 6s 2) cations that often lead to ferroelectric distortions through electronic mechanisms. We observe a ferroelectric-to-paraelectric transition with T C = 410 K for Sr 3 Sn 2 O 7. We also find that the T C increases linearly up to 800 K with increasing the Ca 2+ content, i.e., with decreasing the value of t. Remarkably, this linear relationship is applicable to the suite of all known A 3 B 2 O 7 ferroelectrics, indicating that T C correlates with the simple crystal-chemistry descriptor, t, based on the ionic-size mismatch. This study provides a predictive guideline for estimating T C of a given material, which would complement the grouptheoretical and first-principles design approach. Additional ND and SXRD analyses, first-principles calculation results, and Mössbauer spectroscopy (PDF).

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A thorough crystal structure determination at very low temperature of (CuCl)LaNb₂O₇, originally proposed as a spin-1/2 square-lattice antiferromagnet, is reported thanks to the use of single-crystal x-ray diffraction and powder neutron diffraction. State-of-the-art calculations (maximum entropy method) reveal that (CuCl)LaNb₂O₇ is orthorhombic with Pbam symmetry. First-principles calculations demonstrate that the dominant magnetic interactions are antiferromagnetic between fourth nearest neighbors with a Cu-Cl-Cl-Cu exchange path, which lead to the formation of spin singlets. The two strongest interactions between the singlets are ferromagnetic, which makes (CuCl)LaNb₂O₇ the first system of ferromagnetically coupled Shastry-Sutherland quantum spin singlets.