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

Abstract

Electronic/magnetic transitions and their struc- tural consequences in Fe-based Mott insulators in a re- gime of very high static density are the main issue of this short review paper. The paper focuses on the above-men- tioned topics based primarily on our previous and on- going experimental HP studies employing: (i) diamond anvil cells, (ii) synchrotron X-ray diffraction, (iii) 57 Fe Mossbauer spectroscopy, (iv) electrical resistance and (v) X-ray absorption spectroscopy. It is shown that applying pressure to such strongly correlated systems leads to a number of changes; including quenching of the orbital moment, quenching of Jahn-Teller distortion, spin cross- over, inter-valence charge transfer, insulator-metal tran- sition, moment collapse and volume collapse. These changes may occur simultaneously or sequentially over a range of pressures. Any of these may be accompanied by or be a consequence of a structural phase transition; namely, a change in crystal symmetry. Analyzing this rich variety of phenomena we show the main scenarios which such strongly correlated systems may undergo on the way to a correlation breakdown (Mott transition). To illustrate these scenarios we present recent results for MFeO3 (M ¼ Fe, Ga, Lu, Eu, Pr) and CaFe2O4 ferric oxi- des; FeCl2 and FeI2 ferrous halides, and FeCr2S4 sulfide. Fe3O4 is given as an example case for the impact of Mossbauer Spectroscopy on High Pressure Crystallogra- phy studies.