STRUCTURE REFINEMENT OF THE SUPERCONDUCTING PHASE YNI2B2C AS A FUNCTION OF TEMPERATURE IN THE RANGE 25-300 K

X-ray powder and single crystal diffraction has been used for structur e refinement of various YNi2B2C samples in the temperature range 25-30 0 and 180-300 K, respectively. Linear thermal expansion is strikingly anisotropic. Coefficients vary between alpha(a) = 4.1 X 10(-7) K-(1) ( 25 K)-1.2 X 10(-5) K-1 (300 K) and alpha(c) = -6.1 X 10(-7) K-1 (25 K) -4.5 X 10(-7) K-1 (300 K) while the axial ratio c/a of the unit cell d ecreases with increasing temperature T. Moreover, a small increase of structure parameter z(B) with T was found. Anisotropic mean square ato mic displacements remain almost constant for C and B but increase cons iderably for both Ni and Y atoms. The influence of the starting compos ition on the net content and on structure was studied for a series of samples. Lattice parameters as derived from Rietveld refinement deviat ed significantly among different samples. Relations between structure geometry, superconducting transition temperature, the residual resista nce ratios and composition of the samples were discussed. (C) 1998 Pub lished by Elsevier Science B.V. All rights reserved.