Structural and thermal investigations of Sr2WO5

Article abstract of DOI:10.1016/j.jallcom.2016.11.388

The crystal structure of Sr₂WO₅has been refined using powder X-Ray diffraction (XRD) and neutron diffraction (ND) data. The corner connected WO₆octahedra forms infinite cis-bridged chains along b axis which are further connected by the layer of Sr atoms to give a three dimensional network. Thermogravimetric study revealed that Sr₂WO₅on storage picks up moisture from the surrounding to give a mixture of Sr(OH)₂and SrWO₄. Percentage of Sr(OH)₂in Sr₂WO₅increases with increase of storage time under normal atmospheric condition. The hydrated compound on heating up to 1473?K again yield back Sr₂WO₅. High Temperature X-ray Diffraction (HTXRD) studies of Sr₂WO₅and SrWO₄in vacuum showed positive thermal expansion in the temperature range of 298–1273?K. Thermogram of Sr₂WO₅recorded with Differential Scanning Calorimeter (DSC) showed a reversible phase transition at 423?K. Specific heat capacity of Sr₂WO₅was measured between temperature 463–863?K using heat flux DSC.

Full text of DOI:10.1016/j.jallcom.2016.11.388

Accepted Manuscript
Structural and thermal investigations of Sr WO
2
5
Meera Keskar, S.K. Sali, B.G. Vats, R. Phatak, K. Krishnan, S. Kannan
PII:
S0925-8388(16)33871-3
10.1016/j.jallcom.2016.11.388
JALCOM 39885
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 28 September 2016
Revised Date: 21 November 2016
Accepted Date: 27 November 2016
Please cite this article as: M. Keskar, S.K. Sali, B.G. Vats, R. Phatak, K. Krishnan, S. Kannan, Structural
and thermal investigations of Sr WO , Journal of Alloys and Compounds (2016), doi: 10.1016/
2
5
j.jallcom.2016.11.388.
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ACCEPTED MANUSCRIPT
Structural and thermal investigations of Sr2WO5
Meera Keskar*, S.K. Sali, B.G. Vats, R. Phatak, K. Krishnan, S. Kannan
Fuel Chemistry Division, BARC, Mumbai 400085, India
Abstract
The crystal structure of Sr2WO5 has been refined using powder X-Ray diffraction (XRD) and
neutron diffraction (ND) data. The corner connected WO6 octahedra forms infinite cis-bridged
chains along b axis which are further connected by the layer of Sr atoms to give a three
dimensional network. Thermogravimetric study revealed that Sr2WO5 on storage picks up
moisture from the surrounding to give a mixutre of Sr(OH)2 and SrWO4. Percentage of Sr(OH)2
in Sr2WO5 increases with increase of storage time under normal atmospheric condition. The
hydrated compound on heating up to 1473 K again yield back Sr2WO5. High Temperature X-ray
Diffraction (HTXRD) studies of Sr2WO5 and SrWO4 in vacuum showed positive thermal
expansion in the temperature range of 298–1273 K. Thermogram of Sr2WO5 recorded with
Differential Scanning Calorimeter (DSC) showed a reversible phase transition at 423 K. Specific
heat capacity of Sr2WO5 was measured between temperature 463 to 863 K using heat flux DSC.
Keywords : Alkaline earth tungstates; Powder X-ray diffraction; Thermogravimetric analysis;
structure; Thermal expansion; Heat capacity
* Corresponding author
Email address: mskeskar@barc.gov.in (M. Keskar)

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1.
Introduction
Tungstates, especially alkaline earth tungstates are very interesting class of materials
because of their potential applications in various fields [1-8]. The structural studies of alkali and
alkaline earth tungstates revealed the possibility of doping various cations in the host matrices
for laser applications. Among these tungstates, schellite structured MWO4 (M=Ca, Sr, and Ba)
have numerous industrial applications in quantum electronics, medical field, X-ray phosphors,
cryogenic detectors, catalyst and electron spin resonance studies, in addition to host materials for
laser. [9]. There are several reports on photoluminescence studies of rare earth doped MWO4
systems. [2, 10-12]. MWO4 compounds are also being used as promising materials for
microwave substrate applications [13]. High pressure studies of MWO4 compounds have been
carried and phase transitions in them were discovered using techniques like Raman spectroscopy
and Synchrotron X-ray diffraction [14-17].
The phase equilibrium in SrO-WO3 system is summarized by Grivel and Norby [18]. In
this system, they have reported the formation of SrWO4, Sr2WO5 and Sr3WO6 phases at 1073 K.
Structural and thermal properties of SrWO4 and Sr3WO6 phases are well studied and reported in
literature. Crystal structure of SrWO4 has been carried out by Gurmen et al. using neutron
diffraction data [19]. Study on the growth and thermal properties of SrWO4 single crystal has
been reported by Fan et al. [7]. They have also measured the thermal expansion and specific heat
capacity of the compound using dilatometer and DSC techniques, respectively. Raman study on
the pressure induced structural phase transitions in SrWO4 has been reported by Christofilos et al.
[20]. Study on the phonon properties of SrWO4 has been reported by Goel et al. [21]. The effects
of alkali metal and alkaline earth metal impurities on the synthesis of pure SrWO4 and the
stability of this compound under the influence of a humid atmosphere and water have been
reported by Krazmanc et al. [13]. They have mentioned that materials having significant
tendency to react with the moisture are not a good materials for the applications in electronics,
because of the deterioration of their properties in the presence of a moist atmosphere. Recently,
the structural and vibrational properties of SrWO4 have been studied by angle dispersive
synchrotron X-ray diffraction and Raman spectroscopy measurements and four polymorphs of
the compound were confirmed [14]. Detailed study on structural phase transitions of Sr3WO6 at
different temperatures, has been carried out by King et al. using electron diffraction, synchrotron

