Ionic conductivity and Raman investigations on the phase transformations of Na4P2O7

Article abstract of DOI:10.1016/S0925-8388(02)00046-4

The ionic conductivity and thermo-Raman spectra of anhydrous sodium pyrophosphate Na₄P₂O₇ were measured dynamically in the temperature range from 25 to 600 °C with a heating rate of 2 °C min^-1 to understand the structural evolution and phase transformation involved. The DSC thermogram was also measured in the same thermal process for the phase transformation investigation. The spectral variations observed in the thermo-Raman investigation indicated the transformation of Na₄P₂O₇ from low temperature phase (ε) to high temperature phase (α) proceeded through pre-transitional region from 75 to 410 °C before the major orientational disorder at 420 °C and minor structural modifications at 511, 540 and 560 °C. The activation energies and enthalpies of the proposed phase transformations were determined. The possible mechanism for temperature dependent conductivity in Na₄P₂O₇ was discussed with the available data.

Full text of DOI:10.1016/S0925-8388(02)00046-4

Journal of Alloys and Compounds 340 (2002) 95–100
L
www.elsevier.com/locate/jallcom
Ionic conductivity and Raman investigations on the phase transformations
of Na P O
4
2
7
a
a
a
a
Raje Gangadharan , Jayaraman Kalaiselvi , Devaraj Shanmukaraj , Balan Palanivel ,
b
a ,
*
Sriramulu Mohan , Ramaswamy Murugan
a
Department of Physics, Pondicherry Engineering College, Pondicherry 605 014, India
b
Raman School of Physics, Pondicherry University, Pondicherry 605 014, India
Received 7 December 2001; accepted 14 January 2002
Abstract
The ionic conductivity and thermo-Raman spectra of anhydrous sodium pyrophosphate Na P O were measured dynamically in the
4
2
7
2
1
temperature range from 25 to 600 8C with a heating rate of 2 8C min to understand the structural evolution and phase transformation
involved. The DSC thermogram was also measured in the same thermal process for the phase transformation investigation. The spectral
variations observed in the thermo-Raman investigation indicated the transformation of Na P O from low temperature phase (´) to high
4
2
7
temperature phase (a) proceeded through pre-transitional region from 75 to 410 8C before the major orientational disorder at 420 8C and
minor structural modifications at 511, 540 and 560 8C. The activation energies and enthalpies of the proposed phase transformations were
determined. The possible mechanism for temperature dependent conductivity in Na P O was discussed with the available data.
 2002
4
2
7
Elsevier Science B .V . All rights reserved.
Keywords: Inorganic materials; Optical properties; Ionic conduction; Optical spectroscopy
1
. Introduction
indicated that before melting at 995 8C the Na P O
4 2 7
undergoes the following transformation [2,3].
Phosphates are among the best glass formers. Their
glass formation tendency can be associated with the ability
of phosphates to form one or two-dimensional structures.
To understand the ionic conductivity of mixed pyrophos-
phate systems, the detailed investigations on ionic con-
ductivity and phase transformations of Na P O are very
390 8C
504 8C
512 8C
550 8C
Na P O (´) → (d ) → (g ) → (b) → (a)
4
2
7
However, these two sets were the same.
Although the polymorphic structures and the phase
transformation of Na P O were studied by several tech-
4
2
7
much essential. Sodium pyrophosphate Na P O exhibits
4
2
7
4
2
7
rich variety of phase transformation from room tempera-
ture to 600 8C. Earlier investigations on the polymorphism
of Na P O indicated that there were at least 5–6 poly-
niques the nature and sequence of phase transformation are
not known completely yet. Raman spectroscopy has the
advantage in the identification of compositions and phases
of a solid from vibrational bands. Raman spectra measured
dynamically as a function of temperature is thermo-Raman
spectroscopy (TRS) and has been applied successfully in
the in-situ investigation of solid-state phase transforma-
tions and composition changes [4–9]. Hence in this work
the thermo-Raman, ionic conductivity and thermal analysis
DSC (differential scanning calorimetry) measurements
were utilized extensively to derive a better microscopic
understanding of the phase transformations and sequence
of the phase transformations involved in Na P O .
4
2
7
morphic structures. Farr [1] labeled the five phases of
Na P O as phase V, IV, III, II and I, reported the
4
2
7
transformation temperatures were as:
4
00 8C
510 8C
520 8C
540 8C
Na P O (V) → (IV) → (III) → (II) → (I)
4
2
7
Recent thermal, X-ray and ionic conductivity studies
*
Corresponding author.
E-mail address: moranamurugan@yahoo.com (R. Murugan).
4
2
7
0
925-8388/02/$ – see front matter
 2002 Elsevier Science B .V . All rights reserved.
PII: S0925-8388(02)00046-4

