Crystal chemistry of compounds with lanthanide based microporous heteropolyhedral frameworks: Synthesis, crystal structures, and luminescence properties of novel potassium cerium and erbium silicates

Abstract

Single crystals of compounds based on novel microporous heteropolyhedral frameworks containing Ce and Er were synthesized hydrothermally and their structures were determined. The triclinic unit cell parameters of K7[Ce3(Si12O32)]⋅2H2O and K7[Er3(Si12O32)]⋅2H2O, respectively, are as follows: a = 6.9833(2), 6.8334(3) Å, b = 11.4171(3), 11.4474(3) Å, c = 11.6988(4), 11.4792(3) Å, α = 87.632(2), 88.501(2)°, β = 87.546(3), 89.086(3)°, γ = 78.732(2), 79.711(3)°; V = 913.40(5), 883.16(5) Å3, space group P-1. Structural models were refined to R = 6.52 and 4.74%, respectively. The crystal structures are similar to reported compounds with the general formula K7+x[Ln3(Si12O32)]Øx⋅nH2O and are based on (Si12O32) layers of silicate tetrahedra that are one tetrahedron wide linked by LnO6 octahedra and Ln2O10 dimers of octahedra into a heteropolyhedral framework. The structures contain two systems of parallel channels that contain potassium cations and water molecules. Topological analysis indicates the frameworks are formed by condensation of the sequence of tiles [46][34.43.63]2[34.48.68.82]. Bond valence sums show partial oxidation of Ce3+ to Ce4+, as previously observed by XANES. Luminescence spectra for K7[Er3(Si12O32)]⋅2H2O indicate emission bands are related to the transitions from the 2H11/2 and 4S3/2 exited states to the 4I15/2 ground state. A short review of all potassium containing lanthanide silicate compounds, most of which have heteropolyhedral frameworks, is provided in which twenty-four chemical families of isostructural compounds are grouped according to the type of silicate anion.