Elucidating the physical properties of the molybdenum oxide Mo₄O₁₁ and its tantalum substituted variant Mo₂Ta₂O₁₁

Synthesis

The pure molybdenum oxides were synthesized by the chemical vapour transport (CVT) method. To prevent contamination with oxygen and water vapour, the initial preparations were made in a glovebox. The reactions were performed in evacuated, torch sealed fused silica ampoules (l=250 mm, Ø =25 mm, thickness of the tube walls=1.8 mm). A mixture of 1636.5 mg MoO₃ (11.4 mmol), 363.7 mg MoO₂ (2.8 mmol) and 176.6 mg TeCl₄ (0.7 mmol) yielding a final Mo:O ratio of 1:2.80 was thoroughly ground together in an agate mortar and transferred into a silica ampoule. To yield the phase η-Mo₄O₁₁, the oven was kept at 798–748 K (educt zone – product zone) for 5 days. Afterwards, the oven was subsequently cooled to room temperature. For the phase γ-Mo₄O₁₁, the oven was kept at 923–873 K for 5 days. Due to the inherent nature of the CVT process, the final product forms as a crystalline compound on the ampoule walls as shown in Figure 2. η-Mo₄O₁₁ forms thin layered crystals (up to 3 cm in size). The phase γ-Mo₄O₁₁ on the other hand, yields smaller and thicker crystals as shown in Figure 3. The deep wine-red colour and bronze reflections are typical for both phases of Mo₄O₁₁.

Fig. 2:

Photograph of the final product η-Mo₄O₁₁ retrieved from the ampoule walls. The compound forms large (up to 3 cm) and thin wine-red platelets.

Fig. 3:

Photograph of γ-Mo₄O₁₁ under the microscope. The compound forms large (Ø=0.5 cm) wine-red crystals.

To obtain the tantalum substituted phase Mo₂Ta₂O₁₁, the reaction was performed in an evacuated, torch sealed silica glass ampoule (l=80 mm, Ø==25 mm, thickness of the tube wall=1.8 mm). A mixture of 443.3 mg MoO₃ (3.1 mmol) and 556.7 mg Ta₂O₅ (1.3 mmol) was ground together in a agate mortar and heated up to a temperature of 1123 K and kept there for 15 days. Afterwards, the ampoule was cooled with a rate of 1 K min⁻¹ to room temperature. The product was obtained as a compressed grey-purple powder (Figure 4). In comparison to the synthesis described in [25], the temperature is higher (about 100 K) and the duration of the heating process longer (compared to 3 h). However, it seems that in contrast to an open air reaction, the silica ampoules enable a reaction without much MoO₃ loss (only 20% MoO₃ excess used), while still yielding the phase pure product Mo₂Ta₂O₁₁. The synthesis was therefore very similar to [26], where the phase Mo₂Ta₂O₁₁ was falsely described as Ta₂O₅·3MoO₃ (Mo₃Ta₂O₁₄).

Fig. 4:

Photograph of the product Mo₂Ta₂O₁₁ under the microscope (Ø=0.2 cm).

X-ray structure determination

A STOE Stadi P diffractometer with curved Ge(111)-monochromatized Mo–K_α1 radiation (λ=70.93 pm) was used to characterize the polycrystalline samples by X-ray powder diffraction. Diffraction intensities were measured with a Mythen2 1K (Dectris, Switzerland) microstrip detector with 1280 strips. For the measurement, the bulk material was ground to a powder in an agate mortar and fixed between two polyacetate films.

The Rietveld refinements were accomplished with the software package Diffracplus-Topas^® 4.2 (Bruker AXS GmbH, Karlsruhe, Germany) [27] based on single-crystal data for η-Mo₄O₁₁ (collection code: 82,363) [28], γ-Mo₄O₁₁ (collection code: 76,366) [15], and Mo₂Ta₂O₁₁ (collection code: 247,163) [25] from the ICSD [29], [30], [31]. The reflection shapes were modelled using modified Thompson-Cox-Hastings pseudo-Voigt profiles [32], [33]. Instrumental contributions on reflection profiles were corrected from the refinement of a standard (LaB₆) [34]. The background was fitted with Chebychev polynomials up to the 6^th order [35].

