Metastability of Corundum‐Type In₂O₃

Abstract

The description of structural relations between bixbyite‐ and corundum‐type structures is of particular interest because of the common occurrence of both structures. One of the representative examples of the bixbyite to corundum transition is the high‐pressure high‐temperature synthesis of the corundum‐type indium oxide. The wet chemistry synthesis and stabilisation of the corundum‐type In2O3 under ambient pressure conditions calls for a re‐interpretation of the InO phase diagram as well as for the clarification of the phase transitions in In2O3. One of the questions to be clarified is the stability of the corundum‐type In2O3. In the present work we studied the stability of the corundum‐type In2O3 both theoretically (by density‐functional calculations) and experimentally. The synthesis of the corundum‐type In2O3 was performed by the modified non‐alkoxide sol–gel method based on the ammonia‐induced hydrolysis of indium nitrate in methanol. The corundum‐type In2O3 was subjected to thermal analysis (STA) as well as to structural studies, that is, it was examined using X‐ray powder diffraction (XRPD) including in situ XRPD characterisation upon thermal treatment. For the first time we have undoubtedly demonstrated, both theoretically and experimentally, the metastability of the corundum‐type In2O3 polymorph. The In2O3 polymorph appears to be metastable throughout the entire enthalpy–pressure phase diagram. Upon heating, corundum‐type In2O3 transforms irreversibly into cubic bixbyite‐type In2O3 as shown by STA as well as in situ heating XRPD experiments. Computations indicate the existence of another high‐pressure modification of In2O3 with orthorhombic structure, iso‐typic to Rh2O3‐II. We predict this new phase to form at pressures exceeding 15 GPa from both the cubic bixbyite‐type and the corundum‐type modification of In2O3.

Pbca-Type In2O3: The High-Pressure Post-Corundum phase at Room Temperature.

“…Above 46 GPa the phase transition is completed and the XRD pattern fully corresponds to o1-In 2 O 3 (see XRD pattern at 50.1 GPa in Fig. 3) in good agreement with some previous works [25][26][27][28][29] but in disagreement with other works. 32,33 Note that at 50.1 GPa there is a large experimental broadening of the XRD peaks, likely due to the poor crystalline quality of the Pbcn-type phase at high pressures and RT.…”

“…Instead, two new high-pressure phases were discovered: the orthorhombic Rh 2 O 3 -II (SG Pbcn, No. 60, Z=4), and the orthorhombic α-Gd 2 S 3 phase [25][26][27][28][29] (SG Pnma, No. 62, Z=4).…”

“…62, Z=4). This last phase was initially unexpected and theoretical calculations showed that it was predicted to be the more stable phase 26 Recently, several attempts have been performed in order to obtain in a metastable way o1-In 2 O 3 at room conditions by means of high pressure and high temperature treatments, [27][28][29] and a theoretical work has recently commented on the possible applications of o1-In 2 O 3 . 30 High temperatures have been systematically used in these experimental high-pressure studies in order to induce phase transitions delayed or even impeded in In 2 O 3 at room temperature (RT) due to the presence of kinetic effects.…”

“…30 High temperatures have been systematically used in these experimental high-pressure studies in order to induce phase transitions delayed or even impeded in In 2 O 3 at room temperature (RT) due to the presence of kinetic effects. [25][26][27][28][29]31 In the last years high-pressure studies of In 2 O 3 at RT have also been reported yielding some contradictory results. [32][33][34] A phase transition from c-In 2 O 3 to rh-In 2 O 3 between 12 and 25 GPa in bulk and nanocrystalline In 2 O 3 was reported, 32,33 whereas no sign of phase transition up to 30 GPa neither in bulk nor in nanocrystalline c-In 2 O 3 was found in the latest work 34 in good agreement with previous results.…”

“…The disagreement between experiments and calculations can be explained by the existence of a high kinetic energy barrier for the transformation from c-In 2 O 3 to o1-In 2 O 3 as already commented by several authors. [25][26][27][28][29] These high kinetic energy barriers could be studied in future experiments combining pressure with a moderate heating to help the sample to overcome the kinetic barriers in the structural transition. In order to look for candidate structures of the new phase discovered on downstroke rh-In 2 O 3 and o1-In 2 O 3 , we resorted to ab initio gested to occur in ABO 3 compounds, including sesquioxides ( intermediate between the R-3c and the Pbcn phases, like the monoclinic P2/c, which was suggested to c diffusionless pathway between both R3 petitive phase with the cubic Ia-3, rhombohedral R3 ture, which follows an enthalpy vs. pressure curve that matches with that of the R rhombic Pbca phase, which is isostructural to Rh…”