Manipulation of the Hydration Levels in Minerals of Sodium Cadmium

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Manipulation of the Hydration Levels in Minerals of Sodium Cadmium Bisulfate toward the Design of Functional Materials Dipankar Saha, Giridhar Madras, and Tayur N. Guru Row* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India

bS Supporting Information ABSTRACT:

Several variants of hydrated sodium cadmium bisulfate, Na2Cd2(SO4)3 3 3H2O, Na2Cd(SO4)2 3 2H2O, and Na2Cd(SO4)2 3 4H2O have been synthesized, and their thermal properties followed by phase transitions have been invesigated. The formation of these phases depends on the stochiometry and the time taken for crystallization from water. Na2Cd2(SO4)3 3 3H2O, which crystallizes in the trigonal system, space group P3c, is grown from the aqueous solution in about four weeks. The kr€ohnkite type mineral Na2Cd(SO4)2 3 2H2O and the mineral astrakhanite, also known as bl€odite, Na2Cd(SO4)2 3 4H2O, crystallize concomittantly in about 24 weeks. Both these minerals belong to the monoclinic system (space group P21/c). Na2Cd2(SO4)3 3 3H2O loses water completely when heated to 250 °C and transforms to a dehydrated phase (cubic system, space group I43d) whose structure has been established using ab initio powder diffration techniques. Na2Cd(SO4)2 3 2H2O transforms to R-Na2Cd(SO4)2 (space group C2/c) on heating to 150 °C which is a known high ionic conductor and remains intact over prolonged periods of exposure to moisture (over six months). However, when R-Na2Cd(SO4)2 is heated to 570 °C followed by sudden quenching in liquid nitrogen β-Na2Cd(SO4)2 (P21/c) is formed. β-Na2Cd(SO4)2 takes up water from the atmosphere and gets converted completely to the kr€ohnkite type mineral in about four weeks. Further, β-Na2Cd(SO4)2 has a conductivity behavior comparable to the R-form up to 280 °C, the temperature required for the transformation of the β- to R-form. These experiments demonstrate the possibility of utilizing the abundantly available mineral sources as precursors to design materials with special properties.

’ INTRODUCTION The omnipresent water plays a decisive role in the formation of hydrates of complex inorganic materials, in particular minerals. Bimetallic sulfates are important constituents of Earth’s crust, and they provide an essential link to unravel Earth’s mineral evolution. The variability in the level of hydration generates a variety of naturally occurring minerals like, for example, kr€ohnkite,1 langbeinite,2 Tutton’s salt,3 leonites,4 and astrakhanite.5 Minerals with a hydration level of 2 having a formula A2B(SO4)2 3 XH2O (where A = Liþ, Naþ, Kþ, Rbþ, Csþ, NH4þ, etc. and B = Mn2þ, Fe2þ, Co2þ, Ni2þ, Cu2þ, Zn2þ, Cd2þ, etc.) form the kr€ohnkite phase. With the hydration levels, the complexity of structural framework changes resulting in different types of minerals. For example X = 6 in A2B(SO4)2 3 XH2O forms compounds belonging to the Tutton’s salt family, while X = 4 results in either leonites or astrakhanites. Langbeinites, having a general formula A2B2(SO4)3, r 2011 American Chemical Society

show interesting ferroelectric properties and phase transitions at low temperatures and have no water of hydration. Indeed, several of the naturally occurring minerals also show interesting temperature-related phase transitions with the resulting compounds exhibiting high conductivity, ferroelectricity, and ferromagnetism.2 The possibilities of using minerals as functional materials need to be explored, and, in particular, the changes brought in the structural frameworks with varying temperature provide such an opportunity. Anhydrous Na2Cd(SO4)2 (hereafter referred to as R-Na2Cd(SO4)2) by itself is a material, which exhibits interesting conductivity behavior,6 showing two possible transitions, one at ∼280 °C and the other at ∼552 °C. Received: April 12, 2011 Revised: May 20, 2011 Published: May 24, 2011 3213

dx.doi.org/10.1021/cg2004613 | Cryst. Growth Des. 2011, 11, 3213–3221

Crystal Growth & Design

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Table 1. Crystallographic Refined Parameters Obtained from Single Crystal Data Na2Cd2(SO4)3 3 3H2O

empirical formula

Na2Cd(SO4)2 3 4H2O

Na2Cd(SO4)2 3 2H2O

formula weight

606.96

422.56

crystal habit, color

block, colorless

prismatic, colorless

386.5 block, colorless

crystal size (mm)

0.15 0.12 0.09

0.11 0.10 0.05

0.11 0.10 0.05

temperature (K)

293(2)

293(2)

