FIRST PRINCIPLES CALCULATIONS OF ELASTIC CONSTANTS FOR DEFECTED Na_1/2Bi_1/2TiO₃

ABSTRACT

The dependence of elastic constants on the applied strain for the tetragonal phase of both a defected and an ideal Na_1/2Bi_1/2TiO₃ crystal was calculated from first principles. It was found that the elastic constants show different behavior for compression and elongation. The various values of elastic constants have been calculated for the direction crystallographic c–axis straight to the bismuth layer. Taking into account some defects in an ideal NBT crystal caused a significant decrease in elastic constants values of C ₁₁, C ₂₂, C ₃₃ satisfying Hooke's law in the deformation range from 0.1 per cent to 2.1 per cent. Band gap was reduced by almost a half.

Keywords:

• sodium–bismuth titanate
• first principles
• elastic constants
• point defects
• SIESTA

ACKNOWLEDGMENTS

The authors acknowledge the CPU time allocation at Academic Computer Centre CYFRONET AGH in Cracow. This work was supported by the grants of MNiSW/SGI3700/AP/025/2007 and MNiSW/SGI3700/IFJ/139/2006.