The hydrolysis of solid CaF2

Bontinck, W. 1958

Physica X X I V 650-658

THE

HYDROLYSIS

O F S O L I D CaF~

b y W. B O N T I N C K Laboratorium voor Kristalkunde, Rozier 6, Gent, Belgi~

Summary I t could be shown t h a t water vapor is the active component for the change of the properties of CaFg. after a thermal t r e a t m e n t at higher temperatures. The reaction consists in the diffusion of anion vacancies or oxygen ions. Hydroxylions are only present in the beginning of the reaction. When an equilibrium concentration of anion vacancies is formed, the reaction proceeds further b y coagulation of colloidal CaO particles. These specks precipitate principally along dislocation lines.

I. Introduction. S t o c k b a r g e r 1) found that water vapor had a considerable effect on the growing of artificial fluorite crystals. The U.V. transmission depends on the purity of the atmosphere under which these monocrystals are grown. It was shown ~) that a thermal treatment at 1000°C-1200°C of CaF2 crystals in air, results in a decoration of dislocation lines in the interior of the • crystal. This decoration is due to the precipitation of products formed during the thermal treatment. The work described here is an analysis of the reaction which is responsible for this effect. II. Active components o/ the reaction. W h e n CaF2 crystals ar~ heat treated, even at very high temperature, in an inert atmosphere no change in properties can be observed. This indicates that or oxygen or water vapour must be responsible for the precipitation effects which are found when the same treatment is performed in air. That water vapour is the active component could be proved b y heating CaF2 powder for 96 hrs at 1050°C in air and in carefully dried air. The apparatus is shown in fig. 1. The calcium content of the powder was determined in both cases and compared with the amount present in the original pure product (51,3%). A value of 53,8% was found in the first case, in the second 51,5 to 51,8% was obtained, the small variation in the latter case is due to the impossibility to remove the last traces of water vapour. III. Kinetics o/ the reaction. Most solid state reactions are structure sensitive. To avoid these effects, CaF2 powder (Mallinkrodt p.a.) was used -

-

650

-

-

T H E H Y D R O L Y S I S O F SOLID

CaFg.

651

for our experiments. This was h e a t e d in alumina crucibles at constant temperatures. Samples were t a k e n at different intervals and analysed. The analytical procedure was as follows. The t r e a t e d CaF~ p o w d e r is dissolved in a slightly acidified 3) 4)A1C18 solution (5%). A soluble complex is formed which has the formula (CaF2)sA1CIs. 6H~O. Tartric acid is added a n d the Ca4+ ions are precipitated b y addition of a large excess of oxalic acid a n d ammonia. After filtering, washing a n d acidifying, the calcium c o n t e n t can be determined b y potentiometric titration with ceric sulfate. PT asbest

H2 SOl,

H2

Z

liquid air trap

P20S

/"--'x

Ca F2 ~

oven

!

I

H2 SOl,

Fig. 1. Apparatus used to determine the active component of the thermal reaction of CaF~ in air. "/o Ca

1150 ° C

1095Q C

. - 2 - - " 1030 * C

gSO' C

24

48

92

96

120

144

188

192

216

2/~0

264

288

312

t. in hrs

Fig. 2. Change of the calcium content as a function of reaction time and temperature. The results for the reactions at 950 °, 1030 °, 1095 ° and 1150 ° are shown on fig. 2. The curves consist of two parts, a concave and a linear part. The latter are practically all parallel for t h e d i f f e r e n t reaction temperatures.

652

W, B O N T I N C K

The ,reaction can be represented by the-equation CaFg. + 8H~.O -+ (1 -- 8/2) CaF23/2CaiOH)z + 6HF The reaction product can decompose as shown b y equation (2) (I - -

6/2) CaF2 6/2Ca(OH)z -+ (1 -- 6) CaFzOCaO + 8HF

That reaction (2) takes also place was proved b y X-ray analysis. A sample treated during 216 hrs at 1150 ° showed the (311) (220) (200) and (111) diffraction lines of CaO. 1. I N I T I A L

REACTION

PERIOD.

