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 K 7 [Ce 3 (Si 12 O 32 )]⋅2H 2 O and K 7 [Er 3 (Si 12 O 32 )]⋅2H 2 O, respectively, are as follows: a  = 6.9833(2), 6.8334(3) A, b  = 11.4171(3), 11.4474(3) A, c  = 11.6988(4), 11.4792(3) A, 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) A 3 , 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 K 7+ x [ Ln 3 (Si 12 O 32 )]O x ⋅ n H 2 O and are based on (Si 12 O 32 ) layers of silicate tetrahedra that are one tetrahedron wide linked by Ln O 6 octahedra and Ln 2 O 10 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 [4 6 ][3 4 .4 3 .6 3 ] 2 [3 4 .4 8 .6 8 .8 2 ]. Bond valence sums show partial oxidation of Ce 3+ to Ce 4+ , as previously observed by XANES. Luminescence spectra for K 7 [Er 3 (Si 12 O 32 )]⋅2H 2 O indicate emission bands are related to the transitions from the 2 H 11/2 and 4 S 3/2 exited states to the 4 I 15/2 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.