VO${}_2$ under hydrostatic pressure: Isostructural phase transition close to a critical end-point

Abstract

The high-pressure behavior of monoclinic VO${}_2$ is revisited by a combination of Raman spectroscopy and X-ray diffraction on a single crystal under hydrostatic conditions at room temperature. A soft mode is observed up to P${}_c$ = 13.9(1) GPa. At this pressure, an isostructural phase transition between two monoclinic phases M${}_1$ and M${}_1$' hinders this instability. The features of this transformation (no apparent volume jump) indicate that the compression at ambient temperature passes close to a critical point. An analysis based on the Landau theory of phase transitions gives a complete description of the P-T phase diagram. The M1' is characterized by spontaneous displacements of the oxygen sub-lattice without any strong modification of the VV dimers distances nor the twist angle of vanadium chains. The spontaneous displacements of oxygen and the spontaneous deformations of the ($b_{M1}$, $c_{M1}$) plane follow the same quadratic dependence with pressure and scales with spontaneous shifts of the Raman phonons located at 225, 260 and 310 cm${}^{-1}$. Pressure-induced shifts of the Raman peaks allows for new assignment of several Raman modes. In particular, the A${}_g$(1)+B${}_g$(1) modes at 145 cm${}^{-1}$ are identified as the vanadium displacive phonons. A second transformation in the metallic phase X, which is found triclinic (P$\bar{1}$) is observed starting at 32 GPa, with a wide coexistence region (up to 42 GPa). Upon decompression, phase X transforms, between 20 GPa and 3 GPa, to another phase that is neither the M${}_1$' nor M${}_1$ phase. The structural transitions identified under pressure match with all the previously reported electronic modifications confirming that lattice and electronic degrees of freedom are closely coupled in this correlated material.

arXiv comment12 pages, 12 figures, 2 Tables, submitted to Phys Rev B

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