Reinvestigations of the Li₂O–WO₃ system

Abstract

Reinvestigations of the Li₂O–WO₃ system have been performed with the help of DTA–TG, XRD, IR and UV–Vis–NIR/DRS measuring techniques and WO₃ and Li₂CO₃ as a reactants. Results of investigations have shown that using applied procedure of synthesis four single phases have been obtained: Li₂WO₄, Li₂W₂O₇, α-Li₄WO₅ and β-Li₄WO₅. We failed to obtain pure samples of Li₂W₅O₁₆, Li₂W₄O₁₃ and Li₆W₂O₉, although diffraction reflection characteristic for these phases was identified on powder diffraction patterns of several samples. On the other hand, the formation of Li₆WO₆ has not been corroborated by XRD in our research. Results of DTA–TG investigations have revealed that phases Li₂W₂O₇ and Li₂WO₄ melt congruently at 735 and 745 °C, respectively, whereas α-Li₄WO₅ undergoes a phase transition to β-Li₄WO₅ at 690 °C. Results of DTA–TG and IR investigations indicate that α-Li₄WO₅ can be stabilized by a small amount of carbonate groups. Based on UV–Vis–NIR/DRS investigations, band gap energies were calculated for Li₂WO₄, Li₂W₂O₇, α-Li₄WO₅ and β-Li₄WO₅ and are equal to 4.35, 4.03, 4.00 and 4.12eV, respectively.

Introduction

Literature scan has shown that Li₂O–WO₃ system has been the subject of many studies [1–22]. Phases forming in this system owing to their interesting properties are potential candidates for the production of components of electrodes for lithium batteries, catalysts of oxidative coupling of light hydrocarbons, fluxes for single crystal growing, electrochromic and photochromic devices as well as neutron detectors [1–4]. Previous studies on the Li₂O–WO₃ system have revealed the formation of seven binary compounds: Li₆WO₆, Li₄WO₅, Li₆W₂O₉, Li₂WO₄, Li₂W₂O₇, Li₂W₄O₁₃ and Li₂W₅O₁₆. Synthesis of phases has been conducted in air, oxygen or dry oxygen using Li₂O/WO₃, Li₂O₂/WO₃, LiOH/WO₃, Li₂CO₃/H₂WO₄ and the most frequently Li₂CO₃/WO₃ mixtures. Conducted investigations enabled construction of two variants of phase diagram of the Li₂WO₄–WO₃ system [5, 6] and one of the WO₃–Li₂O system [7]. IR spectra of Li₂W₂O₇, Li₂WO₄, Li₂W₄O₁₃ Li₆WO₆ as well as α and β modifications of Li₄WO₅ are known [7, 8].

Basic crystallographic data of phases forming in the system Li₂O–WO₃ are given in Table 1.

Table 1 Basic crystallographic data of phases forming in the Li₂O–WO₃ system, where CS—crystal system, 3—triclinic, M—monoclinic, O—orthorhombic, R—rhombohedral, T—tetragonal, H—hexagonal, C—cubic

Literature survey has shown that Li₂WO₄ forms four polymorphs: rhombohedral Li₂WO₄-I, of phenacite structure and stable at atmospheric pressure, tetragonal Li₂WO₄-II obtained at 300 MPa, an orthorhombic Li₂WO₄-III, prepared above 300 MPa and at higher temperature than Li₂WO₄-II and monoclinic Li₂WO₄-IV stable at pressure higher than Li₂WO₄-III [9–12]. Crystal structures of rhombohedral, tetragonal and monoclinic structures of Li₂WO₄ were solved [9, 10, 13]. Li₂WO₄ melts at 738 °C [14] 740 °C [7, 11] or 742 °C [6]. If it is heated to well above its melting point at atmospheric pressure, some loss of Li₂O occurs by evaporation, yielding a mixture of Li₂W₂O₇ and Li₂WO₄ [11].

Li₆WO₆ has been obtained as a result of the reaction of Li₄WO₅ with Li₂O at 500 °C or LiOH with WO₃ at 700 °C in dry oxygen [15, 16]; however, Lv and co-workers [4] failed to obtain this phase. At high temperature, Li₆WO₆ has homogeneity range, and below 440 °C, it decomposes into Li₂O and Li₄WO₅ [7]. Reau and co-workers have shown in contrast that it decomposes at 1000 °C yielding β-Li₄WO₅ and volatile Li₂O [15]. Crystal structure of orthorhombic Li₆WO₆ has been solved by Hauck [16].

