Incommensurate Structural and Magnetic Modulations in Potassium-Rich Cryptomelane, KxMn8O16 (x ≈ 1.45)
6 affiliations
- 1Cavendish Laboratory, University of Cambridge, Cambridge CB3 0US, U.K.
- 2Yusuf Hamied Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, U.K.
- 3Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States.
- 4Faraday Materials Laboratory (FaMaL), Materials Research Centre, Indian Institute of Science, C. V. Raman Avenue, Bangalore 560012, India.
- 5Institut Laue-Langevin, 71 Avenue des Martyrs, CS 20156, Cedex 9, Grenoble 38042, France.
- 6Diamond Light Source, Ltd., Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, U.K.
Abstract
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 k⃗struc≈0.74c*⃗ incommensurate modulation parallel to the α-MnO₂ tunnels. Our findings are consistent with a relation previously reported in titanate hollandites, that x≈2|k⃗struc| . 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- k⃗mag structure with a ferromagnetic |k⃗mag|=0 component and an incommensurate k⃗mag≈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 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.
Related
- Mechanism of Incommensurate Magnetic Order in BaCo2(AsO4)2: Interplay of Frustrated Further-Neighbor Exchanges and Bond-Directional Anisotropy
- Polarized neutron diffraction with ex-situ 3He neutron spin filter and two-dimensional detector for noncollinear incommensurate magnetic order
- Angular Modulations in Magnetic Torque Induced by Phase Transitions in the Triangular Supersolid 2H-AgNiO2
- A locally ab initio computational framework for arbitrary incommensurate materials interfaces
- Modulations of the Indian summer monsoon by the hot subtropical deserts: insights from coupled sensitivity experiments
BibTeX
@article{naglecocco2026incommensurate,
title = {{Incommensurate Structural and Magnetic Modulations in Potassium-Rich Cryptomelane, KxMn8O16 (x ≈ 1.45)}},
author = {Liam A V Nagle-Cocco and Joshua D Bocarsly and Krishnakanth Sada and Nicola D Kelly and Mathias A Kiefer and Emmanuelle Suard and Sarah J Day and Cheng Liu and Clare P Grey and Prabeer Barpanda and Clemens Ritter and Siân E Dutton},
journal = {Chemistry of materials : a publication of the American Chemical Society},
year = {2026},
month = oct,
doi = {10.1021/acs.chemmater.6c01980},
url = {https://europepmc.org/article/PMC/PMC13718541},
note = {CC BY},
}
RIS
TY - JOUR
TI - Incommensurate Structural and Magnetic Modulations in Potassium-Rich Cryptomelane, KxMn8O16 (x ≈ 1.45)
AU - Liam A V Nagle-Cocco
AU - Joshua D Bocarsly
AU - Krishnakanth Sada
AU - Nicola D Kelly
AU - Mathias A Kiefer
AU - Emmanuelle Suard
AU - Sarah J Day
AU - Cheng Liu
AU - Clare P Grey
AU - Prabeer Barpanda
AU - Clemens Ritter
AU - Siân E Dutton
PY - 2026
DA - 2026/10/06
JO - Chemistry of materials : a publication of the American Chemical Society
DO - 10.1021/acs.chemmater.6c01980
UR - https://europepmc.org/article/PMC/PMC13718541
AB - 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 k⃗struc≈0.74c*⃗ incommensurate modulation parallel to the α-MnO₂ tunnels. Our findings are consistent with a relation previously reported in titanate hollandites, that x≈2|k⃗struc| . 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- k⃗mag structure with a ferromagnetic |k⃗mag|=0 component and an incommensurate k⃗mag≈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 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.
SN - 0897-4756
SN - 1520-5002
ER -