Article pubs.acs.org/crystal
Flux Growth of [NaK6F][(UO2)3(Si2O7)2] and [KK6Cl][(UO2)3(Si2O7)2]: The Effect of Surface Area to Volume Ratios on Reaction Products Gregory Morrison and Hans-Conrad zur Loye* Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States S Supporting Information *
ABSTRACT: Single crystals of two new salt-inclusion uranium(VI) silicates, [NaK 6 F][(UO 2 ) 3 (Si 2 O 7 ) 2 ] and [KK6Cl][(UO2)3(Si2O7)2], were grown from mixed alkali halide salt fluxes. Both compounds crystallize in the [NaRb6F][(UO2)3(Si2O7)2] structure type in the orthorhombic space group Pnnm with lattice parameters a = 11.0819(5) Å, b = 13.1149(6) Å, and c = 7.8418(4) Å ([NaK6F]) and a = 11.0830(7) Å, b = 13.5850(4) Å, and c = 7.8693(5) Å ([KK6Cl]). The surface area to volume (sa/vol) ratio of the reaction, controlled by the reaction vessel dimensions, has a strong influence on the reaction products with small sa/vol ratios favoring the growth of the salt-inclusion phases. Both compounds exhibit the typical luminescence of the uranyl group with the luminescence of [KK6Cl][(UO2)3(Si2O7)2] being less intense than [NaK6F][(UO2)3(Si2O7)2], likely due to quenching by the Cl− ions.
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INTRODUCTION Alkali and alkaline earth halides have been found to be a versatile flux for the growth of complex oxides.1 In particular, the alkali fluorides are capable of dissolving a wide range of starting materials, including silicon dioxide. Furthermore, the redox neutral nature of these fluxes allows for the growth of materials containing elements in reduced oxidation states.2,3 Finally, the use of mixed or eutectic halide fluxes can greatly reduce the required reaction temperature; for example, CsCl:CsF has a 439 °C eutectic and can allow for the growth of compounds containing multiple alkali and alkaline earth cations.4,5 An interesting class of materials, salt-inclusion compounds, can arise when alkali or alkaline earth halides are used as fluxes. These compounds contain a more covalent metal oxide framework, often composed of SiO4, PO4, AsO4, and/or transition metal oxide units, which contains voids occupied by an ionic salt lattice. In the simplest examples, these saltinclusions consist of a single halide anion surrounded by alkali or alkaline earth cations.6 However, these salt inclusions can become more complex, including dimers,7,8 chains,9−11 and slabs.12 Although the salt inclusion is typically a metal halide, other salt inclusions, such as alkali chalcogenide inclusions,13 have been reported. Salt-inclusion phases are most commonly obtained from salt flux growths but have also been obtained from hydrothermal14 and mild-hydrothermal growth methods.15 The need for new storage media for the immobilization of spent nuclear fuel (SNF) has led to the exploration of new uranium-containing solid state phases. Of particular interest are materials that accommodate other constituents of SNF, such as © 2016 American Chemical Society
the alkali metals. Recently, a new host−guest uranyl chromate, |Li2(H2O)5|[(UO2)(CrO4)2], was reported, which contains uranyl chromate host layers with guest |Li2(H2O)5|2+ units lying between the layers.16 Salt-inclusion compounds have a similar structural motif, i.e., a covalent framework containing electrostatically bonded units within voids and can similarly accommodate alkali metal units. For this reason, it is of interest to grow salt-inclusion uranyl compounds. Uranium silicates are an abundant, diverse group of materials due to the flexibility of the uranium valence,2,17−20 the diversity of possible silicate units,4 and the unique bonding requirements of the uranyl UO22+ unit.21,22 To date, three salt-inclusion uranium silicates have been reported. [Na 9F 2][(UO2)(UO2)2(Si2O7)2]14 was grown under hydrothermal conditions with NaF added to the reaction as the salt-inclusion source. This compound, which contains both U(VI) and U(V), has chains of edge-sharing square pyramidal Na 5F units.14 [K3Cs4F][(UO2)3(Si2O7)2] and [NaRb6F][(UO2)3(Si2O7)2] were grown from mixed alkali fluoride fluxes.6 These two uranyl silicates, although crystallizing in different space groups, are structurally very similar except that the prior contains isolated K2Cs4F salt-inclusion units and the latter contains isolated Rb6F units.6 We have grown two new uranyl salt-inclusion compounds, [NaK6F][(UO2)3(Si2O7)2] and [KK6Cl][(UO2)3(Si2O7)2], which adopt the [NaRb6F][(UO2)3(Si2O7)2] structure type. Received: October 1, 2015 Revised: January 22, 2016 Published: February 2, 2016 1294
DOI: 10.1021/acs.cgd.5b01408 Cryst. Growth Des. 2016, 16, 1294−1299
Crystal Growth & Design
Article
Herein, we report their syntheses, structures, and optical properties.
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Table 1. Crystallographic Data for [AB6X][(UO2)3(Si2O7)2] Compounds [AB6X] formula
EXPERIMENTAL SECTION
[NaK6F]
space group Pnnm a (Å) 11.0819(5) b (Å) 13.1149(6) c (Å) 7.8418(4) V (Å3) 1139.71(9) Z 2 crystal dimensions 0.10 × 0.08 × (mm3) 0.08 temperature (K) 295(2) density (g cm−3) 4.147 θ range (deg) 2.41−28.30 μ (mm−1) 22.682 Data Collection and Refinement collected reflections 14946 unique reflections 1521 Rint 0.0327 h −14 ≤ h ≤ 14 k −17 ≤ k ≤ 17 l −10 ≤ l ≤ 10 Δρmax (e Å−3) 0.711 Δρmin (e Å−3) −1.305 GoF 1.193 extinction coefficient 0.00069(8) R1(F)a for Fo2 > 0.0146 2σ(Fo2) 0.0351 Rw(Fo2)b
Synthesis. U3O8 (International Bio-Analytical Industries, powder, ACS grade), NaF (Alfa Aesar, powder, 99%), KF (Alfa Aesar, powder, 99%), and KCl (Mallinckrodt Chemicals, crystalline, ACS grade) were used as received. SiO2 (Aldrich, fused pieces
