Effect of heat treatment in HF atmosphere on the optical and electrical properties of BaF2 ceramics

Article abstract of DOI:10.1134/S0020168509100197

Using optical spectroscopy and complex impedance measurements, we have studied the transmission spectra and electrical conductivity of different BaF₂ samples: ceramic prepared by vacuum hot pressing (200 MPa, 870°C, 20 min), the same ceramic af

Full text of DOI:10.1134/S0020168509100197

ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 10, pp. 1188–1192. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © N.I. Sorokin, D.N. Karimov, O.N. Komar’kova, E.A. Krivandina, A.N. Smirnov, S.P. Chernov, B.P. Sobolev, 2009, published in Neorganicheskie Materi-
aly, 2009, Vol. 45, No. 10, pp. 1265–1270.
Effect of Heat Treatment in HF Atmosphere on the Optical
and Electrical Properties of BaF Ceramics
2
a
a
a
a
b
N. I. Sorokin , D. N. Karimov , O. N. Komar’kova , E. A. Krivandina , A. N. Smirnov ,
c
a
S. P. Chernov , and B. P. Sobolev
a
b
Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia
Research Institute of Optical Materials Technology, Vavilov State Optical Institute, All-Russia Research Center,
Birzhevaya liniya 14, St. Petersburg, 193131 Russia
c
Moscow State University, Moscow, 119899 Russia
e-mail: sorokin1@mail.ru
Received July 14, 2008
Abstract—Using optical spectroscopy and complex impedance measurements, we have studied the transmis-
sion spectra and electrical conductivity of different BaF samples: ceramic prepared by vacuum hot pressing
2
(
200 MPa, 870°C, 20 min), the same ceramic after heat treatment in a reactive HF atmosphere (1200°C,
1
20 min), and a single crystal grown from the melt in vacuum. Heat treatment of the as-prepared BaF ceramic
2
in a fluorination atmosphere increases its optical transmission in the range 0.2–0.8 µm on average by a factor
of 2 and markedly reduces the selective absorption in the range 6–11 µm. The heat-treated BaF ceramic is close
2
in ionic conductivity to the BaF single crystal.
2
DOI: 10.1134/S0020168509100197
INTRODUCTION
nents of the desired shape. The perfect cleavage of the
crystals leads to a high scrap rate. One way to obviate
Barium fluoride has the fluorite (CaF ) structure and
2
the adverse effect of cleavage is to use BaF in the form
2
crystallizes in space group Fm3 m with a lattice param-
of optical ceramics. Fluoride ceramics based on one-
and multicomponent metal fluorides have long been
eter a = 0.620 nm. BaF crystals are among multifunc-
2
tional materials and are used in a variety of optical considered promising optical structural materials with
applications owing to their high transmittance in the improved mechanical characteristics. At the same time,
range 0.13–14 µm [1], in particular, as fast scintillators hot pressing, widely used in the production of optical
in high-energy physics [2] and as chemical sensors in fluoride ceramics, impairs their transmission. It can be
high-temperature solid-state ionics [3]. On a commer- enhanced by a number of processes [6–9]. The main
cial scale, BaF single crystals are produced by direc- approach to improving the transmission of fluoride
2
tional solidification in vacuum, using PbF additions to ceramics is heat treatment in a fluorination atmosphere.
2
remove oxygen. Under laboratory conditions, use is The products thus obtained may be of interest not only
made of a reactive atmosphere generated by various as optical materials but also as superionic conductors.
volatile fluorination agents.
The objective of this work was to study the effect of
The chemical bonds in barium fluoride have ionic heat treatment in an anhydrous hydrogen fluoride (HF)
character. According to the behavior of its electronic atmosphere on the optical transmission and electrical
conductivity, this compound is a wide-gap dielectric: properties of BaF ceramics produced by hot pressing.
