HIGH PRESSURE PHASE TRANSITIONS OF RARE-EARTH GALLIDES REGa₂ (RE = Tm, Er, Ho). S. Bräuninger, U. Schwarz, Yu. Grin and K. Syassen. MPI für Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany.

We have investigated pressure induced structural changes of the rare-earth gallides REGa₂ (RE = Tm, Er, Ho) by means of x-ray powder diffraction using diamond anvil cell techniques. At ambient pressure ErGa₂ and HoGa₂ crystallize in the AlB₂-type structure (P6/mmm) consisting of six-membered planar Gallium layers forming hexagonal prisms centered by the RE atoms. A common feature of all AlB₂-type compounds REGa₂ is an c/a ratio close to the ideal value of 1.07 for touching spheres. ErGa₂ undergoes a discontinuous transition to the KHg₂-type structure (Imma) near 6 GPa. The interlayer Ga-Ga distance 2.788(5) Å (7.6 GPa) is close to the Ga-Ga distance in TmGa₂ which crystallizes in this structure type at ambient pressure. In the KHg₂-type the Ga atoms form six-membered rings in chair conformation. The layers are connected by Ga-Ga bonds along the cystallographic a-axis thus forming a three dimensional covalent gallium network of distorted tetrahedra with the RE atoms occupying the voids. ErGa₂ and TmGa₂ transform into the UHg₂-type structure at 24 and 22 GPa respectively. The UHg₂-type is a branch of the AlB₂-type having a c/a ratio between 0.59 and 0.87. The pressure induced change of hybridization in these compounds results in a transition of a three-dimensional covalent network (KHg₂) to a layered gallium partial structure (UHg₂). The breaking of the interlayer Ga-Ga bonds was reproduced by LMTO calculations of the electron density and localization. A similar behavior is observed for HoGa₂. However in this case a direct transition from the AlB₂- to the UHg₂-type takes place at about 5 GPa.