Type-I clathrate compounds have attracted a great deal of interest in connection with the search for efficient thermoelectric materials. These compounds constitute networked cages consisting of nano-scale tetrakaidecahedrons (14 hedrons) and dodecahedrons (12 hedrons), in which the group 1 or 2 elements in the periodic table are encaged as the so-called "rattling" guest atom. It is remarkable that, though these compounds have crystalline cubic-structure, they exhibit glasslike phonon thermal conductivity over the whole temperature range depending on the states of rattling guest atoms in the tetrakaidecahedron. In addition, these compounds show unusual glass-like specific heats and THz-frequency phonon dynamics, providing a remarkable broad peak almost identical to those observed in topologically disordered amorphous materials or structural glasses, the so-called Boson peak. An efficient thermoelectric effect is realized in compounds showing these glass-like characteristics. This decade, a number of experimental works dealing with type-I clathrate compounds have been published. These are diffraction experiments, thermal and spectroscopic experiments in addition to those based on heat and electronic transport. These form the raw materials for this article based on advances this decade. The subject of this article involves interesting phenomena from the viewpoint of not only physics but also from the view point of the practical problem of elaborating efficient thermoelectric materials. This review presents a survey of a wide range of experimental investigations of type-I clathrate compounds, together with a review of theoretical interpretations of the peculiar thermal and dynamic properties observed in these materials.

We report structural, transport, and thermal properties of carrier-tuned Ba 8 Ga 16 Sn 30 single crystals with the type-1 clathrate structure ͑␤ phase͒, demonstrating that Ba 8 Ga 16 Sn 30 is a unique thermoelectric clathrate material wherein both the structure type and the carrier type are tunable. The results are compared with the properties of the better known type-8 structure ͑␣ phase͒ and of A 8 Ga 16 Ge 30 ͑A =Sr,Eu͒. Differential thermal analysis and powder x-ray diffraction show that both phases are stable up to their virtually identical melting point of 520Ϯ 3°C. Refinements of single-crystal x-ray diffraction data indicate that the Ba͑2͒ guest ion in the tetrakaidecahedron occupies the off-center 24k sites which are 0.43-0.44 Å away from the centered 6d site. The temperature-linear coefficient of the specific heat is 29 mJ/ mol K 2 for both n-and p-type carriers in the ␤ phase, four times larger than that for the ␣ phase, suggesting contributions from tunneling of the Ba͑2͒ guest ions between off-center minima. Analysis of specific heat with a soft-potential model ͑SPM͒ gives a characteristic energy of 20 K for the Ba͑2͒ vibration, significantly lower than 50 K for the ␣ phase and in fact the lowest among type-1 clathrates. The lattice thermal conductivities L for the ␤ phase with both charge carrier types are very similar and show a glasslike temperature dependence. This behavior in L ͑T͒ is also described using SPM analysis, where it is found that the coupling strength between guest modes and acoustic phonons for the ␤ phase is significantly larger than that for Sr 8 Ga 16 Ge 30 .

An antiferroquadrupolar ordering at T(Q)=0.11  K has been found in a Pr-based superconductor PrIr(2)Zn(20). The measurements of specific heat and magnetization revealed the non-Kramers Γ(3) doublet ground state with the quadrupolar degrees of freedom. The specific heat exhibits a sharp peak at T(Q)=0.11  K. The increment of T(Q) in magnetic fields and the anisotropic B-T phase diagram are consistent with the antiferroquadrupolar ordered state below T(Q). The entropy release at T(Q) is only 20% of Rln2, suggesting that the quadrupolar fluctuations play a role in the formation of the superconducting pairs below T(c)=0.05  K.

The low temperature state of CeRu2Al10 has been studied by neutron powder diffraction and muon spin relaxation (µ + SR). By combining both techniques, we prove that the transition occurring below T * ∼27K, which has been the subject of considerable debate, is unambiguously magnetic due to the ordering of the Ce sublattice. The magnetic structure with propagation vector k=(1,0,0) involves collinear antiferromagnetic alignment of the Ce moments along the c-axis of the Cmcm space group with a reduced moment of 0.34(2)µB . No structural changes within the resolution limit have been detected below the transition temperature. However, the temperature dependence of the magnetic Bragg peaks and the muon precession frequency show an anomaly around T2 ∼12K indicating a possible second transition.

The present controversy over the origin of glasslike thermal conductivity observed in certain crystalline materials is addressed by studies on single-crystal x-ray diffraction, thermal conductivity κ(T ) and specific heat Cp(T ) of carrier-tuned Ba8Ga16X30 (X = Ge, Sn) clathrates. These crystals show radically different low-temperature κ(T ) behaviors depending on whether their charge carriers are electrons or holes, displaying the usual crystalline peak in the former case and an anomalous glasslike plateau in the latter. In contrast, Cp(T ) above 4 K and the general structural properties are essentially insensitive to carrier tuning. We analyze these combined results within the framework of a Tunneling/Resonant/Rayleigh scatterings model, and conclude that the evolution from crystalline to glasslike κ(T ) is accompanied by an increase both in the effective density of tunnelling states and in the resonant scattering level, while neither one of these contributions can solely account for the observed changes in the full temperature range. This suggests that the most relevant factor which determines crystalline or glasslike behavior is the coupling strength between the guest vibrational modes and the frameworks with different charge carriers.