Review: Metastable materials accessed under moderate pressure conditions (P <= 3.5 GPa) in a piston-Cylinder press

Abstract

In this review, we describe different families of metastable materials, some of them with relevant technological applications, which can be stabilized at moderate pressures 2-3.5 GPa in a piston-cylinder press. The synthesis of some of these systems had been previously reported under higher hydrostatic pressures (6-10 GPa), but can be accessed under milder conditions in combination with reactive precursors prepared by soft-chemistry techniques. These systems include perovskites with transition metals in unusual oxidation states (e.g., RNiO_(3) with Ni^(3+), R = rare earths); double perovskites such as RCu_(3)Mn_(4)O_(12) with Jahn-Teller Cu^(2+) ions at A sites, pyrochlores derived from Tl_(2)Mn_(2)O_(7) with colossal magnetoresistance, pnictide skutterudites M_(x)Co_(4)Sb_(12) (M = La, Yb, Ce, Sr, K) with thermoelectric properties, or metal hydrides Mg_(2)MH_(x) (M = Fe, Co, Ni) and AM_(g)H_(3) (A: alkali metals) with applications in hydrogen storage. The availability of substantial amounts of sample (0.5-1.5 g) allows a complete characterization of the properties of interest, including magnetic, transport, thermoelectric properties and so on, and the structural characterization by neutron or synchrotron X-ray diffraction techniques.