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Crystals of Ni(NO3)2.4HzO were prepared by evaporation of acid solutions of nickel nitrate. The unit cell is monoclinic with a= 5-305, b= 27.24, c= 5.705 A, fl= 114 °, Z= 4. The space group is P21/n. The crystal structure was determined by means of the [100] and [001] electron density projections, performed with the help of Von Eller's Photosommateur. Refinement of atomic coordinates by the least-squares method was achieved with the Univac 1107 computer, using 850 independent reflexions. The final R index is 0.074. Hydrogen atoms were located by difference synthesis and their coordinates were refined by the least-squares procedure. Each nickel atom is surrounded by an octahedron composed of four water molecules and two oxygen atoms belonging to the nitrate ions, which are distorted. Nickel nitrate tetrahydrate has a layer structure: two layers of octahedra parallel to the (010) plane are separated by two layers of nitrate ions. Octahedra are linked together by hydrogen bonds, in the layers and between the layers.

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