A. I n f r a - r e d a n a l y s i s . The presence of hydroxylions can only be determined b y infrared analysis. Lamellae cleaved from a same CaFz single crystal (Harshaw), were treated in air at a constant temperature of 1020 °. Transmission in */* tOO

80

~'~(31

|l) Ca FI untreated 12) ~/4u 1020e (4|

40

(3) t h u 1020I (&) 1 u 1020"

SO iS) (6) 171

~

(5) 2 u 1020" t6| 3 u 1020' 17) & U 1020" |8) §u

1020'

(9) 11u 1020" 2O

|0

3600

3200

2800

2400

20'OO

16OO

li00

¢ m "1

Fig. 3. Change of the infra-red spectra at the beginning of the hydrolysis reaction at 1020°C. The infrared spectra after different reaction periods were recorded, fig. 3 summarizes the results. The absorption peaks are very small, which indicates that only a small amount of O H - ions are present, probably in the contactzone. The different peaks reach their m a x i m u m after different time intervals. The absorption peak at 1415 cm -1 due to the deformation vibrations reaches its maximum after one hour and decreases very slowly afterwards. The peak of the asymmetrical valence vibrations (3650 cm-1) reaches i t s m a x i m u m after three hours, and disappears after an 11 hrs treatment. The symmetrical

THE HYDROLYSIS OF SOLID C a F 2

653

v a l e n c e vibrations (3580 c m - 1 ) h a v e their maximum after 1 , 5 h r and disappear after 9 hours. These observations prove that the reaction 13roduct of reaction (I) is only present in the very beginning o f the hydrolysis reaction. The length of this initial reaction period depends upon the temperature. At 1220 ° no more hydroxyl ions are present after a two hours treatment, a n d this. period decreases as the reaction temperature is increased, As can be deduced from fig. 3 the intensity of the deformation vibrations is larger than that of the valence vibrations just as in pure Ca(OH)9.. The absorption peak due to the former vibrations diminishes more rapidly. The deformation vibrations (fig. 4) take place along directions.in a {001} plane. Substitution of a neighbouring F - ion b y a O H - ion must influence these vibrations strongly. This m a y be an explanation for the fact that the absorption peak at 1480 cm -I grows so rapidly to its maximum in the very beginning of the reaction.

0

""

0

F"

•

Ca a +

•

OH"

Fig. 4. One fluorine ion in a CaF2 lattice is substituted by an hydroxyl ion. The deformation vibrations proceed along a <110> direction. B. C h a n g e of t h e a b s o r p t i o n s p e c t r u m d u r i n g t h e i n i t i a l r e a c t i o n p e r i o d . The absorption spectra of small lamellae of CaF2 crystals annealed in air at higher temperature have an absorption peak at 2050 A (fig. 5). The presence of this absorption band is the reason why certain CaF2 crystal prisms cannot be used in the U.V. region. After longer treatments the crystals become opaque. We determined the absorption coefficient k after different intervals and at different temperatures. At a constant temperature, k varies linearly with the time k = A t . The temperature dependence of A is given by A

=

A o e-E/RT

An activation energy E of 38 Kal/mol is found. This is the same value found b y Mollwo for the formation of F-centers in CaF2 crystals 4) 5). C. M e c h a n i s m of t h e i n i t i a l r e a c t i o n p e r i o d . The activation energy for this reaction period, proves that the reaction is determined by the formation of anion vacancies.

654

W. BONTINCK

In the initial stage of the reactiofl the watermolecules will dissociate at the surface into H+ and O H - ions. Gaseous HF will be formed and the hydroxyl ions will substitute the removed F - ions. The reaction can only proceed when reaction (2) takes place, or when one vacancy is formed. It is clear that this surface reaction will be governed by the second reaction step represented by equation 2. After this initial period the vacancies will diffuse into the crystal and the contactzone will catalyze the decomposition of the hydroxyl ions. Log Zo

Z

2,500 2.500 2.&OC 2.300 2.200 2.1010 2.00~ t.90C t.80( t.700 1.600

1.500 1.,;00 1.300 1.200

la)

absorptloncurve of an untreated Ca F:t crystal

1.100 1.0(20

absorptloneurve after 3 hrs

0.900

b)

0.600

thermat

treatment

(755'C)

in air

0.700 0.600

0.500

o.4oo 0.300

tb)

0.200 O.t O0

(a t

200

300

400

500

600

700

800

900

1000 m ]1.

