20 Etching and dissolution of crystals(Application to observation of defects)

Prof. P. N. Kotru

CONTENTS:

20.1 Etching and dissolution of crystals.

20.2. Methods of etching.

20.3. Historicalreview.

20.4. Dislocation etch pits.

20.5. Factors affecting the etching process.

20.6. Correspondence of etch pits and dislocations.

20.7. Information obtainable by etching technique

20.8. Application of etching technique.

20.9. Morphology of etch pits.

20.10. Some examples of etching.

20.11. Summary

20.1 Etching and dissolution of crystals.

Although extremely powerfu l analytical techniques are available, yet etching or dissolution phenomenon is the foremost method of making individual crystals visible in a polycrystalline material and of detecting internal imperfections in a crystalline solid. The term “dissolution”, in general, means It has been often said that there may be strict reciprocity between growth and dissolution of crystals. Crystal dissolution in its earlier stages is represented by the formation of etch figures. The initial dissolution of a crystal on attack by a suitable solvent generally takes place in such a way as to give an indication that it may be related to the underlying structure. The suitable solvent or chemical reagent is called an etchant while the depressions produced by it are called as etch pits. When crystals are placed in a suitable solvent, their faces come in contact with the latter for some suitable time, leading to formation of a number of pits on the faces. These pits are normally bounded by sloping planes along which the solvent has acted rather faster. The solution cavities thus formed are generally having a definite shape and are called as etch figures. Dissolution is required to be carefully controlled in its initial stages in order that there may not be a general retreat of the faces, edges and corners, with rounding of and change of shape. Uncontrolled dissolution leads to unequal solution velocities at corners, edges and faces. The shape of the etch figures depends on parameters like nature and concentration of solvent, time and temperature. In most of the cases the symmetry of the shape and structure of etch pits reflect the symmetry of the underlying structure. In other words, etch patterns produced are related to internal molecular structure. Etching is the result of variations in surface reaction or more specifically dissolution rates, brought about by crystallographic orientation effect, lattice imperfections and chemical composition. Etching is a relatively simple but very powerful tool for studying the structure and composition of materials, in the investigation of crystal defects such as dislocations, vacancies, grain and twin boundaries, slip lines and stacking faults. Inspite of the fact that extremely powerful analytical techniques are now available, etching continues to be indispensable in the study of inhomogeneity (impurity distribution) in single crystals.

Ideally the dissolution should be the exact reverse of crystal growth. All the atoms with energ ies greater than the one half crystal site leave first, then all part ial layers are moved by removal at kink sites and find a perfect crystal surface remains. Dissolution must then occur by two dimensional nucleation of holes in the crystal surface. Imperfections or defects play as large a role in real crystal dissolution as they do in real crystal growth. Further, the surface of any real, imperfect crystal is very likely to contain many dislocation terminat ions, and these terminations may form the nuclei o f etch pits.

Etching processes are generally very co mplex and their detailed mechanis ms are still not quantitatively understood. Etching techniques are still based on qualitative or emp irical bases. Development of an etchant or etching procedure for a given crystalline material is still a “trial and error” process.

20.2.  Methods of etching.

Etching method involves immersing the crystal in a suitable mediu m like, a liqu id, a solution or a gaseous chemical reagent. Tiny pits develop at the sites of intersection on the surface by defects particularly dislocations. There are different ways of etching which may be briefly described as follo ws:

20.2.1. Thermal etching .

In this method the crystalline material is heated to a suitable temperature below its melting point. It is used when one does not have an etchant and the various parts of the surface exh ib it differences in sublimat ion rates. Small pits are produced at the dislocation sites, as for example in silver when the same is heated in an atmosphere of oxygen.

20.2.2   Chemical etchi ng .

This method of etching involves chemical reaction between the solid and the etchant. In this case there is spontaneous chemical reaction between the solid and the etchant. For examp le, calciu m fluoride when dipped in concentrated sulphuric acid results into format ion of etch pits on its surfaces.

