We have investigated the electronic structure and magnetic properties of the new ternary ${\mathrm{Ce}}_{1-x}{\mathrm{La}}_x\mathrm{RhSn}$ system. The $3d$ and $4d$ x-ray photoelectron spectroscopy (XPS) spectra indicate a mixed valence of Ce. Analysis of the ${3d}^9{4f}^2$ weight in the $3d$ XPS spectra using Gunnarsson-Schönhammer theory suggests a hybridization interaction $\Delta$ of about 120 meV for different compositions x of the system. The magnetic measurements indicate the existence of a highly anomalous weak ferromagnetic state below $\sim 220\mathrm{K}$ in ${\mathrm{Ce}}_{1-x}{\mathrm{La}}_x\mathrm{RhSn}$ for $x<\mathrm{}\mathrm{0.6.}\mathrm{}$ This ferromagnetism could originate from the ordering of Ce local moments due to strong Ce $4f$ conduction electron hybridization via the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. The $\chi \mathrm{}\left(T\right)\mathrm{}$ and $\rho \mathrm{}\left(T\right)\mathrm{}$ data are consistent with non-Fermi liquid behavior for $x<~0.5,$ while the magnetic behavior in the range $x>\mathrm{}0.5\mathrm{}$ is dominated by spin fluctuations.

We report on the electrodynamics of a Heusler alloy ${\mathrm{Fe}}_2\mathrm{TiSn}$ probed over 4 decades in energy: from the far infrared to the ultraviolet. Our results do not support the suggestion of Kondo-lattice behavior inferred from specific-heat measurements. Instead, we find a conventional Drude-like response of free carriers, with two additional absorption bands centered at around 0.1 and 0.87 eV. The latter feature can be interpreted as excitations across a pseudogap, in accord with band-structure calculations.

We investigate the effect of enhancement of superconducting transition temperature $T_c$ by nonmagnetic atom disorder in the filled skutterudite-related tetragonal ${\mathrm{Y}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ compound doped with Ca. We documented experimentally that ${\mathrm{Y}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ and its Ca-doped alloys are electronically inhomogeneous at the nanoscale, when Ca content is smaller than $\sim 1.5$ in the ${\mathrm{Y}}_{5-x}{\mathrm{Ca}}_x{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ system, while for $x\ge 1.5$ much stronger chemical phase inhomogeneity is observed with an about 20% volume fraction of the second 3:4:13 cubic phase, which we interpret as a fluctuation of stoichiometry within the bulk sample. Then the enhancement of $T_c$ vs $x$ could be modeled by a mechanism proposed in recent theoretical reports for increasing the mean-field transition temperature $T_c$ in the presence of nonmagnetic disorder. The increase in disorder with doping increases the sample inhomogeneity and causes a systematic increase in $T_c$, while for two-phase $x\ge 1.5$ samples the critical temperature $T_c^{★}\approx 2\times T_c$ rapidly increases. Based on band structure calculations performed under pressure, we demonstrate how the change in density of states would affect $T_c$ of ${\mathrm{Y}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$. We obtained the Grüneisen parameter ${\gamma }_G$ larger for the inhomogeneous phase with respect to ${\gamma }_G$ of the bulk $T_c$ phase and attribute the enhancement of $T_c$ to larger stiffening of the high-temperature $T_c^{★}$ phase.

We investigate the properties of the off-stoichiometric CeNi_1−δSn_1+δ−x Sb _x systems with δ ≈ 0.06 and 0 < x ≤ 0.22. Such a semimetallic CeNi_1−δSn_1+δ−x Sb _x system is shown to transform into a Kondo semiconductor upon the substitution of about 2% of Sb for Sn. The effects of Sb doping on the low-temperature properties of CeNiSn were studied by means of electrical resistivity, magnetic susceptibility and specific heat. We interpret the ground state properties of the series of CeNi_1−δSn_1+δ−x Sb _x compounds as resulting from the change in carrier number acting together with the hybridisation between the 4f electrons and the conduction electrons. For the Kondo semiconductor state we observe the scaling law χρ = const. of the susceptibility χ and the resistivity ρ, which is regarded as the basic feature for this class of materials, as was reported earlier. The magnetic properties of the system gradually evolve from the Kondo semimetallic state to the spin-glass behaviour when the Sb doping increases.