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X-ray powder diffraction and neutron diffraction techniques [8]. Enthalpies, entropies and the
standard Gibbs energies of formations of Sr3WO6, Sr2WO5 and SrWO4 have been calculated by
Levitski and Scolis using electrochemical cells [22]. During the luminescence study of double
perovskite structured Sr3WO6:K1+ doped with Eu3+, an intense red emission was observed by
Zhao et al. [23]. In Sr2WO5 compound, only luminescence study has been carried out. Blasse
and Heuvel [24] have reported that at low temperature, Sr2WO5 when activated with hexavalent
uranium produces efficient luminescence at relatively longer wavelength. Ropp [1] has reported
that Sr2WO5 has a tetragonal structure, however no detailed structural study for this compound
is reported.
In the present work, detailed structural and thermal study of Sr2WO5 has been carried out
and the results are discussed. Structure of Sr2WO5 was refined using powder X-ray diffraction
and neutron diffraction data. Thermal stability of the compound was studied using
thermogravimetric analysis technique. Thermo physical properties such as thermal expansion
and specific heat capacity of Sr2WO5 along with SrWO4 and Sr3WO6 were also derived under
identical experimental conditions using HTXRD and DSC techniques, respectively.
2.
Experimental
SrWO4, Sr2WO5 and Sr3WO6 were synthesized by solid state route, by reacting WO3 with
1, 2 and 3 moles of SrCO3, respectively. Reaction mixtures were heated at 1473 K for 10 h with
intermittent grindings at 1073 and 1273 K. As SrCO3 readily picks up moisture, it was preheated
at 873 K for 4 h and stored in desiccators before use.
Formation of pure product phases was identified by XRD, using Rigaku Miniflex-600 X-
ray diffractometer (Bragg Brentano geometry with θ-θ goniometer) with graphite
monochromatised Cu Kα1 radiation at λ=1.5406 Å at the scanning rate of 2° (2θ)/min. For
structural study, powder XRD data of Sr2WO5 was collected in step scan mode between 2θ = 10
to 100° with step interval of 0.02° and counting time 4 sec. The room temperature powder
neutron diffraction (ND) data was collected with the linear 5-PSD based powder diffractometer
at 100 MW Dhruva Research Reactor, Trombay, Mumbai using neutrons of wavelength 1.2443
Å in the two-theta range of 10–110°. Rietveld refinement was carried out using FULLPROF
program [25]. HTXRD data for SrWO4, Sr2WO5 and Sr3WO6 were collected under vacuum (10-5

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mbar) on a STOE X-ray diffractometer using HDK-2.4 Buhler high temperature attachment from
298-1273 K. The experimental conditions and calibration of HTXRD instrument are discussed
elsewhere in detail [26].
The structural and thermal stability of the compounds were checked by recording
Thermogravimetric (TG) and Differential Thermal Analysis (DTA) curves simultaneously.
Thermograms of the compounds were recorded on Mettler Thermoanalyzer
͙
(model:TGA/SDTA851 /MT5/LF1600) in dry air from room temperature to 1273 K at the
heating rate of 10 K min-1. Around 100 mg sample was heated in alumina crucible (Vol. 150µL)
with gas flow rate of 100 mL min-1. All the weight changes were corrected for buoyancy
correction, obtained under identical experimental conditions.
Heat capacity measurements of the compounds were carried out using a heat flux-type
Differential Scanning Calorimeter (DSC, model number DSC 1/700 of M/s. Mettler Toledo
GmbH, Switzerland) from 303 to 863 K. For measurement of heat capacity, classical three-step
method was used for blank, sapphire and sample runs in continuous heating mode under identical
experimental conditions. A thin disc of sapphire (SRM 720) was used as the heat capacity
standard. Heat flow as a function of temperature was measured at the heating rate of 10 K min-1
in high purity argon atmosphere with a flow rate of 20 mL min-1. Powder samples (120-150 mg)
were loaded in 70 µL Pt crucible for heat capacity measurements. The experimental conditions
are discussed in details elsewhere [27]. Average of six heat flow runs were taken for heat
capacity measurements of Sr2WO5(s) and Sr3WO6 (s) and are within ± 2%.
3.
Results and discussion
3.1. Structural study
Powder XRD patterns for the phases SrWO4, Sr2WO5 and Sr3WO6 agreed well with the
reported phases of the compounds and thereby confirming the formation of these phases.
However, 2-3% phase impurity of SrWO4 was observed in Sr2WO5 and Sr3WO6 compounds. As
no structural data is available for Sr2WO5, structural study of Sr2WO5 has been carried out using
powder XRD and ND data. Ba2WO5 was used as a starting model for refinemnt of Sr2WO5
structure [28]. Fullprof software was used for carrying out Rietveld refinement [25]. The profile

