Solid State Ionics 239 (2013) 41–49
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Solid State Ionics
journal homepage: www.elsevier.com/locate/ssi
Fast fluoride ion conducting materials in solid state ionics: An overview
L.N. Patro a, b,⁎, K. Hariharan a
a
b
Solid State Ionics Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai-600 036, India
Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea
a r t i c l e
i n f o
Article history:
Received 25 June 2012
Received in revised form 1 March 2013
Accepted 8 March 2013
Available online 10 April 2013
Keywords:
Fast ion conductors
Fluorides
Synthesis methods
Transport properties
Impedance spectroscopy
a b s t r a c t
This review article presents a brief overview of synthesis and application aspects of fast fluoride ion
conducting materials in the field of Solid State Ionics. Possible synthesis methodologies known in the literature for the preparation of various fluoride materials have been outlined. Further, their transport characteristics with respect to their crystal structure have been discussed. Various approaches influencing their ionic
conductivity value in the light of synthesis methodology, composition, dopant concentration, crystallite
size, microstructure etc. have been reviewed. At the end, possible technological applications of fast fluoride
ion conducting materials in different solid state ionic devices have been presented.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Materials, which allow the macroscopic movement of ions
through their structure leading to high value of ionic conductivity
(10−1–10−5 S/cm) and negligible electronic conductivity at ambient
temperature or at temperatures much below their melting point, are
known as fast ion conductors. These materials can be classified
according to the type of the mobile ion responsible for conduction
namely, cation conductors (Li+, Na+, Cu+, Ag+, H+) and anion conductors (F−, O 2−). Among them, fluoride ion conducting materials exhibit
high value of ionic conductivity with negligible electronic conductivity
among the anionic conducting systems relatively at lower temperatures
due to the small size of the anion with single charge and thus offer potential for use in many solid state ionic devices [1–5]. Fluoride materials
have also been introduced as electrode materials for rechargeable lithium
batteries due to the high electronegativity value of fluorine and high
free energy of formation [6]. The use of fluoride based electrode materials for advanced energy devices has been reviewed [7]. The present
review article mainly highlights the synthesis and transport behavior
of various fast fluoride ion conducting materials. In the field of solid
state ionics in general and concerning the research on fluoride systems
in particular, the challenges for the various groups are mainly to develop fast ion conducting materials that will allow fluoride ion conduction
at ambient temperature for the materials to be technically useful. The
composition, phase and microstructure of the materials are mainly
⁎ Corresponding author at: Department of Materials Science and Engineering, Seoul
National University, Seoul 151-744, Republic of Korea. Tel.: +91 44 22574856; fax: +91
44 22574852.
E-mail address: [email protected] (L.N. Patro).
0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ssi.2013.03.009
engineered in order to improve the various factors influencing their
conduction characteristics. This leads to two major classes of fast fluoride ion conducting systems such as (i) Glassy electrolytes and (ii)
Framework crystalline/polycrystalline electrolytes with open channels
for ion migration.
2. Fluoride glasses
Glassy electrolytes are realized by arresting the disordered phase
of a normal ion conductor or fast ion conductor in a glass network.
For the preparation of the fluoride glasses, different routes such
as melt quenching, mechanochemical milling and sol-gel are
often used. Fluoride ion conduction has also been realized in
fluorozirconate, fluoroaluminate, fluoroberyllate, fluoroindate and
oxyfluoride (fluorophosphate/fluoroborate/fluorosilicate) glassy systems [8–14]. Various reports including review articles have also
shown the fast ion conducting behavior of these fluoride glasses and
concluded that the conductivity is only due to F− ions [15–17]. The fluoride ion motion in glasses is however lower than that of the most polycrystalline fluoride materials such as PbF2, PbSnF4 [1,9]. However, the
optical properties of fluoride glasses are rather extensively studied
with the emphasis on their applications in infrared optical components,
ultra low loss optical fibers, lenses, filters and high power laser hosts
[9,18–20]. Some reports are also known discussing the optical properties of oxyfluoride based glass ceramics [21,22].