9
6
R. Gangadharan et al. / Journal of Alloys and Compounds 340 (2002) 95–100
2
.3. Ionic conductivity measurements
The anhydrous sample from desiccator was compacted
as pellets of 1 cm diameter and 0.6 mm thick; the applied
pressure was 5.5 kbar. To ensure good homogeneity and to
decrease the inner porosity of the pellets, a heat treatment
at 250 8C for 48 h was applied. The pellet was introduced
between silver electrodes of high purity. The contacts
between pellet and electrode was improved by pressure
applied through a steel compression spring arrangement.
The bulk resistance of the sample from 25 to 600 8C was
measured using Keithley meter. The bulk conductivity of
Na P O was calculated using the relation s 5 t/RA,
4
2
7
where t is the thickness of the sample, A is area of cross
section of the electrode electrolyte contact. The tempera-
ture dependent conductivity plot of Na P O in the form of
log s vs. 1000/T is shown in Fig. 2. The activation energy
is calculated based on Arrhenius dependence of the
conductivity from the slope of the log s vs. 1000/T plot.
4
2
7
Fig. 1. DSC thermogram of Na P O .
4
2
7
2
. Experimental
2
.4. Thermo-Raman studies
2
.1. Sample preparation
Thermo-Raman spectra were measured by the excitation
Anhydrous Na P O was prepared from analytical grade
4
2
7
of the laser light (30 mW) at a wavelength of 514.5 nm
from an argon ion laser (Coherent, Innova 100-15). The
scattered light was collected at right angles, dispersed by a
spectrometer (Spex, 0.5 m) and detected by a CCD camera
Na P O 10H O (Showa Chemicals) by heating up to
4
2
7
2
2
1
1
50 8C with a heating rate of 1 8C min . The sample was
kept at 150 8C for 1 h for complete removal of hydrated
water. Then this powder has been desiccated under vac-
uum. This anhydrous powder has been used for all the
measurements.
(
Princeton Instrument, 102431024 pixels). The sample
from desiccator was packed in the sample holder and kept
in a homemade oven. The thermo-Raman spectra were
recorded under nitrogen atmosphere. For the phase trans-
formation studies, the temperature range was set from 25
2
.2. Thermal analysis
2
1
to 600 8C with a heating rate of 2 8C min . The tempera-
ture was monitored by a thermocouple and controlled by a
programmable controller. The positions of the spectral
bands were calibrated using an argon spectral tube and the
The DSC (Seiko ISSC 5040) thermogram of anhydrous
Na P O measured from 25 to 600 8C with the heating rate
4
2
7
2
1
of 2 8C min in a flow of nitrogen with a flow rate of 100
2
1
2
1
resolution was about 2 cm . The typical thermo-Raman
spectra of Na P O observed in this investigation are
ml min
indicated five endothermic peaks at 407, 517, 523, 548 and
57 8C.
is shown in Fig. 1. The DSC thermogram
4
2
7
presented in Fig. 3.
5
3
. Results
3
.1. Thermal analysis
The DSC thermogram of Na P O shown in Fig. 1
4
2
7
indicates five endothermic peaks at 407, 517, 523, 548 and
57 8C. The enthalpy variation calculated for the endo-
therms in DSC at these temperatures were 9.4, 1.33, 0.54,
5
2
1
0
.98 and 1.44 mJ mg , respectively.
3
.2. Ionic conductivity measurements
The temperature dependent conductivity plot of
Na P O shown in Fig. 2 indicates several temperature
4
2
7
Fig. 2. Temperature dependence conductivity of Na P O .
domains. Between room temperature and 75 8C the con-
4
2
7

