R A M A N S P E C T R U M OF P O T A S S I U M C H L O R A T E A N D ITS T E M P E R A T U R E V A R I A T I O N

BY C. SHANTA KUMARI (From the Department of Physics, Indian Institute of Science, Bangalore)

Received June 26, 1950 (Communicated by Prof. R. S. Krishnan, r.A.sc.)

1. INTRODUCTION

THE Raman spectrum of potassium chlorate was first investigated by Krishna- murthy (1930) in the form of crystal powder and later by Venkateswaran (1938) and Hibben (1939). Hibben recorded the maximum number of Raman lines including two low frequency lines. The observed frequency shifts are 125, 179, 478, 493, 620, 915, 930, 975 and 1025 cm. -1 From a com- parison of the Raman spectrum of potassium chlorate as reported by the earlier workers with that of sodium chlorate (Shanta Kumari, 1948), ir is evident that the data on the lattice spectrum of the former are very meagre. Hence it appeared desirable to reinvestigate the Raman spectrum of potassium chlorate using a single crystal. As potassium chlorate is found to be transparent to ultra-violet, the ~,2537 mercury resonance radiation has been used as exciter for this study. The temperature vafiation of the Raman spectrum is also investigated and the resuits are prcsented in this paper.

Specimens of sing!e crystals of potassium chlorate used in the present study were obtained by the method of slow evaporation Ÿ aqueous solution of the pure chemical. They were in the forro of flat plates of size lO ;< 5 • 2 mm., parallel to the c (001) plane. The exciting radiation was incident on the c (001) face of the crystal and the transversely scattered light was taken through the edge. Using a medium quartz spectrograph and a slit-width of 0.025 mm., exposures of the order of two hours were sufficient to get reasonably intense spectrograms. Photographs were taken with expost, res of the order of 36 hours but they did not reveal any new features. During long exposures a disturbing effect wa.q noticed, namely, the portion of the crystal facing the intense portion of the arc became frosted due to heating and consequent decomposition in spite of the efficiem cooling by fan. Hence, while taking long exposure photographs, fresh crystals were used once every six hours. In replacing the crystals, care was taken to see that the same orientation was adopted.

177

178 C. Shan ta K u m a r i

2 . R E S U L T S

A photograph of the Raman spectrum of potassium chlorate taken at room temperature and its microphotometric record are reproduced in Fig. 1. The positions of the lines are marked in the figure for clarity. The frequency shifts are listed in Table I. The figures given in brackets repre- sent the visual estimate of the relative intensities of the fines. The spectrum recorded by the author exhibits five lines due to the internal oscillations of

TAaLE I The Raman spectrum of potassium cklorate

In te rna l oscillations Lat t ice oscillations

KCIO a KCIO a NaCIO~

4s6 (6)

62o (1)

920 (1)

930 (10)

975 (7)

54 (3)

82 (3)

98 (5)

136 (4) 1 127 145

70

83 103

122- 7 131

179

the chlorate ion and five lines due to the lattice oscillations. The appear- ance of the lines 179, 493 and 1025 cm. -1 with zero intensities reported by Hibben could not be confirmed by the author. In the lattice region, four new lines 54, 82, 98 and 145 cm. -1 have been recorded. Of these, the doublet 127-45 is broad. The sharp line 82 cm. -x is more clearly seen on the anti- stokes side than on the stokes side. The lines 54 and 620 cm. -~ and the doublet nature of the lines 136 and 930 cm. -~ were more easily discernible in a lightly exposed picture taken with a finer slit width.

3. DISCUSSION

Potassium chlorate crystallises in the monoclinic prismatic class without water of crystallisation. According to Zachariasen's analysis (1930) of tt,.e crystal structure of potassium chlorate, the smallest unit cell contains two molecules of KC103. The three oxygen atoms ate unsymmetrically situated around the chlorine atom forming a distorted CIOa group. The correct space group is C~2~. For this structure, there should be nine lattice oscilla- tions in the Raman spectrum of potassium chlorate as shown below.

T' and R' represent respectively the translatory type and rotatory type of lattice oscillations and T represents the translation of the lattice a s a

Ramau Spectrum o/ Potassium Chlorale

TABLE II

179

A l

A2

B1

B 2

[ Infra

C 22jŸ E C z i r i T T" red

1 1 1 1

1 - 1 1 - 1

1 1 - 1 -1

1 - 1 - 1 1

!

