MS28-P04 Crystal chemistry features and physical properties of three Mn phosphates Larisa Shvanskaya (M.V. Lomonosov Moscow State University, Moscow, Russia) Olga Yakubovich (M.V. Lomonosov Moscow State University, Moscow, Russia) Galina Kiriukhina (M.V. Lomonosov Moscow State University, Moscow, Russia) Alexander Vasiliev (M.V. Lomonosov Moscow State University, Moscow, Russia)email: lshvanskaya@mail.ruThe complex phosphates with manganese cations in low oxidation state (Mn2+) attract attention as potential catalysts for industrially important oxygen reduction reactions [1]. For example, Mn3(PO4)2*3H2O are found to display interesting electrocatalytic properties for water splitting [2]. Besides catalytic activity, Mn-based phosphates can demonstrate rare magnetic grounds states, which make these compounds attractive both for experimental physical property investigation and for theoretical calculation.
Three manganese phosphates, namely RbMn4(PO4)3(I), CsMnPO4(II) and Mn2PO4OH(III) were studied by single crystal X-ray diffraction  and characterized by measuring of the magnetic susceptibility, magnetization, and heat capacity. The RbMn4(PO4)3 belongs to the morphotropic series of oxo-salts with general formula AM4(TO4)3. The Mn2PO4OH presents a manganese end-member of the triplite-triploidite family. The (I,III) crystal structures can be described as three-dimensional frameworks of corner and edge-sharing MnO5 and MnO6 polyhedra which are further straightened by PO4 tetrahedra. CsMnPO4 presents a novel polymorphic modification with zeolite ABW-type framework consisting of tridimite-like sheets of six-membered rings of manganese and phosphorus tetrahedra sharing vertices. The magnetic ground states of (I) and (II) are canted antiferromagnet below TN = 3.1 K and TN = 4.7 K, respectively. The Mn2PO4OH compound orders antiferromagnetically at TN = 4.6 К. In general, the magnetic behavior of CsMnPO4 is similar to that of structurally related RbMnPO4 [3]. Despite three dimensional magnetic structures of RbMn4(PO4)3 and Mn2PO4OH,  both phosphates demonstrate low dimensional magnetic behavior. Their specific feature is the high ratio of Weiss and Neel temperatures |Θ| / TN, called frustration parameter. This value is equal to 10 for RbMn4(PO4)3 and is twice as high for Mn2PO4OH. The origin of high frustration degree in the second compound is associated with the twisted saw tooth chain geometry of corner sharing triangles of manganese polyhedra (Fig.1, left), which may be isolated within tubular fragments of its crystal structure. The chain of another topology (Fig.1, right) is seemingly responsible for significant frustration in RbMn4(PO4)3. The finite magnetization of RbMn4(PO4)3 observed for T < TN might be explained by a Dzyaloshinskii–Moriya interaction.
Our investigation led us to establish the relation between the crystal structure and thermodynamic properties.
The work was supported by the Russian Foundation for Basic Research (grant No. 18-03-00908).

[1] Zhan, Y. M. et al. (2016). ChemCatChem. 8, 372 – 379.

[2] Jin, K. et al. (2014). J. Am. Chem. Soc. 136, 7435−7443.

[3] Nénert, et al. (2013). Inorg. Chem. 52, 9627−9635.

Keywords: phosphates, crystal chemistry, antiferromagnet