Growth and structural characterization of Tutton salt mixed of Co and Ni

Abstract The family of crystals known as Tutton's salts plays a significant role in physics and chemistry; because they are used in phase transition studies and to define models applied to materials. The importance of salts in material engineering is recent, as in applications in adiabatic degaussing refrigerators and solid-state anodes. Studies of the (NH4)2Ni(SO4)2∙6H2O and (NH4)2Co(SO4)2·6H2O are widely found in literature but do not occur for the mixture of both. In this research, we studied mixed crystals of the general chemical formula (NH4)2NixCo(1-x)(SO4)2·6H2O with x ranging from 0 to 1, utilizing x = 0.7. The objective is to study the modifications caused owing to the ion's weighted composition in the formation of the solid solution and compare it with the pure salts. For this, the growth of these crystals is discussed based on ICP-OES results and optical microscopy concerning the crystal growth theory. The discussion also relates the Raman spectra of the salts with molecular changes according to structured group theory, qualitatively characterizing its crystalline structure. Finally, a Single-crystal X-ray study solves and confirms the structure of pure salts and mixed salt, quantitatively characterizing their crystal structure.

Many of these crystals have applications: as reagents of considerable reliability and spectroscopic standards (Ganesh et al., 2013); in low-temperature nuclear orientation experiments (Bleaney et al., 1954); in dynamic polarization experiments on dielectric solids used in nuclear physics (Atsarkin, 1978); in Nearest-neighbor-interaction model in the coupled-optical-phonon-mode theory of the infrared dispersion in monoclinic crystals (Ivanovski and Ivanovski, 2010); and high-frequency Electronic Paramagnetic Resonance (EPR) based studies applied to bio-inorganic systems (Reijerse et al., 1998), among others.These applications characterize the importance of these crystals in physics and chemistry.Some of them are used in technological applications, such as hydrated salt pellets, used to produce a low temperature in refrigerators of adiabatical-demagnetization (Shirron and Mccammon, 2014), and in the elaboration of new materials used in solid-state anodes for solar cells with electrodes of proton conduction (Telli et al., 2002), among others.These applications characterize the importance of these crystals in materials engineering.
It is possible to obtain mixed singlecrystals in the form of solid solutions, such as (NH 4 ) 2 Ni x Co (1-x) (SO 4 ) 2 •6H 2 O, with x ranging from 0 to 1 (Fei & Straws, 1995), for the family of Tutton's salts.We found Single-crystal X-ray studies and spectroscopic studies for pure (NH 4 ) 2 Ni(SO 4 ) 2 •6H 2 O and (NH 4 ) 2 Co(SO 4 ) 2 •6H 2 O compounds, but scarce is literature for the mixed ones.
Pure salts are isostructural, crystallize in the monoclinic system, belong to the space group P2 1/c (C 5 2h ), and have twice the formula in the unit cell (Z = 2).The structure is also described from distorted octahedra of [M(H 2 O) 6 ] 2+ and tetrahedra of SO 4 2-and NH 4 + , shown in Figure 1 (Li & Li, 2004;Tahirov & Lu, 1994).The cation M = Ni or Co occupies the octahedron center, and six water molecules occupy your vertices.Only three water molecules are crystallographically independent.These octahedra and tetrahedra interact with each other using hydrogen bonds.

