Chernaya, T. S.

Muradyan, L. A.

Simonov, V. I.

Revistes Catalanes amb Accés Obert

STRUCTURAL FEATURES THAT CAUSE
DIFFERENT IONIC CONDUCTIVITY
IN RbNbWO AND T1NbWO
T. S. CHERNAYA, L. A. MURADYAN, AND V. 1. SIMONOV
Institute of Crystallography , USSR Acad. Sci., Moscow
The compounds RbNbWO6 and TINbWO1, belong to a group with-
idealized chemical formula AB2O6. The structural basis is a pyrochloro-
like three-dimensional framework built up of BO octahedra occupied by
W atoms, the latter being partially relaced by Nb atoms. This results
in nostoichiometry of the composition. Large Rb and T1 cations are
positioned in the framework voids which can be statistically occupied.
The crystals of this type are interesting because of their peculiar
optical and electric properties. At temperatures below 330 and 360 K such
crystals exhibit electrical polarity, while upon heating they undergo a
peculiar phase transition of the ferroelectic-ionic superconductor-type. A
structural study of both these compounds is of special interest because
TINbWO6 crystals exhibit significantly higher spontaneous polarization
in the low-temperature ferrolectric phase as well as higher ionic conducti-
vity in the high-temperature superionic state, as compared to rubidium
tungsten-niobate. Ionic conductivity in these compounds is due to univa-
lent Tl and Rb cations which occupy large framework voids being are
comparatively weakly linked to its atoms and can relatively easily migrate
along the channels within the structure.
Experimental neutron diffraction data were obtained at temperatures
423 and 573 K, that exceed the phase transition point, i.e. the samples were
studied at two temperatures in the superionic state.
It was established that the frameworks of both compounds have prac-
tically identical geometry, while the arrangement of Rb and Tl atoms in
equal octahedra differ significatly. As is shown in Figs la, b, 2a, b, 3a, b,
Rd cation is located at the octahedra centre, while Tl cation is shifted from
this centre towards octahedral faces by 0.6 A and statistically occupies
[Butll. Soc. Cat . Cicn.], Vol. XI I, Num. 2, 1991
428 Y.S. CIIERNAYA - L.A. JfURAI)I"AA' - V.I. SIM0.ti0V
a
3/8 a +b
3/8 -!
C CI
B
3/4
C
3/8 Cr
Fig. I. Difference nuclear density maps in structures of RbNbWO6 (a-diagonal section) and
TINbWO6 (b-diagonal, c-coordinate sections). T = 423 K. Isoline scale is p-0.1 OmA '.
Fig. 2a. Three- dimensional difference density map in the RbNbWO6 structure.
Fig. 2b. Three-dimensional difference density map in the TINhWO6, structure.
a+b
[Butll. Soc. Cat . Cien.], Vol. XII, Num. 2, 1991
DIFFERENT IONIC CONDUCTIVITY
a
Fig. 3
429
eight sites that correspond to two crystallographically independent sites in
the Fd3m space group. Such a situation is unusual for classifical crystal
chemistry of inorganic compounds.
It was found that at 573 K there is a significant deviation of thermal
atomic vibrations of Rb ions from the harmonic law. Difference map of
Rb cation probability distribution obtained by removal of the harmonic
component indicates preferable Rb ion shifts towards the `windows' bor-
dering octahedral sites (Fig. 4). Appropriate sections of this map are simi-
lar to maps of statistical Tl ion distribution in TINbWO6.
Cationic diffusion paths during the ionic transport in the studied
structures are different. The difference in the conductivity is due to the
character of Rb and Tl ion arrangement in the structures.
J/8
4,8
C
a+b
Fig. 4. Diagonal section of zero nuclear density synthesis of RbNbWO6 structure. Con-
tributions of atoms in harmonic approximation are substracted. Isoline level scale
p-0.02 OmA-'.
[Butll. Soc Cat. Cien.], Vol. XI I, Num.2, 1991
430 T.S. CHERNAYA - L.A. MURADYAN - 1'.1..S1h10N01
Table 1. Neutron diffractions data and crystals data.
