Twinning in crystals of the isomorphous compounds [Me(II)(N2H4)2]Cl2

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Abstract
The monoclinic crystals of the isomorphous compounds with general formula [ Me (II)(N 2 H 4 ) 2 ]Cl 2 , where Me (II) = Cd, Zn, Mn, form twins according to different laws. The twin law (001) was observed in crystals of [Cd(N 2 H 4 ) 2 ]Cl 2 , of [Zn(N 2 H 4 ) 2 ]Cl 2 and of [Mn[N 2 H 4 ) 2 ]Cl 2 ; the twin law (110) in crystals of [Cd(N 2 H 4 ) 2 ]Cl 2 and of [Mn(N 2 H 4 ) 2 ]Cl 2 ; the twin law (100) only in crystals of [Mn(N 2 H 4 ) 2 ]Cl 2 . Twinning was deduced from Weissenberg photographs, taking into account the symmetries of the reciprocal layers of the twins. A large number of crystals was examined and their statistical distribution over the twin laws was calculated. The compounds can be assigned to the possible space groups C 2 h 3 – C 2/ m ; C 2 3 – C 2; C s 3 – Cm . From the dimensions of the unit cells it can bo inferred that molecules of hydrazine form bridge-complexes between two metal-ions, as in the compounds with general formula [ Me (II)(N 2 H 4 ) 2 ](NCS) 2 . Obliquities of the twins and twin indices were calculated. Due to the geometrical rather than structural character of the quasi-normal axis, no sure relation can be found between the obliquities, or the twin indices, and the statistical frequencies with which the twin laws appear. More likely the formation of the twins is due to chains of bridge complexes [unk], which form “clusters“ opposing the adjustment of single atoms in the previous structure.

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