High temperature thermoelectric properties of the type-I clathrate Ba₈Ni_xGe_46-x-ysquare_y

Polycrystalline samples of the type-I clathrate Ba8NixGe46-x-y square(y) were synthesized for 0.2 <= x <= 3.5 by melt quenching and for 3.5 < x <= 6.0 by melting with subsequent annealing at 700 degrees C. The maximum Ni content in the clathrate framework at this temperature was found to be x approximate to 4.2 atoms per unit cell. Thermoelectric and thermodynamic properties of the type-I clathrate were investigated from 300 to 700 K by means of electrical resistivity, thermopower, thermal conductivity and specific heat measurements. As the Ni content increases, the electronic properties gradually evolve from a metallic character (x < 3.5) towards a highly doped semiconducting state (x >= 3.5). Below x approximate to 4.0 transport is dominated by electrons, while further addition of Ni (x approximate to 4.2) switches the electrical conduction to p-type. Maximum value of the dimensionless thermoelectric figure of merit ZT approximate to 0.2 was achieved at 500K and 650K for x approximate to 2.0 and x approximate to 3.8, respectively