Structural, Thermal, and Vibrational Effects of Low Level Rb+ Incorporated in (K1-xRbx)2Ni(SO4)2·6H2O Mixed CrystalsAbstract
Mixed crystals of (K1-xRbx)2Ni(SO4)2·6H2O with x = 0, 0.01, 0.02, and 0.05 were synthesized by the isothermal evaporation method. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), Raman spectroscopy, Fourier transform mid-infrared spectroscopy (FT-MIR), Fourier transform near infrared spectroscopy (FT-NIR), and thermogravimetric analysis (TGA). Powder XRD combined with Rietveld refinement confirmed that all compositions crystallize in the monoclinic symmetry (P21/c) space group, with no evidence of a structural phase transition upon Rb⁺ incorporation. Shifts of Bragg reflections toward lower angles and small variations in lattice parameters indicate a gradual lattice expansion consistent with the lattice expansion typically induced by the substitution of K+ with larger Rb+ cation. SEM-EDS revealed that increasing Rb+ content leads to enhanced surface roughness, morphological heterogeneity, and irregular fracture features, consistent with the accumulation of local strain and microstructural disorder. SEM EDS analysis confirmed the progressive substitution of K+ by Rb+ across the series, with minor compositional deviations attributed to surface sensitivity effects. Vibrational spectroscopy (Raman and mid infrared) showed splitting of sulfate ion modes and hydration related modes, reflecting local distortions of the hydrogen bonded framework. FT-NIR results reveal absorption bands attributed to the d-d transitions of the octahedral nickel complex Ni(H2O)62+ and bands attributed to combination involving ν(O-H) stretching and δ(H-O-H) bending modes of water molecules. Thermal analysis demonstrated changes in dehydration behavior and thermal stability correlated with Rb+ concentration.
Keywords:
Tutton salts; Raman spectroscopy; X-ray diffraction; thermal analysis; nickelpicromerite crystals
Introduction
Hexahydrated double sulfate crystals with the general formula A2B(SO4)2·6H2O (A = monovalent cation; B = divalent metal) occur naturally and are commonly referred to as Tutton salts. Among them K2Ni(SO4)2·6H2O is known as nickelpicromerite and crystallizes in the monoclinic system, exhibiting a hydrogen bonding network that plays a central role in its structural stability and physical behavior.1-3
In addition to their structural simplicity, Tutton salts exhibit well-defined and tunable optical properties, making them relevant for emerging technological applications. Their optical behavior, particularly selective transmission in the ultraviolet (UV) and infrared regions (IR), has motivated their integration into optical filtering systems and radiation detection devices.4-7 It is important to note that these materials bridge the gap between fundamental and applied research, as their response to compositional modifications can be directly correlated with changes in optical and vibrational behavior. More fundamentally, these materials serve as model platforms for probing how subtle stochiometric perturbations influence lattice dynamics, electronic structure, and intermolecular interactions in hydrated crystalline systems.
A key feature of Tutton salts is the presence of monovalent cations occupying interstitial sites, which play a critical role in stabilizing the lattice through electrostatic interactions and indirect modulation of the hydrogen-bonding network. Since these cations are only weakly bound within the structure, isovalent substitution provides a controlled pathway to engineer lattice perturbations without disrupting the overall symmetry. This makes Tutton salts particularly suitable for systematic structure-property studies, where small compositional changes can be used to probe local distortions without inducing phase transitions. In particular, substitution with larger alkali ions is expected to induce local strain, modify hydrogen-bond geometries, and consequently alter vibrational and thermal responses, while preserving the long-range crystallographic framework.
The crystal structure of Tutton salts consists of SO42- tetrahedra and M(H2O)62+ octahedra interconnected through an extended network of hydrogen bonds mediated by coordinated water molecules, where M are monovalent cations such as K+, Rb+, Cs+, or NH4+ occupy interstitial sites and contribute to the overall lattice cohesion. Small variations in cation size or local coordination can induce changes without altering the crystal symmetry.
Previous investigations6,8,10 have demonstrated that chemical substitution in Tutton salts can significantly affect vibrational signatures associated with sulfate groups, coordinated water molecules, and metal-centered octahedra. These effects have been interpreted in terms of changes in hydrogen bonding strength, lattice distortions, and symmetry reduction. Complementary experimental and theoretical studies, including density functional theory calculations, have further highlighted the sensitivity of these systems to compositional variations.11,12 Despite most reported studies focus on either high substitution levels or different cationic systems, leaving the regime of low-level isovalent substitution largely unexplored. This represents a significant knowledge gap, as the early stages of lattice distortion often dictate the onset of functional property tuning. To the best of the knowledge of the authors, very low doping concentrations have not been discussed. This is a critical limitation, since the initial stages of ionic substitution are often responsible for triggering local distortions that ultimately govern the properties of the materials.
