Synthesis, crystal structure, and magnetic properties of NH₄CuPO₄·H₂O

Abstract

In this paper is described the synthesis and characterization of a new layered phosphate, NH₄CuPO₄ ·H₂O, and its magnetic properties. Its crystal structure has been solved at room temperature. It crystallizes in the P2₁ /a monoclinic space group, with a=7.3907(8), b=7.5191(6), c=8.651(1) Å, β=94.54(1)° and Z=4. The compound presents a layered structure, with copper phosphate sheets linked by NH₄⁺ cations. These layers are parallel to the (001) plane and are interconnected by hydrogen bonds with the NH₄⁺ cations. The layers are formed by centrosymmetric dimers of CuO₅ edge-sharing distorted square pyramids, crosslinked by corner-sharing phosphate tetrahedra. Each copper(ii) cation is bonded to three phosphate oxygens and one water molecule forming the base of the square pyramid, and a symmetry related phosphate oxygen in the axial position. The crystal structure of NH₄CuPO₄ ·H₂O is related to the layered dittmarite (NH₄MgPO₄ ·H₂O) type structure. However, in the dittmarite family there are MO₆ octahedra separated by the phosphate groups. The title compound is the first one related to the dittmarite family which exhibits layers formed by centrosymmetric Cu₂O₈ dimers crosslinked by phosphate tetrahedra. The IR data of this layered phosphate are in good agreement with the symmetry observed in the phase. EPR data and magnetic studies show the existence of antiferromagnetic interactions and the presence of predominant short range interactions. In spite of the layered structure exhibited by the compound, the magnetic study indicates that the magnetic behaviour is consistent with a dimeric structure with a J/k value of 4.93 K. A 2D ordering, as exhibited by other related compounds, may be reached at temperatures lower than 1.8 K.

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