β-Na2TeO4: Phase Transition from an Orthorhombic to a Monoclinic Form. Reversible CO2 Capture

Article abstract of DOI:10.1021/acs.inorgchem.8b00993

The present work concerns the tellurate Na₂TeO₄ which has a 1D structure and could then present a CO₂ capture ability. It has been synthesized in a powder form via a solid-state reaction and structurally characterized by thermal X-ray diffraction experiments, Raman spectroscopy, and differential scanning calorimetry. The room temperature structure corresponds to the β-Na₂TeO₄ orthorhombic form, and we show that it undergoes a reversible structural transition near 420 °C toward a monoclinic system. Ab initio computations were also performed on the room temperature structure, the Raman vibration modes calculated, and a normal mode attribution proposed. In agreement with our expectations, this sodium oxide is able to trap CO₂ by a two-step mechanism: Na⁺/H⁺ exchange and carbonation of the released sodium as NaHCO₃. This capture is reversible since CO₂ can be released upon heating by recombination of the mother phase.

Full text of DOI:10.1021/acs.inorgchem.8b00993

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
β‑Na2TeO4: Phase Transition from an Orthorhombic to a Monoclinic
Form. Reversible CO2 Capture
Cyrille Galven,† Thierry Pagnier,‡ Noel Rosman,‡ Francoise Le Berre,†
̧
̈
and Marie-Pierre Crosnier-Lopez*,†
†Institut des Molec
Mans CEDEX 9, France
́
ules et Mater
́
iaux du Mans (IMMM), UMR CNRS 6283, Le Mans Universite,
́
Avenue Olivier Messiaen, 72085 Le
‡Universite
́
Grenoble Alpes, Universite Savoie Mont Blanc, CNRS, Grenoble INP, LEPMI, 38000 Grenoble, France
́
S
* Supporting Information
ABSTRACT: The present work concerns the tellurate Na2TeO4
which has a 1D structure and could then present a CO2 capture
ability. It has been synthesized in a powder form via a solid-state
reaction and structurally characterized by thermal X-ray diffraction
experiments, Raman spectroscopy, and differential scanning
calorimetry. The room temperature structure corresponds to the
β-Na2TeO4 orthorhombic form, and we show that it undergoes a
reversible structural transition near 420 °C toward a monoclinic
system. Ab initio computations were also performed on the room
temperature structure, the Raman vibration modes calculated, and a
normal mode attribution proposed. In agreement with our
expectations, this sodium oxide is able to trap CO2 by a two-step mechanism: Na+/H+ exchange and carbonation of the
released sodium as NaHCO3. This capture is reversible since CO2 can be released upon heating by recombination of the mother
phase.
The tellurate Na2TeO4 which has a 1D structure with the
Na+ ions located between [TeO4]n2n− chains can be therefore a
good candidate for the CO2 capture via a Na+/H+ exchange.
This mechanism has been detailed in works on lithium oxides
for which the CO2 capture ability was associated with a Li+/H+
exchange and the formation of Li2CO3.10−18 In Na2TeO4, the
Na+/H+ exchange has been proved to be reversible by
Kratochvil and Jensovsky:19 They obtain their Na2TeO4 crystals
by slow dehydration of Na2H4TeO6 in an NaNO3 melt from
300 °C, and those Na2TeO4 crystals spontaneously rehydrate in
wet atmosphere or in water to give Na3H5Te2O10. This
indicates clearly that Na+ ions are mobile in the structure, a
prerequisite for the CO2 capture through the formation of
NaHCO3.
INTRODUCTION
■
The capture and recycling of atmospheric CO2 is a major goal
in the fight for a cleaner world.1 Numerous technologies have
been described for this purpose in the literature (absorption
with a solvent, selective membrane separation, CO2 absorbents
for example). Among CO2 absorbents, oxides containing alkali
metals can be used as CO2 traps,2−4 trapping which can also be
reversible,5 allowing thus cyclic processes of CO2 capture and
use. If a few lithium oxides have been studied for this
application in the last 10 years,6−10 then some studies11,12 have
shown that sodium use is preferable to lithium: For example,
Na2ZrO3 presents much better properties for CO2 capture than
those of lithium zirconate. Moreover, sodium is more abundant
than lithium and therefore cheaper. Then, in the search for new
materials able to capture CO2, we focused on sodium-based
oxide and in particular compounds in which the sodium ions
must be able to leave the oxide structure. This will be best done
with materials of low dimensionality, containing chains or
planes of structure oxide groups. This is the reason why we
were interested in tellurates containing Te with a +VI formal
oxidation level. Indeed, these compounds, which are nowadays
extensively investigated as new LTCC (low-temperature co-
fired ceramics) dielectric systems for microwave applications,13
crystallize in various dimensionality, from 0D (isolated
octahedra, H6TeO6) to 1D (chains of TeO6 octahedra,
Na2TeO4), to 2D (planes of TeO6 octahedra, H2TeO4) or
even 3D (β-TeO3). Reference 13 lists many of these materials.
