High pressure-temperature Raman spectroscopy of H$_{2}$-H$_{2}$O clathrate.

The melting curve of the C2 clathrate H$_{2}$-H$_{2}$O has been determined by \textit{in-situ} Raman spectroscopy measurements in an externally heated diamond anvil cell. We have determined the melting curve to a maximum pressure of 27 GPa. These are the first measurements on the melting line in this clathrate. Depending on the stoichiometry of the starting mixture of H$_{2}$ and H$_{2}$O, we are able to study either a mixture of C2 and H$_{2}$O or C2 and H$_{2}$. In either case, we were able to pinpoint the melting of the clathrate from the measurements of the molecular stretching mode (vibron) in the clathrate. In the case of C2 + Ice VII, we observe the vibron in the clathrate at a frequency higher than in pure H$_{2}$ at the same pressure. We have cross-calibrated the melting temperatures using the Stokes-anti Stokes ratio of the diamond first order and Raman active TO phonon of cubic Boron Nitride. We find that the clathrate melts well above the H$_{2}$ melting at all pressures studied indicating that the stabilization of this clathrate at high pressures is indeed due to interactions between the host and guest molecules.