Structural, Thermal, Optical and Dielectric Properties of New Synthesized Keggin-Type Lacunary Polyoxometalates Cs₅PMMo₁₁(H₂O)O₃₉ (M = Cu and Zn)

Abstract

New lacunary Keggin-type polyoxometalate salts with the formula Cs₅PMMo₁₁(H₂O)O₃₉ (M = Cu, Zn) were synthesized via the inorganic solution condensation method. X-ray diffraction and FT-IR spectroscopy confirmed the preservation of the Keggin structure. The surface morphology and elemental composition were characterized using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. Thermal analysis, performed by differential scanning calorimetry coupled with thermogravimetry, demonstrated a significant enhancement in thermal stability upon the incorporation of the transition metals into the heteropolyacid framework. Specifically, the substitution of protons by cesium and of molybdenum by copper or zinc positively influenced the crystallographic configuration of the salts, raising their thermal resistance (up to 526 °C). Furthermore, optical and dielectric measurements revealed promising electronic properties in the synthesized lacunary salts. Notably, the compound Cs₅PZnMo₁₁(H₂O)O₃₉ exhibited a substantially increased dielectric constant at low frequency, underscoring the synergistic effect of zinc addition on its dielectric performance.

1. Introduction

Polyoxometalates (POMs) represent a rich and important class of inorganic molecular compounds with unique electronic versatility [1]. These are complexes composed of [M_nO_r]ⁿ⁻ isopolyanions (M = W^VI, Mo^VI, V^V, Nb^V, and Ta^V) centered around a heteroatom X to form a heteropolyanion [X_xM_mO_y]^p− (x ≤ m) [2]. Nearly 70 elements of the periodic table, such as Si, P, S, B, Ge, Cr, and As, can function as the heteroatom X. This confers a wide variety of structures and interesting properties on these materials, which make them relevant for potential applications in various fields, including materials science [3], medicine [4], catalysis [5], electrochemistry [6], magnetism, and electronics [7].

Among the most studied POMs are Keggin-type heteropolyanions, which form a vast class of inorganic metal-oxide cluster compounds with transition metals in their highest oxidation states [8]. They have the general formula X_nM₁₂O₄₀ⁿ⁻, where an XO₄ tetrahedron is surrounded by 12 MO₆ octahedra. These octahedra share edges to form M₃O₁₃ trimetallic groups, which are interconnected via shared corners. The accumulation of transition-metal centers within a single molecular unit, as nearly all transition metals can be and have been incorporated into POMs, confers a diversity of physicochemical properties [9]. However, the binding stability constant of these materials with electrophiles is relatively low. This is largely attributed to the weakly basic surface oxygens and the lack of suitable inner-sphere coordination sites [10,11]. To enhance the reactivity of the oxo ligands, three empirical strategies are typically adopted: (i) increasing the overall charge by total or partial reduction of the addenda atoms, although post-reduced POMs are generally not air-stable; (ii) substituting one or more addenda M^VI ions with lower-valent metal ions (e.g., Ti^IV, V^V et Nb^V), but the alternatives are limited due to strict coordination geometry and ionic radius requirements; (iii) creating one or more vacancies in the POM structures, followed by the incorporation of targeted transition-metal ions into these vacancies, leading to a subclass of Keggin POMs known as transition-metal-substituted polyoxometalates (TMPOMs) [12]. This pathway allows for the tuning of structure and properties at the molecular level, as well as a degree of control over the final cluster architectures. TMPOMs have attracted considerable interest in recent years due to their remarkable structures and characteristics, such as thermodynamic stability towards oxidation, acidity, redox properties, and solubility in various media. The substitution of a transition metal in the POM structure has been studied as a promising route to extending the application range of these compounds [11].

Extensive research has been dedicated to the study of TMPOMs. Day and Kemperer [13], Chen and Zubieta [14], and other researchers have focused their work on investigating structural properties. Lachquer et al. [15] studied the electrocatalytic and optical properties of Keggin POM salts for use as components of photo-cathodes in microbial fuel cells. Chada et al. [16] proposed the use of a POM blend in semiconductors. Tsigdinos and Hallada [17] examined the proton conductivities of POMs for their potential integration as proton exchange membranes in fuel cells. Other researchers have investigated the dielectric properties of materials based on polyurethane/POM, polyimide/POM, and silane-modified POMs [18]. From a practical standpoint, it is advantageous to have a variety of properties within a single material for multiple applications. Recently, the need for materials possessing high optical and dielectric properties has significantly increased in the electronics industry for use in various applications.