Relevant details of the data collections and the refinements using the Rietveld method are listed in Table 1.

Thermal investigations

Magnetic investigations

The single crystals of γ-Mo₄O₁₁, η-Mo₄O₁₁, and Mo₂Ta₂O₁₁ obtained via chemical vapour transport were ground into fine powders in an agate mortar and subsequently packed into polyethylene (PE) capsules and attached to the sample holder rod of a Vibrating Sample Magnetometer (VSM) for measuring the magnetization M(T,H) in a Quantum Design Physical Property Measurement System (PPMS). The samples were investigated in the temperature range of 3–300 K with an applied external magnetic field of 10 kOe. The recorded susceptibilities were corrected by the diamagnetic contributions caused by the PE capsules.

XPS measurements

X-ray photoelectron spectroscopic (XPS) measurements were carried out using a Thermo Scientific MultiLab 2000 spectrometer with a base pressure in the low 10⁻¹⁰ mbar range. The instrument is equipped with a monochromated Al-K_α X-ray source, an Alpha 110 hemispherical sector analyzer as well as a flood gun for charge compensation, providing electrons with a kinetic energy of 6 eV. Wherever possible, the C 1s peak (set to 284.8 eV) was used to calibrate the energy axis shift. Survey spectra were collected to determine the surface purity of the samples, whereas high-resolution spectra of the O 1s, C 1s, Ta 4d, and Mo 3d regions were used to derive information of the relevant oxidation states of the respective elements. Deconvolution was carried out by fitting mixtures of Gaussian and Lorentzian functions (30% Lorentzian character) as peak shapes for each component, limited by several constraints in the fitting process: for each component, the peak splitting was set to a fixed value, depending on the transition and the element. The peak width for the respective spin-orbit components were restricted to the same value. The respective peak area ratios were set to 3:2 for the d_5/2:d_3/2 component and 4:3 for the f_7/2:f_5/2 component. A Shirley-type background was used for baseline correction. For calculation of the surface ion concentration, relative sensitivity factor (RSF) and electron mean free path corrections have been applied. The mean free path correction was based on values from the NIST database.

Raman spectroscopic measurements

The Raman spectroscopic measurements were done on a LabRam HR 800 spectrometer equipped with a 1024×256 CCD detector (Peltier-cooled) combined with an Olympus BX41 microscope. All measurements were carried out using a laser wavelength of 532 nm and a total laser power of approximately 12 mW. The laser wavelength was chosen to avoid resonant effects on Mo₄O₁₁, as observed in [37]. A 300 L mm⁻¹ grating (spectral resolution approximately 3.5 cm⁻¹) was used and all spectra were baseline corrected using a second order polynomial and normalized by the unit vector method (both LabSpec 6).

The measurements of Mo₂Ta₂O₁₁ were carried out on a powder sample using an Olympus ×50 LNPlanFLN objective (NA=0.5). The measurements of both the monoclinic and orthorhombic η/γ-Mo₄O₁₁ modification were done on a platelet sample using an Olympus ×10 MPlanN objective (NA=0.25). The two different orientations of the sample relative to the laser polarisation were achieved by turning the sample by 90°. For the measurement of the third orientation of monoclinic η-Mo₄O₁₁, the sample was embedded upright in an epoxy resin and polished to give a smooth surface.

Elemental analysis

Elemental analyses were conducted for all three compounds to determine the composition with regard to molybdenum and oxygen for the binary compounds and with regard to molybdenum, tantalum, and oxygen for Mo₂Ta₂O₁₁. The analyses were executed via the Mikroanalytisches Labor Pascher (Remagen, Germany).