20.2.3.   Solution etching.

Generally, no distinction is made between solution etching and chemical etching. Etching by solvents involves no chemical reactions. Liquid etchants are most commonly employed; however, gaseous media are also used. Examp le of solution etching is that of sodium chloride wh ich when dipped in for one second in anhydrous methyl alcohol gets etched and produces symmetrical etch pits on its surface. Lithiu m fluoride is etched by dipping it in aqueous solution of ferric fluoride.

20.2.4.   Preferential oxidation.

This is also considered as an etching method. As for example, copper gets oxid ised preferentially at dislocation sites.

20.2.5.    Electrolytic etching.

This is also a means of etching and is known also as anodic dissolution. This method has been used by Jacquet in 1954 on specimens of α-brass.

20.2.6.   Cathodic sputtering or ion bo mbar dment.

In this method, the surface atoms of a given specimen are removed by gas ion bombard ment . Cathodic sputtering in an atmosphere of mercury vapour has been used for revealing emergence points of dislocations in german iu m.

20.3.       Historical review.

Widmann Statten was the first who conducted etching experiment in the year 1808, by producing characteristic etch patterns on meteorites due to corrosion with acids. Wollstan was the first to carry out studies on etching in a scientific manner in the year 1816. In the same year Dariell tried to correlate the nature of etch figures with the mo lecular structure of the crystalline solids. In the later half of the nineteenth century there was an increasing interest in the phenomenon of etching. Tschermark (1882), Bau mhauer (1889), Wulff (1898) and several others contributed significantly in understanding theory and application of etch methods. In the first two decades of the 20th century application in the methods of etching for deriving informat ion in the fields of crystallography, mineralogy and in materials science received lot of attention. Important contribution s were made by Go ldschmidt, Wright, Ko llar, Gaubert, McNairn and many others who carried out goniomet ric examinat ions of etch figures. Miers (1904) used a special goniometer to study etch pits while they were developed in the solution. Utilisation of x-rays for analysing crystal structures generated further interest in the methods of etching. Uncertainties about the molecular arrangement of a crystal and its symmet ry could sometimes be satisfactorily exp lained and decided upon by the nature of etch pits.

Goldschmidt (1904) was the first to offer explanation regarding process of etching. He exp lained the formation of etch pits and etch hillocks as a result of the movements developed in the solvent. According to him the following suggestions should hold true for the etching process.

i. The locations of etch pits are at places where the current starts in the corrosive.

ii. Preferential etching takes place along the scratches.

iii. Minute particles of dust on the substance become po ints of first attack by the corrosive.

iv. Bunching of the etch pits takes place on the strained parts of the crystal.

v. The presence of inclusion or impurity is likely to result into a starting point for etching.

According to McNairin (1916) the lines of selective pitting are also the lines of weak cohesion. It was followed by Desch who in 1934 carried out etching experiments on alu m. The main drawback of the theory put forth on the basis of experiments performed by these investigators was its failure to exp lain satisfactorily the distribution of the etch features over the surface.

In 1927 Honess gave an admirab le account of etching work. According to him it is the more mature etch figures which most reliab ly indicate the crystal face symmetry. The impo rtance of etching technique in furthering the knowledge of crystallography is indicated by the extensive and ever-growing literature on the application of etch techniques , in deriving informat ion regard ing various aspects of crystal symmetry, defect studies and the history of growth o f the crystal.

20.4.   Dislocation etch pits.

The initiat ion of etch pit has been treated as a nucleation process analogous to crystal growth. Quantitative treatments have been developed relating the free energy change associated with etch pit formation to the energetics of the surface and the bulk. So far as dislocation etch pit formation is concerned these exp ressions include the energetic (strain and/or core energy) of dislocations; i.e., the free energy change for nucleation is reduced by the strain or core energy of the dislocation released during dissolution.

