Reinvestigations of the Li2O–WO3 system
Reinvestigations of the Li2O-WO3 system
Piotr Tabero 0
Artur Frackowiak 0
0 Szczecin, Faculty of Chemical Technology and Engineering, Department of Inorganic and Analytical Chemistry, West Pomeranian University of Technology , Al. Piastow 42, 71-065 Szczecin , Poland
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, a-Li4WO5 and bLi4WO5. 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 aLi4WO5 undergoes a phase transition to b-Li4WO5 at 690 C. Results of DTA-TG and IR investigations indicate that a-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, a-Li4WO5 and b-Li4WO5 and are equal to 4.35, 4.03, 4.00 and 4.12eV, respectively.
Li2O-WO3 system; UV-Vis-NIR/DRS; Band gap
& Piotr Tabero
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 Li2
CO3/WO3 mixtures. Conducted investigations enabled
construction of two variants of phase diagram of the Li2
WO4?WO3 system [5, 6] and one of the WO3?Li2O system
. IR spectra of Li2W2O7, Li2WO4, Li2W4O13 Li6WO6 as
well as a and b modifications of Li4WO5 are known [7, 8].
Basic crystallographic data of phases forming in the
system Li2O?WO3 are given in Table 1.
Literature survey has shown that Li2WO4 forms four
polymorphs: rhombohedral Li2WO4-I, of phenacite
structure and stable at atmospheric pressure, tetragonal Li2
WO4-II obtained at 300 MPa, an orthorhombic
Li2WO4III, 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
 740 C [7, 11] or 742 C . If it is heated to well
above its melting point at atmospheric pressure, some loss
of Li2O occurs by evaporation, yielding a mixture of Li2
W2O7 and Li2WO4 .
Unit cell parameters
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 
failed to obtain this phase. At high temperature, Li6WO6
has homogeneity range, and below 440 C, it decomposes
into Li2O and Li4WO5 . Reau and co-workers have
shown in contrast that it decomposes at 1000 C yielding
b-Li4WO5 and volatile Li2O . Crystal structure of
orthorhombic Li6WO6 has been solved by Hauck .
Another phase forming in the system Li2O?WO3, Li2
W5O16, melts incongruently at 820 C [4, 5, 17], whereas
Li2W4O13 melts incongruently at 805 C , 800 C
[7, 13] or at 750 C  with the deposition of Li2W2O7
and WO3 .
Pistorius  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 ,
745 C [5?7] or 754 C . Crystal structure of Li2W2O7
was solved by Okada and co-workers .
Permentier and co-workers  conducing synthesis in
the temperature range of 450?500 C have obtained Li6
W2O9. According to Authors, Li6W2O9 decomposes at
550 C with the formation of Li2WO4 and b-Li4WO5.
Literature survey has shown that Li4WO5 forms two
polymorphic modifications: low-temperature modification,
crystallizing in cubic system a-Li4WO5 and
high-temperature modification, crystallizing in triclinic or
orthorhombic system b-Li4WO5 [7, 15, 20, 22]. At 690 C, a-Li4WO5
undergoes to phase transition to b-Li4WO5 [7, 15].
According to Hauck , b-Li4WO5 melts at 1350 C, but
Rau and co-workers  have shown that it decomposes at
1100 C yielding Li2WO4 and volatile in these conditions
Li2O. a-Li4WO5 can be obtained in different degrees of
order?disorder depending on temperature and time of
synthesis . 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 .
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
The following materials were used for the research: WO3,
99.9% (Fluka AG, USA), and Li2CO3, a.p. (POCh,
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
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
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
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
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, Li2
WO4, a-Li4WO5 and b-Li4WO5 obtained samples were
single phase. In accord with literature data [7, 15] in the
temperature range of 650?700 C, a-Li4WO5 undergoes to
phase transition to high-temperature modification, b-Li4
WO5. 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
Table 2 Results of X-ray phase analysis after consecutive stages of
heating, where Li?Li2CO3, W?WO3, O4?Li2WO4, a?a-Li4WO5,
b?b-Li4WO5, O6?Li6WO6, O7?Li2W2O7, O9?Li6W2O9, O13?
Li2W4O13, O16?Li2W5O16, X?unknown phase
Li2O/% mol Products detected after heating stage at
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, a-Li4WO5 and poor-quality X-ray
data in some other cases makes X-ray phase analysis in the
Li2O?WO3 system very difficult.
