High-pressure behaviour of crystalline silane compared with that for SnH4

[J.S. Tse, Y. Yao, and K. Tanaka, Phys. Rev. Lett. 98 (2007), p. 117004.] predict a high-pressure metallic phase of SnH4 with a layered structure of Sn intercalated by H2 units. Following some suggestions concerning H2 sandwiched between two parallel graphene sheets, we consider the Tse et al.'s prediction on SnH4 in relation to results of [C.J. Pickard and R.J. Needs, Phys. Rev. Lett. 97 (2006), p. 045504] on high pressure phases of crystalline silane. As a result, we propose that the separation of the protons from the Si atoms in silane under pressure could occur in two stages. First, a single H2 molecule is removed, leaving a layer of stoichiometric composition SiH2, and the second step leaves Si layers bridged by H2 molecules. We believe high-pressure experiments on SnH4 below 70 GPa would be of interest, to ascertain whether SnH2 chains or layers along with H2 form a lower pressure phase below that proposed by Tse et al.