Ag18(SiO4)2(Si4O13), the first mixed-anion silver(I) silicate

The system Ag2O/SiO2 has been investigated at Ag:Si = 3:1 by solid state reactions. Ag6Si2O7(II) forms from the binary components applying elevated oxygen pressures (p(O2) = 0.2 GPa) at 560--580 degC. By adding conc. NH3-solution to the same starting mixtures Ag6Si2O7(I) forms at 500--550 degC. Hitherto unknown (Ag4SiO4)2.Ag10Si4O13 is the product if the temperature is kept below 470degC. (Ag4SiO4)2.Ag10Si4O13 transforms in an irreversible phase transition to Ag6Si2O7(I) at T = 495 ± 10degC. The crystal structure of (Ag4SiO4)2.Ag10Si4O13 (C2/m; a = 1201.7(1), b = 1060.7(1), c = 607.4(1) pm, β = 106.7(1)deg; 1927 independent reflections; R = 0.06; Rw = 0.05) is constituted from orthosilicate and catena-tetrasilicate anions. The catena-tetrasilicate anions exhibit 2/m point symmetry. The intense red colour indicates the existence of weak Ag -- Ag bonding interactions (shortest Ag+ -- Ag+ separation: 282.1 pm)