Fine powder SHG technique for the determination of polar distortion in substances and its application to the study of systems with ferroelectric phases (SbNbO₄–BiNbO₄and SbNbO₄–SbSbO₄

Abstract

PHYSICAL PROPERTIES AND STRUCTUREb) Transition metal carbides with large lattice constants and strong covalent bond energy exhibit small deviations from x = 1 stoichiometry and form low temperature phases at x = 0.5 and x = 1. ZrCo o . 65 e.g. orders in Ti 2 C structure rather than in a M Z C 3 structure because of the gain of covalent bond energy for a reduction of Zr-C bond distance by 0. The ternary metal borides of RE T4B4 -type (RE=rare earth metal, T = Os, Ir) crysta~lize in a tetragonal structure with the space group P42/n and are isotypic with NdC0 4 B 4 " The tetragonal unit cell parameters 'a' and 'c' of RE OS4B4 (RE = La,Ce,Pr,Nd and Sm) and of RE Ir 4 B 4 (RE = La,Pr,Nd and Sm) have been determined accurately over the temperature range 300-800 K by X-ray powder diffraction techniques. Using the high temperature lattice parameter data the axial thermal expansion coefficients c('a and c("c have been evaluated at different temperatures. It has been found that the thermal expansion coefficients are anisotropic (oC a >~) and this anisotropy increases with decreasing axial ratio cia. These results are discussed in relation to the structure and other physical properties of these materials. Ge and Nb 1 Sn have been studied by oulsed neu!ron no~der diffraction. These comnounds reoresent hlQh, medi um and 10\'1 temoera ture suoerconductors, The aim of the experiments ~as to stud~ the relationship between suoerc~nductivitv and structural instabilitv at low te~perature. The"marterisitic transformation' (cubi.c to tetragonal) was observed in Nb 3 Sn.The mean squares disolacements of the thermal vibra~ tions were ~~asured as' a function of temoerature. The decree of anisotroov of the transition metal vibrations wi'l be related to'~tructural and suoerconducting properties. Quantitative estimations of noncentrosymmetric distortions in crystals by means of the Second Harmonic Generation (SHG) in powders ~e usually unsatisfactory because of the strong dependence of the intensity of the signal (I2W) on the coherent length (Lc) which for various crystals varies considerably. However in the case of SHG in fine pOl-lder s wi th the size of par ticles ~2.;u it is possible to exclude Lc from the expression for I 2W • Then the only parameter, on which the intensity depends, becomes the nonlinearity of the sample (d). For ferroelectrics this dependence is I 2w "" d 2 '" P~ , where the sponti3-neous polarization P s is a measure of polar distortion of the structure. 05.2-13 FINE POWDER SHG-TECHNIQUE FOR THE DETERMINATION OF POLA R DISTORTION IN SUBSTANCES AND ITS APPLICATION TO THE STU D Y OF SYSTEMSFor the detection of very weak SHG-signals, reflected from a fine-powder or ceramic sample we use a high-sensitive measuring system which also enable us to follow the changes of I 2w (and P s ) vs. temperature. The results obtained by this technique when combined with the data of the usual X -r ay powder analysis prove to be especially useful for the investigation of phase diagrams when ferroelectric or other noncentrosymmetric pha...