Solid State Ionics 177 (2006) 1307 – 1315
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Interstitial oxide positions in oxygen-excess oxy-apatites
Laura León-Reina, J. Manuel Porras-Vázquez, Enrique R. Losilla, Miguel A.G. Aranda ⁎
Departamento de Química Inorgánica, Cristalografía y Mineralogía, Universidad de Málaga, 29071-Málaga, Spain
Received 27 February 2006; received in revised form 10 May 2006; accepted 24 May 2006
Abstract
Several oxy-apatites materials, La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9Sr1(Si5.5Al0.5O24)O2.25, La9.67□0.33(Si5.5Al0.5O24)
O2.25, La8.5Sr1.5(Si5.5Al0.5O24)O2 and La9.5□0.5(Si5.5Al0.5O24)O2 have been prepared as crystalline phases. The impedance study showed that all
samples are oxide ion conductors and their conductivities are similar to those previously reported for related oxy-apatites. A thorough study on the
oxygen sublattices for oxygen excess samples has been carried out using neutron powder diffraction data by the Rietveld method. The structural
study shows the presence of interstitial oxide anions close to the periphery apatite channels. However, the interstitial oxide position in cation
stoichiometric silicates, f. i. La8.65Sr1.35(Si6O24)O2.32, is different from that in germanates, f. i. La8.65Sr1.35(Ge6O24)O2.32. This is likely due to the
different structural flexibility of the two tetrahedral groups and it explains the previously reported evidence of higher oxygen contents for
germanates oxy-apatites. The structural characteristics of these oxide anion conductors are discussed.
© 2006 Elsevier B.V. All rights reserved.
Keywords: SOFC; Oxide ion conductor; Apatite; Synchrotron; Neutron diffraction
1. Introduction
Solid oxide fuel cells (SOFCs) are all-solid-state electrochemical devices that allow the direct conversion of chemical to electrical energy with low emission of pollutants, low noise and high
energy-conversion efficiency [1,2]. The three basic components in
a SOFC unit are: a porous cathode, a porous anode and a dense
electrolyte. The electrolyte must have high oxide conductivity,
negligible electronic conductivity, good chemical compatibility
with the electrode materials and low thermal expansion coefficient
similar to those of the electrodes [3]. Traditionally, the SOFC
electrolyte in commercial systems is a self-supported (≈ 10 μm
thick) yttria stabilized zirconia (YSZ) film. This material
exhibits high oxide ion conductivity at elevated temperatures
(1173–1273 K) and good chemical stability under reducing and
oxidizing atmospheres. However, this high operating temperature
may cause problems such as difficulties in cell sealing which
enforces the use of expensive materials and low lifetime of the
components. Hence, there is a huge research effort in the development of new oxide ion conductors to reduce the operating
⁎ Corresponding author. Tel.: +34 952131874; fax: +34 952132000.
E-mail address: [email protected] (M.A.G. Aranda).
0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ssi.2006.05.034
temperature to 873–1073 K [4]. With the aim of realizing the
intermediate temperature SOFCs (IT-SOFCs), ionic conductors that
have high conductivities at lower temperatures are strongly desired.
Therefore, other electrolytes with higher oxide conductivities are
being intensively studied, for instance: Ce0.8Gd0.2O1.9 (GDC) [5];
(Bi2O3)0.75–(RE2O3)0.25 [6]; perovskite-type oxides such as La0.9Sr0.1Ga0.8Mg0.2O3 [7] and BIMEVOX (Bi2V0.86Ni0.14O5.29) [8].
Rare earth oxy-apatites are also attracting considerable interest due
to their high oxide ion conductivities and low activation energies
[9]. Research interest in this area has grown following the exciting
reports by Nakayama et al. [10,11] about the high oxide
conductivities of rare earth silicates, Ln10−x□x(Si6O24)O3−1.5x.
A key aspect of oxy-apatite materials is the wide range of
substitutional possibilities [9]. The general formula for an apatitetype material can be expressed as A10(T6O24)X2, where A is
generally a large cation (alkali, alkaline earth, rare earth metals
[RE]), TO4 is a tetrahedral group (T = P, V, As, Si, Ge, etc.) and X is
an anion like O, OH, F, Cl, Br, I. In the terms of oxide ion
conductivity, the best properties have been observed in materials
where T = Si, Ge and A corresponds to rare earth and alkaline earth
cations. The initials works on oxy-apatite conductors were focused
on RE10−x□x(Si6O24)O3−1.5x systems, these phases have been
prepared as polycrystalline powders (by standard solid state
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L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315
techniques [12] and sol–gel synthesis [13]) and as single crystals
by the floating zone method [14]. There have also been several
studies with Ge [15], and the La10−x□x(Ge6O24)O3−1.5x series has
been studied by neutron and synchrotron X-ray powder diffraction
and impedance spectroscopy [16]. Mixed Ge–Si oxy-apatites [17]
and derivatives [18] are known. There are also other compounds
with the same framework, but with atomic substitutions at the A
and T sites [9,11], some including transition metals [19]. For
example, the conductivity of Ln10−x□x(T6O24)O3−1.5x (T = Si and
Ge) has been enhanced by partial substitution of Si/Ge by Al
[20,21]. A particularly interesting feature of the oxy-apatite
materials is the effect of cation vacancies on the conductivity. Srdoped oxygen stoichiometric systems, La9.33−xSr3x/2□0.67−x/2
(T6O24)O2 (T = Si and Ge) [22–25] have shown that as the Sr
content increased and hence the number of cation vacancies
decreased, the conductivity decreased.
