Dynamical stability of the hardest known oxide and the cubic solar material:: TiO2

The authors have studied dynamical stability of different polymorphs of TiO2 using ab initio phonon calculations based on density functional theory in conjunction with force-constant method. Rutile TiO2 was found stable at ambient pressure, but unstable at high pressure. The calculated Raman frequency and phonon density of states (PDOS) of rutile TiO2 are in a good agreement with experiment. Concerning two cubic phases (solar materials), fluorite stabilized under pressure, whereas pyrite showed instability throughout the whole pressure range. Furthermore, the PDOS of cotunnite (the hardest known oxide) phase confirmed that it exists at high pressure and can be quenched down to a low pressure limit. (c) 2007 American Institute of Physics.