CRYSTAL STRUCTURE DETERMINATION OF TWO POLYMORPHIC PHASES OF LANTHANUM NITRATE TETRAHYDRATE FROM X-RAY POWDER DIFFRACTION. M. Louër, A.-E. Gobichon, J. P. Auffrédic, D. Louër, Laboratoire de Cristallochimie, CSIM (URA CNRS 1495), Université de Rennes, 35042 Rennes cedex, France

Two polymorphic varieties of La(NO₃)₃.4H₂O have been identified from temperature dependent X-ray diffraction experiments of the hexahydrate carried out in different water pressures. The two phases [[alpha]] and [[beta]] are obtained for P(H₂O) greater and lower than 4 Torr, respectively. Powder diffraction data for the two phases were collected with a high resolution powder diffractometer using monochromatic radiation ([[lambda]] = 1.5406 Å). The indexing of the powder patterns was performed by the successive dichotomy method (DICVOL91), the structure solutions were obtained from Patterson maps and subsequent Fourier analyses and, finally, the models were refined by the Rietveld method (FULLPROF).

[[alpha]]-La(NO₃)₃.4H₂O: P2₁/m, a = 6.778(1), b = 11.367(2), c = 6.585(1) Å, [[beta]] = 90.639(5)deg., V = 507.3 ų, Z = 4, M₂₀ = 28, F₃₀ = 59(0.011,44). The Rietveld refinement (49 varying parameters) converged to R_F = 0.05 and R_wp = 0.08. The structure consists of infinite chains, formed by 11-coordinated La atoms, running along [001]. The La polyhedra, formed by three bidentate, one monodentate nitrate groups and four water molecules, are connected through a bridging nitrate group. The cohesion of the structure is ensured by a network of hydrogen bonds. The 11-coordination of lanthanum has also been reported for the structure of the hexahydrate precursor.

[[beta]]-La(NO₃)₃.4H₂O: Pbca, a = 13.531(1), b = 11.834(1), c = 12.973(1) Å, V = 2027.3 ų, Z = 8, M₂₀ = 60, F₃₀ = 85(0.0048,74). The Rietveld refinement (71 varying parameters) converged to R_F = 0.06 and R_wp = 0.12. The structure consists of infinite chains, formed by 10-coordinated La atoms, running along [001]. The polyhedra are formed by two bidentate, two bridging monodentate nitrate groups and four water molecules.