The complex interplay between crystal structure and electron correlation in driving phase transitions in ${\mathrm{VO}}_2$, as a function of parameters such as temperature, pressure, doping, and strain, has long been a central problem in condensed matter science. In this work, we present a temperature-hydrostatic pressure structural phase diagram of the $M4$ phase in 8% Cr-doped ${\mathrm{VO}}_2$, based on temperature-dependent, high-pressure synchrotron powder x-ray diffraction measurements using a diamond anvil cell. In addition to the ambient-pressure, temperature-driven monoclinic $M4$-tetragonal $R$ transition, we observe the $M4\to R\to {\mathrm{CaCl}}_2$-type orthorhombic $O$ phase transition with increasing pressure at room temperature. The high-temperature $R$ phase also transforms into the $O$ phase at higher pressures through a second-order transition, where the spontaneous strain in the orthorhombic structure acts as the order parameter. Under intermediate temperature-pressure conditions, several mixed-phase regions are observed. These structural findings provide a robust framework for developing an understanding of electronic structure and the underlying mechanisms of phase transitions in 8% Cr-doped ${\mathrm{VO}}_2$ within the temperature-pressure phase space.

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• Crystal structure
• High-pressure studies
• Structural phase transition