Powder diffraction data

The Rietveld refinements of the experimental products are shown in Figure 6. The experimental data is shown in black, the best fit profiles in red, and the difference curves in blue. The experimental data agrees well with the single crystal data from the ICSD, a short comparison of these phases is given in Table 3.

Fig. 6:

Rietveld refinement data. Top to bottom: the observed powder patterns are given in black (η-Mo₄O₁₁, γ-Mo₄O₁₁, and Mo₂Ta₂O₁₁), the best fit profiles in red, and the difference curves in blue.

Thermal investigations

Heat capacity

In the initial heating process for the phase η-Mo₄O₁₁, a mass loss of 0.9% was observed. The change was attributed to volatile substances adhered to the surface of the sample. After confirming mass consistency by conducting a second heating run, the heat capacity was measured. The heat capacity for η-Mo₄O₁₁ was determined to be 0.507 J g⁻¹ K⁻¹ at 373 K. The heat capacity of the phase γ-Mo₄O₁₁ showed a non-linear trend with a pronounced peak at 443 K stemming from a phase transition to a yet unidentified phase. This phase transition is fully reversible exhibiting no significant hysteresis and can be classified to be of weakly first order. The heat capacity of γ-Mo₄O₁₁ could therefore only be measured up to 443 K and was determined to be 0.584 J g⁻¹ K⁻¹ at 373 K. For the tantalum substituted phase Mo₂Ta₂O₁₁, the heat capacity was determined to be 0.459 J g⁻¹ K⁻¹ at 373 K. The temperature dependence of the heat capacity for all three substances is depicted in Figure 7. The differential scanning calorimetry (DSC) depicted in Figure 8 indicates the reversibility of the phase transition occurring at 443.4 K, as the peak in the DSC-signal can be observed in the heating curve (red) and the cooling curve (blue).

Fig. 7:

Determined temperature dependence of the heat capacity of η-Mo₄O₁₁ (red), γ-Mo₄O₁₁ (black), and Mo₂Ta₂O₁₁ (blue).

Fig. 8:

Differential scanning calorimetry (DSC) measurement of γ-Mo₄O₁₁. Heating and cooling run through the phase transition at about 445–450 K.

Thermal expansion

High-temperature dilatometry of γ-Mo₄O₁₁

The temperature-induced strain ε₁₁ of γ-Mo₄O₁₁ was directly determined from the longitudinal strain along [100]. Heating and cooling runs exhibit reversible discontinuities at ca. 440 K, which are hints to a phase transition of weakly first order (Figure 9). The thermal expansion of γ-Mo₄O₁₁ is non-linear (Figure 9); thus, second-order polynomials of the type ε₁₁(T)=α₁₁ΔT +β₁₁(ΔT)² with ΔT=T–T₀ are required for a proper approximation of the strain ε₁₁ over a temperature range between 330 and 436 K. T₀=400 K denotes the reference temperature and α₁₁=7.2(4)·10⁻⁶ K⁻¹ and β₁₁=3.4(1)·10⁻⁸ K⁻¹ are the corresponding coefficients of linear and squared thermal expansion referring to 400 K. The reproducibility of the linear thermal expansion coefficient α₁₁ is of the order of 5%.

Fig. 9:

Temperature induced strain along [100] of γ-Mo₄O₁₁ as observed by dilatometry.

High-temperature X-ray powder diffraction of η-Mo₄O₁₁ and Mo₂Ta₂O₁₁

Both oxides undergo an assumable solid-phase decomposition (Figure 10). η-Mo₄O₁₁ decomposes to the oxide MoO₃ due to the oxidation of Mo⁵⁺ to Mo⁶⁺, which occurs at approximately 693 K. The final temperature of the decomposition is equal to 743 K. The difference between the temperatures of the oxidation determined by HT-XRD and thermal analysis can be due to the different experimental conditions. Mo₂Ta₂O₁₁ starts to decompose at approximately 1033 K into the oxide MoTa₁₂O₃₃ and an insignificant amount of an unidentified phase. No structural phase transitions are observed for both compounds. The temperature dependencies of the unit cell parameters of η-Mo₄O₁₁ and Mo₂Ta₂O₁₁ are given in Figure 11. The unit cell parameters for both compounds were approximated using squared polynomials in the temperature ranges from 293 to 673 K (η-Mo₄O₁₁) and from 293 to 1023 K (Mo₂Ta₂O₁₁). The calculated thermal expansion coefficients at some specific temperatures are given in Table 4.

