Crystal structure of La₄Si₂O₇N₂ analyzed by the Rietveld method using the time-of-flight neutron powder diffraction data

The polycrystalline single-phase sample of La₄Si₂O₇N₂ was prepared at 1773 and 1873 K by the gas-pressured sintering method. The structure refinement was carried out by the Rietveld method using time-of-flight neutron powder diffraction data measured at 297 K. La₄Si₂O₇N₂ is isostructural with the Y₄Al₂O₉ high-temperature phase and crystallizes in a monoclinic cell, P2₁/c (No. 14-1), Z = 4. The lattice parameters refined for the sample prepared at 1773 K were α = 8.0375(2) Å, b = 10.9900(2) Å, c = 11.1115(2) Å, and β = 110.9214(14)°. Nitrogen atoms statistically occupy the bridging site and the terminal sites of Si₂O₅N₂ ditetrahedra in the La₄Si₂O₇N₂ structure. The nonmetal sites surrounded by La atoms only are fully occupied by oxygen atoms.