Another phase forming in the system Li₂O–WO₃, Li₂W₅O₁₆, melts incongruently at 820 °C [4, 5, 17], whereas Li₂W₄O₁₃ melts incongruently at 805 °C [6], 800 °C [7, 13] or at 750 °C [18] with the deposition of Li₂W₂O₇ and WO₃ [18].

Pistorius [11] has found that Li₂W₂O₇ undergoes to sharp and reversible phase transition at 666 °C with a large latent heat. This phase melts congruently at 660 °C [18], 745 °C [5–7] or 754 °C [13]. Crystal structure of Li₂W₂O₇ was solved by Okada and co-workers [19].

Permentier and co-workers [17] conducing synthesis in the temperature range of 450–500 °C have obtained Li₆W₂O₉. According to Authors, Li₆W₂O₉ decomposes at 550 °C with the formation of Li₂WO₄ and β-Li₄WO₅.

Literature survey has shown that Li₄WO₅ forms two polymorphic modifications: low-temperature modification, crystallizing in cubic system α-Li₄WO₅ and high-temperature modification, crystallizing in triclinic or orthorhombic system β-Li₄WO₅ [7, 15, 20, 22]. At 690 °C, α-Li₄WO₅ undergoes to phase transition to β-Li₄WO₅ [7, 15]. According to Hauck [7], β-Li₄WO₅ melts at 1350 °C, but Rau and co-workers [15] have shown that it decomposes at 1100 °C yielding Li₂WO₄ and volatile in these conditions Li₂O. α-Li₄WO₅ can be obtained in different degrees of order–disorder depending on temperature and time of synthesis [7]. Ordered form obtained at higher temperature cannot be transferred to disordered form by heating at lower temperatures. Blasse suggests that cubic modification has disordered rock salt structure [20].

Above-presented literature survey has shown that despite numerous works published, until now there are still controversies concerning the number and composition of forming phases and conditions of their synthesis. The aim of this work was to verify literature data on Li₂O–WO₃ system.

Experimental

The following materials were used for the research: WO₃, 99.9% (Fluka AG, USA), and Li₂CO₃, a.p. (POCh, Poland).

For the experiments, seven samples were selected with contents corresponding to Li₂W₅O₁₆, Li₂W₄O₁₃, Li₂W₂O₇, Li₂WO₄, Li₆W₂O₉, Li₄WO₅ and Li₆WO₆. They represented all described in literature phases forming in the system Li₂O–WO₃. Mixtures of Li₂CO₃ and WO₃ weighed in suitable proportions enabling preparation of 5 g of final product were homogenized in an agate mortar and calcinated at 450, 500, 550, 600, 650 and 700 °C in 24-h stages in an air atmosphere. After each heating stage, the samples were cooled down to room temperature with furnace, powdered in mortar and examined with the help of XRD. The pure phases obtained in this work were examined additionally by the DTA–TG, UV–Vis–IR/DRS and IR methods. These measuring methods were selected because they allow determination of phase composition of samples, establishing their melting temperatures as well as melting behaviour [23–30].

X-ray phase analysis (XRD) of the samples was performed using an Empyrean II diffractometer (PANalytical, The Nederlands, copper radiation filtered with a graphite monochromator) with the help of Highscore + software (PANalyticak, The Nederlands) and PDF4 + ICDD database.

The DTA–TG examinations were made with the aid of an apparatus of Paulik–Paulik–Erdey type (MOM, Hungary). Samples of 500 mg were investigated in air up to the 1000 °C at the heating rate of 10 °C min⁻¹ using quartz crucibles.

The IR spectra were registered by Specord M80 spectrometer (Carl Zeiss, Jena, Germany) in the wavenumber region of 1500–200 cm⁻¹ using halide discs technique (pellets in KBr at a mass ratio 1:300).

The NIR/DRS measurements were performed using a Jasco V670 spectrometer matched with integrating sphere PIN 757 (Jasco, Japan) with Spectralon as a reference material.

