STACKING OF THE SQUARE-LATTICE ANTIFERROMAGNETIC PLANES IN CA2CUO2CL2

Single-crystal neutron elastic and quasielastic scattering and magneti zation studies of the spin-1/2 square-lattice Heisenberg antiferromagn et Ca2CuO2Cl2 indicate an unusual stacking of the antiferromagnetic pl anes in the Neel state (T-N = 247 +/- 5 K) which is different from the collinear stacking in isostructural Sr2CuO2Cl2. A generalized magneti c model is presented to describe the data. Elastic scattering along th e (1/2 1/2 l) rod is observed above T-N, suggesting that the CuO2 plan es, although uncoupled, maintain some degree of two-dimensional Order up to a crossover temperature (T-co approximate to 280 K) to the isotr opic Heisenberg state. No structural phase transition from the 14/mmm symmetry was observed in the temperature range 10-300 K.