Recently, our team synthesized mixed crystals of K2M(SO4)2·(H2O)6, with M being a mixed composition of Ni/Co and Tutton (NH4)2(SO4)2·Y(H2O)6 crystals (Y = Ni, Mg) doped with H3BO3 and AgNO3 and studied their spectroscopic properties.13,14 Studies on mixed ionic systems have demonstrated that partial replacement of alkali metal ions with larger counterparts can introduce local lattice distortions driven by steric effects and compositional disorder, leading to measurable changes. While such effects have been reported in sulfate-based materials, their presence in hydrated nickel based Tutton salts remains more study. In particular the influence of low concentration of Rb+ on the coupled response of K2Ni(SO4)2·6H2O has not yet been systematically investigated. For solid solutions of Tutton salts of the series (K1-xRbx)2Ni(SO4)2·6H2O, the concentration threshold at which the volumetric expansion of the unit cell becomes regular and begins to linearly follow the law of Vegard15 is not yet known.
The present work aims to fill this gap by providing a comprehensive experimental study of the series (K1-xRbx)2Ni(SO4)2·6H2O (with x = 0-0.05), focusing on the effects of low-level isovalent substitution on the crystal structure, lattice dynamics, and thermal stability. To achieve this, structural characterization was performed using powder X-ray diffraction (XRD) combined with Rietveld refinement, enabling precise evaluation of lattice parameters and structure stability. Vibrational properties were investigated using a synergistic approach involving Raman spectroscopy (RS), Fourier transform mid-infrared spectroscopy (FT-MIR), and Fourier transform near-infrared spectroscopy (FT-NIR) techniques. RS, in particular, serves as a highly sensitive probe of local structural distortions, providing insight into symmetry changes and hydrogen-bond interactions through the analysis of SO42- internal modes and lattice vibrations.10,11,16,17 Infrared techniques complement this analysis by probing both fundamental and overtone regions, including electronic transitions associated with the [Ni(H2O)6]2+ complex, thereby offering a more complete description of the vibrational and electronic behavior of the system.
Thermal behavior was assessed via thermogravimetric analysis (TGA), allowing detailed monitoring of dehydration processes and stability as a function of composition. By integrating these complementary techniques, this study establishes direct correlations between ionic substitution, local structural perturbations, and macroscopic physicochemical properties. The findings provide new insights into the role of low-level compositional disorder in hydrated sulfate crystals and demonstrate how subtle ionic substitution can be used as a strategy to tailor material properties in Tutton-type systems.
Experimental
Materials and reagents
Mixed crystals of (K1-xRbx)2Ni(SO4)2·6H2O (x = 0, 0.01, 0.02, and 0.05) were synthesized using the isothermal evaporation method.7 All reagents were of analytical grade and used without any further purification. Potassium chloride (KCl, purity ≥ 99.5%, Dinâmica Química), nickel sulfate hexahydrate (NiSO4·6H2O, purity ≥ 99.0%, Dinâmica Química), and rubidium nitrate (RbNO3, purity ≥ 99.95%, Sigma-Aldrich). Deionized water was used for all solution preparations.
Methods
Stoichiometric amounts corresponding to the nominal compositions were prepared based on a total precursor content of 10 g per batch. Specifically for x = 0 sample, 6.3825 g of NiSO4·6H2O, 3.6175 g of KCl, for x = 0.01 sample 6.3465 g of NiSO4·6H2O, 3.5262 of KCl and 0.1275 g of RbNO3, for x = 0.02 sample 6.3110 g of NiSO4·6H2O, 3.4364 of KCl and 0.2526 g of RbNO3, for x = 0.05 sample 6.2068 g of NiSO4·6H2O, 3.1748 of KCl and 0.6184 g of RbNO3, were accurately weighed using an analytical balance (Shimadzu AY 220, ± 0.0001 g).
The reagents were dissolved in 200 mL of deionized water, resulting in a precursor solution with an approximate total concentration of 0.12 mol L-1. The solution was kept under magnetic stirring for 120 min at 25 °C until complete dissolution was achieved.