Fig. 5. A b s o r p t i o n curves of an u n t r e a t e d and t h e r m a l t r e a t e d CaF2 crystal. 2. S E C O N D

REACTION

PERIOD.

A. During this period the reaction can be represented by the equations CaF2 + ~HzO -+ (1 - - ~) CaF~ OCaO + 2OHF

(3)

~H20 -+ ~ 0 - - + ~ [ ] - + 2(~H+

(4)

or

D - is the Rees symbol 6) for an anion v a c a n c y . E a c h watermolecule can provide an o x y g e n ion which replaces t w o fluorine ions, so t h a t one v a c a n c y is formed. I t is easier to s t u d y the course of the reaction when we follow the change of the v a c a n c y c o n c e n t r a t i o n as a function of the reaction time. T h e a m o u n t of vacancies per cc. can be calculated from the relation

C[]_ = 6.6.1023 .d/M

T H E H Y D R O L Y S I S OF SOLID

CaF9

655

where (I -- 6) CaF96CaO represents the composition Of the reaction product. 6 can be deduced from the calciumcontent, d is the density and 29/ the molecular weight of the reaction product. C(n-}

I0

......

.-.-

It50 ° C

tO(J5t C

2.10

g50' C

~ ......................................

t.1o 0

24

48

72

96

|20

14/~

166

192

2[6

t.ihrsl

F i g . 6. C h a n g e of a n i o n v a c a n c y c o n c e n t r a t i o n a s a f u n c t i o n of reaction time a n d t e m p e r a t u r e .

Fig. 6 represents the calculated results. The meaning of the linear parts will be given further. The curved parts obey the relation C = Kt°, 5

(5)

which is similar to the equations obtained for the oxidation reactions of metals. The activation energy for this reaction period can be calculated from the temperature dependence of K, given by K=

K0

e-gIRT

For e a value of 18 Kal/mol was found in the temperature range 950 °1200°C. In table I the activation energies for the diffusion of negative carriers in CaF2 as determined by U r e 7) are summarized. TABLE I o" (with 0 - - ions impurities) . . . . . . . . . . e (due to anion vacancies) . . . . . . . . . . . (due to interstitial fluorine ions) . . . . . . . .

13 4- 2 Kal/mol (350°-600°C) 12 4- I Kal/mol (600°C) 38 + 8 Kal/mol (690°-920°C)

Lattice parameter measurements of CaF2 crystals additively colored at very high temperatures a) showed that at temperatures beyond 1000°C more and more interstitial fluorine ions are formed. From this we can conclude that the conductivity in the I000 °-1200°C temperature range will be influenced by the diffusion of the interstitial fluorine ions. This explains the observed increase of the activation energy. It must be noted that equation (5) is a solution of the diffusion law of Fick for spherical particles. K has then the same temperature dependence as the diffusion coefficient of the negative carriers, n~

w. BONTINCK

656

We consider now the vacancy concentration at the beginning of the linear part of the curves in fig. 6. A s can be seen on fig. 7 these concentrations Ce follow the law C, = Co e-~'/R~ with an activation energy of 33 Kal/mol. This law differs only very slightly from the experimental curve of Mollwo, which gives the amount of F-centers (or vacancies) in equilibrium at different temperatures. From this we m a y conclude that the second period of the reaction ends when a certain concentration of vacancies or oxygen ions is reached. to9

Ce

ve of

Mottwo

10::

10 21

10 21 o,9

o.s

07

o.s

1/i. lo.~

Fig. 7. Variation with reaction temperature of the vacancy concentration at the~heginning of the final reaction period. I I I 10-10"

Surface

2(F'11-1")

E]"

lI le-

2e" X

Fig. 8. Electrical double layer at the surface of the crysta[.

B. R e a c t i o n m e c h a n i s m d u r i n g t h e s e c o n d r e a c t i o n p e r i o d . At the surface of a crystal two fluorine ions will be replaced b y the oxygen ion of the reacting watermolecule and b y an anion vacancy. Both will diffuse into the bulk of the crystal while two fluorine ions will diffuse to the surface. In the contactzone an electrical double layer is formed (fig. 8).

