Investigation of the α-BiO y F3 - 2y -based tysonite-type solid solutions by X-ray diffraction and impedance spectroscopy

Article abstract of DOI:10.1134/S003602360801018X

Tysonite solid solutions Bi_{1 - x} M _x (O, F) _{3 - d} (M = Na, Sr, or Nd) based on α-BiO _y F _{3 - 2y} were prepared by solid-state synthesis at 873 K with subsequent quenching to ice-cold water. Aliovalent substitutions in both the cation and anion sublattices (M ⁿ⁺ → Bi³⁺ and O ^2- → F^-) made it possible to vary the anion-vacancy density. The solid solutions were characterized by X-ray diffraction and impedance spectroscopy. The homogeneity regions for the tysonite solid solution were determined; triangulation schemes at 873 K were suggested for the systems BiF₃-BiOF-NaBiF₄ and BiF₃-BiOF-SrF₂, and a scheme of the subsolidus phase diagram for the system BiO _0.1F_2.8-NdF₃ was suggested. In the system BiO_0.1F_2.8-NdF₃, the transition temperature from the low-symmetry tysonite phase (phase II, space group P c1, Z = 6) to the high-symmetry one (phase I, space group P6₃/mmc, Z = 2) decreases with increasing anion-vacancy density. Conductivity measurements were performed in the temperature range 300-523 K and the frequency range from 5 to 1 × 10⁶ Hz. The conductivity of samples in the system BiO _0.1F_2.8-NdF₃ increases with increasing bismuth-ion and anion-vacancy concentrations.

Full text of DOI:10.1134/S003602360801018X

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 1, pp. 129–135. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © V.A. Prituzhalov, R.Ya. Zakirov, E.I. Ardashnikova, V.A. Dolgikh, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 1, pp. 135–141.
PHYSICAL METHODS
OF INVESTIGATION
Investigation of the a-BiO F -Based Tysonite-type
y 3 – 2y
Solid Solutions by X-ray Diffraction
and Impedance Spectroscopy
a
b
c
c
V. A. Prituzhalov , R. Ya. Zakirov , E. I. Ardashnikova , and V. A. Dolgikh
a
Materials Science Department, Moscow State University, Vorob’evy gory, Moscow, 119992 Russia
b
Russian Research Center Kurchatov Institute, pl. Kurchatova 1, Moscow, 123182 Russia
c
Chemistry Department, Moscow State University, Vorob’evy gory, Moscow, 119992 Russia
Received March 12, 2007
Abstract—Tysonite solid solutions Bi1 – xå (O, F)
(M = Na, Sr, or Nd) based on α-BiOyF3 – 2y were prepared
x
3 – d
by solid-state synthesis at 873 K with subsequent quenching to ice-cold water. Aliovalent substitutions in both
n+
3+
2–
–
the cation and anion sublattices (M
Bi and O
F ) made it possible to vary the anion-vacancy den-
sity. The solid solutions were characterized by X-ray diffraction and impedance spectroscopy. The homogeneity
regions for the tysonite solid solution were determined; triangulation schemes at 873 K were suggested for the
systems BiF –BiOF–NaBiF and BiF –BiOF–SrF , and a scheme of the subsolidus phase diagram for the system
3
4
3
2
BiO F –NdF was suggested. In the system BiO F –NdF , the transition temperature from the low-symme-
0.1 2.8
3
0.1 2.8
3
try tysonite phase (phase II, space group ê3Ò1, Z = 6) to the high-symmetry one (phase I, space group P6 /mmc,
3
Z = 2) decreases with increasing anion-vacancy density. Conductivity measurements were performed in the
6
temperature range 300–523 K and the frequency range from 5 to 1 × 10 Hz. The conductivity of samples in the
system BiO F –NdF increases with increasing bismuth-ion and anion-vacancy concentrations.