2
its band gap is 10.6 eV [4]. At sufficiently high temper-
atures, BaF exhibits ionic conduction due to the forma-
2
tion of mobile intrinsic defects in its anion sublattice by
the Frenkel mechanism (fluorine interstitials and
vacancies). Above the temperature of their broad Fara-
day transition (t ), all of the MX (Hal = F, Cl) halides
EXPERIMENTAL
Preparation of BaF optical ceramics. Ceramic
2
samples for this investigation were prepared by vacuum
tr
2
hot pressing of fine BaF powder. As the starting mate-
2
with the fluorite structure and M O oxides with the anti-
2
rial, we used pieces of BaF optical crystals grown from
2
fluorite structure have an abnormally high ionic con-
the melt in vacuum.1
–1
ductivity (~10 to 1 S/cm) and become superionic con-
ductors. The t of BaF is 960–1000°C [5].
tr
2
1
The crystals were grown at the Research Institute of Optical
Materials Technology, Vavilov State Optical Institute, All-Russia
Research Center, St. Petersburg, Russia.
In their main application area, BaF single crystals
are mechanically treated to produce optical compo-
2
1
188

EFFECT OF HEAT TREATMENT IN HF ATMOSPHERE
1189
The average oxygen content as determined using
vacuum melting was 0.012 wt % [10]. Pieces of BaF2
single crystals were first crushed in a steel mortar and
then ground in a Pulverisette planetary ball mill
(
Fritsch, Germany) for 10 min. The resultant powder
was loaded into a molybdenum die, which was then
pumped to ~1 Pa and heated at 1200°ë for 90 min
before pressing. The powder was pressed at 870°ë and
2
00 MPa. The load was applied for 20 min. The density
of the ceramic samples was 95% of theoretical density.
Heat treatment. To improve the transmission of the
BaF ceramic samples, these were heat-treated at
2
1
200°C for 120 min in a graphite crucible mounted in a
(
b)
(‡)
KRF-1 crystal growth unit (produced at the Specialized
Design Bureau, Shubnikov Institute of Crystallogra-
phy, Russian Academy of Sciences). After pumping
–2
down to ~10 Pa, the working space of the unit was
filled with helium (99.9999 vol %). A reactive fluorina-
tion atmosphere (anhydrous hydrogen fluoride, HF)
Fig. 1. Hot-pressed BaF2 ceramic: (a) as-prepared, (b) heat-
treated in an HF atmosphere.
was generated via thermal decomposition of BaF · HF,
2
which was placed in the crucible together with the BaF2
ceramic.
where h is the sample thickness and S is the electrode
area. The temperature dependence of conductivity was
Optical transmission spectra. The optical trans- found to exhibit Arrhenius–Frenkel behavior,
mission spectra of the ceramic samples were measured
in the range 0.2–0.8 µm on a Specord M 40 spectropho-
σT = Aexp(–E /(kT)),
a
tometer and in the range 2–16 µm on an AF-1 Fourier- where A is the preexponential factor and E is the acti-
a
transform spectrophotometer (designed at the Science vation energy for ionic transport.
and Technology Center for Unique Instrument Making,
Russian Academy of Sciences). For comparison, we
also measured spectra of commercially available BaF2
single crystals (FBU).
RESULTS AND DISCUSSION
Effect of annealing in an anhydrous hydrogen
fluoride atmosphere on the transmission of BaF2
Electrical properties. The electrical properties of
ceramics. After hot pressing, the BaF ceramic samples
the BaF ceramics were studied before and after heat
2
2
were tinged with gray and strongly scattered light. A
disk cut from a ceramic sample was then cut into sev-
eral parts.
treatment, using plane-parallel plates 2 mm in thickness
2
and 20 mm in area. Electrical contacts were made with
Dag-580 graphite paste (Netherlands). Electrical con-
ductivity σ was determined by complex impedance
Figure 1 shows the unannealed (control) part of a
measurements in a vacuum of 0.1–1 Pa at frequencies disk (region a) and the part heat-treated in an HF atmo-
5
from 5 to 5 × 10 Hz and temperatures from 293 to sphere (region b). As seen, heat treatment in an HF
8
27 K using a Tesla BM-507 impedance meter. Electri- atmosphere markedly increases the transmission of
cal measurements on a BaF single crystal were taken in a translucent BaF
ceramics.
