Orthorhombic In2O3: A Metastable Polymorph of Indium Sesquioxide

Structural, Electronic Properties, and Relative Stability Studies of Low-Energy Indium Oxide Polytypes Using First-Principles Calculations

Materials made of indium oxide (In₂O₃) are now being used as a potential component of the next generation of computers and communication devices. Density functional theory is used to analyze the physical, electrical, and thermodynamical features of 12 low-energy bulk In₂O₃ polytypes. The cubic structure In₂O₃ is majorly used for many of the In₂O₃-based transparent conducting oxides. The objective of this study is to explore other new stable In₂O₃ polytypes that may exist. The structural properties and stability studies are performed using the Vienna ab initio simulation package code. All the In₂O₃ polytypes have semiconductive properties, according to electronic band structure investigations. The full elastic tensors and elastic moduli of all polytypes at 0 K are computed. Poisson's and Pugh's ratio confirms that all stable polytypes are ductile. The phonon and thermal properties including heat capacity are obtained for mechanically stable polytypes. For the first time, we report the Raman and infrared active modes of stable polytypes.

A Room-Temperature Verwey-type Transition in Iron Oxide, Fe5 O6

Functional oxides whose physicochemical properties may be reversibly changed at standard conditions are potential candidates for the use in next‐generation nanoelectronic devices. To date, vanadium dioxide (VO₂) is the only known simple transition‐metal oxide that demonstrates a near‐room‐temperature metal–insulator transition that may be used in such appliances. In this work, we synthesized and investigated the crystals of a novel mixed‐valent iron oxide with an unconventional Fe₅O₆ stoichiometry. Near 275 K, Fe₅O₆ undergoes a Verwey‐type charge‐ordering transition that is concurrent with a dimerization in the iron chains and a following formation of new Fe−Fe chemical bonds. This unique feature highlights Fe₅O₆ as a promising candidate for the use in innovative applications. We established that the minimal Fe−Fe distance in the octahedral chains is a key parameter that determines the type and temperature of charge ordering. This model provides new insights into charge‐ordering phenomena in transition‐metal oxides in general.

Pressure-Induced Phase Transitions in Sesquioxides

Pressure is an important thermodynamic parameter, allowing the increase of matter density by reducing interatomic distances that result in a change of interatomic interactions. In this context, the long range in which pressure can be changed (over six orders of magnitude with respect to room pressure) may induce structural changes at a much larger extent than those found by changing temperature or chemical composition. In this article, we review the pressure-induced phase transitions of most sesquioxides, i.e., A2O3 compounds. Sesquioxides constitute a big subfamily of ABO3 compounds, due to their large diversity of chemical compositions. They are very important for Earth and Materials Sciences, thanks to their presence in our planet’s crust and mantle, and their wide variety of technological applications. Recent discoveries, hot spots, controversial questions, and future directions of research are highlighted.

Scaled-up solvothermal synthesis of nanosized metastable indium oxyhydroxide (InOOH) and corundum-type rhombohedral indium oxide (rh-In2O3)

Phase pure metastable indium oxyhydroxide (InOOH) with crystallite size in the range ca. 2–7 nm is synthesized by a nonaqueous solvothermal synthesis route in ethanol. The influence of synthesis parameters such as temperature, basicity (pH), synthesis time, and water content is carefully addressed. T-pH maps summarize the impact of synthesis temperature and pH and reveal that phase pure InOOH is obtained in water-free solutions at mild temperatures (150–180°C) in highly basic conditions (pH>12). Subsequent calcination of InOOH at 375–700°C in ambient air atmosphere results in metastable nanoscaled rhombohedral indium oxide (rh-In2O3). The synthesis protocol for phase pure nanocrystalline InOOH material was successfully upscaled allowing for obtaining ca. 3 g of phase-pure InOOH with a yield of ca. 78%. The upscaled InOOH and rh-In2O3 batches are now available for a detailed in-situ characterization of the mechanism of decomposition of InOOH to rh-In2O3 to c-In2O3 as well as for the characterization of the functional properties of InOOH and rh-In2O3 materials.

Surface chemistry and stability of metastable corundum-type In2O3

Highly correlated surface- and electrochemical characterization is linked to the metastability of rh-In₂O₃ for explanation of sensing and catalytic properties.

Effects of microstructures on the compression behavior and phase transition routine of In2O3 nanocubes under high pressures

In addition to size and morphology, the microstructure may play an important role and induce differences in the compression and phase transition behaviors of nanomaterials under high pressures.

Anomalous compression and new high-pressure phases of vanadium sesquioxide, V2O3

We report results of a powder x-ray diffraction (XRD) study of vanadium sesquioxide, V2O3, under pressurization in a neon pressure-transmitting medium up to 57 GPa. We have established a bulk modulus value for corundum-type V2O3 of B0 = 150 GPa at B' = 4. This bulk modulus value is the lowest among those known for the corundum-type-structured oxides, e.g. Al2O3, α-Fe2O3, Cr2O3, Ti2O3, and α-Ga2O3. We have proposed that this might be related to the difference in the electronic band structures: at room temperature V2O3 is metallic, but the above corundum-structured sesquioxides are semiconducting or insulating. Around ∼21-27 and ∼50 GPa we registered changes in the XRD patterns that might be addressed to phase transitions. These transitions were sluggish upon room-temperature compression, and hence we additionally facilitated them by the laser heating of one sample. We have refined the XRD patterns of only the first high-pressure phase in an orthorhombic lattice of a Rh2O3(II)-type. Our findings significantly extend the knowledge of the P-T phase diagram of V2O3 and advance the understanding of its properties. We speculate that the elastic properties of V2O3 can be closely linked to its electronic band structure and, consequently, we propose that slightly doped V2O3 (e.g. with Cr) could be a potential candidate for systems in which the bulk modulus value may be remarkably switched by moderate pressure or temperature.