Fig. 10:

The X-ray diffraction patterns of η-Mo₄O₁₁ (left) and Mo₂Ta₂O₁₁ (right) given at different temperatures.

Fig. 11:

The temperature dependencies of the unit cell parameters for η-Mo₄O₁₁ (top) and Mo₂Ta₂O₁₁ (bottom).

Tab. 4:

The eigenvalues of the thermal expansion tensors of η-Mo₄O₁₁ and Mo₂Ta₂O₁₁ at given temperatures.

α (106 K−1)	Temperature/K
	293	473	673
η-Mo4O11
α11	7.11(1)	8.12(3)	9.31(5)
α22=αb	7.31(1)	4.83(2)	2.13(4)
α33	2.12(3)	1.66(7)	1.14(2)
αa	4.27(7)	4.51(2)	4.76(6)
αc	4.56(8)	4.81(2)	5.11(7)
μc3=α33∧c (°)	44.5	44.2	44
αβ	−3.1(3)	−3.9(6)	−4.9(2)
αV	16.5(2)	14.6(6)	12.5(2)
	293	673	1023
Mo2Ta2O11
αa=αb	−4.93(1)	−3.21(3)	−1.57(1)
αc	12.44(6)	12.98(2)	13.48(6)
αV	2.6(3)	6.6(6)	10.3(3)

The monoclinic oxide η-Mo₄O₁₁ expands anisotropically and the degree of the anisotropy increases as the temperature increases (Figure 12a). A mixed-anion framework is composed of the MoO₄ tetrahedra and MoO₆ octahedra, which are connected to each other through common vertices. The maximum thermal expansion α₁₁ is along the direction that is close to the short diagonal of the ac parallelogram (Figure 12a; top), which is consistent with the theory of hinges deformation in monoclinic and triclinic crystals [51], [52].

Fig. 12:

The η-Mo₄O₁₁ (a) and Mo₂Ta₂O₁₁ (b) crystal structure projection and the figures of thermal expansion coefficients (the solid/green line – 293 K, the dashed/red line – the highest temperature), and a structural fragment (two hinge cells) of the Mo₂Ta₂O₁₁ layer (c).

The layered trigonal Mo₂Ta₂O₁₁ expands highly anisotropically and the negative expansion (contraction) is observed within the layers. The Mo atoms are in the tetrahedral coordination of the oxygen atoms and the tantalum atoms are in an octahedral coordination environment. The polyhedra are connected to each other through common vertices forming the thick layers. The maximum expansion is along the c axis (α_c=12.44(6)·10⁻⁶ K⁻¹ at 293 K) (Figure 12b). The expansion can also be described in terms of hinges deformation. The hinge cell is Ta1–O3–Mo1–O3–Ta1–O2–Ta1. The O2 atoms are on special positions and the Ta1–O2–Ta1 angle equals 180° (Figure 12c), whilst the Ta1–O3–Mo1 angle is 166.4° at room temperature [25] and could increase up to 180° under heating. If the Ta1–O3–Mo1 angle increases, the Mo1 atom could shift in the direction of the O1 atom while two Ta1 atoms come closer to each other. The angles between the polyhedra can vary more than the interior polyhedral angles. Thus, the layer expands along the c-axis and contracts along the a-axis.

