Available online at www.sciencedirect.com
Solid State Ionics 178 (2008) 1811 – 1816
www.elsevier.com/locate/ssi
Proton conduction and characterization of an
La(PO3)3–Ca(PO3)2 glass–ceramic
Guojing Zhang a , Rong Yu a,1 , Shashi Vyas a,c , Joel Stettler a,c , Jeffrey A. Reimer b,c ,
Gabriel Harley a,b , Lutgard C. De Jonghe a,b,⁎
a
Materials Sciences Division, Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
b
Department of Materials Science and Engineering, University of California, Berkeley, CA94720, USA
c
Department of Chemical Engineering, University of California, Berkeley, CA94720, USA
Received 22 August 2007; received in revised form 19 November 2007; accepted 26 November 2007
Abstract
A dual phase glass–ceramic composite is produced by heating a CaLa(PO3)5 glass at 800 °C for 20 h. The glass–ceramic consists of
intertwined crystalline La(PO3)3 and amorphous Ca(PO3)2 with an overall conductivity of 1.52 × 10− 5 S/cm− 1 at 550 °C in humidified air. For the
water vapor treated glass, the vibration modes of the incorporated water, as well as of P–OH groups, are detected by infrared spectroscopy in the
range 3440–1660 cm− 1. The conductivity of the glass–ceramic is higher in humidified air than in dry air, consistent with proton conduction.
Published by Elsevier B.V.
Keywords: Rare earth phosphates; Glass–ceramic; Proton conduction
1. Introduction
Proton conduction has been found in a wide variety of systems,
such as perovskite oxides [1–4], solid acids [5], phosphates and
doped phosphates [6–16]. Among these materials, acceptordoped La(PO3)3 glass and glass–ceramics, and related doped rare
earth phosphates offer the possibility of leading to interesting
proton conducting electrolytes. For example, Amezawa et al.
found that after treatment in water vapor at 1125 K for 25 h, the
conductivity of 1 mol% Sr doped La(PO3)3 glass–ceramic was
almost ten times higher that of an undoped one [17], showing a
proton conductivity of 1× 10− 4 S/cm at 700 °C, comparable to that
of some perovskites at similar temperatures. His XRD results
indicated the presence of crystalline La(PO3)3 and LaPO4 after heat
treatment. Since crystalline Sr-doped LaPO4 was reported to have a
lower conductivity than crystalline Sr-doped LaP3O9 [18],
⁎ Corresponding author. Materials Sciences Division, Ernest Orlando
Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
Tel.: +1 510 486 6138; fax: +1 510 486 4881.
E-mail address: [email protected] (L.C. De Jonghe).
1
Presently at Department of Materials Science and Metallurgy, University of
Cambridge, Cambridge CB2 3QZ, U.K.
0167-2738/$ - see front matter. Published by Elsevier B.V.
doi:10.1016/j.ssi.2007.11.038
Amezawa et al. argued that the enhanced conductivity in the
1 mol% Sr doped La(PO3)3 glass–ceramic was associated with the
Sr-doped LaP3O9 phase. The conductivities of Sr-doped LaP3O9
ceramics increased with increasing Sr contents up to 5 mol% [19].
In the present study, the system La(PO3)3–Ca(PO3)2, for
which a phase diagram is available [20], was explored.
Transmission electron microscopy with energy dispersive Xray spectroscopy was used to characterize the morphology and
composition of a glass and glass–ceramics. The local environment of phosphate groups and the possible incorporation of
protons were probed by infrared spectroscopy. 31P NMR was
used to characterize the different chemical environments of
phosphorus in the glass. The conductivity of the glass and glass–
ceramic was compared with literature values of related materials.
2. Experimental
2.1. Synthesis of CaLa(PO3)5 glass and glass–ceramic
A CaLa(PO3)5 glass was prepared as follows. Powders of
CaCO3, La2O3 and (NH4)2HPO4 were mixed in a platinum
crucible in the molar ratio 1: 0.5: 5, and heated on a hot plate
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G. Zhang et al. / Solid State Ionics 178 (2008) 1811–1816
while stirring. The mixture released copious amounts of gas,
presumably water, as well as NH3 as CO2, eventually forming a
viscous paste. The paste was brought to 1300 °C, in air, and held
at that temperature for 0.5 h. The resulting melt was poured onto
a preheated stainless steel plate, in air, forming a clear glass.
