Synthesis and Characterization of a New Family of Thermally Stable Open-Framework Zincophosphate/ Arsenate Phases: M_3Zn_4O(XO_4)_3·nH_2O (M = Na, K, Rb, Li, ...; X = P, As; n = ～3.5-6). Crystal Structures of Rb_3Zn_4O(PO_4) _3·3.5H_2O

摘要

The syntheses, representative crystal structures, and some properties of a new family of microporous zincophosphate/arsenate materials, denoted M_3Zn_4O(XO_4)_3·nH_2O (M = Li, Na, K, Rb, Cs, ...; X = P, As; n = ～3.5-6), are reported. All these materials are based on a flexible, anionic Zn_4O(XO_4)_3 network built up from vertex-linked tetrahedral ZnO_4 and XO_4 units. Four ZnO_4 units share a vertex, resulting in novel OZn_4 centers. The use of mixed four-coordinate anion types (XO_4~(3-), O~(2-)) results in a number of interesting structural and stability properties. The M_3Zn_4O(XO_4)_3·nH_2O network contains only pairs of 3-rings (a spiro-5 unit) and 8-rings, and no 4-rings or 6-rings: it encloses roughly spherical cavities connected by a three-dimensional network of 8-ring channels propagating in the orthogonal [100], [010], and [001] directions (cubic unit-cell axes). Extraframework cations and water molecules occupy these cavities and channels. These phases display typical "zeolitic" dehydration/ rehydration and ion-exchange reactions with thermal stabilities up to 600℃, the highest known for this type of open-framework Zn(P/As)O material. The included guest cation (Na, Rb, K, Li, ...) has a crucial effect on crystal structure, by ordering in and modifying the configuration of the intercavity 8-ring channels. The crystal structures of Rb_3Zn_4O(PO_4)_3· 3.5H_2O, K_3Zn_4O(AsO_4)_3·4H_2O, and Na_3Zn_4O(PO_4)_3·6H_2O are presented. Crystal data: Rb_3-Zn_4O(PO_4)_3·3.5H_2O, M_r = 881.40, cubic, space group F43c (No. 219), a = 15.3423(9) A, V = 3611.4(6) A~3, Z = 8, R_p = 5.12%, R_(wp) = 6.78%, 27 parameters, 1874 data points (X-ray Rietveld refinement). K_3Zn_4O(AsO_4)_3·4H_2O, M_r = 883.63, cubic, space group F43c (No. 219), a = 15.4638(2) A, V = 3697.9(2) A~3, Z = 8, R_p = 8.69%, R_(wp)= 11.42%, 24 parameters, 2124 data points (X-ray Rietveld refinement). Na_3Zn_4O(PO_4)_3·6H_2O, M_r = 739.49, trigonal, space group R3c (No. 161), a = 10.7489(3) A, α = 60.114(2)°, V = 880.4(4) A~3, Z = 2, R = 2.49%, R_w = 2.61%, 180 parameters, 846 observed reflections with I > 3σ(I) (single-crystal data).