Some important experimental contributions to the knowledge of
the relationship between structure and oxide conductivity in the
apatite-type compounds are those works using neutron powder
diffraction [16,23,25–28]. Furthermore, a recent work has been
published where a correlation between the silicon environment,
studied by 29Si NMR spectroscopy, and the oxide conductivity
values has been found [22]. We have carried out a systematic work
to study the presence of interstitial oxygen in highly crystalline
single phases of oxygen-stoichiometric apatites including
La9.33□0.67(Si6O24)O2, La9.5□0.5(Ge5.5Al0.5O24)O2 and La8Sr2
(Si6O24)O2 [23,26]. These oxygen stoichiometric samples had
interstitial oxygens only if they also contained cation vacancies. For
example, La8Sr2(Si6O24)O2 which don't have vacancies in the
lanthanum sublattice, don't have interstitial oxygens. We also
studied the position and population of interstitial oxygen in oxygenexcess apatites with cation vacancies, f.i. La9.55□0.45(Si6O24)O2.32
and La9.60□0.40(Ge6O24)O2.40 [23]. The position of the interstitial
oxide anions in all these cation-deficient samples is placed very
near to the location earlier predicted by atomistic simulations for
La9.33□0.67(Si6O24)O2 [29,30]. This position for the interstitial
oxide anion is very close (∼ 1.0 Å) to the average position of one
oxygen atom of a silicate/germanate tetrahedron but the atomistic
calculations showed that the lattice could accommodate the interstitial oxygen through a local lattice relaxation. This local relaxation is the displacement and tilting of the tetrahedral group towards
the cation vacancy of the lanthanum sublattice [29,30].
The goal of the present work is to study the presence, location
and population of interstitial oxygens in oxygen-excess apatites
without cation vacancies. To do so, three samples, La8.65Sr1.35
(Si 6 O 2 4 )O 2 . 3 2 , La 8 . 6 5 Sr 1 . 3 5 (Ge 6 O 2 4 )O 2. 3 2 and La 9 Sr 1
(Si5.5Al0.5O24)O2.25 have been structurally characterized by
joint refinement of neutron and X-ray powder diffraction data.
Oxide conductivities for these, and other related samples, are also
given and discussed.
2. Experimental
2.1. Synthesis
Six compounds, La 8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35
(Ge 6 O 24 )O 2.32 , La 9 Sr 1 (Si 5.5 Al 0.5 O 24 )O 2.25 , La 9.67 □ 0.33
(Si5.5Al0.5O24)O2.25, La8.5Sr1.5(Si5.5Al0.5O24)O2 and La9.5□0.5
(Si5.5Al0.5O24)O2 have been prepared by the ceramic method in Pt
crucibles using high purity oxides: La2O3 (Alfa, 99.999%), GeO2
(Aldrich, 99.998%), SiO2 (ABCR, quartz powder 99.31%), γAl2O3 (Alfa, 99.997%) and SrCO3 (Alfa 99.99%). Lanthanum
oxide was precalcined at 1273 K for 2 h in order to achieve
decarbonation.
The synthesis of La8.65Sr1.35(Si6O24)O2.32 and La8.65Sr1.35
(Ge6O24)O2.32 was carried out as previously reported [23]. For
the other samples the oxides were ground using an agate mortar
for 20 min, pelletized (200 MPa, ca. 20 mm diameter and ca.
3 mm thickness), and heated at 1773 K for 24 h. The resulting
powders were ground for 40 min using a Fritsch ball mill
(model Pulverisette 7, 45 cm3 agate vessel containing seven
agate balls each with a diameter of 15 mm) at 200 rpm with
reverse rotation every 5 min. A second thermal treatment was
carried out at 1773 K for 48 h. Finally, the samples were reground, pelletized and heated at the same temperature for 36 h.
2.2. Powder diffraction
All samples were characterized by laboratory X-ray powder
diffraction (LXRPD) at room temperature to check for possible
impurity phases. The powder patterns were collected on a
Siemens D5000 automated diffractometer using graphite-monochromated CuKα1,2 radiation. To carry out Rietveld studies, the
compounds were scanned between 15 and 110° (2θ) in 0.03°
steps, counting 18 s per step.
Room temperature neutron powder diffraction patterns (RTNPD) were collected on HRPT diffractometer [31] [SINQ
neutron source at Paul Scherrer Institut, Villigen, Switzerland]
for La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9Sr1
(Si5.5Al0.5O24)O2.25 and La9.5□0.5(Si5.5Al0.5O24)O2 with the
samples loaded in a vanadium can. High temperature neutron
powder diffraction patterns (HT-NPD) data were also collected
for La9Sr1(Si5.5Al0.5O24)O2.25 at 1073 K and for La9.5□0.5
(Si5.5Al0.5O24)O2 at 773 and 1173 K. The neutron wavelength,
∼ 1.886 Å, was selected by the (511) reflection of the vertically
focusing Ge monochromator. The overall measuring time was
≈ 6 h per pattern to have good statistics over the 2θ angular
range of 5–165° [21−0.95 Å] with 0.05° step size.
High resolution synchrotron X-ray powder diffraction patterns
at room temperature (RT-SXRPD) were collected on ID31 diffractometer [European Synchrotron Radiation Facility (ESRF),
Grenoble, France] for La8.65Sr1.35(Si6O24)O2.32 and La8.65Sr1.35
(Ge6O24)O2.32. The samples were loaded in a borosilicate glass
capillary (ϕ = 1 mm) and rotated during data collection. A penetrating wavelength, λ = 0.40084(5) Å (30.93 keV), was selected
with a double-crystal Si (111) monochromator and calibrated with
Si standard from NIST (a = 5.431195 Å). The overall measuring
time was ≈ 1 h to have very good statistics over the angular range
2–30° (in 2θ) [11.5–0.77 Å]. The data from the multi-analyzer Si
(111) stage were normalized and summed up to 0.003° step size
with local software to produce the final raw data.
