In the Playfair Cipher we create a 5*5 matrix with a specified key to Encrypt the Plaintext. Is there any specific reason behind concatenating the alphabets I and J into single cell while creating the Matrix or we can choose any pair of Alphabets???
First of all, we have only 25 blocks to fill 26 letters so we have to put any two Letters in single block.
Now we have to select two letters which is easily differentiated in natural language.
For example, consider the statement I love my Job.
Now when we decrypt the encrypted text we get _ love my _ob. Now, we have to fill I or J to get the original text. We can easily figure it out. In the case of other pairs of letters, it may be complicated in some conditions. That's why only 'I' & 'J' are chosen to put in single block.
I also like to mention that you can use any letters instead of I & J. Conceptually it is right but the standard suggest that the I & J should be chosen to put in single block because of the reason that is mentioned above.