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

Three caesium-bearing borophosphates, Cs[BPO(OH)] (I), CsMn(HO)[BPO]·0.45HO (II) and CsMn[BPO(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 Mn,Mn member of a known family characterized by [BPO] helical chains running along [001]. These chains are connected through MnO(HO) 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 MnO(HO) 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 AM[BPO(OH)] family and is characterized by a 3D framework built by open-branched borophosphate chains and MnO semi-octahedra sharing vertices. The measurements of thermodynamic properties, i.e. magnetization M and specific heat C, to 2 K and 30 T, provide evidence that (II) orders antiferromagnetically at the Néel temperature T = 4.6 K and exhibits a plateau-like feature under the action of an external magnetic field accompanied by a pronounced magnetocaloric effect.