20.4.1 For mation of dislocation etch pits.

In 1951 Burton, Cabrera and Frank proposed a theory of growth and dissolution  in terms of advance and retreat of mono-molecular steps across the surface of the crystal. The active sites are the places along the steps where single-molecular rows end. These positions, called “kinks”, are individual molecules which may be deposited or removed. When a perfect crystal face gets exposed to a solvent, dissolution begins by the nucleation of “unit pits” one molecule depth which may be called as “mono- molecular” pit. These unit pits grow as steps retreat across the crystal through the action of the kinks. Such a process is described through schematic representation in figures 20.1 (a,b). a solvent, dissolution begins by the nucleation of “unit pits” one molecule depth which may be called as “mono- molecular” pit. These unit pits grow as steps retreat across the crystal through the action of the kinks. Such a process is described through schematic representation in figures 20.1 (a,b ).

Figures 20.1: Schematic   representat io n showing growth of unit pit as steps retreat across  the crystal through action of the kinks

Strained regions on the surface of a well-o rdered crystal will store energy. When such a surface reacts with a suitable med iu m, the stored energy will increase the dissolution of the strained part. Structural defects in crystals are the store houses of energy and so are the preferential sites for the attack of the etchant. It is obvious, therefore, that dislocations which are line imperfections intersecting the real crystal surface may become p referential sites for the nucleation of unit pits, and repeated nucleation at a dislocation leads to the formation of an etch pit.

The formation of etch pits by evaporation was described by Cabrera in 1957 in terms of dislocation energy which, he said, has a role in the nucleation of pits. The energy of dislocation which plays an important role in the nucleation of a unit pit is called as “localized energy” near a dislocation line. The “localized energy” of a dislocation consists of the core energy and a small fraction of the total elastic strain energy. The formation of visible pits nucleated at the point of emergence of dislocation depends on the nucleation rate for unit pit at a dislocation and rate at which steps move across the surface of the crystal. The cross-section of a pit depends on the ratio of vl/vn, where vl is the lateral d isplacement velocity of surface steps and vn is the rate at which dissolution occurs perpendicular to the surface , i.e ., the rate at which pit deepens. If vn is << vl, very, shallow p its would be formed which wou ld not be visible when illu minated for lack of contrast. The condition for which dissolution occurs in a d irect ion parallel to the surface .

Dislocation etch pits are generally pyramidal with the apex of the pyramid lying on the dislocation line. The tips of such pits follow the dislocation line as etching is prolonged. An established dislocation etch pit remains pyramidal (i.e., point-bottomed) so long as the dislocation line remains at its bottom. If for one reason or the other the dislocation line moves or shifts fro m its original position, the pit only increases its lateral dimensions but stops deepening. The pyramidal shape of the pits gets truncated or “flat-bottomed”. In this case, prolonged etching would continue increasing its lateral dimensions without deepening it which eventually leads to disappearance of the etch pit. Point-bottomed pit suggests continuation of a dislocation whereas flat-bottomed pit suggests its origin to be as a result of some defect confined to a few atomic layers of the crystal. The situation is explained by a schematic diagram o f figures 20.1 (c&d).

Figures 20.1: Schemat ic diagram showing c) formation of point bottomed dislocation etch pit and d)fo rmat ion of flat bottomed etch pit at a superficial defect

20.5.  Factors affecting  the etching process.

Dissolution  is affected by a number of factors such as crystallographic  orientation , crystalline perfection  , purity of the solid and concentration  and composition  of the etchant . Some of the factors are given here as follo ws:

i) Crystallographic orientation .

Crystallographic  orientation  of a surface has significant  influence  on  its  etching  behaviour. As a rule the closed packed planes are mo re easily etched than others; deviations from this orientation may result in non-preferential etching for some specific etchant. A proper choice of the etchant may allo w etching of higher index faces.

ii) Impurities in crystals

In metals some of impurity segregation is necessary before dislocations can be reliab ly etched. In ionic crystals the presence of impurit ies does not seem to play an important role. On clean cleavage faces of such crystals fresh dislocations (i.e., newly introduced) as well as old ones (grown – in) are made observable. The characteristics of the pits may differ for fresh and for grown-in dislocations. Etchants that will reveal only fresh dislocations have also been developed.

As already said the visibility of etch pits is controlled by the ratio vl/vn and a good etchant is the one for which vl/vn is small. The quality of an etchant can, therefore, be improved by changing either vn or vl. The usual method of decreasing vl is to add impurities to the solvent, which poison the kink sites in the surface steps.

iii) Adsorption of chemical species.

