Crystal structure and physical properties of Na ₂ Mn ₃ (Si ₃ O ₁₀ ) – a new representative of the series of trisilicates A ₂ M ₃ (Si ₃ O ₁₀ )

Single crystals of a manganese-containing member in the series of trisilicates A₂M₃(Si₃O₁₀), where A = Na and K, and M = Ca, Cd, and Mn, were obtained by mild temperature hydrothermal synthesis at 280 °C and 50 atm. Na₂Mn₃(Si₃O₁₀) crystallizes in the monoclinic space group <i>C</i>2/<i>c</i>, with <i>a</i> = 16.1195(9), <i>b</i> = 5.0089(3), <i>c</i> = 10.7158(6), <i>β</i> = 94.078(6), and <i>Z</i> = 4. Its crystal structure is built from trimers of edge-sharing Mn-centered polyhedra forming MnO₅-MnO₆-MnO₅ linear clusters arranged in a staggered pattern and linked together <i>via</i> 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°. 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 <i>b</i> and <i>c</i> axes (2.6 eV along the <i>a</i> axis). The theoretical specific capacity for Li₂Mn₃(Si₃O₁₀) is 126.74 mAh g^-1 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 <i>T</i>_N1 = 29 K and <i>T</i>_N2 = 39 K.