R. Gangadharan et al. / Journal of Alloys and Compounds 340 (2002) 95–100
97
afterwards an appreciable increase in the conductivity was
observed up to 390 8C and the activation energy in this
temperature range was found to be 0.90 eV. After this, a
jump in the conductivity curve was observed at around
4
20 8C with the drop of activation energy to 0.76 eV. Then
a gradual increase in the conductivity was observed up to
45 8C except very slight jumps in the conductivity curve
5
at around 505 and 515 8C. A change in the conductivity
value was again observed at around 550 8C with the drop
of activation energy to 0.92 eV. With further increase of
temperature a slight deviation in the conductivity curve
was observed in the range from 560 to 575 8C.
3
.3. Thermo-Raman studies
4
2
The diphosphate ion P O consists of two PO tetra-
2
7
4
hedra with a central P–O–P bridge bond [10]. The internal
4
7
2
21
modes of P O are in the range from 300 to 1500 cm
.
2
The stretching modes of PO usually occur in the range
4
2
1
from 1250 to 975 cm and the bridge stretching modes of
2
1
PO are in the range from 975 to 675 cm . Other bands
4
2
1
can be grouped in to two regions from 450 to 620 cm
and around 340 cm corresponding to the deformation
2
1
modes of PO and P–O–P, respectively. The symmetric
4
2
1
stretching mode of PO observed at around 1027 cm is
4
the most intense Raman band for the room temperature
Raman spectrum of anhydrous Na P O . The band posi-
4
2
7
tions of Raman bands measured at various temperatures
and their tentative vibrational assignments are given in
Table 1.
Fig. 3. Typical thermo-Raman spectra of Na P O .
4
2
7
Thermo-Raman spectra measured in the temperature
interval from room temperature to 75 8C did not show any
modification in the spectral profile. However, the spectral
variation observed in the thermo-Raman spectra in the
temperature interval from 75 to 415 8C as shown in Fig. 4
indicates the continuous but minute change in the spectral
profile from 75 to 235 8C and appreciable modifications
ductivity of Na P O does not vary much and the activa-
tion energy calculated in this range was 0.049 eV, which is
4
2
7
very low, the characteristics of ´-phase of Na P O . With
4
2
7
further increase in temperature it is clear that there is a
gradual increase in the conductivity up to 260 8C and
Table 1
2
1
The band positions (in cm ) for different phases of Na P O
4
2
7
a
a
a
a
a
Phase ´
Phase ´PT
412 8C
Phase d
518 8C
Phase b
535 8C
Phase a
547 8C
Band assignment
2
5 8C
1
1
1
145
130
116
1128
1111
1023
1110
1024
1109
1022
987
1108
1021
PO3 asym. str.
PO3 sym. str.
2
1027
9
87
881
732
616
580
567
536
523
493
338
327
875
725
614
579
566
535
524
489
342
327
722
720
720
P–O–P sym. str.
588
571
531
590
568
529
567
529
PO3 sym. bend1P–O–P bend
484
347
481
343
479
342
PO3 asym. bend
PO2 twist
a
Tentative assignment.

9
8
R. Gangadharan et al. / Journal of Alloys and Compounds 340 (2002) 95–100
Fig. 4. Thermo-Raman spectra of Na P O measured in the temperature
4
2
7
interval from 75 to 415 8C corresponding to the transformation from
-phase to ´-pretransitional phase.
´
Fig. 5. Thermo-Raman spectra of Na P O measured in the temperature
4
2
7
interval from 416 to 424 8C corresponding to the transformation from
-pretransitional phase to d-phase.
´
afterwards up to 415 8C. The Raman spectrum measured at
15 8C clearly indicates the shift in the band positions of
symmetric stretching mode of PO from 1027 to 1023
4
around 511 8C. In addition, the bands at 1110, 1024, 588,
4
2
1
21
cm . During this course of change, three bands at 1145,
484 and 347 cm shifted to 1108, 1022, 590, 481 and 342
2
1
21
1
130 and 1116 cm became overlapped and appeared as a
cm , respectively. The temperature range covered for this
2
1
broad band centered at 1111 cm with a shoulder at 1128
transformation was from 509 to 513 8C.
2
1
21
21
cm . The band at 881 cm shifted to 875 cm and
became broad and weak. The remaining weak bands were
also broadened as shown in the Raman spectrum measured
at 415 8C.
As shown in Fig. 6 the thermo-Raman spectra measured
in the temperature interval from 526 to 552 8C revealed the
2
1
disappearance of the hump at 987 cm and the band at
2
1
590 cm and signaling another clear phase transforma-
tion. The temperature range covered by this transformation
was from 532 to 548 8C and the transformation tempera-
ture was 540 8C. Thermo-Raman studies also indicate
minor spectral variations, i.e. the broadening the bands at
Thermo-Raman spectra observed in the temperature
range from 416 to 424 8C as shown in Fig. 5 indicated a
clear phase transformation. The most distinct variation in
this temperature interval was the appearance of a small
2
1
21
hump at 987 cm close to the symmetric stretching band
720, 573, 533 and 480 cm in the temperature interval
2
1
of PO at 1024 cm at around 420 8C. In addition to this
from 550 to 570 8C. However, it was difficult to conclude
these modifications were due to phase transformation, as
there were only broadening of bands and no distinct
variation in spectra.
4
major variation some more minor change also observed in
the spectral profile. The bands observed at 579, 489 and
2
1
21
3
42 cm shifted to 588, 484 and 347 cm , respectively.
21
The bands at 875 and 614 cm had disappeared during
this transformation. The temperature range covered for this
transformation was from 418 to 423 8C.
4. Discussion and conclusion
The spectral variation in the temperature range from 509
to 515 8C indicated another phase transformation. The
According to Leung et al. [11] Na P O crystallizes at
4
2
7
2
1
hump at 987 cm
became weak and observed as a
ambient conditions in orthorhombic space group P2 2 2 .
1 1 1
shoulder to the symmetric stretching band of PO4 at
Some studies also reported that it belongs to hexagonal