Parameter
a, A
0, cm
p, cm
(sin A-'
Number of observed reflections
Number of independent reflections
Number of equivalent reflections
R averaged, of equivalent reflections, `Y.
RbNbWO,, TINbWO6
423 K 573 K 423 K 573 K
Fd 3m Fd 3m
10.382 10.392 10.396 10.401
0.30 0.40
0.020 0.014
0.78 0.78 0.78 0.78
1150 597 676 682
85 86 97 99
24 12 12 12
3.01 2.30 2.35 2.51
Remarkable overlapping of density peaks naturally results in a rather
high degree of the correlation between the occupancies of 32 and 32'sites
of T11 and T12 atoms and their thermal parameters obtained after least-
squares refinement of the structural model, TI atoms being statistically po-
sitioned at two independent sites. It is impossible to determine unambi-
guously the most probable values of the strongly correlating parameters
during the LSM refinement because, in fact, there is no evident global
minimum of the discrepancy funcion R for there parameters . In our case,
the coefficients of correlation between the occupancies and the thermal
parameters of TI1 and T12 atoms were 0.95. Our attempts to refine these
parameters led to different solutions, depending on the choice of the initial
approximation and the refinement procedure, R-factor being practically
the same: R = 1,55%.
In this case the scatter in the parameters essentially exceeded the stan-
dard deviations calculated by inverting the matrix of normal equations.
In order to solve the problem we applied compulsory scanning of oc-
cupancies . the procedure of the refinement of all the parameters was re-
peated several times at various fixed occupancies q, 1/8 + Aq and q, =
1/8 - Aq in the range of possible changes of q from -1/8 to + 1/8. In the
region of minimum R-factor values was 0.01. It should be noted that a
strong correlation was observed only between q occupancies and thermal
parameters. In all the refinement procedures all the other parameters,
including the redaction coefficient and the extinction parameter , remained
stable, to an accuracy not worse than 0.1 % for the whole of low R-factor
values.
[Butil. Soc. Cat. Cien.], Vol. XII, Num. 2, ! 991
DIFFERENT IONIC CONDUCTIVITY 431
Table 2. Position and isotropic thermal atomic parameters and main interatomic
distances (T = 573 K).
RbNbWO6 TINbWO6
Atom Parameters
423 K 573 K 423 K 573 K
x=y=z 0 0 0 0
Nb, W B3KB (A2) 1.85 1.94 1.78 1.85
B11 = B22 = B33 0.0041 0.0045 (1) 0.0040 0.0043 (1)
B12 = B13 = B23 -0.0012 -0.0009(1) -0.0010 -0.0008(1)
x 0.3123 0.3121 (1) 0.3120 0.3120(t)
y = z 0.125 0.125 0.125 0.125
O BaKB 1.25 1.90 1.31 1.70
B11 0.0027 0.0037(l) 0.0029 0.0039 (1)
B22 = B33 0.0030 0.0040 (1) 0.0031 0.0040 (1)
B23 0.0011 0.0016(l) 0.0013 0.0016 (1)
x=y=z 0.375 0.375
BH30 6.5 7.6(l)
Rb C123 103 0.004 (1)
D1111 104
-0.0055 (6)
D 1122. 104 -0.0012(4)
T11
T12
x=y=z
BaKB
B11=B22=B33
B12 = B23 = B13
x = y = z
BaKB
B11 = B22 = B33
B12=B23=B13
0.412 0.412(3)
5.2 5.5
0.012 0.013(2)
-
0.0014 -0.0012(8)
0.348 0.347 (7)
5.2 5.6
0.012 0.013 (2)
-0.0018 -0.0012(8)
1.9 1.3 1.9 1.6
1.6 1.1 1.5 1.5
The results of the above refinement procedure of the structural model
with compulsory occupancy scanning at 573 K are shown in Fig. 5. As for
T = 423 K the difference consists only in the fact that the plateausize of
minimum values is less by some q interval. The use of the standard proce-
dure of least -squares refinement for unambiguous determination of the
[Butll. Soc. Cat . Cien.], Vol. X11, Num . 2, 1991
432 T.S. CHERNAYA - L.A. MURADYAN - V.I. SIMONOV
T 4^
n
0,045 0,125 0.205
Fig. 5. Dependence of R-factor (curve 1) and termal parameters B, for Tl, (curve 2) and Tiz
atoms (curve 3) on the occupancy qi of site by Tl, atom; qz = 1/8 - q1; T = 543 K.
structure parameters and the estimation of their confidence intervals in
this case were, in fact, impossible. Additional information is needed. This
