Tubular Chains, Single Layers, and Multiple Chains in Uranyl Silicates

Article pubs.acs.org/crystal

Tubular Chains, Single Layers, and Multiple Chains in Uranyl Silicates: A2[(UO2)Si4O10] (A = Na, K, Rb, Cs) Hsin-Kuan Liu,† Chun-Chi Peng,† Wen-Jung Chang,† and Kwang-Hwa Lii*,†,‡ †

Department of Chemistry, National Central University, Zhongli, Taiwan 320 Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115

‡

S Supporting Information *

ABSTRACT: Four new uranyl silicates with the same framework composition but different crystal structures, Na2[(UO2)Si4O10]·0.5H2O (1), K2[(UO2)Si4O10] (2), Rb2[(UO2)Si4O10] (3), and Cs2[(UO2)Si4O10] (4), have been synthesized under supercritical hydrothermal conditions at 590 °C and 1620 bar and characterized by single-crystal Xray diffraction and infrared and photoluminescence spectroscopy. Crystal data for compound 1: tetragonal, I4/mcm (No. 140), a = 18.0332(7) Å, c = 7.7752(5) Å, V = 2528.5(2) Å3, Z = 8, R1 = 0.0447; compound 2: monoclinic, P21/n (No. 14), a = 8.2970(17) Å, b = 7.7403(15) Å, c = 8.7906(18) Å, β = 104.83(3)°, V = 545.74(19) Å3, Z = 2, R1 = 0.0140; compound 3: monoclinic, P21/c (No. 14), a = 10.3483(3) Å, b = 23.7297(6) Å, c = 7.6278(2) Å, β = 110.482(1)°, V = 1754.68(8) Å3, Z = 6, R1 = 0.0194; compound 4: orthorhombic, Cmca (No. 64), a = 7.7157(3) Å, b = 19.8719(8) Å, c = 24.0160(10) Å, V = 3682.3(3) Å3, Z = 12, R1 = 0.0277. The silicate anions in the structures of these compounds form tubular chains, single layers, and multiple chains which are linked by UO6 tetragonal bipyramids to form three-dimensional frameworks. The tubular chains and single layers are of the new type with respect to the sequence of vertex orientations. The multiple chain is observed for the first time and contains Q2, Q3, and Q4 Si. The structures of these silicate anions can be correlated with the sizes of alkali metal cations.

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the first U(V) silicate, K(UO)Si2O6,17 and a good number of mixed-valence compounds such as U(IV,V) silicate,18 U(IV,VI) germanate,19 U(V,VI) germanates,20 a salt-inclusion U(V,VI) silicate,21 and a U(IV,V,VI) silicate.22 The latter one is extraordinary in that three unique sites corresponding to three different valence states coexist in the structure. Two U(IV) compounds have also been synthesized.23,24 All members in the system of uranium silicates and germanates with the valence states of uranium from IV to VI and their combinations have been observed. In this work we report the synthesis and characterization of four new uranyl silicates which have the same framework composition but different crystal structures. The silicate tetrahedra are linked together by sharing vertices to form tubular chains, single layers, and multiple chains. The correlation of the sizes of alkali metal cations and the structures of silicate anions is discussed.

INTRODUCTION Silicates containing the uranyl ion which is an oxycation of uranium in the oxidation state +6 with the chemical formula UO22+ are important uranium minerals in the altered zones of many uranium deposits.1 These compounds also form when used nuclear fuel interacts with silicon-containing water in oxygen-rich environments.2 Numerous uranyl silicates including an organically templated compound have been synthesized.3−9 The crystal structures of uranyl silicates are diverse and interesting. Most of them have three-dimensional (3D) framework structures, and a few adopt two-dimensional (2D) sheet structures. Recently, the synthesis of the first uranyl silicate with a one-dimensional (1D) chain structure has been achieved by introducing mixed alkali and alkaline earth metal cations into the structure.10 The methods of synthesis of uranium silicates include hydrothermal and flux methods. Several uranium silicates including salt-inclusion compounds were synthesized from molten metal fluoride fluxes.11−15 The majority of uranium silicates were synthesized under hypothermal conditions at 100−200 °C and at temperatures considerably above the critical temperature of water (374 °C). The supercritical hydrothermal method has proven to be very useful to crystal growth. Our long-term exploratory synthesis under hydrothermal conditions at 500−600 °C has led to a great number of new silicates and germanates of uranium in unusual valence states.16−23 In addition to U(VI) compounds,10,16 we reported © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Synthesis and Initial Characterization. Caution: Whereas the uranium oxide used in this study contains depleted uranium, standard precautions for handling radiative chemicals should be followed. Hydrof luoric acid is an extremely dangerous chemical and can cause tissue damage. Extreme caution is required when working with HF, and all Received: May 30, 2016 Revised: July 13, 2016