The crystal growth followed a precise protocol where the solution was divided into 10 mL aliquots and transferred to individual 50 mL beakers, covered with perforated parafilm to ensure slow solvent loss and controlled supersaturation at 60 °C for 10 days per sample in a muffle furnace. A schematic representation of the experimental setup is shown in Figure 1.
The prolonged evaporation time at 60 °C is explained by the fact that the containers were partially covered to ensure slow solvent loss and controlled supersaturation. This approach was intentionally adopted to favor the growth of well-defined crystals and to minimize rapid nucleation, thereby improving crystal quality. Figure 2 shows optical images of the samples, where typical crystal dimensions can be inferred from the 1 mm grid background.
Optical image of (K1-xRbx)2Ni(SO4)2·6H2O crystals grown by isothermal evaporation, showing the size, morphology, and transparency. (a) x = 0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.05.
The pure sample (x = 0) shown in Figure 2a exhibits monoclinic prismatic outline, characterized by the prominent development of the pinacoidal shape, with well-defined faces. Samples with x = 0.01 to 0.05 shown in Figures 2b-2d also display monoclinic prismatic crystals, characterized by a pyramidal shape with stratified faces.
Instrumentation
Powder XRD measurements were performed using a Bruker AXS D8 Advanced da Vinci diffractometer equipped with Cu-Kα radiation source (λ = 1.5418 Å), a Ni filter, operating at 40 kV and 40 mA. Diffraction patterns were collected in the 2θ range of 10-60° with step size of 0.02°, using a Lynxeye linear position sensitive detector. Structural analysis was carried out by using the Le Bail method followed by Rietveld refinement implemented in the MAUD software package (version 2.99993).18
TGA was conducted using a Netzsch STA 449 F3 Jupiter® thermal analyzer under a nitrogen atmosphere, with a heating rate of 10 °C min-1, from 30-1200 °C. Prior to analysis, the crystals were gently ground in a cleaned agate mortar and sieved to obtain particle sizes between 75 and 117 μm. Approximately 20 mg of each sample was used, ensuring consistent response and reliable determination of dehydration steps.
FT-MIR spectra were acquired using a Bruker VERTEX 70 spectrophotometer operating in attenuated total reflectance (ATR) mode at a spectral resolution of 0.4 cm 1. Spectra were collected in the 400-4000 cm-1 range, with 64 scans per sample, focusing on sulfate and hydration related vibrational modes.
FT-NIR spectra were recorded to investigate overtone and combination vibrational modes sensitive to crystal lattice changes and hydrogen bonding. The measurements were carried out using a Bruker FT-NIR spectrometer, operating at a spectral resolution of 0.4 cm-1, equipped with OPUS 6.5v software. The data were acquired in absorbance mode over the extended spectral range of 3500-15000 cm-1.
Raman spectra were obtained using a Micro Raman T64000 (Horiba Jobin-Yvon) equipped with a charge-coupled device (CCD) detector and an Olympus BX41 confocal microscope. A 488 nm excitation laser was employed under low power conditions to avoid local heating, using a 50 × objective, an acquisition time of 60 s, and spectral range of 60-1600 cm-1.
Scanning electron microscopy combined with SEM EDS measurements was employed to investigate the surface morphology and elemental composition of the crystals. The samples were mechanically fractured during sample preparation, mounted on carbon double side tape supports, and coated with a thin gold layer to improve electrical conductivity and image quality. Analyses were performed using a Hitachi TM3030 benchtop SEM equipped with a SwiftED3000 EDS module. SEM-EDS analyses were performed in triplicate for all samples.
Results and Discussion
SEM-EDS results
Figure 3 presents the SEM images and the corresponding EDS spectrum of the x = 0 sample. The SEM-EDS analysis confirms that the elemental composition of the samples is consistent with the expected stoichiometry of the Tutton salt structure, in agreement with previous reports on nickel based Tutton salts and mixed alkali sulfate systems.1,16
SEM-EDS characterization of pure nickelpicromerite sample (x = 0). (a) Representative EDS spectrum confirming the presence of K, Ni, S, and O as the primary constituent elements, (b) low-magnification SEM image showing the morphology of fractured crystals (× 100), and (c) high-magnification SEM image revealing irregular grain surfaces after sample preparation (× 1k).
