On Why the Two Polymorphs of NaFePO4 Exhibit Widely Different Magnetic Structures: Density Functional Analysis.

Triphylite-NaFePO4 is a cathode material for Na(+)-ion batteries, whereas its alternative polymorph maricite-NaFePO4 is not. These two different polymorphs exhibit widely different magnetic structures; the ordered magnetic structure of triphylite-NaFePO4 below ∼50 K is described by the propagation vector q1 = (0, 0, 0) with collinear spins, and that of maricite-NaFePO4 below ∼13 K is described by q2 = (1/2, 0, 1/2) with noncollinear spins. We probed the causes for these differences by calculating the spin exchange interactions of the two polymorphs and determining the preferred orientations of their high-spin Fe(2+) (d(6), S = 2) ions on the basis of density functional calculations. Our study shows that maricite-NaFePO4 is not spin-frustrated, which is also the case for triphylite-NaFePO4, that the ordered magnetic structure of triphylite-NaFePO4 is determined mainly by spin exchange, whereas that of maricite-NaFePO4 is determined by both spin exchange and magnetic anisotropy, and that the preferred spin orientations in the two polymorphs can be explained by perturbation theory using spin-orbit coupling as the perturbation.