Adsorption of chemical species on solids often decreases their dissolution in etching solutions and enhances the format ion of dislocation etch pits. Through extensive studies on LiF it was shown by

processes, which are not necessarily affected by the energetics of dislocations.

20.6.    Corres ponde nce of etch pits and dislocations.

Pits formed during etching may not necessarily correspond to dislocation intersections with the surface. Point defects clusters, precipitates, impurity inclusions, surface damage and foreign particles on the surface may lead to the formation of etch pits. Etch pits which are not associated with with individual dislocations , on the other hand , reappear upon repeated polishing and etching , since the dislocation lines cannot terminate within the crystal ; in fact , individual dislocations can be traced through the entire crystal. For crystals which exhibit cleavage, the etch patterns should appear as mirror images on the matched cleaved surfaces. An etched thin plate of a crystal should exhibit one to one correspondence on its opposite sides , and if etching is carried out for a sufficiently long time the dislocation pits should finally develop into individual holes through the crystal plate. Configuration of dislocation net work within the body of a crystal can be traced by following the tip of the point-bottomed pit on prolonged successive etching .Planar defects like small-angle grain boundaries and twin boundaries can also be made observable by etching.

Vogel and h is co-workers demonstrated that the distance between etch pits in a tilt boundary corresponds within the experimental error, to the distance between the dislocation D as given by the Burger’s model D = b/θ. The misorientation angle θ was determined with a refined x-ray method. In this way they proved both the correctness of the Burger’s model o f tilt boundary and the reliability of etch method in revealing d islocations in crystals.

20.7.   Information obtaina ble by etching  technique

Etch method is able to provide the fo llo wing info rmation  in a very effective manner:

1.  Dislocation etch pits provide a means of direct measure of dislocation densities in a given crystal.

2.  Since the etch pits have a certain depth and follow the track o f d islocation line, they can give some

indication regarding the general direction of the dislocation lines. If the dislocation line intersects the

crystal surface perpendicularly, a symmetrical p it results; if the line is oblique (inclined ) the pits become

asymmetrical and fro m this asymmetry one can deduce inclination of the dislocation line.

3.  Dislocation configurations in the body of a given crystal can be exposed by gradual removal of the surface

layers alternated with etching. This technique can be used in mapping dislocation loops. By polishing and re-

etching technique it is possible to study spatial arrangements of isolated dislocations and grain boundaries

in single crystals. If the dislocation line is perpendicular to the surface and has a helical shape, the etch pits

acquire the form of a conical spiral. Helical dislocations have been shown to produce spiral etch pits.

4.  Etching techniques can be employed for studying the movement of dislocations in crystals.

5. Dislocation half-loop may lead to the format ion of two coupled pits, which on continued etching

approach one another and finally coalesce. A helix intersected along its axis is, in fact ,equivalent to a

sequence of such half-loops; therefore helices can easily be recognized in an etch pattern.

6.  Tracks due to fission frag ments can be etched. Also, “debris” left by moving dislocation etches, but

discontinuously. The ‘debris’ may consist either of agglomerates of point defects o r o f dislocation ‘dipoles’

broken up in small elongated loops.

7.  Dislocations parallel to and close to the surface may produce grooves along their length on etching the crystal.

20.8. Application of etching  technique.

Etching techniques have been applied to several dislocation problems. A few typical examp les of the application of etch techniques may be briefly described as follo ws:

i) To decide whether a given solid is a single crystal or not.

ii) To distinguish between different faces of a crystal.

iii) To reveal the growth h istory of a crystal

iv) To determine the density of dislocat ions.

v) To assess impurity distribution in crystalline solids.

vi) To study stress-velocity relation for indiv idual dislocations. Velocity of dislocations is found to be an

extremely sensitive function of the applied stress.

vii) To study origin of dislocations in as-grown crystals.

viii) To study deformation pattern like p ile -up and polygonization .

ix) To study dislocation multip lication  and its movement. For examp le, according to calculat ions of

Eshelby and his co-workers (1951), the d istribution of the d islocations in the pile-ups follo w the relation :

Where i is the etch pit number (which is equal to zero at the locked d islocation); σ0 is the applied shear stress,

n is the number of dislocations in the slip plane,

x is the etch pit distance from the origin.