The SEM micrographs reveal fractured crystals with irregular surfaces, a morphology commonly observed in hydrated sulfate salts after sample preparation for electron microscopy.16,19 The associated EDS analysis confirms the presence of K, Ni, S, and O as the main constituent elements. Trace amounts of chlorine are detected and attributed to residual surface species from the growth solution rather than structural incorporation into the lattice, as previously reported for sulfate crystals grown from chloride-containing solutions.16,19 The quantitative elemental composition of the x = 0 sample is summarized in Table 1 and serves as a reference for comparison with the Rb containing samples.
Rubidium-containing mixed crystals exhibit notable changes in surface morphology when compared to the x = 0 sample. Figure 4 shows the SEM images and EDS spectrum of the x = 0.02 sample. The SEM micrographs indicate fractured crystals with subtle modifications in surface texture and grain organization, suggesting that even low levels of Rb+ substitution influence the crystal growth process and microstructural arrangement. Such behavior is consistent with earlier studies on alkali-ion substitution in ionic crystals, where the incorporation of larger cations introduces local lattice distortions and alters growth dynamics due to ionic size mismatch.16,19,20 The EDS results shown in Figure 4 confirm the presence of rubidium together with K, Ni, S, and O, indicating effective substitution of K+ by Rb+ within the material. The corresponding elemental concentrations are listed in Table 1.
SEM-EDS characterization of the mixed crystal sample with (x = 0.02). (a) Representative EDS spectrum confirming the effective substitution of K+ by Rb+, indicating the presence of Rb, K, Ni, S, and O as constituent elements, (b) low-magnification SEM image showing the morphology of fractured crystals (× 100), and (c) high-magnification SEM image revealing subtle modifications in surface texture and grain organization associated Rb+ incorporation (× 1k).
At higher substitution levels, more pronounced morphological changes are observed. Figure 5 presents the SEM images and EDS spectrum of the x = 0.05 sample. The SEM micrographs reveal increased surface roughness, a higher degree of heterogeneity, and more irregular fractured features compared to the matrix and the x = 0.02 sample. These microstructural characteristics suggest that higher Rb+ content enhances structural perturbations, likely associated with local strain accumulation and disruption of the hydrogen-bonded network characteristic of Tutton salts.1,2,10 Similar morphological evolution has been reported in other alkali-metal-substituted crystalline systems, where increasing dopant concentration leads to enhanced disorder and surface heterogeneity.19-21
SEM-EDS characterization of the mixed crystal sample with (x = 0.05). (a) Representative EDS spectrum confirming the presence of Rb, K, Ni, S, and O as constituent elements, consistent with the progressive substitution of K+ by Rb+, (b) low-magnification SEM image showing the morphology of fractured crystals, and (c) high-magnification SEM image revealing increased surface roughness and a higher Rb+ degree of morphological heterogeneity associated with higher incorporation.
The elemental composition values presented in Table 1 were obtained by averaging the values from the three points studied in each sample. The data reveal an increase in rubidium content accompanied by a corresponding decrease in potassium concentration, confirming the progressive substitution of K+ by Rb+ across the series. Minor variations in sulfur, oxygen, and nickel contents are attributed to local compositional fluctuations and surface sensitivity inherent to EDS measurements, as discussed in previous studies16,17 on hydrated sulfate crystals. Chlorine is detected at low concentrations (ca. 1 wt.%), supporting its attribution to residual surface contamination rather than lattice incorporation.
Overall, the SEM-EDS results demonstrate that rubidium incorporation significantly affects both the microstructural and compositional characteristics of the K2Ni(SO4)2·6H2O system. The morphological evolution observed in Figures 3-5 and the compositional trends summarized in Table 1 corroborate the structural and spectroscopic results discussed in other sections of this work, highlighting the strong sensitivity of Tutton salts to alkali-ion substitution and their potential for tunable physicochemical properties through controlled compositional modification.1,16
XRD results
Figure 6 shows the powder X-ray diffraction patterns of the (K1-xRbx)2Ni(SO4)2·6H2O samples with x = 0, 0.01, 0.02, and 0.05, together with the corresponding fitted profiles (red lines). All diffractograms were refined using the ICSD 207397 structural model. The pure compound (x = 0) was used without modification, while for the samples with x = 0.01, 0.02, and 0.05, the corresponding substitutions of K+ by Rb+ were introduced at the same position, confirming that all compositions crystallize in the monoclinic system with space group P21/c characteristic of Tutton salts.
Powder XRD patterns of the (K1-xRbx)2Ni(SO4)2·6H2O mixed crystal for (a) x = 0, (b) x = 0.01, (c) x = 0.02, and (d) x = 0.05. The experimental data are represented by open circles, the Rietveld refinement fits by red lines, and the difference plots Yobs - Ycal by blue lines. Vertical bars indicate the Bragg reflection positions for the monoclinic P21/c phase.