293(2)

radiation

MoKR

MoKR

MoKR

monochromator

graphite

graphite

graphite

wavelength (Å)

0.71069

0.71069

0.71069

crystal system space group

hexagonal P3c

monoclinic P21/c

monoclinic P21/c

a (Å)

15.6033(2)

5.5314(2)

5.9085(3)

b (Å)

15.6033(2)

8.4624(3)

13.0165(5)

c (Å)

8.8392(1)

11.3048(4)

5.5636(2)

volume/Å3

1863.70(4)

522.04(3)

410.86(3)

β (°)

90

99.411(4)

106.215(5)

Z

3

2

2

density/g cm3 F(000)

3.245 1716

2.688 412

3.125 372

Scan mode

ω scan

ω scans

ω scans

θmax (°)

32.86

27.0

26.0

hmin,max, kmin,max, lmin,max

(23, 23), (23, 23), (13, 13)

(7, 7), (10, 10), (14, 14)

(7, 7), (16, 16), (6, 6)

no. of reflns measured

37997

5734

4330

no. of unique reflns

4368

1133

814

no. of parameters

228

96

78

absorption correction μ (mm1)

numerical from crystal shape 4.081



refinement

F2

F2

F2

R_all, R_obs

0.0231, 0.0213

0.0210, 0.0180

0.0194, 0.0189

wR2_all, wR2_obs

0.0536, 0.0530

0.0444, 0.0430

0.0532, 0.0528

GOF

1.039

1.069

1.127

max/min ΔF e/Å3

0.893, 0.855

0.426, 0.351

0.723, 0.665

One of the main features of hydrated minerals is that the water molecules get readily incorporated into the crystal lattice because of their small size and their ready hydrogen bonding capabilities. In fact, water molecules have been observed to stabilize crystal structures when there is an imbalance in charge distribution in framework moieties. It is this property which provides pathways for the polymorphic modification in the hydrated minerals. From the crystal engineering point of view based on the studies related to hydrated compounds, it has been recognized that water molecules can serve as a design element7 to maneuver supramolecular assemblies. The formation of hydrates essentially causes the water molecules to act as a structural glue,8 and hence the level of hydration governs the overall structural complexity. It may be agreed upon that the prediction of hydrated structures is almost impossible from the point of view of their preparation, their stoichiometry, and their stability9 and as a consequence the predictability of the structure of the dehydrated compound is almost impossible. However, it can be safely assumed that the abundance of hydrated minerals would provide a useful source for generation of new functional materials. In this article, we have explored the phase transition followed by the removal of water of hydration in several new phases of sodium and cadmium bisulfate in an effort to produce materials to show high conductivity at elevated temperature. The dehydration and rehydration of these compounds also provide the pro-

pensity of forming hitherto unexplored compounds. This analysis opens up new possibilities of using the large number of naturally occurring mineral hydrates as possible precursors for generating new materials.

’ EXPERIMENTAL SECTION Materials and Methods. Synthesis of Na2Cd2(SO4)3 3 3H2O (HNACD). The trihydrated compound was crystallized from a solution in water containing a 2:3 molar ratio of Na2SO4 (MERK, 99%) and 3CdSO4 3 8 H2O (MERK, 99%). The stoichiometric amount of the starting materials were dissolved in water and kept for crystallization in 5 mL glass beakers. Block shaped colorless crystals were obtained after approximately four weeks. Crystals of HNACD were taken out from the beaker prior to complete evaporation of water solvent. A good quality crystal was chosen under the polarizing microscope for single crystal X-ray diffraction (XRD) studies. Single crystals of HNACD were crushed to form a polycrystalline powder for further characterization. The composition of the crystals was confirmed by preliminary EDAX (FEI Quanta scanning electron microscope) measurements (see Supporting Information, Figure S1). Synthesis of Na0.7Cd0.6SO4 (DHNACD). The trihydrated compound HNACD was heated up to 700 °C and kept for 1 h at this temperature resulting in the formation of a new phase DHNACD. Synthesis of β-Na2Cd(SO4)2, Na2Cd(SO4)2 3 2H2O, and Na2Cd(SO4)2 3 4H2O. β-Na2Cd(SO4)2 was synthesized by quenching the 3214