3. Framework-crystalline/polycrystalline fluoride materials
These materials provide an essentially more or less rigid frame
work structure with channels along which the mobile ionic species
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L.N. Patro, K. Hariharan / Solid State Ionics 239 (2013) 41–49
of the solid can migrate. The conduction process mainly involves the
hopping of the ions from site to site along the channels. The ionic conductivity of a material is mainly determined by the concentration of
mobile charge carriers and their mobility. Thus these two parameters
can be modified in an ion conducting material to further influence
its conductivity value. The following sub-sections mainly highlight
various approaches/strategies for enhancing the conductivity value
of various normal fluoride ion conducting materials, which includes
(i) Homogeneous doping, (ii) Heterogeneous doping, (iii) Ternary
systems, (iv) Grain size (nanoionics) and (v) Thin films.
3.1. Homogeneous doping
This involves the creation of extrinsic defects such as anion vacancies or interstitials in the lattice for charge compensation by chemical
doping with aliovalent dopant cations. The effect of doping mainly
depends on the content and ionic radius of the dopant ion. The high
ionic conductivity value in bivalent fluorides can be realized by generating F − vacancies or interstitials through doping with monovalent
(anion deficient solid solutions) and trivalent fluorides (anion excess
solid solutions) respectively, which provides easier path ways for the
fluoride ion conduction [23–26]. Fig. 1(a) shows the temperature
dependent conductivity plots of both SnF2 as such and 10 mol% KF
doped SnF2 indicating a break in conductivity around 443 K corresponding to α to γ phase transition temperature of SnF2 [27]. The
conductivity of SnF2 gets enhanced by one order by doping with KF
concentration of 10 mol%. Similarly, PbF2 homogeneously doped
with either trivalent fluoride dopant like LaF3 or monovalent fluoride
dopant like NaF also show higher conductivity in comparison to pure
PbF2 (Fig. 1 (b)) [28]. The high value of conductivity in PbF2 based
doped systems are explained in terms of an enhancement in interstitials and vacancies concentrations upon doping with LaF3 and NaF
respectively.
There is also a great interest in investigating the influence of the rare
earth ions on the conduction characteristics of the fluoride materials for
further enhancement of their conductivity [29]. In addition to enhancing the ionic conductivity value, rare earth ion doped fluoride materials
are widely investigated due to their potential applications in many
optoelectronic devices [30]. The conductivity of trivalent fluoride materials such as CeF3, LaF3 etc exhibiting tysonite structure are influenced
by doping with bivalent fluoride materials [31].
3.2. Heterogeneous doping (dispersed phase solid electrolytes)
These electrolytes are mainly prepared by dispersing insoluble, insulating, ultra fine particles like Al2O3, SiO2, CeO2, TiO2, MgO etc., into
different normal ion conducting matrices. They are commonly known
as dispersed phase solid electrolytes or composite solid electrolytes.
These composites generally exhibit an enhancement in conductivity
in comparison to the host ionic conductor. In addition, they offer better mechanical characteristics [32]. Various reports have shown that
the enhancement in conductivity of composite solid electrolytes
mainly depends on the following factors (i) Nature of dispersoids
(ii) Dispersoid concentrations (iii) Particle size of dispersoids and
(iv) Method of preparation [33–39]. The conductivity value increases
with decrease in particle size of the filler of a particular type. The conductivity of the composites is also found to increase with increase in
filler concentration and achieves its maximum value at a certain concentration. However, further increase of the volume fraction of the
fillers results in a decrease in the conductivity probably due to
blocking effect. Fig. 2(a) shows the variations of the conductivity of
(1 − x)NaSn2F5 − xAl2O3 (0 ≤ x ≤ 0.15) composite materials as a
function of Al2O3 concentration(x) at a typical temperature (373 K)
[40]. The conductivity of NaSn2F5 increases with increase in Al2O3
concentration up to 10 mol% and further increase of the volume fraction of Al2O3 particles results in a decrease in the conductivity. An enhancement in conductivity of two orders in magnitude is obtained for
NaSn2F5 with Al2O3 dopant concentration of 10 mol%. The conductivity value of composites increases with decrease in particle size of the
filler of a particular type. The temperature dependent conductivity
Fig. 1. Temperature dependent conductivity plots for (a) SnF2 and Sn0.99K0.01F1.99 [27] and (b) PbF2, Pb0.999Na0.001F1.999 and Pb0.999La0.001F2.001 [28].