The diffraction patterns show no additional peaks or peak splitting upon Rb⁺ substitution, indicating the absence of secondary phases or crystallographic phase transitions within the investigated compositional range. This confirms that partial substitution of K+ by Rb+ preserves the overall crystal symmetry and structural framework of nickelpicromerite.
Variations in the relative intensities of some diffraction peaks are observed for the Rb containing samples. These changes are attributed primarily to microstructural effects, such as differences in crystallite orientation or packing conditions during powder preparation, rather than to symmetry lowering or long-range structural rearrangements.
A shift of selected Bragg peaks toward lower 2θ values was observed with increasing Rb+ content, particularly for the x = 0.05 sample (Table 2). However, non-monotonic behavior was observed for reflection (111). This behavior can be attributed to the fact that at low concentrations, below the threshold for which volumetric expansion becomes regular, the introduced Rb+ ions act as isolated point defects. The host potassium lattice locally compresses the hydrogen bond cloud around the Ni(H2O62+) octahedron to accommodate rubidium without necessarily expanding the crystal. This indicates a slight expansion of the unit cell, consistent with the substitution of K+ (R= 1.51 Å) by the larger Rb+ (R = 1.75 Å).
The observed trend is consistent with the larger unit cell volume reported for Rb2Ni(SO4)2·6H2O (V = 677.73 Å3)7 when compared to K2Ni(SO4)2·6H2O (V = 649.9 Å3), as well as with previous studies on mixed alkali ion.1 Systems such as LiK1-xRbxSO422 and KxRb1-xPb2Br5.24 These results support a continuous and isostructural incorporation of Rb⁺ into the nickelpicromerite lattice.
The nickelpicromerite crystal structure is composed of [Ni(H2O)6]2+ octahedral units linked via hydrogen bonds to (SO4)2- tetrahedra, forming an alternating octahedral and tetrahedral framework, as can be seen in Figure 7.
Crystal structure of nickelpicromerite obtained using the ICSD 207397 model and the Vesta 3.5.8 software. The model illustrates the monoclinic unit cell with space group P21/c, highlighting the arrangement of the molecular building blocks and the hydrogen-bonding network.
The refined parameters and unit cell volumes obtained from Rietveld refinement are summarized in Table 3. Minor variations in lattice parameters were observed for x = 0.01 and x = 0.02 samples, while a more pronounced increase in unit cell volume is detected for x = 0.05. At lower concentrations (x = 0.01), decreases in volume were observed, suggesting a slight rearrangement of atoms to occupy the empty spaces of the structure and a stabilization of the structure stabilized by an extensive network of strong hydrogen bonds and electrostatic interactions.
This crystallographic behavior confirms that Rb incorporation does not significantly alter the structural properties of the material. Volume variations of approximately 0.5% were observed when the Rb concentration was increased by 0.05. To the best of our knowledge, this is the first time a structural characterization has been performed under these conditions.
Thermal analysis
Figure 8 shows the TGA (solid lines) and derivative thermogravimetric curve (DTG) curves (dashed lines) for all the samples. The first mass loss event occurred between approximately 150 and 250 °C and was attributed to dehydration resulting from the release of structural water molecules. This stage was marked by a sharp DTG peak, indicating a rapid mass loss process. The second event, observed from ca. 800 to 1100 °C, corresponds to the decomposition of residual sulfate species and the breakdown of the crystalline framework.24
TGA (solid lines) and DTG (dashed lines) curves of the crystals (K1-xRbx)2Ni(SO4)2·6H2O with x = 0, 0.01, 0.02, and 0.05.
As detailed in Table 4, the total mass loss increased with rising Rb+ concentration, ranging from 42.03% in undoped samples to 46.58% in those with x = 0.05. Additionally, a systematic shift in the DTG peak positions and a reduction in their intensities were noted at higher doping levels, suggesting a change in the thermal decomposition kinetics and, possibly, an increased in structural strain. These effects are likely associated with modifications in the bonding strength and lattice enthalpy, resulting from the partial substitution of K+ with the larger Rb+ ion.22
Thermogravimetric mass loss for (K1-xRbx)2Ni(SO4)2·6H2O crystals with x = 0, 0.01, 0.02, and 0.05 in different temperature ranges
This behavior aligns with previous reports on doped ionic crystals, where alkali-metal incorporation affects both thermal and electrochemical properties through alterations in lattice energy and ion coordination. For example, Patel et al.26 demonstrated that doping-induced structural changes can improve the thermal resistance of Ni:ZnS systems owing to enhanced lattice bonding and stability.