In 1977, the crystallographic structure of Na2TeO4 was
determined concurrently by two groups with two different
results. While Kratochvil and Jensovsky19 found an ortho-
rhombic structure (Pbcn; 5.798, 12.24, and 5.214 Å), Daniel et
al.20 obtained a monoclinic one (P21/c; 10.632, 5.161, and
13.837 Å, β = 103.27°) under hydrothermal conditions from a
mixture of NaOH and TeO2 at 600 °C. In 1985, Lapasset et al.
21
confirmed the dimorphism of Na2TeO4 and concluded that
the transition from one form (α-Na2TeO4, monoclinic) to the
other (β-Na2TeO4, orthorhombic) was difficult and irreversible,
due to a need for large rotations and translations of some of the
Received: April 12, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00993
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
sample has been heated from RT to 550 °C at a rate of 10 °C min−1
and cooled from 550 °C to RT at the same rate.
TeO6 chains, despite their probably very close energy. In 2000,
another preparation way was published:22 It corresponds to a
ceramic route which consists of heating at 600 °C of a mixture
of Na2CO3 and TeO2. Unfortunately, no structural details were
given except the cell parameters of a new orthorhombic cell
(10.602, 10.622, and 8.506 Å). In addition, the authors give one
table with 12 hkl and their corresponding dobs and dcal, but our
refinement performed from these few data does not lead to
their published parameters. More recently, a new Na2TeO4
form (denoted as γ-form, orthorhombic Pnna: 13.5390, 8.4169,
and 6.7781 Å) was obtained by Weil et al.23 as minority phase
from treatment of Ba[TeO2(OH)4] in a NaNO3 melt.
Nevertheless, whatever the synthesis mode (except that of
the ref 22), the three Na2TeO4 polymorphs contain edge-
The TGA curve has been registered between RT and 550 °C on a
TGA Jupiter STA 449 F3 Netzsch instrument coupled with a mass
spectrometer Aelos QMS 403C Netzsch allowing the analysis of gas
̈
emitted by the sample during heating.
Ab Initio Calculations. Ab initio calculations were performed with
the ABINIT software.27 A Perdew−Burke−Ernzerhof GGA func-
tional28 was used for the exchange-correlation calculations. FHI
pseudopotentials29 were obtained from the Abinit Web site. The
energy cutoff was held at 45 Ha (1224 eV) and the k-points grid was
that proposed by the software, containing 4 independent k-points.
Raman vibrational frequencies and normal modes were calculated
using generalized forces as described in the Supporting Information. A
correcting factor of 1.077 was applied to the calculated frequencies to
take into account the underestimation inherent to the technique. This
factor was chosen to minimize the distance between observed and
calculated Ag modes.
2n−
sharing TeO6 octahedra forming [TeO4]n chains and could
then be interesting candidates toward CO2 capture.
In addition to the CO2 capture property, we present in this
paper the thermal stability of the β-Na2TeO4 orthorhombic
form. Furthermore, except for a room temperature Raman
spectrum appearing in Cornette’s thesis,13 there was no work
on Na2TeO4 vibrational properties. The Raman spectrum was
then measured as a function of temperature while ab initio
computations were performed on the crystallographic struc-
tures that we determined in order to compare their energy.
Scanning Electron Microscopy (SEM) Analysis. SEM images
were obtained using a JSM 6510 LV microscope (JEOL) equipped
with an energy-dispersive X-ray (EDX) OXFORD detector (AZtec
software) to perform elementary microanalyses. For SEM observa-
tions, the particles were coated with gold.
RESULTS AND DISCUSSION
■
Characterization of the Room Temperature Phase.
The different PXRD patterns registered have rapidly revealed
that this compound presented a strong preferential orientation.
This is clearly showed in Figure 1 where two PXRD patterns
EXPERIMENTAL SECTION
■
Synthesis. We chose to prepare our β-Na2TeO4 sample via a solid-
state reaction. Powders of TeO2 (Acros Organics, 99.8%) and dried
Na2CO3 (Alfa Aesar, 99.99%) in stoichiometric ratio were intimately
mixed in an agate mortar and pressed into pellets. The pellets (1 g;
diameter: 10 mm) were placed in an alumina crucible and heated two
times at 650 °C for 12 h in air. Powder X-ray diffraction (PXRD)
analysis was used after each heating cycle to check the completeness of
the reaction as well as the crystallinity and the purity of the product.
After the synthesis, the obtained white powder was kept immediately
in a dryer to prevent any unwanted hydration taking into account the
study of Kratochvil and Jensovsky.19
Powder X-ray Diffraction. For the structural room temperature
(RT) study, powder X-ray diffraction (PXRD) data were collected in
air with a PANalytical X’pert Pro diffractometer using Cu Kα radiation
equipped with a X’Celerator detector with the following experimental
conditions: angular range, 2θ = 13.55−150.00°; step scan increment,
2θ = 0.0084°; counting time = 12 h.
For the thermal analysis, a PANalytical Empyrean diffractometer
equipped with a high-temperature attachment HRK900 was used with
the Cu Kα radiation. The data were collected under dry air flow every
50 °C between 50 and 600 °C with the following experimental
conditions: angular range, 2θ = 13.00−148.00°; step scan increment,
2θ = 0.0131°; counting time = 6 h.
