Specific electronic properties of metallic molecules MX, correlated to piezoelectric properties of solids MX

Highly correlated ab initio approaches are used here to show that, already at the molecular level, all the diatomic MX compounds, involving a di- or trivalent metal and parents of the systems presenting piezoelectric effect in the solid phase, exhibit with their eight valence electrons, electronic characteristics which can be considered as precursors of the piezoelectric effect. Piezoelectric effects are generally observed in ionic semiconductor materials involving a metal of Group IIa, IIb or III, and a non-metal of Group V or VI: typical examples are ZnO and AlN. The diatomic MX systems investigated in the present paper (ZnO, ZnS, CdO, CdS, HgO, HgS, BeO, BeS, MgO, MgS, BN, AlN and AlP) possess two low lying, very close electronic states, a 1Σ+ and a 3Π states or a 3Π and a 3Σ− states, with different electric properties and just slightly different equilibrium geometry. Consequently, the ground state of the molecule changes abruptly with a small modification of the geometry, and this results in a large change of the charge distribution.