On the crystal structure of chlorine

ON T H E CRYSTAL STRUCTURE OF CHLORINE b y W. H. KEESOM and K. W. TACONIS Communication No. 240e from the Kamerlingh Onnes L a b o r a t o r y at Leiden

Summary T h e c r y s t a l s t r u c t u r e of c h l o r i n e a t - - 1 8 5 ° C w a s d e t e r m i n e d w i t h t h e X - r a y g o n i o m e t e i - . W e f o u n d a t e t r a g o n a l l a t t i c e w i t h a = 8.56 & a n d c = 6.12 ~ ; d e n s i t y 2.09 w i t h 8 m o l e c u l e s p e r u n i t c e l l T h e i n t e n s i t i e s of the Debij e-Scherrer lines m e a s u r e d b y K 6 h l e r agree with the 16 in w h i c h t h e c o - o r d i n a t e s of t h e C l - a t o m are x = 0.125 ; s p a c e g r o u p D4h, y = 0.167; z = 0.107. I n t h i s g r o u p t h e d i s t a n c e of t h e a t o m s in t h e CIm o l e c u ! e is 1.99 A.

§ 1. Introduction. The structure of solid chlorine was determined b y means of an X-ray goniometer with moving film in which a single crystal is irradiated just as was done in the treatment of ethylene and T-oxygen 1) 2). We had at our disposal the data from D e b i j e - S c h e r r e r diagrams made b y K 6 h 1 e r 3). Moreover the density of solid chlorine had been measured b y H e u s e; he found at --185°C 2.12 *). As a result of his optical res.earch, W a h 15) concluded that the structure might be rhombic and, as iodine e) was found to be rhombic, this was not altogether improbable. K 6 h 1 e r was able to mention several rhombic structures agreeing with his measured spacings. Chlorine having a great tendency to attack other substances, we availed ourselves of the experience gained b y K 6 h 1 e r in preparing the pure substance from KMn04 and HC1. We kept the chlorine in the same manner, in a little glass bulb, to be opened in vacuum b y smashing, b y means of an iron bullet, the thin glass separation between the bulb and the tube connecting it with the camera (as we learned from Philips Lampworks). This was done b y introducing the bullet into the tube b y means of a magnet and --

237

--

238

W. H. KEESOM AND K. W. TACONIS

dropping it on to the glass separation b y removing the magnet. To prevent the mercury meniscus in the thinwalled glass tube in the camera from being attacked b y the chlorine, some sulphuric acid was put on top of the meniscus.

§ 2. Apparatus. When dealing with the structure of y-oxygen *), we mentioned already that the apparatus originally used 1) was somewhat modified b y using a glass cryostat. We shall briefly describe this new cryostat here (fig. 1). The copper cryostat used before, and which was designed originally 7) for obtaining powder diagrams, caused some trouble as, in cooling, any slight asymmetrical contraction of the walls displaced the thinwalled glass tube, in which the crystal was to be produced, from the middle of the camera. In the glass cryostat we used, this displacement was avoided b y the rigidity of the inner wall, the stress in the outer wall being intercepted b y an elastic brass case (v) 8). The b o t t o m b of the cryostat was made of copper and welded to the glass b y means of a chrome-iron ring. The copper arm a forming the connection with the cup filled with mercury m was soldered to the bottom• The cryostat fits in the rotating copper cone c (cf. fig. 1, Commun. No. 235b) of the camera b y means of the ground joint g. We irradiated with CuK~-rays. For cooling we used liquid air. We estimate the temperature to have been about --175°C. § 3. Results• It was extremely difficult to obtain perfect single crystals of chlorine. All the films show weak D e b ij e-S c h e r r e r lines too. We reproduce an X-ray diagram in which the crystal was rotated over 189 ° (fig. 2a). Fig. 2b shows the gnomonic Fig. 1. projection constructed from the diagram in the way we described previously 1). The projection plane was perpendicular to the axis of the camera at' a distance of 2 cm above the crystal.