0 4

0 2

1 1

2 2

R' Raman

I 2 } p

I 2 ! f

1 • l

UR(8)

UR(s-v)

kpXp'(T) hpXp' (T') hpXp'(R')

~

4 0 0 4

2 0 0 2

3 - 1 - ~ 1

9 1 3 3

6 0 0 - 2

whole. Of the nine Raman active modes, five come under the symmetric class A~ and four under class A2. Again, six should belong to the trans- latory type and three to the rotatory type. The recorded spectrum of potassium chlorate shows only five !attice lines of which the line 98 cm. -1 is most intense. The Raman spectrum of the crystal was photographed for three different orientations of the crystal. The line 98 cm. -1 exhibits marked variations in intensity with different orientations and is also found to be highly polarised. This fact suggests that this lattice line should be attributed to the only rotatory oscillation which comes under the symmetric class Av The doublet 127-45 cm. -~ has a mean value of 136 cm. - t which also coincides with the sum of the other two lattice lines 54 and 82 cm. -~ It seems probable that there is only one lattice oscillation with this frequency in the spectrum of potassium chlorate which splits into 127 and 145 cm. -1 by Fermi resonance. In Table I ate also included the lattice lines of sodium chlorate for comparison. It can be seen that the ratio between the highest frequency shifts in the two cases, namely 136 and 179 cm. -1, is inversely pro- portional to the square root of the mass of the cations. The widths of these two tines are also appreciable. Hence they may be attributed to the mutual oscillations of the chlorate ions against the metallic ions in both cases.

Internal .frequencies.--As is well known, the chlorate ion in the free state possesses four fundamental vibrations active in Raman effect, namely 930, 975, 615 and 478 cm. -1 with degeneracies 1, 2, 1 and 2 respectively. These four lines are observed in the spectrum of the crystal with nearly the same frequency. !t is surprising to note that the doubly degenerate lines

180 C. Shanta K u m a r i

478 and 975 of the free chlorate ion are not split in the spectrum of the crystal in spite of the lower symmetry of the crystal. This suggests that the influence of the potassium ion on the vibrations of the chlorate ion is small in the crystalline state. The line at 930 cm. -z due to the symmetric oscilla- tion, is found to have in the crystal a feeble companion at 920cm. -z This splitting may be attributed to the fact that there are two molecules in the unit cell.

4. EFFECT OF TEMPERATURE

Using the same ultraviolet technique, the Raman spectrum of potassium chlorate was photographed at a series of ten different temperatures, namely, 90, 258, 297, 343, 398, 420, 453, 483, 520, 563 and 603 ~ K. and the variations in the position and width of the prominent Raman lines were studied. With a slit-width of 0.025 mm., exposures of the order of two hours each were given. In order to demonstrate clearly, the effect of temperature on the Raman spectrum, two pictures, one with the crystal held at 603~ and the other at 90 ~ K. were taken side by side under identical conditions on the same photographic plate using a Har tmann diaphragm. This spectrum is reproduced in Fig. 2. The variations in the position and width of three prominent Raman tines 98, 136 and 975 cm. -z are graphically represented in Figs. 3, 4 and 5. Unlike the case of sodium chlorate (Shanta Kumari , 1950), the symmetric oscillation with fi'equency shift 930cm. -z shows a definite shift in position. The line 486cm. -z does not show any appre- ciable shift.

~o

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�9 ~ , ' 0 r:

t . ! I I ~ I

; 0 0 2 0 0 ~00 ~t00 $ 0 0 6G ~,

T e m ~ e r a � 9 1 i~~ ~K.

F!~. 3. Temperature dependence of the position and width of 98 Raman line

O

;O

20

R a m a n Speclru nz o f Po tass ium Chlora/e 181

S

b,, r

.~ 10

F]G. 4.

I I I T , I

! 0 0 200 ,~00 400 SO" 600 Te,'nporc~~ur, e in *K

Temperature dependence of the position and width of 136 Raman line

I 0

2 0

3 0

r

~ o "4

S

b~

~ o

Fin . 5.

o , e w/dd.~

t I ! i i , , i

I 0 0 2 0 0 ,300 4 0 0 S O 0 60, t Te~pe. ' .acure in *K

Temperature dependcnce of the position and width of 975 Raman linq

,0

=70 "~

~o ~"

182 C. Shanta Kumari

The total changes in frequency shiff and approximate width of these lines together with the proportional change in the two ranges of temperature from 90 to 297 ~ K. and from 297 to 603 ~ K. are entered in Table III.