Results and discussion

Samples obtained after consecutive stage of heating have been subjected to XRD investigations. Results of X-ray phase analysis are given in Table 2. Analysis of data presented in Table 2 shows that in all cases synthesis starts at 450 °C, but is very slow at this temperature. Moreover, syntheses processes are complex and run with formation of several intermediates. Only in the case of Li₂W₂O₇, Li₂WO₄, α-Li₄WO₅ and β-Li₄WO₅ obtained samples were single phase. In accord with literature data [7, 15] in the temperature range of 650–700 °C, α-Li₄WO₅ undergoes to phase transition to high-temperature modification, β-Li₄WO₅. We failed to obtain pure samples of Li₂W₅O₁₆, Li₂W₄O₁₃ and Li₆W₂O₉, although diffraction reflection characteristic for these phases was identified on powder diffraction patterns of several samples. It was very characteristic for Li₆W₂O₉, whose diffraction reflections have been detected on diffraction patterns of all samples. On the other hand, the formation of Li₆WO₆ has not been corroborated by XRD in our research. We have encountered some problems with X-ray phase analysis of investigated samples. The lack of structural data in the cases of Li₂W₅O₁₆, Li₂W₄O₁₃, α-Li₄WO₅ and poor-quality X-ray data in some other cases makes X-ray phase analysis in the Li₂O–WO₃ system very difficult.

Table 2 Results of X-ray phase analysis after consecutive stages of heating, where Li—Li₂CO₃, W—WO₃, O4—Li₂WO₄, α—α-Li₄WO₅, β—β-Li₄WO₅, O6—Li₆WO₆, O7—Li₂W₂O₇, O9—Li₆W₂O₉, O13—Li₂W₄O₁₃, O16—Li₂W₅O₁₆, X—unknown phase

Powder diffraction patterns of single-phase samples of Li₂W₂O₇, Li₂WO₄, α-Li₄WO₅ and β-Li₄WO₅ were subjected to indexing. Calculated unite cell parameters are given in Table 3. Table 4 presents result of indexing of powder diffraction pattern of the triclinic β-Li₄WO₅ obtained in this work. Despite the fact that indexing results are in good agreement with literature data [15, 22] (Tables 1, 3) we turn our attention to powder diffraction pattern of high-temperature modification of Li₄WO₅, β-Li₄WO₅. Figure 1 shows fragments of powder diffraction patterns of orthorhombic Li₄WO₅ (generated on the basis of ICDD PDF 00-021-0530) (a), triclinic β-Li₄WO₅ obtained in this work (b) and triclinic Li₄WO₅ (generated on the basis of ICDD PDF 04-010-6772) (c). Analysis of the number of diffraction lines, their angular positions and relative intensities have revealed that diffraction pattern of triclinic β-Li₄WO₅ (Tables 1, 3) obtained by us is very similar to diffraction pattern of high-temperature orthorhombic modification of Li₄WO₅ [15] and differs to some extent from PDF 04-010-6772 calculated on the basis of structural data of β-Li₄WO₅ [22]. The differences consist in splitting or overlapping of certain pairs of reflections, like (−110)–(001), (110)–(−1−11) or (101)–(1−21) and measurable shift of diffraction lines on powder diffraction pattern of sample obtained by us towards lower 2θ angles. As a consequence of it, unit cell parameters of β-Li₄WO₅ obtained by us are somewhat larger than these presented by Hoffmann and Hoppe [22] (Tables 1, 3). The differences in unit cell parameters are responsible for splitting or overlapping of certain reflections. It is worth to mention that single crystal which was used in the structure solving of β-Li₄WO₅ was obtained by heating a mixture containing components in atomic ratio Li/W = 4.4:1 at 950 °C for 28 days in gold tube. It is possible that lithium content in single crystal of β-Li₄WO₅ obtained by Hoffmann and Hoppe [22] was higher than assumed (Li/W = 4:1) or that temperature of 950 °C is necessary for ordering of ions in the lithium and tungsten sublattices. To clarify this problem, sample of β-Li₄WO₅ obtained at 700 °C was additionally heated for 2 h at 1000 °C and next cooled to room temperature and subjected to XRD phase analysis. Figure 2 shows fragments of powder diffraction patterns of β-Li₄WO₅ recorded after heating stage at 700 °C (a) and after additional heating at 1000 °C for 2 h (b). Results of phase analysis have revealed that sample after additional heating at 1000 °C except the β-Li₄WO₅ contains also Li₂WO₄ and Li₂O. It is in accord with literature data informing that at 1100 °C β-Li₄WO₅ decomposes yielding Li₂WO₄ and Li₂O [15]. As there were no evidences of splitting and overlapping of reflections as a result of heating at 1000 °C, this problem requires further investigations.