Raman spectroscopy
Raman spectra were analyzed in the 400-1200 cm-1 range, which encompasses the internal vibrational modes of sulfate groups. The experimental spectra and their corresponding peak deconvolutions are presented in Figure 9. The fitting procedure accurately reproduced the experimental profiles, indicating that the observed spectral features were intrinsic to the crystalline lattice. No additional bands associated with secondary phases were detected, confirming that the Tutton-type framework was preserved throughout the investigated compositional range, in agreement with previous Raman studies of substituted and mixed Tutton salts.13,16,27
Raman spectra of the (K1-xRbx)2Ni(SO4)2·6H2O mixed crystals for (a) x = 0, (b) x = 0.01, (c) x = 0.02, and (d) x = 0.05 in the 400-1200 cm-1 range with the corresponding peak deconvolutions. The experimental data (open circles) are shown alongside the individual peak deconvolution (blue lines) and the cumulative fits (red lines).
The free sulfate ion (SO₄2-) has tetrahedral symmetry (Td) and possesses four Raman-active modes with irreducible representations A1 + E + 2F2.28 These modes are described as the non-degenerate symmetric stretching mode ν1(SO42-) represented by A1, the doubly degenerate symmetric bending mode ν2(SO42-) represented by E, the triply degenerate asymmetric stretching mode ν3(SO42-) represented by F2, and the triply degenerate asymmetric bending mode ν4(SO42-) represented by the second F2. These frequencies appear in the Raman spectrum at approximately 1000, 460, 1100, and 615 cm-1, respectively.11,16,29
The peak positions obtained from the deconvolution of the Raman spectra, along with their respective assignments, are summarized in Table 5, enabling a detailed analysis of the evolution of the sulfate vibrational modes as a function of composition.
The ν₂(SO42-) mode can be deconvolved into two components for all compositions, and the ν₄(SO42-) asymmetric bending mode exhibits splitting into three components, requiring a three-peak model to reproduce the experimental profiles (Figure 9). This behavior reflects the lifting of degeneracy associated with the low-symmetry local environment of the sulfate tetrahedra. Similar features for the ν4 mode have been reported in structurally related sulfate systems and are attributed to symmetry-lowering effects.11,13,16
The most pronounced splitting was observed for the ν3(SO42-) asymmetric stretching mode. As shown in Figure 9 and quantified in Table 5, three distinct components are systematically required to fit the 1099.60 1172.41 cm-1 region, indicating a complete lifting of degeneracy in the crystal field. The use of peak deconvolution to resolve these closely spaced components follows the same methodological approach adopted in recent Raman studies of vanthoffite-type sulfates, in which sulfate units occupy low-symmetry sites and all components of the F₂ representation become Raman active.29 The splitting of the ν3(SO42-) and ν4(SO42-) vibrational modes in the three components supports the concept of symmetry reduction to C2v.
Nevertheless, the splitting of the ν3(SO42-) and ν4(SO42-) modes of sulfate provides a measure of the distortion of the tetrahedron formed by sulfate. Therefore, the magnitude of the energy distortion of the SO4 ions can be deduced from the values of ∆νi and ∆νmax: where ∆νi = νimax - νimin.30 The values of ∆ν3 and ∆ν4 are presented in Table 5.
It can be seen that ∆ν3 initially decreases for samples with x = 0.01 and 0.02 but increases for the sample with x = 0.05, a behavior similar to that observed for the unit cell volume. On the other hand, ∆ν4 is quite sensitive to the introduction of Rb, showing a large difference between x = 0 and the samples doped with Rb, in particular the sample with x = 0.05 that presents the highest value of ∆ν4. This result is consistent with the XRD results, which indicated an expansion of the unit cell for the sample with x = 0.05. In nickel Tutton salts, the crystal structure contains three main building blocks: octahedral Ni(H2O)62+ complexes, K+ ions, and SO42- tetrahedra. These components are linked by an intense network of hydrogen bonds. When the volume of the unit cell increases (for example, by partial replacement of K+ with larger ions), it reduces the confinement imposed on the SO42- tetrahedra, allowing them to relax and altering their local distortion, characterized by a deviation from the ideal tetrahedral symmetry Td to smaller symmetries.