Despite extensive research on transition-metal-substituted POMs, to the best of our knowledge, no previous work has reported the combination of three structural and compositional features in a single POM salt. In this work, we investigated, for the first time, new lacunary Keggin-type POM salts combining three modifications simultaneously: (i) cesium counter-cation substitution, (ii) lacunary framework (PMo₁₁O₃₉⁷⁻), and (iii) transition-metal incorporation (Cu²⁺ or Zn²⁺), reported with the formula Cs₅PMMo₁₁(H₂O)O₃₉ (M = Cu and Zn). These salts were prepared via inorganic solution condensation method and subsequently characterized by various analytical techniques to confirm their composition and structure. The effect of transition-metal substitution (Cs, Cu, and Zn) on the structural, optical, and dielectric properties of lacunary Keggin-type POMs was investigated. To elucidate their electronic properties, measurements of optical and dielectric behaviors were performed.

2. Materials and Methods

2.1. Materials Preparation

All commercial products used in this work were obtained from Sigma-Aldrich (St. Louis, MO, USA). The prepared materials are lacunary Keggin-type POM (L-POM) salts modified by transition metals, with the formula Cs₅PMMo₁₁(H₂O)O₃₉ (M = Cu²⁺, Zn²⁺). The synthesis was performed using the inorganic solution condensation technique [19]. The heteropolyacid H₃PMo₁₂O₄₀ (HPMo) was employed as the starting material and dissolved in distilled water. The pH of the solution was maintained at 4.8 using sodium hydroxide (NaOH). Subsequently, 1 equivalent of the metal cation, obtained from MCl₂, nH₂O (M = Cu²⁺, Zn²⁺), was added to the initial solution. The mixture was heated at 80 °C under vigorous magnetic stirring for 1 h. Following this, an appropriate quantity of CsCl, 5H₂O was added to the mixture, the final pH of the solution was 4.8. The excess water was removed using a rotary evaporator. The obtained salts Cs₅PCuMo₁₁(H₂O)O₃₉ (CsPCuMo) and Cs₅PZnMo₁₁(H₂O)O₃₉ (CsPZnMo) were washed several times with distilled water to remove any sodium residues, and the resulting powders were dried at 80 °C for 4 h.

2.2. Characterization Methods

The synthesized materials were characterized using various techniques. The crystalline structure was verified by X-ray diffraction (XRD) using a PANalytical diffractometer (Malvern Panalytical B.V., Almelo, The Netherlands) with Cu-Kα radiation over a 3–60° 2θ range, with a step size of 0.02° and a scanning speed of 0.05° s⁻¹. The Keggin structure was confirmed by Fourier-transform infrared spectroscopy (FT-IR) performed on an ATR-JASCO FT/IR-4100 spectrometer (JASCO Corporation, Tokyo, Japan). Spectra were recorded in transmission mode as transmittance from 4000 to 400 cm⁻¹ with a resolution of 4 cm⁻¹ and 64 scans. Samples were prepared using the KBr pellet technique. Surface morphology and composition were investigated by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX) using a Thermo Fisher QUATTRO S-FEG instrument (FEI Company, Hillsboro, OR, USA). To evaluate the thermal stability of the materials, differential scanning calorimetry coupled with thermogravimetric analysis (TGA/DSC) profiles were obtained using a LINSEIS STA PT1600 thermogravimetric analyzer (LINSEIS, Selb, Germany). Measurements were conducted from 0 to 700 °C at a heating rate of 10 °C min⁻¹ under a dry air atmosphere. UV-Vis diffuse reflectance spectra were collected using a PerkinElmer Lambda 900 spectrophotometer (PerkinElmer, Waltham, MA, USA) with barium sulfate (BaSO₄) as a reference standard. Dielectric experiments were performed using an impedance analyzer (Agilent 4294A) (Agilent Technologies, Santa Clara, CA, USA). The synthesized powders were pressed into circular pellets with a diameter of 10 mm. The thickness of each pellet was individually measured using a micrometer prior to analysis. Conductive silver paste was then applied to both flat surfaces of each pellet to serve as electrodes and ensure good electrical contact during impedance measurements. The dielectric constant (ε′) was recorded across a frequency range of 1 kHz to 1.5 MHz with an applied voltage of 500 mV at room temperature (25 °C). This frequency range was selected to probe the low-frequency relaxation phenomena commonly observed in polyoxometalate-based materials, such as interfacial polarization and charge carrier hopping mechanisms.