RT full structural characterization was carried out for
La8.65Sr1.35(Si6O24)O2.32 and La8.65Sr1.35(Ge6O24)O2.32 from a
joint Rietveld analysis [32] of NPD and SXRPD data. The RT
L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315
Table 1
Pellets compaction percentages, bulk and total conductivities (S cm− 1) at 773 K
and bulk and total activation energies (eV)
Composition
%C
σT
σbulk
Ea(T)
Ea(bulk)
La8.65Sr1.35(Si6O24)O2.32
76
–
1.05(2)
––
La8.65Sr1.35(Ge6O24)O2.32
74
–
1.05(2)
––
La9Sr1(Si5.5Al0.5O24)O2.25
65
0.81(2)
79
0.66(1)
0.82(4)
La8.5Sr1.5(Si5.5Al0.5O24)O2
76
0.72(1)
0.88(2)
La9.50□0.5(Si5.5Al0.5O24)O2
73
9.5(3)
10− 5
3.3(6)
10− 3
4.1(4)
10− 5
1.7(2)
10− 3
0.70(1)
La9.67□0.33(Si5.5Al0.5O24)O2.25
1.2(1)
10− 4
1.8(1)
10− 4
4.2(3)
10− 5
3.4(1)
10− 4
8.2(2)
10− 6
1.7(3)
10− 4
0.64(1)
0.82(1)
1309
METALOR® 6082 platinum paste and gradually heating to
1073 K at a rate of 10 K·min− 1 in air to decompose the paste and
harden the Pt residue. Successive treatments were made to
achieve an electrical resistance of both pellets faces lower than
1 Ω. The impedance data were collected using a Hewlett-Packard
4284A impedance analyzer over the frequency range 20 Hz to
1 MHz from 473 to 1273 K. Measurement processes were
electronically controlled by the winDETA package of programs
[34]. The pellets were mounted in a home made alumina conductivity jig, with four Pt wires shielded in two alumina tubes,
which were placed in a tubular furnace. Electrical data were taken
every 50 K. A delay time of 60 min at each temperature was
selected to ensure thermal equilibrium. Temperatures were reproducible to ±1 K.
3. Results and discussion
structures for La 9.5 □ 0.5 (Si 5.5 Al 0.5 O 24 )O 2 and La 9 Sr 1
(Si5.5Al0.5O24)O2.25 have been obtained from joint Rietveld
analysis [32] of NPD and LXRPD data. HT structures have been
derived only from NPD data. The GSAS suite of programs [33]
was used for all calculations.
2.3. Impedance studies
Green pellets (∼ 10 mm diameter and ∼ 1 mm thickness) were
obtained by pressing the fine powder at 400 MPa for 2 min. These
pellets were sintered in order to have dense materials with high
mechanical strength: La8.65Sr1.35(Si6O24)O2.32 was heated at
1673 K for 6 h; La8.65Sr1.35(Ge6O24)O2.32 at 1573 K for 6 h and
La9Sr1(Si5.5Al0.5O24)O2.25, La9.67□0.33(Si5.5Al0.5O24)O2.25,
La8.5Sr1.5(Si5.5Al0.5O24)O2 and La9.5□0.5(Si5.5Al0.5O24)O2 at
1823 K for 6 h. No weight losses were detected in these sintering
steps. Electrodes were made by coating opposite pellet faces with
Fig. 1. Complex impedance plane plot for La9Sr1(Si5.5Al0.5O24)O2.25 at 673 K
(873 K in the inset) as square points. The full line is the data fit using the
equivalent circuit described in the text. Selected frequency and capacitance
points are highlighted.
3.1. Synthesis and single phase existence
Six materials have been prepared as highly crystalline phases.
Four silicates were single phases. La8.65Sr1.35(Ge6O24)O2.32 contained 1.2(1) wt.% of La2GeO5 and La9.5□0.5(Si5.5Al0.5O24)O2
contained 1.2(1)% of LaAlO3 (derived from the Rietveld analyses
of the powder patterns). The unit cell volumes from LXRPD data
for La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9Sr1
(Si5.5Al0.5O24)O2.25, La9.67□0.33(Si5.5Al0.5O24)O2.25, La8.5Sr1.5
(Si5.5Al0.5O24)O2, and La9.5□0.5(Si5.5Al0.5O24)O2 were 589.24
(2), 622.00(3), 590.48(2), 590.71(1), 591.68(2) and 590.16(2) Å3,
respectively. As expected, germanates have higher volumes than
silicates. The unit cell volume variation is complex due to the
interplay of three parameters, i) the amount of cation vacancies, ii)
Fig. 2. Arrhenius plots of logσT for La8.65Sr1.35(Si6O24)O2.32 (+), La8.65Sr1.35
(Ge6O24)O2.32 (⁎), La9Sr1(Si5.5Al0.5O24)O2.25 (Δ), La9.67□0.33(Si5.5Al0.5O24)
O2.25 (○), La8.5Sr1.5(Si5.5Al0.5O24)O2 (∇) and La9.5□0.5(Si5.5Al0.5O24)O2 (□),
and of logσbulk for La9Sr1(Si5.5Al0.5O24)O2.25 (▴), La9.67□0.33(Si5.5Al0.5O24)
O2.25 (●), La8.5Sr1.5(Si5.5Al0.5O24)O2 (▾) and La9.5□0.5(Si5.5Al0.5O24)O2 (n).
