Transport and magnetic properties of rare-earth containing clathrates and clathrate-like compounds

The main aim of the doctoral work described in this thesis was to search for strongly correlated electron behavior in clathrates. Two Ba8Ga16xGe30+x samples synthesized in Ga-flux have been investigated. As expected, they have p-type charge carrier properties since the Ga content exceeds 16 and thereby x < 0. It is shown that m* ∝ 3.5 m0 for the charge carriers in the valence band of Ba8Ga16-xGe30+x. Combining the analysis of the electrical transport properties of p-type Ba8Ga16-xGe30+x and n-type α- and β-Eu8Ga16-xGe30+x (from this thesis with literature data of both p- and n-type Ba8Ga16-xGe30+x) and n-type Sr8Ga16-xGe30+x reveals that a relatively large concentration of impurity-like states are located close to the edge of the valence band in these compounds. Although α- and β-Eu8Ga16-xGe30+x do not show strongly correlated electron behavior, the thermoelectric properties are relatively good. A β-Eu8Ga16-xGe30+x sample with n = 0.431 e-/u.c. has ZT ∼ 0.4 at 400 K which is about half of the ZT of the state of art material around that temperature. All measurements of the magnetic properties of α- and β-Eu8Ga16-xGe30+x show that they are both local moment ferromagnets with ordering temperatures (Tc) close to the previously published values of 10.5 K and 36 K for the α and β phase, respectively. According to the low temperature saturation magnetization and temperature dependent susceptibility (χ) Eu is in a 3+ state. In the second experimental part of this thesis two new rare-earth containing clathrate-like systems were investigated. The first is RE2Ni36P15, with RE = La, Ce and Pr and the second is RE3Pd20P6 with RE = La - Lu. The Hall coefficient (RH) and resistivity (p) of these compounds indicate that these compounds are semimetals or a very small band gap is located around the Fermi level. χ(T) of Pr2Ni36P15 has a normal local moment paramagnetic temperature dependence, with Pr in a +3 state, combined with crystal electric fields (CEF) effects. Ce2Ni36P15 is a heavy fermion system with TK ∼ 1 K - 2 K, with non-Fermi liquid like behavior at lowest temperatures. Above TK the compound shows normal paramagnetic behavior that can be explained by CEF effects and Ce in a 3+ state. KL(T) of all three RE2Ni36P15 compounds is very similar. (orig.)