Figure 1. PXRD pattern of Na2TeO4 at room temperature showing
the strong preferential orientation in the case of a crushed sample.
All the refinements were performed with the Rietveld method24
using the Fullprof program,25 and a pseudo-Voigt function was chosen
to model the peak shape. The background points were determined
manually before being refined.
are presented: one with a crushed sample and a second one
with a sprinkled sample. We indeed observe significant
differences in the relative intensities of the hkl lines on the
two diagrams. This preferential orientation can be explained by
the elongated platelet aspect of the grains evidenced by SEM
image (see Figure S1). This lead us to perform the structural
study at room temperature with the data obtained from a
carefully sprinkled sample on grease to promote a random
orientation of the crystallites. The hkl lines are narrow,
indicating a good crystallization of the powder sample. In
addition, the purity of the sample is quite good since all the
lines could be indexed in the orthorhombic Pbcn (no. 60) space
group with refined cell parameters (a = 5.75845(2), b =
Raman Spectroscopy. The Na2TeO4 Raman spectra were
collected on a pellet with a Renishaw InVia Raman spectrometer in
backscattering mode. The exciting source was an Ar laser operating at
the 514.5 nm green light. The sample was illuminated through a ×50
objective with a long (8 mm) working distance. Raman spectra down
to about 7 cm−1 were obtained with the use of superNotch filters. The
detector was a Peltier-cooled CCD. For the temperature measure-
ments, a Linkam THMS600 cell was used. In order to accurately
measure the sample temperature, we placed a small silicon piece close
to the sample. The temperature was then calculated according to the Si
520 cm−1 band position as described in ref 26.
Thermal Analysis. Differential scanning calorimetry (DSC, 92
Setaram) was used in order to precisely determine the thermal
behavior of Na2TeO4. For this study performed under N2 flow, the
B
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Figure 2. Observed, calculated, and difference PXRD patterns of β-Na2TeO4 (sprinkled sample) at room temperature in the Pbcn SG. Vertical bars
are related to the Bragg reflections positions.
a
Table 1. Room Temperature Structure Data of Na2TeO4 Obtained by Rietveld Refinement
space group Pbcn (no. 60)
a = 5.75845(2) Å, b = 12.22603(5) Å, c = 5.18998(2) Å
[a = 5.757 Å, b = 12.207 Å, c = 5.229 Å]
Rp = 11.1, Rwp = 10.9, Rexp = 5.19, χ2=4.39, Bragg R-factor = 5.36
η = 0.615(6); half-width parameters: u = 0.0121(3), v = −0.0111(3), w = 0.0086(1)
atom
site
x
y
z
Biso
Te
4c
4c
4c
8d
8d
0
0
0
0.07152(4) [0.0723]
0.7717(2) [0.7746]
0.3610(2) [0.3610]
0.3262(3) [0.3258]
0.5476(3) [0.5506]
1/4
1/4
1/4
1.15(1)
2.62(4)
2.62(4)
1.47(5)
1.47(5)
Na1
Na2
O1
0.3012(6) [0.2982]
0.3190(5) [0.3202]
0.6150(6) [0.6117]
0.5723(6) [0.5710]
O2
a
The results of the ab initio calculations are given in square brackets.
12.22603(5), and c = 5.18998(2) Å) in agreement with the β-
Na2TeO4 form of ref 19.
value is consistent with the sum of the ionic radii (1.96 Å) from
Shannon’s table31 and leads to a bond valence sum equal to 5.7
instead of the formal value of 6. Classically, the two shortest
Te−O distances correspond to the terminal O2− ion (O1)
We decided then to start the Rietveld calculations with the
Na2TeO4 structural model published by Kratochvil and
Jensovsky.19 With one isotropic temperature factor attributed
for each kind of atom, the agreement between calculated and
experimental patterns is good with the final reliability factors:
Rp = 11.1, Rwp = 10.9, Rexp = 5.19, χ2=4.39, Bragg R-factor =
5.36 (Figure 2). The structural refinement results are given in
Table 1 with atomic coordinates and isotropic temperature
factors, while selected interatomic distances and bond valence
sums30 are gathered in Table 2.
2n−
which is not linked to the neighboring Te of the [TeO4]n
chain. In the distorted NaO6 octahedra, Na−O distances range
from 2.221(4) to 2.611(4) Å for Na1 and from 2.340(3) to
2.653(4) Å for Na2, leading to mean distances respectively
equal to 2.381 and 2.532 Å which can be compared to the
corresponding sum of the ionic radii:31 2.42 Å. For the two
sodium sites, the calculated bond valence sums are respectively
1.4 and 0.9 for Na1 and Na2 (expected value: 1). All these
values are very close to those obtained from the single crystal
study: The bond valence sums of Te, Na1 and Na2 calculated
from the atomic positions of ref 19 are respectively equal to 5.6,
1.4, and 0.9. We can note here that the two sodium ions are
clearly not equivalent, which may have an effect on their
mobility thereafter.
