Temperature Dependence of Raman Scattering and the Electro-Optic Properties of CuCl

The absolute Raman-scattering efficiencies, the clamped electro-optic coefficient r₄₁, and the dielectric constant κ of CuCl have been measured as functions of temperature. We confirm previous observations of four Raman lines: the longitudinal (LO), transverse (TO), and two second-order modes. The TO and a second-order mode have comparable strengths and are adjacent in frequency, giving the appearance of a double peak. The scattering efficiencies are nearly two orders of magnitude weaker in CuCl than the LO and TO modes in GaAs. Proper mode assignments and relative scattering efficiencies are discussed. Both κ and r₄₁, measured at 0.633 and 3.39 {μ}m, decrease appreciably as the temperature is reduced from 300 to 90 ^∘{}K. It is shown that the magnitude and sign of the nonlinear optical coefficient d₁₄⁰ as well as the ratio C of the lattice and electronic contributions to r₄₁ can be determined from these temperature-dependent r₄₁ and κ measurements alone. Attempts to determine d₁₄⁰, r₄₁, and C from the Raman measurements alone, as had been done in GaAs, are complicated by the double-peaked TO mode, but such a correlation is possible. The parameters of a simple bond-charge model (including effective charge, spring constant, and Raman polarizability) for CuCl and GaAs are calculated and compared. The signs of the effective charges on Cu and Ga are found to be opposite. The results are discussed in terms of the relative ionicities (low for GaAs and high for CuCl) of the two compounds.