Salt-Inclusion Synthesis of Ba2MnSi2O7Cl. A Fresnoite-Type Polar Framework Containing the Acentric [Si2O7]^6- Polyanion in the Anti-ReO3 Type [(Ba2Mn)Cl]^6+ Cage

A novel non-centrosymmetric (NCS) solid, Ba2Mn(Si2O7)Cl (CU-13), was isolated via high-temperature, salt-inclusion reactions. This manganese(III) silicate chloride adopts the fresnoite structure exhibiting pseudo-one-dimensional channels in which the Ba^2+ cations reside. The framework can be viewed alternatively as made of a fascinating anti-ReO3 type (Ba2Mn)Cl lattice centered on the acentric Si2O7 unit. This new compound crystallizes in one of the 10 NCS polar crystal classes, 4mm (C4v), which is cooperatively attributed to the MnO4Cl2 tetragonal distortion and the oriented Si2O7 polyhedral unit. This discovery once again demonstrates the utility of salt inclusion with the formation of NCS frameworks.