Structural and electronic properties and optical absorption of oxygen vacancy cluster defects in KDP crystals: hybrid density functional theory investigation

Oxygen vacancies, hydrogen vacancies and potassium vacancies are common intrinsic point defects in potassium dihydrogen phosphate (KDP) crystals and can combine to form cluster defects. In this work, the stability of V-O(2+) + 2V(K)(-), V-O(2+) + V-K(-) + V-H(-) and V-O(2+) + 2V(H)(-) cluster defects, as well as the defect-induced structural and electronic properties and optical absorption in KDP crystals, is investigated by hybrid density functional theory. Cluster defects composed of oxygen vacancies and their nearest hydrogen vacancies or potassium vacancies can be stable in KDP crystals. V-O(2+) + 2V(K)(-), V-O(2+) + V-K(-) + V-H(-) and V-O(2+) + 2V(H)(-) cluster defects introduce large relaxation to the O-H bonds linking to the PO4 group in KDP crystals, and the structural instability of the crystal could be quickly enhanced with the increase of these cluster defects. This structural instability is detrimental to the laser induced damage thresholds of KDP crystals. The band gaps of KDP crystals are shortened to 6.6 eV, 6.4 eV, and 6.4 eV by V-O(2+) + 2V(K)(-) cluster defects, V-O(2+) + V-K(-) + V-H(-) cluster defects and V-O(2+) + 2V(H)(-) cluster defects, respectively. Under a 355 nm laser, this shortening may cause two-photon absorption, resulting in the reduction of the laser induced damage threshold of the KDP crystal. The electron transition between oxygen vacancies and hydrogen vacancies introduces an extra absorption peak at 6.4 eV. This work provides a good basis for deeply understanding the structural and electronic properties and optical absorption of oxygen vacancy cluster defects in the KDP crystal.