Incommensurate structural and magnetic modulations in potassium-rich cryptomelane, KxMn8O16 (x ~ 1.45)

Cryptomelane is a hollandite-like material consisting of K+ cations in an α-MnO2 tunnel-like crystallographic motif. Cryptomelane with stoichiometry KxMn8O16 (x ≈ 1.45) has been synthesized and its magnetic properties investigated using variable temperature magnetic susceptibility, heat capacity, and neutron powder diffraction. Three distinct transitions at T1 = 184 K, T2 = 54.5 K, and T3 = 24 K are observed. At T1 there is a subtle tetragonal→monoclinic transition associated with emergence of a set of non-magnetic superstructure peaks indexable to a kstruc ≈ 0.74c incommensurate modulation parallel to the α-MnO2 tunnels. Our findings are consistent with a relation previously reported in titanate hollandites, that x ≈ 2kstruc. Magnetic Bragg peaks emerge below T2 = 54.5 K, and their positions indicate an incommensurate modulated magnetic structure. The model consistent with the data is a dual-kmag structure with a ferromagnetic kmag = 0 component and an incommensurate kmag ≈ 0.37c, with the latter most likely to be helical. The period of oscillation of the incommensurate magnetic component is in line with predictions based on a Heisenberg spin Hamiltonian [Mandal et al. Phys. Rev. B 90, 104420 (2014)]. Below T3 = 24 K, there is a magnetic transition, which gives rise to a different set of magnetic Bragg peaks indicative of a highly complex magnetic structure.