2
2
7
nitrogen atmosphere at frequencies from 1 to 1–10 Hz
The ultraviolet/visible transmission spectra of the
and temperatures from 293 to 1073 K with a Solartron
ceramic samples and a BaF single crystal are presented
2
1
260 instrument. The sample was 1 mm in thickness
in Fig. 2. The as-prepared ceramic (spectrum 1) had low
transmission and was essentially opaque for λ < 0.2 µm.
Annealing in an HF atmosphere increased its transmis-
sion in the range λ < 0.5 µm by almost a factor of 2
2
and 28 mm in area. Electrical contacts were made with
Leitsilber silver paste (Germany).
From impedance spectra, we evaluated the total
(
spectrum 2). Nevertheless, the transmission of the flu-
resistance of the ceramic, R , or the bulk resistance of
cer
orinated ceramic in the range λ = 0.2–0.8 µm remained
the single crystal, R . The relative uncertainty in the
cr
lower than that of the BaF single crystal (spectrum 3).
2
resistances obtained was within 5%. The observed
blocking effect of the inert (C, Ag) electrodes in the
impedance spectra indicated that the samples had pre-
dominantly ionic conductivity. The conductivity was
determined as
This may be due to contamination of the starting pow-
der with the mortar and die materials during grinding
and pressing.
The infrared transmission spectrum of the as-pre-
pared ceramic (Fig. 3, spectrum 1) shows considerable
absorption in the range λ = 6–11 µm, due to anion
σ = h/(RS),
INORGANIC MATERIALS Vol. 45 No. 10 2009

1
190
00
SOROKIN et al.
–
–
2–
2
OH + F
O + 2HF↑.
1
The formation of carbonate species is due to reactions
of the forming hydroxide with atmospheric carbon
dioxide:
3
2
8
6
4
2
0
0
0
0
1
–
2–
2
éç + ëé
CO3 + H O
2
2
and
–
3
–
éç + ëé
HCO .
2
The most energetically favorable state of oxygen
incorporated into MF is the oxide ion rather than the
2
hydroxide ion [14]. MF crystals with the fluorite struc-
2
ture dissolve small amounts of MO oxides. For exam-
0
0
ple, the CaO solubility in CaF at 773 K is 0.014 mol %
2
.2
0.3
0.4
0.5
0.6
0.7
0.8
[
15]. Accordingly, the absence of light scattering by
Wavelength, µm
CaF crystals points to rather low oxygen content. Such
2
data for BaF are not available in the literature.
2
Annealing in a reactive fluorination atmosphere
(HF) enables almost complete removal of oxygen-con-
Fig. 2. Transmission spectra of 5-mm-thick BaF samples
in the range 0.2–0.8 µm: (1) as-prepared ceramic, (2) fluor-
inated ceramic, (3) single crystal.
2
taining impurities from the as-prepared BaF ceramic
2
samples. This is accompanied by changes in the spec-
troscopic characteristics of the ceramics (Figs. 2, 3).
2
–
–
3
Fluorination effects on the electrical properties of
impurities such as CO3 and HCO . Heat treatment in
BaF optical ceramics. Figure 4 shows the Arrhenius
a hydrogen fluoride atmosphere (spectrum 2) markedly
2
increased the transmission of the ceramic in this spec- plots of ionic conductivity for the as-prepared ceramic
tral range. The fluorinated ceramic and BaF single (σcer 1), heat-treated ceramic (σcer 2), and BaF single
2
2
crystal have essentially identical infrared transmission crystal (σ ). At 827 K, the highest temperature at which
cr
spectra (spectra 2, 3).
the conductivity of the three materials was measured,
they differ in conductivity only slightly:
An annealing-induced increase in optical transmis-
sion was also observed previously in MF (M = Ca, Ba)
ceramics produced by hot pressing and then heat-
treated in a gaseous atmosphere of the pyrolysis prod-
ucts of polytetrafluoroethylene (C F ) [7–9].