0.1 2.8
3
DOI: 10.1134/S003602360801018X
Materials with tysonite-type (LaF ) structures are likely, stabilizes the high-symmetry hexagonal phase
3
now the best solid electrolytes with fluoride-ion con- [10]. Transitions II
I from the low-symmetry to
ductivity. The tysonite structure has been studied and high-symmetry phase during heating have been
discussed repeatedly and is comprehensively described described for LnF
(Ln = La–Eu) [11] and Sr Yb1 – xF
x 3 – x
3
for two very similar phases: the high-symmetry hexag- [12].
onal phase (phase I, space group ê6 /mmc, Z = 2 [1];
3
The high fluoride-ion mobility is due to anionic
natural mineral LaF ) and the low-symmetry trigonal
3
vacancies. Aliovalent substitutions of alkaline-earth for
phase (phase II, space group ê3 Ò1, Z = 6 [2–4]; synthe- rare-earth cations (M2+
Ln ) increase the conduc-
3+
tivity by one to two orders of magnitude because of the
generation of extra anion vacancies [8]. Thus, the dop-
sized LaF single crystals).
3
X-ray diffraction patterns for the low-symmetry ing cation concentration offers a means for affecting the
trigonal phase II, in addition, display several weak transport properties of the solid electrolyte.
superstructural lines, indicating its increased unit cell
Tysonite solid electrolytes during performance vir-
volume as distinct from the high-symmetry hexagonal
tually always evolve small amounts of oxygen ions,
phase I. The unit cell parameters of these phases are
which substitute for part of the fluoride ions via pyro-
related through ‡ = ‡ / 3 and Ò = Ò . Tysonite-type hydrolysis and generate anion vacancies. Thus, even
I
II
I
II
structures are rather abundant among the fluoride and small amounts of oxygen ions can change the conduc-
oxofluoride stoichiometric compounds and solid solu- tivity parameters of solid electrolytes. The literature
tions: apart from the tysonite mineral, the following does not highlight the influence of the aforementioned
compounds crystallize in the high-symmetry structure substitutions on the transport characteristics of tysonite
of phase I: ThOF [5], BiO F
[6], Ln1 – xCaxF3 – x, and solid solutions.
2
y 3–2y
Ln1 – xSrxF3 – x (Ln = Tb–Lu, Y) [7, 8]; in the low-sym-
metry structure of phase II, LnF , its solid solutions
3+
Highly polarizable cations, such as Bi , usually
3
considerably improve the transport characteristics of
solid electrolytes [13]. Bismuth trifluoride forms solid
Ln1 – xMxF
3 – 2x
(M = Ca or Sr; Ln = La–Gd) [8], and
Bi1 – xBaxF3 – x [9] crystallize. The defect structure, solutions through aliovalent substitutions not only in
1
29

1
30
PRITUZHALOV et al.
the cation sublattice but also in the anion one, generat-
Conductivity measurements were carried out with a
ing tysonite-like α-BiO F
tions of O for F ions (O
through partial substitu- two-point probe using a Solarton HF FRA 1255 fre-
y
3 – 2y
2–
2–
–
–
6
F ) [14].
quency-response analyzer (frequency range, 5 to 10 Hz)
on disk samples (d = 8 mm, h = 1–2 mm) with copper
or gold electrodes from 300 to 523 K under an argon
atmosphere. Gold electrodes were sputtered in vacuum.
Copper electrodes were formed during molding as fol-
lows: to a mold (d = 8 mm), a thin layer of powdered
copper, a layer of a powdered test sample, and another
layer of powdered copper were placed. The test disks
prepared in this manner (under a load of 1.0 t) with cop-
This work was intended to search for and study tyso-
3
+
nite solid solutions containing Bi with the goal of elu-
cidating relationships between their structural features
and ionic conductivity.