Results
According to Cotton et al. (1993), Tahirove & Lu (1994), Cotton et al. (1994), and Li & Li (2004), the structure of these salts exhibits a complex network of hydrogen bonds allowing the whole series to be isomorphic.lations (Barashkov et al., 2000)).The objective of this article is to present and discuss the method of crystal growth and its structure using the techniques of Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) characterization, optical microscopy, Raman spectroscopy, and Single-crystal X-ray.This study compares the spectra of the pure salts containing Ni and Co, widely found in literature, to the mixed salt.Formulated using Li and Li (2004).
The solvent used for crystal growth was water.The solutes used were the analytical reagents: NiSO  Single crystals obtained using the growth process are exhibited in Figure 2, with adequate morphology and high purity.Samples 1, 2, and 3 correspond to (NH 4 ) 2 Co(SO 4 ) 2 •6H 2 O, ( N H 4 ) 2 N i ( S O 4 ) 2 • 6 H 2 O , a n d (NH 4 ) 2 Co 0,3 Ni 0,7 (SO 4 ) 2 •6H 2 O, respectively.In the study, the adsorption of impurities is insignificant because there was scientific rigor in its preparation.
The size of the crystalline samples is related to the growth time variable, ranging from micron to centimeters scales.Micrographs of the crystals are in Figure 3.
The crystal growth method by isothermal evaporation of the solution in water allows for growing crystals with natural faces and small concentrations of structural defects (Andreeta, 1999).The solution was prepared with pre-established quantities of the reagents solubilized in deionized water until it reached a volume of 80×10 -6 m 3 at a temperature of 343 K, filtered, and placed in the growth oven at a constant temperature and equal to 313 K (±1) for periods of up to two weeks.
It used Agilent 725 ICP-OES, with 99.996% purity Argon gas for plasma formation.The selected samples were observed in the Leica DMRX optical microscope and taken to the Raman without any preparation.Only then were they dissolved in 25×10 -6 m 3 of distilled water, with constant stirring to expedite dissolution.
Used was a Horiba / Jobin-Yvon LABRAM-HR spectrometer with the 632.8 nm line of a helium-neon laser (power of 6×10 -3 W applied on the sample surface) as an excitation source, a diffraction grating 600×10 3 slots/m (for all spectrum) and 1800×10 3 slots/m (for better resolution of the low wavenumber region), Peltiercooled CCD detector, Olympus confocal microscope (100x objective), experimental resolution of typically 10×10 -3 m -1 (10 accumulations in 30 s).
Drop-shaped crystals (≈200×10 -6 m) generated X-ray patterns, collected at a temperature of 293(2) K in the Oxford Diffraction Gemini A Ultra equipped with a CCD detector, using Mo-Kα radiation (λ = 0.71073×10 -10 m).Used WinGX -Version 2014.1 software (Farrugia, 1999) to solve and refine the structure, CrysAlisPro software for data collection and reduction, jointly with the SHELX package (Sheldrick, 2008), and Diamond Version 4.0.1 and Mercury 3.5.1 as a software of graphic design.
Equation 1 -Chemical equation of mixed salt preparation.The results of the ICP-OES analysis with direct quantitative information on the concentration of the elements (in mg/kg), considering the analytical lines that best fit the model, are presented in Table 1, together with the theoretical values stoichiometrically calculated according to Lide and Baysinger (2009).
The result of the Raman analysis is shown in Figure 4. We divide the spectrum between 50 and 4000 cm -1 in: low wavelength region (a) between 50 cm -1 and approximately 400 cm -1 ; region intermediate (b) between 400 cm -1 and 1300 cm -1 ; region of high wavenumbers (c) as from 1300 cm -1 .+ with SO 4 2-and a mixed character with contributions from the three ions designated TO, TA and TI, respectively.The wavenumbers describing this region are in Table 2.
In region (b), four bands of the SO 4 2-stand out and some low-intensity bands.The low-intensity bands can be associated with libration modes of the H 2 O (Marinova et al., 2009and, Georgiev et al., 2010).Finally, in region (c), there are several weak bands between 1300 cm -1 and 2700 cm -1 and a cluster between 2700 cm -1 and 4000 cm -1 that correspond mainly to NH 4 + and H 2 O (Dong et al., 2007).The wavenumbers describing these two regions are in Table 3.