A plot of the square root of the distance versus the dislocation index is expected to be a straight line. It has been shown to be true for pile-ups in SiC and in barium fluoride crystals.

x) To distinguish between fresh and as-grown dislocations.

xi) To study plastic flow around indentation.

xii) To distinguish between positive and negative dislocations.

xiii) To study kink, configuration and inclination of a dislocation.

xiv)     To delineate grain and twin boundaries.

xv )   To study radiation damage in crystals.

20.8.1. Theory of etching.

Etching and evaporation are phenomena which are reverse of growth. In crystal growth, crystal is made thicker whereas etching means to make it thinner by successive removal of surface layers.

Etch pits will be visible only if dislocation lines are attacked by the solvent in which the crystal is dissolved.

According to Cabrera and Levine (1956) the radius of the critical two-dimensional dissolution nucleus is given by:

where γ is the surface energy of the crystal-solution interface , Ω is the molecular volu me,

c0 is the equilibriu m concentration of the crystalline substance in the solvent and c is the actual concentration .

If undersaturation is large enough i.e., if ρc is small enough, a visible etch pit will result .

This theory is required to be modified for etching at edge dislocations. Here two-dimensional nucleation is necessary and we are concerned with the preferential nucleation of pits at the points of edge dislocations. This follows because the activation energy for two-dimensional nucleation is locally decreased since strain energy is gained by dissolving deformed material in the immediate vicinity of the d islocations.

20.9.   Morphology of etch pits.

Honess in 1927 performed large number of experiments and came to the conclusion that etch pits produced by different solvents on the same face or by the same solvent at different concentrations may change form , but they invariably reveal the symmetry of the face on which they occur .

According to him, the shape of the pit is more directly connected with the intermolecular forces within the crystal, which may be readily overcome by one solvent, causing dissolution in a given direction while for another solvent this dissolution is min imu m. In preferential etchants the morphology of dislocation etch pits depends on orientation, structure and on angle at which the dislocation line intersects the surface. Apart fro m internal factors associated with segregation of impurities and character and configuration of dislocations , the morphology of etch pits is also sensitive to a number of external factors such as nature and concentration of the solvent , the nature of the additive salt or complexing reagent and its concentration , the temperature of etching and stirring of the etching system.

20.10.    Some examples  of etching.

As already explained the growth of crystals is due to advance of growth layers from the nucleus centre, thus growing the crystal, whereas etching is retreat of atomic layers from the nucleation centre thus thinning the crystal surface. In general, etching / dissolution is the reversal of growth.

20.10.1  Example of etching as re ve rs al of growth.

One example in support of this may be cited here. Rhombohedral habit face of quartz crystal generally has a triangular outline. One finds triangular growth hillocks on such habit faces as is shown in figure 20.2 (a, b). The growth hillocks are oriented as shown in the figures. When rhombohedral face of quartz is etched by “hydrothermal etching” (etching conducted by placing the crystal in a sealed steel tube called as “bomb” filled with water and then placed in a furnace maintained at a higher temperature for some hours), the etch pattern produced is as shown in figure 20.3. Here also, the orientation of the figures is indicated. That the growth process is reverse of etching is clearly suggested by these two figures. The situation is convincingly explained by figure 20.4 which is a schematic diagram revealing the fact that the growth hillock is due to advance of triangular layers from the nucleus centre whereas triangular etch pit is as a result of retreat of such layers thus producing a triangular etch pit whose orientation is just opposite to that of the growth hillock.

The orientation of growth hillocks is just opposite to that of etch pits. This example is representative of several such cases where one can draw an inference that growth process is reverse of dissolution process. Etching is just a controlled process of dissolution.

Figure 20.3: Etch patterns on a rho mbohedral surface of quartz crystal due to hydrothermal etching for 8 hours.

