Co(NH3)2Cl2 and Co(ND3)2Cl2:: Order-disorder behaviour of N(H, d)3 and antiferromagnetic structure

Diammine cobalt(II) chloride, Co(N(H, D)(3))(2)Cl-2 was prepared by decomposition of the corresponding hexaammines at 120 degreesC in dynamical vacuum. Crystal structures and magnetic properties of these materials were characterised by X-ray and neutron powder diffraction, and heat capacity measurements. At ambient temperatures Co(N(H, D)(3))(2)Cl-2 crystallises in the Cd(NH3)(2)Cl-2 type structure: space group Cmmm, Z = 2, a = 8.0512(2) Angstrom, b = 8.0525(2) Angstrom, c = 3.73318(9) Angstrom (X-ray data of the H compound). This structure consists of chains of edge-sharing octahedra (1)(infinity)[CoCl4/2(NH3)(2)] running along the c-axis. Neutron diffraction confirms that that the ND3 groups are rotationally disordered at ambient temperatures. At 1.5 K and 20 K neutron diffraction data reveal rotational ordering of the ND3 groups leading to doubling of the c-axis and to Ibmm symmetry: a = 7.9999(6) Angstrom, b = 7.9911(5) Angstrom, c = 7.4033(3) Angstrom (Z = 4, values for T = 1.5 K). Furthermore, antiferromagnetic ordering is present at these temperatures. It is caused by a ferromagnetic coupling of the magnetic moments at Co2+ (3.60(5) mu (B) at 1.5 K, 3.22(5) mu (B) at 20 K) along the octahedra chains (1)(infinity)[CoCl4/2(NH3)(2)] and antiferromagnetic coupling between neighbouring chains. According to heat capacity measurements the phase transition antiferromagnetic-paramagnetic takes place at T-N = 26 K.