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DOI: 10.1021/acs.cgd.6b00817 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 1. Reaction Conditions for Na2[(UO2)Si4O10]·0.5H2O (1), K2[(UO2)Si4O10] (2), Rb2[(UO2)Si4O10] (3), and Cs2[(UO2)Si4O10] (4)a compound 1 2 3 4

reactants, length of gold ampule

products

UO3/SiO2/NaOH(1 M)/HF(10 M) (15.5 mg/9.5 mg/525 μL/42 μL; molar ratio: U:Si:Na:F = 1:2.9:9.7:7.8), 6.4 cm UO3/SiO2/KOH(1 M)/HF(10 M) (15.3 mg/13.6 mg/515 μL/41 μL; molar ratio: U:Si:K:F = 1:4.2:9.6:7.7), 6.3 cm UO3/SiO2/RbOH(50 wt %))/HF(10 M) (15.5 mg/13.1 mg/252 μL/192 μL; molar ratio: U:Si:Rb:F = 1:4:38.6:35.4), 5.1 cm UO3/SiO2/CsOH(50 wt %)/HF(10 M) (12.5 mg/11.2 mg/296 μL/153 μL; molar ratio: U:Si:Cs:F = 1:4.3:38.9:35), 4.8 cm

yellow acicular crystals of 1 (minor), yellow cubic crystals of Na2(UO2)2SiO4F2 (major) yellow bladed crystals of 2 (minor); unidentified yellowgreen material (major) yellow acicular crystals of 3 (major), yield: 73%; colorless quartz crystals (minor) yellow acicular crystals of 4 (major), yield: 60%; colorless quartz crystals (minor)

Each reaction mixture was heated at 590 °C for 2 days, cooled to 350 °C at 5 °C/h, and then cooled to r.t. by removing the pressure vessel from the tube furnace. The pressure at 590 °C was estimated to be 1620 bar.

a

Table 2. Crystallographic Data for Na2[(UO2)Si4O10]·0.5H2O (1), K2[(UO2)Si4O10] (2), Rb2[(UO2)Si4O10] (3), and Cs2[(UO2)Si4O10] (4) compound

1

2

3

4

chemical formula formula weight crystal system space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z T, °C λ(Mo Kα), Å Dcalc, g·cm−3 μ(Mo Kα), mm−1 R1a wR2b

Na2USi4O12.5H 598.38 tetragonal I4/mcm (No. 140) 18.0332(7) 18.0332(7) 7.7752(5) 90 90 90 2528.5(2) 8 23 0.71073 3.139 13.35 0.0447 0.1167

K2USi4O12 620.59 monoclinic P21/n (No. 14) 8.2970(17) 7.7403(15) 8.7906(18) 90 104.83(3) 90 545.74(19) 2 23 0.71073 3.777 16.14 0.0140 0.0345

Rb2USi4O12 713.33 monoclinic P21/c (No. 14) 10.3483(3) 23.7297(6) 7.6278(2) 90 110.482(1) 90 1754.68(8) 6 23 0.71073 4.050 22.61 0.0194 0.0692

Cs2USi4O12 808.21 orthorhombic Cmca (No. 64) 7.7157(3) 19.8719(8) 24.0160(10) 90 90 90 3682.3(3) 12 23 0.71073 4.374 19.52 0.0277 0.0639

R1 = Σ∥Fo| − |Fc∥/Σ|Fo|. bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2, w = 1/[σ2(Fo2) + (aP)2 + bP], P = [Max(Fo2,0) + 2(Fc)2]/3, where a = 0.0451 and b = 67.43 for 1, a = 0.0172 and b = 0.84 for 2, a = 0.0280 and b = 6.93 for 3, a = 0.0221 and b = 63.85 for 4.

