Overview of Experimental Results on Filled Skutterudites with Pt-Ge framework

Filled skutterudites are compounds with the body-centred (space group Im-3, a = 9 Å) crystal structure and total chemical formula MT₄X₁₂ (structure type LaFe₄P₁₂), where M = alkaline, alkali-earth, rare-earth or actinide metal, T is a transition metal of Fe-subgroup and X is a pnictogen-atom. Their structural arrangement belongs to the group of the so-called cage-compounds, where positively charged filler-atoms (i.e. M-component) are situated inside large voids formed by a covalently bonded atomic framework (i.e. T₄X₁₂). Such a combination of different types of chemical bonding together with vibrational dynamics of the filler results in numerous exciting physical phenomena (e.g. superconductivity, heavy fermion and Kondo behaviours, charge-density wave, thermoelectric properties etc.). In the past decade we succeed in the preparation and study of the physical properties of filled skutterudites with the new Pt-Ge framework and total formula MPt₄Ge₁₂ (M = Sr, Ba, La, Ce, Pr, Nd, Eu, Sm). {Sr, Ba, La, Pr}Pt₄Ge₁₂ are superconductors with high transition temperatures (i.e. up to ∼8 K). The complete characterization of their properties, which included μSR and penetration depth measurements, revealed LaPt₄Ge₁₂ to be a multi-gap superconductor, while the PrPt₄Ge₁₂ is an unconventional superconductor with nodes in the energy gap. CePt₄Ge₁₂ is found to be a system at the very border between typical intermediate valence and true Kondo lattice behaviour. SmPt₄Ge₁₂ is a high-pressure phase with the moderate heavy fermion behaviour. Filled skutterudites {Nd, Eu}Pt₄Ge₁₂ show Curie-Weiss paramagnetism in the temperature range 10-400 K and complicated magnetic structures with multiple orderings below 2 K.