ISSN 1063�7745, Crystallography Reports, 2013, Vol. 58, No. 5, pp. 755–759. © Pleiades Publishing, Inc., 2013.Original Russian Text © D.N. Karimov, N.A. Ivanovskaya, N.V. Samsonova, N.I. Sorokin, B.P. Sobolev, P.A. Popov, 2013, published in Kristallografiya, 2013, Vol. 58, No. 5,pp. 744–748.

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INTRODUCTION

Crystals of heterovalent solid solutions with fluoritestructure M1–xRxF2+x (M = Ca, Sr, Ba, Cd, or Pb andR is a rare�earth element) are promising polyfunc�tional materials with physical properties that can becontrolled in wide ranges by varying the qualitative (Mand R) or quantitative composition [1].

In the SrF2–CeF3 system [2], a vast region of theSr1 – xCexF2 + x solid solution (0 < x < 0.5) is formed.The congruent melting of this solution at х ~ 0.3 allowsgrowing homogeneous crystals without a cellular sub�structure from melt [3]. Crystals of the M1 – xСexF2 + x

solid solutions with a high content of Ce3+ ions are ofinterest for special optical instrument engineering,since they ensure high UV absorption at high transpar�ency in the visible spectral range.

In fluoride crystals, absorption related to theallowed 5d–4f transitions in Ce3+ ions corresponds tothe wavelength range λ < 320 nm [4]. Transmission inthe visible range is determined by the transparency ofthe fluoride matrix MF2, in which Ce3+ ions areembedded.

In practice, the color brown is often observed dur�ing the growth of fluoride crystals with a high contentof Ce3+ ions, e.g., Ba0.73Ce0.27F2.27 and Ca0.70Sr0.11Ce0.19F2.19

crystals grown in the atmosphere of polytetrafluoroethyl�ene pyrolysis products [5] and in a rapidly quenchedCeF3 melt held preliminarily in an atmosphere ofthese products [6].

In the Sr1 – xCexF2 + x (x ~ 0.3) crystals obtained byvertical directional crystallization in a fluorinating

atmosphere (CF4 or polytetrafluoroethylene pyrolysisproducts), a yellowish brown color is also observed [7].

Coloring is related to additional absorption in thecrystals in the blue visible spectral range. This effectsignificantly reduces the mean transmittances ofcerium�containing fluoride crystals and, as a whole,degrades the spectral characteristics of optical ele�ments fabricated from them. The aim of this study wasto select technological parameters for growingSr1 – xCexF2 + x crystals without spurious absorption inthe visible spectral range, which hinders their applica�tion in optics, and investigate some properties of thesecrystals.

EXPERIMENTAL

Crystals were grown by the vertical directionalcrystallization on a KRF facility (Special DesignBureau, Shubnikov Institute of Crystallography) in amulticellular crucible and heating unit (both made ofgraphite). The temperature gradient in the growthzone was ∼45°C/cm, the crucible lowering rate was5 mm/h, and the crystal cooling rate was no more than100°C/h. No additional heat treatment of the crystalswas used.

As raw material we used SrF2 powder (purity of99.995 wt % with respect to metals; oxygen content nomore than 0.02 wt %, Sigma�Aldrich) and CeF3 pow�der (purity of 99.99 wt % with respect to metals; oxy�gen content no more than 0.05 wt %, Lankhit).

To purify the raw material from oxygen�containingimpurities, it was preliminarily calcined in vacuum ata residual pressure of ~10–2 Pa at temperatures up to

CRYSTAL GROWTH

Coloring Elimination in Sr1 – xCexF2 + x Crystals in the Visible Spectral Range during Growth from Melt

D. N. Karimova, N. A. Ivanovskayaa, N. V. Samsonovaa, N. I. Sorokina, B. P. Soboleva, and P. A. Popovb

a Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russiab Bryansk State University, ul. Bezhitskaya 14, Bryansk, 241036 Russia

e�mail: dnkarimov@gmail.comReceived January 30, 2013

Abstract—Crystals of the Sr1 – xCexF2 + x compositions close to the congruent one (x ~ 0.3) are fabricated bythe vertical directional crystallization. It is shown that the use of CF4 to form a fluorinating atmosphere dur�ing growth leads to additional spurious absorption in the crystals in the range 350–600 nm. The use of PbF2and ZnF2 for fluorination makes it possible to obtain colorless Sr1 – xCexF2 + x crystals of the desired opticalquality from melt. The thermal conductivity of crystal with x ~ 0.28 in the temperature range 80–500 K lieswithin 1.50 ± 0.03 W m–1 K–1. High ionic conductivity makes the Sr1 – xCexF2 + x crystals promising for appli�cation in solid�state ionics.

