A. J. C. WILSON 183

for (p less than ½g. For zc greater than ½re they repeat the same numerical values, so that cos ~0, csc2 q)etc. should be replaced by [cos ~01, [csc2 ~01 etc.

As was shown in general by Stokes & Wilson (1944), the apparent particle sizes depend only on the relation of the diffraction vector to the external form of the crystal, and not on the dimensions and orientation of the unit cell. In the past they have usually been given in terms of the indices of reflexion relative to a cell bearing a simple orientation with respect to the external form, and this may often be the most convenient re- presentation for practical use. It does, however, in- troduce at least one parameter more than is mathema- tically necessary. In order to calculate the integral- breadth Scherrer constant it is necessary to determine V(t) for all values of t, whereas the variance Scherrer constant and taper parameter require only the initial derivatives. In the examples considered above, and

probably in many other cases, the initial derivatives are easier to evaluate. The Scherrer constants etc. appro- priate for use with the Fourier-component method (Warren, 1959) are the same as those for the variance method.

The author is indebted to Dr D. Lou~r and Dr J. I. Langford, whose experimental work suggested the problem, and to Mr H. J. Edwards for checking the calculations.

References

LELE, S. & ANANTHARAMAN, T. R. (1966). Proc. Indian Acad. Sci. A64, 261.

STOKES, A. R. & WILSON, A. J. C. (1944). Proc. Camb. phil. Soc. 40, 197.

WARREN, B. E. 0959). Progr. Metal Phys. 8, 147. W I L S O N , A. J. C. (1962). X-ray Optics. London: Methuen.

J. Appl. Cryst. (1969). 2, 183

A New Method of Etching Diamond

BY A. R. PATEL AND S. M. PATEL

Physics Department, Sardar Patel University, Vallabh Vidyanagar, Gujarat State, India

(Received 6 August 1968 and in revised form 2 January 1969)

A new method of etching diamond is described which produces rapid etching and etch patterns which are decidedly better and more easily interpreted than those produced by the conventional method. It is also shown that the new method produces etch patterns having the same morphology as that of trigons observed on natural (II1) faces and quadrons on (100) natural faces of diamond.

Introduction

It is well known that, when a crystal is etched in a suitable solvent, the etch figures produced are strictly related to the molecular structure of the crystal. It is also well known that etching reveals the sites and the structures of dislocations in the body of the crystals. However, the selection of the etchant and the method of etching play an important part in producing etch fig- ures which can be interpreted.

Earliest etching experiments on diamond were re- ported by Luzi (1892) who exposed the crystals for 20 to 30 minutes in a Kimberlite flux at 1770°C and obtained very poor results. Fersmann & Goldschmidt (1911) heated diamond at about 900°C in potassium nitrate or soda fluxes and produced triangular and rounded etch marks on (111) faces. Williams (1932) re- ported that by etching diamond in fused potassium ni- trate at 900°C for two hours, the crystal lost one third of its weight. Since then many etching experiments have been carried out by a number of investigators; chief amongst them are Custers (1954), Omar, Pandya & Tolansky (1954), Patel & Tolansky (1957), Evans &

Sauter (1961), and Patel & Ramanathan (1962). These investigators used the conventional method for etching the crystals i.e. they heated their crystals in a melt of oxidizing agents in a nickel or platinum crucible at temperatures varying from 400°C to 1300°C. Produc- tion of appreciable etch patterns by this method takes a considerable amount of time which depends upon the etchant used and its temperature. Sometimes it be- comes difficult to interpret the etch patterns produced in this way. A number of experiments were, therefore, carried out to find a suitable method of etching dia- mond. We describe here a method we have evolved by which good etch patterns are produced within only a few seconds.

