Ab Initio Structure Determination of New Mixed Zirconium Hydroxide Nitrates ZrM(OH)2(NO3)3 (M=K, Rb) from X-Ray Powder Diffraction Data

Two new mixed zirconium hydroxide nitrates ZrM(OH)2(NO3)3 (M=K, Rb) have been synthesized through a wet chemical process. The two crystal structures have been solved ab initio from powder diffraction data collected with conventional monochromatic X-rays. ZrK(OH)2(NO3)3 crystallizes with a monoclinic symmetry [a=16.569(3) ?, b=5.791(1) ?, c=9.813 (2) ?, BETA=90.17(2)°, P21/n, Z=4) and ZrRb(OH)2(NO3)3 with an orthorhombic symmetry [a=10.126(3) ?, b=16.492 (3) ?, c=5.855(2) ?, Pbcn, Z=4]. The heavy atoms have been located from an interpretation of Patterson functions. The coordinates of the remaining light atoms have been determined from successive three-dimensional Fourier maps. The final Rietveld refinement indicators were RF=0.042, Rp=0.077 (M=K) and RF=0.064, Rp=0.115 (M=Rb). Like the structures of ALPHA-Zr(OH)2(NO3)2·1.65H2O and BETA-Zr(OH)2(NO3)2·H2O, the structures of the mixed basic zirconium nitrates are built from edge-sharing ZrO8 polyhedra to form infinite neutral zigzag chains of chemical composition [Zr(OH)4/2(NO3)2]n. The main difference with respect to the hydrated phases is the nature of the cohesion in the structures based on ionic contacts involving intercalated K+ or Rb+ and NO-3 species in the mixed compounds and on a complex hydrogen-bonding network in the hydrated phases. The crystal chemistry of the zirconium hydroxide nitrates is discussed and three structure types are identified.