To elucidate the discrepancies in low-temperature data reported on the quantum critical heavy fermion compound Ce 3 Pd 20 Si 6 and reveal the compound's intrinsic properties, single crystals of varying stoichiometry were grown by various techniques-from the melt and from high-temperature solutions using fluxes of various compositions. The resulting stoichiometry of the crystals as well as their physical properties show sizable dependence on the different growth techniques. The Ce content ⌬Ce varies by more than 3 at. % among all grown single crystals. We have revealed a systematic dependence of the residual resistance ratio, the lattice parameter, the ͑lower͒ phase-transition temperature T L , and the maximum in the temperature dependent electrical resistivity T max with ⌬Ce. This clarifies the sizable variation in the values of T L reported in the literature. We discuss the physical origin of the observed composition-property relationship in terms of a Kondo lattice picture. We predict that a modest pressure can suppress T L to zero and thus induce a quantum critical point.

Thermoelectric properties of the type-I clathrates Ba$_8$Cu$_x$Si$_{46-x}$ ($3.6 \leq x \leq 7$, $x$ = nominal Cu content) are investigated both experimentally and theoretically. The polycrystalline samples are prepared either by melting, ball milling and hot pressing or by melt spinning, hand milling and hot pressing techniques. Temperature-dependent electrical resistivity, $\rho(T)$, and the Seebeck coefficient, $S(T)$, measurements reveal metal-like behavior for all samples. For $x = 5$ and 6, density functional theory calculations are performed for deriving the enthalpy of formation and the electronic structure which is exploited for the calculation of Seebeck coefficients and conductivity within Boltzmann's transport theory. For simulating the properties of doped clathrates the rigid band model is applied. On the basis of the density functional theory results the experimentally observed compositional dependence of $\rho(T)$ and $S(T)$ of the whole sample series is analyzed. The highest dimensionless thermoelectric figure of merit $ZT$ of 0.28 is reached for a melt-spun sample at $600^{\circ}$C. The relatively low $ZT$ values in this system are attributed to the too high charge carrier concentrations.Comment: 11 pages, 13 figures, submitted to Phys. Rev.

High temperature thermoelectric (TE) properties for triple-filled skutterudites (Sr(x)Ba(x)Yb₁₋₂x)(y)Co₄Sb₁₂ were investigated for alloy compositions in two sections of the system: (a) for x = 0.25 with a filling fraction y ranging from 0.1 to 0.25 and (b) for 0 < x < 0.5 and y = 0.11 + 0.259x. The representation of the figure of merit, ZT, as a function of skutterudite composition, defined the compositional range (0.25 < x < 0.4; 0.18 < y < 0.24) with ZT over 1.4 at 800 K. It was shown that an enhanced TE performance for these triple-filled skutterudites is caused by low electrical resistivities and low lattice thermal conductivities, as well as by a fine tuning of the chemical composition. Low temperature measurements for the samples with the highest ZT values showed that even a small change of the filler ratios changes the contribution of scattering effects, the carrier concentration and the mobility.

In order to disentangle the effects of the two different crystallographic Ce sites in Ce3Pd20Si6 on the intriguing low‐temperature properties of this compound, we have investigated polycrystalline samples of Ce3−xLuxPd20Si6 ($x = 1,2$) and Ce3−yLayPd20Si6 ($y = 1,2$). The lattice parameters and electrical resistivity measurements in the temperature range 2–300 K are presented. Lu‐substitution leads to a considerable decrease in the unit cell volume, corresponding to positive chemical pressure, while substitution with La expands the lattice much more gradually. We interpret the observed shifts in the characteristic temperatures for the onset of incoherent and coherent Kondo scattering, Tmin and Tmax, respectively, by competing effects of chemical pressure and density of Kondo scatterers. We also investigate the important issue of preferential site occupation by Lu and La and present evidence for Ce2LaPd20Si6 being an essentially ordered compound. The incoherent Kondo scattering dominating the electrical resistivity of Ce3Pd20Si6 between 20 and 300 K appears to be caused by Ce at the 8c‐site only.

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