T A B L E I I I

Total shift and proportional change in the position of Raman lhTes in potassium chlorate

�9 F r o m 90 ~ to 297 ~ K F r o m 297 ~ to 603 ~ K Breadth03~ R a m a n - -

I line cm. -• S h i f t 1 bv S h i f t 1 ~~ A t 290 ~ K K c m . - 1 X = -- . . . . j, ~ c m - x X = - ~ ~--~. c m . - 1 c m . - 1

F 98

136

930

975

4

3

1 . 2

2

1 9 7 . 2 x 10 - 6

1 0 6 . 5 x l 0 - ~

6 . 2 7 x 10 - 6

5 . 5 x 10 -6

I1 3 6 6 . 8 X 10 -6

8 . 5 2 0 4 . 3 X 10 -6

3 1 0 . 6 X 10 'e~

5 . 5 1 8 . 4 5 X 10 TM

10

1 7 - 2

12

7

2 2

36

28

22

As in the case of other crystals it is found that the proportional change 1 3v X = - - ~ " ~~ is different for different Raman lines and for a single Raman

line, it is higher at higher temperatures. The graphs show that the varia- tions in frequency shift and width for the individual Raman lines ate similar in nature. This suggests that the two effects of temperature, namely, decrease in frequency shift and increase in width of the Raman lines are closely connected to one another as indicated by Sir C. V. Raman (1947).

According to Madan (1886), a change of structure is supposed to take place when potassium chlorate is heated to about 250 ~ C. But Lord Rayleigh (1922) who investigated the optical properties of the crystal at this temperature was of the opinion that the change of structure brought about by heat was macroscopic in character and did not in any way alter the crystal symmetry. In order to settle this issue, the author photographed the Raman spectrum of potassium chlorate with the crystal maintained at the tempe- ratures 225, 247 and 280 ~ C. respectively. The recorded spectra did not indicate any change of crystal structure, thus supporting the conclusions arrived at by Lord Rayleigh.

I wish to place, on record my deep indebtedness to Professor R. S. Krishnan for many helpful suggestions and encouragernent,

Raman Spectru m o/" Potassiu m Chlorale 180

5. SUMMARY

Using the ;~ 2537 radiation of mercury a r e a s exciter, the Raman spectrum of a single crystat of potassium chlorate has been photographed. The spectrum exhibits 10 Raman lines of which 4 lattice lines 54, 82,98 and 145 cm. -~ have been recorded for the first time.

The variations in the position and width of the prominent Raman lines have been studied at ten different temperatures ranging from 90 ~ to 603 ~ K. The value of the proportional change is found to be different for different Raman lines and is higher at higher temperatures. No change in the crystal symmetry is noticed throughout the temperature range investigated.

REFERENCES

Hibben . . R a m a n E f f e c t a n d i ts C h e m i c a t A p p l i c a t i o n s , 1939. Krishna Murthy . . Ind. J . P h y . , 1930, 5, 633. Madan . . Na tu re , 1886, 34, 66. C. V. Raman . . Proc . Ind. A c a d . Sc i . , 1947, 26, 511. Rayleigh . . Proc . R o y . S o c . , 1922, 102, 668. Shanta Kuma¡ . . Proc . Ind. Acad . Sc i . , 1948, 28, 500 ; t950, 31, 348.

Venkateswaran . . I b id . , 1938, 7-8, 144. Zachariasen " .. Z . f . K~ist. , 1930, 71, 513.

C. Shau/a Ku,zari Proc, Z~d. Acad. Sci., ,q, w / X?Cu 1u X U [

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FIa. 1. (a) Ramanspectrumof Potassiumchlorate takenwith amedium quartzspectrograph. (b) Its microphotometer record.

,,, I I . . . . I l

i! �84 .~ o

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0 1 3 K

9 )~.2

t.~m. 2. Ramanspectruna ofpotassiumchlorate takenwith Hartmai:ndiaphragnl