Structural Series in the Ternary A-Mn-As System (A = Alkali Metal): Double-Layer-Type CsMnAs and RbMnAs and Tunnel-Type KMnAs.

New manganese arsenides CsMnAs, RbMnAs, and KMnAs were synthesized by solid-state reaction. They consist of edge-sharing MnAs tetrahedra, which are a building block similar to those of Fe-based superconductors. CsMnAs and RbMnAs adopt the KCuS-type structure (tetragonal P4/ mmm space group, No. 123) with a MnAs double layer, while KMnAs has the CaFeAs-type structure (orthorhombic Pnma space group, No. 62) with a MnAs tunnel framework. The structural change from CsMnAs and RbMnAs to KMnAs as well as the structural trend of the other ternary A-Mn-As (A = alkali metal) and AE-Mn-As (AE = alkaline-earth metal) compounds is understood as a consequence of reduction of the coordination number around the A and AE sites owing to the decrease of the ionic radius from Cs to Mg. Electrical resistivity measurements confirm that the three new phases are Mott insulators with band gaps of 0.52 (CsMnAs), 0.43 (RbMnAs), and 0.31 eV (KMnAs). Magnetic and heat capacity measurements revealed that CsMnAs and RbMnAs are antiferromagnets without apparent phase transitions below 400 K, which is similar to the magnetism of LaMnAsO and BaMnAs, while the existence of the ferromagnetic component was indicated in KMnAs with a magnetic transition at 179 K.