Chain caesium borophosphates with B:P ratio 1:2: synthesis, structure relationships and low-temperature thermodynamic...

Three caesium-bearing borophosphates, Cs[BP₂O₆(OH)₂] (I), Cs_0.51Mn_1.17(H₂O)₂[BP₂O₈]·0.45H₂O (II) and CsMn[BP₂O₈(OH)] (III), were synthesized by a hydrothermal method at 473-523 K. Their crystal structures have been studied by means of single-crystal X-ray diffraction; all three structures comprise borophosphate chain anions with a B:P ratio of 1:2. The unique construction of (I) is based on four-membered-ring chains running parallel to the [010] direction. These protonated borophosphate chains are linked via hydrogen-bond interactions to form a 3D framework with caesium cations incorporated. (II) is the first Cs and Mn²⁺,Mn³⁺ member of a known family characterized by [BP₂O₈]_∞ helical chains running along [001]. These chains are connected through MnO₄(H₂O)₂ octahedra to form a 3D framework. The caesium cations are disordered over two independent positions in the channels, which they occupy together with water molecules. An additional MnO₂(H₂O)₃ bipyramid statistically shares a common edge and two corners with three main Mn octahedra to form tetrameric clusters. The topological relation between the chain anionic fragments of (I) and (II) as well as the structural relation between (I) and previously studied boro- and berillophosphates are discussed. Compound (III) presents the first Mn member of the A^IM^III[BP₂O₈(OH)] family and is characterized by a 3D framework built by open-branched borophosphate chains and MnO₅ semi-octahedra sharing vertices. The measurements of thermodynamic properties, i.e. magnetization M and specific heat C_p, to 2 K and 30 T, provide evidence that (II) orders antiferromagnetically at the Néel temperature T_N = 4.6 K and exhibits a plateau-like feature under the action of an external magnetic field accompanied by a pronounced magnetocaloric effect.