Table 3 Unit cell parameters and volumes of Li₂WO₄, Li₂W₂O₇, α-Li₄WO₅, β-Li₄WO₅ phases obtained in this work

Table 4 Result of indexing of X-ray powder diffraction pattern of the triclinic β-Li₄WO₅ obtained in this work

Fig. 1

Comparison of fragments of powder diffraction patterns of (a) orthorhombic Li₄WO₅ (generated on the basis of ICDD PDF 00-021-0530), (b) triclinic β-Li₄WO₅ obtained in this work and (c) triclinic Li₄WO₅ (generated on the basis of ICDD PDF 04-010-6772)

Fig. 2

Comparison of fragments of powder diffraction patterns of (a) β-Li₄WO₅ recorded after heating stage at 700 °C and after additional heating at 1000 °C for 2 h (b), where O4 stands for Li₂WO₄ and L stands for Li₂O

The single-phase samples of Li₂WO₄, Li₂W₂O₇ and α-Li₄WO₅ obtained after heating stages at 650 °C as well as β-Li₄WO₅ obtained after heating stage at 700 °C were subjected to the DTA–TG investigation up to 1000 °C. Figure 3a shows DTA–TG curves of Li₂WO₄ and Fig. 3b DTA–TG curves of Li₂W₂O₇. On each DTA curve was recorded only one endothermic effect, with their onsets at 745 °C for Li₂WO₄ and 735 °C in the case of Li₂W₂O₇.

Fig. 3

DTA (light line) and TG (dark line) curves of pure: (a) Li₂WO₄ obtained after last heating stage at 650 °C and (b) Li₂W₂O₇ obtained after heating stage at 650 °C

It was in accord with literature data where these endothermic effects were attributed to the melting of these phases [5–7, 11, 13, 14, 18]. TG curves of both phases did not contain any mass change effects. In order to explain melting behaviour of Li₂WO₄ and Li₂W₂O₇, samples of these compounds were additionally heated for 3 h at 780 °C, i.e., at temperature close to the extremum temperature of the endothermic effects registered on the DTA curves.

At temperature 780 °C, samples were liquid, colourless and transparent. After heating at 780 °C, samples were cooled rapidly to room temperature. The X-ray phase analysis of the melted and next quenched samples showed that they comprised only Li₂WO₄ and Li₂W₂O₇, respectively, which suggests congruent melting in both cases. On the other hand, Fig. 4 shows the DTA–TG curves of β-Li₄WO₅ (3a) and α-Li₄WO₅ (3b). On the DTA curve of α-Li₄WO₅ was recorded one small endothermic effect with onset temperature at 690 °C which was accompanied by small mass loss effect (2%). TG curve of α-Li₄WO₅ includes also another small mass loss effect (0.8%) with onset temperature at 230 °C, which was not accompanied by any thermal effects. The first mass loss effect the most probably can be attributed to desorption of water adsorbed by α-Li₄WO₅. The endothermic effect with onset at 690 °C can be connected with phase transition to β-Li₄WO₅. This is in accord with results of our XRD investigations (Table 2) indicating run of phase transition leading from α-Li₄WO₅ to β-Li₄WO₅ in the temperature range of 650–700 °C and literature data [7, 15]. However, the nature of the second mass loss effect is unknown and cannot be explained using only results of DTA–TG investigations. On the DTA curve of β-Li₄WO₅ was recorded only one endothermic effect with onset temperature at 210 °C, which was accompanied by small mass loss effect (2%). This mass loss effect was, however, greater than this recorded in the same temperature range on TG curve of α-Li₄WO₅. This effect can also be connected with desorption of water adsorbed, this time, by β-Li₄WO₅ (Fig. 4).

Fig. 4

DTA (light line) and TG (dark line) curves of pure: (a) β-Li₄WO₅ obtained after heating stage at 700 °C and (b) α-Li₄WO₅ obtained after heating stage at 650 °C

In order to know better properties of obtained phases and to explain the nature of mass loss effects recorded on TG curves of α-Li₄WO₅ and β-Li₄WO₅, single-phase samples of Li₂WO₄, Li₂W₂O₇, β-Li₄WO₅ and, α-Li₄WO₅ were subjected to an investigation with the help of IR and UV–Vis–NIR/DRS spectroscopy. Figure 5 shows the IR spectra of Li₂WO₄ (curve a), Li₂W₂O₇ (curve b), β-Li₄WO₅ (curve c), α-Li₄WO₅ (curve d) and for comparison Li₂CO₃ (curve e), Fig. 6 shows UV–Vis–NIR/DRS spectra of β-Li₄WO₅ (square) and α-Li₄WO₅ (circle), whereas Fig. 7 shows UV–Vis–NIR/DRS spectra of Li₂WO₄ (circle) and Li₂W₂O₇ (square).

