Multiple Competing Magnetic Interactions in Na₄Ni₇(PO₄)₆

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.