Advances in Ma terials Phys ics and Chemistry, 2012, 2, 71-73
doi:10.4236/ampc.2012.24B020 Published Online December 2012 (htt p://
Copyright © 2012 SciRes. AMPC
Ion Mobility in the Fluorit e Solid Sol ut ions
50PbF2–30BiF320K(Na)F According to 19F, 23Na NMR Data
V. Ya. Kavun1, A. B. Slobodyuk1, I. A. Telin1, R. M. Yaroshenko1, I. G. Ma sle nnikov a1,
V. K. Goncharuk1, V.I. Kharchenko1,2
1Institute of Chemistry, Far Eastern Branch of RAS, Vladivostok, Russia
2Far-Eastern Federal University, Vladivost ok, Russ ia
Received 2012
Ion mobility in solid solutions of the fluorite structure 50PbF230BiF320KF (I) and 50PbF230Bi F320NaF (II) was studied by
NMR method. Analysis of 19F, 23Na NMR spect ra made it possible to reveal th e character o f ion motions in the fluor ide and sodium
subl attices with temperat ure variat ion, to determine t he types and te mper ature ranges in which they took pl ace. It was found that the
dominant form of ionic mobility in the samples I and II above 380 K was the diffusion of fluoride and sodium ions. According to
preliminary results of electro-physical studies, the conductivity reached values of ~ 2×10–210–3 S/cm above 500 K. The solid solu-
tions I and II can be recommended as a basis for u s e in the develo pment of new fu nctional material s..
Keywords: Solid Solutions 50PbF230BiF320K(Na)F; Ion Mobility; Ionic Conductivity; 19F, 23Na NMR Sp ectra; Fun ctional Mate-
1. Introduction
The fluorite-related solid solutions are known to exhibit fluo-
rine ionic conductivity and may be used as solid electrolytes
[1-4]. Solid electrolytes based on PbF2 [5] and BiF3, such as
MBiF4 (M = K, Rb, Tl) [6,7] and Pb1–xBixF2+x [1,6,8-10] , a re o f
special interest due to the high ionic conductivity of about 10–2
S/cm at 500 K. There are a few papers [1,3,7,11] on the con-
ductivity of fluorite solid solutions formed in the KFBiF3
system. Contrary to earlier data [7], authors of [3] observed
phase transitions in the solid solutions xKF(1х)BiF3 (0.35 х
≤ 0.50) attributed to transformation of the meta-stable fluo-
rite-structured phase [12] into the stable form of a structure
being similar to NaNdF4 compound. Under solid-phase interac-
tion in the BiF3KF system, it was found a formation of KBi2F7,
KBiF4, and KBi3F10 compounds, as well as of a phase of varia-
ble composition in the range of 48-70 mol.% BiF3 [13]. X-ray
analysis of the BiF3NaF system sho wed a pr esence of the Na-
BiF4 compound and a phase of variable composition of the
fluorite structure in the range of 65-73 mol.% BiF3 [14]. The
diffusion of fluoride and sodium ions in the solid solutions
Na1xBixF1+2х of the fluorite structure and NaBiF4 were studied
by 19F, 23Na NMR [15,16]. It was noted in [15] that there was
no motion of sodium ions in the NaBiF4 compound, whereas in
the solid solutions, the diffusion of Na+ ions was observed.
Ionic mobility in the fluorine sublattice of NaBiF4 compound
was attributed to reorientation of fluorine-containing groups,
and the number of high-mobile fluoride ions increased in the
solid solutions with temperature increase. Thus, in PbF2BiF3,
KFBiF3, and NaF–BiF3 systems, the fluorite solid solutions
of high ionic conductivity are formed, but there are no pub-
lished data on ion mobility and transport properties in solid
solutions in the ternary PbF2B iF3NaF system. As concern s
the study of solid solutions in the PbF2BiF3KF system,
some data were presented in ou r recen t paper [ 17].
The purpose of this work was to consider ion mobility in the
solid solution 50PbF230BiF320NaF and to compare ob-
tained data with our earlier similar results for the solid solution
50PbF230BiF320KF [ 17].
2. Experimental
Original materials for solid-phase synthesis of solid solutions of
the fluorite structure were the preliminarily vacuum dried bis-
muth trifluoride, lead difluoride, potassium and sodium fluoride
(grade che micall y pure). Th e mixtures of powdered fluorides
were melted in a closed glassy carbon crucible in a dry box
filled by argon at temperature of 700-800°C for 15 minutes.
