First Principles Calculations of Linear and Second-Order Optical Responses in Rhombohedrally Distorted Perovskite Ternary Halides, CsGeX3 (X = Cl, Br, and I)

Systematic studies based on first-principles calculations of second. order optical susceptibilities as well as the dielectric function of CsGeX3 (X = Cl Br, and I, CGX) are presented The relationship between structural properties and optoelectronic responses are examined The structural factors Delta alpha, and d(Ge), d(x) are proposed to describe the degree of distortion from an ideal perovskite structure. Delta alpha and dGe increase when halide anions are changed from Cl to I, while halide anion displacement, dx, decreases. The structural distortion effect on these rhombohedral CGX crystals is analyzed by first-principles calculations The dielectric function and the second harmonic generation (SHG) response coefficient also increase with increasing Delta alpha and d(Ge). The direct band gaps (E-c) of CsGeX3 ail occur at the R-point, Delta E-R The experimental band gaps of CGX crystals become smaller. i e, E-G(CGI) (= 3 67eV) > E-G(CGB) (=2 32 eV) > E-G(CGI) (= 1 53eV), as Delta alpha and el increase, i e. d(Ge)(CGC) < d(Ge)(CGR) < d(GB)(CGI) Partial density of states (PDOS) analysis revealed that the valence band maximum (VBM) and conduction band minimum (CBM) are mainly contributed by the p-orbitals of germanium. The calculated magnitudes of chi((2))(ijk) are close to some reported experimental values near the band gap (C) 2009 The Japan Society of Applied Physics