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was fitted with Pseudo-Voigt profile function and the background was fitted with six parameters
polynomial. Initially the scaling factor, sample displacement and background parameters were
refined with the unit cell parameters to fit the observed X-ray diffraction data. Subsequently half
width parameters and peak asymmetry correction were also included in the refinements. After
getting a proper match in the profile model, the positional parameters and overall thermal
parameters for Sr and W cations were refined. The refined model was than used for fitting the
neutron diffraction data. The position parameters and overall thermal parameters of the lighter
oxygen atoms were refined. The goodness of the refinements was observed by the residuals (R-
values). The details of the various profile parameters and structural parameters are discussed
below.
Sr2WO5 has orthorhombic crystal lattice with a space group Pnma (No. 62). Lattice
parameters obtained from Rietveld refinement of the powder XRD data are: a = 7.2491(3) Ǻ, b
= 5.5488(2) Ǻ, c = 10.8916(4) Ǻ and volume = 438.1016(1) Ǻ3, which are comparable with
those reported in ICDD card [29]. The density of Sr2WO5 mesured pyknometrically using
toulene as a solvent (ρmeas.=6.63 gm/cc) is comparable with the X-ray density (ρcal.=6.65 gm/cc)
with 4 molecules per unit cell (Z=4). The XRD and ND Rietveld refinement plots for Sr2WO5
are shown in Fig. 1 [A] and [B], respectively. In figures 1A, reflections due to impurity phase
SrWO4 (2-3%) are observed at 2θ =17.96° and 27.64° and are shown by solid circles. The
refined crystal structural parameters and bond lengths along with the average Bond Valance Sum
(BVS) for Sr2WO5 are listed in Tables 1 and 2, respectively. The structure of Sr2WO5 is shown in
Fig. 2 [A]. Structure contains distorted octahedra, which are linked by common corners and form
infinite cis-bridged chains along the b- axis. These extended chains are further connected by the
layers of Sr atoms as shown in Fig. 2 [B]. Sr atoms connect the WO6 octahedra chains and form
10 co-ordinated sphenocorona with oxygen atoms from the adjacent layers. The average BVS
calculated using VaList program for Sr is 2.01 and for tungsten is 6.08 matched well, with the
expected oxidation states for these cations [30].
3.2. Thermal study
3.2.1. Thermal stability

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TG curves of freshly prepared SrWO4, Sr2WO5 and Sr3WO6 recorded in dry air did not
show any weight change up to 1473 K. Moreover, the DTA does not show any indication of an
endothermic or an exothermic peak. These observations confirmed the thermal as well as the
structural stabilities of the compounds. During the structural study of Sr3WO6, King et al. [8]
have stated that the high content of Sr in the compound and its sensitivity to moisture results in
gradual decomposition of the material. In the present study also, it was observed that when
Sr2WO5 and Sr3WO6 were left open in static air (average relative humidity ~ 70 % at 298 K),
they were un stable and readily picked up moisture on storage. XRD patterns of freshly
synthesized Sr2WO5 and Sr3WO6 along with XRD of their stored samples for different durations
are shown in Fig. 3 and Fig. 4, respectively. It is clear from the XRD patterns in the stored
Sr2WO5 and Sr3WO6 additional lines are due to the formation of SrWO4. Sr3WO6 showed the X-
ray lines of additional phase Sr(OH)2, which could not be seen in the XRD pattern of stored
Sr2WO5, probably because, the Sr(OH)2 formed may be below the detection limit of XRD ( 5
wt. %). To confirm the formation of Sr(OH)2, thermogravimetric curves of SrWO4, Sr2WO5 and
Sr3WO6 stored in air for 80 days were recorded and shown in the Fig. 5. From the figure it can be
confirmed that SrWO4 is stable and did not show any weight change up to 1273 K. This is in
agreement with the results reported by Krzmanc et al. [13]. However, Sr2WO5 and Sr3WO6
showed weight changes in two steps between 300 to 700 K and 800 to 1100 K. The first weight
loss step is, due to the loss of moisture whereas the second step corresponds to the
decomposition of strontium hydroxide to strontium oxide. The TG curves of 80 days stored
Sr2WO5 and Sr3WO6 sample showed hydroxide weight loss of 0.55% and 1.1%, respectively.
Also, weight loss due to moisture present in the sample, is more in Sr3WO6 than in Sr2WO5. This
may be explained on the basis of higher strontium content in Sr3WO6 as compared to that of
Sr2WO5. Thus from XRD and TG studies it can be interpreted that on storage, Sr2WO5 and
Sr3WO6 are picking up moisture and partially getting converted into SrWO4 and Sr(OH)2. The
partial conversion of Sr2WO5 and Sr3WO6 after picking up of moisture under ambient conditions
can be given as:
Sr2WO5 + H2O
ꢀ
ꢀ
SrWO4 + Sr(OH)2
(1)
(2)
Sr3WO6 + 2 H2O
SrWO4 + 2 Sr(OH)2

