The Raman spectra, the clamped electro-optic coefficient ($r_{41}$), the dielectric constant ($\kappa$), and refractive index ($n$) of CuBr have been measured as functions of temperature. The longitudinal (LO) and transverse (TO) Raman modes are easily identifiable at low temperatures and can be followed continuously to room temperature where two weak second-order modes are present. The TO mode has a strong temperature dependence which may be related to the high-temperature phase transition by a temperature-dependent ionicity. Both $\kappa$ and $r_{41}$ have a strong positive temperature dependence, whereas $n$ is nearly independent of temperature. Fitting of a Sellmeier relation to refractive-index data allows a good value of $n_{\infty }$ to be calculated. With $n_{\infty }$, the temperature-dependent Raman frequencies allow a Lyddane-Sachs-Teller calculation of the temperature-dependent dielectric constant for comparison with the directly measured dielectric constants. The thermal behaviors of $r_{41}$, $\kappa$, and $n$ are then used to derive nonlinear optical coefficients. The derived coefficients and directly measured ones agree reasonably well in magnitude, have the same algebraic signs, and show similar wavelength dependence.

• Received 25 September 1973

DOI:https://doi.org/10.1103/PhysRevB.9.2524