To achieve the above goal, we used simultaneous
aliovalent cation and anion doping: in the solid solution
α-BiOyF3 – 2y, bismuth was partially substituted by
sodium, strontium, and neodymium cations. In choos- per leads on their ends were sintered at 723 K in welded
ing these dopant ions, we were guided not only by their copper ampoules under a dry argon atmosphere for sev-
different oxidation numbers but also by the closeness of eral hours. X-ray diffraction did not signify any reac-
their sizes to those of bismuth and fluorine ions: RF
–
=
tion between the lead material and test samples or any
change in the composition of test samples. Test disks
were transferred to a silica cell; the cell was degassed in
dynamic vacuum (423 K, 2 h) and filled with dry argon.
The resulting impedance loci were processed using the
Zview2.1 program (1998 Scribner Associates, Inc.,
written by Derek Johnson).
1
.17 Å, RO2– = 1.24 Å (CN = 4), RBi3+ = 1.31 Å, RNa
+
=
1.32 Å, RSr2+ = 1.40 Å, and RNd3+ = 1.25 Å (CN = 8) [15].
EXPERIMENTAL
The starting chemicals used were commercially
available samples of strontium, sodium, and neody-
mium fluorides, and bismuth oxides (all of pure for
analysis grade) and bismuth fluoride (chemically pure
grade). All starting chemicals were pretreated to
remove trace water and other impurities.
RESULTS AND DISCUSSION
BiF –BiOF–NaF and BiF –BiOF–SrF System
3
3
2
X-ray powder diffraction data for annealed samples
of the system BiF –BiOF–MF (where M = Na or Sr;
3
n
n = 1 or 2) show α-BiOyF3 – 2y-base tysonite solid solu-
Sodium and strontium fluorides were dried to con-
tions Bi1 – xå (O, F)3 – d: Bi1–xNa O F
(Fig. 1a)
x
x
y
3 – 2x – 2y
stant weight at 423 K; NdF and BiF were exposed to
3
3
and Bi1 – xSr O F (Fig. 1b). The X-ray diffraction
x
y 3 – x – 2y
flowing gaseous HF at 573 K for 3 h and degassed in
dynamic vacuum (~10 Pa) at 473 K for 2 h.
patterns for all single-phase tysonite samples were
indexed assuming the hexagonal crystal system (phase I)
by analogy with the α-BiOyF3 – 2y tysonite phase [6].
The unit cell parameters are listed in Tables 1 and 2.
In addition to the commercial bismuth fluoride sam-
ple, we used a sample prepared in a laboratory from bis-
muth hydroxide (chemically pure grade), as in [16]. At
Figures 1a and 1b display the experimentally deter-
mined homogeneity regions with account for the litera-
ture data on α-BiOyF3 – 2y and β-BiOyF3 – 2y [14, 16, 17].
the first stage of this synthesis, Bi(OH) was concen-
3
trated with 40% HF in a Teflon beaker; next, the precip-
itate was dehydrated in a copper-and-nickel reactor at
73 K for 3 h in flowing gaseous HF.
5
Figure 1a shows the triangulation scheme for the
system BiF –BiOF–NaBiF at 873 K suggested pro-
3
4
X-ray diffraction identification of the starting chem-
icals verified the absence of impurity phases. Ready-
ceeding from the results of this work and [18]. There
are three solid solutions (ss) in this system, namely flu-
orite solid solutions (F ss), tysonite solid solutions
T ss), and β-BiOyF3 – 2y-base ones, as well as five two-
phase fields and four three-phase ones.
for-use starting chemicals were stored over P O in a
2
5
desiccator.
(
Test samples were prepared by solid-state synthesis.
Homogenized and pelletized blends of powdered
chemicals were transferred to copper ampoules.
Figure 1b shows our suggested triangulation scheme
1
Ampoules with test samples were degassed in dynamic for the system BiF
–BiOF–SrF
at 873 K. There are
3
2
vacuum at 473 K for 1 h, filled with dry argon, and her- three solid solutions (ss) in this system, namely F ss,
metically welded. Ampoules were annealed (873 ± 10 K, T ss, and β-BiOyF3 – 2y ss, as well as six two-phase
3
h) and quenched to cold water.
fields, and three three-phase ones. The fields designed
in terms of the phase rule and unverified experimentally
are indicated by dashed lines in both phase diagrams.