The sample single-phase and characterization as compounds of
the fluorite structure were confirmed by X-ray diffraction anal-
ysis perfor med o n a B ru ker D8 ADV ANCE di ffractometer with
CuKα radiation.
19F, 23Na NMR spectra were reco rded on a multinuclear d ig-
ital spectrometer Bruker AV-300 at Larmor frequencies of
282.404 and 79.4 MHz, respectively, in the temperature range
of 150-450 K. The temperature accuracy was ± 2 K. Calcula-
tions of a rms width of NMR spectra (or the second moment S2,
in G2) were performed using an original code by formulas given
in [18]. The error in S2 value did not exceed 10%. The width
ΔH of lines (at half height, in kHz), chemical shift (CS, δ in
ppm) and integral intensity of 19F NMR spectrum components
were measured with the accuracy of 3%. The CS values of 19F
NMR signals were determined relative to the reference C6F6
(CS of C6F6 is -589 ppm relative to gaseous F2, for which δ(F2)
= 0 ppm), and of 23Na NMR signals relative to the aqueous
solution NaCl.
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3. Results and Discussion
Parameters of 19F NMR spectra and temperature ranges, in
which some particular type of ion motions took place in the
fluoride sublattice of fluorite solid solutions in the ternary
PbF2–BiF3K(Na)F systems, were determined by nature of the
alkali cation and its concentration. The shape of BiF3 line and
the almost constant second moment (S2(F) = 55 G2) under tem-
perature variations within 290-420 K were the evidences that
BiF3 was characterized by the absence of F ion motion with
frequencies ωc > γΔH (≈ 104 Hz) in this temperature range.
Contrary to BiF3, in the lead difluoride β-phase there was di ffu-
sion of F ions at temperatures above 370 K [19]. Introduction
of potassium (sodium) fluoride into the PbF2BiF3 system
caused a sh arp decrease of th e activation energy of a local (di f-
fusion) motion in the fluoride sublattice relatively to β-PbF2, as
evidenced by nature of ΔH1/2(F) temperature dependences
(Figure 1).
19F NMR spectrum of the solid solution 50PbF230BiF3
20KF (I) at 150 K was a single, slightly asymmetric, line with
ΔH = 42.7 kHz and CS = 126 ppm (Figure 2). For solid solu-
tions of fluorite structure, the CS anisotropy should be practi-
cally absent, and, hence, the observed spectrum asymmetry
could be attributed to a presence of different fluoride ions in the
lattice. Indeed, the experiment al spectrum of th is sample at 150
K could be decomposed into two Gaussian components with
CS of 157 and 92 ppm indicated a presence of at least two dif-
ferent types of fluoride ions in the lattice. The same deconvolu-
tion procedure, done for 19F NMR spectra of fluorite solid solu-
tions Na1xBixF1+2x at 175 K, allowed to authors of [16] to
attribute two revealed components to two different fluoride
sublattices, formed by the cubic and cubic-octahedral clusters.
Considering the absence of a plateau in the dependence ΔH1/2 =
f(T ) in the range of 200-150 K (Figure 1), it may be concluded
that the rigid lattice for fluoride subsystem was observed below
150 K. The temperat ure in crease to 20 0 K caused a N MR spec-
trum transformation and registration of a new "narrow" com-
ponent with CS = 118 ppm.
The observed changes in NMR spectra were due to an ap-
pearance of high-mobile fluoride ions (Ea < 23 kJ/mol) in-
creased with temperature. As it follows from a ratio of the
components integral intensities in the NMR spectrum, at 300 K,
the number of high-mobile fluoride ions was about 95%. At
Fi g ure 1. Temperature dependences of 19F NMR spectrum wi dth of
solid solutions 50PbF230BiF320MF (M = Na, K).
Figure 2. Transformation of the 19F NMR spectra of solid solutions
50PbF230BiF 320MF (M = Na, K) with temperature variation.
420 K, the NMR spectrum was a single line (ΔH = 2 kHz, S2 =
0.05 G2), its shap e with accu rac y up to 3% may be descr ib ed b y
a weakly resolved pent tent (with a small share of the Lorent-
zian function) being characteristic for nuclei with an axially
symmetric tensor of magnetic shielding. As a result of the
NMR spectrum simulations at 450 K, the parameters of this
tensor components were determined: δ11 = δ
22 = 121.1, δ33 =
114.75, and δiso = ⅓( δ11 + δ22 + δ33) = 119 ppm. The observed
ΔH and S2(F) values indicated a dominant role of diffusion in
the fluoride sublattice of solid solution 50PbF230BiF320KF
at temperatu res above 400 K.