Magnetic investigations

For all compounds, the magnetic susceptibility was investigated in zero-field-cooled mode (ZFC) with an applied external field of 10 kOe in the temperature range of 3–300 K (Figure 13). All curves show almost temperature independent behaviour with small upturns below 25 K, which can be linked to traces of paramagnetic impurities. The susceptibilities at 300 K are: χ(γ-Mo₄O₁₁)=+9(1)×10⁻⁶ emu mol⁻¹, χ(η-Mo₄O₁₁)=+429(1)×10⁻⁶ emu mol⁻¹ and χ(Mo₂Ta₂O₁₁)=−155(1)×10⁻⁶ emu mol⁻¹, indicating the expected diamagnetism for the latter compound, since all atoms exhibit close shell electron configurations according to (Mo⁶⁺)₂(Ta⁵⁺)₂(O²⁻)₁₁. The observed value is in line with the calculated diamagnetic increment of χ(Mo₂Ta₂O₁₁)=−172(1)×10⁻⁶ emu mol⁻¹ using the tabulated values of the respective ions (χ(Ta⁵⁺)=−14×10⁻⁶, χ(Mo⁶⁺)=−7×10⁻⁶, χ(O²⁻)=−12×10⁻⁶ emu mol⁻¹) [53].

Fig. 13:

Temperature dependence of the magnetic susceptibilities χ of η-Mo₄O₁₁ (red), γ-Mo₄O₁₁ (black), and Mo₂Ta₂O₁₁ (green) measured with a magnetic field strength of 10 kOe.

The binary molybdenum oxides in contrast, exhibit a very different magnetic behaviour. A weak positive but also temperature independent susceptibility is observed, contradicting a potentially expected paramagnetism caused by the Mo⁵⁺ (4d¹) cations according to (Mo⁵⁺)₂(Mo⁶⁺)₂(O²⁻)₁₁. The present magnetism is more in line with band-paramagnetism, caused by a metallic-like behaviour of the material. This, however, implies that no ordering of the Mo⁵⁺/Mo⁶⁺ cations but rather a delocalization is present. Studies of the electrical resistivity and the band structures of γ- and η-Mo₄O₁₁ confirm this assumption. The Mo 4d states are filled and therefore lower in energy, the Fermi level therefore is found in the conduction band leading to the metallic character [54], [55], [56]. The absence of charge ordering has been already predicted in structural studies [28].

XPS measurements

The relevant Mo 3d spectra of the phases η- and γ-Mo₄O₁₁ are shown in the panels a and b of Figure 14, respectively. The extended region between 230 and 290 eV binding energy features the C 1s region (mainly adventitious carbon and surface-adsorbed oxygenate species in agreement with the deconvoluted O 1s spectra) and a Mo 3d peak region with at least three different oxidation states of Mo in surface-near regions. Each Mo component (corresponding to Mo(IV), Mo(V), and Mo(VI)) has been accordingly fitted with a spin-orbit split component. For η-Mo₄O₁₁, the Mo 3d_5/2 component of Mo(IV) is measured at 230.5 eV, the Mo 3d_5/2 component of Mo(V) at 231.8 eV and the one of Mo(VI) at 233.1 eV. The assignment is corroborated by literature-reported values of Mo in different oxidation states [54], [57], [58].

Fig. 14:

XPS analysis of η-Mo₄O₁₁ (Panel a), γ-Mo₄O₁₁ (Panel b), and Mo₂Ta₂O₁₁ (Panels c and d). The relevant regions (C 1s, O 1s, Mo 3d, and Ta 4f) have been deconvoluted into different components wherever applicable. The O 1s spectra are shown in Panels e (η-Mo₄O₁₁) and f (Mo₂Ta₂O₁₁), respectively.

The O 1s region (Panel e), besides organic residues and lattice oxygen, shows a well-developed OH species at 531.9 eV, indicative of protonated Mo₄O₁₁. Mo(VI) is the majority species, followed by Mo(V) and Mo(IV). The respective at.-% Mo(VI):Mo(V) ratio is 6:1, the one for Mo(VI):Mo(IV) is 15:1. Transferring the analysis to γ-Mo₄O₁₁, the relevant XP spectra of the phase Mo₄O₁₁ shows strikingly similar features, both in the Mo 3d and O1 region, as well as in the qualitative and quantitative distribution of the existing Mo species.

