Structure Determination of Ca3HfSi2O9 and Ca3ZrSi2O9 from Powder Diffraction

The crystal structure of Ca3HfSi2O9 has been determined from X-ray powder diffraction data using a recently developed software package in order to find individual intensities from overlapping reflections. The Hf atoms were found from the Patterson map, while Ca atoms were found in a subsequent heavy atom Fourier map. The other atoms were determined by trial and error using the Rietveld method. The atomic parameters for Ca3ZrSi2O9 were determined from neutron powder diffraction data with the structure of Ca3HfSi2O9 use as the trial model. Ca3HfSi2O9, a = 7.3517(4)Å, b = 10.1489(11) Å, c = 10.4319(12)Å, β = 91.(184(7)°, P21/c, and Z = 4; Ca3ZrSi2O9, a = 7.3603(1)Å, b = 10.1766(3) Å, c = 10.4514(3) Å, β = 90.875(2)°, P21/c, and Z = 4. The structure contains ribbons of edge-sharing octahedra parallel to [100]. The structure of these compounds 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.