can be crystal-chemical, physical or some other information. In our case
we have chosen the solution that leas to equal Beqv. for TI1 and Tl2 atoms.
If we ignore this circumstance, the least-squares technique in an automat-
ed regime will lead randomly to one of possible solutions, and the calcu-
lated deviations of the correlating parameters will yield significantly
lowered values of their confidence intervals.
From the obtained thermal parameters for a double-site model of
TINbWO6 we calculated probability density functions (PDF) P(x) of Tl1
and Tl2 atom displacements from their equilibrium sites. We also obtained
distributions of single-atom potentials near these sites (Fig. 6a, b, c).
The character of the distortion of a harmonic PDF by anharmonic
corrections for RbNbWO61 obtained as a result of the refinement, also
indicates preferable Rb atom shifts towards the above "windows" of con-
ductivity (Fig. 7) during the thermal motion.
The TI ion displacement from 8(b) to 32(e) site is due to specific fea-
tures of the electronic structure of these ions, such as a lone electron pair.
A comparison of thermal atomic vibrations at T 423 and 573 K sug-
gests that the phase transition leading to loss of conductivity as the tempe-
rature falls, results from the displacements of the main framework cations
from three-fold symmetry axes (Table 3).
[Butll. Soc. Cat . Cien.], Vol. XI1, Num. 2, 1991
DIFFERENT IONIC CONDUCTIVITY
V, a8
433
Fig. 6. Single -atom potentional of TI atoms in the TINbWO6 structure . Directions: TI, -
Tlz(a), T12 - Tl'z (b), TI, - Tl'z (c).
I./
1
98
0.4 -
Fig. 7. Behaviour of single-atom potentive of Rb atom in the RbNbWO6 structure, along the
directions II1(1 ) and 111(2) T = 573 K.
3/B
v
Fig. 8. Mutual arrangement of Tl ions occupying statistically 32 e, and 32 e2 sites near the
8b (3/8, 3/8, 3/8) site denoted by a cross. Distances: e, - e; = 1.09 A, e2 - e2 = 0.78 A, e, -
ez = 0.68 A. Tetrahedral vertices are 16 d sites.
[Butll. Soc. Cat . Cien.], Vol. XII, Num . 2, 1991
434 T.S. CHERNAYA - L.A. MURADYAN - V.I. SIMONOV
Table 3. Ellipsoids of thermal vibration of basis atom in RbNbWO6 and TINbWO6.
Atom and RbNbWO6 TINbWO6
point i = 1 2 3
simmetry
, ,
423 K 574 K 423 K 573 K
(Nb, W) 1 0 . 100 (3) 0 . 120 (3) 0 . 106 (3 ) 0.120 (3)
m 2,3 0.174 (1) 0.173 ( 1) 0.168(l) 0 .167(l)
0 1 0.102 ( 2) 0.115(2 ) 0.100(1) 0 . 113(l)
2 0.122 (2) 0.142 (2) 0.126 (1) 0.146 (1)
2mm 3 0 . 149(1) 0.176 ( 1) 0.155(1) 0.176(1)
Rb
42m 1,2,3 0.287 ( 3) 0.308 (3)
TI, 1 0.22 ( 2( 0.24 (2)
3m 2,3 0.27(1) 0.28(1)
I1, 1 0.21 (1) 0.24 (2)
3m 2,3 0.28 (1) 0.28 (1)
ABSTRACT
At temperatures below 330 an 260 K RbNbWD and TINbWO exhibit electric
polarity, while upon heating they undergo a peculiar phase transition of the ferroelec-
tric-ionic superconductor -type. TINbWO crystals exhibit significantly higher spon-
taneous polarization in the low- temperature ferroelectric phasse es well es higher io-
nic conductivity in the high - temperature superionic state, as compared to rubidium
tungsten -niobate.
Experimental neutron diffraction data were obtained at temperatures 422 and 572
K, that exced the phase transition point . It was established that the frameworks of
both compounds have practically identical geometry, while the arrangement of Rb
and TI atoms in equal octahedra differ significantly . Pb cation is located at the octahe-
dra centre, while TI cation is shifted from this centre towerds octahedral faces by 0.6
A and statistically occupies eight sites that correspond to two crystallographically in-
dependent sites in the Fdam space group.
[Butll. Soc. Cat. Cien.], Vol. X1 I, Num. 2, 1991


Structural features that cause different ionic conductivity in RbNbWO and TlNbWO

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