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Abstract

The recently discovered clathrate Ba6Ge25 undergoes a two-step first-order phase transition at the temperatures T(S1,S2)approximate to215, 180 K. The first-order nature of the transition is evidenced from the hysteretical temperature dependences of the electrical resistivity rho(T), the Hall coefficient R-H(T), and the magnetic susceptibility as well as from the temperature dependence of the specific heat. rho(T) increases drastically below T-S1,T-S2, but the charge-carrier concentration, as determined from R-H(T), is virtually unaffected by the phase transition. Thus, it is the charge- carrier mobility which is strongly reduced below T-S1,T-S2. Taking these observations together with results from a recent structural investigation we conclude that the "locking-in" of "rattling" Ba atoms to off-center positions in the Ge cages is responsible for the mobility reduction of the conduction electrons. It is due to this strong electron-phonon interaction that, while the concept of a "phonon glass" appears to be fulfilled, the concept of an "electron crystal" is heavily violated, in contrast to other filled-cage systems. In the phase Ba6-xEuxGe25 (xless than or equal to0.6), T-S1,T-S2 is quickly suppressed with increasing x and, in Ba4Na2Ge25 [Ba6- xNaxGe25 (x=2)], the locking-in transition is absent alltogether at temperatures below 400 K.