Structure and Magnetism of VSB-2, -3, and -4 or Ni4(O3P-(CH2)-PO3)2·(H2O) n (n = 3, 2, 0), the First Ferromagnetic Nickel(II) Diphosphonates:  Increase of Dimensionality and Multiple Coordination Changes during a Quasi Topotactic Dehydration

• Published in: Chemistry of materials 1999-10, Vol.11 (10), p.2937-2947
• Main Authors: Gao, Qiuming, Guillou, Nathalie, Nogues, Marc, Cheetham, Anthony K, Férey, Gérard
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Domain effects, magnetization curves, and hysteresis; Exact sciences and technology; Inorganic compounds; Magnetic properties and materials; Magnetization curves, hysteresis, Barkhausen and related effects; Magnetization curves, magnetization reversal, hysteresis, barkhausen and related effects; Metal complexes; Physics; Structure of solids and liquids; crystallography; Structure of specific crystalline solids
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm9910714

[Ni4(O3P-CH2-PO3)2·(H2O)3, or VSB-2 was prepared in a pure form under hydrothermal conditions (4−6 days, 453 K, autogenous pressure). It dehydrates in two steps, giving successively the dihydrate VSB-3 at 275 °C and the anhydrous methyl diphosphonate VSB-4 at 350 °C.The structures of the three solids were solved ab initio from X-ray powder data. The three compounds are monoclinic (space group Cc for VSB-2 with a = 19.177(3) Å, b = 8.0930(9) Å, c = 9.1824(8) Å, β = 102.387(9)°, and V = 1391.9(2) Å3, space group C2/c for VSB-3 and VSB-4 with a = 18.683(4) Å, b = 8.152(2) Å, c = 8.936(2) Å, β = 107.65(1)°, V = 1297.0(3) Å3; and a = 17.647(2) Å, b = 8.0727(6) Å, c = 8.7741(7) Å, β = 106.339(7)°, V = 1199.5(1) Å3, respectively). The layered VSB-2 is built from sheets of trimeric edge sharing units of Ni2+ octahedra onto which Ni octahedra and diphosphonate groups are grafted. During the first loss (−1 H2O), the migration of a nickel cation toward a tetrahedral site leads to the connection of the layers and renders the dihydrate VSB-3 three-dimensional. In the totally dehydrated compound VSB-4, stable up to 575 °C, the trimers consist of a central octahedron with two edge-shared square pyramids. The Ni coordinations follow the unique sequence:  4 oct (VSB-2) → 3 oct + tetra (VSB-3) → 1 oct + 2 SQ + tetra (VSB-4) which has drastic effects on the magnetic properties discussed in this paper. Below 4 K, VSB-2 and VSB-4 are indeed canted ferromagnets, whereas VSB-3 is a pure ferromagnet. These open framework phases do not exhibit any porosity.