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Figure 1. Asymmetric unit view of HNACD. R-Na2Cd(SO4)2 from 570 °C to liquid nitrogen. The crystals of Na2Cd(SO4)2 3 2H2O, a representative of the kr€ohnkite type family of minerals, were grown from quenched samples of R-Na2Cd(SO4)2 dissolved in water at room temperature. The starting compound Na2Cd(SO4)2 was prepared based on the literature method.6 Na2Cd(SO4)2 3 2H2O was also synthesized from the stoichiometric solution of Na2SO4 and 3CdSO4 3 8H2O. The stoichiometric amounts of the starting materials were dissolved in water and kept for crystallization in a 500 mL glass beaker. Block shaped crystals were obtained after 6 months. Prismatic Na2Cd(SO4)2 3 4H2O crystals were obtained from the same water solution after 6 months concomitantly with the mineral kr€ohnkite in the beaker (see Supporting Information, Figure S2). Phase Characterization. Differential scanning caloriemetry (DSC) (Mettler Toledo STARe) measurements and thermogravimetric analysis (Mettler Toledo) were carried out at a heating/cooling rate of 5 oC min1 under N2 atmosphere in the temperature range 25650 °C. Powder X-ray diffraction (PXRD) data were collected on a Philips, X’pert Pro powder diffractometer in reflection geometry. Single crystal XRD data were collected on Xcalibur Mova E diffractometer with four circle kappa goniometer and Eos CCD detector in CrysAlis CCD, Oxford Diffraction with X-ray generator 50 kV and 1 mA, using MoKR radiation (λ = 0.7107 Å). The cell refinement and data reduction were accomplished using CrysAlis RED.10 The structures were solved by direct method using SHELXL9711 using the program suite WINGX (version 1.70.01).12 The molecular diagrams were generated using ORTEP-3.213 and packing diagrams were generated using package Diamond, Version 2.1c.14 Profile refinements were carried out using Jana2000.15 Ionic Conductivity from AC-Impedance Spectroscopy. AC impedance measurements were carried out using pelletized powder samples. Prior to measurement, the pellets were annealed at 120 °C for 12 h for HNACD, while for β-Na2Cd(SO4)2 pellets of R-Na2Cd(SO4)2 were annealed at 570 °C and quenched in liquid nitrogen to obtain β-Na2Cd(SO4)2 pellets. Silver paste was applied on both sides of the pellets for better ohmic contact. The coated pellets were heated at 120 °C for 1 h to remove the organic component of the Ag-paste. Diameter and thickness of the pellet were approximately 10 mm and 1 mm, respectively. The annealed pellets were then placed between two steel electrodes of a homemade conductivity cell. AC-impedance measurements were carried out (Novocontrol Alpha-A) in the frequency range (13) 106 Hz (signal amplitude = 0.05 V) from 30700 °C at an interval of 20 °C up to 200 °C and an interval of 50 °C up to 700 °C . The temperature of samples was equilibrated for 30 min at each temperature point ((1 °C, programmable Thermolyne furnace) prior to the impedance measurements.

Figure 2. (ah) xy section of the difference Fourier map around different metal centers. Contour lines in the interval of 0.3 e Å3.

’ RESULTS AND DISCUSSION Crystal Structure of HNACD. A single crystal XRD study reveals that HNACD crystallizes in a trigonal system with the space group P3c and cell parameters a = 15.6033(2), c = 8.8392(1) Å; V = 1863.70(4) Å3 and Z = 3 (Table 1). The positions of Cd, S, and Na were obtained by direct methods using SHELXL97.11 All oxygen atoms were located from the subsequent difference Fourier synthesis. A view of asymmetric unit is given in Figure 1 where atoms O13, O14, and O15 belong to the water molecules. Initially, four Cd atoms, two Na atoms, along with three sulfate units were assigned full occupancy during refinement cycles. The resulting final R factor was 0.048, and the corresponding residual electron density map shows significant 3215



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Table 2. Atomic Coordinates, Occupancies, and Isotropic Thermal for Metal Centers x

atoms

y

z

occupancy

Ueq (Å2)

Wyckoff position

Cd1

0

0

0.44038(4)

1

0.016(1)

2a

Na1

0.23157(10)

0.00399(10)

0.05100(16)

1

0.022(1)

6d

Cd2

0.09934(2)

0.45244(2)

0.37273(3)

0.819

0.018(1)

6d

0.015(1)

2b

Na2

0.181

Cd3

0.33333

0.66667

0.27238(7)

0.423

Na3

0.577

Cd4

0.69756(4)

Na4 Cd5

0.12446(2)

0.66667

0.33333

0.17310(4)

0.665

0.022(1)

6d

0.0405(3)

0.335 0.132

0.039(1)

2c

Na5

0.868

Table 3. Coordinates, Occupancies, and Isotropic Thermal Parameters of the Disordered Oxygen Atom Associated with the Sulfates Moiety occupancy

Ueq (Å2)

0.2027(11)

0.529

0.040(2)

0.1609(9)

0.471

0.043(3)

0.3883(6)

0.2957(16)

0.527

0.073(4)