The new method of etching

In the conventional method the diamond to be etched is placed in the hot oxidizing melt kept at the required temperature; in the new method, the diamond is first heated to the required temperature and the etchant is then poured on it. For this purpose, the crystal to be etched was placed with its face upward in a hole drilled in the lower electrode of a carbon arc. The arc was

J A C I - 3*

184 A N E W M E T H O D O F E T C H I N G D I A M O N D

then struck and kept running until the diamond reached the required temperature. The arc was then switched off and the etchant was immediately poured over the hot crystal surface. Solutions of potassium nitrate and so-

dium nitrate in water were used as etchants. Although both the etchants produced good results, we found that a solution of sodium nitrate in water (50 g dis- solved in 100 ml) was found to be more suitable. It may

".. "/,~_n - J 7 ~ . / ~ ; ~ : - ~ ~-, ,- ; , ~ * . , S ~ I

~'.~l~' ,~. ~' ~ " ~ q,'~. "" " ' -22" ,,,t'.,4~'~,:'~'* ,K~:',E"

, . J r ' . ,.." ,,:% .% ,, ~ ' Lb ; ~ "%,,:'~-~~.*.:~.~ d i r - ~ - ~ ' ~ ° ,;~ : - , ~ ~.',, ~ , ~ :. ~ / , ; , , ,:-. ! : t~ ,~:

~" " ~ 2 ~ - . ~ ' A ~ ," ~ , A ' . ~ - , ' . ~ . . : ' A ~ , ~ riP'. ~_~

(#)

(a)

i: i! .. :

, ~ ~ ~ ~ ~ "..\-,,

• - - . ' ~ . ~ ' A :'~

, .,~ , ' . - ~. ~\ :

I (c)

Fig. 1. (a) An etch pattern produced by the conventional method. (Etching time 4 hours.) x 280. (b) an etch pattern produced by the new method. (Etching time 1 second); x 280. (c) etch pattern produced by heating in a carbon arc and then cooling in the atmosphere, x 280.

A. R. P A T E L AND S. M. PATEL 185

(a)

(b) Fig.2. (a) Further etching of the region shown in Fig. l(a); × 280. (b)~further etching of the region shown in Fig.2(b). × 280.

be mentioned that when drops of the solution at room temperature were dropped on the heated crystal sur- faces for etching, the solution suddenly vaporized and the crystals were ejected from the cavity. Thus the etching took place for only about one second, at the most. It may also be mentioned that, when the crystals were heated in the core of carbon arc and allowed to cool in the atmosphere, they were etched but the etch patterns produced were not very clean and sharp. The tempera- ture of the crystal was not raised to such an extent that the diamond would be graphitized.

Observations and results

Etching of natural (111) faces For comparison of the etch patterns the natural octa-

hedral faces were selected and etched by both methods. Thus Fig. l(a) represents the etch patterns produced by the conventional method, i.e. by etching the crystal in hot potassium nitrate for 4 hours at 570°C, while that shown in Fig. l(b) is produced by the new method in about one second. Attention is drawn to the following:

(1) The pattern produced by the conventional meth- od consists of developed micro cracks and some scrat- ches along with the triangular etch pits crowded to- gether with rounded corners and edges, while in the etch pattern produced by the new method only the triangular pits having sharp corners and edges are produced.

(2) The triangular pits in both the figures have posi- tive orientation.

(3) The pits produced by the conventional method are all nearly the same size, while those produced by the new method are of different sizes.

The crystals were further successively etched by their respective methods and the etch patterns were photographed. Thus Fig.2(a) and (b) represents the etch patterns produced by re-etching the region shown in Fig. l(a) and (b), respectively. It may be noted that the etch pattern in Fig 2(a) produced by the conventional method has become more complicated, the etch pits have become more rounded and the overlapping has increased. The etch pattern shown in Fig. 2(b), produced by re-etching by the new method, has become more

186 A NEW M E T H O D OF E T C H I N G D I A M O N D

distinct, showing pits with sharp corners and rectilinear sides which do not overlap.

The etch pattern shown in Fig. l(c)is produced on the natural (I1 I) face of a diamond octahedron by heating the crystal in the core of a carbon arc and then allowing it to cool in the atmosphere. Comparison of the etch patterns in Fig. l(c) and Fig. l(b) reveals that the pattern produced in (b) is more distinct and clear than that shown in (c).

