Influence of the bcc to tetragonal transformation on superconductivity in "La3X4"(X = S or Se)

Yeh, Ying-Cheun
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The superconducting LaSe(,x) (1.333 (LESSTHEQ) x (LESSTHEQ) 1.500) alloys were studied to investigate the influence of the bcc to tetragonal phase transformation on their superconducting properties. The results were compared to the published data of LaS(,x) alloys. It is found that x less than a critical composition (x(,c) = 1.362 for sulfides and 1.363 for selenides) the alloy transforms from bcc to bct phase at low tem- perature. This bcc to tetragonal phase transformation temperature (T(,M)) decreases as the sulfur or selenium to metal ratio increases toward the x(,c). The optimum superconducting properties in both selenides and sulfides exist at the composition x(,c). The effect of substitution of small amounts (2 to 6%) of Mg, Ca, Y, Ce and Th for La and the substitution of S by Se in LaS(,x) alloys on T(,c) (super- conducting transition temperature) and T(,M) was also studied. The compositional dependences of the electronic specific heat parameter (gamma) and the upper critical magnetic field at 0 K (H(,cz)(0)) in pseudobinary sulfides containing Th are discussed in connection with the phase transformation. For both selenides and pseudobinary sulfides, the electrical resistivity of the alloys has been measured from 4.2 to 300 K. The T(,M) value is indicated by a drop in the resistivity upon cooling. The a.c. magnetic susceptibility measurement has been used routinely to determine T(,c). The low temperature heat capacity of the alloys has been measured from 1.4 to 20 K in magnetic fields up to 10 T. The alloys are shown to be high field superconductors (H(,cz)(0) ('(TURN)) 16 T);From the heat capacity results, the values of (lamda) (electron-phonon coupling parameter), H(,c)(0) (thermodynamic critical field at 0 K), (DELTA)C (jump in the heat capacity at T(,c)), D(t) (deviation function), 2(DELTA)(0)/kT(,c) (reduced gap parameter) and (kappa)(,1), (kappa)(,2), (kappa) (generalized Ginzburg-Landau parameters) have been calculated. The results are compared to the BCS predictions and the theory for strong-coupling type-II super-;conductors. The results show that the alloys (with x < 1.37) are strong-coupling type-II superconductors; *DOE Report IS-T-1137. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.

Metals science and engineering, Metallurgy (Physical metallurgy), Physical metallurgy