0.5948(6) 0.4532(7)

0.3971(7) 0.1556(13)

0.2072(10) 0.4219(9)

0.473 0.440

0.039(2) 0.043(2)

0.5162(7)

0.1780(14)

0.4425(7)

0.560

0.059(3)

atoms

x

y

z

O9a

0.4382(5)

0.1660(10)

O9b

0.4796(8)

0.1910(15)

O11a

0.5927(7)

O11b O12a O12b

deviation with the value of ΔF ranging from þ2.60 to 4.43 e Å3. This clearly indicates that the metal atoms share their position, and refinements need to be carried out by assigning appropriate occupancy values for the metal atoms. The difference Fourier approach to unequivocally establish crystal structures from either PXRD or single crystal XRD has been described in our earlier studies,16,17 and the same methodology is used in subsequent analysis. The following describes the refinement strategy adopted to unravel the correctness of the structure. Initially, the occupancies of metal atoms were allowed to refine keeping their corresponding isotropic thermal parameters fixed. This protocol was followed by refining the occupancies of each atom separately. It was observed that the occupancies of Cd1 and Na1 do not change significantly, while the other occupancies associated with Cd2, Cd3, and Cd4 decrease and that of Na5 increases. Figure 2 shows the difference Fourier maps at the site of all four metal centers Cd2, Cd3, Cd4, and Na5. It is to be mentioned that the difference Fourier accounts for the number of electrons associated with each site, and if it is in excess the maps show negative contours for the electron density and positive contours are for deficit density. Figure 2a shows that the Cd2 site has negative contours suggesting that the assignment of the electron density to this site is in excess. This Wyckoff 6d site was hence identified to share both Cd2 and Na2, and the refinements of occupancies converge to 0.819 for Cd2 and 0.181 for Na2, respectively. Further refinements were carried out on occupancies and isotropic thermal parameters alternatively leading to the convergence. The difference Fourier map computed after these refinements (Figure 2b) is almost flat supporting the correctness of the occupancy assignment at this site. Similar refinement strategies were followed, and Figure 2ch shows the residual electron density maps for Cd3, Cd4, and Na5 before and after the occupancy refinement. Table 2 lists the final coordinates along with the occupancies at each site and the corresponding refined Ueq values for all metal atoms. All contour levels are drawn at

Figure 3. Octahedra and tetrahedra arrangement in HNACD.

0.3 e Å3. Further refinements were carried out keeping the occupancies fixed at all these sites and allowing anisotropic thermal parameters to alter. The final R factor is 0.021 with ΔFmin, max = 0.893, 0.855 e Å3. Of the three sulfate moieties, one depicts larger thermal vibrations associated with the oxygen atom O9, O11, and O12, respectively. However, refinements clearly suggest that each of these atoms has two identifiable sites (Table 3), and refinement of occupancies indicate nearly equal population at each site. Even though the refinements incorporate anisotropic thermal parameters for all non-hydrogen atoms, the location of the associated hydrogen atom with molecular water is not justified. Figure 3 depicts the polyhedral arrangement of HNACD. SO4 tetrahedra are isolated from each other and are corner shared with two different kinds of CdO6 octahedra; one them is connected via corner sharing with SO4 tetrahedra units while the other shares edges (Figure 3) with other octahedra. Figure 4 shows the three-dimensional (3D) repetitive unit which propagates itself in all three directions where corner shared SO4 tetrahedra and CdO6 octahedra act as linkers. Thermal Analysis. Figure 5a,b depicts the thermogravimetric analysis (TGA) and differential scanning clorimetry (DSC) studies of HNACD. The TGA shows that the structure loses all three lattice water molecules after 230 °C. In the first step, two water molecules are lost at about 125 °C readily, while the third water molecule is removed in a sluggish manner around 210 °C. The DSC plot (Figure 5b) correlates with these observations with a first sharp peak at 150 °C corresponding to the loss of two water molecules and a second broad peak at 215 °C due to the loss of the third water molecule. This feature can be explained on the basis of the structure. The representation 3216



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Figure 4. Repetitive unit of HNACD.