1310
L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315
Table 2
Refined unit cell values, Rietveld disagreement factors, atomic positional parameters and occupation factors, at different temperatures for La8.65Sr1.35(Si6O24)O2.32,
La8.65Sr1.35(Ge6O24)O2.32, La9Sr1(Si5.5Al0.5O24)O2.25 and La9.5□0.5(Si5.5Al0.5O24)O2 in P63/m space group
La8.65Sr1.35(Si6O24)O2.32
a
a (Å)
c (Å)
V(Å3)
N
X
/RWP
(%)
RWP
X
RN
/R
P P (%)
X
RN
F /RF (%)
La(1), 6h, (x y 1/4)d
x
y
Uiso × 100 (Å2)
La/Sr(2), 4f, (1/3 2/3 z)d
z
Uiso × 100 (Å2)
Ge/Si/Al, 6h, (x y 1/4)e
x
y
Uiso × 100 (Å2)
O(1), 6h, (x y 1/4)
x
y
Uiso × 100 (Å2)
O(2), 6h, (x y 1/4)
x
y
Uiso × 100 (Å2)
O(3), 12i, (x y z)
x
y
z
Uiso × 100 (Å2)
O(4), 2a, (0 0 1/4)
Uiso × 100 (Å2)
Occ. factor.
O(5), 12i, (x y z)f
x
y
z
Occ. factor
La8.65Sr1.35(Ge6O24)O2.32
a
La9Sr1(Si5.5Al0.5O24)O2.25
b
c
La9.5□0.5(Si5.5Al0.5O24)O2
RT
RT
RT
1073 K
RTb
773 Kc
1173 Kc
9.7100(1)
7.2254(1)
589.973(2)
3.57/8.17
2.78/5.56
0.83/1.50
9.9120(1)
7.3236(1)
623.130(6)
2.86/7.98
2.23/5.74
0.97/1.95
9.7111(2)
7.2290(1)
590.40(2)
3.57/9.19
2.77/7.07
0.86/2.17
9.7870(3)
7.2715(2)
603.19(5)
3.22/–
2.52/–
1.47/–
9.7260(1)
7.2002(1)
589.85(2)
3.59/11.63
2.77/8.36
1.30/2.55
9.7665(3)
7.2227(3)
596.63(5)
4.08/–
2.97/–
1.52/–
9.8100(4)
7.2482(3)
604.08(6)
3.66/–
2.63/–
1.67/–
0.2289(1)
− 0.0125(1)
0.53
0.2305(1)
− 0.0112(1)
0.85
0.2299(1)
− 0.0120(1)
0.39
0.2285(3)
− 0.0125(3)
1.59
0.2293(1)
− 0.0105(2)
0.58
0.2274(3)
− 0.0113(4)
1.27
0.2275(3)
− 0.0109(4)
1.98
− 0.0007(1)
0.74
0.0008(1)
1.41
− 0.0009(2)
0.54
0.0001(4)
1.95
− 0.0014(2)
0.69
0.0002(5)
1.04
0.0008(6)
1.92
0.4020(1)
0.3718(1)
0.24
0.4008(1)
0.3723(1)
0.54
0.4020(2)
0.3720(2)
0.05
0.4015(5)
0.3727(5)
1.34
0.4016(2)
0.3723(2)
0.45
0.4019(5)
0.3725(5)
0.77
0.4014(6)
0.3725(5)
1.36
0.3224(1)
0.4847(1)
1.20
0.3117(1)
0.4866(1)
1.90
0.3238(2)
0.4863(2)
0.90
0.3240(4)
0.4867(4)
3.07
0.3253(2)
0.4866(2)
2.22
0.3260(5)
0.4878(5)
3.04
0.3273(5)
0.4885(5)
4.50
0.5956(1)
0.4731(1)
0.64
0.6029(1)
0.4755(1)
1.46
0.5972(2)
0.4730(2)
0.53
0.5946(4)
0.4712(4)
2.55
0.5965(2)
0.4722(2)
0.90
0.5946(4)
0.4717(5)
1.91
0.5937(5)
0.4717(5)
3.13
0.3443(1)
0.2549(1)
0.0701(1)
1.52
0.3408(1)
0.2478(1)
0.0615(1)
3.24
0.3446(1)
0.2550(1)
0.0691(1)
1.32
0.3430(3)
0.2565(3)
0.0703(3)
3.84
0.3469(1)
0.2564(1)
0.0684(1)
2.30
0.3460(4)
0.2577(3)
0.0690(3)
3.24
0.3450(4)
0.2589(3)
0.0701(3)
4.73
5.34
1.00(– –)
2.83
0.929(4)
3.95
1.00(– –)
6.33
1.00(– –)
5.76
0.927(8)
8.28
0.94(2)
10.26
0.96(2)
0.014(5)
0.071(3)
0.555(4)
0.023(1)
0.009(2)
0.213(2)
0.598(1)
0.045(1)
0.015(18)
0.064(8)
0.537(9)
0.020(2)
0.051(12)
0.083(10)
0.556(12)
0.023(4)
− 0.009(18)
0.221(17)
0.570(13)
0.012(1)
0.01(5)
0.23(4)
0.57(3)
0.010(3)
0.01(7)
0.23(6)
0.58(5)
0.006(3)
a
Joint RT-SXRPD and RT-NPD refinements. bLXRPD and RT-NPD refinements. cHT-NPD refinements. dAll the Sr was located in the position of La(2), and the
occupation factors of La and Sr were fixed to the nominal values. eThe occupation factors for Ge and Al were fixed to the nominal ones. fThe Uiso for O(5) was fixed to
0.03 Â2 at all the temperatures.
the amount of oxygen excess, and iii) the size of the element at the
center of the tetrahedral group.