L.N. Patro, K. Hariharan / Solid State Ionics 239 (2013) 41–49
43
Fig. 2. (a) Influence Al2O3 concentrations on the conductivity of NaSn2F5–Al2O3 composites (373 K) [40]. (b) Temperature dependent conductivity plots for β–PbF2–SiO2 composites
of different silica concentrations and its grain size [28]. (c) Arrhenius plots for (i) β–PbF2, (ii) β–PbF2–10 m/o SiO2 (grain size: 0.014 μm) - co-precipitation and β–PbF2–10 m/o SiO2
(grain size: 0.014 μm) - solid state diffusion (ssd) [28]. (d) Comparative temperature dependent conductivity plots for (i) CaF2, (ii) CaF2–4 m/o Al2O3, (iii) CaF2–4 m/o CeO2 [42].
plots of pure PbF2 and PbF2 doped with 5 mol% and 10 mol% silica of
different particle sizes (0.007 μm, 0.014 μm and 0.03 μm) is shown in
Fig. 2(b). It is clear from the figure that the conductivity of PbF2 increases with increase in SiO2 concentration and decrease in particle
size of the filler [28].
The choice of the synthesis methodology adopted for the preparation of the composite materials also plays an important role for the enhancement in conductivity value as it affects the contact surface area
and the nature of distribution of dispersoid particles in the host matrix.
The conductivity of PbF2–SiO2 composite prepared by precipitation
method shows better conductivity in comparison to the same composite material synthesized by solid state reaction method (Fig. 2(c)) [28].
The influence of submicron size particles of insulating oxides like
Al2O3, SiO2, MgO on the transport characteristics of various fluoride
materials such as PbF2, CaF2 and BaF2 has been studied [28,41–43].
The addition of 30 mol% MgO dispersoids on CaF2 has caused an enhancement in conductivity of two orders as compared to pure CaF2
[43]. The influence of the synthesis methodologies on the transport
behavior of SiO2 and Al2O3 dispersed CaF2 composites and their application to the solid oxide galvanic cells have been reported [44,45].
Earlier studies on fluoride based composites have demonstrated that
the isoelectric points (CeO2: 7, ZrO2: 4, SiO2: 2 and Al2O3: 9) and cationic
radii of oxide inclusions (CeO2: 0.87 Å, ZrO2: 0.59 Å, SiO2: 0.26 Å and
Al2O3: 0.39 Å) mainly influence the conductivity enhancement behavior. The maximum enhancement should occur for the oxide inclusion
with greatest ionic radii and smallest isoelectric point in the following
order, ZrO2 > SiO2 > CeO2 > Al2O3 [28,42,46]. The temperature dependent conductivity plots of pure CaF2, CaF2 doped with 4 mol%
Al2O3 and CaF2 doped with 4 mol% CeO2 are shown in Fig. 2(d) indicating
the high ionic conductivity value in CaF2 dispersed with CeO2 composite
system [42].
The development of various theories concerning the conduction
mechanism of these composites is well addressed in the literature
[47–49]. Among them, “Adsorption and Desorption model” commonly
known as Maier's model is the most extensive and suitable one
explaining the conduction behavior of various fluoride based composites [49]. In this model, it is assumed that the ions can be trapped at
the interfaces due to the driving force provided by the chemical affinity
of dispersoids. According to the charge neutrality principle, the counter
species of opposite sign is then accumulated forming space charge regions with high conductivity near the interface. Hence the increase in
the defect concentration in the space charge region appears to be a
major contributing factor for the enhanced electrical conduction in dispersed solid electrolytes [49]. However, the influence of the mechanical
strain on the enhancement of the conductivity of composite materials
cannot be ruled out. The strain effects mainly emerge inside the crystal
lattice near the interface due to the mismatch in lattice parameters of
the contacting phases [50].
3.3. Ternary systems
Earlier literature on the transport characteristics of different fluoride systems have shown the best electrical performance in stoichiometric compounds such as PbSnF4, RbPbF3, KBiF4, KSn2F5, TlZrF5 etc.