3. Results and Discussion

3.1. XRD Results

Figure 1 shows the characteristic pattern for HPMo, which crystallizes in the triclinic system (JCPDS: 01-075-1588) [20]. The principal diffraction peaks at 2θ = 7.01°, 7.77°, 8.68°, 9.06°, 10.98°, 17.58°, 18.32°, 27.51°, 28.06°, and 28.70° are indexed to the crystallographic planes (001), (110), (0-11), (-101), (121), (-202), (-141), (3-11), (312), and (-312), respectively. Significantly, the cesium salts exhibited more numerous and better-defined diffraction peaks than the pristine HPMo. The incorporation of transition metals had a clear effect on the diffraction peak positions, which can be attributed to changes in the hydration state and local environment around the lacunary POM units. Crucially, these characteristic peaks were preserved in the patterns of the synthesized salts, confirming that crystallinity was maintained and that the POM salts are isostructural with HPMo. XRD refinement using JCPDS: 01-075-1588 (

Table 1. Lattice parameters of HPMo, CsPCuMo and CsPZnMo.

Compound	Crystal Structure (Space Group)	Parameters	Volume (Å3)	dXRD (nm)
HPMo	Triclinic (P-1)	a = 15.342 Å	4216	92
		b = 15.156 Å
		c = 14.478 Å
		α = 112.87°
		β = 111.12°
		γ = 60.04°
CsCuPMo		a = 15.902 Å	4568	35
		b = 15.787 Å
		c = 16.356 Å
		α = 113.91°
		β = 114.08°
		γ = 59.95°
CsZnPMo		a = 17.312 Å	4824	45
		b = 17.534 Å
		c = 14.708 Å
		α = 114.94°
		β = 114.05°
		γ = 58.98°

) revealed an increase in the HPMo lattice volume upon cations incorporation. This expansion can be attributed to the larger ionic radii of Cs⁺ (1.88 Å), Cu²⁺ (0.87 Å), and Zn²⁺ (0.88 Å) compared to H⁺ (0.37 Å) and Mo⁶⁺ (0.59 Å), which leads to an overall enlargement of the POM framework. The average crystallite size (d_XRD) was determined using the Debye–Scherrer equation. The synthesized salts, CsPCuMo (35 nm) and CsPZnMo (45 nm), exhibited significantly smaller crystallite sizes than HPMo (92 nm), confirming the dispersive effect of NaOH during synthesis, which effectively nanostructured the polyacid framework into finer, salt-based materials.

3.2. FT-IR Spectroscopy

FT-IR spectra of the synthesized POMs are presented in Supplementary Materials Figure S1 and Figure 2 . All POMs exhibited similar spectra. In the high-wavenumber region (Figure S1), two bands were detected at approximately 3400 and 1600 cm⁻¹, attributed to the O–H stretching vibration and the H–O–H bending vibration, respectively. HPMo displayed four characteristic bands at 1067, 964, 868, and 786 cm⁻¹ which correspond to the vibrational modes of the Keggin structure: the symmetric and asymmetric stretches of ν_as(P–O_a), ν_s(Mo=O_t), ν_as(Mo–O_b–Mo), and ν_as(Mo–O_c–Mo), respectively (

Table 2. IR frequencies (cm⁻¹) of HPW and prepared L-POM salts.

