The results of heat capacity measurements for the lanthanum orthoniobate substituted with 10, 20 and 30mol% of antimony (LaNb0.9Sb0.1O4, LaNb0.8Sb0.2O4 and LaNb0.7Sb0.3O4) are presented and discussed. Temperature dependence of low temperature heat capacity was analyzed within the Debye and Einstein models. The Debye temperature decreased, whereas the Einstein temperature increased with antimony content. The decrease of the Debye temperature with increasing antimony content was correlated with decreasing scheelite -fergusonite transition temperature. The increase of the Einstein temperature of LaSbxNb1-xO4 with increasing antimony content may indicate increasing frequency of optical vibrations of Nb(Sb)-O4 -2 polyhedra relative to La +3 cations. Using the heat capacity data, standard entropies of the phases were calculated and combined with previously measured enthalpies of formation to obtain Gibbs energies of formation. Standard thermodynamic properties were tabulated.

We report a study on tuning the charge density wave (CDW) ferromagnet SmNiC2 to a weakly coupled superconductor by substituting La for Sm. X-ray diffraction measurements show that the doped compounds obey Vegard’s law, where La (Lu) alloying expands (shrinks) the lattice due to its larger (smaller) atomic size than Sm. In the series Sm1−xLaxNiC2, CDW transition (TCDW = 148 K) for SmNiC2 is gradually suppressed, while the ferromagnetic (FM) ordering temperature (TC) at 17 K slightly increases up to x = 0.3. For x > 0.3, TC starts to decrease and there is no signature that could be related with the CDW phase. Electrical resistivity, magnetic susceptibility and specific heat measurements point toward the possible presence of a FM quantum critical point (QCP) near x = 0.92, where the TC is extrapolated to zero temperature. Superconductivity in LaNiC2 (Tsc = 2.9 K) is completely suppressed with small amount of Sm inclusion near the proposed FM critical point, indicating a competition between the two ordered phases. The tunable lattice parameters via chemical substitution (La,Lu) and the ensuing change among the ordered phases of ferromagnetism, CDW and superconductivity underscores that SmNiC2 provides a rich avenue to study the rare example of a FM QCP, where the broken symmetries are intricately correlated.

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La 3 Co and La 3 Ni are reported superconductors with transition temperatures of 4.5 and 6 K, respectively. Here, we reinvestigate the physical properties of these two intermetallic compounds with magnetic susceptibility χ, specific heat C p and electrical resistivity ρ down to 1.9 K. Although bulk superconductivity is confirmed in La 3 Co, as observed previously, only trace superconductivity is found in La 3 Ni, indicating that the superconductivity in La 3 Ni originates from an impurity phase. Superconducting state parameters for La 3 Co, including lower and upper critical fields and the superconducting gap, are estimated. Results of the theoretical calculations of the electronic structure for both materials are also presented, and comparison of the Fermi level location in La 3 Co versus La 3 Ni explains its larger superconducting T c . A major discrepancy between band structure calculations and the experimentally measured Sommerfeld coefficient is found. The measured electronic density of states is about 2.5 times larger than the theoretical value for La 3 Co. This effect cannot be explained by the electron-phonon interaction alone. This renormalization of g, as well as an ~T 2 behavior of the resistivity, suggests the presence of spin fluctuations in both systems.

The ferromagnetic superconductor Y 9 Co 7 was chemically doped to yield the solid solution Y 9 Co 7-x Pd x for 0 < x < 0.4. The lattice parameter a does not depend on x, whereas c increases with increasing Pd content up to x = 0.2, the palladium solubility limit. The transition from ferromagnetism (T C = 4.25 K) to superconductivity (T sc = 2.4 K) was observed only for the parent Y 9 Co 7 compound. For the lowest tested Pd doping level (x = 0.05), ferromagnetism is enhanced strongly (T C = 9.35 K) and superconductivity is not seen above 1.8 K. The Curie temperature rapidly increases from 4.25 K to about 10 K for a Pd concentration of x = 0.1 and remains almost unchanged for Y 9 Co 6.8 Pd 0.2 .