Influence of Polymorphism on the Magnetic Properties of Co5TeO8 Spinel

This study presents the influence of polymorphism on the magnetic properties of Co5TeO8. This compound with a spinel-like structure [Co-2](A)[Co3Te]O-B(8) was synthesized into two polymorphs: one disordered within a cubic Fd (3) over barm structure, where Co2+ and Te6+ ions are randomly distributed on the octahedral B sites [the disordered polymorph can also be presented as an inverse spinel of the formula Co(Co1.5Te0.5)O-4] and the other ordered with a cubic P4(3)32 structure where Co2+ and Te6+ ions are ordered on the B sites. The macroscopic magnetic measurements showed that both polymorphs present a ferrimagnetic ordering, below similar to 40 K, and a second transition is also observed at 27 K for the ordered polymorph. Neutron powder diffraction data between room temperature and 1.7 K showed as well the presence of short-range magnetic ordered clusters, which appears for both polymorphs below 200 K. At lower temperature, these short-range orders are transformed into long-range ferrimagnetic orders. Below T-C = 40 K, the colinear ferrimagnetic structure of the disordered polymorph is described with the I4(1)/am'd' space group. The ordered polymorph undergoes an incommensurate ferrimagnetic spiral spin ordering below T-C1 = 45 K, followed by a second magnetic phase transition at T-C2 = 27 K. This last transition is associated with the emergence of an additional ferrimagnetic component and an abrupt change in the magnitude of the magnetic propagation vector k = [0, 0, gamma] from gamma = 0.086 at T = 30 K to gamma approximate to 0.14 in the range between 27 and 1.7 K. The magnetic symmetry of the ordered polymorph is described with the P4(3)(00 gamma)0 magnetic superspace group. We evidenced that the ordering of Co2+/Te6+ on the B sites changes all of the Co-Co and Co-O distances and thus all J(AB), J(AA), and J(BB) exchange interactions, between the A and B sites, which are able to stabilize the incommensurate spin modulation in the ordered polymorph.

Automated summary

This study investigates the influence of polymorphism on the magnetic properties of Co5TeO8 compound. Two polymorphs were synthesized, showing ferrimagnetic ordering at low temperatures and different magnetic symmetry. The ordering of Co2+/Te6+ ions on the B sites affects the exchange interactions and stabilizes incommensurate spin modulation in the ordered polymorph.