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SiO4F2,25 structurally related to soddyite, whereas that for 2 is an unidentified yellow-green material. Optimal reaction conditions to improve their yields have not been found. Compounds 3 and 4 were obtained as major products. A phase pure sample of 1 was obtained via crystal picking for the determination of water content. Thermogravimetric analysis (TGA) data were measured using a PerkinElmer Pyris 1 analyzer on a powder sample and a sample of small pieces of crystals of 1 in a Pt container that was heated from 40 to 900 °C at 5 °C/min under N2 (Figure S1). Acicular crystals of compounds 3 and 4 were separated from the side products by hand and ground into powder samples. As shown in Figures S2 and S3, the measured X-ray powder patterns agree well with the simulated ones, which are indicative of pure phases. The differences between the observed and calculated intensities of 00l reflections for 4 can be ascribed to preferred orientation. The yields of 3 and 4 were 73% and 60% based on uranium, respectively. X-ray powder patterns were measured at room temperature using a Bruker D2 PHASER diffractometer with Cu Kα radiation. The IR spectra of 3 and 4 confirmed the presence of UO22+ and silicate groups (Figures S4 and S5).26 Photoluminescence data were collected on a powder sample of 4 at room temperature using a fluorescence spectrophotometer (Hitachi F-4500, excitation wavelength: 365 nm). As shown in Figure S6, the spectrum is typical of UO22+ ion and consists of a wide band ranging from 500 to 600 nm. The peaks in the band arise from a transition between electronicvibrational levels at about 508 nm followed by a coupling of the O UO vibrations.

work should be done in a chemical f ume hood. Supercritical hydrothermal reactions were conducted using a Leco high pressure vessel with a cold seal in which the pressure was generated by water. Reactants which include alkali metal hydroxide aqueous solution (NaOH, KOH, Merck, reagent grade; RbOH(aq), CsOH(aq), Aldrich, 50 wt %), UO3 (Cerac, 99.9%), SiO2 (Alfa Aesar, 99.995%), and HF(aq) (Acros, 48−51%) were welded shut in gold tubes (i.d. = 0.48 cm) which were placed in the vessel and counter-pressured with water at a fill level of 50%. HF(aq) was added to adjust the pH of the reaction mixture. The molar ratios of alkali metal hydroxide to HF are 1.2 for 1 and 2, and 1.1 for 3 and 4, indicating that the pH values of these reaction mixtures are about the same. The pressure vessel was heated to 590 °C, held at the temperature for 2 days, cooled to 350 °C at 5 °C/h, and then rapidly cooled to r.t. by removing the vessel from the tube furnace. The pressure at 590 °C was estimated to be 1620 bar according to the pressure−temperature phase diagram of H2O. The solid products were separated by suction filtration, washed with water, rinsed with ethyl alcohol, and dried in a desiccator at ambient conditions. Yellow acicular or bladed crystals of the four compounds were obtained. The reaction conditions and products are summarized in Table 1. Qualitative X-ray fluorescence (XRF) analyses of the crystals using a Bruker S2 RANGER energy dispersive XRF spectrometer confirmed the presence of U, Si, and Na (or K, Rb, Cs). A single crystal of each product was chosen to determine its crystal structure and chemical formula by X-ray diffraction. Compounds 1 and 2 could only be obtained as minor phases. The major product of the reaction for 1 is a uranyl silicate, Na2(UO2)2B

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Single-Crystal X-ray Diffraction. A single crystal of each compound was chosen for indexing and data collection at 296 K on a CCD diffractometer (Bruker Kappa Apex II). The unit cell parameters and integrated intensities were determined by using the SAINT program, and the absorption effect was corrected by the SADABS program. The initial models of these structures were obtained by direct methods, and the rest of atoms were located from successive difference Fourier maps. The H atoms of the water molecule in the structure of 1 could not be located. For all four structures the final least-squares refinement included coordinates and anisotropic atomic displacement parameters for all atoms. SHELXTL version 6.14 were used for all calculations.27 The crystallographic data and structure refinement results are given in Table 2.