of the repeat unit shown in Figure 4 supports the observation made from thermal studies. The inner core surface represented by the blue circle has one water molecule represented as O15, while the outer core of the repeat unit outlined by the red circle consists of two water molecules O13 and O14. In addition, the coordinating distance of O13Cd2 and O14Cd2 bonds are 2.340(1) and 2.312(2) Å, respectively, whereas the coordinating distance of O15Cd4 is shorter [2.291(1) Å]. After removal of the water molecules, HNACD forms an anhydrous stable phase in the temperature range of 250650 °C. The DSC plot shows a possible phase transition at 526 °C. Further, this phase transition is reversible and the corresponding temperature is 450 °C (Figure 5b). The structure of the dehydrated phase has been determined from high resolution PXRD. It is to be noted that the two endothermic peak indicating transitions after the transition at 526 °C shows the presence of R-Na2Cd(SO4)2 with dehydrated HNACD. Crystal Structure of Dehydrated HNACD (DHNACD) from Powder X-ray Diffraction. Figure 6 shows the fitted profile (Rp = 0.0552, Rwp = 0.0727) for the dehydrated phase. The powder data were recorded after heating HNACD up to 700 °C and then cooling it to room temperature. The diffractogram clearly indicates the coexistence of two phases after removal of water. The parent phase (DHNACD) was indexed in a cubic cell with a = 9.8951(2) and space group I43d, while the second phase is identified with R-Na2Cd(SO4)2.6 Peaks of the impure phase were subtracted by using X’pert Highscore software, and the resulting pattern was subjected to ab initio structure determination. The starting model was chosen by matching the powder pattern with the ICDD database. The best match was Sr3La(PO4)3 which is an eulytite type structure.18 Coordinates of La and P from Sr3La(PO4)3 were taken as the coordinates for Cd and S for preliminary Rietveld refinement using the GSAS19 program. At this stage, the occupancy of the Cd1 atom refined to a lower value suggesting that the Na atom might be located at the same site satisfying the overall stoichiometry. Hence, Na and Cd were put at the same Wyckoff site 16c, and the occupancy was allowed to refine. A subsequent difference Fourier map revealed the position of the remaining oxygen atom which was used for the final Rietveld refinement. It was also observed that occupancy

Figure 5. (a) Thermogravimetric analysis (TGA) and differential thermal analysis (DTA), (b) differential scanning calorimetry (DSC) for HNACD.

of O1 refined to a lower value indicating the positional disorder of oxygen atom. At this stage, a difference Fourier analysis revealed yet another possible position for oxygen hereafter referred to as O2. Occupancies of O1 and O2 were constrained during further refinements to unity (see Supporting Information Table S4 and S5 for coordinates of atoms and refined crystallographic parameter). The profile was fitted using the pseudo-Voigt function. A Chebyshev function consisting of 12 coefficients was used to define the background. Figure 7 shows the fitted profile (Rp = 0.0781, Rwp = 0.10.42) after Rietveld refinement of DHNACD. Isotropic thermal parameters of all atoms were refined independently at the early stage of refinements. The formula unit at the end of the refinement is Na0.7Cd0.6SO4 for the structure of DHNACD. DHNACD consists of a 3D packing of Cd/NaO6 octahedra and SO4 tetrahedra. The octahedra form a 3D network by sharing edges among themselves, while the SO4 tetrahedra are completely independent and share four corner oxygen atoms with different adjoining Cd/NaO6 octahedra. Indeed, the packing of SO4 tetrahedra forming pentagonal channels is a characteristic of any eulytite structure (Figure 8). Further, eulytite structures20,21 also possess disordered oxygen atoms in building up the structural framework. In both HNACD and DHNACD Na/CdO6 octahedra are connected by the corner shared SO4 unit. While HNACD consists of both Na/CdO6 and CdO6 octahedra, DHNACD consists of Na/CdO6 octahedra only. Both structures have similar Na and Cd disorder at same crystallographic site in the corresponding structure. The water molecules in HNACD 3217



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connect between the inner and outer core (Figure 4). However, the absence of water in DHNACD results in corner shared octahedra with sulfate tetrahedra. Both structures have a similar disordered sulfate moiety. Indeed, the disorder in the Na/Cd sites results in extensive disorder in the oxygen atom positions in both structures. Ionic Conductivity from AC-Impedance Spectroscopy. The measured conductivity versus temperature of HNACD is shown in Figure 9. The value of the bulk ionic conductivity was calculated from the intercept of the single semicircular arcs obtained in the complex impedance plots of Re (Z0 )Im (Z00 ) plots (Z: impedance). The room temperature conductivity is on the order of 1011 Ω1 cm1, and this value does not significantly change with an increase in temperature up to 140 °C. The behavior of the ionic conductivity with temperature measured using AC-impe-

dance spectroscopy closely correlates with the observed thermal behavior of HNACD. Two of the water molecules are removed around 140 °C, and then the third water molecule is removed at 210 °C. The variation in the conductivity between 140 and 210 °C correlates with the sluggishness shown in both DSC and TGA measurements. The conductivity gradually increases from 210 °C which represents the DHNACD phase with the phase R-Na2Cd(SO4)2 and until the phase transition temperature ∼550 °C is reached. Indeed the conductivity value is on the order of 104 Ω1 cm1 at 600 °C. Since the structural feature clearly depicts the presence of Naþ and Cd2þ at the same crystallographic site, it may be concluded that the conduction mechanism

Figure 6. Full profile refinement of the mixed phase upper one being DHNACD and lower one being R-Na2Cd(SO4)2.