3.2. Impedance study
The sintering conditions for the pellets led to specimens not
fully dense with compactions ranging between 70 and 80% of the
theoretical value (see Table 1). No weight losses were detected
during the sintering process. Representative impedance data for
La9Sr1(Si5.5Al0.5O24)O2.25 at two temperatures are shown as
impedance complex plane plots in Fig. 1. Similar plots were
obtained for the remaining compositions. At low temperatures,
i.e. 673 K, a set of overlapping semicircles can be observed. This
set of semicircles indicates that several effects are involved in the
electrical response of the pellets, including bulk and grain boundary contributions. At intermediate temperatures, 873 K, a well-
developed spike can be observed with an associated capacitance
of 0.1 μF cm− 1 (20Hz). Since it is inclined to the Z′ axis at
roughly 45°, it indicates a partial-blocking electrode response that
allows limited diffusion. At higher temperatures the spike
collapses to a semicircular arc (not shown), indicating that oxygen molecules are able to diffuse through the entire thickness of
the electrode. In summary, the conducting species appear to be
oxide ions as for many other oxy-apatites. Impedance data for
La8.65Sr1.35(Si6O24)O2.32 and La8.65Sr1.35(Ge6O24)O2.32 were
previously reported [23]. Bulk conductivities for these materials
must be close to the total values given in Table 1, as there is an
almost non-deformed semicircle in the impedance complex plane
plots. Bulk results obtained by least squares equivalent circuit
fittings are not given because the associated errors were high.
Total pellet conductivities (σT) are obtained from the intercept of the spikes and/or the arcs (low frequency end) on the Z′
L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315
1311
Table 3
Refined anisotropic thermal parameters at room temperature for selected samples
Atoms
La/Sr(1), 6h, (x y 1/4)
La8.65Sr1.35(Si6O24)O2.32
La8.65Sr1.35(Ge6O24)O2.32
La9Sr1(Si5.5Al0.5O24)O2.25
La9.5□0.5(Si5.5Al0.5O24)O2
La/Sr(2), 4f, (1/3 2/3 z)
La8.65Sr1.35(Si6O24)O2.32
La8.65Sr1.35(Ge6O24)O2.32
La9Sr1(Si5.5Al0.5O24)O2.25
La9.5□0.5(Si5.5Al0.5O24)O2
Ge/Si/Al, 6h, (x y 1/4)
La8.65Sr1.35(Si6O24)O2.32
La8.65Sr1.35(Ge6O24)O2.32
La9Sr1(Si5.5Al0.5O24)O2.25
La9.5□0.5(Si5.5Al0.5O24)O2
O(1), 6h, (x y 1/4)
La8.65Sr1.35(Si6O24)O2.32
La8.65Sr1.35(Ge6O24)O2.32
La9Sr1(Si5.5Al0.5O24)O2.25
La9.5□0.5(Si5.5Al0.5O24)O2
O(2), 6h, (x y 1/4)
La8.65Sr1.35(Si6O24)O2.32
La8.65Sr1.35(Ge6O24)O2.32
La9Sr1(Si5.5Al0.5O24)O2.25
La9.5□0.5(Si5.5Al0.5O24)O2
O(3), 12i, (x y z)
La8.65Sr1.35(Si6O24)O2.32
La8.65Sr1.35(Ge6O24)O2.32
La9Sr1(Si5.5Al0.5O24)O2.25
La9.5□0.5(Si5.5Al0.5O24)O2
O(4), 2a, (0 0 1/4)
La8.65Sr1.35(Si6O24)O2.32
La8.65Sr1.35(Ge6O24)O2.32
La9Sr1(Si5.5Al0.5O24)O2.25
La9.5□0.5(Si5.5Al0.5O24)O2
U11 × 100
U22 × 100
U33 × 100
U12 × 100
0.7(1)
1.2(1)
0.4(1)
0.8(1)
0.4(1)
0.4(1)
0.4(1)
0.6(1)
0.5(1)
0.9(1)
0.3(1)
0.3(1)
0.7(1)
1.9(1)
0.5(1)
0.3(1)
0.7(1)
1.9(1)
0.5(1)
0.3(1)
0.1(1)
0.6(1)
− 0.3(1)
0.9(1)
U13 × 100
U23 × 100
0.2(1)
0.4(1)
0.3(1)
0.3(1)
0
0
0
0
0
0
0
0
0.7(1)
0.5(1)
0.6(1)
1.4(1)
0.4(1)
0.9(1)
0.2(1)
0.2(1)
0
0
0
0
0
0
0
0
0.3(1)
0.4(1)
0.2(1)
0.0(1)
0.3(1)
0.6(1)
0.2(1)
0.5(1)
0.2(1)
0.3(1)
0.1(1)
0.9(1)
0
0
0
0
0
0
0
0
1.3(1)
2.4(1)
0.9(1)
2.5(1)
1.4(1)
1.8(1)
1.3(1)
2.8(1)
0.9(1)
1.5(1)
0.5(1)
1.3(1)
1.2(1)
2.0(1)
1.0(1)
2.6(1)
0
0
0
0
0
0
0
0
0.6(1)
0.6(1)
0.4(1)
0.9(1)
0.4(1)
0.5(1)
0.4(1)
− 0.1(1)
0.9(1)
3.3(1)
0.8(1)
1.9(1)
0.2(1)
0.4(1)
0.1(1)
0.2(1)
0
0
0
0
0
0
0
0
2.7(1)
5.2(1)
2.4(1)
4.3(1)
1.2(1)
2.5(1)
1.1(1)
1.6(1)
0.7(1)
2.0(1)
0.4(1)
1.0(1)
1.3(1)
2.7(1)
1.2(1)
1.5(1)
−0.9(1)
−2.2(1)
−0.9(1)
−1.4(1)
− 0.5(1)
− 1.1(1)
− 0.4(1)
− 0.7(1)
0.7(1)
− 0.6(1)
0.4(1)
0.1(1)
0.7(1)
− 0.6(1)
0.4(1)
0.1(1)
14.6(2)
9.7(1)
11.0(3)
17.1(5)
0.3(1)
− 0.3(1)
0.2(1)
0.1(1)
0
0
0
0
0
0
0
0
axis, and are given in Fig. 2, in the traditional Arrhenius format.
