Wickersheim, Kenneth A. Physica 25 Hanking, Bet ty M.. 569-57.0 1959

LETTER TO THE EDITOR

i n f r a r e d a b s o r p t i o n i n c a l c i u m f l u o r i d e

B o n t ~n c k x) has discussed the reactions which lead to the decoration of dislocations when crystaUi~, e eatclum, fluoride is heated strongly in moist air: l~.e reports 5our infrared absorption bands (at 3650, 3580, 1~480 and 1415 ¢m-1) , which appear in CaF2 crystals so t~eated anal assigns these bands to ~ibrations and toz~iens of the hydroxyl ion subs~i.tut~ng 5¢~ fluorine iu the calcium fluoride l~ttice. The purpose o5 this ~et~er is to present facts which point to a different interpretation of some o~ these b a n ~

The isotated OH- ion should exhibit only. one. internal vibration, freq~eney in the vicinity o~ 350~) cm ~! provided there is ~ one type ot site for i,~ to oceul~y when it replaces a fluorin~ ion. For. ~xamp~e ht l~um fluoride gre.wn in air exMbits a stTong band presumably caused l~y a substi tut ional OIrl- impu~ity a~ 3583 cm-~. Hindered rotations of the hydroxyl ion should be expected to give rise to fundamental irequeneies between 200, and 400 cm-1 whi~le, h/~dered translations of the io~ will' probably possess stil~ lower frequencies.

If the hydroxyl ion concentration in the orystat builds u~ unt.il hydroxyl ions are in proximity with one another they can interaet to produce additional vibration frequencies. This si tuation is carried t~ the extreme when. enough hydroxyl ions accumulat~ to produce a local region of Ca(OH)2. However pure Ca(OH)s also exhibits only one infrared active OH stretching fundamental . Other frequencies exist, bu t the COZTesponding infrared transitions are forbidden by orystat selection rules. The infra~e¢l fundamental o/Ca{OH>~ occurs at or near 3650 em~k.~) ~) In the single crystal, combi- nat ion bands are also observed in. ~he ~icinity o5 this fundamental , hut as in the case o/ 1V~g(OH)s ~) the combination bands are so much weaker than the OH fundamental tha t they are not normally observed when spectra are obtained from small samples consisting of finely divided, randomly oriented particles of the hydroxide.

The spectrum of pure Ca(OH)~ shows no strong absorption between 3000 and 700 cm-Z.a) Bands in tlle neighborhood of 1~00 em-~ have been erroneously reported for Ca(OH)s apparently as the resul± o5 a CaCOs impurity. Ca(O.H)2 alters readily to CaCOs when exposed to CO~ in the air. The infrared spectrum of CaCOs shows a strong double absorption band between 1400 and 1500 cm-1 and a weaker sharp band at about 880 cm-Z. ~)

These considerations lead us to make the 5ollowing assignments for the bands observed by Bontinck. The ba~d at 3580 cm-Z is identified with the stretching vibration of hydroxyl ions occurring ~ubstitutionally for fluorine in CuFf. The band at 3650 cm-~ is assigned as the infrared active O~-I stretching fundamental of Ca(OH)2 occurring in or on the CaF2 crystal. The bands at 1480 and 1415 cm-1 are assigned as vibrations of the carbonate ion arising from a CaCOs impuri ty in or on the CaF2 crystal. In order to strengthen these assignments (in particular tha t of the carbonate impurity) we have heated thin crystals of CaF2 obtained from the Harshaw Chemical Company to approximately 1020 degrees centigrade for varying periods of time in atmospheres of room air, H~O vapor, D20 vapor and CO2. The samples were heated in a closed quartz tube inserted in a combustion tube furnace. Prior to heating, the tube with

Physica 25 - - 569 - -

570 INFRARED ABSORPTION IN CALCIUM FLUORIDE

sample in place was evacuated and the clesired atnmosphere was admitted. The infrared spectra of the samples were taken with a Perkin-Elmer Model 21 spectrometer equipped with NaCI and CaF2 prisms.

When CaF2 was heated in room air all the bands reported by Bontinck were observed, although with the resolution available the band at 3580 cm-1 was not always seen. In addition to the bands seen by Bontinck a weak sharp band was observed a t 880 cm -1 just short of the CaFs cutoff and imagreement .with the CaCOs spectrum. When CaFs was heated in water vapor alone only the two bands near 3600 cm -1 appeared. When CaFs was heated in DsO vapor onlybands in the vicini ty of 2700 cm -1 appeared. When traces of room air were i e f t in the tube with either HsO or D~.O vapor the two bands a t ,I415 and 1480 cm=l ~lso.appeared weakly. %%-hen CaFs was heated in COn alone no bands appeared. But when CaFs was heated first in HsO vapor and then in COn the bands at 1480, 1415 and 880 cm- ! appeared with greatly increased intensity. When the outer faces were removed ~ (by chipping or sand ing) f rom samples of CaFs heated in air, the 1480, 1415 and 880 cm -1 bands disappeared whi le t h e bands near 3600 cm-1 remained. -- . . . . . . . . . .

I t thus appears t h a t after CaFs Crystals h a v e been heated in, air hydroxyl ions can occur both substit~tionally for fluorine and in regions of Ca(OH)s. Ca(OH)2 which forms on the Surface of the cry~tal is converted to CaCOs by the COn in the air, and the resultant surface layer o f CaCOs gives rise to the infrared absorption bands at 1480; 1415 and 880 cm -1.

In contradiction to Bont inck ' s contention i t is not clear tha t the hydroxyl, content of the crystals decreases again after prolonged heat ing a t 1020°C. No such trend.was observed by us, al though scattering resulting from precipitation of particles Within the CaFs t rys ta l increased with heating t ime reducing the general transmission at shorter wavelengths and causing the 3650 cm -1 band to decrease in apparent size, But the ratio of transmission at the band center to transmission just outside the band did not increase~with extended heating. This fact m a y also be noted in Bontinck's spectra. There too the strength of the absorpt ion band at 3650 cm -1 zelat4w to background transmission is at least as great after six.hours heating as .after four hours. Fur thermore the shape of Bontinck's final curve (11 hours at 1020 °) suggests strongly tha t because of scattering losses his sample is not t ransmit t ing at all above 3500 cm -1 and tha t t he apparent five percent residual transmission is caused by stray light in the instrument. If this is true, the f inal-curve gives no information regarding the strength of the hydroxyl bands. .:

The occurrance of Ca(OH)z as a distinct phase within the CaFs crystal as indicated by the spectrum coupled with the inference just made tha t the Ca(OH)s content of the CaFs does not decrease after prolonged heating raises the question of whether the precipitated particles observed within the CaFs crystal after heating in moist air are Ca(OH)2 rather than CaO as previously assumed. I t does not seem to us tha t this question has been answered satisfactorily as yet.

KENNETH A. WICKERSHEIM BETTY M. HANKING

Research Laboratories, Hughes Aircraft Company Received 18-5-59. Culver City, California

REFERENCES .

1) Bontinck, W., Physica 24 (1988) 650. 2) Woefle, J. B., Thesis, McMaster University (1958). 3) Busing, W. R. and Morgan, H. W., J. Chem. Phys. 2B (1958) 998. 4) Benesi, H. A., J. chem. Phys. 30 (1959) 852. 5) Miller, F. A. and Wilkins, C. H., Anal. Chem. 24 (1952) 1262.