Stress-induced phase and structural changes in KDP crystals

Potassium dihydrogen phosphate (KDP) is a nonlinear optical material which has been widely used in optical modulators and frequency converters. In making optical KDP components, however, the mechanical stresses in a manufacturing process often bring about unexpected surface integrity problems such as phase changes and cracks. This paper explores some possible phase transformations in monocrystalline tetragonal KDP under uniaxial, biaxial and triaxial compressive/tensile stresses externally applied through the use of ab initio Density Functional Theory (DFT) calculations. It shows that the change in crystal symmetry/phase transitions is due to hydrogen dynamics. Under a uniaxial tensile stress of similar to 4 GPa along the < 110 > direction, the paraelectric phase II, which has a tetragonal structure belonging to the I42d (D-2d(12)) space group, will undergo an irreversible transformation to an orthorhombic (C-2v) structure. (C) 2015 Elsevier B. V. All rights reserved.