Investigation by the use of profile refinement of neutron powder diffraction data of the geometry of the [Si2O7]6− ions in the high temperature phases of rare earth disilicates prepared from the melt in crucible-free synthesis

Powders and X-ray size single crystals of the rare earth disilicates RE2Si2O7 (RE = La to Nd and Gd to Lu and Y) were synthesized from the melts in a fast crucible-free process. The purity and unit cell parameters of the disilicates were determined from Guinier photographs. Previously reported structure types of the high temperature phases of RE2Si2O7 (RE = La, Ce, Ho, Er, Yb, and Y) were confirmed by profile refinement of neutron powder diffraction data. The high temperature modifications of La2Si2O7 and Ce2Si2O7 have the type G-structure. They are monoclinic, P21/c with Z = 4 and a = 5.4109(6), b = 8.796(8), c = 14.287(1) Å, β = 112.74(1)° for La2Si2O7, and a = 5.401(1), b = 8.723(1), c = 14.169(2) Å, β = 112.61(2)° for Ce2Si2O7, respectively. The high temperature modifications of Ho2Si2O7 and Y2Si2O7 have the type E-structure. They are orthorhombic, Pna21 with Z = 4 and a = 13.681(4), b = 4.997(3), c = 8.187(2) Å for Ho2Si2O7, and a = 13.6650(1), b = 5.0166(1), c = 8.1494(1) Å for Y2Si2O7, respectively. Er2Si2O7 and Yb2Si2O7 have the type C-structure at high as well as at low temperatures. They are monoclinic, C2/m with Z = 4 and a = 6.8491(9), b = 8.9395(10), c = 4.7222(5) Å, β = 108.810(9)° for Er2Si2O7, and a = 6.7988(3), b = 8.8759(4), c = 4.7101(3) Å, β = 101.982(4) for Yb2Si2O7, respectively.