The crystal structure can be described as built up from
infinite chains of TeVIO6 octahedra sharing edges parallel to the
2n−
c axis (Figure 3). The chains of [TeO4]n
formulation are
linked together by the sodium cations in a very distorted
octahedral coordination. The Te−O distances range from
1.835(4) to 2.014(3) Å with a mean distance of 1.953 Å. This
C
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Table 2. Selected Interatomic Distances (Å) and Bond Valence Sums (Σν) for Na2TeO4 at Room Temperature (S.G.: Pbcn) and
a
at 550°C (P21/c)
Te octahedra
Na octahedra
RT
550 °C
RT
550 °C
Te−O1: 2 × 1.835(4)
Te−O2: 2 × 1.993(3)
Te−O2: 2 × 2.014(3)
⟨Te−O⟩ = 1.953 Å
Te−O2: 1.892(6)
Te−O4: 1.944(7)
Te−O3: 1.951(6)
Te−O1: 1.983(6)
Te−O3: 2.096(6)
Te−O4: 2.148(7)
⟨Te−O⟩ = 2.00 Å
Na1−O1: 2 × 2.221(4)
Na1−O1: 2 × 2.311(3)
Na1−O2: 2 × 2.611(4)
⟨Na1−O⟩ = 2.381 Å
Na1−O1: 2.074(8)
Na1−O2: 2.247(9)
Na1−O2: 2.287(9)
Na1−O3: 2.534(8)
Na1−O1: 2.699(8)
⟨Na1−O⟩ = 2.37 Å
(Na1−O4: 2.948(8))
Σν = 5.7, Σνexpected = 6
Σν = 1.4, Σνexpected = 1
Na2−O2: 2 × 2.340(3)
Na2−O1: 2 × 2.603(3)
Na2−O1: 2 × 2.653(4)
⟨Na2−O⟩ = 2.532 Å
Na2−O4: 2.259(8)
Na2−O3: 2.357(8)
Na2−O2: 2.512(7)
Na2−O1: 2.523(8)
Na2−O4: 2.772(8)
⟨Na2−O⟩ = 2.49 Å
(Na2−O2: 2.930(8))
Σν = 0.9, Σνexpected = 1
a
Mean distances are given in italic.
2n−
Figure 3. Projection of the Na2TeO4 structures showing the [TeO4]n chains.
Figure 4 shows the Raman spectrum of Na2TeO4 at room
temperature together with its decomposition into Lorentzian
bands. It has to be pointed out that band intensities varied from
point to point, indication of local preferential orientations of
the grains. Apart of this, the experimental spectrum is identical
to the one published by Cornette in her thesis.13 From the
group theory, 42 vibrations are expected to be Raman-active (9
Ag, 12 B1g, 9 B2g, 12 B3g). Twenty-three bands have been
observed on the experimental spectrum. The band wave-
numbers obtained experimentally are listed in Table 3, together
with the calculated values obtained by Cornette with the LADY
software13 and those obtained by our ab initio calculations. All
these results agree fairly well, which confirms the orthorhombic
structure of β-Na2TeO4 at room temperature prepared by the
solid-state reaction from TeO2 oxide and Na2CO3 carbonate.
The band attributions are also provided in Table 3. In many
cases, two bands belonging to two different symmetries appear
very close to each other, Ag with B2g and B1g with B3g. This is
particularly clear at high frequencies. The highest frequencies
correspond to Te-non bridging oxygen O1 stretching.
Surprisingly, the second group (647.7−673.0 cm−1) corre-
sponds to a O2−O2 stretch, suggesting a rather strong bond
between these two oxygen atoms. Te−O2 stretching
frequencies appear slightly below, near 520 cm−1. The Na1
translations appear between 300 and 236 cm−1, while those of
Na2 appear at lower frequencies, below 230 cm−1. For the
atomic displacements ab initio calculated for all modes; see
Tables S1−S4.
Structure evolution with temperature. Figure 5 shows
the evolution of the PXRD patterns obtained when heating the
sample up to 600 °C. A change is highlighted between 350 and
450 °C corresponding simultaneously to the appearance of a set
of new hkl lines and a peak splitting (for example near 2θ =
31.5°) leading to a lowering of the symmetry. This trans-
D
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Figure 4. Raman spectrum of Na2TeO4 at room temperature. Insert: expanded view of the low intensity region.
formation is reversible since the PXRD pattern obtained at
room temperature after this heating can be indexed in the
mother cell with the initial Pbcn (no. 60) space group. In
addition, this phase transition is also well observed by thermal
analysis: While no weight loss can be depicted between RT and
650 °C on the TGA curve, the DSC graph clearly shows a
reversible peak at 421 °C on heating and 420 °C on cooling
(Figure 6), a peak which can be unambiguously associated to
the structural transition observed on PXRD patterns.