–4
2
σcer 1 = σcer 2 = σ = (1.2 ± 0.1) × 10 S/cm.
cr
Comparison of the temperature dependences of σ
for the as-prepared ceramic and the single crystal dem-
2
4
onstrates that σcer 1 and σ are equal only in the narrow
cr
It is reasonable to assume that the observed changes
in the transmission spectra of the ceramics upon
annealing in an HF atmosphere are associated with bar-
ium oxide removal. BaO results from pyrohydrolysis, a
reaction between barium fluoride and water vapor at
high temperatures of hot pressing. Such reactions are
typical of many metal fluorides (see, e.g., Ref. [11]) and
are main sources of oxygen-containing defects in fluo-
ride materials. Partial pyrohydrolysis of the as-prepared
barium fluoride ceramic is facilitated by the use of fine-
particle starting material, which readily absorbs atmo-
spheric moisture. During hot pressing, the moisture
reacts with the fluoride.
range 773–827 K. Beyond this temperature range, the
conductivity of the as-prepared ceramic substantially
exceeds that of the BaF single crystal.
2
The higher σ of the ceramic is also attributable to the
pyrohydrolysis reaction. Not only does the oxygen
impurity in the partially hydrolyzed as-prepared
ceramic reduces its optical transmission (Fig. 2), but it
also gives rise to an additional conduction mechanism
with an activation energy of ~0.2 eV in the temperature
range 293–773 K, which masks the bulk conductivity
(
Fig. 4). A similar effect was observed previously in
partially hydrolyzed Cd Ce F and Na Y F sin-
0.9
0.1 2.1
0.4 0.6 2.2
gle crystals [16, 17].
MF (M = Ba, Ca) pyrohydrolysis can be repre-
2
According to current views [12, 13], the pyrohydrol-
sented by the scheme [12, 13]
ysis of MF (M = Ca, Ba) single crystals is an ion-dif-
2
MF + H O
MO + 2HF↑.
2
2
fusion-limited solid-state reaction. The activation
energy for cation diffusion in these materials is substan-
tially higher than that for anion diffusion, and the diffu-
sion velocity of the cations is much lower. For this rea-
The intermediate steps of the reaction with water dur-
ing heating are
–
–
–
2–
F + H O
OH + HF↑
and
son, the diffusion species are the F and O anions. In
2
2–
the structure of MF , O ions diffuse substantially
2
INORGANIC MATERIALS Vol. 45 No. 10 2009

EFFECT OF HEAT TREATMENT IN HF ATMOSPHERE
1191
1
00
2
, 3
80
60
40
20
0
1
2
4
6
8
10
12
14
16
Wavelength, µm
Fig. 3. Transmission spectra of 5-mm-thick BaF samples in the range 2–16 µm: (1) as-prepared ceramic, (2) fluorinated ceramic,
2
(3) single crystal.
–
more slowly than F ions. In CaF crystals, the activa- by their microstructure and mobile defects in the grain
2
2–
bulk and at grain boundaries. The electrical properties
tion energy of O diffusion is ~2 eV [18], whereas that
–
of F diffusion is 0.79–0.92 eV for the interstitial mech- of the grain-boundary phase and grain phase can be
anism [19] and 0.39–0.51 eV for the vacancy mecha- characterized by resistances Rgb and R , respectively.
g
nism [19].
The total resistance of a sample is the sum of these two
Annealing of the as-prepared BaF ceramic samples contributions: R = R + R .
2
cer
g
gb
in a reactive fluorination atmosphere (HF) is accompa-
nied by changes in their electrical properties due to the
The fact that σcer 2 is equal to σ between 624 and
removal of oxygen-containing species. Comparison of 827 K indicates that, in this temperature range, the con-
cr
the temperature dependences of conductivity for the
annealed ceramic and the single crystal indicates that
ductivity of the annealed ceramic is determined prima-
rily by the electrical transport processes in the bulk of
the temperature range where σcer 2 = σ is markedly
cr
the crystalline grains (Rcer ≈ R ), and the contact resis-
g
broader: 624–827 K. The activation energy for ionic
transport is 1.6 ± 0.1 eV (624–827 K) in the annealed
ceramic and 1.64 ± 0.05 eV (623–1073 K) in the single
tance R is insignificant. R depends significantly on
gb
gb
the ceramic preparation parameters, which influence
crystal, in good agreement with earlier data for BaF2 defect formation processes and the structure of grain
crystals [19, 20].