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RESULTS AND DISCUSSION All of these four compounds contain UO6 tetragonal bipyramids with two short U−O bonds about 1.8 Å forming the uranyl unit [OUO]2+, which is typical for U(VI) in crystal structures, and four longer U−O bonds about 2.2 Å in the equatorial plane. Bond-valence sums for alkali metals, uranium, and silicon atoms were calculated, and their values are consistent with the valences (Table S1). As shown in Figure 1, the structure of Na2[(UO2)Si4O10]· 0.5H2O (denoted as 1) contains silicate tubular chains along the c axis which are formed by condensation of unbranched eight-membered rings, in which SiO4 tetrahedra are oriented alternately up and down with respect to the ring plane. The tubular chain can also be formed by the linkage of four unbranched single chains with a periodicity of four. The shortest oxygen−oxygen distance across the circular eight-ring window is 6.20 Å. Lateral six-ring windows are formed on the wall of the eight-ring channel. These tubular chains are linked through the equatorial oxygen atoms of the UO6 tetragonal bipyramids to form a 3D architecture. The Na+ cations are situated in the space between the tubes. The Na(1) site is fully occupied, whereas the site occupancy of Na(2) is 0.5. A water molecule with a site occupancy of 0.5 which is located between the tubes is coordinated to Na+ cations at 2.318 and 2.505 Å. The water molecule also forms weak hydrogen bonds with the uranyl oxygen, O(6), as indicated by the O(1w)···O(6) distance of 3.05 Å. The highest peak in the final difference Fourier maps within the tubular chain is 2.5 e/Å3, which is too small for a sodium or an oxygen atom. The deepest hole (−2.0 e/Å3) in the final difference maps is near a uranium atom. Several structures containing tubular chains with eight-ring windows have been reported.28 Compound 1 differs from them in the sequence of vertex orientation. The TGA data of powder sample of 1 show a broad weight loss of 4.47% between 30 and 500 °C, which is equivalent to the loss of about 1.5 H2O per formula unit. The weight loss is considerably greater than the calculated value of 1.5% for the loss of 0.5 H2O because the sample absorbed water during grinding. To minimize the amount of absorbed water TGA data were also measured on a sample containing pieces of acicular crystals. The weight loss of 1.81% between 30 and 500 °C is close to the calculated value for 0.5 H2O per formula unit. The weight loss above 700 °C is due to decomposition with the loss of oxygen. The major product of the reaction for 1 is the uranyl silicate Na2(UO2)2SiO4F2, which is structurally related to the mineral soddyite and consists of infinite chains of edge-sharing UO6F pentagonal bipyramids.25 The structure of K2[(UO2)Si4O10] (2) contains silicate single layers in the (101̅) plane which are connected through equatorial atoms of the UO6 tetragonal bipyramids to form a

Figure 1. (a) Structure of 1 viewed parallel to the c axis. Key: yellow tetragonal bipyramid, UO6; green tetrahedron, SiO4; red circle, Na+; blue circle, H2O. (b) A tubular silicate chain.

3D architecture (Figure 2a). The K+ cations are situated in the channels surrounded by two UO6 tetragonal bipyramids and six SiO4 tetrahedra. The SiO4 tetrahedra share three of their four oxygen corners with three SiO4 tetrahedra to form single layers with the composition of [Si2O5] which contain four and eight rings. The fourth corner is coordinated to a uranium cation. The silicate layer can be formed by successive linkage of a fundamental chain which is indicated by heavy lines in Figure 2(b). This layer is classified as “loop-branched dreier layer” according to Liebau.29,30 The single layers in Na2[(UO2)C

DOI: 10.1021/acs.cgd.6b00817 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 3. (a) Structure of 4 viewed parallel to the a axis. The red circles are Cs+ cations. (b) An unbranched vierer triple chain. The three chains are shown in different colors. Figure 2. (a) Structure of 2 viewed parallel to the b axis. Red circles are K+ cations. (b) A silicate single layer in which one fundamental chain is indicated by heavy lines.