DOI: 10.1134/S1063774513050027

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200°С for 3–5 h and then, for deeper purification,fused in a CF4 atmosphere (purity 99.999 vol %) for 2–3 h with the melt overheating of no more than 100°Crelative to the melting point (Tmelt) of the compounds.The thus obtained transparent polycrystalline ingotswere used to grow crystals.

The crystallization chamber was preliminarilyevacuated to a residual pressure of ∼10–2 Pa and filledwith helium 7.0 (water vapor content no more than0.0001 vol %).

To form a fluorinating atmosphere, the followingsubstances were used:

(i) Tetrafluorocarbon СF4 (purity 99.9999 vol %;oxygen and water vapor content no more than0.0002 vol %);

(ii) Solid fluorinating agents (so�called deoxidizersor scavengers), which react with the basis melt andextract oxygen in the form of volatile compounds fromit. The conditions for applying such compounds aretheir intrinsic high volatility combined with the vola�tility of oxygen�containing products of the purifica�tion reaction. The scavenger excess (against the sto�ichiometry of the purification reaction) and the prod�ucts of all the reactions, except for the basis fluoridemelt, are eliminated by evaporation.

The mechanisms of the interaction of the melt ofrefractory fluorides with scavengers are understudied;it is well�known, however, that the latter lower the oxy�gen content in the melt. This concerns both themechanical particles of refractory metal oxides andoxygen dissolved in the melt.

It was decided to study the effect of the two follow�ing well�known scavengers on the Sr1 – xCexF2 + x melt:

(i) PbF2, which has been frequently used for a longtime to grow refractory fluorides [8, 9] (powder of (7–3) special�purity grade);

(ii) ZnF2, which is rarely mentioned as successfullyused [10, 11] (powder of (18–2) special�purity grade).

Both of these reagents in amounts up to 5% of theloading weight were poured on the bottom of thegrowth container prior to charge loading.

Sr1 – xCexF2 + x crystals (0.28 < x < 0.31), 20 mm indiameter and 70�mm long, were grown. The evapora�tion loss during the growth was no more than 2% of theinitial charge mass.

Transmission spectra of the crystals were recordedat room temperature with a Varian Cary 5000 spectro�photometer in the wavelength range λ = 0.2–0.8 µm.Polished plane�parallel plates with thicknesses of 2–10 mm were used for analysis.

The conductivity of the colorless Sr0.7Ce0.3F2.3

crystals was measured by impedance spectroscopy invacuum at a residual pressure of ∼10–1 Pa in the tem�perature range 300–900 K (Tesla BM�507 impedancemeter; frequency range 5–5 × 105 Hz). In the electri�cal measurements, electrodes made of Leitsilber silverpaste (Germany) were used and the crystal sample

(cubic syngony, sp.gr.) was not oriented (on theassumption of isotropic conductivity).

Thermal conductivity k of the Sr0.72Ce0.28F2.28 crys�tal in the temperature range 323–573 K was investi�gated by the dynamic method on an IT�λ�400 thermalconductivity meter with an error of ±10%. The mea�surements were performed on the crystal sample in theform of a pellet 15 mm in diameter and 5 mm thick.For low�temperature measurements, a parallelepipedsample was cut from this pellet. Thermal conductivityin the temperature range 50–300 K was studied by theabsolute stationary longitudinal thermal flux methodwith an error of no more than 5%. The experimentaltechnique was described in [12]. To ensure thermalfront flatness, the resistive heater specifying the mea�sured temperature gradient along the long (14.5 mm)side of the sample was pasted along its end surface(5 × 5 mm square).

