We report structural, transport, and thermal properties of carrier-tuned ${\text{Ba}}_8{\text{Ga}}_{16}{\text{Sn}}_{30}$ single crystals with the type-1 clathrate structure ($\beta$ phase), demonstrating that ${\text{Ba}}_8{\text{Ga}}_{16}{\text{Sn}}_{30}$ 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 ($\alpha$ phase) and of $A_8{\text{Ga}}_{16}{\text{Ge}}_{30}$ $\left(A=\text{Sr},\text{Eu}\right)$. Differential thermal analysis and powder x-ray diffraction show that both phases are stable up to their virtually identical melting point of $520\pm 3°\text{C}$. Refinements of single-crystal x-ray diffraction data indicate that the Ba(2) guest ion in the tetrakaidecahedron occupies the off-center $24k$ sites which are $0.43–0.44\text{Å}$ away from the centered $6d$ site. The temperature-linear coefficient of the specific heat is $29\text{mJ}/\text{mol}{\text{K}}^2$ for both $n$- and $p$-type carriers in the $\beta$ phase, four times larger than that for the $\alpha$ phase, suggesting contributions from tunneling of the Ba(2) guest ions between off-center minima. Analysis of specific heat with a soft-potential model (SPM) gives a characteristic energy of 20 K for the Ba(2) vibration, significantly lower than 50 K for the $\alpha$ phase and in fact the lowest among type-1 clathrates. The lattice thermal conductivities ${\kappa }_L$ for the $\beta$ phase with both charge carrier types are very similar and show a glasslike temperature dependence. This behavior in ${\kappa }_L\left(T\right)$ is also described using SPM analysis, where it is found that the coupling strength between guest modes and acoustic phonons for the $\beta$ phase is significantly larger than that for ${\text{Sr}}_8{\text{Ga}}_{16}{\text{Ge}}_{30}$.

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 $\kappa \left(T\right)$, and specific heat $C_p\left(T\right)$ of carrier-tuned ${\mathrm{Ba}}_8{\mathrm{Ga}}_{16}{X}_{30}$ $(X=\mathrm{Ge},\mathrm{Sn})$ clathrates. These crystals show radically different low-temperature $\kappa \left(T\right)$ 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, $C_p\left(T\right)$ above $4\mathrm{K}$ and the general structural properties are essentially insensitive to carrier tuning. We analyze these combined results within the framework of tunneling scattering, resonant scattering, and Rayleigh scattering models, and conclude that the evolution from crystalline to glasslike $\kappa \left(T\right)$ is accompanied by an increase both in the effective density of tunneling 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 that determines crystalline or glasslike behavior is the coupling strength between the guest vibrational modes and the frameworks with different charge carriers.

Samples of aligned ${\mathrm{MgB}}_2$ crystallites have been prepared, allowing for the first time the direct identification of an upper critical field anisotropy $H_{c2\mathrm{}}^{\mathrm{ab}}{/H}_{c2\mathrm{}}^c={\xi }_{\mathrm{ab}}/{\xi }_c\simeq 1.7$, with ${\xi }_{o,ab}\simeq 70Å$, ${\xi }_{o,c}\simeq 40Å$, and a mass anisotropy ratio $m_{\mathrm{ab}}{/m}_c\simeq 0.3$. A ferromagnetic background signal was identified, possibly related to the raw materials purity.

We present a systematic study of thermal conductivity, specific heat, electrical resistivity, thermopower, and x-ray diffraction measurements performed on single-crystalline samples of the pseudoquaternary type-I clathrate system ${\mathrm{Sr}}_8{\mathrm{Ga}}_{16}{\mathrm{Si}}_{30-x}{\mathrm{Ge}}_x$, in the full range of $0⩽x⩽30$. All the samples show metallic behavior with $n$-type majority carriers. However, the thermal conductivity and specific heat strongly depend on $x$. Upon increasing $x$ from 0 to 30, the lattice parameter increases by 3%, from $10.446\text{to}10.726\mathrm{\AA }$, and the localized vibrational energies of the Sr guest ions in the tetrakaidecahedron (dodecahedron) cages decrease from $59\left(120\right)\mathrm{K}\text{to}35\left(90\right)\mathrm{K}$. Furthermore, the lattice thermal conductivity at low temperatures is largely suppressed. In fact, a crystalline peak found at $15\mathrm{K}$ for $x=0$ gradually decreases and disappears for $x⩾20$, evolving into the anomalous glasslike behavior observed for $x=30$. It is found that the increase of the free space for the Sr guest motion directly correlates with a continuous transition from quasi-on-center harmonic vibration to off-center anharmonic vibration, with consequent increase in the coupling strength between the guest’s low-energy modes and the cage’s acoustic phonon modes.

Raman active phonons in the type-I clathrate compounds of $X_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ $(X=\mathrm{Eu},\mathrm{Sr},\mathrm{Ba})$ have been fully assigned by the combination of polarized Raman scattering and first principles calculations. The dynamical motions of guest ions in the $6d$-site cage have been clearly identified. The energy of the modes, related to a rattling motion, decreases with decreasing temperature. The energy-decrease at low temperatures is a common property for thermal rattling in $X_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$. The anomalous decreases are originated from the quartic anharmonic potential, and its contribution has been experimentally determined. A mixed state between thermal rattling and quantum tunneling has been observed below $10\mathrm{K}$. A soft mode due to the guest ion motion has been observed, but the transition due to this soft mode is somewhat different from the ordinary structural transition because of the shielding by carriers. In addition, it is found that the elastic dispersion of ${\mathrm{Sr}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ is caused by the interference with the rattling motion of the guest ion in the cage, and that the rattling at the off-center location plays an important role to suppress a lattice thermal conductivity and to achieve the phonon-glass state in $X_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$.

For the past decade, intermetallic clathrates have been rattling their way into mainstream research in thermoelectrics. The unusual vibrations of their guest ions inside oversized cages interfere with the cage phonons while leaving the electronic flow intact, providing an exotic way to achieve a best-of-both-worlds scenario in terms of electrical and thermal transport in the same material. Here, we present the structural and thermoelectric properties of ${\mathrm{Ba}}_8{\mathrm{Ga}}_{16}{\mathrm{Sn}}_{30}$ single crystals grown in the type-I clathrate structure $\left(\beta \text{-}\mathrm{BGS}\right)$ , showing one of the lowest recorded thermal conductivities $\kappa \left(T\right)$ for any bulk compound, while still behaving electronically as a heavily doped $n$ -type semiconducting crystal.