Single-crystal X-ray study confirms the structure obtained by Li & Li (2004) and Tahirov and Lu (1994) for pure cobalt and nickel compounds, respectively.The geometric parameters of the metal coordination octahedron are in Table 4, and the hydrogen bonds and parameters of the mixed salt are in Table 5.The formation of nuclei is the initial stage of crystallization.This step is possible only when kinetic energy barriers for the agglomeration of molecules are overcome (Saito, 1996).These barriers are supersaturation conditions.Evaporation of the solvent at a constant temperature of 313 K increases supersaturation.Increasing the supersaturation increases the velocity of the nucleation.Thus, it forms a sufficient number of smaller nuclei for crystallization.In the isothermal process, nucleation happens when there is the continuous exchange of the solidificationlatent heat between the core and the solution, as described in Saito (1996) by the homogeneous nucleation theory.
After the nucleation stage, the crystal goes on to the growth phase.This phase occurs as a successive and periodic ordering of atoms or molecules to form a standard structure, producing the repetition of a unit cell.This process occurs according to the theory of kink formation described in Pimpinelli and Villain (1999), which uses diffusion as its principal mechanism.The diffusion fields are formed, which give rise to surfaces that grow due to the presence of layers associated with defects in the crystal and, in particular, screw dislocations, as described in Pimpinelli & Villain (1999).Figure 3 contains micrographs with the sample surfaces that prove the growth model discussed.
Liquid inclusions present in crystals can be viewed in Figure 3a.Volumes -where growth units forming the diffusion fields were not indexed, that is -not filled by the solution, cause these inclusions.In Figure 3b, we observe the two-dimensional growth mechanism known as Kink's formation (Pimpinelli & Villain, 1999), and in Figure 3c, the three-dimensional growth mechanism (formation of growth layers).Surface defects such as irregularities and holes are in Figure 3d.
Analyzing the quantities of the chemical elements (Table 1) obtained by the ICP-OES method, the element S has experimental values approximated to the theoretical values in pure and mixed samples.The same is true for Ni and Co elements in the pure salts.In mixed salt, however, we can observe in the experimental result that the quantity of Co is smaller, and that of Ni is higher than predicted in the theoretical model.Therefore, the concentrations of these metals in the crystal differ from the stoichiometric values used for the synthesis.
Thermodynamic models describe the growth process as a binary system because it is always possible to obtain a single solid phase in the entire concentration interval.So, we acquire mixed monocrystals with the form Co (1-x) Ni x for 0 ≤ x ≤ 1, producing compounds with the same crystal structure (Pimpinelli & Villain, 1999).
At 343 K, we have the solution in the liquid phase.As the temperature decreases, we have a biphasic region.In this region, the first crystals in the solution begin to nucleate, coexisting with solid and liquid.Because it is a practical procedure, concentrations are shifted, creating a metastability region until crystals form and solids remain with divergences in their concentrations.Under these conditions, the solid must homogenize through the kink's forming diffusion mechanisms to originate a solid solution.Thus, a balance deviation occurs (displacement of the solidus line), causing the system to assume metastable conditions.
In the negative deviation of the solidus line, there is no tendency for phase separation.At low temperatures, when the thermal energy of atoms becomes smaller, there is a tendency for randomness in the distribution of octahedral molecules.Believed this trend is caused to distortions in the octahedral complexes induced by the Jahn-Teller effect (Cotton et al., 1993).
The experimental values in Table 1 reveal an amount of Co = 30% and Ni = 70% where it should be Co = 50% and Ni = 50%.Crystal growth by the isothermal evaporation of the solution in water gives rise to crystals consisting of a regular solution (unbalanced cooling) that favors [Ni(H 2 O) 6 ] 2+ ions.Therefore, the growth method must consider the metastability conditions to obtain the entire concentration range.
The mixed salt spectrum is the stoichiometric sum containing the pure salt spectra.Any different value within the concentration range converges to  the same result, as we will always have intermediate values.