Figure 8. Packing diagram of DHNACD showing the pentagonal arrangement of disordered SO4 tetrahedra.

Figure 7. Rietveld refinement of DHNACD. 3218



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Figure 11. CdO4(H2O)2 chain arrangement in the Kr€ohnkite type mineral.

Figure 9. Ionic conductivity from AC-impedance spectroscopy for HNACD (conductivity cell configuration: Ag| electrolyte |Ag, Ag = silver (cell constant = 0.083 cm1).

Figure 10. Comparison of powder pattern of R-Na2Cd(SO4)2 and quenched β-Na2Cd(SO4)2.

may be Naþ ion hopping. The presence of Cd2þ hinders the mobility associated with Naþ ion and therefore reduces the conductivity. Crystal Structures of β-Na2Cd(SO4)2 and Na2Cd(SO4)2 3 2H2O. R-Na2Cd(SO4)2 is a novel fast ionic conductor,6 crystallizing in a space group C2/c with cell parameters a = 10.059(2) Å, b = 8.2250(18)Å, c = 8.6557(19) Å; β = 114.620(2)°, V = 651.0(2) Å3. This is a stable phase and does not pick up water of crystallization when soaked in water or when exposed to air for extended periods of time (weeks to month). During the study of DHNACD, described in the previous section, R-Na2Cd(SO4)2 was the accompanying surrogate phase. Single crystals of R-Na2Cd(SO4)2 were grown independently and were crushed to form a polycrystalline powder. This sample was heated to 570 °C, which is well beyond the structural phase transition temperature of 554 °C. After keeping the sample at this temperature for an hour, it was immediately quenched in liquid nitrogen. The powder diffraction pattern of the quenched sample at room temperature shows significant differences from that of the R-phase (Figure 10). Indeed, this phase can be uniquely indexed in a monoclinic system and is identified as a new polymorph,

β-Na2Cd(SO4)2. The cell dimensions are a = 15.114(2) Å, b = 9.051(1) Å, c = 12.770(2) Å, β = 126.549(5)°, V = 1405.67(7), space group P21/c. When β-Na2Cd(SO4)2 is dissolved in water and kept for crystallization at ambient temperature, transparent block type crystals were obtained after two weeks. Single crystal XRD reveals that the resulting phase crystallizes in a space group P21/c with cell parameters a = 5.9085(3) Å, b = 13.0165(5) Å, c = 5.5636(2) Å, β = 106.215(5)°, V = 410.86(7) corresponding to the observed parameters22,23 for Na2Cd(SO4)2 3 2H2O (Table 1). The structure forms infinite chains, comprised of CdO6 octahedra sharing four of its corners with SO4 tetrahedra and the other two with the two water molecules (Figure 11). These chains referred to as kr€ohnkite chains are interconnected through OH 3 3 3 O hydrogen bonds. It is of interest to note that this structure has been reported earlier, wherein the authors directly recrystallized the mineral from water. We have also synthesized the mineral from the stoichiometric solution of Na2SO4 and 3CdSO4 3 8H2O. Of the several crystallization dishes, it was observed that crystals which appeared within few weeks were not of the mineral but were crystals of Na2SO4 3 H2O and 3CdSO4 3 8H2O. However, the crystals of the mineral, Na2Cd(SO4)2 3 2H2O, were obtained after 6 months in some of the crystallization dishes. This feature indicates that the mineral formation is somewhat sluggish and follows several steps. It is also to be pointed out that β-Na2Cd(SO4)2 absorbs water from the atmosphere and gets converted to Na2Cd(SO4)2 3 2H2O slowly over long periods of storage. For example, around 60 mg of quenched powder takes about 30 days to covert to the kr€ohnkite type mineral completely. DSC studies (Figure 12a) reveal that the mineral loses water around 100 °C and is completely dehydrated after 150 °C. After the loss of water molecules, it forms the R-Na2Cd(SO4)2 phase. The peak around 133 °C corresponds to the water loss, and peaks at 540 and 554 °C are characteristic peaks due to phase transition in the RNa2Cd(SO4)2 phase.6,24 DSC of β-Na2Cd(SO4)2 shows an exothermic peak around 280 °C (Figure 12b) converting to RNa2Cd(SO4)2, thus demonstrating that β-Na2Cd(SO4)2 is a metastable polymorphic form of R-Na2Cd(SO4)2. The measured conductivity versus temperature of β-Na2Cd(SO4)2 is shown in Figure 13. The room temperature conductivity is on the order of 1011 Ω1 cm1. The features associated with the conductivity behavior show two possible transitions in β-Na2Cd(SO4)2 one at 280 °C representing the transformation to R-Na2Cd(SO4)2 RNa2Cd(SO4)2 and the other at 550 °C which has a one-to-one correlation with the reported literature value for the R-phase. Crystal Structure of Na2Cd(SO4)2 3 4H2O. Single crystal XRD study reveals that Na2Cd(SO4)2 3 4H2O crystallizes in a monoclinic 3219