ON THE CRYSTAL STRUCTURE OF CHLORINE

239

The numbers 1 to 7 in the projection are corresponding to normals on the planes which have caused spots on the film, in such a way, that each normal has the same number as the corresponding reflection in the diagram. From the projection we concluded the lattice to be a tetragonal one with c/a = 0.715. In order to check this unit cell we first constructed in the projection a few other normals (a--g). The point a was constructed as being the intersection of the normal on the plane through 2 and 5. After that we placed letters at the inter-

13= iI

12j

12 13.

it

~¢m

i,

T, T, J

0 t 2 ~CI~

I,,,I,i

Fig. 2a. X - r a y diagram. Fig. 2b. Gnomonic projection. D e t e r m i n a t i o n of the crystal s t r u c t u r e of chlorine.

sections of planes laid through a few normals already represented in the projection. In table I we gave indices to the planes, corresponding with the normals mentioned above. Finally in table II we show the agreement which is obtained, when we compare the angles measured in the projection (to measure these angles we used the method, given

240

W.H.

KEESOM

AlqD K. W. TACONIS

TABLE

TABLE I

The indices of the planes corresponding to the normals indicated in projection normal

plane

,

12T

2

002 22T

31 3s

221

4 5

3iT 400

6

7

4oV 72l

a

040

b

o4~-

c

3 I 0 30"[

d

e f

-i10

g

021

n

Measured and calculated angles normal between

a--2 5--a 5--2 6--5 e--a b--a g--a c--a

d--5 1--b f--5

measured in the projection 90 89 91 35 46 35 345 72 24 21 29

calculated with c

ig

=0.715 90 90 90 356 45 35 355 72 25 22 s 27

4~o

in Commun. No. 235b) with those calculated from the above proposed unit cell. A very exact determination of the absolute dimensions of the cell from our single crystal diagrams being difficult, we availed ourselves of the spacings measured b y K 6 h 1 e r and published for the greater part ~in his thesis 3). We added some spacings not published b y K 6 h 1 e r, as t h e y originated from very weak D e b ij e-S c h e rr e r lines which were not observed b y him on all the films and the reality of which was therefore not very certain. W e marked these spacings in table III with an asterisk. Moreover we added the spacing 2.03 A, which could not be noticed b y him because this line coincided on his films with the 200 line of silver, this being the material of the rod on which, in his experiments, the chlorine was deposited. In the diagrams made b y us this spacing could be observed to be relatively strong. In table n I the 2nd column contains the spacings measured b y K 5 h 1 e r, the 3rd column those we calculated, starting from the tetragonal cell with a ---- 8.56 A and c = 6.12 A. The calculated density at - - 1 8 5 ° C with 8 molecules per unit cell we found to be 2.09, which is somewhat smaller t h a n t h a t found b y H e u s e (2.12 a t - - 1 8 5 ° C ) .

§ 4. The space group. To obtain the space group for chlorine we again used the intensities of D e b ij e-S c h e r r e r lines measured

ON THE

CRYSTAL

STRUCTURE

TABLE

OF CHLORINE

241

III

Spacings and intensitiesas determined by K 6 h I e r compared with those calculated f r o m t h e u n i t cell w i t h a = 8.56 A, c = 6.12/~, indices of t h e p l a n e

210 201 211 002 220 102 300 112 221 310 301 202 311 212 320 321 222 4OO 3O2 410 OO3 312 330 103 411 I l 3 331 420

s p a c i n g s in A measured by K 8 h 1e r

3.26 3.07

2.685

2.455 2.39* 2.21 2.15

2.03*

1.93* 1.91"

322 203 42 2 l 402 412 430 500 223 332 510 303 43 801 313 422 5 l 520 323 521 004 440 104 502 432 403 114 53O 441 5 l

l 3

1,82 1.755

1.70

s p a c i n g s in ,/t calculated 3.83 3.51 3.25 3.06 3.03 2.88 2.85 s 2.73 2.71 2.70 s 2.58 2.49 2.47 2.39 2.375 2.2i 2.15 2.14 2.09 2.08 2.04 2.03 2.02 1.985 1.97 1.93 1.92 1.915 1.875 1.84 1.825 1.80 1.755 1.715 1.71 1.71 1.69

intensities

W in

W W.W. W S

w w.w. m %v

1.685 l 1.64" l

1.59"

