Valence and magnetic instabilities in Sm compounds at high pressures

We report on the study of the response to high pressures of the electronic and magnetic properties of several Sm-based compounds, which span at ambient pressure the whole range of stable charge states between the divalent and the trivalent. Our nuclear forward scattering of synchrotron radiation and specific heat investigations show that in both golden SmS and SmB₆ the pressure induced insulator to metal transitions (at 2 and ∼4-7 GPa, respectively) are associated with the onset of long-range magnetic order, stable up to at least 19 and 26 GPa, respectively. This long-range magnetic order, which is characteristic of Sm³⁺, appears already for a Sm valence near 2.7. In contrast to these compounds, metallic Sm, which is trivalent at ambient pressure, undergoes a series of pressure induced structural phase transitions which are associated with a progressive decrease of the ordered 4f moment.