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[Display omitted] The Potassium dihydrogen phosphate (KH 2 PO 4 : KDP) of ferroelectric phase is a polarized structure that can be used for nonlinear optical transformation. Using the generalized gradient approximate (GGA), we study and predict the crystal structural trends, electronic structures, charge densities, and optical properties of KDP crystals with paraelectric (PE) and ferroelectric (FE) phases under pressures ranging from 0 to 20 GPa. We have studied how pressure affects the structural and electronic properties of KDP. It is found that under high pressure, KDP undergoes a transition from Fdd 2 structure to I -42 d structure. The calculations indicate that when the hydrostatic structural pressure of the ferroelectric KDP is applied to 11.36 GPa, the KDP has a paraelectric phase structure. The 11.36 GPa is the critical pressure value at which the structure undergoes phase transition. We further showed that as the pressure increases, the lattice size and ion displacement undergo a strong distortion. Surprisingly, the band gap of KDP greatly increases with the increase of pressure. The results show that high pressure enhances the compactness of the KDP system and enhance the laser damage threshold, which can open a new way for high pressure manipulation of frequency doubling materials in optoelectronic applications. [ABSTRACT FROM AUTHOR]

[Display omitted] The electronic structure and optical properties of Na 2 Bi 2 (SeO 3) 3 F 2 crystal are calculated under uniaxial, epitaxial, and hydrostatic stress based on the first principles of density functional theory. The crystal structures of the materials are composed of isolated [BiO x F y ] polyhedrons and [SeO 3 ] tetrahedrons. The stability of the crystal was confirmed by the cohesive energies and Ab initio molecular dynamics (AIMD). The band gap is 3.93 eV without strain, the maximum band gap is 4.04 eV and the minimum band gap is 2.33 eV under epitaxial stress of −2.61 % and hydrostatic pressure of −9 GPa, respectively. It is found that there is a band gap transition from direct to indirect when uniaxial strain and hydrostatic pressure are applied. This change has been explained by analyzing the near-band-edge orbitals energy. The strain also affects the optical properties of the crystal Na 2 Bi 2 (SeO 3) 3 F 2. In particular, hydrostatic pressure of −9 GPa causes a minimum increase of transmittance of about 10 % and a maximum increase of about 30 %. The optical properties of the crystal can be effectively improved by applying strain, which can be universal for other crystals. [ABSTRACT FROM AUTHOR]