Sodium Filling in Superadamantoide Na_1.36(Si_0.86Ga_0.14)₂As_2.98 and the Mixed Valent Arsenidosilicate-Silicide Li_1.5Ga_0.9Si_3.1As₄

Abstract

Na1.36(Si0.86Ga0.14)2As2.98 and Li1.5Ga0.9Si3.1As4 were synthesized by heating mixtures of the elements at 950 °C. The crystal structures were determined by single crystal X-ray diffraction (Na1.36(Si0.86Ga0.14)2As2.98: I41/a, Z = 100, a = 19.8772(4) Å, c = 37.652(1) Å; Li1.5Ga0.9Si3.1As4: C2/c, Z = 8, a = 10.8838(6) Å, b = 10.8821(6) Å, c = 13.1591(7) Å). Na1.36(Si0.86Ga0.14)2As2.98 crystallizes similar to NaSi2P3 with interpenetrating networks of vertex-sharing T4 and T5 supertetrahedra. Gallium substitution at the silicon sites increases the charge of the cluster network, which is compensated for by a 36% higher sodium content. Since in contrast to NaSi2P3, all sodium sites are now fully occupied, there is no significant ion mobility, as indicated by 23Na-NMR. Consequently, the total sodium-ion conductivity of Na1.36(Si0.86Ga0.14)2As2.98 amounts to only 1.6(1) × 10−7 S cm−1 and is therefore three orders of magnitude lower than in NaSi2P3. Li1.5Ga0.9Si3.1As4 crystallizes in a new structure type with layers of edge-sharing (Si1−xGax)As4 tetrahedra alternating with layers that contain infinite Sin zigzag chains. Lithium ions reside in channels between the chains, and thus, the structure does not provide three dimensional pathways for ion conduction and the measured total Li-ion conductivity amounts to only 1.3(1) × 10−7 S cm−1.

Keywords

• superadamantoid
• ion conduction
• arsenidosilicate
• crystal structures