R. Gangadharan et al. / Journal of Alloys and Compounds 340 (2002) 95–100
99
rearrangements of atoms i.e. an onset rotation of P O , the
2
7
skeleton structure was nearly the same as at room tempera-
ture.
2
1
The appearance of small but distinct hump at 987 cm
close to the strong symmetric stretching band of PO at
4
2
1
1
024 cm in the thermo-Raman spectra in the tempera-
ture range from 416 to 424 8C signaled the clear disorder
in the framework of P O . An appreciable jump in the
2
7
conductivity value and an enthalpy variation of 9.4 mJ
2
1
mg for an endothermic peak in DSC in this temperature
range also confirms the order disorder transformations. The
observation of minor spectral variation in thermo-Raman,
slight jump in conductivity value and weak endothermic
2
1
peak with enthalpy variation of 1.33 mJ mg
in DSC
implied moderate order disorder transformation in the
temperature interval from 509 to 515 8C.
The major spectral variation of disappearance of the
2
1
21
hump at 987 cm
and the band at 590 cm
in the
thermo-Raman, an appreciable conductivity jump and an
2
1
endothermic peak with enthalpy variation of 0.98 mJ mg
in the temperature at around 540 8C indicates an another
possible order disorder transformation. The absence of
distinct spectral variation except broadening of all the
Raman active bands in the temperature interval from 550
to 570 8C indicated the modifications might be due to
thermal effect.
The typical thermo-Raman spectra shown in Fig. 3 were
probably from ´-phase (at 72 8C), ´-pretransitional phase
(at 412 8C), d-phase (at 435 8C), b-phase (at 518 8C) and
a-phase (at 547 8C) of Na P O . The transformation
4
2
7
Fig. 6. Thermo-Raman spectra of Na P O measured in the temperature
4
2
7
observed in this work were for ´-phase to ´-pretransitional
in the temperature intervals 75 to 410 8C, ´-pretransitional
phase to d-phase at around 420 8C, g-phase to b-phase at
interval from 526 to 552 8C corresponding to the transformation from
b-phase to a-phase.
5
11 8C, b-phase to a-phase at 540 8C and a-phase to high
structure [12]. Although the exact crystal structures of
other phases were not well known, the available crystallo-
graphic data signaled that they were more similar to each
other and belong to the ordered version of hexagonal
phase.
temperature phase at 560 8C. Although the earlier studies
predicted the phase d to g-transformation at around
505 8C, no distinct spectral variation was observed in the
thermo-Raman for the phase transformation in this tem-
perature range.
The DSC thermogram of Na P O showed continuous
The ionic conductivity measurement in this work indi-
cates the Na P O materials behave as medium ionic
4
2
7
deviation in the base line from 75 to 400 8C with a slight
dip at around 260 8C. The ionic conductivity measurements
carried out in this work also distinguish two temperature
ranges in conductivity measurement below 260 8C with
low activation energy (0.049 eV) and between 260 and
4
2
7
conductors even at high temperature. The thermo-Raman
spectra and activation energy measured in different tem-
perature domains in this work may be useful to propose the
mechanism of temperature dependent conductivity in this
material. The temperature dependent conductivity of
Na P O depends on the number of mobile charge carriers
3
90 8C with high activation energy (0.90 eV). The thermo-
Raman studies of Na P O indicated the continuous but
4
2
7
4
2
7
minute change in the spectral profile from 75 to 235 8C and
appreciable modifications afterwards up to 415 8C. The
observation of gradual modification in the vibrational
bands in the P O region in this temperature range before
the first transformation was an indication of a pretransi-
tional region in advance of the first structural transforma-
tion at around 420 8C. The pretransition is considered to be
a preparatory stage caused by the structural changes not
associated directly with the formation of the new phase. In
the pretransitional region however there has been some
available and possible sites for them. In Na P O structure
4 2 7
the P O group consists of two PO4 tetrahedra with a
2
7
central P–O–P bridge bond constitute relatively rigid
structure. Hence, for the diffusion of charge carriers, i.e.
for the diffusion of sodium ions, the presence of defect is
essential. The absence of dramatic spectral variation in the
thermo-Raman during the transformation from ´-phase to
´-pretransition phase indicates the skeleton structure of
Na P O remains the same, except for the relative position
2
7
4
2
7
and orientation of P O group. During the ´-pretransition
2
7

