CRYSTAL STRUCTURE DETERMINATION OF Ag2Ga BY SINGLE CRYSTAL X-RAY DIFFRACTION AND ANALYTICAL ELECTRON MICROSCOPY
Arne Olsen1, Anette E. Gunnas1, Ole Bjorn Karlsen2, Bernt Klewe2, Agnar Aasen2 and Przemyslaw T. Zagierski1
1 Centre for Materials Science/Department
of Physics, University of Oslo, Gaustadalleen 21, 0371 Oslo,
Norway,
2 Department of Chemistry, University of
Oslo, 0315 Oslo, Norway
Keywords: Structure, Ag2Ga, P2m, single crystal X-ray diffraction, electron microscopy
Interest in gallium alloys as dental materials has recently increased due to its possible replacement for amalgam as filling material. In a previous analytical scanning and transmission electron microscopy (SEM and TEM) study of some dental gallium alloys, one of the identified phases was z'-Ag2Ga (Gunnas, Olsen and Hero, 1997). The equilibrium phase diagram of the Ag-Ga system at temperatures below 300 oC is not well known and structure reports on z'-Ag2Ga differ markedly from each other. The crystal structure of z'-Ag2Ga has previously been studied with powder X-ray diffraction by Stratton and Kitchingman (1964). They determined the structure to be trigonal with space group P and cell dimensions a = 0.77677 nm and c = 0.28778 nm. The present paper reports a new structure determination of this phase. Crystals of z'-Ag2Ga were grown from melt and studied by single crystal X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The single crystal X-ray diffraction data were recorded with the SMART CCD SYSTEM (Siemens) at two different temperatures: 150 K and room temperature (293 K). The cell dimensions are a = 0.77680(16) nm, c = 0.28759(6) nm at 293 K and a = 0.77460(16) nm, c = 0.28704(5) nm at 150K. Although the cell dimensions were close to the values previously reported in the literature, the crystal structure and atomic positions differ from the previously published structure by Stratton and Kitchingman (1964). The space group is the non-centrosymmetric hexagonal P2m (no. 189) instead of the trigonal P. z'-Ag2Ga was found to have the InMg2 type of structure with all 9 atoms in the unit cell (6 Ag and 3 Ga) distributed on four different atomic sites (3f, 3g, 1b and 2c). The Ag atoms fully occupy the 3f and 3g positions and the Ga atoms the 1b and 2c positions. The space group P2m was found to be consistent with convergent beam electron diffraction (CBED) by using transmission electron microscopy. The composition of the individual single crystals used in the X-ray diffraction experiments was determined with SEM and energy dispersive X-ray spectroscopy (EDS).