In order to quantitatively estimate the bulk and grain boundary
conductivities of these samples, complex impedance spectra
were analyzed by nonlinear least squares fittings of equivalent
circuits using the program Zview [35]. The pellets behavior has
been modeled using an equivalent circuit formed with the series
association of two parallel circuits: one corresponding to
the bulk response (a resistor and an ideal capacitor), and the
other to the grain boundary region (a resistor and a constant
phase element). These parameters have been determined in the
473–673 K temperature range for all compositions. The calculated spectra (solid line) for La9Sr1(Si5.5Al0.5O24)O2.25 at
673 K is shown in Fig. 1 as an example of this type of fits. These
calculations gave the bulk conductivities, which are also
plotted against the inverse of temperature in Fig. 2. This figure
also includes the total conductivities for La8.65Sr1.35(Si6O24)
O2.32 and La8.65Sr1.35(Ge6O24)O2.32. Arrhenius plots for the
total and bulk conductivities fall on a set of approximately
parallel lines, showing a maximum in conductivity for
La9.67□0.33(Si5.5Al0.5O24)O2.25. From these plots, the total
and bulk conductivities at 773 K and intrinsic activation
energies have been determined, Table 1.
As it can be seen in Fig. 2 and Table 1, La8.5Sr1.5(Si5.5Al0.5O24)
O2 has the lowest conductivity of reported compounds.
La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9.67□0.33
(Si5.5Al0.5O24)O2.25 and La9Sr1(Si5.5Al0.5O24)O2.25 have higher
conductivities because of their excess-oxygen content. However
La9.5□0.5(Si5.5Al0.5O24)O2 has the same overall oxygen content
than La8.5Sr1.5(Si5.5Al0.5O24)O2 but much higher conductivity
which must be related to the presence of vacancies in the lanthanum sublattice. This behavior was already reported for others
oxy-apatite compounds [22,23,25]. The cation vacancies are
important as they allow the local lattice relaxation around the
migrating interstitial oxide. However La8.65Sr1.35(Ge6O24)O2.32,
La8.65Sr1.35(Si6O24)O2.32 and La9Sr1(Si5.5Al0.5O24)O2.25 do not
have vacancies in the La sublattice, so a NPD study is important to
understand the conductivity behavior of these stoichiometries and
for the comparison of their crystal structures with those of other
oxy-apatites (with cation vacancies).
3.3. Crystal structures
The main objective of this work is to study the presence,
location and population of interstitial oxygens in oxygen-excess
1312
L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315
Table 4
Selected bond distances (Å) for La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9Sr1(Si5.5Al0.5O24)O2.25 and La9.5□sub 0.5(Si5.5Al0.5O24)O2 at different
temperatures
La8.65Sr1.35(Si6O24)O2.32
a
A(1)–O(1)
A(1)–O(2)
A(1)–O(3) × 2
A(1)–O(3) × 2
A(1)–O(4)
<A(1)–O>
A(2)–O(1) × 3
A(2)–O(2) × 3
A(2)–O(3) × 3
<A(2)–O>
T–O(1)
T–O(2)
T–O(3) × 2
<T–O>
O(5)···O(3)
O(5)···O(4)
La8.65Sr1.35(Ge6O24)O2.32
a
La9Sr1(Si5.5Al0.5O24)O2.25
b
c
La9.5□0.5 (Si5.5Al0.5O24)O2
RT
RT
RT
1073 K
RTb
773 Kc
1173 Kc
2.744(1)
2.510(1)
2.482(1)
2.603(1)
2.285(1)
2.53
2.495(1)
2.550(1)
2.871(1)
2.64
1.626(1)
1.629(1)
1.630(1)
1.63
2.37(2)
1.54(3)
2.745(1)
2.525(1)
2.455(1)
2.624(1)
2.342(1)
2.54
2.486(1)
2.578(1)
2.937(1)
2.67
1.751(1)
1.735(1)
1.746(1)
1.74
1.15(2)
2.33(1)
2.759(2)
2.502(2)
2.476(2)
2.605(2)
2.293(1)
2.53
2.491(1)
2.540(1)
2.868(1)
2.63
1.628(3)
1.642(2)
1.637(2)
1.63
2.46(5)
1.64(7)
2.783(5)
2.549(4)
2.495(2)
2.635(3)
2.300(2)
2.55
2.500(3)
2.565(3)
2.912(2)
2.66
1.633(5)
1.637(5)
1.636(3)
1.63
2.39(9)
1.58(9)
2.790(3)
2.518(2)
2.477(1)
2.606(2)
2.283(1)
2.53
2.492(2)
2.533(2)
2.853(2)
2.63
1.617(2)
1.642(2)
1.631(2)
1.63
1.13(15)
2.54(12)
2.809(6)
2.550(5)
2.487(3)
2.629(4)
2.278(2)
2.55
2.488(4)
2.557(4)
2.879(3)
2.64
1.628(5)
1.630(6)
1.629(4)
1.63
1.0(4)
2.6(3)
2.835(7)
2.561(5)
2.643(4)
2.450(3)
2.287(2)
2.55
2.493(4)
2.575(5)
2.907(4)
2.66
1.628(6)
1.634(6)
1.622(4)
1.63
1.0(6)
2.5(4)
a
Joint RT-SXRPD and RT-NPD refinement. bJoint LXRPD and NPD refinement. cHT-NPD refinement.
A = La, Sr and T = Si, Ge, Al.
apatites without cation vacancies. The interstitial oxygen has
already been experimentally found in oxygen-excess and oxygenstoichiometric samples with vacancies at the La site [23,26].
