Efficient full relaxation of crystal structures with quasiharmonic approximation: Application to pyroelectricity of GaN and ZnO*

We develop an efficient calculation scheme of quasiharmonic approximation (QHA) which enables the simultaneous optimization of all the structural degrees of freedom, i.e., the shape of the unit cell and the internal coordinates. We employ the IFC renormalization [1], which efficiently calculates the harmonic phonon dispersion of updated crystal structures without additional expensive DFT calculations.

Our method removes the constraints on the internal coordinates or the anisotropy of the thermal expansion, which are conventionally imposed to reduce computational cost.

We implement the methodology to the ALAMODE package [2], an open-source package for first-principles phonon calculation.

 

We apply the method to the pyroelectricity of wurtzite materials GaN and ZnO, which shows good agreement with experiments. By comparing the results with conventional methods, we show that simultaneous optimization of the unit cell and the atomic positions is crucial to the quantitative prediction of anisotropy in thermal expansion. Furthermore, we show that relatively small changes in the anisotropic thermal expansion lead to a significant difference in the pyroelectricity.

 

[1] R.Masuki, et al arXiv:2205.08789 (2022)

[2] T. Tadano, et al. J. Phys.: Condens. Matter 26, 225402 (2014)