RESULTS AND DISCUSSION

Crystal Growth in the CF4 Atmosphere

The growth experiments showed that the growth ofthe Sr1 – xCexF2 + x (0.28 < x < 0.31) crystals in a CF4�containing atmosphere always yields crystals of abrownish color. No essential dependence of crystalcoloring intensity on the ratio of volume fractions ofgas components in the CF4/He growth atmosphere inthe range 0.1–0.3 was observed. The transmissionspectra of the colored crystals contain a wide absorp�tion band in the range 350–600 nm (Fig. 1, curve 1).

Fm3m

100

80

60

40

20

0500400300 600 700

2

1

λ, nm

T, %

Fig. 1. Transmission spectra of (1) colored and (2) color�less Sr0.7Ce0.3F2.3 crystals with a thickness of 10 mm.

CRYSTALLOGRAPHY REPORTS Vol. 58 No. 5 2013

COLORING ELIMINATION IN Sr1 – xCexF2 + x CRYSTALS 757

Crystal Growth with the Use of Scavengers

The use of PbF2 or ZnF2 fluoranating agents inamounts of up to 5 wt % stably yields colorless crystals(Fig. 2). The crystals have no light�scattering cracks orinclusions.

The transmission spectra of the crystals exhibit noadditional bands (Fig. 1, curve 2). The UV transpar�ency boundary of the Sr0.7Ce0.3F2.3 crystals is deter�mined by the 5d configuration edge in the Ce3+ ionand corresponds to the range 315–320 nm, dependingon the sample thickness.

When PbF2 is used, the following reactions occurin the crystallization chamber:

1. Interaction between PbF2 and oxide impuritiesin the melt:

SrO + PbF2 → SrF2 + PbO↑Ce2O3 + 3PbF2 → 2CeF3 + 3PbO↑

2. Pyrohydrolysis of PbF2 and charge components:PbF2 + H2O → PbO↑ + 2HFSrF2 + Н2O → SrO + 2HF↑

2CeF3 + 3H2O → Ce2O3 + 6HF↑3. Interaction between the formed HF and oxide

impurities:SrO + 2HF → SrF2 + 2H2O↑

Ce2O3 + 6HF → 2CeF3 + 3H2O↑These chemical reactions are of the exchange type.

The reaction products do not exhibit oxidizing proper�ties. Similar interactions occur when ZnF2 is used.

In the first approximation, no dependence of thegrown�crystal quality on the specific type of additiveand its amount in the aforementioned limits wasobserved. To find the optimal scavenger concentrationin the melt, it is necessary to quantitatively study themelt purification.

Thus, having analyzed the growth of Sr1 – xCexF2 + x

crystals, we established a correlation between the crys�tal coloring and the type of fluorinating agents. Thenature of the observed coloring of the Sr1 – xCexF2 + x

crystals grown in a fluorocarbon atmosphere isunclear. Its oxidative character was mentioned in [13].The effect of this growth atmosphere on cerium ionswith variable valence requires additional investiga�tions.

Properties of the Crystals Grown

The table contains data on some properties of theSr1 – xCexF2 + x (x ∼ 0.3) crystals under study, which aresolid, isotropic, and low�refractive�index materialstransparent in a wide spectral range. Compared withSrF2, density ρ of the Sr0.7Ce0.3F2.3 crystals is higher by15%, the cubic lattice parameter а is larger by 0.02 Å,refractive index nD is higher by 4%, microhardness Hµ

is higher by a factor of more than 2, and the IR�trans�mission boundary λb is slightly blue�shifted.

