Heat capacities of the spin quantum number S = 1/2 two-dimensional Heisenberg antiferromagnet bis(2-amino-5-chloropyridinium) tetrabromocuprate(II) [(5CAP)2CuBr4] crystal and its nonmagnetic analogue, the bis(2-amino-5-chloropyridinium) tetrabromozincate(II) [(5CAP)2ZnBr4] crystal, were measured by adiabatic calorimetry. For the (5CAP)2ZnBr4 crystal, single-crystal X-ray diffraction was also performed. The (5CAP)2ZnBr4 crystal belongs to the orthorhombic space group Pbca, with a = 16.074(2) Å, b = 7.688(2) Å, c = 30.538(6) Å, and Z = 8. In the (5CAP)2CuBr4 crystal, an antiferromagnetic phase transition occurred at T N = 5.08 K, and a thermal anomaly arising from the short-range order characteristic of two-dimensional magnetic substances was found above T N; the heat capacity of the (5CAP)2ZnBr4 crystal showed no thermal anomaly. The enthalpy and entropy gains due to the magnetic transition were estimated to be 49.3 J mol-1 and 5.65 J K-1 mol-1, respectively. The value of the entropy gain coincides well with the R ln 2 (5.76 J K-1 mol-1, R stands for the gas constant) expected for S = 1/2 spin systems. The thermal anomaly observed above T N is well accounted for in terms of the S = 1/2 two-dimensional antiferromagneic Heisenberg model of a square lattice with J/k B = −4.3 K. Spin wave analysis of the magnetic heat capacities below T N suggests that the (5CAP)2CuBr4 crystal is in a three-dimensional antiferromagnetic state, which is realized by a weak interlayer magnetic interaction between the two-dimensional layers.

The magnetic susceptibility and magnetization of two new layered S= 1 2Heisenberg antiferromagnets with moderate exchange are reported.The two isostructural compounds, (2-amino-5-chloropyridinium)2CuBr4 ((5CAP)2CuBr4) and (2-amino-5-methylpyridinium)2CuBr4 ((5MAP)2CuBr4), contain S= , Cu(II) ions related by C-centering, yielding four equivalent nearest neighbors. The crystal structure of newly synthesized compound, (5CAP)2CuBr4, shows the existence of layers of distorted copper(II)-bromide tetrahedra parallel to the ab plane, separated by the organic cations along the c axis. Magnetic pathways are available through the bromide-bromide contacts within the layers and provide for moderate antiferromagnetic exchange. Susceptibility measurements indicate interaction strengths to be 8.5(1) K, and 6.5(1) K and ordering temperatures of 5.1(2) K and 3.8(2) K for (5CAP)2CuBr4 and (5MAP)2CuBr4 respectively. High field magnetization experiments on both compounds show upward curvature of M(H,T). Magnetization measurements made at T = 1.3 K show saturation occurs in (5MAP)2CuBr4 at 18.8 T and in (5CAP)2CuBr4 at 24.1 T. The magnetization curves are consistent with recent theoretical predictions. Single crystal magnetization measured at 2.0 K indicate a spin flop transition at 0.38 T and 0.63 T for (5CAP)2CuBr4 and (5MAP)2CuBr4 respectively.

The magnetic materials ͑5CAP͒ 2 CuCl 4 and ͑5MAP͒ 2 CuCl 4 form effective square planar lattices of CuCl 4 2Ϫ anions. The Cu 2ϩ ions have weak antiferromagnetic interactions in the plane through Cl-Cl contacts. These planes are well separated by the 5MAP and 5CAP organic groups. High temperature susceptibility measurements give Jϭ0.57 K for ͑5CAP͒ 2 CuCl 4 and Jϭ0.38 K for ͑5MAP͒ 2 CuCl 4 . (T,Hϭ0) for these materials has a rounded maximum at T peak ϭ1.1 K for ͑5CAP͒ 2 CuCl 4 and T peak ϭ0.62 K for ͑5MAP͒ 2 CuCl 4 . A kink in (T,Hϭ0) for the ͑5MAP͒ 2 CuCl 4 suggests a phase transition to three-dimensional long range order at T C ϭ0.44 K. Measurements of (T,H) show that the saturation field H C for the transition to a fieldinduced saturated state is 3.78 T for ͑5CAP͒ 2 CuCl 4 and 2.17 T for ͑5MAP͒ 2 CuCl 4 . Thus these materials are excellent model systems in which to explore the full H -T phase diagram of the two-dimensional Sϭ1/2 Heisenberg antiferromagnet.

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