New low-dimensional antiferromagnetic compounds based on Cu-pyz chain

Two new Cu2$+$ coordination complexes, [CuCl(pyz)2](BF4) and [CuBr(pyz)2](BF4) (pyz $=$ pyrazine), were synthesized and their structures determined with single crystal X-ray diffraction. These tetragonal compounds are isostructural and crystallize in space group P4/nbm. The magnetic susceptibility shows a broad maximum around 8 K for both compounds, indicating predominantly two-dimensional (2D) antiferromagnetic interactions localized within the [Cu(pyz)2]2$+$ layers. A fit of a 2D Heisenberg model to the magnetic susceptibility data results in J $=$ 9.6 K for [CuCl(pyz)2](BF4) and 9.1 K for [CuBr(pyz)2](BF4). Towards lower temperature kinks are observed in the magnetic susceptibility at 4 K for the chloride and 3.6 K for the bromide compound, indicating the onset of long-range 3D magnetic order. The magnetic structures were determined by neutron diffraction. Bragg peaks due to long-range 3D magnetic order are observed below TN $=$ 3.9(1) K for the chloride and 3.7(1) K for the bromide compound. The magnetic unit cell is doubled along the c-axis, the moments are antiferromagnetically coupled both in the a-b plane and along the c-axis.