h i g h l i g h t s Identification of (0112) system twins in LiNbO 3 films by means of XRD. Stress relaxation by mechanical twinning in LiNbO 3 thin films. Tunning of in-plane and out-of-plane thermal expansion of thin films. Better understanding of the relaxation mechanisms and residual stresses. Relationship between clamped thermal expansion and film thickness.: Thin films Chemical vapour deposition (CVD) Raman spectroscopy and scattering Thermal expansion Domain structure Deformation a b s t r a c t LiNbO 3 films of high epitaxial quality and with thicknesses of 120e500 nm were deposited at 650 C on C-sapphire by atmospheric pressure metal-organic chemical vapour deposition. Li nonstoichiometry, residual stresses, twinning, and thermal expansion of the films as a function of the film thickness were investigated by means of Raman spectroscopy and X-ray diffraction. The relaxation of residual stresses, Li 2 O loss, inelastic deformation and elastic hysteresis during cycles of heating up to 860 C and cooling down to room temperature were studied, as well. The residual stresses and thermal expansion of films were highly thickness dependent. It was shown that the f0112g twinning contributed to the stress relaxation in the thick LiNbO 3 films.