K.T. Jacob et al.: Measurement of Gibbs energy of formation of LaGaO3 using a composition-graded solid electrolyte
II. EXPERIMENTAL METHODS
A. Materials
Both oxide and fluoride solid electrolytes can be used
to measure the standard Gibbs energy of formation of
LaGaO3 at high temperatures. The oxygen chemical poten-
tials established by phase mixtures Ga + La2O3 + La4Ga2O9
and Ga + La4Ga2O9 + LaGaO3 can be determined using
a cell incorporating yttria-doped thoria as the solid elec-
trolyte. At the low oxygen potentials established by these
mixtures, stabilized-zirconia solid electrolytes exhibit
significant electronic conductivity (te > 0.01). For the
design of a suitable solid-state cell based on a fluoride
solid electrolyte, it is important to know the composition
of the fluoride phase in equilibrium with LaGaO3 and
Ga2O3. The aim is to measure the activity of La2O3 in the
two-phase mixture containing LaGaO3 and Ga2O3.
Presented in this article is the measurement of the
Gibbs energy of formation of LaGaO3 from its com-
ponent oxides using a solid electrolyte based on CaF2.
Preliminary experiments indicated that the fluoride
phase that coexists with LaGaO3 and Ga2O3 was LaF3
for T < 1010 K and an oxyfluoride solid solution
LaO1−xF1+2x at T > 1010 K. Therefore, working elec-
trodes consisting of LaGaO3 + Ga2O3 + LaF3 and
LaGaO3 + Ga2O3 + LaO1−xF1+2x were used in a solid-
state cell incorporating single-crystal CaF2 as the solid
electrolyte and CaO + CaF2 as the reference electrode.
Both electrodes were placed under flowing oxygen gas.
However, there was significant interaction between the
working electrode and the solid electrolyte, with migration
of La3+ from the electrode into the electrolyte. A com-
Optical-grade single crystals of CaF2, in the form of
disks 1.5 cm in diameter and 0.2 cm thick, were obtained
from Harshaw Chemical Company. Ultrapure anhydrous
powders of CaF2, LaF3, CaO, La2O3, and Ga2O3 were
supplied by Johnson Matthey Ltd., London, UK. The
oxides were heated at 1273 K in dry inert gas for 10 ks
before use. For the preparation of LaGaO3, stoichiomet-
ric amounts of La2O3 and Ga2O3 were ball milled, pel-
letized, and reacted at 1673 K for 36 ks. The pellet was
reground, repelletized, and reacted again at the same tem-
perature for an identical period. The formation of
LaGaO3 was confirmed by powder x-ray diffraction
(XRD). At room temperature, LaGaO3 has an orthorhom-
bic unit cell (space group Pbnm) with a ס
0.5524, b ס
0.5492, and c ס
0.7775 nm. At 1023 K, XRD revealed a
unit cell with rhombohedral symmetry (space group
R3c), with lattice parameters a ס
0.5561 and c ס
1.3567 nm in the hexagonal setting.
Solid solutions, LaO1−xF1+2x, were prepared by solid-
state reaction. Fine powder of La2O3 was mixed with
LaF3 in the required ratio. The fully homogenized mix-
tures were compacted into pellets, buried in a powder
mixture of the same composition, and reacted in a pre-
purified inert-gas atmosphere first at 1275 K for 36 ks
and subsequently at 1475 K for 18 ks:
a LnF3 + b Ln2O3 → (a + 2b) LnO1−xF1+2x , (1)
position-graded solid electrolyte (LaF3)y и (CaF2)(1−y)
,
with axial variation of y, was designed to minimize the
effect of La3+ diffusion on cell performance. The com-
position of the graded electrolyte in equilibrium with
LaF3 was defined by y ס
0.32. The composition of the
graded electrolyte in equilibrium with LaO1−xF1+2x was
found by trial and error. The cells can be represented as:
with x ס
(a − b)/(a + 2b). The high-purity Ar gas
(99.999%) used was first dehydrated by passing through
anhydrous Mg(ClO4)2 and P2O5, and then deoxidized by
passing through Cu turnings at 750 K and Ti granules at
1150 K. Pellets were pulverized after heat treatment. The
formation of LaO1−xF1−2x was confirmed by XRD. The
oxyfluoride solid solution has cubic structure at high
temperature and tetragonal structure at low temperature.
The high-temperature phase can be retained by rapid
quenching into chilled mercury. The lattice parameters of
the tetragonal phase are plotted as a function of compo-
sition in Fig. 1. The lattice parameters of the cubic phase
at room temperature vary linearly from a ס
0.5765 nm
for x ס
0 to a ס
0.5793 nm for x ס
0.3.
Lanthanum oxyfluoride solid solutions of various
compositions were equilibrated at different tempera-
tures with an equimolar mixture of LaGaO3 and
Ga2O3. The fluoride phase in equilibrium was identi-
fied by XRD. Below 1010 K, LaF3 was the equilibrium
phase. At higher temperatures, the oxyfluoride solid
solution was stable. The equilibrium composition of
the oxyfluoride phase was obtained from its lattice
parameter as shown in Fig. 1. The equilibrium compo-
sition of the lanthanum oxyfluoride phase is defined
(Cell I) Pt, O2, CaO
+ CaF2 CaF2 (LaF3)y и (CaF2)(1−y) LaGaO3
y = 0
y = 0.32
+ Ga2O3 + LaF3, O2, Pt
(Cell II) Pt, O2, CaO
+ CaF CaF LaF ͒ и CaF ͒
LaGaO3
͑
͑
2
2
3 y
2
͑
1−y
͒
y = 0
y = 0.28
+ Ga2O3 + LaO1−xF1+2x, O2, Pt
with the right-hand electrodes positive. Single-crystal
CaF2 was coupled with a polycrystalline composition-
graded solid electrolyte to form a bielectrolyte combina-
tion. The single-crystal CaF2, placed adjacent to the
CaO + CaF2 reference electrolyte, prevented grain-
boundary diffusion of CaO into the solid electrolyte.
J. Mater. Res., Vol. 15, No. 12, Dec 2000
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