The ν1(SO42-) symmetric stretching mode remained narrow and essentially unsplit across the entire compositional range (Figure 9), consistent with its non-degenerate character. The small shifts and limited linewidth variations observed for this mode (Table 5) are attributed to subtle changes in the average S-O bond strength rather than symmetry-related effects, as commonly reported for sulfate-based hydrated crystals.23,32
An additional broad spectral contribution was observed in the intermediate region between 700 and 900 cm-1, as shown in Figure 9. This region does not correspond to the fundamental internal vibrations of the SO42- tetrahedron and was attributed to the isolated modes of H2O (twisting-tw and rocking-ρ modes) identified at wavelengths between 689.5-899.25 cm-1.28 In accordance with interpretations reported for vanthoffite-type and related sulfate systems, this broad band is attributed to overtone and combination modes, as well as to vibrational coupling between sulfate internal modes and the hydrogen-bond network involving coordinated water molecules.30 Its diffuse character and gradual evolution with composition indicate sensitivity to local structural disorder, without evidence of new vibrational species or changes in global symmetry.
Overall, the Raman results demonstrate that the isovalent substitution K+ → Rb+ preserves the global Tutton-type structure and deep measurable local distortions in the sulfate environment. These distortions are evidenced by the lifting of the degeneracy of the ν2, ν4, and ν3 modes and by composition-dependent variations in peak positions and relative intensities (Figure 9 and Table 5). Such local vibrational rearrangements are expected to directly affect the infrared-active modes, particularly those associated with sulfate bending vibrations and hydrogen-bond interactions, providing a consistent basis for the complementary FT-MIR analysis discussed in the following section.
FT-MIR results
The FT-MIR spectra were acquired over the range (400 3900 cm-1) as shown in Figure 10. In the spectral window (400-1200 cm-1) shown in Figure 10a the first absorption around 411 cm-1 band is attributed to H2O vibrations with minor contributions from coupled motion of the ν1(SO42-) symmetric stretching mode. The prominent and clearly resolved absorption features concentrated in the 570-640 cm-1 range is dominated by the ν4(SO42-) asymmetric bending modes of the sulfate tetrahedra, displaying splitting into three components. These assignments are consistent with the infrared studies of alkali and transition metal sulfates and Tutton salts reported in the literature and Raman results above. In this region, the spectrum exhibits three distinct bands attributed to the splitting of the triply degenerate ν4(SO42-) mode.11,16,19,33
FT-MIR spectra of the (K1-xRbx)2Ni(SO4)2·6H2O series. (a) FT-MIR in the 400-1200 cm-1 range, (b) FT-MIR in the 1400-3900 cm-1 range.
The two modes between 740-796 cm-1 are characteristic of libratory modes of the H2O water molecule: twisting-tw(H2O) and rocking-ρ(H2O) modes.34 The isolated mode around 982 cm-1 is attributed to the symmetric stretching mode ν1(SO42-), which remained narrow and without splitting throughout the composition range (Figure 10a), consistent with its non-degenerate character.23,32
The three intense absorption bands between 1050 and 1150 cm-1 are assigned to the triply degenerate asymmetric stretching mode ν3(SO42-), whose degeneracy was raised by tetragonal distortion according to the Raman results.
In the spectral range of 1400 to 1800 cm-1 shown in Figure 10b two vibrational modes were observed, associated with two H2O wagging vibrations (wag(H2O)), with minor contributions from coupled motion of the (SO4)2- tetrahedra. The bands between 2800 and 3400 cm-1 are described as characteristic contributions of the anti symmetric stretching of H2O molecules ν1(H2O) and the absorption bands observed in the range between 3600 3800 cm-1 are associated with symmetric stretching of H2O molecules ν3(H2O).28,32
Taken together, the combined Raman and FT-MIR results provided a consistent vibrational picture of the (K1-xRbx)2Ni(SO4)2·6H2O system. The isovalent substitution of K+ → Rb+ preserved the global crystal structure while inducing local distortions in both the sulfate tetrahedron and the hydrogen-bonded water network.
FT-NIR results
The FT-NIR spectra were recorded over the 3500 15000 cm-1 range and are presented in Figure 11. The spectra are characterized by broad and overlapping absorption bands, which is typical of near-infrared spectra of hydrated sulfate systems. These bands originate primarily from the first overtones and combination modes of O-H stretching and bending vibrations associated with the six water molecules coordinated to the Ni2+ ions and involved in the extended hydrogen-bond network of the Tutton salt structure.
