Studies on Tetragonal Na2CoP2O7, a Novel Ionic Conductor

Crystals of the tetragonal form of Na2CoP2O7 have been grown and characterized by single crystal X-ray diffraction. This compound crystallizes in the tetragonal system, space group P42/mnm with a=7.7058(12), c=10.301(2) ?, and Z=4. The structure was solved by Patterson and Fourier methods, and refined to final agreement factors R(F2)=5.8%, Rw(F2)=12.4%, S(F2)=1.07. Their structure can be described as layered since it is formed by slabs of [CoP2O7] with Na cations lying between the layers. The Co2+ cation adopts a tetrahedral coordination geometry unusual in inorganic solids. CoO4 tetrahedra share four oxygens with P2O7 pyrophosphate groups and the Na+ cations appear in a NaO8 quadrangular prismatic coordination geometry. Magnetic measurements reveal the presence of antiferromagnetic interactions in the Co2+ sublattice, at about 10 K. Magnetic correlations are also studied from the analysis of possible superexchange pathways in the structure. Impedance measurements (frequency range: 1–5×106 Hz; temperature range: 90–540°C) show Na2CoP2O7 an ionic conductor being the conductivity at 300°C 2×10-5 Scm-1 and the activation energy 0.61 eV. The possible motion of the Na+ ions in the interlayer surface plane and through adjacent layers (c axis) is discussed.