Pressure-Driven Reversible Phase Transitions and Thermoelastic Behavior in Cubic Mn 0.7 Zn 0.3 O Alloys

Metastable oxide phases synthesized under high pressure offer unique opportunities to tailor material properties, yet their reversible switching behavior and thermoelastic response under extreme conditions remain poorly understood. Here, we report the synthesis of single-phase cubic rock-salt Mn0.7Zn0.3O alloys under a high pressure and temperature (6.5 GPa and 1100 K, respectively) and demonstrate their fully reversible transition to a hexagonal wurtzite structure below ∼2.8 GPa across a broad temperature range (300–1100 K). This transition is accompanied by a pronounced volume collapse of ∼17% at room temperature, indicative of a strongly first-order transformation. Through in situ synchrotron X-ray diffraction under simultaneous high-pressure and high-temperature conditions, we present a complete thermoelastic analysis of the cubic phase. Fitting the pressure–volume–temperature data to a third-order high-temperature Birch–Mürnaghan equation of state yields a bulk modulus of 191.2 ± 3.0 GPa, its temperature derivative ∂B/∂T = −0.074 ± 0.012 GPa/K, and evidence of anisotropic lattice behavior. These results advance the understanding of phase stability and thermoelastic systematics in Mn–Zn–O solid solutions and establish a design platform for reversible phase-transition oxides that can be applied in extreme environments.