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The above equations reveal that the formation of hydroxide is double in the case of
Sr3WO6 as compared to that of Sr2WO5.
To understand the thermal decomposition of Sr(OH)2, the compound was prepared by
mixing aqueous solutions of Sr(NO3)2 and NaOH in 1:1 molar proportion. The precipitate was
dried with ethyl alcohol. The composition of the precipitate was ascertained from XRD and TG
analysis. XRD analysis showed the composition of the precipitate as anhydrous Sr(OH)2. TG
curve of Sr(OH)2 shown in Fig. 6 indicates the weight losses in three steps between 310 to 400
K, 720 to 880 K and 880 to 1040 K. The first weight loss step is due to the loss of loose
moisture. The second and third steps showed decomposition of molten Sr(OH)2 to SrO (Obs.
weight loss = 14.34) which is in agreement with the calculated weight loss of 14.8% and also
reported by Dinescua and Preda [31].
In order to get the quantitative data on the formation of Sr(OH)2 during storage of
Sr2WO5, synthetic mixtures of SrWO4 containing 10 and 40 mol. % of Sr(OH)2 were prepared
and thermograms of the mixtures were recorded in dry air which are included in Fig. 6. As
discussed above, SrWO4 is a stable compound and does not pick up any moisture on storage.
Thus, TG curves of synthetic mixtures of SrWO4 and Sr(OH)2 showed weight loss between 720
to 1200 K which corresponds to the decomposition of anhydrous strontium hydroxide to
strontium oxide. The observed weight losses for both the compositions matched well with the
expected weight loss and are given in Table 3.
3.2.2. Estimation of Sr(OH)2 formation during storage of Sr2WO5
During the structural study of Sr3WO6 king et al. [8] have mentioned that the compound
absorbs moisture as strontium is moisture sensitive which results in gradual decomposition of the
compound. Since moisture sensitivity of Sr3WO6 is already reported, a systematic study was
carried out for the evaluation of the sensitivity of Sr2WO5 towards moisture using XRD and
thermogravimetric techniques. For this, a freshly prepared Sr2WO5 was exposed to static
atmosphere having average relative humidity ~ 70 % at 298 K. XRD and thermograms of the
stored samples were recorded after 0, 18, 80, 120 and 180 days. TG curves of stored samples
indicate that the weight loss takes place in two steps due to the presence of loose moisture and
decomposition of Sr(OH)2. TG curves obtained by heating the stored Sr2WO5 in dry air are

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shown in Fig. 7. The figure clearly indicates that as Sr2WO5 is stored for longer duration, the
weight loss due to moisture as well as the decomposition of Sr(OH)2 also increases. Thus, TG
and XRD studies of that the stored Sr2WO5 consist of lose moisture and mixture of SrWO4 ,
Sr(OH)2 and Sr2WO5 phases.
The quantitative data on formation of Sr(OH)2 on storage of Sr2WO5 was obtained from
thermogravimetric studies. Table 4 summarizes thermogravimetric data of freshly prepared and
stored Sr2WO5 samples for 0 to 180 days. The weight loss in the temperature range of 800 to
1150 K was used to calculate the formation of hydroxide during storage. Pure anhydrous
Sr(OH)2 showed 14.34 % loss forming SrO as final product as given in Table 3. Assuming the
stored samples as Sr(OH)2, the expected weight loss (X) for complete conversion to SrO was
calculated. By comparing the observed weight loss of stored samples (Y) with (X), the % of
Sr(OH)2 formed was calculated as [(Y)*100]/(X). Equation (1) clearly indicates that during
partial decomposition of Sr2WO5, equal moles of Sr(OH)2 and SrWO4 are formed whose
molecular weights are in 1:2.76 proportions. From the amount of Sr(OH)2 formed, the weights
of SrWO4 was calculated. The quantity of unreacted Sr2WO5 can be obtained by subtracting
amount of [Sr(OH)2 + SrWO4] from the weights of anhydrous samples. Table 4 clearly indicates
that in Sr2WO5 the formation of Sr(OH)2 increases from 0 to 4.8 wt. % on storage up to 180 days
which is equivalent to 15 mole % of Sr(OH)2. Fig. 8 shows the observed % weight loss of
Sr2WO5 vs. number of days of storage at 298 K. From the figure it can be said that the moisture
pick up is very fast initially and then gradually reaches to a saturation value with storage of time.
The figure also shows that within 180 days the hydroxide formation reaches a maximum value.
This study clearly indicates that properties of Sr2WO5 will degrade with storage time and thus
special care needs to be taken to avoid the contact of moisture with the sample.
It was observed that stored Sr2WO5 and Sr3WO6 compounds when reheated at 1473 K,
form respective pure tungstate compounds. This can be explained as, the stored Sr2WO5/Sr3WO6
on absorption of moisture gets partially converted to a mixture of SrWO4 and Sr(OH)2. This
mixture, on heating above 1000 K, decomposes to SrO, which combines in-situ with SrWO4 to
form Sr2WO5/Sr3WO6. XRD patterns shown in Fig. 3 and 4 confirm the above observations.
3.3. Thermal expansion studies

