First principles lattice dynamics study of SnO2 polymorphs

The structural properties of SnO2 polymorphs in the sequential order of observed phases in experiments are determined by the density functional theory (DFT) calculations based on local density approximation (LDA) of ultra soft pseudo potentials (US-PPs). Phonon dispersion relations are calculated by the lattice dynamics calculations. Shifts in the infrared (IR) active optical modes due to polarization (LO/TO splitting) are also calculated. Moreover, softening of B1g mode at the rutile-CaCl2 second-order ferroelastic phase transition is confirmed. Thermal properties, such as temperature behavior of bulk modulus and thermal expansion in the rutile phase are obtained by employing quasiharmonic approximation (QHA). They are in good agreement with the available experimental results. Dynamic stabilities of SnO2 polymorphs except for the rutile phase are checked for the first time by using phonon dispersions. The rutile, CaCl2, pyrite, ZrO2 and cotunnite type structures have shown thermodynamical stability. The cause of α-PbO2 phase showing nearly stability is discussed in the light of experimental studies. However, the fluorite type structure is definitely instable even at different pressures. It may not be one of SnO2 polymorphs.