Synthesis and crystal structure of a new microporous zirconosilicate, MCV-2

We report the hydrothermal synthesis and crystal structure of a new microporous, octahedral-tetrahedral zirconosilicate, Ba2Na(Na0.5,(H2O)0.5)2Zr2Si6O19 · 3H2O (simplified Ba2(Na,H2O)3Zr2Si6O19 · 3H2O), which we designate as MCV-2. It represents the first example of a Ba-Na-zirconosilicate. The compound has orthorhombic symmetry, space group Cmca (no. 64), with a = 16.258(3), b = 20.806(4), c = 12.339(2) Å, V = 4173.8(13) Å3, Z = 8. The complex crystal structure has been solved from single-crystal X-ray diffraction data (Mo-Kα radiation, CCD area detector, 293 K, 2θmax = 60°, Rint = 3.9%) and refined to R1(F) = 5.03% and wR2all = 10.84% for 2458 'observed' reflections with Fo > 4σ(Fo). The structure of MCV-2 is built up from two crystallographically non-equivalent, isolated ZrO6 octahedra (dav(Zr-O) = 2.085 and 2.084 Å) sharing all their corners with isolated, non-linear (horseshoe-shaped) Si3O10 trisilicate groups and Si6O18 groups of six-membered rings of SiO4 tetrahedra. Average Si-O bond lengths range from 1.617 to 1.628 Å. The resulting open framework hosts channels oriented in different directions. These channels are filled with one ten-coordinated Ba site, two Na sites (eight- and four-fold coordinated; the latter site has an ideal occupancy of Na0.5H2O0.5) and two water molecules. MCV-2 contains two different PME ('polyhedral microensembles') types (C-2 and G as defined by [G.D. Ilyushin, V.A. Blatov, Acta Crystallogr. B58 (2002) 198]), a very unusual feature for a mixed octahedral-tetrahedral framework.