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As no data is available for thermal expansion behavior of Sr2WO5, the same has been
studied in vacuum (10-5 mbar) from 298 to 1273 K using HTXRD technique. Fan et al. [7] have
studied thermal expansion behavior of SrWO4 single crystal up to 773 K using dilatometer.
Thermal expansion behavior of Sr3WO6 has been studied by King et al. [8] using synchrotron
based HTXRD. They have reported phase transitions in Sr3WO6 between 464 and 506 K, but
have mentioned that the lattice parameters increases linearly up to 1373 K without anomalies at
the transitions temperature. However, no thermal expansion coefficients are reported. Therefore,
thermal expansion studies of SrWO4, Sr3WO6 were also carried out in vacuum from ambient to
1273 K. Room temperature XRD data of SrWO4, Sr2WO5 and Sr3WO6 were indexed on
tetragonal, orthorhombic and triclinic systems, respectively. HTXRD patterns of all the three
compounds recorded from ambient to 1273 K were similar except for the shift in the X-ray line
positions to lower 2θ values, indicating the expansion of lattice with rise in temperature. Lattice
parameter and unit cell volume of SrWO4, Sr2WO5 and Sr3WO6 were plotted against the
temperatures and are shown in Fig. 9 [A], [B] and [C], respectively. Percentage thermal
expansion of lattice parameters and cell volumes were calculated using the formula,
Expansion (%) = (aT – a298 ) × 100 / a298
(3)
Where, aT and a298 represents the lattice parameter or volume at temperature T and at 298 K,
respectively.
The variation in lattice parameter ′a′, ′b′, ′c′ and volume (Vol.) of the oxides were fitted to
second order polynomial expressions of the form
a = x1 + y1T + z1T2
b = x2 + y2T + z2T2
c = x3 + y3T + z3T2 and
vol.= x4 + y4T + z4T2
(4)
(5)
(6)
(7)
where T denotes the absolute temperature in K. The coefficients x1, y1 and z1, x2, y2 and z2, x3, y3
and z3 and x4, y4 and z4 of the fitted equations of the lattice parameters a, b, c and vol.,
respectively against the temperature, T (K) for the three compounds are given in Table 5.

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The axial (αa) and volume thermal expansion coefficient (αv) were derived from the
equations ; αa = (∆aT/a298) × (1/∆T) and αv = (∆vT/v298) × (1/∆T), respectively where ∆aT/∆vT is
the difference between lattice parameter/cell volume at any given temperature T (aT/vT) and
lattice parameter/cell volume at room temperature (a298/v298); ∆T is the corresponding
temperature difference. The average axial as well as volume thermal expansion coefficients for
SrWO4, Sr2WO5 and Sr3WO6 are given in Table 6.
3.4. Heat capacity measurements
Specific heat capacity of SrWO4 has been already measured using DSC by Fan et al. [7].
Heat capacities of freshly prepared Sr2WO5(s) and Sr3WO6(s) samples were measured using heat
flux type Differential Scanning Calorimeter (DSC) from 303-863 K. Averages of six runs were
carried out. During collection of the Cp measurement data of Sr2WO5, first heating cycle showed
two endothermic peaks at 362 and 423 K. The cooling cycle showed a single exothermic peak at
417 K. However, all subsequent heating and cooling cycles showed only a single peak at 423 and
417 K, respectively. It has been discussed earlier that Sr2WO5 is moisture sensitive, and thus the
endothermic peak obtained during first heating cycle at 362 K can be assigned to the loss of
moisture. In further heating cycles, the peak due to lose moisture was absent. The peak obtained
at 423 K, observed in all heating cycles indicates structural phase transition in the compound.
During cooling cycle, peak was obtained at 417 K, confirmed reversibility of the phase
transition. Heating and cooling DSC curves obtained during 1st and 2nd cycle are shown in Fig.
10 [A]. ∆H of the phase transition of Sr2WO5 at 423 K was determined from DSC peak area
measurement during heating and cooling cycles using indium (m.p = 429.7 K, ∆H fusion = 3.28
kJ/mol) and zinc (m.p = 692.7 K, ∆H fusion = 7.32 kJ/mol) as standards and was found to be 285
± 15 J/mol. The low value of enthalpy of transition indicates that there is not much change in the
structure during phase transition. The observed DTA and HTXRD studies also could not detect
any phase transition. Multiple phase transitions have been reported for Sr3WO6 from 303 to 1373
K which were detected in temperature programmed synchrotron x-ray powder diffraction,
neutron diffraction and electron diffraction [8]. It has been mentioned that room temperature
triclinic phase transforms to monoclinic phase above 464 K without drastic change in structural
parameters.

