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

Two new alkaline earth copper(II) germanates were hydrothermally synthesized: ^CaCuGeO4·^H2O (1) and ^BaCu2Ge3O9·^H2O (2), and their structures determined by single crystal X-ray diffraction. Compound (1) crystallizes in space group P2₁/c with a=5.1320(2) Å, b=16.1637(5) Å, c=5.4818(2) Å, β=102.609(2)°, V=443.76(3) ų and Z=4. This copper germanate contains layers of composition [^CuGeO4]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) ų 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 χT plot seems to indicate the onset of interchain antiferromagentic correlations. However, no ordering temperature is detected in the susceptibility data.