Magnetostructural correlations in the antiferromagnetic Co2−x Cux(OH)AsO4 (x=0 and 0.3) phases

The Co2−xCux(OH)AsO4 (x=0 and 0.3) compounds have been synthesized under mild hydrothermal conditions and characterized by X-ray single-crystal diffraction and spectroscopic data. The hydroxi-arsenate phases crystallize in the Pnnm orthorhombic space group with Z=4 and the unit-cell parameters are a=8.277(2) Å, b=8.559(2) Å, c=6.039(1) Å and a=8.316(1) Å, b=8.523(2) Å, c=6.047(1) Å for x=0 and 0.3, respectively. The crystal structure consists of a three-dimensional framework in which M(1)O5-trigonal bipyramid dimers and M(2)O6-octahedral chains (M=Co and Cu) are present. Co2(OH)AsO4 shows an anomalous three-dimensional antiferromagnetic ordering influenced by the magnetic field below 21 K within the presence of a ferromagnetic component below the ordering temperature. When Co2+ is partially substituted by Cu2+ions, Co1.7Cu0.3(OH)AsO4, the ferromagnetic component observed in Co2(OH)AsO4 disappears and the antiferromagnetic order is maintained in the entire temperature range. Heat capacity measurements show an unusual magnetic field dependence of the antiferromagnetic transitions. This λ-type anomaly associated to the three-dimensional antiferromagnetic ordering grows with the magnetic field and becomes better defined as observed in the non-substituted phase. These results are attributed to the presence of the unpaired electron in the dx2−y2 orbital and the absence of overlap between neighbour ions.