1.54 s 1.50 1.49

2

Physica I I I

1168 1.66 1.65 1.65 1.63 1.63 1.62 1.59 1.54 ~ 1.54 1.53 1.51 1.507 1.49 s 1.493 1.48 1.48 1.47 1.47 1.47

w.w. w

w w

r e l a t i v e intensities c a l c u l a t e d 0 0 191 . 264 0 54 0 102 1206 1474 0 34 184 170 0 310 0 630 47 0 0 144 480 0 220 560 0 100 81 0 145 0 105 0 0 0 840 117 0 0 0 0 84 18 190 0 5 17 100 90 4 216 126 0 40 8 0 0 16

242

ON T H E C R Y S T A L S T R U C T U R E OF C H L O R I N E

b y K 6 h 1 e r. W e concluded it to be a group D ~ in which the coordinates of the Cl-atom are x = 0.125 y = 0.167, z = 0.107. W e r e p r e s e n t the arr a n g e m e n t of the C1O÷ ®oka t o m s in fig. 3. I t shows a projection of the unit 4 ®o ®½cell on ~he horizontal plane as used in the I n t e r national Tables for the D e t e r m i n a t i o n of Crystal 0o½ ÷ S t r u c t u r e s 9). T h e distance C1-C1 in the chlorine molecule in this molecular lattice is f o u n d to be 1.99 A. This value most satisfactorily ®~" tagrees w i t h 2 × the v a l u e F i g . 3. A r r a n g e m e n t o f a t o m s i n t h e u n i t cell (0,99 A) given b y P a uwith x=0,125, y=0,167, z=0,107. i i n g lo) for C1 in his table of the n o r m a l e l e c t r o n - p a i r - b o n d radii of atoms. T h e 4th and 5th column of table I I I contain the e x p e r i m e n t a l and the calculated relative intensities. F o r this calculation we used the formula I + cos 2 2 0 Ih ~-- sin2 0 cos 0 v, I Sh I2. Sh = the s t r u c t u r e factor for the amplitude, vh = n u m b e r of planes c o n t r i b u t i n g to the same line. J a m e s a n d B r i n d I e y's a t o m fact.ors were used 11). Received February 22, 1936. REFERENCE 1) W. H. K e e s o m and K. W. T a c o n i s , Commun. Kamerlingh Onnes Lab., Leiden No. 235b; Physica, 's-Gray. 2, 463, 1935. 2) W . H . K e e s o m a n d K . W. T a c o n i s , Commun. K a m e r l i n g h 0 n n e s L a b . , L e i den No. 240d; Physica, 's-Gray. 3, 141, 1936. 3) J. W. L. K 6 h l e r , Thesis Leiden, 1934. 4) W. H e u s e, Z. physik. Chem. (A) 147, 226, 1930. 5) W. W a h 1, Proc. roy. Soc. London (A) 88, 348, 1913. 6) P . M . H a r r i s , F. M a c k and F. C. B l a k e , J. amer. chem. Soc. 50, 1583,19~.8. 7) J. D e S m e d t , W.H. Keesom and H. H. M o o y , Commun. Kamerlingh Onnes Lab., Leiden No. 217e; Proc. roy. Acad. A m s t e r d a m 35, 25, 1932. 8) -H. K a m e r l i n g h Onnes and W. H e u s e , Commun. Kamerlingh Onnes Lab., Leiden No. 85; Proc. roy. Aead. A m s t e r d a m 7, 674, 1905. 9) International Tables for the Determination of Crystal Structures, BeHin 1935. 10) L. P a u l i n g , Proe. nat. Acad. U.S.A. 1 8 , 2 9 3 , 1932. I11 R . W . J a m e s a n d G . W. B r i n d l e y , Phil. Mag.(7) 1 2 , 8 1 , 1931.