[51–55]. Among them, SnF2 based systems are of much importance
due to their high ionic conductivity value at ambient temperature
44
L.N. Patro, K. Hariharan / Solid State Ionics 239 (2013) 41–49
with fluoride ion transport number close to unity. SnF2 forms a number of solid electrolytes of type MSn2F5 (M: Na, K, Rb, Cs, NH4 and Tl)
and MSnF4 (M: Pb, Ba and Sr), which are found to be useful for various
solid-state ionic device applications [56,57]. MSnF4 (M: Pb and Ba) systems are known to be high performance fluoride ion conductors due to
their high ionic conductivity value (~10 −3 S/cm) at ambient temperature. Several allotropic forms of PbSnF4 are known in the literature
[58–60]
h 0
i
α−PbSnF 4 ðmonoclinic; P2=nÞ α ðmonoclinic; P21 =nÞ →353K β
ð1Þ
653K
623K 0
−PbSnF 4 ðtetragonal; P 4 =nmmÞ ⇄ β −PbSnF 4 tetragonal; P 42 =nmm ⇄ γ
663K
−PbSnF 4 ðcubic; Fm3mÞ ⇄ Melt
Sorokin et al. have listed out the temperature dependent conductivity values of various phases of PbSnF4 material synthesized by different
methods in addition to the earlier known values [58]. Ahmed et al. have
synthesized PbSnF4 by mechanochemical milling and reported its giant
dielectric constant properties [61]. The influence of various synthesis
methods on the crystallite size, microstrain and transport characteristics of BaSnF4 have been investigated by the present authors [62].
Investigation on the synthesis and transport properties of KSn2F5
[63], RbSn2F5 [64,65], CsSn2F5 [66,67], TlSn2F5 [68], NH4Sn2F5 [69,70]
and NaSn2F5 [71] exhibiting high ionic conductivity at ambient temperature (10 − 6–10 - 4 Scm − 1) have been reported by several groups.
Recently Podgorbunsky et al. have compared the conduction characteristics of various fluoride materials and reported high conductivity
in 0.9PbSnF4–0.1LiF (σ = 6.3 × 10 − 2 S/cm, 463 K) and RbSn2F5
(σ = 1.25 × 10 − 1 S/cm, 473 K) [72].
Fig. 3 presents the temperature dependent conductivity plots for
some typical SnF2 based ternary systems synthesized by different
methods [27,58,62,71]. It is observed from the figure that KSn2F5,
PbSnF4 and BaSnF4 exhibit high value of ionic conductivity ~10−4 S/cm
at room temperature. However NaSn2F5 exhibits a lower conductivity
value probably due to the polarization of F− ions by sodium ions. It is
seen that synthesis methodology also influences the ionic conductivity
value of the chosen materials.
3.4. Nanoionics in fluoride systems
Material with dimensions in the nanoscale range often exhibits different characteristics in comparison to its bulk form and hence it offers
new and technological applications. The influence of crystallite size control as a promising way for tuning the transport properties of fast ion
conducting materials has been discussed in this section. In the case of
ionic conductors, the conductivity results are known to vary with both
crystallite size and strain [73–76]. An increase in interface surface area
is resulted by decreasing the crystallite size and therefore it often influences the various physical properties of the material. Wu Da-Xiong has
reported the ionic conductivity of nano-crystalline LaF3 material of
grain size 16 nm as 1.5 × 10 −5 S/cm which is about one order of magnitude higher than that of the LaF3 single crystal (10−6 S/cm) [77]. In
the case of CaF2, some reports are known indicating an enhanced conductivity value in its nano-crystalline form over its micro-crystalline
counterparts [76]. Nano-crystalline CaF2 of grain size 9 nm exhibits
higher ionic conductivity compared to its microcrystalline form (grain
size: 200 nm) (Fig. 4(a)) [76]. An enhancement in conductivity has
also been observed in SnF2 after milling of about 10 h and the enhanced
value is mainly attributed to the smaller crystallite size [78]. The conductivity results of mechanically milled SnF2 are found to be closely
related to both the crystallite size and microstrain in such a way that
the conductivity values are inversely proportional to the crystallite
size (Fig. 4(b)). SnF2 of two different crystallite sizes 81 nm and
36 nm respectively exhibits conductivity values of 3.74 × 10−6 S/cm
and 1.61 × 10−4 S/cm [78]. In the case of nano-crystalline materials
the enhancement in conductivity value can be mainly understood in
terms of large fraction of interface regions so that the conduction process is interface controlled [74,75,77] and the conductivity is mainly
governed by the properties of the surface. The fractional cross sectional
Fig. 3. Temperature dependent conductivity plots for (a) NaSn2F5 [71], (b) KSn2F5 [27], (c) BaSnF4 [62] and (d) PbSnF4 [58] materials synthesized by different methods (SSR: Solid
state reaction, MM: Mechanochemical milling).