Parts of this CaLa(PO3)5 glass were transformed at 800 °C for
20 h, in air, to form a glass–ceramic. Additionally, samples were
ground and held at 550oC for 24 h in dry air (designator: “dry”)
or for 24 h at 550 oC in humidified air with pH2O ~ 4 kPa
(designator: “wet”).
2.2. Characterization of glass and glass–ceramics
X-ray diffraction (XRD- CuKα) was used to detect crystallization phases. Transmission electron microscopy (TEM) with
energy dispersive spectroscopy (EDS) was used to characterize
the morphology and composition of the glass and glass–
ceramic. The local environment of metaphosphate groups
and the possible presence of protons were probed in the range
400–4000 cm− 1 with a Nicolet 6700 FTIR spectrometer on
ground specimens mixed with KBr, and on 0.3 mm thick
polished “wet” bulk samples, free of KBr.
Solid state 31P NMR spectra were collected on a LapNMR
(Tecmag) spectrometer operating at 26.56069 MHz with a Doty,
magic angle spinning (MAS) probe DSI-945. All NMR spectra
were collected at a spinning rate of 11 kHz. The 90° pulses were
2 µs and the recycle delays between subsequent scans were set
to 600 s to ensure complete relaxation. All chemical shifts are
reported with respect to 85% phosphoric acid.
2.3. Conductivity measurement of glass and glass–ceramics
The glass or glass–ceramic samples were cut, polished, and
platinum mesh was attached with Pt paste for electrical
conductivity measurements. The total conductivity of the
glass or glass–ceramic was determined from Cole–Cole plots
obtained by a Solartron 1260 impedance analyzer. The
frequency range was 1 MHz to 0.1 Hz, with an alternating
voltage amplitude of 100 mVp−p. The temperature range was
300 to 550 °C. The air gas flow rate was 50 sccm. Humidified
air was obtained by flowing dry air through a water bubbler held
at 30 °C (pH2O ~ 4 kPa).
structure of the composite. One component was crystalline,
while the other component was amorphous. Based on local EDS
analysis of the different phase regions, the structure could be
characterized as consisting of amorphous Ca(PO3)2 intertwined
with crystalline La(PO3)3 of a grain size of a few hundred
nanometers. The features within the LaP3O9 phase are
presumably twins, but a definitive analysis was not possible
due to the high radiation sensitivity of these phosphates. The Ca
(PO3)2 did not have a detectable La content, while the La(PO3)3
showed about 4 cation% Ca (i.e. Ca/(La+Ca) ~ 0.04). The EDS
results show that the double phosphate, LaCa(PO3)5, expected
on the basis of the phase diagram of Jungowska and
Znamierowska [20], did not form.
The as-quenched CaLa(PO3)5 glass held at 800 °C for 20 h in
dry air, produced the dual phase glass–ceramic shown in Fig. 2.,
ground, and heated at 550 °C in dry or in humidified
(pH2O ~ 4 kPa) air, for 24 h. The as-quenched glass was similarly
crushed into powders and annealed at 550 °C, in dry and in
humidified (pH2O ~ 4 kPa) air, for 24 h. X-ray analyses with a
CuKα source were subsequently performed on all samples. The
results are shown in Fig. 3. Fig. 3-a shows the X-ray
diffractogram for the as-quenched glass powders, while Fig.
3-b is the XRD pattern for the as-quenched glass powder heated
in wet air for 24 h at 550 °C. Both diffractograms are very
similar, showing very few features, as expected for a glass. The
minor peak at 28.7° in the as-quenched ground glass
diffractogram, Fig. 3-a, and also in the ground glass following
exposure to water at 550oC, Fig. 3-b, corresponds to the 202
reflection of LaP3O9, and may be attributed to some incipient
crystallization of LaP3O9 following this treatment; no other
features were evident in these X-ray diffractogram. The
similarity of the diffractograms for both the “dry”, 3-a, and
the “wet”, 3-b, treated samples indicating that the presence of
water during the heat treatment did not significantly change the
character of the sample. This would indicate that the incipient
crystallization, while possibly a surface effect, is not catalyzed
3. Results and discussion
3.1. TEM and XRD characterization
No significant crystallization was detected by either XRD or
TEM in the as-quenched bulk glass samples. As shown in
Fig. 1, EDS results show that the molar ratio of La, Ca and P in
the quenched glass sample was about 14:14:72, which matches
the initial nominal composition of CaLa(PO3)5. After heating at
800 °C for 20 h, the molar ratio of La, Ca, and P was found to be
14:15:71, indicating that the potential loss of P2O5 at 800 °C
was negligible.
