Stability and electronic structure of hydrogen vacancies in ADP: hybrid DFT with vdW correction

The formation energies, charge transition levels, and electronic structures of positively charged, neutral, and negatively charged hydrogen vacancies in the NH₄H₂PO₄ (ADP) crystal are investigated in the framework of density functional theory with local and hybrid exchange-correlation functionals. The inclusion of nonlocal exchange opens the ADP fundamental band gap by nearly 1 eV and well reproduces the experimental value. The van der Waals (vdW) interaction is found to have a major influence on the energetics of charged hydrogen vacancies in ADP. The calculated relative stability of and with vdW interaction could well explain the break point on the measured conductivity curve of the ADP crystal in the high temperature region. On the other hand, a missing H atom in the (H₂PO₄)^- group is found to be more energetically preferable than NH₄ ⁺. It could capture a hole carrier to form a molecular-type polaron with its adjacent two O atoms, and be responsible for the optical absorption under irradiation by a high-intensity laser beam.