Figu re 20.4: Schmeticdiagram showing the orientat ions of t riangu lar growth figures and etch figures w.r.t the t riang u lar outline of a rho mboh ed ral face of quartz crystal

20.10.2 Example of etch pit unfolding symmetry & revealing of dislocations

Next, we come to examples of morphology of etch pits in accordance with the symmetry of the surface and the system to which the crystal belongs.

Figu re 20.5 (a, b, c): He xagon al etch pits on (1000) plane of strontium Hexaferrite crystal due to etching in 85% H3 P O4 at 120⁰C for 30 min utes.

Figures 20.5(a, b, c) show hexagonal etch pits on the cleaved basal (0001) plane o f St rontiu m hexaferrite ( Sr Fe12 O19 ) due to etching the crystal in 85% H3 P O4 at 120⁰C fo r 30 minutes. Figure 20.6 is a schematic diagram showing the structure and orientation of the etch pits thus produced.

Figu re 20.6: Schematic diagram showing the structure and orient action of the etch pits on (0001) plane of strontium Hexaferrite crystal due to H₃PO₄ each ant.

The etch patterns consist of hexagonal point-bottomed as well as flat-bottomed pits. The point-bottomed pits are attributed to line defects (dislocations) whereas the flat-bottomed pits are examples of some superficial defects which do not penetrate into the body of the crystal. In this figure there are also examples of geometrically centred hexagonal pits, pits with regularly spaced terracing and eccentric hexagonal pits with irregularly spaced terracing. Symmetric pits are as a result of dislocations perpendicular to the surface whereas asymmetry of etch pits is due to oblique (inclined ) dislocations .The above said examples are indicative of normal, inclined, stepped and bending dislocations in SrFe12O19.The situation can be explained by referring to schematic diagram of figure 20.7 (a,b,c,d ).

Figu re 20.7 (a, b, c, d): Schematic diagram exp lain ing the formation of geometrically centred, symmetrical, regularly and irreg u larly terraced pits on (0001) pits plane of strontium  Hexaferrit e crystals due to etching  in H₃ P O₄ .

The presence of impurity segregation affects the rate of nucleation of an etch pit along a linear defect . In case the impurity is such as to slow down the dissolution on account of its poor solubility, the rate of etch pit nucleation will be high only till an impurity precipitate is encountered ( as seen in figure 20.7 b) and obviously , thereafter, the nucleation rate will fall. During the time the nucleation rate is low, the step formed during the period of high nucleation rate will advance away from the linear defect. When the impurity precipitate gets removed as a result of either its own dissolution or by dissolution around it, the nucleation rate will increase. A repetition of this process will give rise to a terraced pit as explained by a schematic diagram of figure 20.7 (b ). In case the linear defect does not encounter an impurity and are normal to the surface of observation, a smooth centred pit is expected as explained by the schematic diagram of figure 20.7 (a).However, if the linear defect meets the impurities at regular intervals, a centred and regularly terraced pit with regular spacings is expected (see figure 20.7 b). In the event the linear defect meets impurit ies at irregular intervals, on account of its non-uniform segregation, one expects pits with irregularly spaced terracing but geometrically centric (see figure 20.7 d). A linear defect inclined to the surface, encountering impurit ies at regular intervals should lead to eccentric and non-uniformly terraced p it (figure 20.7 c).

Let us take up example of associating point-bottomed etch pits to linear defects (dislocations). As explained in previous sections , if a crystal is cleaved/fractured along a suitable crystallographic direction /plane , a pair of surfaces thus obtained , if etched under identical conditions is expected to produce etch patterns such as to mirror images of one another. There has to be a one-to-one correspondence of point bottomed etch pits on match cleaved/fractured surfaces, in terms of number, shape and structure.