Fig. 2 is a bright field TEM image of the glass–ceramic after
the 800 °C heat treatment in dry air, evidencing the two-phase
Fig. 1. Average EDS spectrum of CaLa(PO3)5 glass after transformation at
800 °C for 20 h. The average composition is La:Ca:P = 14:14:71, which is within
experimental error, the same as that of the as quenched glass, La:Ca:
P = 14:15:72, indicating that the heat treatment did not lead to P2O5 loss.
G. Zhang et al. / Solid State Ionics 178 (2008) 1811–1816
1813
Fig. 2. Bright field image of glass–ceramic La(PO3)3–Ca(PO3)2 with EDS results.
by humidity for that time–temperature range. Fig. 3-c shows the
XRD pattern for the as-quenched glass after 20 h at 800 °C in
dry air. As expected from the TEM observations, a well-defined
diffraction peak developed, Fig. 3-c, corresponding to the 002
reflection of orthorhombic LaP3O9 (at 2θ ~ 24o) [21]. Related
crystal structures have also been determined by Hong (the
isomorphous NdP3O9) [22], and by Botto and Baran [23].
The second heat treatment of the composite glass-ceramic
sample, at 550 °C in humidified air for 24 h, led to a dramatic
increase in the relative LaP3O9 - 002 intensity, which is
reflected in the strong comparative diminution of the other
reflections, Fig. 3-d, suggesting that the water vapor assisted in
reducing the lattice structural imperfection of the crystallized
LaP3O9, since there was no corresponding increase in the
LaP3O9 phase content. No shift was associated with this peak of
strongly increased relative intensity. A tentative explanation
may be that the high local lattice distortions associated with the
formation of additional P–O–P linkages produced in the
LaP3O9 upon Ca2+ substitution on La 3+ sites, are relieved by
hydrolysis, as would also have to be the case in Sr-doped LaPO4
[9]. Due to different synthesis process and the incorporation of
Ca, the peak positions and relative intensities do not match
perfectly with the reflections listed in the JCPD file for pure
LaP3O9 (33-0717).
3.2. IR characterization of glass CaLa(PO3)5 and glass–ceramic
La(PO3)3–Ca(PO3)2
Fig. 3. XRD results of ground glass CaLa(PO3)5 and ground glass–ceramic
La(PO3)3–Ca(PO3)2 before and after water vapor treatment at 550 °C for 24 h.
The local environment of the phosphate groups and the
possible incorporation of protons in the metaphosphate CaLa
(PO3)5 glass may be deduced from its IR spectroscopic features,
Fig. 4-a and b. As expected, for the as-quenched glass, all the
vibration bands are very broad. According to the literature [24],
the bands of metaphosphate glasses are observed at about 1295,
1085, 900 cm− 1, The vibration modes of CaLa(PO3)5 glass
shown in Fig. 4-a were assigned similar to those of calcium
metaphosphate. The band at 1270 cm− 1 is attributed to the
asymmetric stretching mode of P = O bonds, while the band
at 1080 cm− 1 would belong to the stretching mode of an ionic
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G. Zhang et al. / Solid State Ionics 178 (2008) 1811–1816
Fig. 4. IR spectra of (a) ground glass CaLa(PO3)5 for the range 1300–400 cm− 1,
and (b) a polished 0.3 mm slab after a 24 h exposure at 550 oC with pH2O ~ 4 kPa
in the 4000–1500 cm− 1 range.
P–O− bond. The band at 905 cm− 1 is due to the asymmetric
stretching mode of P–O–P bonds in linear chains. The band at
740 cm− 1 has been associated to the symmetric stretching mode
of P–O–P bonds in small metaphosphate rings. The band at
475 cm− 1 is due to deformation modes of PO2 fragments. These
vibration modes all belong to Q 2 group, in which the
phosphorous is connected by two bridging oxygens, one
double-bonded oxygen and one non-bridging oxygen.
Fig. 4-b shows the IR spectrum of the 0.3 mm thick solid
CaLa(PO3)5 glass wafer, after the water vapor treatment at
550 °C for 24 h. Three primary absorption band maximums at
3042 cm− 1, 2349 cm− 1, and 1684 cm− 1 are noted. Following
Abe [25,26], the band at νOH 3042 cm− 1 is assigned to the
mobile proton as it correlates with weak hydrogen bonding.
Though OH stretches may also be observed at 2350 cm− 1, this
frequency has overlap with both P–O–P vibrations and CO2
[25,27], and is thus not as useful to analyze the water content.