The phase composition of test samples was studied
by X-ray powder diffraction. X-ray diffraction patterns
were recorded in a Guinier monochromator camera
with the effective diameter 228 mm using CuK radia-
1
The authors thank E.N. Dombrovskii, a student of the Chemistry
α1
Department, Moscow State University, for participation in the
experimental refinement of some fields of the phase diagram.
tion.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 1 2008

INVESTIGATION OF THE α-BiOyF3 – 2y-BASED TYSSONITE-TYPE SOLID SOLUTIONS
131
(
‡)
NaBiF4
Na0.5 – xBi0.5 + xF2 + 2x
F ss
F ss + BiF + T ss
3
F ss + β + BiOF
1
0% NaF
5
% NaF
T ss
BiOF
BiF3
β-BiOyF3 – 2y
α-BiOyF3 – 2y
SrF2
(
b)
Sr1 – xBi O F
x
y
2 + x – 2y
F ss
Single-phase samples
Two-phase samples
Three-phase samples
F ss + BiF + T ss
3
T
BiF
3
BiO1.5
BiOF
β-BiOyF3 – 2y
α-BiOyF3 – 2y
Fig. 1. Panel (a): the triangulation scheme for the BiF –BiOF–NaBiF phase diagram at 873 K. Panel (b): the same for the
3
4
BiF −BiOF–SrF phase diagram at 873 K.
3
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 1 2008

1
32
PRITUZHALOV et al.
Table 1. Hexagonal unit cell parameters (Å) and logσ
Figure 2 shows the ionic conductivity versus tem-
(
conductivity, S/cm) in the tysonite-like solid solution perature plots for some samples of tysonite solid solu-
Bi1 − xNa O F
x
y
3 – 2x – 2y
tions of the systems studied. These plots obey the
Arrhenius equation σ = σ exp(–E /k T). The activa-
T
0
a
B
y
x
tion energies of conductivity E found with an accuracy
of 0.02 eV by the least-squares fits are in the range
‡
0
.09
0.12
0.15
0
0
0
.03 a = 4.096(3)
c = 7.274(7)
a = 4.102(3)
c = 7.272(4)
a = 4.119(2)
c = 7.28(2)
0
.56–0.59 eV. The logσ values for 293, 373, and 473 K
are displayed in Tables 1 and 2.
logσ303 K = –7.05 logσ303 K = –7.29 logσ303 K = –6.84
logσ373 K = –5.05 logσ373 K = –5.27 logσ373 K = –4.88
logσ473 K = –2.63 logσ473 K = –2.84 logσ473 K = –2.55
The unit cell parameters and conductivity vary only
insignificantly inside the fields of homogeneous solid
solutions Bi1 – xNa O F
and Bi1 – xSr O F
.
x
y
3 – 2x – 2y
x
y
3 – x – 2y
.05 a = 4.102(1)
c = 7.272(2)
a = 4.109(6)
c = 7.28(2)
Conductivity versus concentration plots are nonmono-
tone: they have minima in ı = const and y = const sec-
tions.
logσ303 K = –7.75 logσ303 K = –7.57
logσ373 K = –5.84 logσ373 K = –5.63
logσ473 K = –4.09 logσ473 K = –3.37
The small sizes of the homogeneous fields make it
impossible to interpret the nonmonotony of the proper-
ties. To increase the extent of the homogeneous field, we
studied solid solutions in the system BiO F –NdF
.08 a = 4.101(2)
a = 4.096(3)
c = 7.26(1)
c = 7.267(4)
0.1 2.8
3
logσ303 K = –7.03
logσ373 K = –5.08
logσ473 K = –3.18
with aliovalent anion substitutions and with isovalent
substitutions of Bi for Nd , a cation with a similar
radius.
