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An investigation on the crystal structure and thermoelectric properties of type-I clathrate compounds in the Ba-Ga-Ge system is conducted as a function of Ga content. The absolute values of electrical resistivity and Seebeck coefficient increase, while that of lattice thermal conductivity decreases with the increase in the Ga content.

Ba(sub 8)Ga(sub 16)Si(sub x)Ge(30-x) were synthesized and their structural and electrical properties compared to those of similar type-I clathrates (Ba(sub 8)Ga(sub 16)Si(sub x)Ge(30-x)and Sr(sub 8)Ga(sub 16)Si(sub x)Ge(30-x)). The increasing Si content is accompanied by a slight increase in the absolute Seeback coefficient even as the carrier concentration increases and the resistivity decreases.

The synthesis of A(sub 8)(sup II)B(sub 16)(sup III)B(sub 30)(sup IV) clathrates, study of their formation mechanisms and measurements of their transport properties over the temperature range 100-870 K is reported. Results show that the calculated effective masses of the clathrates range from 0.9 to 3 of the free electron mass.

Single-crystalline samples of type-VIII clathrate [Ba.sub.8][Ga.sub.16-x][Al.sub.x][Sn.sub.30] are grown from Sn flux to analyze the structural and thermoelectric properties from 300 to 600 K. The Seebeck coefficients of all samples are largely negative and the large thermopower has coexisted with the metallic behavior in the electrical resistivity.

Several studies are conducted to explain the thermal expansion taking place in the different series of thermoelectric type-I-clathrates. The thermal expansion is shown to be highly dependent on the temperature and the thickness of the clathrates.

An ab initio study is used for analyzing the structural stability of carbon clathrates at high pressure and identifying fcc-[C.sub.135] clathrate as the third most stable carbon phase after cubic diamond and hexagonal graphite. Phonon dispersion calculations have confirmed the dynamic stability of fcc-[C.sub.136] along with diamond.

The type-VIII [Sr.sub.8][Al.sub.x][Ga.sub.y][Ge.sub.46-x-y] samples with various concentrations are prepared to examine their thermoelectric properties. The type-VIII clathrate had a smaller effective mass, a higher mobility and a higher lattice thermal conductivity than those of the type-I clathrate and the difference in transport properties between the type-I and type-VIII clathrates is discussed.

Powder inelastic neutron scattering (INS) is used for determining the guest atom 'rattling' energy in thermoelectric clathrates [Ba.sub.8][Y.sub.x][Ge.sub.46-x] ([Y.sub.x] = [Ni.sub.6], [Cu.sub.6], [Zn.sub.8], [Ga.sub.16]) under different applied conditions. Chemical pressure is exerted by the atomic substitution and the volume reduction induced by the physical pressure has increased the energy of the guest atom rattling mode.

First principles calculations are used for examining both type I and II silicon clathrates. The mean free paths and thermal conductivities of the clathrates are larger than those of the diamond phase Si (d-Si) at low temperature and the Seebeck coefficients (S) of the clathrates are higher than that of d-Si in the temperature interval 300-1000 K.

The crystal structure and thermoelectric properties of type-III clathrate compounds in the Ba-In-Ge system are evaluated as a function of In content. The findings demonstrate the changes in lattice thermal conductivity in terms of the low rattling frequency of the guest atom as well as the low Debye temperature of the cage framework.

The solid-state reaction is combined with the spark plasma sintering method in order to synthesize Zn-doping [Ba.sub.8][Ga.sub.16][Ge.sub.30-x] clathrates with different Zn contents. The effects of Zn content on thermoelectric transport properties are examined in order to develop the high thermoelectric performance p-type clathrate.

The thermoelectric properties of type-III clathrate compounds in the Ba-Al-Ge system were examined as a part of Al content. One of the results concluded that solid solubility of Al in the type-III clathrate compounds was found to be less than X=12.

Polycrystalline samples of [Sr.sub.8][Ga.sub.x][Ge.sub.46-x] clathrates with various carrier concentrations are prepared and their thermoelectric and transport properties are measured. The band gap of the samples is estimated to be 0.4-0.5 eV, their effective mass is a constant value of 3.1 [m.sub.e] at room temperature which decreases with increasing temperature and their electron Hall mobilities are 8-13 [cm.sup.2] [V.sup.-1] [s.sup.-1] at room temperature, increasingly slightly with decreasing carrier concentration.