Hydrostatic and Anisotropic Pressure Responses in Tetragonal Zirconia Nanoparticles

Tetragonal zirconia is a metastable phase that is typically hidden at room temperature but becomes observable when the particle size is reduced to the nanoscale. Unlike typical phase transition materials, zirconia exhibits an unusual thermodynamic relationship in which the metastable tetragonal phase possesses a smaller volume than the thermodynamically stable monoclinic phase. In this study, we investigate the pressure‐induced phase transition behavior of tetragonal zirconia nanoparticles under hydrostatic and anisotropic pressure conditions. Tetragonal zirconia nanoparticles were synthesized and subjected to anisotropic and hydrostatic pressure. X‐ray diffraction measurements reveal that the tetragonal phase remains largely stable even under hydrostatic pressure up to ~1000 MPa. Conversely, anisotropic pressure readily induces a tetragonal‐to‐monoclinic transformation. Under strong anisotropic pressure, approximately half of the phase transforms to the monoclinic phase at pressures as low as ~50 MPa, indicating a high sensitivity to nonhydrostatic stress components. This pressure‐response behavior is interpreted by considering the isotropic stress generated under hydrostatic pressure and the local strain arising from the inhomogeneous stress distribution under anisotropic loading. These findings indicate that tetragonal zirconia nanoparticles can serve as sensitive probes for detecting nonhydrostatic pressure components in the MPa range, thus providing an approach for evaluating pressure anisotropy under practical pressure environments.