[en] Full text: Intermetallic clathrates have low thermal conductivities κ but high electrical conductivities σ ('phonon glass - electron crystal') which leads to a large ratio of σ/κ. The introduction of strong electronic correlations which is expected if adequate rare earth atoms are incorporated would in addition enhance the value of the thermopower S. To date the only rare earth containing clathrate is Eu₈Ga₁₆Ge₃₀. However, it shows no sizeable interaction of 4f electrons with the conduction electrons. In our search for ways to substitute Eu with other rare earth elements we have found an unconventional synthesis procedure s. The poster of A. Prokofiev et al., 'Rapid quenching as an unconventional synthesis technique for Eu₈Ga₁₆Ge₃₀-based clathrate phases'. The bonding situation in a clathrate Eu₈Ga₁₆Ge₃₀ can be understood in a first approximation with the Zintl concept. Ga and Ge build a covalently bonded tetrahedrally coordinated cage structure. The guest Eu atoms are ionically bonded to the framework. The electronegative Ga and Ge receive electrons from the electropositive Eu in accordance with the octet rule. Since all valence electrons are used in local covalent bonds, one expects clathrates to be semiconducters. Here we present the results of the magnetic and electric transport investigations. The resistivity and Hall-resistivity were measured over a broad temperature range from 350 to 2 K at magnetic fields from 0 to 9 Tesla. In contrast to conventional procedures a semiconducting compound has been produced by quenching of the melt. (author)

[en] Full text: Due to their low and 'glass-like' thermal conductivities intermetallic clathrates have a large potential for thermoelectric applications. The introduction of strong electronic correlations would further enhance the thermoelectric figure of merit Z = F²σ/κ; Kondo insulators are known to exhibit giant values of the thermopower. To date only europium clathrates were known which show however no sizeable interaction of 4f electrons with the conduction electrons. All previous attempts to incorporate other rare earth elements in clathrates based on the conventional synthesis techniques (crystallization from the melt followed by long annealing) have failed because of higher thermodynamic stabilities of other competing non-Eu rare earth ternary phases. A synthesis procedure that could use different formation kinetics of the participating phases was searched. Rapid quenching was tried in order to obtain the Yb substituted Eu_8-xYb_xGa₁₆Ge₃₀ phase (0 < x < 1.6) in a metastable state. Surprisingly well formed crystalline clathrate phases have yielded. The quenching procedure and the results of the characterization of the quenched phases by the x-ray powder diffraction, SEM/EDX and TEM are presented. The quenched clathrates show unusual electrical transport properties - unlike the conventionally synthesized Eu-clathrates which show metallic behavior (contrary to expectation from the Zintl concept), our materials are semiconductors (more detailed about the electrical properties see poster of S. Laumann et al 'Electric transport and magnetization investigation of the rapidly quenched clathrate Eu₈Ga₁₆Ge₃₀'). (author)

[en] A promising route to highly efficient thermoelectric materials is to introduce the effect of strong electronic correlations into cage compounds which contain weakly bound ions in oversized cages of the framework structure. By this approach the low thermal conductivity of cage compounds shall be combined with giant thermopower values of strongly correlated systems, thus doubly optimizing the thermoelectric figure-of-merit. We present here results on a new candidate material, Ce₄Pt₁₂Sn₂₅. There is a unique Ce site in the center of a polyhedron cage made up of 6 Pt and 12 Sn atoms. Synthesis, structure and various physical properties measured between room temperature and 2 K are presented.

[en] The heavy fermion compound Ce₃Pd₂₀Si₆ is one of the rare examples of a cubic system with a readily accessible quantum critical point. Ce atoms at the two different sites 4a and 8c of the crystal structure have recently been shown to have different crystal field ground states (Γ₇ and Γ₈, respectively) and are assumed to be responsible for the two different low-temperature phase transitions at T_L and T_U, which have been tentatively attributed to antiferromagnetic and antiferroquadrupolar order, respectively. Here we present electrical resistivity measurements in a wide temperature range (50 mK-300 K) on two new representatives of the La substitution series Ce_3-xLa_xPd₂₀Si₆, x= 1/3 and 2/3 . Put in the context of previously published data our results indicate that La preferentially occupies the 4a site and that Ce ions at the 8c site have a sizably larger Kondo temperature. Low-temperature resistivity measurements in applied magnetic fields suggest that (disorder smeared) quantum critical points exist in the x=1/3 and 2/3 samples at fields below 1 T.

[en] High temperature thermoelectric (TE) properties for triple-filled skutterudites (Sr_xBa_xYb_1-2x)_yCo₄Sb₁₂ 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.

[en] The release of unburnt coal particles and associated polycyclic aromatic hydrocarbons (PAHs) may cause adverse impacts on the environment. This study assessed variations in the concentration and composition of PAHs in a set of fifty coal samples from eleven coal basins worldwide. The maximum PAH concentrations at high volatile bituminous rank were recorded in samples from a single basin. Considering the entire sample set, the highest PAH concentrations were in fact found outside of this rank range, suggesting that the maceral composition and thus the coal's origin also influenced PAH concentrations. The examination of the PAH compositions revealed that alkylated 2-3 ring PAHs remain dominant compounds irrespective of coal rank or origin. Multivariate analysis based on PAH and maceral content, bulk and maturity parameters allowed the recognition of seven groups with different rank and origin within the coal sample set. - Highlights: → The concentration of extractable PAHs depends on both, coal rank and origin. → Alkylated 2-3 ring PAHs dominate in all coals independently of their rank and origin. → In coals of higher rank and different origin monomethylated PAHs predominate. → PCA performed using PAHs separates coals in 7 groups according to the rank and origin. - The origin of coal has a strong influence on the concentration and composition of indigenous PAHs.

[en] Ce₃Pd₂₀Si₆ has recently been shown to undergo two successive low-temperature phase transitions. The suppression of one of these by a magnetic field induces quantum critical phenomena. Here we present the first results of both elastic and inelastic neutron scattering on poly- and single-crystalline Ce₃Pd₂₀Si₆