The Resolution of Crystal Lattices 



89 



Fig. 3. Crystal habit ofcoppcr plitlialocsanine (after Robert- 

 son, 1934). 



with intermediate molecular weights. Among these 

 the phthalocyanines appeared a promising group to 

 study and most of our work so far has been carried 

 out on copper and platinum phthalocyanine. 



The metal phthalocyanines have a number of 

 favourable properties. The molecule of platinum 

 phthalocyanine is shown in figure I, and the unit cell 

 of the crystal lattice in figure 2. From these it is 

 apparent that we may idealise the structure into 

 widely spaced planes of heavy metal atoms (^^o,,, 

 1 1.94 A) embedded in a matrix of the light elements 

 carbon, nitrogen and hydrogen. We would expect 

 to obtain strong diffracted beams from these planes 

 even in very thin crystals since the scattering from 

 the heavy metal atoms swamps that from the organic 

 parts of the unit cell. The crystals grow as long thin 

 ribbons with (001) as the habit plane of the ribbon 

 surface as shown in figure 3, so that crystals sup- 

 ported on a specimen grid will be oriented with (001) 

 perpendicular to the electron beam and (20T) almost 



Fig. 4. Schematic representation ol"(20T) planes in platinum 

 phthalocyanine in relation to crystal habit. ABCD is a (001) 

 plane. The almost vertical sheets such as ABEF arc (20T) 

 planes. 



parallel to the beam, since (001)A(20T) =88". The 

 (201) planes are thus in a favourable orientation for 

 diffraction, which is essential in order to form an 

 image of the planes. A schematic representation of 

 the nearly vertical (20T) planes is shown in figure 4. 

 In copper phthalocyanine the corresponding para- 

 meters are (001)A(20T) 80 , f/201 = 9.8 A. 



Preparation of specimens and method of examina- 

 tion. — The crystals were prepared by sublimation 

 from the powder after the method of Barrett, Dent & 

 Linstead (I). After irechanical breaking down to 

 reduce their size they v\ere suspended in ethyl alcohol, 

 and a drop of the suspension dried down on to a 



Fig. 6. Single edge dislo- 

 cation in platinum phtha- 

 locyanine crystal 

 ( 1,000.000). 



Fig. 7. Guide to figure 6 

 showing exact position of 

 edge dislocation. 



Fig. 5. Portion of platinum phthalocyanine crystal showing perfect structure of (20T) planes ( < 1,500,000). 



