High-pressure behavior of structural, optical, and electronic transport properties of the golden Th2S3-type Ti2O3

Abstract

Recently, a golden colored, dense polymorph of titanium sesquioxide, Ti${}_{2}$O${}_{3}$ with a Th${}_{2}$S${}_{3}$-type structure, has been synthesized at high-pressure high-temperature conditions. In this paper, we present results of investigations of structural, optical, and electronic transport properties of this unusual golden polymorph of Ti${}_{2}$O${}_{3}$ under high pressure. Several experimental techniques were used, including x-ray diffraction studies using synchrotron radiation, Raman spectroscopy, electrical resistivity, and thermoelectric power. The structural studies showed that the Th${}_{2}$S${}_{3}$-type lattice is conserved under pressure, while it is subjected to an isostructural phase transition with a \ensuremath{\sim}0.7$%$ volume drop at 38.5 GPa. We speculated that this transition could be driven by the $s\ensuremath{\rightarrow}d$ electron transfer in the Ti atoms. For the Th${}_{2}$S${}_{3}$-type Ti${}_{2}$O${}_{3}$, we have established a bulk modulus value, ${B}_{0}$ $=$ 258.3 GPa at ${B}_{0}^{\ensuremath{'}}$ $=$ 4.1. A full profile analysis of the diffraction patterns allowed us to discover anomalies in the compression behavior of the Th${}_{2}$S${}_{3}$-type structure. The bond valence sums method suggested that at ambient conditions the Ti cations have predominantly Ti${}^{3+}$ oxidation state, but applied pressure stimulates a partial charge disproportionation between the Ti1 and Ti2 sites achieving the maximal effect---reduction of the Ti1 cations to \ensuremath{\sim}Ti${}^{2.5+}$ and oxidation of the Ti2 ones to \ensuremath{\sim}Ti${}^{3.5+}$ near 14 GPa. Pressure evolution of Raman spectra across the above crossovers showed distinct changes corroborating the above findings. The high-pressure electronic transport studies confirmed that the Th${}_{2}$S${}_{3}$-type Ti${}_{2}$O${}_{3}$ remains semiconducting up to 21 GPa at ambient and low temperatures down to 4.2 K. These studies found additional features, e.g., in the activation energy curve near 7 GPa, that is accompanied by inversion of the dominant conductivity type from electron to hole. The intriguing high-pressure behavior of Ti${}_{2}$O${}_{3}$ with the Th${}_{2}$S${}_{3}$-type structure can contribute to better understanding of high-pressure properties of transition-metal sesquioxides.