  Birnessite compounds are stable across a wide range of compositions that
produces a remarkable diversity in their physical, electrochemical and
functional properties. These are hydrated analogues of the magnetically
frustrated, mixed-valent manganese oxide structures, with general formula,
NaxMnO2. Here we demonstrate that the direct hydration of layered rock-salt
type a-NaMnO2, with the geometrically frustrated triangular lattice topology,
yields the birnessite type oxide, Na0.36MnO2 0.2H2O, transforming its magnetic
properties. This compound has a much-expanded interlayer spacing compared to
its parent a-NaMnO2 compound. We show that while the parent a-NaMnO2 possesses
a Neel temperature of 45 K as a result of broken symmetry in the Mn3+
sub-lattice, the hydrated derivative undergoes collective spin-freezing at 29 K
within the Mn3+/Mn4+ sub-lattice. Scaling-law analysis of the frequency
dispersion of the AC susceptibility, as well as the temperature-dependent,
low-field DC magnetization confirm a cooperative spin-glass state of strongly
interacting spins. This is supported by complementary spectroscopic analysis
(HAADF-STEM, EDS, EELS) as well as by a structural investigation
(high-resolution TEM, X-ray and neutron powder diffraction) that yield insights
into the chemical and atomic structure modifications. We conclude that the
spin-glass state in birnessite is driven by the spin-frustration imposed by the
underlying triangular lattice topology that is further enhanced by the in-plane
bond-disorder generated by the mixed-valent character of manganese in the
layers.