L.N. Patro, K. Hariharan / Solid State Ionics 239 (2013) 41–49
area of grain boundaries increases considerably in nano-crystalline
form and thus offer fast path ways for the ions leading to the high
ionic conductivity value.
3.5. Thin film ionics in fluoride systems
Fast ionic conducting materials in thin film forms are of much importance in a variety of low power (micro-watt) applications due to
their low value of internal resistance. It also helps in areas where
miniaturization is necessary. The use of thin film electrolytes also reduces the impedance of the material and thus not only improves the
response time but also reduces the operating temperature of the
sensor.
Several reports on fluoride materials such as LaF3, CeF3, BaF2, and
PbF2 in thin films are known in the literature [79–88]. However
major groups focus on the transport properties of LaF3 thin films deposited by different techniques, which may be due to its high ionic
conductivity value [79,81,89,90]. Vijay et al. have reported the influence of substrate temperature on the transport behavior of LaF3 thin
films deposited on glass substrates by thermal evaporation technique
[81]. The application of LaF3 thin films in oxygen sensor and biosensors has also been reported [91,92].
Enhanced interfacial conductivity has been reported in nanometer
size multilayer structures of BaF2–CaF2 heterostructures on alumina
substrates at 773 K by molecular beam epitaxy in which F − exhibits
space charge layer conduction at the interfaces [93,94]. The lattice
mismatch between the two adjacent phases might result the strain
at the interfacial region. Hence, the influence of strain on the enhanced conductivity cannot be ruled out.
45
known for the preparation of fluoride materials and their influence on
the electrical properties.
Several preparatory methods such as physical methods, which include
solid state reaction, ball milling etc., and chemical methods, which include precipitation from solutions, [62,77], sol-gel, [95], hydrothermal
[96], sonochemical/sonication [97] etc., are reported for the preparation
of the fluoride materials. The influence of synthesis methodology on
the ionic transport properties of fast ion conducting materials such as
KSn2F5, BaSnF4, NaSn2F5, PbSnF4 and BaLiF3 has been demonstrated
by several authors [27,58,62,71,98]. To achieve a quality product with
respect to purity, homogeneity, reactivity, crystallite size etc., each
method finds its own advantages and disadvantages. The following
section discusses some possible methods adopted in the literature for
the preparation of fast ion conducting fluoride materials.
4.1. Solid state reaction
This is the most widely used method for the preparation of polycrystalline materials. It is carried out in the solid state form from a mixture of
starting solid materials at higher temperatures. In the case of fluoride
materials, the reaction is generally carried out in either nitrogen (N2)
atmosphere or in vacuum in order to avoid oxidation. In the case of
some typical fluoride materials such as BaSnF4 and SrSnF4, the reaction
at high temperature (773 K) has been carried out in a specially designed
evacuated copper tube [99]. The use of copper tubes mainly provide an
inexpensive and convenient alternative to gold and platinum for solid
state reactions at high temperatures under non-oxidizing atmospheres
[100,101]. Loss of fluorine is sometimes expected, if the preparation
temperature is high enough which affects the stoichiometry of the material hence the conductivity. The fluorine loss is generally minimized
by the addition of ammonium bifluoride during the preparation [74].
4. Synthesis methodologies
4.2. High energy ball milling (mechanochemical milling)
From the above discussions, it is observed that the processing
techniques have profound effects on the transport properties of materials. The present section discusses different synthesis methodologies
The term mechanochemical milling is widely used to refer
“reactions” induced by mechanical energy in starting materials to
Fig. 4. (a) Comparative conductivity vs. temperature plots for both microcrystalline CaF2 (grain size: 200 nm) and nano-crystalline CaF2 (grain size: 9 nm) [76]. (b) Crystallite size
dependent conductivity plot of SnF2 [78].
46
L.N. Patro, K. Hariharan / Solid State Ionics 239 (2013) 41–49
produce a homogeneous product. The energy is provided by the ball
mill due to increase in the contact surface area among the reactants as
the particle size of the starting materials decreases. The operation of a
typical planetary ball mill is shown in Fig. 5. Each bowl sits on an independent rotatable platform and the entire assembly of bowls rotated in
a direction opposite to the direction of the bowl platform rotation
(planetary motion). Recently, James et al. have critically reviewed the
historical development, mechanical aspects, opportunities and challenges of the mechanical milling technique [102]. This method has
also attracted much attention in the area of solid state ionics for the synthesis of both crystalline as well as amorphous superionic conductors.