Powder diffraction patterns of single-phase samples of
Li2W2O7, Li2WO4, a-Li4WO5 and b-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 b-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,
bLi4WO5. Figure 1 shows fragments of powder diffraction
patterns of orthorhombic Li4WO5 (generated on the basis
of ICDD PDF 00-021-0530) (a), triclinic b-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 b-Li4WO5 (Tables 1, 3) obtained by us is very
similar to diffraction pattern of high-temperature
orthorhombic modification of Li4WO5  and differs to
some extent from PDF 04-010-6772 calculated on the basis
of structural data of b-Li4WO5 . 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
2h angles. As a consequence of it, unit cell parameters of
bLi4WO5 obtained by us are somewhat larger than these
presented by Hoffmann and Hoppe  (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 b-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 b-Li4WO5 obtained by
Hoffmann and Hoppe  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 b-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 b-Li4WO5 recorded after heating stage at
700 C (a) and after additional heating at 1000 C for 2 h
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 b-Li4WO5 obtained in this work and
(c) triclinic Li4WO5 (generated on the basis of ICDD PDF
(b). Results of phase analysis have revealed that sample
after additional heating at 1000 C except the b-Li4WO5
contains also Li2WO4 and Li2O. It is in accord with
literature data informing that at 1100 C b-Li4WO5
decomposes yielding Li2WO4 and Li2O . As there were no
evidences of splitting and overlapping of reflections as a
result of heating at 1000 C, this problem requires further
The single-phase samples of Li2WO4, Li2W2O7 and
aLi4WO5 obtained after heating stages at 650 C as well as
b-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. 2 Comparison of fragments of powder diffraction patterns of
(a) b-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
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
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
bLi4WO5 (3a) and a-Li4WO5 (3b). On the DTA curve of
aLi4WO5 was recorded one small endothermic effect with
onset temperature at 690 C which was accompanied by
small mass loss effect (2%). TG curve of a-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 a-Li4WO5. The endothermic effect with onset at 690 C
can be connected with phase transition to b-Li4WO5. This
is in accord with results of our XRD investigations
(Table 2) indicating run of phase transition leading from
aLi4WO5 to b-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 b-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
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
Fig. 4 DTA (light line) and TG (dark line) curves of pure: (a)
bLi4WO5 obtained after heating stage at 700 C and (b) a-Li4WO5
obtained after heating stage at 650 C
Fig. 5 IR spectra registered for pure samples obtained by the solid
state reaction: (a) Li2WO4, (b) Li2W2O7, (c) b-Li4WO5, (d)
aLi4WO5 and (e) Li2CO3
recorded in the same temperature range on TG curve of
aLi4WO5. This effect can also be connected with desorption
of water adsorbed, this time, by b-Li4WO5 (Fig. 4).
In order to know better properties of obtained phases
and to explain the nature of mass loss effects recorded on
TG curves of a-Li4WO5 and b-Li4WO5, single-phase
samples of Li2WO4, Li2W2O7, b-Li4WO5 and, a-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), b-Li4
WO5 (curve c), a-Li4WO5 (curve d) and for comparison
Li2CO3 (curve e), Fig. 6 shows UV?Vis?NIR/DRS spectra
of b-Li4WO5 (square) and a-Li4WO5 (circle), whereas
Fig. 6 UV?Vis?NIR/DRS spectra of a-Li4WO5 (circle), b-Li4WO5
Fig. 7 UV?Vis?NIR/DRS spectra of Li2W2O7 (square), Li2WO4
Fig. 7 shows UV?Vis?NIR/DRS spectra of Li2WO4
(circle) and Li2W2O7 (square).
Analysis of the number and positions of absorption
bands recorded in the IR spectra of Li2WO4, Li2W2O7,
bLi4WO5 and a-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 a-Li4WO5 (Fig. 5, curve d) weak absorption
bands with maxima at 1430 and 1480 cm-1, characteristic
for carbonates . 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 a-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 a-Li4WO5, but not detectable by
XRD. These both assumptions raise question concerning
real composition of a-Li4WO5.
On the other hand, analysis of the UV?Vis?NIR/DRS
spectra of a-Li4WO5 and b-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 b-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 a-Li4WO5 and b-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, a-Li4WO5 and b-Li4WO5 equal to
4.35, 4.03, 4.00 and 4.12 eV, respectively, were calculated.
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,
aLi4WO5 and b-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
Results of DTA?TG and XRD investigations have
revealed that phases Li2W2O7 and Li2WO4 melt
congruently at 735 and 745 C, respectively, whereas
a-Li4WO5 undergoes a phase transition to b-Li4WO5 at
Results of DTA?TG and IR investigations indicate that
crystal structure of a-Li4WO5 can be stabilized by a
small amount of carbonate groups.
Based on UV?Vis?NIR/DRS investigations, band gap
energies for Li2WO4, Li2W2O7, a-Li4WO5 and,
bLi4WO were calculated.
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