H. Paulus

Crystal structure and magnetic properties of CuMoO4 at low temperature (γ-phase)

Abstract The crystal structure and magnetic properties of a new modification of copper molybdate (γ-CuMoO 4 ) are reported, which is stable below 200 K. Single-crystal X-ray diffraction revealed triclinic symmetry ( P 1 ) with a = 9.699(9) A , b = 6.299(6) A , c = 7.966(7) A , α = 94.62(4)°, β = 103.36(4)°, γ = 103.17(4)° at 180 K and Z = 6. All Cu- and Mo-ions are surrounded by distorted oxygen octahedra. In contrast to the high-pressure modification CuMoO 4 -II, which is of a triclinic distorted wolframite structure, the octahedra are not closed packed. This is also in contrast to the triclinic ( P 1 ) room-temperature phase (α) at standard pressure with tetrahedrally coordinated Mo-ions and with a = 9.901(3) A b = 6.786(2) A , c = 8.369(3) A , α = 101.13(1)°, β = 96.88(1)° and γ = 107.01(1)°. The new crystal structure of γ-CuMoO 4 is extensively discussed and compared with those of AlNbO 4 and α-CoMoO 4 . The transition between α- and γ-phase is of first order and a wide temperature range of coexistence is observed in powder experiments. The relative amount of both phases depends on both temperature and cooling rate. No long range magnetic ordering was observed down to 1.8 K by SQUID measurements. The magnetic properties of γ-CuMoO 4 can be explained by a chain-like arrangement of copper-moments deduced from the crystal structure. If only interactions within such chains are considered, the calculations based on an Ising-model are in good agreement with experimental data.

Structural Studies on Substituted N-(phenyl)-2,2-DichIoroacetamides, 2/4-XC6H4 NHCO CHCI2 (X = H, Cl or CH3)

Abstract The effect of substitution in the phenyl ring on the crystal systems of amides of the type, 2/4-XC6H4-NHCO-CHCl2 (X = H, Cl or CH3), has been studied by determining the crystal structures of the compounds, iV-(phenyl)-2,2-dichloroacetamide, C6H5-NHCO-CHCl2 (PhDCA); N-(2-chlorophenyl)-2,2-dichloroacetamide, 2-ClC6H4-NHCO-CHCl2 (o-ClPhDCA), jY-(4-chlorophenyl)-2,2-dichloroacetamide, 4-ClC6H4-NHCO-CHCl2 (p-ClPhDCA) and N -(4-methylphenyl)-2,2-dichloroacetamide, 4-CH3C6H4-NHCO-CHCl2 (p-CH3PhDCA) and analysed the data along with our earlier crystal structures of the compounds, N-(phenyl)-2,2,2-trichloro-acetamide (PhTCA), N-(2-chlorophenyl)-2,2,2-trichloroacetamide (o-ClPhTCA), N -(4-chloro-phenyl)-2,2,2-trichloroacetamide (p-ClPhTCA), N-(4-methylphenyl)-2,2,2-trichloroacetamide (p-CH3PhTCA); N-chloro-N-(phenyl)-2,2-dichloroacetamide (NClPhDCA), N-chloro-N-(phenyl)-2,2,2-trichloroacetamide (NClPhTCA) and N-(phenyl) acetamide (PhA). The crystal type, space group, formula units and lattice constants in A of the new structures are; PhDCA: monoclinic, P 21/c, Z = 4, a = 8.785(3), b = 11.139(4), c = 9.521(3), ß = 97.47(2)°; o-CIPhDCA: monoclinic, P 21/c, Z = 4, a = 4.711(2), b = 11.234(6), c = 19.191(8), ß = 98.12(2)°; p-CIPhDCA: monoclinic, P 21/c, Z = 8, a = 18.627(5), b = 11.533(3), c = 9.583(3), ß = 102.43(2)°, and /?-CH3PhDCA: orthorhombic, Pbca, Z = 8, a = 9.464(3), b = 9.894(3), c = 21.973(7). The compound p-ClPhDCA shows two molecules in its asymmetric unit. This is in agreement with the multiple lines observed in the 35Cl NQR spectra of the compound. The crystal systems of the chlorosubstituted and methyl substituted chloroacetamides get interchanged on replacement of the side chain -CHCl2 by -CCl3. C(i)-C(j) ring distances show no significant variations with the substitution either at the side chain or in the phenyl ring. C(ring)-N and C (0)-N bond distances are also not much affected by either ring or side chain substitution, but are affected by N-chlorination, while the C -0 bond length is slightly shortened by the replacement of the -CH3 group by -CCl3, the introduction of electron withdrawing group into the phenyl ring or by N-chlorination. Deviations of C2(ring)-Cl(ring)-N, C6(ring)-Cl(ring)-N and C (ring)-N-C(0) bond angles from 120° narrow down on substitution either in the phenyl ring or in the side chain, but the latter increases on W-chlorination of the compounds.