The stretch at 1684 cm− 1 is near the reported stretch of
1674 cm− 1 for an interstitial water molecule bending mode
[28]. The presence of these peaks confirms the presence of
protons in the system. Protons absorbing IR at lower frequency
are not thought to be mobile though may still play a role in
proton conduction.
When CaLa(PO3)5 glass is exposed to water vapor, water
attacks the phosphate chains or rings and breaks them into
smaller structural units with hydrogen-bonded protons at the
end [29], allowing an increase in the intermediate-range order.
This may facilitate protons and molecular water transport
through the modified phosphate network.
After treatment at 800 °C for 20 h, the CaLa(PO3)5 glass
decomposed into crystalline La(PO3)3 and amorphous Ca
(PO3)2, as shown by TEM examinations (Fig. 2). The transition
from glass to glass–ceramic obviously caused the local
environments of phosphorous to change, which is reflected in
the IR spectra, Fig. 5. The asymmetric stretching mode of P = O
bonds changes from 1270 to 1265 cm− 1, and the stretching
mode of P–O− bonds at 1080 cm− 1 splits into four peaks: 1153,
1120, 1060 and 1005 cm− 1. The asymmetric stretching mode of
P–O–P bonds in Q2 tetrahedra shifts the position of the
905 cm− 1 in the as quenched glass, to 945 cm− 1 in the glass–
ceramic. The symmetric stretching mode of P–O–P bonds at
740 cm− 1 splits into 770, 719 and 682 cm− 1. The deformation
mode of PO2 fragments at 475 cm− 1 splits into 565, 523 and
472 cm− 1. In CaLa(PO3)5 glass, the phosphorous has an
homogenous chemical environment. This homogeneity disappears when the CaLa(PO3)5 glass decomposes into amorphous Ca(PO3)2 and crystalline La(PO3)3. In amorphous Ca
(PO3)2, the PO4 tetrahedra are connected by two sets of Ca
polyhedra, whose coordination number is 8 or 7. In crystalline
La(PO3)3 [21], oxygen atoms link with phosphate group to form
corner sharing tetrahedra or link with lanthanium atoms to form
edge sharing LaO8 polyhedra. When P atoms are connected
with O atoms which coordinate with La atoms, the formed two
P–O bonds are shorter while the two P–O bonds formed in the
Fig. 5. IR spectra for range 1300–400 cm− 1 ground glass CaLa(PO3)5 after
treatment at 550 °C for 24 h with pH2O ~ 4 kPa (1), and for ground glass–ceramic
after treatment at 550 °C for 24 h either “dry” (2), or with pH2O ~ 4 kPa (3).
G. Zhang et al. / Solid State Ionics 178 (2008) 1811–1816
1815
P–O–P bridges are longer. Compared with that of CaLa(PO3)5
glass, P–O–P bridge angle in crystalline La(PO3)3 and
amorphous Ca(PO3)2 also changes, which affects the chain
configuration and related vibration modes [30]. Therefore,
when La(PO3)3 crystallizes, the frequencies of the vibration
modes shifts and band splitting occurs.
3.3. Conductivity of glass CaLa(PO3)5 and glass–ceramic La
(PO3)3–Ca(PO3)2
Fig. 6 shows A.C. conductivities of glass CaLa(PO3)5 and
glass–ceramics La(PO3)3–Ca(PO3)2 in dry and wet air (pH2O ~
4 kPa). Amezawa [31] synthesized undoped and 1 mol% Cadoped LaP3O9 by calcining the starting materials at 1173 K for
2 h, followed by spark plasma sintering at 1323 K for 5 min.
The conductivity of the resulting Ca-doped LaP3O9 was almost
independent of oxygen partial pressure in wet conditions.
These conductivities obtained in wet oxygen (pH2O ~ 4.2 kPa),
may thus be compared to with that of our glass–ceramic treated
in wet air.
When the CaLa(PO3)5 glass decomposed into glass–ceramic
La(PO3)3–Ca(PO3)2 after heat treatment at 800 °C for 20 h, the
conductivity increased approximately ten-fold, Fig. 6. The
increased conductivity is associated with the crystallization of
La(PO3)3, as follows from the TEM and XRD results. The
bright field image of glass–ceramic, Fig. 2, also shows that the
La(PO3)3 grains with a size of a few hundred nanometers, are
interconnected and thus can provide a percolation path for
conduction. The small size (nanometer or submicron) La(PO3)3
grains may be one reason why the conductivity of our glass–
ceramic is slighter higher than 1 mol% Ca doped LaP3O9 of
Amezawa et al. [31]. The conductivity of the present glass–
ceramic is around 1.52 × 10− 5 S/cm− 1 at 550 °C in wet air while
Amezawa's conductivity of 1 mol% Ca doped LaP3O9 was
around 6.3 × 10− 6 S/cm− 1 in wet oxygen. The conductivity of
glass–ceramic is higher in wet air than that in dry air, which
Fig. 7. A deconvolution of the 31P NMR spectrum for heat treated La(PO3)3–Ca
(PO3)2 glass shows the original spectrum overlaid on the individual simulated
peaks. The residual difference between the original and the simulated spectrum
is shown as a dashed line at the bottom.
