Multiple Competing Magnetic Interactions in Na4Ni7(PO4)6

Abstract

The low-temperature magnetic behavior and ground state of the candidate sodium-ion battery cathode compound Na4Ni7(PO4)6, have been investigated by physical property measurements and neutron powder diffraction. On cooling, Na4Ni7(PO4)6 undergoes three successive long-range spin ordering transitions to Phase I (below TN = 17 K), Phase II (below TN’ = 9.1 K) and Phase III (below TN’’ = 4.6 K) with ordering vectors [0, 1, 1/2], [0, 2/3, 1/2], and [~0.076, 2/3, 1/2], respectively. All three magnetic phases can be described in terms of ferromagnetic Ni2+ stripes with antiferromagnetic interactions between them. The moment amplitude of all stripes is the same in Phase I but varies in Phase II, while Phase III is an incommensurate variation on Phase II. Phases I and II both feature a crystallographically unique Ni site with no ordered magnetic moment due to geometric frustration, the resolution of which may be the driving force behind the final transition to Phase III. Even among transition metal phosphates, which typically show complex spin ordering due to competition between superexchange and super-superexchange (through PO4 linkers), Na4Ni7(PO4)6 has one of the richest magnetic phase diagrams explored so far.