Figu re 20.8 (a, b): One-to-one correspondence of point bottomed triangular etch pits on match rhomb ohed ral cleavages of natu ral quartz crystal after 8 hours of hydrothermal etching

Figures 20.8 (a, b) represent etch patterns on match rhombohedral cleavages of natural quartz crystal after 8 hours of hydrothermal etching. Corresponding to each point-bottomed triangular etch pit on one cleaved surface there is a point-bottomed etch pit on the other cleaved match surface. Prolonged etching leads to continuation of the point-bottomed on these match surfaces whereas the flat-bottomed pits either get washed off or some new ones appear at different positions. The point-bottomed pits are associated with dislocations whereas the flat-bottomed pits are associated with some superficial defects confined to a few atomic layers only. The same is expected if matched cleaved pairs are etched in dissimilar etchants as is illustrated by figures 20.9 (a, b) and 20.10 (a, b).

Figure 20.9 (a, b): One-to-one correspondence of etch pits on match rhombohedral cleavages of natural quartz crystal etched by a) Hyd rother mal method b) in NaOH

Figure 20.10 (a, b): One-to-one correspondence on match rhombohedral cleavages of natural quartz crystal etched by a) Hydrothermal method b) in HF

The etch patterns on figure 20.9 (a) is the hydro thermally etched rhombohedral surface of a natural quartz crystal and 20.9 (b) is its corresponding match surface etched in sodium hydroxide at an elevated temperature. Fig. 20.10 (a) shows a rhombohedral surface etched by hydrothermal method whereas its match cleaved surface is etched in HF and shown in figure 20.10 (b). The correspondence of each pattern on the match cleaved faces etched in the same etchant or dissimilar etchants is a reliability test of etching linear defects which penetrate into the body of the crystal. One has, however, to note that to every etch pit a dislocation may be associated but to every dislocation an etch pit may not be associated. Etching is a very simple technique with the help of which one can estimate the density of dislocations by counting etch pits spread over a particular area of surface of the crystal.

20.10.3.    Revelation of planar defects.

20.10.3.1. Delineation of tilt boundaries

It was through etching of germanium crystal that Vogel and his co-workers could provide a proof in support of Burger’s dislocation model of low angle grain boundaries. The angle of tilt given by measurement of distance d between consecutive etch pits in a row of equidistant etch pits on application of the formula θ = tan─1 b/d, where b is Burger’s vector, matched with the value of θ as measured by x-ray technique. One example of row of equidistant etch pits along a tilt boundary as revealed by etching rhombohedral face of quartz crystal on hydrothermal etching is shown in figure 20.11. A grid of parallel edge dislocations along a tile boundary is revealed by the each method.

Figure 20.11: Low angle tilt boundary as revealed by etching rhomb ohed ralface of natural quartz crystal by hydrothermal method

20.10.3.2.  Detection of twin boundaries.

When a single crystal is etched in a suitable solvent, it produces etch patterns which consist of strictly and crystallographically oriented etch pits. However, it is not so for twinned crystals. Figure 20.12 shows a prism (1010) face of a quartz crystal etched in KOH at an elevated temperature. One finds series of etch pits along a boundary which separates two regions. In each region, the etch pits are crystallographically and strictly oriented but the etch pits of one region are oppositely oriented with respect to the etch pits of the other region. The row of etch pits is formed along a twin boundary which separates the two regions oriented at 180⁰ with respect to each other. It provides an example of how etching can reveal twinning in crystals.

Figure 20.12: Series  of etch pits along  a twin-boundary and  exactly oppositely oriented  etch pits on the two sides of the boundary  on a prism face of natural quartz crystal as revealed by etching  in KOH

20.11. SUMMARY:

• Etching and dislocation of crystals as phenomena concerned with obtaining information regarding symmetry and perfection of crystals are described and discussed.
• Various types of etching viz, thermal chemical solution, electrolytic etching and also preferential oxidation and ion bombardment are explained.
• Theoretical understanding of formation of dislocation etch pits is given correspondence of etch pits and dislocation and crystallographic information obtainable by employing etching technique are explained.
• Types of dislocation problems that can be effectively dealt with by application of etching as a tool are specified.
• Etching as a phenomenon of reversal of growth is established by illustration of some examples. Examples of etch pits unfolding symmetry revertion of dislocation and their spatial configuration in crystals and delineation of planar defects, like low angle tilt boundaries and twin boundaries in crystals, are discussed.