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 VK +Hi defect models in KDP crystals have been studied based on DFT. Lattice dynamics methods give the most reasonable compensation mechanism for VK, namely compensation with the third-nearest Hi neighbor from VK for the paraelectric (PE) phase and compensation with the fourth-nearest neighbor Hi from VK for the ferroelectric (FE) phase. The defect formation energies indicate that the (VK + Hi)x (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 Hi in the (VK + Hi)' system. For (VK + Hi)x system, one electron is accommodated in VK. There is not a new defect state in the band gap. The Hi bonds with the O ion (0.99 & Aring;) form a hydroxyl. For (VK + Hi)' system, the hydroxyl is broken, the Hi 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 VK +Hi can significantly impact the optical damage threshold of the KDP.