Effect of high pressure on the vibrational modes and the energy gap of ZnP(2).

The pressure dependence of the vibrational modes in ZnP(2) has been investigated by Raman Spectroscopy using a diamond anvil cell, up to 150 kbar pressure. The intrachain phosphorus modes exhibit a strong pressure dependence whereas the low frequency Zn-P modes soften very slightly under pressure. For a crystal which is treated as a molecular crystal this is an unexpected result. It is suggested that the behavior may be due to a buckling of the phosphorus chain, or due to a double bond promotion between P atoms, or a charge transfer under pressure. The shift in the energy gap has also been measured to 100 kbar hydrostatic pressure. There is a small initial blue shift which gradually changes over to a red shift. However the whole shift in 100 kbar is quite small. Combining the (dEg/dP)(T) with the published (dEg/dT)(P) the thermal expansion contribution and the electron-phonon interaction contribution were evaluated. The latter dominates the total (dEg/dT)(P) of ZnP(2).