It is reported by Tolansky & Patel (1957) that, if eaching is carried out very slowly, the etch pits having rectilinear sides and sharp corners can be produced after prolonged etching but the overlapping cannot be avoided. Fig. 3 shows an etch pattern produced by heat- ing a crystal in a melt of potassium nitrate for 24 hours at 475 °C. Comparison of even this pattern with the pattern in Fig.2(b) reveals the superiority of the etch pattern produced by the new method. A careful comparison of the etch patterns shown in Figs. l(b) and 2(b) reveals that the density of etch pits changes on successive etching. It has been observed that the density of the etch pits in the first etching is greater and it slow-

ly decreases with further etching to a minimum value (a detailed study of this phenomenon is under investi- gation and will be published when completed).

The etch patterns produced by the new method of etching were examined by transmission electron micro- scopy of replicas. The electron micrograph in Fig.4 re- veals the typical morphology of the etch patterns pro- duced by this method.The electron micrograph reveals that the etch pattern more or less resembles the trigon pattern on natural (111) faces of diamond having the following characteristics:

(1) The etch pits are equilateral triangles in shape, having rectilinear sides and sharp corners.

(2) The etch pits are flat bottomed as well as point bottomed and they do not overlap each other.

(3) The corners of one pit just touch the sides of the neighbouring pits.

Etching of(100) polished faces Etching of (100) polished faces was also tried by the

new method and it was found that even on these faces Fig. 5(a) represents the electron micrograph of the etch

.,. ,~, • - , : . - - ~ - ~ - . ~ . , . , . .

(a)

, v ~ t ~ •

(b)

Fig. 3. An etch pattern produced by heating the crystal in a melt of potassium nitrate, 24 hr at 475 °C. x 280.

A. R. P A T E L AND S. M. P A T E L 187

(a)

n

~)

Fig.4. Electron micrographs (replica technique) of the patterns produced by the new method, x 2000.

pattern produced on a (100) polished face of a diamond. Fig.5(b) represents the electron micrograph of the microstructures on a natural (100) face of another crys- tal, and is given here only for comparison. Comparison of the two electron micrographs reveals a good deal of similarity between the quadrons (Tolansky & Suna- gawa, 1960) observed on natural (100) faces and the etch pattern produced on polished (100) faces by the new method.

Discussion

It appears that the new method produces etching at a high rate in comparison with the conventional meth- od. Because of the fast etching, the etch patterns pro- duced at superficial imperfections such as surface scrat- ches and ring cracks are rapidly washed out, as they are confined to the few topmost layers. As the etch pat- terns produced by the new method are free from devel-

188 A N E W M E T H O D O F E T C H I N G D I A M O N D

(a)

(b) Fig.5. (a) Electron micrograph of the etch pattern produced on a (100) polished face; x 2000. (b) electron micrograph of the

microstructures on a natural (100) face. x 2000.

oped ring cracks and surface scratches, and as the etch pits produced do not overlap, the pat terns are clearer, more distinct and easier to interpret than are patterns produced by the old method.

References

CUSTERS, J. F. H. (1954). Physica, 30, 183. EVANS, J. & SAUTER, D. H. (1961). Phil. Mag. 6, 429. FERSMANN, A. & GOLDSCHMIDT, V. (1911). Der Diamant.

Heidelberg: K. Winter.

Luzl, W. (1892). Berichte deutsch, chem. Ges. 25, 248. OMAR, M., PANDYA, N. S. & TOLANSRY, S. (1954). Proc.

Roy. Soc. A 225, 33. PATEL, A. R. & RAMANATHAN, S. (1962). Phil. Mag. 7,

1305. PATEL, A. R. & TOLANSKV, S. (1957). Proc. Roy. Soc. A243,

33. TOLANSKYo S. d~ PATEL, A. R. (1957). Phil. Mag. 2, 1003. TOLANSKY, S. • SUNAGAWA, I. (1960). Indi. Diamond Rev.

20, 7. WILLIAMS, A. T. (1932). The Genesis of Diamond. London:

Benn.