In good agreement with those observations, while the PXRD
patterns obtained between RT and 400 °C can be refined
successfully in the initial orthorhombic cell using the Pbcn space
group, the PXRD patterns registered at higher temperature
(450−600 °C) require the use of a monoclinic cell to index all
the observed hkl lines. The Figure 7 gives the evolution versus
temperature for each cell parameter: If the cell parameter
parallel to the chains axis (close to 5.2 Å) remains nearly
constant, then the two others increase leading to a continuous
evolution of the cell volume. Nevertheless, a careful
examination of these four evolutions shows an accident at the
transition temperature. The refined cell parameters and the cell
volume for each temperature are gathered in Table S5.
Figure 8 shows the Raman spectra obtained during heating in
dry synthetic air. The high-frequency modes do not vary much,
except for a band widening evidenced clearly above 400 °C. At
low frequency, however, changes are more drastic, with an
apparent splitting of the band near 200 cm−1 and the
appearance of a very strong band at low frequency, close to
18 cm−1. All these changes are fully reversible, with no
hysteresis.
Characterization of the High Temperature Structure.
As it is impossible at high temperature to use grease to limit the
preferential orientation, we tried in a first time to quench the
sample to stabilize the high temperature form at RT.
Unfortunately, even the quench in liquid nitrogen (sample
placed in a sealed platinum tube) did not allowed to retain the
monoclinic structure. The structural study has then been
performed with a diffractometer equipped with a high
temperature attachment from data collected at 550 °C on a
sample carefully sprinkled. All the hkl lines of the
corresponding PXRD pattern could be successfully indexed in
a conventional unique axis b monoclinic cell (a = 5.18890(4), b
= 5.85867(5), and c = 12.4260(1) Å, β = 90.2085(4)°) and the
study of the systematic absences leads to the P21/c space group
(no. 14). We notice here that this group is that used by Daniel
et al.20 for the structural description of α-Na2TeO4 prepared by
hydrothermal conditions from a mixture of NaOH and TeO2 at
600 °C (10.632, 5.161, and 13.837 Å, β = 103.27°). If those two
parameters sets are different, then we can nevertheless remark
2n−
that the parameter corresponding to the [TeO4]n chains is
preserved. The refined angle β is very close to 90°, indicating a
very small distortion from the orthorhombic structure. Indeed,
replacing (aM, bM, cM) by (−cO, aO, −bO) and neglecting β gives
the orthorhombic structure. The Rietveld calculations are then
started in the P21/c space group with only one Te position
transposed simply from Pbcn to P21/c according to the
following:
Pbcn, 4c site = 0, y, 0.25 → P21/c, 4e site = 0.25, 0, y
In this P21/c space group, the three atomic coordinates of
that 4e site must be refined according to the international
tables. After their refinement, Fourier difference calculations
revealed clearly six new 4e sites. From the calculated distances
between these sites and the Te one, the two first can be
attributed to the Na+ ions, while the last four ones,
corresponding to less intense peaks, are related to O2−
positions. With this atomic distribution and assuming an
isotropic thermal motion for each kind of atom as for the RT
study, the refinement (Figure 9) converged quickly to good
reliability factors: Rp = 11.5, Rwp = 9.49, Rexp = 6.26, χ2=2.30,
Bragg R-factor = 4.71. Table 4 gathered the structural results at
550 °C for Na2TeO4, while selected interatomic distances are
given in Table 2. As in the RT part, the crystal structure at 550
°C (Figure 3) can also be described as built up from infinite
[TeO4]n2n− chains of TeVIO6 octahedra sharing edges parallel to
the a axis (cell parameter close to 5.2 Å). In these chains, the
Te−O distances are larger than the room temperature ones,
ranging from 1.892(6) to 2.148(7) Å (mean distance: 2.00 and
1.95 Å at RT). Moreover, the TeVIO6 octahedra are quite
distorted with edge length varying from 2.52 to 3.03 Å (for
comparison: 2.50−2.85 Å at RT), the two shortest (2.50 Å at
RT and 2.52 Å at 550 °C) corresponding to the O−O edge
2n−
shared by two successive TeO6 octahedra. The [TeO4]n
chains can be described from Te−O2−Te rings (Figure 10)
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Table 3. Normal Mode Frequencies, Calculated and Experimental, and Attributions for Room Temperature Orthorhombic
a
Na2TeO4
calcd
ref 13
796
exp
attribution
800.0 (B2g)
790.3 (Ag)
760.7 (B1g)
747.4 (B3g)
673.0 (B1g)
670.9 (B3g)
649.5 (B2g)
647.7 (Ag)
529.6 (B1g)
523.9 (B3g)
522.1 (B2g)
519.4 (Ag)
422.4 (B3g)
410.1 (B2g)
400.7 (B1g)
399.9 (Ag)
371.0 (B2g)
345.3 (Ag)
794
784
759
sym. stretch Te−O1
asym stretch Te−O1
796
784
784
653
653
624
624
540
539
529
529
431
666
642
522
O2−O2 stretch out of phase along Te−O22−Te chains
O2−O2 stretch in phase along Te−O22−Te chains
Te−O2 stretch
O1−Te−O1 wagging
O2−Te−O2 scissor + O1−Te−O1 scissor
O1−Te−O1 wagging
O2−Te−O2 scissor + O1−Te−O1 scissor
O1−Te−O1 twisting
411
403
371
340
343 (B2g)
324
313
295
306
301
284
256
222
242
221
187
236
155
160
165
176
151
137
140
48
390
356
334
324.1 (B3g)
319.7 (B1g)
300.3 (Ag)
295.2 (B2g)
295.2 (B1g)
283.7 (B3g)
279 (B1g)
