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

Electrical-transport, magnetic and structural properties of the ferrimagnetic semiconductor FeCr ₂S ₄ (T _N = 170K) have been studied by electrical resistance, R(P, T), 57Fe Mössbauer 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 ∼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 ∼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 Cr ₃S ₄ (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 ∼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 Cr ₃S ₄-like structure. The onset of HP1 is accompanied by the Fe2 ⁺ 4 → 6 coordination number increase resulting in an additional ∼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 Cr ₃S ₄-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°C.