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The values of heat capacity of Sr2WO5(s) or Sr3WO6(s) were calculated from heat flow
signals measured by DSC using following relation:
Cp(T )sample = {(HFsample – HFblank)/(HFref. – HFblank)}*(Mref./Msample)*(Cp(T )ref.)
(8)
where HFblank, HFref. and HFsample represent heat flow during first (blank), second (reference) and
third (sample) runs, respectively. Cp(T)sample and Cp(T)ref. are the heat capacities, and Msample and
Mref. are the molar masses of [Sr2WO5(s) or Sr3WO6(s)] and sapphire (s), respectively. Measured
molar heat capacity data for Sr2WO5(s) or Sr3WO6(s) are given in Table 7 and plotted in Fig. 10
[B]. The fitted curves are also shown in the figure as solid lines. The inception in Fig. 10 [B]
indicates a phase transition for anhydrous Sr2WO5 at 421 K and thus Cp data for the same was
collected from 463 to 863 K.
The specific heat capacities of Sr2WO5 and Sr3WO6 fitted as a function of temperature in
the form of Cp = A + BT + C/T2 and are shown below:
Sr2WO5
Cop,m (JꢀK−1ꢀmol−1) = 208.061-18.27×10-3 (T/K)-48.80×105(K/T)2 (463 ≤ T/K ≤ 863)
(9)
Sr3WO6
Cop,m (JꢀK−1ꢀmol−1) = 240.282+5.86×10-3 (T/K)-40.192×105(K/T)2 (303 ≤ T/K ≤ 863)
(10)
The values of constants A, B and C are given in Table 6. As Sr2WO5 shows phase
transition, Cp data for the same was collected from 463 K to 863 K. The inset shown in Fig. 10
[B] depicts the reversible phase transition in Sr2WO5. The relative standard deviations between
observed and fitted values of heat capacity are in the range of 1-2 %.
4.
Conclusions
Crystal structure of Sr2WO5 was derived from powder XRD and neutron diffraction data
using Rietveld profile method. Structure consists of WO6 octahedra forming infinite chains
which are connected by the layers of SrO10 atoms. Compound picks up moisture and partially

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getting converted to form SrWO4 and Sr(OH)2. Strontium hydroxide content increases with the
storage of time and was estimated using thermogravimetric technique. Maximum 15 mol % of
Sr(OH)2 was formed on 180 days of storage. DSC study confirmed reversible phase transition in
Sr2WO5 at 423 K with the enthalpy of transition 285 ± 15 J/mol. Thermal expansion and heat
capacities of the compound were measured in the temperature range of 298-1273 K and 463-863
K, respectively. Thermal expansion of SrWO4 and Sr3WO6 were also studied in vacuum and both
compounds showed positive expansion from ambient to 1273 K.

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(1973) 465–473.

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Table 1
Crystal structure parameters for Sr2WO5 using X-ray and neutron diffraction data.
Lattice Parameters: a= 7.2491(3) Å, b= 5.5488(2) Å, c= 10.8916(4) Å, V= 438.10(3) Å3
Space group: Pnma (No. 62)
XRD :Rp (%) = 8.8, Rwp (%) = 13.3, Rf-factor (%) = 3.11, GoF (χ2) =1.71
Neutron: Rp(%) = 3.46, Rwp (%) = 4.43, Rf-factor (%) = 4.96, GoF (χ2) = 4.4
Atom Site
x
y
z
Biso( Å2) Occupancy BVS*
Sr1
Sr2
W
4c
4c
4c
4a
4c
4c
8d
0.3140(6)
0.0163(5)
0.1882(3)
0
0.25
0.25
0.75
0.5
0.9143(4) 0.68(4)
0.2179(4) 0.65(4)
0.0666(2) 0.13(8)
1
1
1
1
1
1
1
1.882
2.144
6.082
2.008
2.044
2.042
2.007
O1
O2
O3
O4
0
1.58(9)
0.0413(9)
0.2759(7)
0.3364(5)
0.75
0.75
0.2021(5) 1.99(2)
0.8992(5) 0.95(8)
-0.0007(1) 0.1134(3) 1.85(7)
Note: The refinement of crystal structure was done by initially refining the lattice parameters,
atomic positions and thermal parameters Biso of heavy elements (Sr and W) using XRD data. The
obtained model was then used for the refinement of the neutron diffraction data, where refinement
of atomic positions and thermal parameters of lighter atom oxygen was taken into consideration.
* Bond Valance Sum (BVS) was calculated using VaList program [26].