O1−Te−O1 rocking, O2−Te−O2 rocking, Na (x, z) translation
314
290
Na1 y translation + O2−Te−O2 scissor + O1−Te−O1 twisting
O2−Te−O2 scissor + O1−Te−O1 twisting
Na1 x translation + O2−Te−O2
Na1 z translation + O2−Te−O2 rocking + O2−Te−O2 rocking
Na1 (x, z) translation + O2−Te−O2 rocking
Na1 y translation
274
259
249
242
224
206
255.3 (B2g)
251.3 (B3g)
236.8 (B3g)
209.7 (Ag)
206.7 (B1g)
198.9 (B1g)
196.0 (B3g)
189.1 (B2g)
189.0 (Ag)
178.0 (B3g)
166.6 (B2g)
165.6 (B1g)
140.2 (Ag)
112.2 (B1g)
91.8 (B3g)
52.7 (B1g)
31.7 (B3g)
Na1 (x, z) translation
Na1 (x, z) translation + Na2 (x, z) translation
Na2 y translation + Te y translation + O2−Te−O2scissor + O1−Te−O1 scissor
Na2 (x, z) translation
Na1 (x, z) translation + Na2 z translation + O1−Te−O1 wagging
Na2(x, z) translation + O1−Te−O1 wagging
Na1 y translation + Na2 y translation + O2−Te−O2 y translation
Na1 y translation
182
Na1 (x, z) translation + Na2 (x, z) translation
Na2 y translation
Na1 (x, z) translation + Na2 (x, z) translation
Na2 y translation
137
112
71
106
103
81
Na2 (x, z) translation
Na2 (x, z) translation
46
Na2 (x, z) translation
58
28
Te x translation + O1−Te−O1 rocking + Na1 (x, z translation) + Na2 (x, z) translation
The calculated wavenumbers have been multiplied by a factor of 1.077. Earlier calculations by Cornette13 are also included.
a
that are quite similar in the two RT and HT forms: All the
characteristic distances and angles are close. Whatever the
symmetry (orthorhombic or monoclinic), each ring belongs to
a plane and the angle between two successive planes is about
It has been impossible to obtain the same monoclinic
distortion by ab initio calculations. Starting from the
experimental values, we obtain a stable structure very close,
not to say identical, to the orthorhombic low temperature one
(Table 4). Attempts were also made to start with a more
deformed structure, by increasing the β angle, but calculations
converged always toward a structure very close to ortho-
rhombic, with identical Te−O2−Te bridges.
Concerning the counting of the Raman-active modes in the
monoclinic structure, 42 modes (21 Ag and 21 Bg) are expected
as in the orthorhombic one. The Ag and B3g modes of the
orthorhombic structure transform into Ag, and B1g and B2g
transform into Bg modes. Therefore, no real band splitting is
expected at the transition. However, because of the close value
2n−
88° (88.53 at RT and 87.34° at 550 °C). The [TeO4]n
chains are linked together by the sodium cations in an
octahedral coordination that is even more distorted than at
room temperature: the Na−O distances are ranging from 2.07
to 2.95 Å for Na1 and from 2.26 to 2.93 Å for Na2. The two
distances 2.95 and 2.93 Å being very long, one can consider
here an (5 + 1) environment instead of an octahedral one. All
these results mean that the two forms are very close from a
structural point of view.
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very low wavenumbers, near 20 cm−1. A band exists in this
region in the orthorhombic structure spectrum, but we have at
the moment no explanation for this sudden intensity change at
the transition.
Study of the CO2 Capture. According to our previous
works on garnets and Ruddlesden−Popper phases and the
observations of Kratochvil et al.,19 we decided to test the ability
of β-Na2TeO4 toward the CO2 capture. This test can be rapidly
realized by using this following procedure: A platinum crucible
with about 0.3 g of Na2TeO4 powder is placed in a reactor (125
cm3) containing 10 drops of water.10 A CO2 pressure of 10 bar
is applied, and the reactor is heated at 140 °C for 14 h. After a
natural cooling of the furnace, the powder is first dried at 60 °C
in an oven before recording a PXRD pattern. This PXRD
pattern reveals first the complete disappearance of the mother
form β-Na2TeO4 (Figure 11a,b). It also shows the presence of
NaHCO3 (Powder Diffraction File no. 17−3645, Joint
Committee on Powder Diffraction Standards, 2017) and two
sets of hkl lines, one fine and the other broad. The resulting
mixture is then heated under argon flow in a TG analyzer
coupled with a mass spectrometer (Figure 12). On heating
between RT and 550 °C, the TG curve shows a total mass loss
close to 16% with a three steps process: the first step around
120 °C, the second one near 420 °C, and the third one around
475 °C. Each one is clearly associated with simultaneous
departure of CO2 and H2O. After the TG/MS analysis, the
PXRD pattern registered showed only the presence of β-
Na2TeO4 (Figure 11c) indicating thus that the β-Na2TeO4
transformation in humid CO2 is reversible and that a heating at
550 °C allows to recover entirely the mother phase. At this
stage, we tried to find a cell able to index all the hkl lines of the
PXRD pattern excluding the NaHCO3 ones. This was possible
with an orthorhombic cell of parameters a ≈ 4.829, b ≈ 12.822
and c ≈ 5.200 Å (see Figure S2). As expected due to the PXRD
pattern characteristics (coexistence of large and thin hkl lines),
the reliability factors were not satisfying; however, we can
nevertheless note that the cell parameters are relatively close to
those of the mother Na2TeO4: a = 5.75845(2), b =
12.22603(5), and c = 5.18998(2) Å. Moreover, the preserved
cell parameter corresponds to the one running along the
infinite TeO6 octahedra chains (≈5.19 Å for the both forms).