(a) FT-NIR spectra in absorbance mode to (K1-xRbx)2Ni(SO4)2·6H2O series in the 3,500-15,000 cm-1 range, (b) normalized absorbance as a function of wavenumber for all samples.
The most intense spectral contributions are observed between approximately 4500 and 5500 cm-1 (Figure 11a), which are commonly attributed to combination bands involving ν(O-H) stretching and δ(H-O-H) bending modes of water molecules. The peak centered at 4100 cm-1 can be attributed to the combinational modes of (SO42-) ion. Additional broad absorptions extending from ca. 5600 to 7000 cm-1 are associated with the first overtone of the O-H stretching vibration.
The band with a peak at 4800 cm-1 can be assigned to the combinations of vibrations of water and water bending bonding modes. These assignments are consistent with previous FT-NIR studies of hydrated sulfate crystals and other hydrogen-bonded inorganic materials.11,19,33
The band with two peaks at 6000 and 6500 cm-1 can be considered as the overtones of water molecules.31 The broad absorption bands with maxima at 9000 and 13500 cm-1 can be attributed to the 3A2g-3T2g and 3A2g-3T1g transitions, respectively, of the Ni2+ ion in the octahedral nickel complex Ni(H2O)62+.28,34
A systematic evolution of the FT-NIR spectral profiles is observed as a function of Rb content (Figure 11a). With increasing Rb substitution, gradual changes in band shape, relative intensity, and bandwidth are detected, particularly in the regions associated with O-H overtones.
An interesting behavior of these spectra can be observed around 7000 cm-1, where the absorbance tends to zero. This means that the radiation incident at this wavelength is completely transmitted through the medium. This behavior may be important in temperature devices based on the detection of the first overtone of the O-H stretching vibration mode of water (6170-7690 cm-1).35,36Figure 11b shows the graphs of normalized absorbance as a function of wavenumber, where characteristics of a band-pass filter can be noted in the region around 7000 cm-1 for samples with low Rb concentration, up to x = 0.02.
Conclusions
In this study, the Tutton (K1-xRbx)2Ni(SO4)2·6H2O salt was successfully crystallized from an aqueous solution using the slow solvent evaporation method.
The experimental findings presented in this study demonstrate that the partial substitution of potassium with rubidium ions in (K1-xRbx)2Ni(SO4)2·6H2O crystals preserves the monoclinic symmetry (P21/c) across all samples but alters their morphological, structural, vibrational, and thermal properties. XRD analysis revealed lattice distortion across the series. All samples retained the monoclinic space group P21/c, including the x = 0.05 sample, which displayed an increase in the unit cell, without a true crystallographic phase transition.
Raman and FT-MIR spectroscopy provided complementary insights into vibrational dynamics. Raman results evidenced the lifting of degeneracy of the ν2, ν4 and ν3 modes of sulfate ion (SO42-) and the appearance of twisting-tw and rocking-ρ modes of H2O and FT-MIR emphasizes changes in dipole-active bending vibrations. These results reinforce the spectroscopic evidence of lattice perturbation and local environmental modification. FT-NIR results reveal absorption bands attributed to d-d transitions of the Ni(H2O)62+ complex and a broad band attributed to first overtone of the O-H stretching vibration mode of water (6170-7690 cm-1).
TGA/DTG revealed enhanced thermal resistance in Rb-containing samples, with decomposition temperatures shifting upward and total mass loss increasing from approximately 42% (matrix) to over 46% (x = 0.05). The combined TGA/DTG results showed systematic smoothing of dehydration and decomposition peaks, suggesting stronger bonding and increased lattice enthalpy.
Compared to other alkali-substituted Tutton salts, such as Csor Li-based compounds, the Rb-incorporated samples exhibit intermediate behavior: moderate lattice distortion combined with improved thermal resilience and vibrational asymmetry. This balance between structural stability and tunability positions Rb+ as a particularly effective substituent for the tailored design of crystalline materials. These findings underscore the potential of Rb-containing Tutton salts for use in thermally stable optoelectronic devices, ionic conductors, and sensor technologies. The ability to fine-tune structural and vibrational characteristics via controlled substitution offers a valuable route for materials design.
Acknowledgments
This research was supported by CAPES, CNPq, FAPEMIG, FINEP, and FADEPE.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon request.
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Edited by
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Editor handled this article:
Juliano Alves Bonacin (Associate)