Among the physical properties of theSr1 – xCexF2 + x crystals, we should distinguish ionicconductivity σ. Significant deviation from fluoritestoichiometry in the structure of the heterovalentSr1 – xCexF2 + x solid solutions leads to the disorderingof the anion sublattice and a sharp increase in the flu�orine�ion conductivity. Temperature dependences ofthe ionic conductivity of the Sr0.7Ce0.3F2.3 crystal and,for comparison, the SrF2 matrix are shown in Fig. 3.The experimental data for Sr0.7Ce0.3F2.3 were repro�duced upon thermal cycling and satisfied the Frenkel–Arrhenius equation

σT = Aexp[–ΔH/kT],

where А = 5 × 105 S K/cm is a preexponential factorand ΔH = 0.6 eV is the ion�transport activationenthalpy. The fluorine�ion conductivity of the crystalsat 573 K (300°С) is 5 × 10–3 S/cm, which exceeds σ ofthe nominally pure SrF2 matrix by a factor of ∼104. At873 K (600°С), the conductivity is as high as 0.2 S/cm.The high ionic conductivity of Sr1 – xCexF2 + x crystals

Fig. 2. Sr0.7Ce0.3F2.3 crystal boule obtained using PbF2.

Some physical properties of Sr0.7Ce0.3F2.3 crystals

Tmelt, °C 1552 ± 10 [2]

ρ, g/cm3 4.90 [14]

а, Å 5.825 [7]

nD 1.499 [15]

λb, µm 11.20 [16]

Hµ, GPa 3.4 [14]

σ, S/cm 0.2 (873 K)*

ΔH, eV 0.6 [17]

k, W m–1 K–1 1.50 (300 K)**

* Our data.** Our data for the Sr0.72Ce0.28F2.28 crystal.

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makes them promising superionic conductors in theintermediate temperature range.

The measured thermal conductivity k(T) of theSr0.72Ce0.28F2.28 crystal is shown in Fig. 4. The lowthermal conductivity and the behavior of the temper�ature dependence k(T) indicate the presence of effi�cient phonon scattering in the material studied. Onlyat the lowest temperatures (in the range under consid�eration) are there weak traces of the low�temperaturemaximum of k(Т) being approached, which is charac�teristic of crystalline material. In the temperature

range 80–500 K, the experimental points of k(T) lie inthe range 1.50 ± 0.03 W m–1 K–1; i.e., thermal conduc�tivity fluctuations do not exceed 2%. As follows fromthe fundamentals of the heat�transport theory, this sta�bility of the thermal conductivity in dielectric crystals[18] can be related to the mutual compensation of theincrease in the specific heat of this crystal and thereduction of the phonon free path with an increase intemperature. The established temperature indepen�dence of the thermal conductivity of Sr0.72Ce0.28F2.28

crystals in the range 80–500 K indicates that they canbe used as a reference material with a constant thermalconductivity in this temperature range.

CONCLUSIONS

The use of tetrafluorocarbon to grow Sr1 – xCexF2 + x

crystals leads to the formation of color centers in therange 350–600 nm. The nature of these centersrequires additional study.

When growing refractory (above 1500°С)Sr1 – xCexF2 + x crystals, additives of low�melting andvolatile fluorine agents PbF2 (822°С [19]) or ZnF2

(872°С [10]) in amounts up to 5 wt % can be effec�tively used to fluorinate the melt; these agents removecrystal coloring.

ACKNOWLEDGMENTS

We are grateful to B.V. Nabatov for his help in theexperiments.

This study was supported in part by the Ministry ofEducation and Science of the Russian Federation, theFederal Target Program “Research and Developmentin Priority Areas of the Scientific and TechnologicalComplex of Russia for 2007–2012” (state contractno. 16.523.11.3005, July 12, 2011). Equipment of theMixed�Use Center of the Shubnikov Institute of Crys�tallography was used.

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0600 300 200 100 20

T, °C

12

–1

–2

–3

–4

–5

–6

–71.0 1.4 1.8 2.2 2.6 3.0 3.4

103/T, K–1

logσ

[S

/cm

]

Fig. 3. Temperature dependence of the ionic conductivityof the Sr0.7Ce0.3F2.3 (1) and SrF2 (2) crystals: (�) heatingand (+) cooling cycles.

1.7

1.6

1.5

1.4

1.3

100 200 300 400 500 600T, K

k, W m–1 К–1

Fig. 4. Temperature dependence of the thermal conductiv�ity of Sr0.72Ce0.28F2.28 crystals.

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COLORING ELIMINATION IN Sr1 – xCexF2 + x CRYSTALS 759

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Translated by E. Bondareva