Hypersonic studies and refractive indices of CsTiOAsO4 and KTiOAsO4 single crystals
Schmidt, V. Hugo
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The longitudinal Brillouin backscattering spectra along the  phonon direction have been measured as a function of temperature (24-896 °C) for both CsTiOAsO4 (CTA) and KTiOAsO4 (KTA). As the temperature approaches the ferroelectric transition temperature Tc, the acoustic phonon frequencies of both CTA and KTA show a strong softening (which reaches a minimum at Tc∼688°C for CTA). For both CTA and KTA, a broad damping peak was observed and can be attributed to the dynamic order-parameter fluctuations. This anomaly implies that an η2μ-type coupling is the dominant mechanism for the acoustic soft modes in CTA and KTA. Here η is the order parameter and μ is the strain. The refractive indices (nx,ny,nz) and Cauchy’s equations were obtained as a function of wavelength (0.4–1.7 μm) for KTA. The phase matching angles (θ,φ) of second harmonic generation for the fundamental wavelengths of 1.064 (type I) and 1.32 (types I and II) were also calculated.
C.-S. Tu, Y.-L. Yeh, V.H. Schmidt, R.-M. Chien, R.S. Katiyar, Ruqian Guo, Ruyan Guo, and A.S. Bhalla, “Hypersonic studies and refractive indices of CsTiOAsO4 and KTiOAsO4 single crystals,” Phys. Rev. B 56(13), 7988-7992 (1997).