All these results suggest a strong structural relationship between
Figure 5. Evolution of β-Na2TeO4 PXRD pattern under heating
showing the appearance of new hkl lines (★) and the splitting of a hkl
▲
line due to the transition orthorhombic → monoclinic. ( : Kα3, Kα4,
and Kβ contributions). Insert: zoom of the peak splitting resulting of
the structural transition. For more clarity, we decided to show only half
of the patterns recorded (every 100 °C).
of modes with different symmetries in the orthorhombic
structure which transform into modes with the same symmetry,
apparent splitting can occur; two bands with the same
symmetry and energy interacting to give two new bands
separated by a larger gap. This seems to appear in the 400 and
500 cm−1 ranges and more clearly near 200 cm−1. A major
2n−
change in the [TeO4]n chains observed at the transition is
the appearance of two different Te−O2−Te bridges, which in
turn give rise to two different O−O distances: 2.527 Å for the
O3−O3 bond and 2.597 Å for the O4−O4 one. In the
orthorhombic structure, we have attributed the band group
near 640 cm−1 to the stretching of these bonds. We thus expect
that two bands will replace the single one at the transition.
What we observe is in fact a strong enlargement of the most
intense band, from 17−20 cm−1 below 400 °C to 30 cm−1
above the transition temperature. This could be due to the
small difference between both O−O distances. Another feature
observed at the transition is the existence of a strong band at
Figure 6. DSC curve showing the endothermic peak associated to the structural transition orthorhombic → monoclinic of β-Na2TeO4.
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Figure 7. Evolution of the cell parameters (Å) and of the volume (Å3) of Na2TeO4 versus temperature from PXRD data refinements.
Figure 8. Raman spectra of Na2TeO4 at various temperatures (°C) in dry synthetic air.
H
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Figure 9. Observed, calculated, and difference PXRD patterns of Na2TeO4 at 550 °C in the P21/c SG. Vertical bars are related to the Bragg reflection
positions.
a
Table 4. 550 °C Structure Data of Na2TeO4 Obtained by Rietveld Refinement
space group P21/c (no. 14)
a = 5.18890(4) Å, b = 5.85867(5) Å, c = 12.4260 (1) Å, β = 90.2085 (4)°
[a = 5.2288 Å, b = 5.7566 Å, c = 12.2066 Å, β = 90.0009°]
Rp = 11.5, Rwp = 9.49, Rexp = 6.26, χ2 = 2.30, Bragg R-factor = 4.71
η = 0.475(9); half-width parameters: u = 0.044(1), v = −0.0287(7), w = 0.0094(1)
atom
site
x
y
z
Biso
Te
4e
4e
4e
4e
4e
4e
4e
0.2507(2) [0.25000]
0.743(2) [0.75000]
0.209(1) [0.25000]
0.373(2) [0.38813]
0.869(2) [0.88811]
0.435(2) [0.42916]
0.949(2) [0.92915]
0.0329(1) [0.00000]
0.0419(8) [0.00000]
0.0165(8) [0.00001]
0.191(2) [0.29815]
0.121(1) [0.20182]
0.149(2) [0.17992]
0.208(2) [0.17992]
0.07141(5) [0.07230]
0.2356(3) [0.22540]
0.3842(3) [0.36099]
0.6487(5) [0.67427]
0.8046(5) [0.82719]
0.9319(6) [0.94949]
0.0291(6) [0.05050]
0.67(3)
3.47(8)
3.47(8)
2.6 (1)
2.6 (1)
2.6 (1)
2.6 (1)
Na1
Na2
O1
O2
O3
O4
a
The results of the ab initio calculations are given in square brackets.
the mother phase β-Na2TeO4 and the one formed after the
ROR experiment. In order to obtain more information on this
new phase formed during the CO2 test, we decide to observe it
using a SEM and to perform an EDX analysis. First, we do not
observe two types of grains with the retrodiffused electrons
detector, thus implying that this sample is homogeneous versus
the chemical composition despite the presence of two phases
on the corresponding PXRD pattern. We think that each
crystallite of Na2TeO4 releases Na+ cations on its entire surface
leading to the presence of NaHCO3 on the whole surface of
each grain. This seems to be confirmed by the SEM image (see
Figure S3) showing a particle consisting of a layer-coated core.
