Magnetic structure and properties of the Na2CoP2O7 pyrophosphate cathode for sodium-ion batteries: a super-superexchange driven non-collinear antiferromagnet

The crystal and magnetic structure and properties of the Na2CoP2O7 sodium-ion battery cathode material have been characterized by magnetic susceptibility, specific heat, and variable temperature neutron powder diffraction measurements. Na2CoP2O7 crystallizes in the orthorhombic space group Pna21 with a = 15.4061(3) Å, b = 10.28854(9) Å and c = 7.70316(15) Å, having a layered structure with slabs of [CoP2O7]∞ separated by Na cations. The magnetic property measurements and neutron diffraction data analysis reveal that the material undergoes long-range ordering to a non-collinear antiferromagnetic G-type structure below TN ~ 6.5 K. The magnetic structure is rationalized as a result of super-superexchange between Co2+ atoms linked by phosphate groups