Compound	νas (P–Oa)	νs (Mo=Ot)	νas (Mo–Ob–Mo)	νas (Mo–Oc–Mo)	ν (M–O)
HPMo	1054	949	883	717	-
CsCuPMo	1047	934	858	718	533
CsZnPMo	1034	910	827	720	480

). The spectra of the prepared salts were similar to that of the HPMo, confirming the preservation of the Keggin structure. However, for both salts, the transition-metal substitution induced a reduction in the symmetry of the P–O_a bonds evidenced by the formation of a shoulder at approximately 1078 cm⁻¹ on the ν_as(P–O_a) vibration. This shift indicates that the PO₄ tetrahedron evolves from a perfect tetrahedral symmetry to a lower C₃ symmetry due to the elongation of one of the P–O_a bonds [21]. This symmetry lowering is mainly associated with the formation of the lacunary transition-metal-substituted Keggin structure itself. Specifically, the creation of the vacant site and the incorporation of Cu²⁺ or Zn²⁺ induce a local distortion around the PO₄ tetrahedron, reducing its ideal Td symmetry toward a lower C₃-like environment. Furthermore, the incorporation of the metal ions led to the appearance of new bands in the 400–800 cm⁻¹ region, which are attributed to metal-oxygen vibrations [22].

3.3. Morphology and Composition

The influence of metal cation substitution on the surface morphology of L-POMs was examined by SEM-EDX (Figure 3). HPMo exhibits a morphology with large, well-defined grains of non-uniform size. The grains appear highly agglomerated, with relatively smooth surfaces and sharp fractures, suggesting a dense crystalline structure and anisotropic crystal growth. In contrast, the prepared salts reveals a highly disaggregated and more porous structure with agglomerates of fine particles, displaying a rough and heterogeneous texture. This arrangement suggests an advanced disaggregation of the initial structure, leading to enhanced porosity. The surface chemical composition is presented in Figure S2 and

Table 3. Elemental analysis of HPMo, CsPCuMo and CsPZnMo using EDX analysis (using spectra of Figure S2).

Compound	Weight %					Atom %
	P	Mo	O	Cs	M	P	Mo	O	Cs	M
HPMo	2.0	63.0	35.0	-	-	2.2	22.6	75.2	-	-
CsCuPMo	1.1	44.1	19.2	33.6	1.9	1.9	23.2	60.6	12.8	1.5
CsZnPMo	1.1	46.8	17.8	33.4	1.0	1.8	25.7	58.5	13.2	0.8

. Both HPMo and the prepared salts revealed the presence of all the elements of the starting heteropolyacid (P, Mo, and O) with a homogeneous distribution, as evidenced by the elemental mapping (Figure 4). Furthermore, new peaks corresponding to the incorporated Cs⁺, Cu²⁺, and Zn²⁺ cations were detected in the spectra of the salts. No sodium signal was detected, indicating that the residual Na⁺ content is negligible. This confirms that the repeated washing step with distilled water during the preparation effectively removed sodium species originating from the pH adjustment step. It should be noted that the elemental ratios obtained from EDX are semi-quantitative and mainly representative of the surface composition. The higher experimental Cs/P ratios compared to the theoretical values may result from the lower accuracy of phosphorus quantification and possible surface enrichment of cesium species during the precipitation and drying processes.

3.4. Thermal Stability

Figure 5 displays the TGA-DSC profiles of the studied materials. HPMo exhibits a high degree of hydration, with a mass loss of 11.77% between 25 and 175 °C associated with the two endothermic peaks at 73 and 115 °C and attributed to the removal of crystallization water. A subsequent, smaller mass loss of 1.47% occurs between 300 and 430 °C, accompanied by an exothermic peak at 425 °C, which corresponds to the decomposition of the Keggin structure into constituent oxides (P₂O₅ and MoO₃). In contrast, the l-POM salts demonstrate significantly enhanced thermal stability. CsPCuMo shows a mass loss of 12.9% extending up to 300 °C, linked to the endothermic peaks at 118 and 283 °C that indicates the gradual elimination of a significant amount of water, including water coordinated to the Cu²⁺ ion. Two exothermic peaks at 486 and 638 °C indicate a multi-stage decomposition leading to stable oxide phases. CsPZnMo presents lower hydration (5.28% mass loss) with endothermic peaks shifted to higher temperatures (192 and 342 °C), reflecting stronger interactions with water structure. Two exothermic peaks mark its thermal decomposition at 496 and 526 °C. These multi-step decomposition pathways in the salts can be interpreted as follows: the first exothermic event is likely due to a crystallographic reorganization involving the transition metals (Cu or Zn) and the compensating Cs⁺ cation [23,24]. This reorganization is a key step that contributes to the improved thermal stability of the L-POM salts. The second exothermic stage presumably results from the final decomposition of the salts into their corresponding oxides (e.g., Cs₂O, P₂O₅, MoO₃, and MO) [15]. The proposed decomposition sequence for the studied materials is summarized in Scheme 1.

