STRUCTURE DETERMINATION OF CA3HFSI2O9 AND CA3ZRSI2O9 FROM POWDER DIFFRACTION

The crystal structure of Ca3HfSi2O9 has been determined from X-ray pow der diffraction data using a recently developed software package in or der to find individual intensities from overlapping reflections. The H f atoms were found from the Patterson map, while Ca atoms were found i n a subsequent heavy atom Fourier map. The other atoms were determined by trial and error using the Rietveld method. The atomic parameters f or Ca3ZrSi2O9 were determined from neutron powder diffraction data wit h the structure of Ca3HfSi2O9 use as the trial model. Ca3HfSi2O9, a = 7.3517(4) Angstrom, b = 10.1489(11) Angstrom, c = 10.4319(12) Angstrom , beta = 91.084(7)degrees, P2(1)/c, and Z = 4; Ca3ZrSi2O9, a = 7.3603( 1) Angstrom, b = 10.1766(3) Angstrom, c = 10.4514(3) Angstrom, beta = 90.875(2)degrees, P2(1)/c, and Z = 4. The structure contains ribbons o f edge-sharing octahedra parallel to [100]. The structure of these com pounds is nearly the same as that of BeY2O4, substituting Si2O groups for 2 Be and doubling the a-axis. The mineral cuspidine (Ca4Si2O7(F,OH )(2)) has a very similar structure as well. (C) 1995 Academic Press, I nc.