3
+
3+
Table 2. Hexagonal unit cell parameters (Å) and logσ (conductivity, S/cm) in the tysonite-like solid solution
Bi1 − xSr O F
x
y 3 – x – 2y
y
x
0.03
0.06
0.09
0.12
0.15
0
0
0
.03
.05
.08
a = 4.114(2)
c = 7.273(4)
log σ293 K = –5.35
logσ373 K = –3.26
logσ473 K = –1.64
a = 4.114(1)
c = 7.268(3)
a = 4.114(1)
c = 7.273(3)
a = 4.111(1)
c = 7.271(4)
a = 4.113(2)
c = 7.274(5)
a = 4.113(1)
c = 7.273(2)
log σ293 K = –4.88
logσ373 K = –2.69
logσ473 K = –1.00
log σ293 K = –5.38
logσ373 K = –3.29
logσ473 K = –1.68
log σ293 K = –5.29
logσ373 K = –3.12
logσ473 K = –1.43
a = 4.111(1)
c = 7.274(3)
log σ293 K = –5.44
logσ373 K = –3.31
logσ473 K = –1.65
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 1 2008

INVESTIGATION OF THE α-BiOyF3 – 2y-BASED TYSSONITE-TYPE SOLID SOLUTIONS
133
Sr0.05Bi0.95O0.06F2.83
logσ [S/cm]
Sr0.05Bi0.95O0.12F2.71
Sr0.05Bi0.95O0.15F2.65
Sr0.03Bi0.97O0.12F2.73
–
–
–
–
–
–
–
2
3
4
5
6
7
8
Sr0.08Bi0.92O0.12F2.68
Na0.08Bi0.92O0.04F2.76
Na0.05Bi0.95O0.04F2.82
2
.0
2.2
2.4
2.6
2.8
3.0
3.2
1
3.4
000/T, K–
1
Fig. 2. Conductivity vs. temperature for the tysonite solid solutions Bi1 – ıå (O, F)
(M = Na or Sr).
ı
3 – d
System BiO F –NdF
3
field of the high-symmetry phase I increases, that of the
low-symmetry phase II decreases, and the two-phase
0
.1 2.8
X-ray powder diffraction of annealed (873 K) sam-
ples showed two tysonite solid solutions in the system
(
I + II) field narrows with rising temperature. With
account for the II I phase-transition temperature in
NdF , which is equal to 1280 K [11], for the BiO F –Nd
3
2
BiO F –NdF : BiO F3 – 2y-based ss Bi1 – xNd (O, F)
0
,1 2.8
3
y
x
3 – d
3
0.1 2.8
(
(
0 < x < 0.20) and NdF -based ss Bi Nd (O, F)
3 1 – x x
3 – d
system, we suggested a scheme of the subsolidus region
0.78 < x < 0.1) (Fig. 3). A two-phase field is sus-
pended between these two solid-solution fields.
of the phase diagram for the BiO F –NdF system
0,1 2.8
3
(
Fig. 4). One component of this system (NdF ) is poly-
3
Bi1 − xNd (O, F) ss (0 < x < 0.20; 0 < d < 0.10 at 873 K)
x
3 – d
morphic, and the high-symmetry hexagonal phases
was identified with the high-symmetry phase I because
of the absence of superstructural reflections in its X-ray
powder diffraction patterns. The parameters ‡ and Ò of
(phase I) are completely miscible.
