Structural, magnetic, and electrochemical properties of the high pressure form of Na2Co[PO4]F.

The new compound HP-Na2Co[PO4]F was synthesized by high pressure solid state reaction and its crystal structure was determined from single crystal X-ray diffraction data. The physical properties of HP-Na2Co[PO4]F were characterized by magnetic susceptibility, specific heat capacity, galvanometric cycling, and electrochemical impedance spectroscopy measurements. HP-Na2Co[PO4]F crystallizes with the space group P63/m, a = 10.5484(15), c = 6.5261(9) Å, V = 628.87(15) Å(3) and Z = 6. The crystal structure consists of infinite chains of edge-sharing CoF2O4 octahedra. The latter are interconnected through the PO4 tetrahedra forming a 3D-Co[PO4]F-framework. The six coordinated sodium atoms are distributed over three crystallographic sites (2b, 6h, and 4f). The structure of HP-[Na11/3Na23/3Na32/3]Co[PO4]F is similar to [Na11/3Na23/3Sr1/3□1/3]Ge[GeO4]O. There is only one difference; Na3 occupies the 4f (1/3, 2/3, 0.0291) atomic position, whereas the Sr occupies the 2c (1/3, 2/3, 1/4) atomic position. The magnetic susceptibility follows a Curie-Weiss behavior above 50 K with Θ = -21 K indicating predominant antiferromagnetic interactions. The specific heat capacity and magnetization measurements show that HP-Na2Co[PO4]F undergoes a three-dimensional magnetic ordering at TN = 11.0(1) K. The ionic conductivity σ, estimated at 350 °C, is 1.5 × 10(-7) S cm(-1). The electrochemical cycling indicates that only one sodium ion could be extracted during the first charge in Na half-cell; however, the re-intercalation was impossible due to a strong distortion of the structure after the first charge to 5.0 V.