Heat capacity in superconducting and normal-stateLaSx(1.333<~x<~1.500) compounds

The low-temperature heat capacity of the superconducting ${\mathrm{LaS}}_x$ ($1.333<~x<~1.500$) compounds with the ${\mathrm{Th}}_3$ ${\mathrm{P}}_4$-type structure were measured from 1.3 to 20.0 K in magnetic fields of 0, 2.50, 5.39, 7.62, and 9.98 T. The compositional variation of the electronic specific-heat parameter $\gamma$ and the Debye temperature ${\Theta }_D$ in $\mathrm{La}{\mathrm{S}}_x$ are discussed in connection with the electric properties of these alloys. The results also show that the $\mathrm{La}{\mathrm{S}}_x$ alloys near $x=1.333\mathrm{}$ are strong-coupling type-II superconductors having large values of the electron-phonon coupling constant values, $\lambda =0.71\mathrm{}$, and upper critical field values of $H_{c2\mathrm{}}\mathrm{}\left(0\right)\mathrm{}$, e.g., 16.2 T for ${\mathrm{LaS}}_{1.366}$. The ${\mathrm{LaS}}_{1.346}$ and ${\mathrm{LaS}}_{1.355}$ alloys undergo a cubic-to-tetragonal phase transition at 85 and 55 K, respectively. The former composition has the intrinsic coherence length ${\xi }_0$ of 158 Å, which gives a large value for the intrinsic Ginzburg-Landau parameter, ${\kappa }_0=11\mathrm{}$. However, this tetragonal phase, which forms only in the narrow composition region of $1.333<~x<~1.362$, has a lower $\gamma$ value than the cubic-phase alloys with $x\simeq 1.37$. Furthermore, the superconducting transition temperatures of $\mathrm{La}{\mathrm{S}}_x$ compounds hardly have any compositional variation for $x<~1.36$. Finally, there is a large drop in $H_{c2\mathrm{}}\mathrm{}\left(0\right)\mathrm{}$ from 16.2 T for the cubic phase at $x$ = 1.366 to 9.7 T for the tetragonal phase at $x=1.346\mathrm{}$. This study also clearly illustrates that lattice instability is an important factor governing the superconducting behavior of these alloys, but the best superconducting properties are found in the stable phase (bcc) just before it becomes unstable, rather than in the phase which actually undergoes the transformation.