T H E HYDROLYSIS O F "SOLID

CaF2

657

The current I through the element dx is given'by I =

tF-. tO-(2~F- + t0__)e

d dx [~-(~) + Pc~-(x) + Po-- (z)] #x

where tF_ and to-- are the transport numbers for the fluorine and oxygen ions, /~F-, P0--, /~D- the chemical potentials of the same ions and the anion vacancies and # the conductivity. As ~ and the diffusion coefficient have the same temperature dependence, we m a y conclude that the reaction is determined by a diffusion process. 3. THIRD REACTION PERIOD. Once an equilibrium concentration of vacancies is reached, the reaction can only proceed b y a coagulation of these defects, which causes a precipitation of a CaO phase. X-ray analysis showed that the (200) and (220) reflections of CaO appear at a time corresponding with the beginning of the third period. This period is represented b y two lines. The horizontal line parallel to the t axis (fig. 6) indicates that the concentration of the vacancies remains constant. The difference between the two lines gives the amount of vacancies which precipitate. The growth of the colloids corresponds to a linear law. Other authors have also observed a constant growth rate with CuS04.5H9.0, NiS04.7H20, alums s) and BaNs 10). The nuclei of the colloidal specks will form at jogs of dislocations, which have a charge of 4- 1,Se in CaF2 4). Experiments of Gayler n) have shown that an excess of vacancies will favour the precipitation of a new phase. The small growth rate and the temperature independence of this process can be explained by the small amount of cation vacancies present and their small mobility at these temperatures.

IV. Change o/ the lattice parameter during hydrolysis reaction. When the proposed reaction mechanism is accepted, the lattice constant must change in the same w a y as by additive coloration. We determined at different temperatures the lattice constant at the beginning and during the third reaction period. For these experiments a Philips X-ray diffraetometer was used. Table 2 which summarizes the results indicates that only above 1100 ° a considerable change of lattice parameter occurs. TABLE II

Physica XXIV

Duration in hours

Temperature in °C

24 96 96 144 144

950 lO00 1050 I095 I150

Lattice parameter in A 5,452 5,448 5,455 5,453 5,439

-4- 0,002 4- 0,001 4- 0,004 4- 0,002 4- 0,004

658

THE HYKROLYSIS OF. SOLID C a F 2

The lattice parameter after additive coloration at 1150 ° is 5,438 4- 0,004 A, a value which fits well with the value obtained after hydrolysis. At lower temperatures no change can be observed. In table 3 the lattice parameters are given in function of the duration of the hydrolysis reaction at 1150 °. TABLE III Time in ~ - - - - ~ A ~ I 5,454 424 72 ] 5,456 496 5,443 4144" 5,439 4216 5,438 4-

0,003 0.004 0,002 0,004 0,003

The asterisk marks the beginning of the third reaction period, when the coagulation of vacancies starts. The lattice parameter remains constant when the reaction proceeds further which is a proof that the vacancy concentration does not change anymore.

Acknowledgment.

This work is part of a research program (C.E.S.) sponsered by I.R.S.I.A. (Brussels). I thank Prof. W. D e k e y s e r for m a n y helpful discussions and Mr P. De B i ~ v r e for the measurements of the infra-red spectra. Received 23-4-58

REFERENCES 1) S t o c k b a r g e r , D. C., J. opt. Soc. Amer. 39 (1949) 731. 2) B o n t i n c k , W. and A m e l i n c k x , S., Phil. Mag. 2 (1957) 94. B o n t i n c k , W., Phil. Mag. 2 (1957) 561. 3) H e m i n s k a y a , U. I., Tsement 14, no. 6 (1943) 21. 4) B o n t i n c k , W., Thesis, University Ghent (1958). 5) Mollwo, E., Nachr. Ges. Gottingen, math. phys. Klasse, Fachgruppe II (1934) 79. A m e l i n c k x , S., B o n t i n c k , W., D e k e y s e r , W. and Seitz, F., Phil. Mag. 2 (1957) 1254. 6) Rees, A. L. G., Chemistry of the defect solid state, Methuen & Co., London: (1954). 7) Ure, R. W., J. chem. Phys. 26 (1957} 1363. 8) B o n t i n c k , W., Physica 24 (1958) 639. 9) C o o p e r and G a r n e r , Trans. Farad. Soc. 32, (1936) 1739. 10) W i s h i n , Proc. roy. Soc. A 172 (1939) 314. 11~ G a y l e r , M. L. U., Journ. Inst. Metals 72 (1946) 243.