Crystallography and composition of some new potassium-neodymium silicates

Hydrothermal methods have been used to synthesize a rich variety of K-Nd silicates as single crystals up to 3mm in length. The nutrient charges (usually glasses) had compositions close to the ideal phyllosilicate stoichiometry, Si₂O₅^2-. Eight phases, six of which have not previously been reported, were obtained at temperatures ranging 350-600°C and pressures 300-1400 bar. The composition of each phase was determined with electron microprobe analysis and confirmed, in some instances, by a crystal structure determination. The structure of the prototype compound, K₃NdSi₆O₁₅, that had attracted our attention to the system as a source of potential fast-ion conductors, was refined to a residual of 8.2% using 1495 observed reflections, I ≥ 2σ(I), to 2θ = 60° with MoKα radiation. The structure, with 19 atoms in the asymmetric unit, contains a novel corrugated Si₂O₅^2- sheet with 4-, 6-, and 8-membered rings. The structure undergoes a reversible phase transformation at 180°C. A new phase K₈Nd₃Si₁₂O₃₂OH, triclinic, P1̄, a = 6.9660(6), b = 11.4550(10), c = 11.6070(10) angstrom, α = 87.677(8)°, β = 87.491(9)°, γ = 79.083(8)°, was determined with 5668 observable reflections and refined to a residual of 4.5%. The asymmetric unit of the structure contains 30 atoms that form a flat silica layer with 6-, 8-, and 12-membered rings. The new phase K₃NdSi₃O₈(OH)₂, refined to a residual of 4.80%, contains discrete trisilicate units, Si₃O₈(OH)₂^6-. The set of phases that were synthesized were obtained as crystals large enough to permit measurement of ionic electrical conductivity along at least one crystallographic direction for six of the eight phases, and the full conductivity tensor for four phases.