Heuristic Engineering of Magnetic Mn₄Ta₂O₉ Phases from High-Pressure and High-Temperature Synthesis

Rational design and precise high-pressure and high-temperature (HPHT) synthesis of metastable oxides, such as polar magnets for desired functions, remain a long-standing challenge, which is largely dominated by thermodynamically controlled atomic-scale engineering of the local structure. Although cutting-edge research can reliably predict the synthetic pressure (P) for the target product, theoretical attempts to estimate the reaction temperature (T) remain elusive to date in HPHT synthesis, thus necessitating further strategies to confine the T and avoid costly trial-and-error. Here, Mn4Ta2O9 (MTO) is adopted as a prototype to reveal the correlation between local crystal structure and formation conditions, namely, the synthetic P and T. Three high pressure polar polymorphs of MTO crystallized in Cc, R3, and R3c were synthesized at 5 to…