bridging atom (Δs > 1) are rare. The connectedness s of an SiO4 tetrahedron is defined as the number of other SiO4 tetrahedra with which it is connected. The structure of the Si6O156− anion in A3[(UO2)1.5Si6O15] (equivalent to Si4O104− in A2[(UO2)Si4O10]) can also be analyzed according to the Zintl− Klemm concept.32 The structures of silicates can be considered as if the Si atoms were behaving as Zintl polyanions, adopting the structure of either main-group elements or Zintl polyanions showing the same connectivity in spite of being embedded in oxygen matrix. The structure of the Si6O156− anion can be interpreted as if the 3 Cs (or Rb) atoms and 1.5 uranyl units transfer 6 (= 3 + 3) electrons to 6 Si atoms. Two of the Si atoms receive four electrons and are transformed into two pseudo-S atoms which are 2-fold connected, and two Si atoms receive two electrons transforming into two pseudo-P atoms with a 3-fold connectivity. The other two Si atoms do not receive any electron and are 4-fold connected. The most often observed dimensionality of silicate anions for the ratio Si:O = 2:5 is 2. Interestingly, the silicate anions in these four uranyl compounds with Si:O = 2:5 exhibit three different topologies, namely, tubular chain, single layer, and multiple chain. In the tubular chains the negative charges can be considered to be localized at the terminal oxygen atoms and evenly distributed over the outer surface of the tubes. The charges of the bridging oxygen atoms are balanced by bonds to two silicon atoms. The volume in the interior of the tubular

Si4O10]·2.1H2O (USH-1) are similar to those in 2, but they have different orientations of tetrahedra.31 The layer in USH-1 is described as an unbranched vierer single layer. To our knowledge compound 2 is the first single layer silicate which contains loop-branched dreier layers. As shown in Figure S7, Rb2[(UO2)Si4O10] (3) and Cs2[(UO2)Si4O10] (4) contain the same type of silicate chains which are linked through equatorial atoms of the UO6 tetragonal bipyramids to form similar 3D architectures. The Rb+ or Cs+ cations are situated at sites in the circular 8-ring and rectangular 11-ring channels. The structure of 4 is shown in Figure 3a. The chain is an unbranched vierer triple chain with the number of linkages between the chains in a repeat unit of the silicate anions being equal to 6. An unbranched vierer triple chain is shown in Figure 3b, and the three chains are shown in different colors. Alternately, the chain can be built by linking loop-branched vierer rings, the planes of which are not perpendicular to the chain direction. These two compounds are the first example of multiple chain silicate with P(periodicity) = 4 and M(multiplicity) = 3. Another unusual feature of the silicate structure is that it consists of three kinds of Si, namely, Q2, Q3, and Q4 Si. Silicates which consist of tetrahedra with connectedness differing by more than one D

