Magnetic Dimensionality of Metal Formate $M[(H_2O)_2 (HCOO)_2]$ Compounds ( $M={}$ Co(II), Cu(II))

The magnetic properties of two metal formate systems with formula M[(H 2 O) 2 (HCOO) 2 ], which are isostructural, are described (M=Co,Cu). These compounds contain two different crystallographic metal centers (M 2+ ), in octahedral coordination, which are connected through (HCOO) bridges, leading to the formation of two types of infinite chains: M(1)-HCOO-M(2) and M(1)-HCOO - M(1). The packing of these chains, along with hydrogen bonds, generates a 3-D framework. Cu-sample presents an usual paramagnetic behavior, following Curie law. On the contrary, Co-sample has an order-disorder magnetic transition, further analyzed by means of magnetization and specific heat. These data lead us to conclude an anisotropic environment for Co ions, as expected for an octahedral neighborhood. Critical exponents of these thermodynamic quantities around the thermal transition are in accordance with 3-D model. This is a very important remark, since a previous work had shown that the magnetic phase transition is 2-D.