Fig. 5

IR spectra registered for pure samples obtained by the solid state reaction: (a) Li₂WO₄, (b) Li₂W₂O₇, (c) β-Li₄WO₅, (d) α-Li₄WO₅ and (e) Li₂CO₃

Fig. 6

UV–Vis–NIR/DRS spectra of α-Li₄WO₅ (circle), β-Li₄WO₅ (square)

Fig. 7

UV–Vis–NIR/DRS spectra of Li₂W₂O₇ (square), Li₂WO₄ (circle)

Analysis of the number and positions of absorption bands recorded in the IR spectra of Li₂WO₄, Li₂W₂O₇, β-Li₄WO₅ and α-Li₄WO₅ phases obtained in this work has shown good agreement with literature data in the wavenumber range of 1000–400 cm⁻¹ [7, 8]. However, analysis of IR spectra in the range of 1600–1000 cm⁻¹, not investigated earlier by other authors, has revealed in spectrum of α-Li₄WO₅ (Fig. 5, curve d) weak absorption bands with maxima at 1430 and 1480 cm⁻¹, characteristic for carbonates [31]. Similar bands, however much stronger, occur in IR spectrum of Li₂CO₃ (Fig. 5 curve e) but are absent in the spectra of other investigated phases. Thus, from the analysis of the envelopes of recorded spectra shown in Fig. 5, let us come to conclusion that mass loss effect recorded at 690 °C on TG curve of α-Li₄WO₅ can be attributed to elimination of carbonate groups from the crystal lattice of this phase. We cannot exclude, however, that carbonate groups are eliminated from amorphous admixture accompanied α-Li₄WO₅, but not detectable by XRD. These both assumptions raise question concerning real composition of α-Li₄WO₅.

On the other hand, analysis of the UV–Vis–NIR/DRS spectra of α-Li₄WO₅ and β-Li₄WO₅ has shown that they contain weak absorption bands with maxima near 1430 nm, characteristic for water [24, 32]. Intensity of this band is higher in the case of β-Li₄WO₅. Such bands do not occur in the spectra of Li₂W₂O₇ and Li₂WO₄ (Fig. 7). It indicates that the small mass loss effects recoded on TG curves of α-Li₄WO₅ and β-Li₄WO₅ in the temperature range 200–230 °C can be connected with elimination of water during heating of these samples.

Based on recorded UV–Vis–NIR/DRS spectra and using procedures described in [33–35], band gap energy values for Li₂WO₄, Li₂W₂O₇, α-Li₄WO₅ and β-Li₄WO₅ equal to 4.35, 4.03, 4.00 and 4.12 eV, respectively, were calculated.

Conclusions

• Reinvestigations of the Li₂O–WO₃ system have been performed with the aid of DTA–TG, XRD, IR and UV–Vis–NIR/DRS measuring techniques and WO₃ and Li₂CO₃ as a reactants.

• Using applied procedure of synthesis, four single phases have been obtained: Li₂WO₄, Li₂W₂O₇, α-Li₄WO₅ and β-Li₄WO₅.

• Diffraction reflection characteristic for Li₂W₅O₁₆, Li₂W₄O₁₃ and Li₆W₂O₉ was identified on powder diffraction patterns of several samples, but it was not possible to obtain these compounds as single phases.

• Formation of Li₆WO₆ has not been corroborated in our research.

• Results of DTA–TG and XRD investigations have revealed that phases Li₂W₂O₇ and Li₂WO₄ melt congruently at 735 and 745 °C, respectively, whereas α-Li₄WO₅ undergoes a phase transition to β-Li₄WO₅ at 690 °C.

• Results of DTA–TG and IR investigations indicate that crystal structure of α-Li₄WO₅ can be stabilized by a small amount of carbonate groups.

• Based on UV–Vis–NIR/DRS investigations, band gap energies for Li₂WO₄, Li₂W₂O₇, α-Li₄WO₅ and, β-Li₄WO were calculated.