This technique produces chemically homogeneous fine powders enhancing a good contact with electrode materials, which makes them
useful solid electrolytes in the fabrication of electrochemical devices.
This synthesis route is also advantageous for the preparation of fluoride
materials, as it is a low temperature (~room temperature) method
and hence the problem of fluorine loss during the preparation can be
eliminated.
Various numbers of articles are known discussing the mechanical
activation and/or mechanochemical reaction of fluoride materials.
Initial reports concern the mechanochemical synthesis of AZnF3
(A: Na, K, NH4) with perovskite structure and the synthesis of ARF4
(A: Na, K and R: rare earth elements) materials [103,104]. Followed
by this study, several groups have published on the preparation and
characterization of various complex fluoride systems such as CaF2
[105], NaSn2F5 [71], NH4Sn2F5 [74], (Ba, Sr)LiF3 [98,106], RbPbF3
[52], Pb(1 − x)SnxF2 [107] etc. In certain cases, the milling of starting
fluoride materials for some hours alone does not produce the required product material. In such cases, the reaction is activated by
sintering the resultant milled powder. BaSnF4 is prepared successfully
after heating at 573 K in the nitrogen atmosphere followed by milling
of the starting materials BaF2 and SnF2 for 10 h. It is seen that the
synthesis temperature has been reduced from 773 K (solid state reaction) to 573 K after mechanical activation [62].
Several reports are also known correlating milling time with
the crystallite size, micro-strain and the ionic conductivity of the
material [71,74,78]. Ruprecht et al. have reported enhanced fluorine
diffusivity in mechanochemically prepared BaF2–CaF2 composites
[108]. In addition, this method is also used for the synthesis of
various fluoride based cathode materials for battery applications
[109,110].
4.3. Chemical methods
The solution based methods provide a simple, low temperature and
cost effective approach in comparison to other techniques. It also overcomes the use of inert gas condition and expensive metal containers.
Many metal complex fluorides are insoluble in water. Hence, the precipitation of the product has been resulted by the exchange reaction of the
metal chlorides/nitrates with the fluorine precursor. Chemicals like HF,
NH4F, trifluoroacetate and NH4HF2 are often used as the precursors for
the fluorine [111–113]. Other less frequently used precursors include
NaBF4, KBF4 and ionic liquids such as 1-butyl-2methylimidazolium
tetrafluoroborate, 1-butyl-3methylimidazolium hexafluorophosphate
and 1-butyl-3-methylimidazolium bromide [114].
The pH value of the solution plays an important role in determining
the mode of the growth. Chemicals like NaOH, NH4OH, KCl and ammonia are often used to control the pH value of the solution. Several groups
have reported the synthesis of different nano-crystalline fluoride systems of various shapes such as BaF2: Eu 3+ cubic nanorods [115],
MgF2, KMgF3 nanorods [111], LaF3-nano-plates [116], SrF2: Eu3+
nanospheres [117], BaF2: Eu 3+ nanospheres [97].
5. Applications
Fast fluoride ion conducting materials have several promising
applications in various solid state electrochemical devices such as
solid state batteries, fuel cells, sensors and electrochromic displays.
Such application aspects of the fluoride materials are briefly discussed
in the following part.
Hagenmuller et al. have measured the open circuit voltage of a typical
secondary fluoride micro battery Bi/Pb|PbF2|BiF3/Bi as 0.34 eV, close
to its theoretical value [118]. The fabrication and characterization
of NaYF4:Dy as electrolyte in a solid state battery of configuration Na/
electrolyte/I2 + C + electrolyte exhibiting an open circuit voltage
ranges from 2.5 V to 2.7 V has been reported [119]. Reddy and Fichtner
have reported the demonstration of secondary fluoride ion batteries,
which are able to deliver high capacities [120]. The use of different fluoride materials as solid electrolytes in different solid state batteries has
been reviewed [5].
Fluoride ion conductors offer high ionic conductivity value with
negligible electronic conductivity over oxygen ion conducting electrolytes relatively at lower temperatures and reactive environments.