For a number of skutterudite-related cage compounds, such as cubic $R_3{M}_4{\mathrm{Sn}}_{13}$ ($R=\mathrm{Ca}$ or La, $M=\text{transition}$ $d$-electron metal) or tetragonal ${\mathrm{Y}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$, we have previously reported the impact of various atomic defects in the crystal lattices on the normal-state and superconducting properties of these clathrates. In these quasiskutterudites the nanoscale disorder and/or local inhomogeneity in composition lead to an abnormal increase in the superconducting transition temperature $T_c$. Here we investigate the impact of atomic defects in the superconductors ${\mathrm{Lu}}_{5-\delta }{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ (tetragonal symmetry $I{4}_1/acd$). We have documented that ${\mathrm{Lu}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ does not crystallize in the assumed stoichiometry but forms a 5-6-18 phase with a Lu deficiency ($\delta \approx 0.5$). Our comprehensive investigations of electronic structure and thermodynamic and electrical transport properties documented a $T_c$ increase in the more disordered sample (sample 1). The quality of two investigated samples (sample 1 and sample 2) with similar stoichiometry but different local inhomogeneity was obtained by microanalysis and electron transmission microscopy observations. The band structure calculations show a hybridization pseudogap in the electronic bands of stoichiometric ${\mathrm{Lu}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ at about 0.3 eV in respect to the Fermi level ${\epsilon }_F$. The ab initio calculations predict the scenario that vacancies could shift this pseudogap towards ${\epsilon }_F$, which is manifested by the Mott variable-range hopping behavior in the resistivity $\rho \sim T^{-1/4}$ of more homogeneous sample 2.

Density functional theory (DFT) calculations confirm that the structurally ordered ${\mathrm{Fe}}_2\mathrm{VAl}$ Heusler alloy is nonmagnetic narrow-gap semiconductor. This compound easily crystallizes in various disordered phases with high concentration of antisite defects. We study the effect of structural disorder on the electronic structure, magnetic, and electronic transport properties of the full Heusler alloy ${\mathrm{Fe}}_2\mathrm{VAl}$ and its off-stoichiometric equivalent ${\mathrm{Fe}}_2{\mathrm{VAl}}_{1.35}$. Data analysis in relation to ab initio calculations indicates an appearance of antisite disorder mainly due to Fe-V and Fe-Al stoichiometric variations. The data for weakly magnetic ${\mathrm{Fe}}_2{\mathrm{VAl}}_{1.35}$ are discussed with respect to ${\mathrm{Ni}}_2\mathrm{VAl}$. ${\mathrm{Fe}}_2{\mathrm{VAl}}_{1.35}$ can be classified as a nearly ferromagnetic metal with a pronounced spin-glassy contribution, which, however, does not have a predominant effect on its thermoelectric properties. The figure of merit $ZT$ is at 300 K about 0.05 for the Fe sample and 0.02 for Ni one, respectively. However, it is documented that the narrow $d$ band resulting from Fe/V site exchange can be responsible for the unusual temperature dependencies of the physical properties of the ${\mathrm{Fe}}_2{\mathrm{TiAl}}_{1.35}$ alloy, characteristic of strongly correlated electron systems. As an example, the magnetic susceptibility of ${\mathrm{Fe}}_2{\mathrm{VAl}}_{1.35}$ exhibits singularity characteristic of a Griffiths phase, appearing as an inhomogeneous electronic state below $T_G\sim 200$ K. We also performed numerical analysis which supports the Griffiths phase scenario.