Order Parameter and Phase Transitions of Stressed SrTiO3

EPR spectra of $\mathrm{F}\mathrm{e}-V_{\mathrm{O}}$ pairs were used to study how the order parameter of SrTi${\mathrm{O}}_3$ varies with uniaxial stress applied to a (111) face. A second-order cubic-trigonal phase boundary appears above the stress-free transition temperature $T_a$. A first-order tetra-gonal-trigonal phase boundary is found below $T_a$. An independently determined Landau potential describes the results.