Intriguing sequence of GaFeO3structures and electronic states to 70 GPa

Abstract

Structural studies of the ferrimagnetic (${T}_{\mathrm{N}}$ = 200 K) Mott insulator GaFeO${}_{3}$ (SG Pc2${}_{1}n$) to 70 GPa, complemented by ${}^{57}$Fe M\"ossbauer spectroscopy and resistance (R) measurements at compression, decompression, and recompression, reveal a fascinating sequence of structures. Starting at \ensuremath{\sim}25 GPa a new structure, an orthorhombic perovskite (Pv) (SG Pbnm), is sluggishly formed followed by a volume V(P) drop of 5.4%. The complete formation of the Pv occurs at 42 GPa. In the 0--33 GPa range ${T}_{\mathrm{N}}$ reaches 300 K and R(P) decreases by one order of magnitude. At 53 GPa an isostructural transition is detected, characterized by a discontinuous drop of V(P) by \ensuremath{\sim}3%. M\"ossbauer spectra (MS) reveal a nonmagnetic component coexisting with the magnetic one at \ensuremath{\sim}60 GPa. Its abundance increases and above 77 GPa no sign of a magnetic hyperfine interaction is detected down to 5 K. Concurrently, one observes a continuous yet precipitous decrease in R(P) taking place in the 53--68 GPa range, leading to an onset of the metallic state at P = 68 GPa. These electronic/magnetic features of the high pressure (HP) Pv are consistent with a Mott transition. With pressure decrease below 50 GPa, the insulating Pv is recovered, and at \ensuremath{\sim}24 GPa a 1st-order structural transition takes place to a LiNbO${}_{3}$-type structure with SG R3c. This structure remains stable down to ambient pressure and with recompression it is stable up to 50 GPa, afterwards it transforms back to the HP Pv structure. It is noteworthy that this transition occurs at the same pressure, regardless of the preceding structures: Pbnm or R3c. The results are compared with hematite (Fe${}_{2}$O${}_{3}$, SG $R\overline{3}c$) and other ferric oxides. The mechanisms of the transitions are discussed.