One can see from this scheme that the temperature
this solid solution decrease with increasing neodymium of the II
I phase transition decreases with increas-
concentration ı (Fig. 3). Bi1 – xNd (O, F)3 – d ss (0.78 < ing BiO0.1F2.8 concentration and, accordingly, anion
x
x < 0.1; 0.02 < d < 0 at 873 K) was identified with the vacancy concentration.
low-symmetry phase II because its X-ray diffraction
Conductivity versus temperature curves were
patterns showed superstructural reflections that are
recorded for solid-solution samples (Fig. 5). The con-
nonexistent in high-symmetry phase I. The parameter ‡
ductivity of BiOyF3 – 2y-based Bi1 – xNd (O, F)
ss (0 <
x
3 – d
of this solid solution insignificantly increases, whereas
the parameter Ò insignificantly decreases with increas-
ing neodymium concentration ı (Fig. 3). Figure 3 dis-
plays the unit cell parameters of the two solid solutions
x < 0.20) is higher than that of Bi1 – xNd (O, F)
ss
x
3 – d
(0.78 < x < 1) over the whole temperature range. The
conductivity versus temperature plots obey the Arrhe-
versus concentration; for the convenience of compari- nius equation σ
son, they refer to an increased unit cell (Z = 6).
= σ exp(–E /k T). The activation
0
a B
T
energies of conductivity E were calculated by the least-
‡
From the X-ray powder diffraction patterns of sam-
ples annealed and quenched from two temperatures
squares fits accurate to 0.02 eV; they fall in the range
0
.56–0.59 eV.
(873 and 923 K), it was found that the homogeneous
in the range 470–500 K, a kink appears on the con-
2
The authors thank T.V. Serov and T.A. Sysoeva, students of the
Materials Science Department, Moscow State University, for par-
ticipation in the experimental refinement of some fields of the
phase diagram.
ductivity–temperature curves (Figs. 2, 5); this kink,
which is typical of tysonite phases, is related to a
change in the conductivity mechanism [19].
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 1 2008

1
34
PRITUZHALOV et al.
Å
000/T, K–1
1
a
7
7
7
7
.08
.06
.04
.02
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4
1
0
1
Bi1 – xNd (O, F)
x
3 – d
x = 0.1
x = 0.7
x = 0.3
x = 0.9
x = 0.2
x = 0.8
–
NdF3 ss
–2
BiO0.1F2.8 ss
–
3
–4
–
–
5
6
7
7
7
7
7
7
7
.31
.29
.27
.25
.23
.21
.19
c
logσ [S/cm]
Fig. 5. Conductivity vs. temperature for the tysonite solid
solution in the system BiO0.1F2.8–NdF3.
BiO0.1F2.8 ss
NdF3 ss
CONCLUSIONS
Our studies of the tysonite solid solutions in the sys-
0
10 20 30 40 50 60 70 80 90 100
NdF mol %
tem BiO F –NdF revealed a phase transition from
the low-symmetry phase II to the high-symmetry phase
I induced by temperature evolution: II I. The
0
.1 2.8
3
3
Fig. 3. Unit cell parameters vs. concentration for the solid
solution in the system BiO0.1F2.8–NdF3.
phase-transition temperature is composition-depen-
dent: it decreases with increasing anion-vacancy con-
centration. For the solid solutions with aliovalent cation
and anion substitutions with close sizes of dopant ions,
the parameters ‡ and Ò in phases I and II behave differ-
ently as a function of anion-vacancy concentration. The
conductivity of the solid solutions in both phases
increases with increasing bismuth-ion concentration,
but in the high-symmetry phase I (in which the anion-
vacancy concentration is higher), the conductivity is
one to two orders of magnitude higher.
T, K
280
X*
1
Phase I
9
8
8
23
73
23
The nonmonotony in the variation of conductivity as
a function of composition in the solid solutions
Bi1 − xå (O, F)
(M = Na or Sr) is likely due to the
x
3 – d
I + II
II
I phase transition in the two-phase (II + I) field,
which is unobservable in X-ray diffraction patterns.
Tysonite-like phase II should be sought in composi-
tions with low anion-vacancy levels, i.e., near bismuth
trifluoride.
Phase II
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research (project no. 07-03-00985).
BiO F
50%
NdF3
0
.1 2.8
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 1 2008

INVESTIGATION OF THE α-BiOyF3 – 2y-BASED TYSSONITE-TYPE SOLID SOLUTIONS
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