For many years, deviations of stoichiometry correlated with the density of point defects; interstitials, vacancies, anti-sites, etc. has been neglected. However, control of these by vapor pressure showed significant effects on the properties of compound materials; moreover, crystal growth under controlled vapor pressure also produced nearly perfect crystals of very high quality. Also, some considerations based on the thermodynamics are described. (C)2000 Elsevier Science S.A. All rights reserved.