Novel alkaline earth copper germanates with ferro and antiferromagnetic S=1/2 chains

Two new alkaline earth copper(II) germanates were hydrothermally synthesized: CaCuGeO4·H2OCaCuGeO4·H2O (1) and BaCu2Ge3O9·H2OBaCu2Ge3O9·H2O (2), and their structures determined by single crystal X-ray diffraction. Compound (1) crystallizes in space group P21/cP21/c with a =5.1320(2) Å, b =16.1637(5) Å, c =5.4818(2) Å, β=102.609(2)°β=102.609(2)°, V =443.76(3) Å3 and Z =4. This copper germanate contains layers of composition [View the MathML sourceCuGeO4]∞2− comprising CuO4 square planes and GeO4 tetrahedra with calcium and water molecules in the inter-layer space. Compound (2) crystallizes in the Cmcm space group with a =5.5593(3) Å, b =10.8606(9) Å, c =13.5409(8) Å, V =817.56(9) Å3 and Z =4. This structure contains GeO6 and CuO6 octahedra as well as GeO4 tetrahedra, forming a three-dimensional network of interconnecting six-membered ring channels. The magnetic susceptibility for both samples can be interpreted as S =1/2 chains, in agreement with the copper topology observed in the crystal structure. The susceptibility of (1) exhibits a Bonner–Fisher type behavior, resulting from antiferromagnetic intra-chain interactions without three-dimensional ordering down to 5 K—the lowest measured temperature. This observation, together with the absence of super-exchange paths between the copper chains, make this system particularly promising for the study of low dimensional magnetism. The magnetic properties of (2) show a very weak ferromagnetic near-neighbor interaction along the chain. In this compound a peak the χTχT plot seems to indicate the onset of interchain antiferromagentic correlations. However, no ordering temperature is detected in the susceptibility data.