3.5. Optical Behavior

Figure 6a presents the UV-Vis DRS collected over the 200–800 nm range. HPMo exhibits two intense bands at 300 and 453 nm, attributed to the dπ–pπ electronic transitions of Mo=O_t bonds and the dπ–pπ–dπ transitions within Mo–O–Mo bridges, respectively [25]. The introduction of Cu and Zn cations induced a notable shift in these bands to lower absorbance values, likely due to the incorporation of the transition metals into the POM framework. Notably, the shift of the molybdenum-related absorbance in the UV region indicates an increase in the oxidation potential of the samples [26]. Based on this result, CsPZnMo is considered more oxidizing than the other studied compound.

The optical and electronic properties of the POM salts were further elucidated using Tauc’s plots derived from the Kubelka–Munk function, assuming an indirect band gap transition (Figure 6b). HPMo exhibits a band gap of 2.34 eV, characteristic of its semiconducting behavior. The band gaps increased significantly upon transition-metal incorporation. CsPCuMo and CsPZnMo exhibited larger band gaps of 3.22 eV and 3.45 eV, respectively. This finding is consistent with a prior study by Glass et al. [27], which demonstrated that the substitution of transition metals in POM materials affects their electronic structure, including changes in the effective band gap associated with the metal-to-POM charge transfer transition. The interaction between the POM framework and the cations raises the energies of both the HOMO and LUMO levels, resulting in the formation of distinct conduction and valence bands [28]. Furthermore, it has been established that the electronegativity of the incorporated cations plays a key role in band gap engineering. In POM compounds, the band gap energy correlates with the cation electronegativity, such that the gap decreases as the cation electronegativity increases. Therefore, the observed band gap trend (Cu < Zn) follows the expected inverse relationship, given that Cu is more electronegative than Zn.

3.6. Dielectric Measurement

The dielectric behavior of the prepared compounds was tested at room temperature across a frequency range of 1 kHz to 1.5 MHz (Figure 7). The graph reveals that the dielectric constant (ε′) decreases with increasing frequency and stabilizes at high frequencies. The high ε′ values observed at low frequencies can be attributed to four polarization mechanisms: electronic, ionic, orientation and space-charge polarizations [29,30,31]. Owing to the heterogeneous surface structure of the prepared POMs, interfacial space-charge polarization plays a dominant role in enhancing the dielectric constant in the low-frequency regime. As demonstrated by Shivaraja et al. [32], this space-charge polarization is associated with grain boundaries within the compounds. At low frequencies, grain boundaries in POM structures are more active, impeding electron hopping between ions. This behavior enhances interfacial polarization by restricting electron movement, thereby leading to a higher ε′ value [33]. Conversely, at high frequencies, the polarization effect diminishes as electron hopping between ions increases, accounting for the lower dielectric constant [34]. Above 0.5 MHz, the dielectric constant becomes frequency-independent and approaches zero, as clearly observed in the enlarged representation of ε′ with a reduced scale in Figure 7b. Among the studied POMs, CsPZnMo exhibits the highest dielectric constant. These results suggest that the association of the cesium and zinc cations with the lacunary polyanion (PMo₁₁O₃₉)⁷⁻ framework may effectively restrict electron hopping between Mo⁶⁺ and Zn²⁺ sites within the crystalline lattice. This restriction enhances polarization and consequently increases the dielectric constant.