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Figure 14. Packing diagram of the mineral astrakhanite [Na2Cd(SO4)2 3 4H2O] viewed along the a direction showing the [Cd(SO4)2 3 4H2O]2 units and the hydrogen bonding pattern between them.

Figure 12. Differential scanning calorimetry (DSC) of (a) kr€ohnkite mineral Na2Cd(SO4)2 3 2H2O (b) β-Na2Cd(SO4)2.

Figure 13. Ionic conductivity from AC-impedance spectroscopy for βNa2Cd(SO4)2 (conductivity cell configuration: Ag| electrolyte |Ag, Ag = silver (cell constant = 0.115 cm1).

space group P21/c and cell parameters a = 5.5314(2) Å, b = 8.4624(3) Å, c = 11.3048(4) Å; β = 99.411(4)°, V = 522.04(3) Å3. The positions of Na, Cd, and S were obtained by direct methods. The oxygen atoms were located from the subsequent difference Fourier synthesis. Hydrogen atoms were located from difference Fourier maps. Atom Cd1 occupies the center of inversion (2c site), whereas all the other atoms are in general positions 4e. The structure consists of a [Cd(SO4)2 3 4H2O]2 repetitive unit along with a distorted NaO(aqua)2O(sulfate)4 octahedra appearing in

general positions resulting in a 3D network with the water molecules bridging the coordinating sulfate groups. [Cd(SO4)2 3 4H2O]2 units are connected by OH 3 3 3 O hydrogen bonds. Figure 14 depicts the packing diagram of Na2Cd(SO4)2 3 4H2O viewed along the a direction showing the [Cd(SO4)2 3 4H2O]2 units and the hydrogen bonding pattern between them. Na2Cd(SO4)2 3 4H2O is isostructural with the mineral astrakhanite, which is also known as bl€odite.5,25 It has a chemical formula Na2Mg(SO4)2 3 4H2O. There are very few reports of isostructural bl€odite where Mg atom is being replaced by Co and Zn, Ni, and Fe.2628 In all three hydrated structures the basic building block is formed by SO4 tetrahedra coordinating (corner shared) with CdO6 octahedra (Figures 4, 11, and 14). Further, these building blocks are interconnected by either coordinated water molecules or by hydrogen bonding involving the hydrogen atom of one of the water molecules with one of the oxygen atoms of SO4 unit. In the case of HNACD having 3H2O units, the corner sharing units of SO4 tetrahedra and CdO6 octahedra propagate in all three directions (Figure 4). In case of the kr€ohnkite mineral Na2Cd(SO4)2 3 2H2O, the building block is similar and these units form a chain along the [001] direction (Figure 11). However, in the case of Na2Cd(SO4)2 3 4H2O, the building blocks are isolated from each other and the hydration is across individual building blocks (Figure 14).

’ CONCLUSIONS The prospect of using differently hydrated minerals to produce materials with possible applications as devices is exciting, and the diversity available among the entire terrestrial and extraterrestrial minerals sources makes this a worthwhile effort. The examples described above like in the case of Na2Cd(SO4)2 3 2H2O generating polymorphic anhydrous forms with very high ionic conductivity and the formation of a hitherto unknown phase from Na2Cd2(SO4)3 3 3H2O displaying reasonable ionic conductivity suggests that minerals serve as an “Aladdin’s Cave” providing an extremely rich supply of compounds to be exploited for the generation of futuristic materials exhibiting exotic properties. 3220



