Intricate relationship between pressure-induced electronic and structural transformations in FeCr 2 S 4

Electrical-transport, magnetic and structural properties of the ferrimagnetic semiconductor FeCr2S4 (T-N = 170 K) have been studied by electrical resistance, R(P, T), Fe-57 Mossbauer spectroscopy (MS), and synchrotron x-ray diffraction to 20 GPa using diamond anvil cells. It was found that the local maximum, R-max(P) on the R(T) curve, corresponding to the colossal magnetoresistance effect, is substantially reduced and broadened with pressure increase accompanied by a shift to higher temperatures and finally disappears at similar to 7 GPa, the highest pressure of the single, high-spin spinel phase designated as LP1. Suppression of R-max(P) precedes a gap closure leading to metallization at similar to 7 GPa. The 7-10 GPa range is a coexistence pressure zone composed of three phases: (i) LP1, a paramagnetic spinel (SG Fd3m); (ii) LP2, a nonmagnetic isostructural spinel; and (iii) HP1, a high-spin Cr3S4 (SG I2/m) type structure. Based on MS and R(P, T) studies it was concluded that the Mott transition is responsible for the onset of metallization (correlation breakdown) coinciding with the collapse of Fe2+ moments. The shortening of the Fe-O bond length due to the electronic transition leads to a volume decrease of the low pressure (LP) phase by similar to 1%. This electronic transition initiates a structural instability of the spinel structure resulting in a first-order phase transition into HP1, a post-spinel with Cr3S4-like structure. The onset of HP1 is accompanied by the Fe2+ 4 -> 6 coordination number increase resulting in an additional similar to 12% volume reduction. In the coexistence zone the post-spinel phase is paramagnetic, but at P > 10 GPa an isostructural transition takes place and Fe2+ becomes nonmagnetic, as evidenced from the large drop of the isomer shift and of the quadrupole splitting. The structural transition is irreversible with the isothermal pressure decrease, and the Cr3S4-like structure remains upon full release of pressure at 300 K. Interestingly at decompression the high pressure (HP) phase undergoes a reverse noncorrelated -> correlated transition recovering its localization features, e. g., insulating state and paramagnetism with T-N <= 6 K. The original room temperature (RT), LP spinel phase is finally recovered following heat treatment at 400 degrees C.