| Summary: | The ferri- and antiferromagnetic structures of a hureaulite-type synthetic compound, Mn<sup>2+</sup><sub>5</sub>(PO<sub>4</sub>)<sub>2</sub>(PO<sub>3</sub>(OH))<sub>2</sub>(HOH)<sub>4</sub>, were elucidated by high-resolution neutron powder diffraction in combination with magnetic susceptibility and heat capacity measurements. At 6.17 K, the paramagnetic phase (space group: <b><i>C</i></b>2/c) transforms to inherit a ferrimagnetic order (magnetic space group: <b><i>C</i></b>2′/c′), followed at 1.86 K by an incommensurately modulated antiferromagnetic order (magnetic superspace group: <b><i>P</i></b>2<sub>1</sub>/c.1′(<i>α0γ</i>)<i>00s</i> with the propagation vector <b><i>k</i></b>(0.523(2), 0, 0.055(1)). In the ferrimagnetic state, antiferromagnetic interactions are dominant for both intra and inter pentamers of Mn<sup>2+</sup>(O, HOH)<sub>6</sub> octahedra. Differently aligned spin-canting sublattices seen in the ferrimagnetic models at 3.4, 4.5, and 6.1 K explain a weak ferromagnetism in the title compound. The observation of magnetic moments vigorously changing in a small temperature range of 6.1–1.5 K adumbrates a high complexity of interplaying structural and magnetic orders in this manganese phosphatic oxyhydroxide.
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