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Crystal Structures and Magnetic Properties of a Novel Layer Perovskite System:  (3-Picoliniumylammonium)CuX4 (X = Cl, Br)

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journal contribution
posted on 02.07.1997, 00:00 by Greg S. Long, Mingyi Wei, Roger D. Willett
The compounds (C6N2H10)CuX4, where the cation is diprotonated 3-picolylamine and X = Cl- or Br-, form an unusual variant of the normal antiferrodistortive A2CuX4 lattice [(C6N2H10)CuCl4, orthorhombic, Pna21, a = 7.747(1), b = 24.960(2), and c = 17.041(1) Å, Z = 12; (C6N2H10)CuBr4, orthorhombic, Pna21, a = 8.133(1), b = 26.129(1), and c = 17.148(1) Å, Z = 12]. The structures contain ribbons of the antiferrodistortive sheets which are six copper atoms wide. The central four Cu(II) ions have the usual elongated octahedral coordination, while on each edge are CuX42- ions which have a distorted tetrahedral geometry. Long Cu···X interactions (4.000 and 4.124 Å for the Cl and Br salts, respectively) between these terminating anions link the ribbons together into sheets. The organic cations provide overall stability to the lattice and link adjacent sheets together, forcing the existence of short two-halide X···X contacts between sheets (average Cl···Cl = 4.629 Å, Br···Br = 4.425 Å). While the changes in the semicoordinate Cu···X distances upon replacement of Cl by Br mirror the change in ionic radius, the X···X distances actually decrease when the larger bromide ion is introduced. This is because these distances are largely dictated by the size of the organic cation. The magnetic data for both salts show the presence of predominant ferromagnetic interactions at high temperature. The Cl salt orders antiferromagnetically at Tc = 5.7 K, and the susceptibility data can be fit to a S = 1/2 Heisenberg model for antiferromagnetically coupled ferromagnetic layers with a mean in-plane exchange constant of J/k = 8.13 K and an interlayer exchange constant of J‘/k = 1.22 K. The corresponding values for the Br salt are Tc = 18.9 K, J/k = 21.3 K, and J‘/k = 5.87 K The increase in magnitude of the exchange constants and ordering temperature of the Br salt relative to the Cl salt is consistent with the increase in ionic radius of Br- relative to Cl-.