Sodium Filling in Superadamantoide Na[sub.1.36][sub.2]As[sub.2.98] and the Mixed Valent Arsenidosilicate-Silicide Li[sub.1.5]Ga[sub.0.9]Si[sub.3.1]As[sub.4]

Na[sub.1.36] (Si[sub.0.86] Ga[sub.0.14] )[sub.2] As[sub.2.98] and Li[sub.1.5] Ga[sub.0.9] Si[sub.3.1] As[sub.4] were synthesized by heating mixtures of the elements at 950 °C. The crystal structures were determined by single crystal X-ray diffraction (Na[sub.1.36] (Si[sub.0.86] Ga[sub.0.14] )[sub.2] As[sub.2.98] : I 4[sub.1] /a , Z = 100, a = 19.8772(4) Å, c = 37.652(1) Å; Li[sub.1.5] Ga[sub.0.9] Si[sub.3.1] As[sub.4] : C 2/c , Z = 8, a = 10.8838(6) Å, b = 10.8821(6) Å, c = 13.1591(7) Å). Na[sub.1.36] (Si[sub.0.86] Ga[sub.0.14] )[sub.2] As[sub.2.98] crystallizes similar to NaSi[sub.2] P[sub.3] 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 NaSi[sub.2] P[sub.3] , all sodium sites are now fully occupied, there is no significant ion mobility, as indicated by [sup.23] Na-NMR. Consequently, the total sodium-ion conductivity of Na[sub.1.36] (Si[sub.0.86] Ga[sub.0.14] )[sub.2] As[sub.2.98] amounts to only 1.6(1) × 10[sup.−7] S cm[sup.−1] and is therefore three orders of magnitude lower than in NaSi[sub.2] P[sub.3] . Li[sub.1.5] Ga[sub.0.9] Si[sub.3.1] As[sub.4] crystallizes in a new structure type with layers of edge-sharing (Si[sub.1−x] Gax )As[sub.4] 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[sup.−7] S cm[sup.−1] .