It has been known for a long time that the low temperature behavior shown by the dielectric constant of quantum paraelectric SrT iO 3 can not be fitted properly by Barrett's formula using a single zero point energy or saturation temperature (T 1 ). As it was originally shown [K. A. Müller and H. Burkard, Phys. Rev. B 19, 3593 (1979)] a crossover between two different saturation temperatures (T 1l =77.8K and T 1h =80K) at T ∼ 10K is needed to explain the low and high temperature behavior of the dielectric constant. However, the physical reason for the crossover between these two particular values of the saturation temperature at T ∼ 10K is unknown. In this work we show that the crossover between these two values of the saturation temperature at T ∼ 10K can be taken as a direct consequence of (i) the quantum distribution of frequencies g(Ω) ∝ Ω 2 associated with the complete set of low-lying modes and (ii) the existence of a definite maximum phonon frequency given by the structural transition critical temperature T tr .

Quasiphase-matching second-harmonic-generation experiments have been performed on the Nd3+-doped strontium barium niobate multiself-frequency-converter nonlinear crystals. Optical microscopy has been used to determine the size dispersion of cylindrical ferroelectric domains appearing in as-grown crystals. A broad distribution of domain sizes (ranging between 1.5 and 8 μm) has been obtained. This gives the possibility of broadband second-harmonic generation in the 800–1200 nm range. The dependence of second-harmonic efficiency on crystal temperature has been found to be correlated with the thermal behavior of the spontaneous polarization, so that second-harmonic efficiency is almost temperature independent in the 25–50 °C range.

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Automatically recorded hysteresis loops and dielectric constant data on pure and partially deuterated triglycine selenate and sulphate are used to characterize the transition evolution from quasi-tricritical to clearly first order character. A quantitative assessment of the transition's displacive degree is made through the first anharmonic contribution to the effective field ( g ), obtained from hysteresis loop data. This characterization is complemented with the assessment by means of a generalized Rhodes-Wohlfarth parameter R = (NkB C /4 P 2 S 0 )1/2 involving the Curie constant C and the saturation spontaneous polarization. P S 0 as done previously by other authors.

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