’ ASSOCIATED CONTENT

bS

Supporting Information. Table S1. Atomic coordinates ( 104) and equivalent isotropic displacement parameters (Å2 103) for HNACD; Table S2. Bond lengths [Å] and angles [°] for HNACD; Table S3. Anisotropic displacement parameters (Å2 103) for HNACD; Table S4. Refined crystallographic parameters for DHNACD; Table S5. Atomic coordinates, isotropic thermal parameters and occupancies of DHANCD; Table S6. Atomic coordinates ( 104) and equivalent isotropic displacement parameters (Å2 103) for Na2Cd(SO4)2 3 4H2O; Table S7. Bond lengths [Å] and angles [°] for Na2Cd(SO4)2 3 4H2O; Table S8. Anisotropic displacement parameters (Å2 103) for Na2Cd(SO4)2 3 4H2O; Figure S1. EDX analysis for HNACD; Figure S2. Images of Kr€ohnkite [Na2Cd(SO4)2 3 2H2O] and Astrakhanite [Na2Cd(SO4)2 3 4H2O] Crystal; CIF files of Na2Cd2(SO4)3 3 3H2O, Na2Cd(SO4)2 3 4H2O and Na2Cd(SO4)2 3 2H2O. This material is available free of charge via the Internet at http:// pubs.acs.org.

ARTICLE

(18) Barbier, J.; Greedan, J. E.; Asaro, T.; McCarthy, G. J. Eur. J. Solid State Inorg. Chem. 1990, 27, 855. (19) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); Los Alamos National Laboratory Report LAUR 86-748, 2000. (20) Sahoo, P. P.; Row, T. N. G. Inorg. Chem. 2010, 49, 10013. (21) Arbib, E. H.; Chaminade, J.-P.; Darriet, J.; Elouadi, B. Solid State Sci. 2000, 2, 243. (22) Stoilova, D.; Wildner, M.; Koleva, V. Vib. Spectrosc. 2003, 31, 115. (23) Wildner, M.; Stoilova, D. Z. Kristallogr. 2003, 218, 201. (24) Pradhan, G. K.; Swain, D.; Row, T. N. G.; Narayana, C. J. Phys. Chem. A 2009, 113, 1505. (25) http://www.mindat.org/min-695.html. (26) Bukin, V. I.; Nozik, Y. Z. Geokhimiya 1974, 1277. (27) Diaz de Vivar, M. E.; Baggio, S.; Ibanez, A.; Baggio, R. Acta Crystallogr. E 2008, 64, i30. (28) Hudak, M.; Diaz, J. G.; Kozisek, J. Acta Crystallogr. E 2008, 64, i10.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: þ91-80-2292796. Fax: þ91-80-3601310.

’ ACKNOWLEDGMENT The authors thank Mr. I. S. Jarali for DSC and TGA measurements and Dr. Aninda J Bhattacharyya for discussions on ionic conductivity. T.N.G. thanks DST for JC Bose fellowship for funding. We also acknowledge funding under DST-FIST (Level II) for single crystal facility. ’ REFERENCES (1) Hawthorne, F. C.; Ferguson, R. B. Acta Crystallogr. B 1975, 31, 1753. (2) Nalini, G.; Row, T. N. G. Chem. Mater. 2002, 14, 4729. (3) Ballirano, P.; Belardi, G. Acta Crystallogr. E 2007, 63, i56. (4) Hertweck, B.; Giester, G.; Libowitzky, E. Am. Mineral. 2001, 86, 1282. (5) Palache, C.; Berman, H.; Frondel, C. The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana, 7th ed.; John Wiley and Sons, Inc: New York, 1951; Vol. II. (6) Swain, D.; Row, T. N. G. Chem. Mater. 2007, 19, 347. (7) Varughese, S.; Desiraju, G. R. Cryst. Growth Des. 2010, 10, 4184. (8) Clarke, H. D.; Arora, K. K.; Wojtas, Å. u.; Zaworotko, M. J. Cryst. Growth Des. 2011, 11 (4), 964. (9) Morris, K. In Structural Aspects of Hydrates and Solvates; Marcel Dekker: New York, 1999; Vol. I. (10) CrysAlis CCD, CrysAlisPro RED, 1.171.33.34d ed.; Oxford Diffraction Ltd.: Abingdon, Oxfordshire, England, 2009. (11) Sheldrick, G. M. Acta Crystallogr. A 2008, 64, 112. (12) Farrugia, L. J. Appl. Crystallogr. 1999, 32, 837. (13) Farrugia, L. J. Appl. Crystallogr. 1997, 30, 565. (14) Brandenburg, K. DIAMOND, 2.1c ed.; Crystal Impact GbR: Bonn, Germany, 1999. (15) Petricek, V.; Dusek, M.; Palatinus, L. Jana2000, 08/11/2007 ed.; Institute of Physics: Praha, Czech Republic, 2007. (16) Saha, D.; Mahapatra, S.; Row, T. N. G.; Madras, G. Ind. Eng. Chem. Res. 2009, 48, 7489. (17) Sahoo, P. P.; Gaudin, E.; Darriet, J.; Row, T. N. G. Inorg. Chem. 2010, 49, 5603. 3221


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