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The structure of strontium barium niobate crystals SrxBa1-xNb2O6 is comprehensively studied in the whole range of the tetragonal tungsten bronze phase (x=0.32-0.82) using both powder and single-crystal X-ray diffraction measurements. Unit-cell parameters, density, site-occupancy factors and interionic distances show an explicit composition dependence which can be consistently explained using simple model calculations. The temperature dependence of the unit-cell parameters exhibits a remarkable anisotropy in a broad temperature region below the phase transition temperature. This proves that the electrostrictive contribution to the thermal expansion plays an important role in strontium barium niobate.

The observation of a novel noncollinear optical second-harmonic generation mechanism is reported. In strontium barium niobate crystals, a circular cone of second-harmonic light is generated when a fundamental beam of intensive laser light is directed along the crystallographic c axis. It can be shown that the effect is caused by the nonlinear polarization of antiparallel ordered ferroelectric microdomains.

%'e measured the re&active indices of undoped and Mg-doped lithium niobate by an interferometric technique in a wavelength range from 400 to 1200 nm. The composition of the undoped samples varied between 47 and 50 mo1% Li&O in the crystal whereas the Mg-doped samples were grown from congruent melt with up to 9 mol%%uo Mg. Our results can be excellently described by a generalized Sellmeier equation which takes into account the defect structure of Li-de6cient and Mgdoped lithium niobate. Calculations of the phase-matching conditions of several nonlinear efFects in Mg-doped lithium niobate reveal good correspondence with the respective experimental results in a temperature range from -50 to 250 'C.

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Abstract. The dielectric constants of rare-earth oxides (RE 2 O 3 ) and rare-earth nitrides, phosphides, arsenides, and antimonides (REX, RE = rare-earth element, X = N, P, As, Sb) are quantitatively estimated. The estimation is based on the approximately linear relationship between the dielectric constant and the atomic number of the constituent atoms. Similar correlations are valid in nearly all series of related compounds; thus the approach presented here can be used as an effective tool in the search for materials with specific properties, e.g. high dielectric constants.