Crystal structure and physical properties of Na2Mn3(Si3O10) - a new representative of the series of trisilicates A2M3(Si3O10)

Abstract

Single crystals of the manganese member in the series of trisilicates A₂M₃(Si₃O₁₀), where A=Na, K, and M=Ca, Cd, Mn, were obtained by mild temperature hydrothermal synthesis at 280^oC and 50 atm. Na2Mn₃(Si₃O₁₀) crystallizes in the monoclinic space group C2/c, with a=16.1195(9), b=5.0089(3), c=10.7158(6), β=94.078(6), Z=4. Its crystal structure is built by trimers of edge-sharing Mn-centered polyhedra forming MnO₅-MnO₆-MnO₅ linear clusters arranged in a staggered pattern and linked together via oxygen vertices in the layers. The layers are further connected by (Si₃O₁₀) groups of tetrahedra in a framework with Na atoms in its channels. The trisilicate assemblages of tetrahedra are close to a linear configuration with an Si2-Si1-Si2 angle of 154.1^o. No mobility of Na⁺ ions through the structural channels was observed. The migration of Li⁺ ions was estimated by assuming a possible Li-analog, Li₂Mn₃(Si₃O₁₀). As a result, biperiodic pathways organized by a network of voids that bends around the trisilicate groups and lies between the layers of Mn-centered polyhedra, were found for Li atoms. The conductivity of Li⁺-ions was confirmed by bond valence energy landscape (BVEL) calculations, which showed energy barriers of 0.4 eV and 0.7 eV along the b and c axes (2.6 eV along the a axis). The theoretical specific capacity for Li₂Mn₃(Si₃O₁₀) is 126.74 mAh/g for both Li atoms per formula unit. The ground state of Na₂Mn₃Si₃O₁₀ is a canted antiferromagnet reached through a sequence of two phase transitions at T_N1 = 29 K and T_N2 = 39 K.