Computational study of K vacancy with an H interstitial defect in KHPO crystal.

The electronic structures, defect formations, defect transition levels and optical properties for V+H defect models in KDP crystals have been studied based on DFT. Lattice dynamics methods give the most reasonable compensation mechanism for V, namely compensation with the third-nearest H neighbor from V for the paraelectric (PE) phase and compensation with the fourth-nearest neighbor H from V for the ferroelectric (FE) phase. The defect formation energies indicate that the [Formula: see text] (The superscript represents the charged state, the 'x' represents neutral, ''' represents -1 charge state and '' represents +1 charge state.) is the main defect type in this kind of defect cluster and a self-trapped electron is located at H in the (V + H)' system. For [Formula: see text] system, one electron is accommodated in V. There is not a new defect state in the band gap. The H bonds with the O ion (0.99 Å) form a hydroxyl. For (V + H)' system, the hydroxyl is broken, the H exists in an atomic form and introduces new defect states in the band gap. As the large relaxation energy leads to a large Huang-Rhys factor, the Stokes red shifts will significantly affect the optical properties. A broad ultraviolet (UV) absorption band and emission band range from UV to visible are obtained originating from the defect cluster. We believe that the far-violet absorption peaks (167 nm and 179 nm) caused by defect cluster V+H can significantly impact the optical damage threshold of the KDP.