1
00
R. Gangadharan et al. / Journal of Alloys and Compounds 340 (2002) 95–100
to d-phase and b to a-phase transformations the thermo-
Raman spectra indicated there is an appreciable disorder in
the P O group, hence, the sodium ions diffuse freely in
ment of Chemistry, NTHU, Hsinchu, Taiwan, ROC for
extending the Raman facilities.
2
7
the lattice and there is an increase in conductivity and drop
in activation energy. However, with the available data it
was difficult to probe the mechanism for the changes
observed in the conductivity and activation energies in the
remaining phases.
References
[1] T.D. Farr, Phosphorus, properties of the element and some of its
compounds, Chemical Engineering Report No. 8, T .V .A, Wilson
Darn, Alabama, 1950, p. 39.
In all the spectral variations observed in thermo-Raman
investigation, ionic conductivity measurement and thermal
analysis indicate the transformation of Na P O from low
[
2] J. Berak, T. Znamierowski, Roczniki Chem. 46 (Part III) (1972)
921.
3] S. Villain, E. Nigrelli, G. Nihoul, Solid State Ionics 116 (1999) 73.
4
2
7
1
temperature (´) to high temperature (a) phase proceeds
through continuous modification in the crystal structure
from 75 to 410 8C, major orientational disorder of P O
[
[4] R. Murugan, A. Ghule, H. Chang, J. Appl. Phys. 86 (1999) 6779.
[5] R. Murugan, A. Ghule, H. Chang, J. Phys. Condens. Matter. 12
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(
2000) 67.
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2000) 2157.
7] C. Bhongale, A. Ghule, R. Murugan, H. Chang, J. Therm. Anal. Cal.
65 (2001) 891.
group at 420 8C and minor structural modification at 511,
40 and 560 8C. The spectral variations observed in the
[
[
5
(
thermo-Raman studies signalled that except the major
orientational disorder at 420 8C the other phase transforma-
tions involved very minor structural deviations. The ther-
mo-Raman investigations carried out on Na P O provided
[8] R. Murugan, H. Chang, J. Chem. Soc., Dalton Trans. 20 (2001)
125.
9] A. Ghule, R. Murugan, H. Chang, J. Inorg. Chem. 40 (2001) 5917.
3
4
2
7
[
better microscopic understanding of the phase transforma-
tions compared to the earlier investigations.
[
[
[
10] D.M. MacArthur, C.A. Beevers, Acta Crystallogr. 10 (1957) 428.
11] K.Y. Leung, C. Calvo, Can. J. Chem. 50 (1972) 2519.
12] Joint Council for Powder Diffraction Standards. Powder Diffraction
File 10-0187, 1997, Inorganic Phases, International Center for
Diffraction Data, 1601 Park Lane, Swarthmore, PA 19081, USA,
Acknowledgements
1
992.
The author is thankful to Professor Hua Chang, Depart-