Incommensurate Structural and Magnetic Modulations in Potassium-Rich Cryptomelane, K _x Mn₈O₁₆ (x ≈ 1.45)

Cryptomelane is a hollandite-like material consisting of K⁺ cations in an α-MnO₂ tunnel-like crystallographic motif. Cryptomelane with stoichiometry K _x Mn₈O₁₆ (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 T ₁ = 184 K, T ₂ = 54.5 K, and T ₃ = 24 K are observed. At T ₁ there is a subtle tetragonal → monoclinic transition associated with emergence of a set of nonmagnetic superstructure peaks indexable to a ${\mathrm{k⃗}}_{struc}\approx 0.74\overrightarrow{c^{*}}$ incommensurate modulation parallel to the α-MnO₂ tunnels. Our findings are consistent with a relation previously reported in titanate hollandites, that $x\approx 2\left|{\mathrm{k⃗}}_{struc}\right|$ . Magnetic Bragg peaks emerge below T ₂ = 54.5 K, and their positions indicate an incommensurate modulated magnetic structure. The model consistent with the data is a dual- ${\mathrm{k⃗}}_{mag}$ structure with a ferromagnetic $\left|{\mathrm{k⃗}}_{mag}\right|=0$ component and an incommensurate ${\mathrm{k⃗}}_{mag}\approx 0.37\overrightarrow{c^{*}}$ , 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 2014, 90, 104420]. Below T ₃ = 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.