Moreover, the EDX analyses of the core and the layer show that
the amount of tellurium is very low in the layer compared to
that of the core.
All these combined results are in favor of the following
mechanism:
Na2TeO + xH2O + xCO2
4
↔ Na2−xHxTeO + xNaHCO3
4
To check this hypothesis, 0.5 g of Na2TeO4 was introduced in
100 mL of water. Under stirring, a nitric acid solution (2 M)
was progressively added until the pH remained acidic and
constant. After 12 h, the solid product was recovered by
filtration, washed with distilled water, and finally dried in an
oven at 60 °C. The filtrate obtained after the washing step has
then been analyzed with an atomic emission spectrometer. The
presence of sodium has been confirmed and corresponded to
71% of the initial sample. A PXRD pattern of the solid has also
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2n−
Figure 10. [TeO4]n chains showing the Te−O−Te rings with their characteristic distances.
DSC peak area as equal to about 7 J g−1. Above 420 °C,
Na2TeO4 crystallizes in the monoclinic P21/c space group, and
its structure has been characterized from Rietveld treatment of
its powder X-ray diffraction pattern. Whatever the temperature,
Na2TeO4 structure can be described as built up from infinite
chains of TeO6 octahedra sharing edges and running parallel to
the preserved cell axis during heating (close to 5.2 Å, c axis in
the orthorhombic form, and a axis in the monoclinic one). The
chains [TeO4]n2n− are linked together by the sodium cations in
an octahedral coordination whose distortion increases with
temperature. The major effect of the transition is to replace
identical Te−O2−Te cycles by two slightly different cycles.
It can be noted here that this transition from an
orthorhombic to a monoclinic symmetry is uncommon since
in general a heating is associated with a symmetry increase. We
can nevertheless find some examples of such unexpected
behavior in the literature. For instance, the 3D perovskite
Li0.75Ba3Nb2.25O9 undergoes around 1300 °C a structural
transition from a cubic to a hexagonal cell due to an ordering
of Li+ and Nb5+ cations, which are distributed on the same
crystallographic site at room temperature.32 More recently, two
polymorphs Li4B2O5 transforms into each other at about 618
°C: The low-temperature form crystallizes in an orthorhombic
cell while that of the high-temperature is monoclinic.33
We have also shown that β-Na2TeO4 orthorhombic form
allows the reversible CO2 capture through the formation of
NaHCO3 by a two-step mechanism:
Figure 11. β-Na2TeO4 PXRD pattern: mother phase (a), after CO2
test (b), after CO2 test and TGA (c), and after 12 h in HNO3 (d).
been registered: It is similar to that of the compound obtained
after the ROR experiment with the coexistence of large and thin
hkl line (Figure 11d). All these results demonstrate that
Na2TeO4 allows the reversible CO2 capture and confirm the
proposed mechanism: an ionic Na+/H+ exchange leading to the
formation of a Na2‑xHxTeO4 phase and NaHCO3.
Na2TeO + xH2O + xCO2
4
↔ Na2−xHxTeO + xNaHCO3
4
This mechanism shows also that β-Na2TeO4 can be used as
humidity sensor. Surprisingly, this reaction of β-Na2TeO4
orthorhombic form with CO2 and H2O is in contradiction
with the study of Kratochvil and Jensovsky: For them, Na2TeO4
crystals spontaneously rehydrate in wet atmosphere or in water
to give Na3H5Te2O10.19
CONCLUSION
■
The structure of the β-Na2TeO4 orthorhombic form (Pbcn
space group), prepared via a solid-state reaction from TeO2 and
dried Na2CO3, have been studied as a function of temperature
between room temperature and 650 °C from PXRD data, DSC
analysis, and Raman spectroscopy. Ab initio calculations have
also been performed. The room temperature orthorhombic
structure has been confirmed and a phase transition occurring
near 420 °C toward a monoclinic structure has been
highlighted. This transition is fully reversible and the
corresponding enthalpy change can be estimated from the
A structural study of the partially Na+/H+ exchanged form
Na2−xHxTeO4 is complex due to the coexistence of large and
thin hkl lines in the PXRD pattern. However, we were able to
propose a set of cell parameters for this compound. As the cell
parameter parallel to the chains axis is preserved, it is
2n−
reasonable to think that the chains [TeO4]n are still present
in the exchanged form Na2−xHxTeO4. This work will be the
subject of a forthcoming paper.
J
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Figure 12. TGA curve and MS results of Na2TeO4 after the CO2 test.
Diffraction des Rayons-X” and “Plateforme de Microscopie
Electronique”.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00993.
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*E-mail: marie-pierre.crosnier-lopez@univ-lemans.fr.
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Marie-Pierre Crosnier-Lopez: 0000-0002-8676-7063
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