boundaries. These parameters include unintentional
impurities, their segregation, the pressing temperature,
ature range where σcer = σ can be interpreted as fol- the composition of the gaseous environment, heat-treat-
The observed considerable extension of the temper-
cr
lows: Ceramic (polycrystalline) BaF samples consist ment conditions, and others. In addition to the removal
2
of individual single-crystal grains (major phase) sepa- of oxygen-containing species, heat treatment of fluo-
rated by grain boundaries, which differ in structure and
physical properties from the grain bulk and, hence, can
be thought of as an individual phase. In the case of
porous ceramics, the air-filled pores should also be
taken into account. In our case, their effect can be
ride ceramics in an HF atmosphere gives rise to recrys-
tallization and grain growth [6, 21]. With increasing
grain size, the contact resistance R in ceramic samples
gb
decreases [22], and the resistance of single-crystal
grains, R , begins to determine the conductivity of the
neglected because the BaF ceramic has high density.
g
2
The electrical conductivity of ceramics is determined annealed ceramic at lower temperatures.
INORGANIC MATERIALS Vol. 45 No. 10 2009

1
192
SOROKIN et al.
logσ [S/Òm]
7. Sobolev, B.P., Krivandina, E.A., Smirnov, A.N., et al.,
USSR Inventor’s Certificate no. 1 513 838, 1987.
–
1
8
. Krivandina, E.A., Smirnov, A.N., Lyamina, O.I., and
Sobolev, B.P., Heat Treatment of Polycrystalline Fluo-
ride Optical Materials in a Reactive Medium, VII Vse-
soyuznoe soveshchanie “Kristallicheskie opticheskie
materialy” (VII All-Union Conf. Crystalline Optical
Materials), Leningrad, 1989, pp. 218–219.
–
3
1
–
–
–
5
7
9
9
. Karimov, D.N., Krivandina, E.A., Chernov, S.P., and
Sobolev, B.P., The Transmission Improvement of the
Optical Hot-Pressed Fluoride Ceramics, Int. Conf.
2
“
Crystal Materials 2007 (ICCM 2007)”, Kharkov, 2007,
p. 123.
1
0. Sobolev, B.P., Nonstoichiometry in Systems ofAlkaline-
Earth and Rare-Earth Fluorides, Doctoral (Chem.) Dis-
sertation, Moscow: Shubnikov Inst. of Crystallography,
Russ. Acad. Sci., 1978.
3
–
11
11. Warf, J.C., Cline, W.D., and Tevebaugh, R.D., Pyrohy-
drolysis in the Determination of Fluoride and Other
Halides, Anal. Chem., 1954, vol. 26, no. 2, pp. 2342–
0
.8
1.2
1.6
2.0
2.4
03/T, K–1
1
2
346.
Fig. 4. Ionic conductivity ofBaF samples: (1) as-prepared
ceramic, (2) fluorinated ceramic, (3) single crystal.
2
1
1
2. Rakov, E.G. and Teslenko, V.V., Pirogidroliz neorgan-
icheskikh ftoridov (Pyrohydrolysis of Inorganic Fluo-
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CONCLUSIONS
The present experimental data on the spectroscopic
and electrical properties of the as-prepared BaF2
ceramic indicate that it undergoes partial pyrohydroly-
sis during vacuum hot pressing.
4. Bollmann, W., Incorporation of O2– and OH– Ions in
1
1
1
CaF Crystals by Reaction with the Surrounding Atmo-
2
Special heat treatment in an anhydrous hydrogen
fluoride atmosphere improves the optical properties of
such ceramics in the spectral region 0.2–16 µm and
brings their electrical properties closer to those of BaF2
single crystals grown from the melt in vacuum.
sphere, Phys. Status Solidi A, 1980, vol. 57, pp. 601–607.
5. Bollmann, W., Solubility and Distribution Coefficient of
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2
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Sobolev, B.P., Ionic Transport in Iso- and Heterovalent
CdF -Based Solid Solutions, Elektrokhimiya, 2005,
2
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