Structure determination of thermal decomposition products from laboratory X-ray powder diffraction

Abstract

Abstract Ab initio structure determination from powder diffraction data of compounds resulting from solid state transformations is still rather limited. Two major factors influence the stages of the structure elucidation of these solids, i.e. the lower precision in peak position for indexing and the strong line overlap for extracting integrated intensities. The present study deals with the consideration of these two problems for solving the crystal structure of two thermal-decomposition inorganic products, Nd(NO3)3 · 4 H2O and Pb3O2(NO3)2. The new features of DICVOL04, as zero shift refinement, a priori zero search and tolerance of spurious lines have been used for indexing several phases obtained during the thermal treatment of neodymium nitrate hexahydrate. The crystal structures of Nd(NO3)3 · 4 H2O and Pb3O2(NO3)2 have been solved with the direct methods and the structure model of Pb3O2(NO3)2 has been completed using a global optimisation approach. The monoclinic structure of neodymium nitrate tetrahydrate [a = 10.1744(4) Å, b = 8.9716(5) Å, c = 11.7295(5) Å, β = 97.384(4)°, V = 1061.80 Å3, S.G. P21/c] is built from isolated ten-fold coordinated Nd polyhedra. The orthorhombic structure of lead oxide nitrate [a = 7.6034(4) Å, b = 5.7691(4) Å, c = 18.5817(9) Å, V = 815.09 Å3, S.G. Pnma] is built from double chains of edge-sharing OPb4 tetrahedra connected by nitrate groups. In order to estimate the chance of solving crystal structure of nanocrystalline powdered compounds, a comparison of the efficiency of direct methods and direct-space approaches is made from powder diffraction patterns simulated for various crystallite sizes from the crystal structure of the decomposition product γ-Zn2P2O7.