The local atomic and magnetic structures of the compounds AMnO2 (A = Na, Cu), which realize a geometrically frustrated, spatially anisotropic triangular lattice of Mn spins, have been investigated by atomic and magnetic pair distribution function analysis of neutron total scattering data. Relief of frustration in CuMnO2 is accompanied by a conventional cooperative symmetry-lowering lattice distortion driven by Néel order. In NaMnO2, however, the distortion has a short-range nature. A cooperative interaction between the locally broken symmetry and short-range magnetic correlations lifts the magnetic degeneracy on a nanometer length scale, enabling long-range magnetic order in the Na-derivative. The degree of frustration, mediated by residual disorder, contributes to the rather differing pathways to a single, stable magnetic ground state in these two related compounds. This study demonstrates how nanoscale structural distortions that cause local-scale perturbations can lift the ground state degeneracy and trigger macroscopic magnetic order. a arXiv:2001.09917v1 [cond-mat.str-el]

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The layered β-NaMnO 2 , a promising Na-ion energy-storage material has been investigated for its triangular lattice capability to promote complex magnetic configurations that may release symmetry restrictions for the coexistence of ferroelectric and magnetic orders. The complexity of the neutron powder diffraction patterns underlines that the routinely adopted commensurate structural models are inadequate. Instead, a single-phase superspace symmetry description is necessary, demonstrating that the material crystallizes in a compositionally modulated q= (0.077(1), 0, 0) structure. Here, Mn 3+ Jahn-Teller distorted MnO 6 octahedra form corrugated layer stacking sequences of the β-NaMnO 2 type, which are interrupted by flat sheets of the α-like oxygen topology. Spontaneous long-range collinear antiferromagnetic order, defined by the propagation vector k= (½, ½, ½), appears below T N1 = 200 K. Moreover, a second transition into a spatially modulated proper-screw magnetic state (kq) is established at T N2 = 95 K, with an antiferromagnetic order parameter resembling that of a two-dimensional (2D) system. The evolution of 23 Na NMR spin-lattice relaxation identifies a magnetically inhomogeneous state in the intermediate T-region (T N2