The sum formula for Mo₄O₁₁ allows for different oxidation states of molybdenum in the structure. With the mean oxidation state being +5.5, Mo₄O₁₁ is at first expected to incorporate an equal amount of Mo(V) and Mo(VI), as indicated by Inzani et al. [54]. Taking into account the crystallographic data provided by Knorr and Müller [28], Mo₄O₁₁ seems to consist of Mo(VI) inside the tetrahedra and Mo(V) and Mo(VI) in a ratio of 2:1 inside the octahedra, resulting in an overall ratio of 2:2. XPS measurements have previously been undertaken, indicating the presence of Mo(IV), Mo(V), and Mo(VI). However, as described in [59], [60], the distribution of Mo(VI), Mo(V), and Mo(IV) depends on the specific temperature, at which the samples were synthesized.

For Mo₂Ta₂O₁₁ (Panels c, d and f of Figure 14), deconvolution of the Mo 3d region is not possible due to strong overlap with the Ta 4d region. The qualitative peak shape (intensity profiles, as well as the low binding energy shoulder) is, by direct comparison to Panels a and b, very similar, inferring an at least qualitatively similar distribution of Mo species. The Ta 4f region shows the presence of three Ta oxidation states between Ta(V) and Ta(III). Ta(V) and Ta(IV) are present in an almost 1:1 ratio, whereas Ta(III) is clearly the minority component (approximately half of the at.-% amount compared to Ta(V) or Ta(IV)). The Ta 4f_7/2 components of Ta(V), Ta(IV), and Ta(III) were measured at binding energies of 29.5, 28.8, and 25.2 eV. Due to charging issues on that particular sample, the Ta 4f binding energies are shifted to too high binding energies with respect to literature data [61]. A protonated OH species is also present at 531.5 eV.

Raman spectroscopic measurements

The Raman spectrum of Mo₂Ta₂O₁₁ with the band positions marked is shown in Figure 15. To the best of our knowledge, no Raman spectrum of Mo₂Ta₂O₁₁ is available in the literature. Therefore, this spectrum was mainly recorded to serve as a future reference for the identification of Mo₂Ta₂O₁₁.

Fig. 15:

Raman spectrum of a powder sample of Mo₂Ta₂O₁₁ with the band positions marked.

In contrast, Raman spectra of both η-Mo₄O₁₁ [5], [62], [63] and γ-Mo₄O₁₁ [37], [63] have been reported in the literature. All of these reports have found little to no difference between the band position, but vast differences in relative band intensities even between the same phase of η/γ-Mo₄O₁₁ [5], [62], [63]. This can in part be explained due to the use of different laser excitations. Also little to no difference was found between the Raman spectra of η-Mo₄O₁₁ and γ-Mo₄O₁₁, especially by Olson [63], which is the only report we are aware of that has measured both phases thus far. The Raman spectra measured of η/γ-Mo₄O₁₁ in this study are shown in Figure 16. The band positions are consistent with the literature and there is little difference between η-Mo₄O₁₁ and γ-Mo₄O₁₁. However, interestingly a strong dependence of the relative band intensities on the orientation of the sample relative to the laser polarization was found. The relative band intensities for the orientation referred to as “pol x” are consistent with what was found by Blume [5] and Borovška et al. [62], whereas the orientation “pol z” is consistent with the relative intensities in [37] and [63]. Thus, orientation effects seem to be a dominated factor in the measurement of Mo₄O₁₁ Raman spectra and need to be considered.

Fig. 16:

Raman spectra of η-Mo₄O₁₁ (top) and γ-Mo₄O₁₁ (bottom) with different orientations of the laser polarisation relative to the crystal (same colour indicates the same orientation).