First-Principles Determination of Slater and Takagi Defect Properties in KDP

Using ab-initio electronic structure calculations, we show that Slater defects in KH ₂ PO ₄ (KDP) are stable only within chains, in accordance with the displacive model of the transition. In contrast, Takagi defects are unstable, with formation energy (53.5 meV) in fair agreement with previous phenomenological estimations. As for Slater defects, their correlated formation energy per formula unit (5.2 meV) agrees well with previous model assignments, while the isolated defect energy comes out ≈3 times larger. Slater defect chains are accompanied by phosphate rotations and a basal lattice contraction. Observations of the latter near T _c could thus be explained.

Keywords:

• KDP
• defects
• first-principles calculations

Acknowledgments

This work was supported in part by the Agencia Nacional de Promoción Científica y Tecnológica and CONICET under Grant PICT99 Nr. 03-07248. R. M. and S. K. acknowledge additional support from CONICET. S. K. thanks supports from Fundación Antorchas (Argentina) and from the Florida State University, Tallahassee (USA), and also acknowledges helpful discussions with O. Gunaydin-Sen.

Paper originally presented at IMF-11, Iguassu Falls, Brazil, September 5–9, 2005;