Stacking of the square-lattice antiferromagnetic planes inCa2CuO2Cl2

Single-crystal neutron elastic and quasielastic scattering and magnetization studies of the spin-$\frac{1}{2}$ square-lattice Heisenberg antiferromagnet ${\mathrm{Ca}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2$ indicate an unusual stacking of the antiferromagnetic planes in the Néel state ($T_N=247\mathrm{}\pm 5\mathrm{K}$) which is different from the collinear stacking in isostructural ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2$. A generalized magnetic model is presented to describe the data. Elastic scattering along the ($\frac{1}{2}$ $\frac{1}{2}$ $l$) rod is observed above $T_N,$ suggesting that the ${\mathrm{CuO}}_2$ planes, although uncoupled, maintain some degree of two-dimensional order up to a crossover temperature ($T_{\mathrm{co}}\approx 280\mathrm{K}$) to the isotropic Heisenberg state. No structural phase transition from the $I4/mmm$ symmetry was observed in the temperature range 10–300 K.