Recently Tolchard et al. [28] have also reported the presence of
interstitial oxygen in an oxygen-excess sample, La10(Si5CoO24)
O2.5, but the results were not conclusive due to the high disorder
imposed by the presence of the cobalt cations. Here, structural
descriptions are only reported for the materials where NPD data
have been collected: La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35
(Ge 6 O 24 )O 2.32 , La 9Sr 1(Si 5.5Al 0.5O 24 )O 2.25 and La9.5 □ 0.5
(Si5.5Al0.5O24)O2.
The starting model for the structure refinements was that earlier
reported [26] for La9.55□0.45(Si6O24)O2.32 in P63/m space group.
The occupation factors for lanthanum and silicon sites were conveniently modified to describe the stoichiometries. For La8.65Sr1.35
(Si 6 O 2 4 )O 2 . 3 2 , La 8 . 6 5 Sr 1 . 3 5 (Ge 6 O 2 4 )O 2. 3 2 and La 9 Sr 1
(Si5.5Al0.5O24)O2.25, the strontium cations have been found to be
located only at the La(2) site, in full agreement with a very recent
report for La8Sr2(Si6O24)O2 [36]. For La9.5□0.5(Si5.5Al0.5O24)O2,
the La vacancies are completely located at La(2) site, in full
agreement with many previous structural studies [9]. The nominal
aluminum content was placed at the tetrahedral site. The usual
parameters: histogram scale factors, background coefficients, unit
cell parameters, zero error and peak shape coefficients were varied.
The positional atomic parameters for the pairs Sr/La(2) and
Si/Al were constrained to be the same. Atomic displacement
parameters (ADP's) for all atoms except interstitial oxygens
have been refined anisotropically. Final unit cell parameters,
Rietveld disagreement factors, atomic positional parameters and
occupation factors are given in Table 2. The anisotropic room
temperature displacement factors are given in Table 3. Selected
bond distances are given in Table 4. As an example of the
excellent fits, the Rietveld plots of RT-SXRPD and RT-NPD for
La8.65Sr1.35(Si6O24)O2.32 are given in Fig. 3.
The refinement of the loosely bonded atoms (oxide anions)
in these materials is vital to understand the oxide conduction
properties and it has been studied very carefully. The joint
refinements for La8.65Sr1.35(Si6O24)O2.32 indicated that does not
present oxide vacancies at the center of the channels and the free
refinement of the occupation factor for the interstitial oxygens
converged to 0.023(1), see Table 2. This value yields the
following refined stoichiometry, La8.65Sr1.35(Si6O24)O2.28(1),
which is very close to the expected one. Analogous refinements
for La9Sr1(Si5.5Al0.5O24)O2.25 gave no oxide vacancies and an
occupation factor for O(5) of 0.020(1), which results in a refined
structural stoichiometry of La9Sr1(Si5.5Al0.5O24)O2.24(2). However, similar joint refinements for La8.65Sr1.35(Ge6O24)O2.32
showed the presence of oxide vacancies at the center of the
channels and the free refinement of the occupation factors for O
(4) and O(5) converged to 0.928(4) and 0.045(1), see Table 2.
These values yield La8.65Sr1.35(Ge6O24)O2.40(2) as refined
structural stoichiometry which is not far from the expected one.
The joint refinements converged to a structural description
with interstitial oxygens at (0.009(2) 0.213(2) 0.598(1)) and
(− 0.009(18) 0.221(17) 0.570(13)) for La8.65Sr1.35(Ge6O24)
O2.32 and La9.5□0.5(Si5.5Al0.5O24)O2, see Table 2. This position
is the same than that predicted theoretically [29,30] and
experimentally found from NPD [23,26]. This position is very
close to the average position of O(3) (∼ 1.1 Å) but the atomistic
calculations showed that the lattice can accommodate the
interstitial oxygen through a local relaxation. The interstitial
oxygens converged to (0.014(5) 0.071(3) 0.555(4)) and (0.015
(18) 0.064(8) 0.537(9)) for La8.65Sr1.35(Si6O24)O2.32 and La9Sr1
(Si5.5Al0.5O24)O2.25, respectively, see Table 2.
It must be underlined that the positions of the interstitial
oxide anions in La8.65Sr1.35(Si6O24)O2.32 and La8.65Sr1.35
(Ge6O24)O2.32 are different and, to the best of our knowledge,
it has not been reported so far. For the silicate apatites, without
L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315
1313
Fig. 3. Observed (crosses), calculated (full line) and difference curve (bottom) of a) RT-SXRPD (λ = 0.40 Å) and b) RT-NPD patterns for La8.65Sr1.35(Si6O24)O2.32
(λ = 1.89 Å). The inset shows a selected RT-SXRPD region (20–30°/2θ) for the same sample.
cation vacancies, the interstitial site is much closer to the center
of the oxide channels and far away from O(3). The average (not
real) O(5)···O(4) and O(5)···O(3) distances for La8.65Sr1.35
(Si6O24)O2.32 are 1.54(3) and 2.37(2) Å, respectively. The
average O(5)···O(4) and O(5)···O(3) distances for La8.65Sr1.35
(Ge6O24)O2.32 are 2.33(1) and 1.15(2) Å, respectively. Two
projections of the crystal structures of La8.65Sr1.35(Si6O24)O2.32
and La8.65Sr1.35(Ge6O24)O2.32 with the interstitial oxygen atoms
highlighted are shown in Figs. 4 and 5. As it can be seen in Fig. 4,
the interstitial oxides are closer to the center of the channel for
La8.65Sr1.35(Si6O24)O2.32 and far away for La8.65Sr1.35(Ge6O24)
O2.32. Fig. 5 highlights the existing difference at the interstitial
oxide positions for the silicate and germanate apatites. Furthermore, the c-axis path for the O(4) and O(5) migration is also
visible in Fig. 5.
