Metastable Materials Accessed under Moderate Pressure Conditions (P ≤ 3.5 GPa) in a Piston-Cylinder Press.

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 with Ni, R = rare earths); double perovskites such as RCuMnO with Jahn-Teller Cu ions at A sites, pyrochlores derived from TlMnO with colossal magnetoresistance, pnictide skutterudites MCoSb (M = La, Yb, Ce, Sr, K) with thermoelectric properties, or metal hydrides MgMH (M = Fe, Co, Ni) and AMgH (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.