supports the existence of proton conduction and agrees closely
with the infrared spectroscopy results.
3.4.
31
P NMR of heat treated glass–ceramic composite
Fig. 7 shows the 31P NMR spectrum representing the various
chemical environments for phosphorus in the La(PO3)3–Ca
(PO3)2 glass–ceramic composite after heat treatment at 800 °C
for 20 h. This is the same material shown in the TEM image,
Fig. 2. Lineshape analysis (Table 1) revealed the coexistence of
partially crystalline LaP3O9 and partially crystalline calcium
phosphate regions. The peaks at − 30.3 ppm, − 33.8 ppm
and − 35.9 ppm ( the narrowest lines) are attributed to partially crystalline calcium phosphate glass while the peaks
at − 37.5 ppm, − 46.61 ppm and − 51.5 ppm (the broadest lines)
arise from regions of lanthanum metaphosphate. All chemical
shift values of the heat-treated composite match well with those
of individually heat treated lanthanum phosphate and calcium
phosphate component glasses, and are consistent with literature
values [32–35]. These results will be discussed in detail in a
future study. The NMR lineshape analysis suggests partial
crystallization in the calcium phosphate regions. Such partial
crystallization was not revealed by TEM, possibly as a result of
beam damage effects or limited statistical sampling.
Table 1
Chemical shifts, linewidths and populations observed for the La(PO3)3–Ca
(PO3)2 composite
Fig. 6. Conductivities of glass–ceramics La(PO3)3–Ca(PO3)2 in dry and
humidified air (pH2O ~ 4 kPa ). Conductivities of 1 mol% Ca doped LaP3O9 in
wet oxygen are adapted from [31].
Chemical Shift
% Population
Linewidth
Environment
−30.3 ppm
−33.8 ppm
−35.9 ppm
−37.5 ppm
−46.61 ppm
−51.5 ppm
7.2%
11.2%
5.2%
44.4%
9%
23.0%
.8 ppm
1.2 ppm
.9 ppm
5.8 ppm
2.5 ppm
4.6 ppm
Calcium phosphate
Calcium phosphate
Calcium phosphate
Lanthanum phosphate
Lanthanum phosphate
Lanthanum phosphate
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G. Zhang et al. / Solid State Ionics 178 (2008) 1811–1816
4. Conclusions
CaLa(PO3)5 glass decomposes into crystalline La(PO3)3 and
amorphous Ca(PO3)2 after being treated at 800 °C for 20 h. The
crystalline La(PO3)3 grains with a size of several hundred
nanometers are interconnected and provide a percolation path
for conduction in the glass–ceramic La(PO3)3–Ca(PO3)2. The
Ca(PO3)2 glass phase was nearly La -free, while the La(PO3)3
crystalline phase contained approximately 1at% (ignoring the
oxygen) of Ca. XRD indicated that annealing of the composite
in wet air at 550 °C, appeared to improve the La(PO3)3 crystal
perfection, possibly due to the hydrolysis and elimination of
some highly strained pyrophosphate defects introduced by the
Ca substitution on La sites. The conductivity of this glass–
ceramic is around 1.52 × 10− 5 S/cm− 1 at 550 °C in wet air,
slighter higher than that reported of bulk, 1 mol% Ca doped
LaP3O9. The conductivity of glass–ceramic La(PO3)3–Ca
(PO3)2 is found to be higher in wet air than that in dry air,
which indicates that it is a proton-conducting material.
Acknowledgements
This work was supported by the Director, Office of Science,
Office of Basic Energy Sciences, Materials Sciences and
Engineering Division, of the U.S. Department of Energy
under Contract No. DE-AC02-05CH11231. The authors
acknowledge support of the National Center for Electron
Microscopy, Lawrence Berkeley Lab, which is also supported
by the U.S. Department of Energy under Contract No. DEAC02-05CH11231.
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