Movement of the supporting disc
Rotation of the grinding bowl
Fig. 5. Schematic diagram for the planetary ball mill.
L.N. Patro, K. Hariharan / Solid State Ionics 239 (2013) 41–49
Fig. 6. Schematic diagram of a chemical sensor.
Hence they find major applications in various chemical sensors. An excellent review on the use of several fluoride electrolytes such as CaF2,
PbF2 and LaF3 based systems in various types of chemical sensors is
reported by Fergus [3]. The schematic diagram of a chemical sensor
using LaF3 as electrolyte for the detection of fluorine is shown in Fig. 6
[3]. Lars Bartholomaus and Werner have reported an investigation on
a chemical sensor of configuration n-Si/SiO2/Si3N4/LaF3/Pt exhibiting
sensitivity of 29 mV/s to 174 mV/s by the varying the thickness of the
metal layer [121].
Electrochromic displays are the devices that emit the visible light
due to color changes induced in certain materials under the action
of electric field. Electrochromism has been observed earlier in the
case of vacuum evaporated anodic iridium oxide films using solid
electrolytes such as PbF2 and PbSnF4 thin films. A response time as
short as 0.1 s was observed and the device could be tested for up to
300 cycles of coloring and bleaching [122].
Apart from the above-mentioned applications, solid fluoride electrolytes also find useful applications in other devices such as specific ion
47
electrodes, piezoelectric gauges, electrochemical fluorine generator
and infrared detectors [1,2,5]. Various thermodynamic parameters associated with a chemical reaction can also be calculated after sandwiching
the electrolyte between two reversible electrodes [123]. Matsuo et al.
have reported the use of PbSnF4 as an electrolyte for the electrochemical
fluorination of various metals such as tungsten, vanadium and molybdenum [124]. Some reports have also demonstrated the use of metal
fluoride-carbon based nanocomposite as potential electrode material
for lithium batteries, where carbon matrix ensures a high electronic
conductivity while metal fluoride particles brings short diffusion paths
for lithium ions [125].
Fluorides are also known to be suitable host matrices for the investigation of spectroscopic properties of transition metals and rare earth ions
due to their low vibrational energies and optical transparency over a
wide wavelength range. They exhibit several advantages over oxides
such as (i) Weaker crystal field effects (ii) Weaker non-radiative probabilities and (iii) Long life time of the excited states [1]. Hence, they offer
various promising applications in many optoelectronic devices [30].
The photoluminescence characteristics of various rare earth ions such
as Eu3+, Nd3+ ions doped fluoride materials (CaF2, BaF2, SrF2, LaF3 etc.)
have been investigated by several groups [126,127].
6. Summary and conclusions
The temperature dependent conductivity plots for typical fluoride
materials that are known in the form of glasses, polycrystalline, nanocrystalline and thin films are summarized in Fig. 7 [13,17,27,28,
40,42,58,62,71,76,86,128–130]. The following points are extracted as
key issues in this review (Fig. 7).
∙ Although fluoride glasses are suitable for different optical device
applications, their conductivity values at the ambient temperature
are far from the conductivity value of the high performance fluoride ion conductors like PbSnF4 and BaSnF4.
∙ Various reports are known discussing the influence of dopants
Fig. 7. Temperature dependent conductivity plots for typical fluoride ion conducting materials in the form of glasses, polycrystalline, nano-crystalline and thin films
[13,17,27,28,40,42,58,62,71,76,86,128–130].
48
∙
∙
∙
∙
L.N. Patro, K. Hariharan / Solid State Ionics 239 (2013) 41–49
(homogeneous and heterogeneous) on the transport behavior of
different fluoride ion conducting materials. In most cases, an enhancement in conductivity is observed.
PbSnF4 is known to exhibit highest fluoride ion conductivity value
and is a potential candidate for different energy related device
applications.
A few reports are available discussing the transport properties of
fast ion conducting fluoride materials in thin film forms. However
the reports on the growth and characterization of PbSnF4 and
BaSnF4 thin films are still scarce.
Most of the recent studies have shown the synthesis of various fast
ion conducting fluoride materials by mechanochemical synthesis
route in comparison to other methods.
Fluoride materials exhibit high ionic conductivity value with ionic
transport number close to unity at ambient temperature in comparison to oxides. However some reports are only available demonstrating the application aspects of fast ion conducting fluoride
materials in different solid state ionic devices.
References
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