DOI: 10.1021/acs.cgd.6b00817 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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(5) Chen, C.-S.; Kao, H.-M.; Lii, K.-H. Inorg. Chem. 2005, 44, 935− 940. (6) Lin, C.-H.; Chiang, R.-K.; Lii, K.-H. J. Am. Chem. Soc. 2009, 131, 2068−2069. (7) Ling, J.; Morrison, J. M.; Ward, M.; Poinsatte-Jones, K.; Burns, P. C. Inorg. Chem. 2010, 49, 7123−7128. (8) Morrison, J. M.; Moore-Shay, L. J.; Burns, P. C. Inorg. Chem. 2011, 50, 2272−2277. (9) Babo, J.-M.; Albrecht-Schmitt, T. E. J. Solid State Chem. 2013, 197, 186−190. (10) Liu, C.-L.; Liu, H.-K.; Chang, W.-J.; Lii, K.-H. Inorg. Chem. 2015, 54, 8165−8167. (11) Lee, C.-S.; Wang, S.-L.; Chen, Y.-H.; Lii, K.-H. Inorg. Chem. 2009, 48, 8357−8361. (12) Jin, G. B.; Soderholm, L. J. Solid State Chem. 2015, 221, 405− 410. (13) Read, C. M.; Smith, M. D.; Withers, R.; zur Loye, H.-C. Inorg. Chem. 2015, 54, 4520−4525. (14) Morrison, G.; Ramanantoanina, H.; Urland, W.; Smith, M. D.; zur Loye, H.-C. Inorg. Chem. 2015, 54, 5504−5511. (15) Morrison, G.; Smith, M. D.; Tran, T. T.; Halasyamani, P. S.; zur Loye, H.-C. CrystEngComm 2015, 17, 4218−4224. (16) Chen, Y.-H.; Liu, H.-K.; Chang, W.-J.; Tzou, D.-L.; Lii, K.-H. J. Solid State Chem. 2016, 236, 55−60. (17) Chen, C.-S.; Lee, S.-F.; Lii, K.-H. J. Am. Chem. Soc. 2005, 127, 12208−12209. (18) Lee, C.-S.; Wang, S.-L.; Lii, K.-H. J. Am. Chem. Soc. 2009, 131, 15116−15117. (19) Nguyen, Q. B.; Liu, H.-K.; Chang, W.-J.; Lii, K.-H. Inorg. Chem. 2011, 50, 4241−4243. (20) Lin, C.-H.; Lii, K.-H. Angew. Chem., Int. Ed. 2008, 47, 8711− 8713. (21) Chang, Y.-C.; Chang, W.-J.; Boudin, S.; Lii, K.-H. Inorg. Chem. 2013, 52, 7230−7235. (22) Lee, C.-S.; Lin, C.-H.; Wang, S.-L.; Lii, K.-H. Angew. Chem., Int. Ed. 2010, 49, 4254−4256. (23) Liu, H.-K.; Lii, K.-H. Inorg. Chem. 2011, 50, 5870−5872. (24) Nguyen, Q. B.; Lii, K.-H. Inorg. Chem. 2011, 50, 9936−9938. (25) Blaton, N.; Vochten, R.; Peeters, O. M.; van Springel, K. N. Jb. Miner. Mh. 1999, 6, 253−264. (26) Cejka, J. Infrared Spectroscopy and Thermal Analysis of the Uranyl Minerals.; In Uranium: Mineralogy, Geochemistry and the Environment; Burns, P. C.; Finch, R., Eds.; Mineralogical Society of America: Washington, D. C., 1999. (27) Sheldrick, G. M. SHELXTL, version 6.14; Bruker AXS GmbH: Karlsruhe, Germany, 2000. (28) Rozhdestvenskaya, I. V.; Krivovichev, S. V. Crystallogr. Rep. 2011, 56, 1007−1018. (29) Liebau, F. Structural Chemistry of Silicates: Structure, Bonding and Classification; Springer-Verlag: Berlin, 1985. (30) To denote a silicate anion according to its periodicity, the German numerals einer, zweier, dreier, ... etc. for periodicities P = 1, 2, 3, ... have been used. (31) Wang, X.; Huang, J.; Liu, L.; Jacobson, A. J. J. Mater. Chem. 2002, 12, 406−410. (32) Santamaria-Perez, D.; Vegas, A.; Liebau, F. Struct. Bonding (Berlin) 2005, 118, 121−177. (33) Hawthorne, F. C. Z. Kristallogr. - Cryst. Mater. 1992, 201, 183− 206.

chain contains almost no negative charges, and therefore, no Na+ cation was found inside the tubes. Charge balance is achieved by surrounding UO22+ and Na+ cations. The space between tubular chains is too small to accommodate larger alkali metal cations, and, therefore, only the sodium compound can adopt the structure of 1. The presence of lattice water molecules in 1 can be understood by considering the valencematching principle based on the work by Hawthorne.33 The Lewis-acid strength of the Na+ cation should match the Lewisbase strength of the structural unit in order to stabilize the structure. The Na+ cation has a higher Lewis acidity than the other alkali metal cations, and its acidity can be moderated by lattice water as a bond-valence transformer. Accordingly, compound 1 contains a lattice water molecule in the region between the tubular chains so that the acidity of Na+ better matches the basicity of the structural unit and thus a stable structure is formed. The water molecule is not only coordinated to Na+ cations but also forms a hydrogen bond to one of the uranyl oxygen atoms. The unique unbranched vierer triple chains in the structures of 3 and 4 contain Q2, Q3, and Q4 Si. Each of the four rings in the chains consists of two Q3 and two Q4 Si atoms. The uncharged region of the chain which is formed of Q4 Si is a narrow cylinder. The Q2 and Q3 Si atoms are apart and distributed around the neutral cylinder, and thus more space is available for cations. The negative charges on the terminal oxygen atoms are balanced by UO22+ and the large alkali metal cations (Rb+, Cs+). Accordingly, the alkali metals are not only counter cations but also structure-directing agents.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00817. TGA curves, PXRD patterns, IR spectra, photoluminescence spectrum, and a table of selected bond lengths and bond-valence sums (PDF) Accession Codes

CCDC 1469464−1469467 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Taiwan for financial support. REFERENCES

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DOI: 10.1021/acs.cgd.6b00817 Cryst. Growth Des. XXXX, XXX, XXX−XXX