Study of Slater and Takagi Defects in KDP from first-principles

S. KOVAL; J. LASAVE; N. S. DALAL

Iguazú

Conferencia; 11th International Meeting on Ferroelectricity - IMF11; 2005

Abstract:We performed ab initio electronic structure calculations to study the stability of the Slater and Takagi protonic defects in the ferroelectric phase of KH2PO4. We show that the Slater defects are stabilized only when they are aligned forming chains of defects in the lattice, in accordance with the displacive model of the transition. In contrast, the Takagi defects are not stable in the lattice. The ab initio energy estimation for the Takagi defect (≈54 meV) is in fair good agreement with previous phenomenological estimates. However, the calculated uncorrelated Slater defect energy (≈17 meV) is larger than that from previous model assignments (5.2 meV), while the latter agrees well with our result for the correlated value per formula unit. This suggests that the earlier estimates contain considerable contribution from correlations between configurations of different H2PO4 groups. Our ab initio results show that the formation of chains of Slater defects is accompanied by phosphate rotations, and a concomitant lattice contraction in the basal plane. Therefore, we propose that the as yet unexplained anomalous lattice contraction observed in the neighborhood of the phase transition is due to an increase of the Slater defect population.