Discussion
In region (b), according to Georgiev et al. (2010), the band corresponding to ν 1 (A1) is single, ν 2 (E) is double, ν 3 (F2) and ν 4 (F2) are triple.This description is consistent with Table 3, but no changes in the pure salt bands occurred compared to the mixed salt spectrum.Therefore, vibrational modes remain degenerate even with changes in composition.
In region (c), NH 4 + exhibits vibrational modes equivalent to SO 4 2- (Georgiev et al., 2010, Frost et al., 2011).ν 1 (A1) and ν 3 (F2) of NH 4 + are in a cluster with ν 1 (A1) and ν 3 (B2) of H 2 O.In the spectrum, it is not possible to associate the ν 2 (A1) bands of the H 2 O, expected at 1595 cm -1 (Best et al., 2006).The intensity versus shift Raman for the ν 2 (E) and ν 4 (F2) of the NH 4 + vibrational modes is low, causing the noise.Even so, the spectrum presents, for the vibrational mode ν 4 (F2), a band at 1424 cm -1 , above the wave number reported by Georgiev et al. (2010), 1400 cm -1 .For the ν 2 (E) vibrational mode, the spectrum has two bands, whose average is 1685 cm -1 , subtly above the wave number reported by Marinova et al. (2009), 1680 cm -1 .The displacement noted for the molecule in a solid may be misunderstood due to the low intensity.Therefore, ν 4 (F2) and ν 2 (E) continue to feature one and two bands, respectively.Even with changes in composition, they remain degenerate.
The details of the vibrational modes related to the cluster are presented in Table 3, as performed by Dong et al. (2007).The values agree well with experimental NH 4 + data but are above the wavenumbers reported for a molecule in solution (Dong et al., 2007).This difference occurs through the interaction with their neighbors in this solid.Therefore, these vibrational modes are also not significantly influenced by changes in composition.
The positions of the SO 4 2-ions in the unit cell are identical, and their geometry shows slight distortions from tetrahedral geometry.The same is true for NH 4 + , whose hydrogen bonds are moderate.The anharmonic force fields and the low polarizability of NH 4 + ions cause the width increase and the low intensity of the bands.The same does not happen with the corresponding SO 4 2-ions, although both belong to the same point group and occupy sites with identical symmetry.Thus, it's possible to infer that SO 4 2-is spinning freely in the structure, which is not the case with NH 4 + (Rajagopal & Aruldhas, 1989).The wavenumbers of the SO 4 2-and NH 4 + vibrational modes in Tutton's salt structure accompany the predictions of group theory (Nakamoto, 1986), having symmetry described by the point group Td.That is, SO 4 2-and NH 4 + are present in the salt structure with the geometrical form of a tetrahedron whose vibrational modes are not sensitive to changes in composition in the solid solution.
The values corresponding to H 2 O do not agree with those reports by Dong et al. (2007) but are in the same region.The coordination of the molecule in the crystal causes the deviation and shifts the bands to the left.The difference in the displacement of vibrational modes is related to the M -O coordination distances, which polarize the water molecules to strengthen the bonds.As the ionic radius of M 2+ decreases, their interaction with water increases.Thus, the spectrum of the solid solution exhibits larger displacements than the others.
The wavenumbers of the H 2 O vibrational modes in the Tutton salt structure accompany the predictions of group theory (Nakamoto, 1986) with symmetry described by the point group C 2v .However, the compound is coordinated, and therefore it does not only address the properties of the point group for free molecules.
In region (a), the active vibrational modes in Raman of the octahedral complex are ν 1 (A1), ν 2 (E), and ν 5 (F2) (Nakamoto, 1986).The vibrational modes of sample 1 present modifications caused by the Jahn-Teller effect, which is more pronounced in this sample.[M(H 2 O) 6 ] 2+ ions are influenced by composition changes due to octahedron shape distortions (Barashkov et al., 2000).According to Table 2, the pseudo-octahedral vibrational modes accompany the predictions of group theory (Nakamoto, 1986), with symmetry described by the point group Oh.That is, [M(H 2 O) 6 ] 2+ is present in the salt structure with the geometric shape of a slightly distorted octahedron.Wyckoff's position is 2a and occupies a high symmetry site, so his local symmetry is C i .
Libration modes arise from rotations about their three main axes constrained by interactions with their neighboring atoms (Nakamoto, 1986, Best et al., 2006) , and H 2 O in Tutton's salt structure accompany the predictions of group theory, its position being Wyckoff 4e and occupying a low symmetry site.Therefore, its local symmetry is C 1 .
Translational modes shift the molecule center of mass into one of three coordinates of 3-D space.These arise from the unit cell containing the octahedron, SO 4 2-, and NH 4 + vibrating as a coupling of molecules (Vandenabeele, 2013).The Jahn-Teller effect causes modifications in sample 1, affecting the interaction of the octahedral complex with SO 4 2-, which affects its interaction with NH 4 + (change in TA), and reflects in TI, inducing its modification.The wavenumbers of the translation modes in Tutton's salt structure accompany the predictions of the group theory (Nakamoto, 1986), confirming that the studied molecules interact with each other and are the most affected by solid solution formation.
Ni and Co atoms present a static occupational disorder in which two different types of metals can alternately occupy the same site (mixed crystals).The refined probabilities in Single-crystal X-ray are 69.3(3)% and 30.3(1)% for Ni and Co, respectively.
Table 4 has the geometric parameters for the octahedral complex.We realize that these have the effect of Jahn-Teller (Cotton et al., 1993), caused by the unfolding of the orbitals d in a degenerated state that suffers this kind of distortion to reduce its symmetry and remove degeneration.As expected, the mixed salt results are the weighted average of pure crystals.
Notice the slight differences in the bonds and the angles O -H of the H 2 O.These distortions can be caused by interaction with the neighborhood because the molecule coordinates the bivalent metal ion and also participates in hydrogen bonds in the crystal lattice, whether donor or recipient in these interactions.
In the structure, the units interact through two types of hydrogen bonding: six O -H ••• O, being the donor water molecule; and four N -H ••• O, being the donor ammonium molecule.Table 5 details these chemical bonds.In either case, the sulfate ion oxygen atoms act as receptors for hydrogen bonds.

Figure 2 -
Figure 2 -Photographs of the crystals.
The twelve hydrogen atoms of the group [M(H 2 O) 6 ] 2+ and the four of the NH 4 + group partici-pate in hydrogen bonds in the structure.The group theory analysis for the active vibrational modes in Raman predicts 114 (57 A g and 57 B g ) vibrational modes: 45 internals, 6 librations, and 6 trans- 4

Table 2 -
Raman spectrum vibrational modes for the region (a).

Table 3 -
Raman spectrum vibrational modes for regions (b) and (c).

Table 4 -
Parameters of bonds of the octahedron.

Table 5 -
Parameters of hydrogen bonding in the crystals.
* Weighted Average with 70% Ni and 30% Co . The bands corresponding to LS and LS+LA have modifications, and the bands LW and LA are unchanged.