Computational study of K vacancy with an H interstitial defect in KH₂PO₄ crystal

The electronic structures, defect formations, defect transition levels and optical properties for V_K+H_i defect models in KDP crystals have been studied based on DFT. Lattice dynamics methods give the most reasonable compensation mechanism for V_K, namely compensation with the third-nearest H_i neighbor from V_K for the paraelectric (PE) phase and compensation with the fourth-nearest neighbor H_i from V_K 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_i in the (V_K + H_i)' system. For [Formula: see text] system, one electron is accommodated in V_K. There is not a new defect state in the band gap. The H_i bonds with the O ion (0.99 Å) form a hydroxyl. For (V_K + H_i)' system, the hydroxyl is broken, the H_i 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_K+H_i can significantly impact the optical damage threshold of the KDP.