Pressure-induced Mott transition in transition-metal iodides (invited)

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Abstract
Many of the transition‐metal (TM) compounds, because of exchange and correlation interactions within the narrow and poorly overlapping d bands, become antiferromagnetic insulators, the Mott insulators (MI). The properties of the MI and their gradual transition into the noncorrelated metallic state (the Mott transition) are of crucial importance for the elucidation of high‐temperature superconducting materials features in particular and to magnetism in general. The transition of the MI into a metal can be achieved either by doping or by high pressure. The first method is definitely inappropriate for studying the nature of the Mott transition; for the narrow‐band materials the electronic and structural disorder inherent in doping has a strongly perturbing effect. To yield the definitive data on the Mott transition, high‐pressure work on well‐characterized materials should be sought. The present studies provide for the first time extensive information on the Mott–Hubbard gap closure induced by high pressure. High‐pressure studies using diamond anvil cells were conducted with several (TM)I2 compounds. They all have layered structures and order antiferromagnetically at ambient pressure. 129I Mössbauer spectroscopy (MS) was used to study the properties of the (TM)2+ sublattice magnetization as a function of pressure and temperature, and x‐ray diffraction was used to look for possible crystallographic transitions and to obtain the equation of state. Results show that the high‐pressure transition at Pc from a magnetic to a nonmagnetic state is not accompanied by crystallographic changes. Previous studies [M. P. Pasternak et al., Phys. Rev. Lett. 65, 790 (1990)] with NiI2 have confirmed the presence of a metallic state at P ≳ Pc. Inherent to the pressure behavior of the magnetic state is the gradual increase of TN in all cases and a slight increase in the TM2+ moments with pressure increase. The collapse of the magnetic state is abrupt for some cases (NiI2) and gradual for others (CoI2), indicative of different band‐overlap mechanisms.