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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.