The frequency dependence of the imaginary part of the permittivity (ε″) and the dielectric loss (tan δ) for the HPMo, CsPZnMo, and CsPCuMo samples were illustrated in Figure 8a,b. These parameters were measured to evaluate the electromagnetic energy dissipated within the POM matrix and to understand the underlying relaxation mechanisms [35]. Both ε″ and tan δ exhibit a characteristic dispersion, they are significantly higher at low frequencies (10³ Hz) and decrease drastically as the frequency increases toward the MHz range. This behavior is primarily attributed to interfacial polarization, also known as the Maxwell–Wagner effect, which promoted the relaxation effect and reduce the resistivity into the POMs matrix at low frequencies [36]. The introduction of transition metals significantly modifies the dielectric profile of the HPMo. The CsPZnMo sample displays the highest dielectric loss at low frequencies (tan δ ≈ 10 at 1 kHz). This suggests a higher density of mobile charge carriers or enhanced interfacial effects compared to the other compositions. The introduction of copper leads to a marked reduction in loss compared to the zinc-substituted analog, reaching tan δ ≈ 7 at 1 kHz. This improvement may be associated with the d⁹ electronic configuration of Cu²⁺, which could promote charge carrier trapping effects and consequently reduce dielectric losses. In contrast, the fully filled d¹⁰ configuration of Zn²⁺ may limit the introduction of additional active electronic states within the POM framework [37].

To further elucidate the dissipation mechanisms, the AC conductivity (σ_ac) was calculated across different frequencies (1 kHz–1.5 MHz) using the following Equation (1) [38], and the data are presented in Figure 9:

σ_ac = 2 π ε′ ε₀ tan δ f,

(1)

where ε′ is the dielectric constant, ε₀ is the vacuum permittivity, tan δ is the loss factor, and f is the frequency.

All L-POM samples showed that AC conductivity increases with frequency. This behavior is attributed to the activated jumping process corresponding to the ionic diffusion mechanism [38,39]. Indeed, the grain boundaries become less active, which improve the electrons jumping process at high frequency. The plots reveal a distinct low-frequency plateau followed by a sharp dispersion at higher frequencies (log f > 5.5). This transition indicates a shift from long-range translational motion to short-range localized hopping. CsPZnMo displays the highest conductivity (>0.004 S/m at 10⁶ Hz), which correlates directly with its higher tan δ values. The lower conductivity of CsPCuMo compared to the zinc-substituted sample confirms that copper, with its d⁹ electronic configuration and strong redox character, acts as an effective trap for mobile charge carriers, which is in agreement with the extended absorption observed in the visible region. This trapping mechanism minimizes energy dissipation and stabilizes the dielectric response across the measured spectrum. Conversely, the d¹⁰ electron configuration of Zn²⁺ does not introduce additional active electronic levels, resulting in lower conductivity and an insulator-like, wide-band gap behavior.

4. Conclusions

In this work, new Keggin-type L-POMs, Cs₅PMMo₁₁(H₂O)O₃₉ (M = Cu, Zn), are successfully synthetized and characterized using XRD, FT-IR, SEM-EDX, ATG-DSC, UV-Vis DSR spectroscopies, and dielectric measurements. The incorporation of transition metals (Cu²⁺, Zn²⁺) coupled with proton substitution by Cs⁺ cations induced modifications in the surface morphology and grain size of HPMo. However, the structure did not change, the prepared salts showed a triclinic structure characteristic of the Keggin structure. Thermal analyses demonstrated enhanced thermal stability of the substituted salts, with decomposition temperatures reaching above 500 °C. Optical investigations exhibited that transition-metal incorporation significantly modified the electronic structure of the lacunary POMs, leading to increased band gap energies compared with heteropolyacid. Dielectric measurements revealed a strong frequency-dependent behavior characteristic of interfacial polarization phenomena. Among the studied compounds, CsPZnMo exhibited the highest dielectric constant (7660) and AC conductivity values within the investigated frequency range (>0.004 S/m at 106 Hz), indicating that Zn substitution strongly influences the dielectric response of the lacunary framework.

These results demonstrate that transition-metal substitution provides an effective strategy for tuning the physicochemical and dielectric properties of lacunary POMs, highlighting their potential interest for future dielectric and electronic materials research.