Dear Huy,as it was pointed out in previous letter by Joerg Bergmann, line broadening is caused by violation of theta-2theta relationship.
More info can be found in the following articles: 1. R. Berthold. Z. Angew. Phys. 7 (1955) 443. 2. H. Toraya, J. Yoshino. J. Appl. Cryst. 27 (1994) 961.
Best regards, Eduard.