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Table 2
Bond length and bond angle for Sr2WO5.
Bond
Bond length (Å) Bond
Bond length (Å)
Bond
Bond length (Å)
Sr1-O1
Sr1-O1
Sr1-O2
Sr1-O2
Sr1-O3
Sr1-O3
Sr1-O4
Sr1-O4
Sr1-O4
Sr1-O4
<Sr1-O>
2.824(4)
2.824(4)
2.87(2)
Sr2-O1
2.751(4)
2.751(4)
2.786(2)
2.786(2)
2.47(3)
W-O1
W-O1
W-O2
W-O3
W-O4
W-O4
<W-O>
2.077(2)
2.077(2)
1.82(2)
1.93(2)
1.824(2)
1.824(2)
1.9254
Sr2-O1
Sr2-O2
Sr2-O2
Sr2-O3
Sr2-O3
Sr2-O4
Sr2-O4
Sr2-O4
Sr2-O4
<Sr2-O>
2.54(2)
2.793(3)
2.793(3)
2.581(2)
2.903(2)
2.903(2)
2.581(2)
2.7612
2.48(3)
2.935(2)
2.648(2)
2.935(2)
2.648(2)
2.7196
Bond angles
O1
O2
O3
O4
<Oi-W-Oi> in
WO6 octahedra
O1
O2
O3
O4
83.82(8) 84.2(1) 83.46(2) 88.7(8)
84.2(1)
83.46(2)
88.7(8)
96.78(9)
94.02(8)
96.78(9) 94.02(8) 98.66(5)
WO6 bond angle variance = 35.7522 deg2

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Table 3
Thermogravimetric data of pure Sr(OH)2 and synthetic mixtures of Sr(OH)2 + SrWO4.
Sr.
Composition
Wt. of sample Temp range
Wt. loss (%)
No.
(mg)
59.17
(K)
Obs.
Cal
1
2
3
Sr(OH)2
760-1040
820-1190
712-1220
14.34 14.80
0.1Sr(OH)2 +0.9SrWO4
0.4Sr(OH)2 +0.6SrWO4
89.42
76.86
0.55
2.70
0.57
2.88

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Table 4
Thermogravimetric data of Sr2WO5 after storage for different time interval in static air at 298 K
and average relative humidity ~ 70 %.
Sr.
Period of
Wt. of
Temp
range
Wt.
loss
(%)
Wt. (%)
Compositions of stored
compound
No.
sample *
storage (d)
(mg)
(K)
Sr(OH)2
0
SrWO4
0
Sr2WO5
363.3
1
2
0
363.3
-
0
Sr2WO5
18
100.98
773-973
0.34
2.30
6.50
91.20
0.84 Sr2WO5+0.08SrWO4
+ 0.08Sr(OH)2
3
4
5
80
134.98
121.56
248.13
800-1000 0.52
800-1053 0.63
800-1123 0.69
3.61
4.36
4.80
9.96
81.43
83.61
81.90
0.76 Sr2WO5+0.12SrWO4
+ 0.12Sr(OH)2
120
180
12.03
13.25
0.72 Sr2WO5+0.14SrWO4
+ 0.14Sr(OH)2
0.70 Sr2WO5+0.15SrWO4
+ 0.15Sr(OH)2
* Weight of anhydrous sample

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Table 5
Coefficients of the second order polynomial equations a(x1, y1, z1), b(x2, y2, z2), c(x3, y3, z3)
and vol. (x4, y4, z4) used to fit the lattice parameters a, b, c and vol. against temperature, T (K)
for SrWO4, Sr2WO5 and Sr3WO6.
Compd.
x1
y1
z1
x2
y2
z2
x3
y3
z3
x4
y4
z4
(×10-5) (×10-9)
(×10-5) (×10-9)
(×10-5) (×10-9)
(×10-5) (×10-9)
SrWO4 5.4122 1.2629 27.079 5.4122 1.2629 27.079 11.8421 24.912 35.520 346.923
873
4804
3554
Sr2WO5 7.2264 6.5144 49.794 10.8605 15.1593 -3.3625 5.5221 7.7580 4.7037 433.289 1509
Sr3WO6 10.092 5.7229 28.618 17.600 29.602 64.534 11.8010 8.5138 7.4623 1701.462 3973
24072

