The temperature dependence of the infrared reflectivity spectra in KNbO3 is reported for the cubic, tetragonal and orthorhombic phases in a temperature range extending from 300 to 1200K. Spectra have been fitted with a model based on the factorised form of the dielectric function. The temperature dependence of the mode frequencies, the dampings and the oscillator strengths through the successive phase transitions is reported. The experimental data exhibit an unstable mode which decreases continuously in frequency upon cooling across the diverse transitions. The ionic effective charges, the spontaneous polarisation and the static dielectric constant are calculated from these data. All results are discussed in terms of a displacive/order-disorder crossover, which appears especially when the transition from the cubic to the tetragonal phase is approached. A particular phase transition mechanism is proposed, which leads to an understanding of the discrepancy between the experimental and the calculated values of the dielectric constant, the large damping of the soft mode, the incomplete softening of the unstable phonon and the special sequence of phase transitions.

https://iopscience.iop.org/article/10.1088/0022-3719/17/3/020/pdf

Infrared spectroscopy in KNbO3 through the successive ferroelectric phase transitions

We have measured the refractive indices of potassium niobate in the orthorhombic phase at six different temperatures between 22 and 180°C in the wavelength range 400–3400 nm. Sellmeier coefficients are determined by using a two-term oscillator model with an IR correction term. The temperature-dependent spontaneous polarization and the quadratic polarization optical coefficients are obtained from the index data. For practical purposes the temperature dependence can be well approximated by a polynomial expansion of degree 2.

Refractive indices of orthorhombic KNbO 3 . I. Dispersion and temperature dependence

Electro-optical properties of KNbO3

Dielectric, piezoelectric and elastic constants have been measured in single domain KNbO3, and electrooptic r-coefficients are estimated. Results allow a thorough verification of Devonshire's theory for the orthorhornbic phase. Good agreement is found for the dielectric and piezoelectric properties whereas fitting the elastic behaviour requires inclusion of terms coupling stiffness to spontaneous strain. The temperature dependence of clamped dielectric constants and elastic stiffness points to the importance of piezoelectric coupling for the low temperature phase transitions. Absorption measurements reveal a nearly isotropic polarization-induced band edge shift of 200 meV. Large anisotropy is observed in the polarized UV reflectance spectra measured up to 10 eV. Comparison of reflectivity in ferroelectric KNbO3 with electroreflectance in cubic KTaO3 gives further evidence for the unified nature of electro-optic effects in oxygen-octahedra compounds. Differences in e''-spectra derived from Kramers-Kronig a...

Dielectric, mechanical and optical properties of orthorhombic KNbO-3

High efficiency second-harmonic generation in KNbO3 crystals

Measurements of birefringence and refractive indices in KNbO3 were performed in the orthorhombic, tetragonal and cubic phases. KNbO3 is phase matchable and collinear phase matching is possible for a Nd-laser. Absorption measurements reveal an exponential law for the absorption coefficient (Urbach tail). The dispersion of the refractive indices is discussed in terms of a single-oscillator model. The parameters obtained are in good agreement with those of other perovskite materials. Relations for the electrooptic effect in an orthorhombic oxygen-octahedron ferroelectric are derived and it is found that this effect can explain the observed temperature dependence of refractive indices. Finally, the coefficients for the linear electrooptic effect are calculated and discussed.

E. Wiesendanger

Papers

Optical properties of KNbO3