Stepwise shock compression with relation to phase diagram of C$_{70}$ fullerite*

Phase transitions of C$_{70}$ fullerite with different initial phase compositions under stepwise shock compression are experimentally studied to pressure of 52 GPa and temperature of about 1700 K. The crystalline phase of fullerite C$_{70}$ with a hexagonal close-packed structure remain practically unchanged under stepwise shock loading up to pressure of 8 GPa. Shock-induced transformation of the hexagonal phase into the face centered cubic phase is observed at pressures in the range 9 to 23.5 GPa. The amount of transformed material increases with the shock intensity. Upon further increase of the shock pressure, the destruction of C$_{70}$ molecules occurs. This destruction is accompanied by a formation of the graphite phase of carbon. Pressure--temperature history of C$_{70}$ specimens is estimated with the use of equation of state C$_{60}$ fullerite [K.V. Khishchenko et al. Diamond and Relat. Mat. 2007 (16) 1204]. Analyzing this pressure-temperature history along with the tentative phase diagram of C$_{70}$ [B. Sundqvist. Advances in Phys. 1999 (48) 1], we conclude that the observed conversion of the hexagonal to the cubic phase can not be described in the framework of this diagram.