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Table 6
Average thermal expansion coefficients (obtained in vacuum) and Cp data (collected in argon
atmosphere) for SrWO4, Sr2WO5 and Sr3WO6.
Compd.
Average thermal expansion
*Cp ( J K-1 mol-1)
coefficient [×10-6] K-1
α
α
α
α
A
B
-
C
a
b
c
vol.
SrWO4 7.06
Sr2WO5 12.87 10.06
Sr3WO6
7.06
22.0
14.07
6.94
35.81
37.00
36.97
-
-
-18.27×10-3
5.86×10-3
-48.80×10-5
208.061
240.282
-40.192×10-5
8.43
20.80
*Cp = A + BT + C/T2

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Table 7
Heat Capacity data for Sr2WO5(s) and Sr3WO6(s)
Sr2WO5
Sr3WO6
T/K
Cop,m(T)/J.K-1.mol-1
T/K
Cop,m(T)/J.K-1.mol-1
463.16
483.16
503.16
523.16
543.16
563.16
583.16
177
178
303.16
323.16
343.16
363.16
383.16
403.16
423.16
193.31
206.54
210.53
213.8
216.51
218.84
220.97
178.94
180.27
181.1
181.9
182.53
603.16
623.16
643.16
663.16
683.16
703.16
723.16
743.16
763.16
783.16
803.16
823.16
843.16
863.16
183.02
183.37
183.52
183.39
184.6
443.16
463.16
483.16
503.16
523.16
543.16
563.16
583.16
603.16
623.16
643.16
663.16
683.16
703.16
723.16
743.16
763.16
783.16
803.16
222.62
224.21
225.79
227.05
228.3
185.5
229.21
230.22
231.2
186.8
187.72
188.13
188.06
187.53
186.12
184.32
181.53
232.17
232.93
233.56
233.91
234.73
237.37
239.18
240.32
241.17
241.54
242.01
823.16
843.16
863.16
240.25
236.91
233.82

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Captions of the Figures
Fig. 1. Rietveld refinement plots for [A] powder X-ray diffraction data; [B] Neutron diffraction
data of Sr2WO5. Iobs., Ical., difference (Iobs. – Icalc.) and Bragg reflection positions has
been shown in red, black, blue and green colours, respectively. Blue solid circles indicate
XRD lines of impurity phase SrWO4.
Fig. 2. [A] Crystal structure of Sr2WO5, color code: Sr-Green, W-Black, O-Red; [B] WO6
octahedral zig-zag chain along b-axis.
Fig. 3. XRD patterns of Sr2WO5 freshly prepared [A], stored in air for 120 days [B], 180 days
[C] and reheated at 1473 K [D]; ′+′ =X-ray lines due to SrWO4.
Fig. 4. XRD of Sr3WO6, freshly prepared [A], stored in air for 80 days [B] and reheated at 1473
K [C]; ′+′ and # represent X-ray lines due to SrWO4 and Sr(OH)2, respectively.
Fig. 5. Thermogravimetric (TG) curves of 80 days stored SrWO4, Sr2WO5 and Sr3WO6.
Fig. 6. TG curves of pure Sr(OH)2 [A], synthetic mixtures of 0.1 Sr(OH)2 + 0.9 SrWO4 [B]
and 0.4 Sr(OH)2 + 0.6 SrWO4 [C].
Fig. 7. Thermogravimetric curves of Sr2WO5 stored for different duration of time.
Fig. 8. Weight loss % due to decomposition of Sr(OH)2 vs No. of days Sr2WO5 stored in static
atmosphere.
Fig. 9. Variations of unit cell parameters and cell volume with temperature for SrWO4 [A],
Sr2WO5 [B] and Sr3WO6 [C].
Fig. 10. [A] DSC curve obtained during heating of Sr2WO5 from 300 K to 523 K and [B] heat
capacity (Cp) of Sr2WO5 and Sr3WO6. Inset shows phase transition in Sr2WO5.

ACCEPTED MANUSCRIPT
[A]
[B]
Fig. 1.

ACCEPTED MANUSCRIPT
[A]
[B]
Fig. 2.

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[D]
[C]
*
*
*
*
*
[B]
[A]
*
*
20
30
40
50
60
2 Theta (°)
Fig.3

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[C]
[B]
+
+
#
#
#
#
+
#
#
+
+
#
#
+
[A]
10
20
30
40
50
60
2 Theta (°)
Fig. 4.

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0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
SrWO
4
Sr WO
2
5
Sr WO
3
6
400
600
800
1000
Temperature(K)
Fig. 5.

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0
-5
[B]
-10
-15
-20
[C]
[A]
400
600
800
1000
1200
Temperature(K)
Fig. 6