Formal inclusion of the interstitial oxygens at the channel
periphery in diffraction data analysis is difficult because the
stable interstitial site lies too close to other atoms (either an
oxygen of the silicate substructure in La8.65Sr1.35(Ge6O24)O2.32
or the loosely bounded oxide of the center of the channels in
La8.65Sr1.35(Si6O24)O2.32). This closeness (1.1–1.5 Å) requires
local relaxation of the structure around the site, which is not
possible to model accurately with diffraction methods. However, some clues about the local relaxation can be indirectly
extracted from the anisotropic ADP's values when studying a
series in a consistent way as previously underlined [23,26].
The main anisotropic ADP to detect the local relaxation of the
silicate groups is U11 of O(3) which is very large (close to
0.05 Å2) for those compositions with the interstitial oxygen
position located very close to the average position of the tetrahedral groups. As it can be seen in Table 3, U11 values of O(3) are
significantly larger for La8.65Sr1.35(Ge6O24)O2.32 (0.052(1) Å2)
and La9.5□0.5(Si5.5Al0.5O24)O2 (0.043(1) Å2) than for La8.65Sr1.35
(Si6O24)O2.32 (0.027(1) Å2) and La9Sr1(Si5.5Al0.5O24)O2.25
(0.024(1) Å2). This finding indirectly supports the structural
result where the oxygen interstitials are closer to the silicate
groups in the two former apatites, and hence, they must be more
displaced (relaxed), i.e. higher ADP values. These results are in
1314
L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315
Hence, La10−(1−x)2/3□(1−x)2/3(Ge6O24)O2+x series is single phase
in the compositional range (0.18 ≤ × ≤ 0.62) being hexagonal
(space group s.g. P63/m) for 0.18≤ × ≤0.33 and triclinic (s.g. P-1)
for 0.33 ≤x≤ 0.62 [16]. On the other hand, La10−(1−x)2/3□ (1−x)2/3
(Si6O24)O2+x is single phase in the compositional range
(0 ≤x≤ 0.40) and the symmetry is hexagonal for all compositions
[26]. Oxy-germanates may incorporate higher amount of oxygens
in the structure because germanate tetrahedra are more flexible
and they undergoes the local relaxation easier, see above. When
the amount of oxygen interstitial reaches a limit, 0.35 mol of
oxide per chemical formula in the former series, the symmetry
changes to triclinic at room temperature. Many other oxy-apatites
series have been prepared with different aliovalent substitution
[9] but the maximum oxide content for germanates is always
larger than for silicates.
HT-NPD data showed that there is no phase transition on
heating. The Rietveld refinements were carried out as previously described using only neutron data. The final results are
given in Tables 2 and 4. The bond distances slightly increase on
heating due to the thermal expansion and the thermal vibration
parameters increase, as expected.
Fig. 4. c-axis view of the crystal structure of a) La8.65Sr1.35(Ge6O24)O2.32 and b)
La8.65Sr1.35(Si6O24)O2.32, showing the TO4 groups as tetrahedra and La(1), La/Sr
(2) and O(4) atom as balls. The cation vacancies are not shown. Interstitial oxygen,
O(5), is highlighted (white-grated balls) and shown at 16% of occupancy for clarity.
agreement with many previous reports. For instance, U11 values
of O(3) for La9.33□0.67(Si6O24)O2 and Nd9.33□0.67(Si6O24)O2,
were 0.048(2) and 0.063(2) Å2, respectively and for La8Sr2
(Si6O24)O2 and Nd8Sr2(Si6O24)O2, were 0.023(1) and 0.021
(1) Å2, respectively [36]. These oxy-apatites with cation
vacancies have high oxide conductivities due to oxygen interstitials close to the silicate tetrahedral that must be locally relaxed
leading to very high U11 values for O(3).
Finally, we must underline the different behavior between
the oxy-germanates and oxy-silicates. La8.65Sr1.35(Ge6O24)
O2.32 has oxide vacancies at the center of the oxide channels
and the interstitial oxygen position is far away from the center of
the channels close to the germanate sublattice. Conversely,
La8.65Sr1.35(Si6O24)O2.32 do not have oxide vacancies at the
center of the oxide channels and the interstitial oxygen position
is very close to these oxide anions. This is an important difference that it justifies their different crystalchemistry properties.
Fig. 5. a-axis view of the crystal structure of a) La8.65Sr1.35(Ge6O24)O2.32 and b)
La8.65Sr1.35(Si6O24)O2.32, with the atoms displayed as in Fig. 4.
L. León-Reina et al. / Solid State Ionics 177 (2006) 1307–1315
4. Conclusions
The reported NPD study have shown an important structural
difference for La8.65Sr1.35(T6O24)O2.32 (T = Si, Ge) materials.
The framework of these compounds is the same but the interstitial oxygen positions are different. La8.65Sr1.35(Ge6O24)O2.32
has vacancies at the center of the oxide channels and a higher
amount of interstitial oxygens which are close to the locally
relaxed germanate sublattice. La8.65Sr1.35(Si6O24)O2.32 do not
have such oxide vacancies and the interstitial oxygen position is
very close to oxide channels which prevents the stabilization of
materials with high oxygen excess.
Acknowledgments
Financial support from the MAT2003-7483-C2-1 research
grants is acknowledged. LLR thanks the Junta de Andalucía for
a studentship. This work was partially performed at the spallation neutron source SINQ, Paul Scherrer Institut, Villigen,
Switzerland. ESRF is thanked for the provision of X-ray synchrotron facilities at ID31. We thank Dr. Sheptyakov for his
assistance with the NPD data collection.
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