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

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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 $d_{\text{Ge}}$, $d_{\text{X}}$ are proposed to describe the degree of distortion from an ideal perovskite structure. $\Delta\alpha$ and $d_{\text{Ge}}$ increase when halide anions are changed from Cl to I; while halide anion displacement, $d_{\text{X}}$, 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 $d_{\text{Ge}}$. The direct band gaps ($E_{\text{G}}$) of CsGeX3 all occur at the R-point, $\Delta E_{\text{R}}$. The experimental band gaps of CGX crystals become smaller, i.e., $E^{\text{CGC}}_{\text{G}}$ ($=3.67$ eV) $>E^{\text{CGB}}_{\text{G}}$ ($=2.32$ eV) $>E^{\text{CGI}}_{\text{G}}$ ($=1.53$ eV), as $\Delta\alpha$ and $d_{\text{Ge}}$ increase, i.e., $d^{\text{CGC}}_{\text{Ge}}<d^{\text{CGB}}_{\text{Ge}}<d^{\text{CGI}}_{\text{Ge}}$. 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.
2009-11-25

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

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