Amorphization of Al32Ge68 in a slow heating run studied by NMR

Amorphization of Al₃₂Ge₆₈ at normal pressure, starting from the thermobarically quenched high-pressure crystalline metallic γ-phase, was studied in a slow heating run by performing NMR field-sweep experiments. In the temperature interval from 77 to 300 K the ²⁷Al central line exhibits a continuous broadening and a positive frequency shift. Close to 300 K rather abrupt changes were observed, where the shift changed its sign, and the line width jumped to its highest value. The discontinuous course of the phase changes indicates that the amorphization process proceeds via a sequence of intermediate metastable states, which can be ‘overheated’ in a slow heating run and consequently transform in an explosion-like manner to the final amorphous state at a well-defined temperature. The frequency shift could be decomposed into the negative second-order quadrupolar shift and the positive Knight shift. The change of the shift sign from positive to negative at 300 K reflects the vanishing tendency of the Knight shift upon heating and is compatible with the conduction-electron localization upon structure amorphization.