Phonons splitting induces the second harmonic of tetragonal KH₂PO₄ by first-principles calculations

First-principles calculations of electrons, phonons and elasticity are used to investigate KH₂PO₄ based on the density-functional theory. Calculated lattice parameters are in agreement with the available theoretical and exprimental data. Electronic structure reveals that the KH₂PO₄ has a direct band gap. Calculated phonons show that the vibration of O₁…H[sbnd]O₂ causes a LO* split, generating the second harmonics generation (SHG) and double refraction of KH₂PO₄ because of the depolarization field of vibrational O₁…H[sbnd]O₂. Born effective charge proves the existence of depolarization field. Optical properties including the dieletric function, refractive index, extinction coefficient, reflectivity, abosrption coefficient, loss function, and optical conductivity are calculated, revealing that KH₂PO₄ has a fine transmission and optical anisotropy. Elastic constants, elastic compliance, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, compressibility, Lamé constants, sound velocity and Debye temperature of tetragonal KH₂PO₄ are calculated to discuss the mechanical perperties.