Intriguing sequence of GaFeO3structures and electronic states to 70 GPa

Abstract

Structural studies of the ferrimagnetic (T N =200 K) Mott insulator GaFeO 3 (SG Pc2 1 n) to 70 GPa, complemented by 57 Fe Mössbauer spectroscopy and resistance (R) measurements at compression, decompression and recompression reveal a fascinating sequence of structures. Starting at ~ 25 GPa a new structure, an orthorhombic perovskite (SG Pbnm), is sluggishly formed followed by a volume V(P) drop of 5.4%. The complete formation of the perovskite occurs at 42 GPa. In the 0 -33 GPa range T 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 ~3%. Mössbauer spectra reveal a non-magnetic component coexisting with the magnetic one at ~ 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) perovskite are consistent with a Mott transition.With pressure decrease, below 50 GPa, the insulating perovskite is recovered, and at ~24 GPa a 1 storder 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 perovskite structure. It is noteworthy that this transition occurs at the same

Structural, magnetic and vibrational properties of multiferroic GaFeO3 at high pressure

“…In the pressure range 0 -21.3 GPa they correspond to orthorhombic crystal structure of Pc2 1 n symmetry (Fig. 2) Pbnm -phase reaches 11.5% at 30.5 GPa and the gradual character of the structural transition is consistent with findings by Arielly et al [20].…”

“…Calculated lattice parameters at ambient conditions are in agreement with that one found in previous studies [13,14,18,20 density functional theory within the local density approximation (LDA) [18] and somewhat less than the value of B 0 = 230(4) GPa, found for stoichiometric GaFeO 3 in [20]. The value of the bulk modulus pressure derivative was found to be B' = 4.0(5).…”

“…The calculated lattice parameters of the Pbnm phase at the highest pressure value achieved in the X-ray diffraction experiments P = 30.5 GPa, a = 4.900(9), b = 5.070(9), c = 7.000(15) Å, are comparable with those obtained at similar pressures in [20].…”

“…Recent high-pressure measurements [20] [20]. The observed relationship between electronic and magnetic properties of the high pressure phase of GaFeO 3 is consistent with the Mott transition driven by a band-width broadening mechanism [21].…”

The Mott insulators at extreme conditions; structural consequences of pressure-induced electronic transitions

“…However, later studies performed on a series of Fecontaining compounds show that the MT is usually accompanied by an appreciable reduction of interatomic distances and crystal volume. Such a transition has been evidently observed in hematite (Fe 2 O 3 ) [17,18], and recently in GaFeO 3 [19]. The studies in GaFeO 3 revealed that the structural instability of the low pressure partially low-coordination Pc2 1 n lattice beyond 25 GPa results in a sturdier perovskite Pbnm structure (Pv), which undergoes at +53 GPa a reversible isostructural transition, characterized by a discontinuous volume V(P) drop of +3% (Fig.…”