High pressure studies on silane to 210 GPa at 300 K: optical evidence of an insulator-semiconductor transition

Silane (SiH4) has been studied in a diamond anvil cell from 7-210 GPa by using optical reflection and absorption techniques at 300 K. The reflectivity and transmission measurements showed a dramatic change in the neighbourhood of 100 GPa. On the basis of reflectivity and absorption experimental data, the pressure dependence of the refractive index (n) of solid SiH4 was derived, which was then used to determine the ratio of the molar refraction (R) to the molar volume (V). There is a large jump in the ratio R/V between 92 and 109 GPa. At 109 GPa and 1.6 eV, n*(SiH4) = 3.62 (the real part of refractive index) and R/V(SiH4) = 0.79, which are similar to the values for silicon at one atmosphere at the same energy. The results indicated that an insulator semiconductor phase transition might have occurred in solid SiH4 between 92 and 109 GPa. Comparing values of the real part of n, n*, and the extinction coefficient k* with that of metals, we conclude that SiH4 is not yet a metal at the maximum pressure investigated (210 GPa), suggesting that a higher pressure is needed for its metallization.