Segregation into Layers: A General Problem for Structural Instability under Pressure, Exemplified by SnH4

When a molecular compound is thermodynamically unstable (but kinetically persistent) with respect to the elements, structures that contain segregated layers of the elements may be favored at moderate pressures, as a compromise between the potential stability of novel electronic configurations and decomposition into the elements (or other stable compounds). We use stannane, SnH4, to approach this quite general problem theoretically, since the heat of formation of SnH4 is so positive. Our ground-state DFT searches for optimal structures begin with slabs formed from 1-4 layers of tin atoms in the beta-Sn and bcc configurations, and also slabs of molecular hydrogen or hydrogen atoms, preserving the overall SnH4 stoichiometry. As argued, segregated layers are an important structural feature in the lower- and moderate-pressure regime (0 and 50 GPa). By 140 GPa (V/V-0 = 0.21) the coordination of tin and hydrogen increases and the slabs disappear, as judged from the optimized structures.