19F NMR sp ectra of th e solid solution 50PbF230BiF320NaF
(II) of fluorite structure in the temperature range o f 150-230 K
consisted of a single, slightly asymmetric, line with ΔH =
41.5-38 kHz (Figure 2). Nature of temperature dependence of
the NMR spectrum second moment (Figure 1) evidenced that,
in this temperature range, the fluoride sublattice of sample II
may be considered as rigid. Taking into account the above
mentioned for 19F NMR spectra of the solid solution I, at 150 K,
the sample II NMR spectrum may be represented as two com-
ponents with CS of 151 and 106 ppm indicated an existence of
at least two different positions of fluorine ions in the lattice.
With the temperature increase from 230 to 300 K, the NMR
spectrum transformation was observed, associated with a nar-
rowing of th e spectru m to 22.5 kHz and an app earan ce of a new
“narrow” component with CS = 122 ppm above 245 K, indi-
cated on a development of the ion local motion in the fluorine
sublattice. The number of high-mobile fluoride ions increased
with temperatu re, and, at 350 K, the ratio of integral intensities
of the narrow and broad components (the number of mobile and
immobile fluoride ions) was 87.5:12.5. At 380 K, the NMR
spectrum consisted of a nearly Lorentzian line with ΔH = 5.3
kHz and CS = 121 ppm. Heating of the sample to 450 K caused
a narro wing of the reso nance line to 2.5 kHz and a d ecrease of
the second moment to 0.15 G2. In this case, the spectral shape
may be described by superposition of a weakly resolved pent
tent, being characteristic for an axially symmetric tensor with
parameters δ11 = 123.8, δ22 = δ33 = 118, and δiso = 119.9 ppm,
and a Lorentzian function. It should be noted that since the 19F
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NMR spectrum shape of samples I and II at 450 K was de-
scribed by various pent tents (the tents were “flipped” relative
to each other), so the tensor parameters were different. For ex-
ample , a similar situation occured for a axially symmetric ten-
sors described the 19F NMR spectra of polycrystalline M2AF6
samples with undistorted octahedrons AF6 [20]: at maximal
shielding along the A-F bond there was a tent with σ11 = σ22, at
minimal shielding a “flipped” tent with σ22 = σ
33. The ob-
served ΔH and S2(F) values indicated a presence of diffusion
in fluorine sublattice of the solid solution 50Pb F230BiF3
20NaF at temperature above 400 K.
23Na NMR spect ra (nu clear spi n I = 3/2) of the solid solution
50PbF230B iF320NaF at temperature below 250 K consisted
of a single symmetr ic li ne with CS = 17.5 ppm (the absence of
satellit es was due to aver aging of th e tensor of an el ectric field
gradient at resonating 23Na nuclei), the resonance line width
was determined by internuclear interactions of Na+ Na+, Na+
F, and Na+ Mn+. Wit h the temperature increase from 150 to
420 K, the resonance line narrowing was observed (from 6.5 to
1.5 kHz), caused by a partial averaging of dipole-dipole inte-
ractions between sodium and fluorine nuclei due to an appear-
ance of ion mobility in the fluoride subsystem, as well as, per-
haps, by a transition of sodium ions to local motion (diffusion).
4. Conclusions
The nature of ion mobility was studied in the solid solutions
50PbF230B iF320K(Na)F with temperature variation. The
introduction of potassium or sodium cations into the fluorite
lattice PbF2BiF3 system caused an improvement of ionic mobil-
ity. The observed transformations of 19F NMR spectra were
associated with a gradual change in ionic dynamics in the fluoride
sublattice with temperature, from the rigid lattice through local
ionic motion to a translational diffusion of fluoride ions.
The presence of diffusion in the fluoride and sodium sublat-
tices of the studied solid solutions indicated the existence of
high ionic conductivity. According to the preliminary data of
electro-physical research, conductivity in the solid solutions I
and II reached values of ~2×10–210–3 S/cm at temperature
above 500 K, that allowed to recommend these systems as a
basis for the development of new functional materials.
5. Acknowledgements
The work was supported by the RFBR project (Grant No. 11-
03-00229) and the FEFU project (Grant No. 3.2360.2011).
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