Pressure-Induced Destabilization and Anomalous Lattice Distortion in TcO 2 .

Tc-based oxides are of interest because of their complex crystalline structures. In this work, the phonon dispersions, lattice distortions, and elastic constants of TcO2 at external pressures up to 120 GPa were comprehensively studied using first-principles calculations. It is found that the lattice dynamic stability of TcO2 can be assessed by fitting the Γ-Z acoustical phonon branch. The applied external pressure can be divided into three ranges: the low-pressure stable range, the middle-pressure buckling range, and the high-pressure unstable range. Interestingly, the variation tendency of the low-pressure stable range is very close to that of the high-pressure unstable range. On the other hand, the TcO2 lattice responds intensely to external pressure in the middle-pressure buckling range, which can be sustained under about 71 GPa pressure. More importantly, we have unraveled the pressure-induced lattice distortion in TcO2 , which leads to anomalous behaviors for the lattice constants, Tc-O bond lengths, and elastic constants at 10 and 20 GPa external pressures.