Order and disorder in the modulated phase hp-Mg₉Ge_5.1

Single crystals of Mg₉Ge_5.1(2) were obtained at a pressure of 5 GPa in the temperature range 723 to 1273 K. The metastable high-pressure phase exhibits a crystal structure, which may be described as a host-guest variety of an UCl₃-type framework. The atomic arrangement is characterized by a host crystal structure Mg₉Ge₃ with disordered germanium guests (≈2 Ge per formula unit) in the channels. The composite arrangement undergoes incommensurate modulation. Upon heating at ambient pressure, the phase transforms into the thermodynamically stable compounds Mg₂Ge and Ge at 496(5) K. Measurements of the magnetic susceptibility indicate diamagnetic behavior between 75 and 350 K.