Two micrometer thick AlxGa1-xN layers with 0 < x < 0.4 were grown by low-pressure metal organic chemical vapour deposition on sapphire (0001) substrates. For Al concentrations 0.18 < x < 0.25 the layers are found to be nearly strain-free as determined by high resolution X-ray reciprocal space mapping around the (0002), (20 - 24), and (20 - 20) Bragg reflections in conventional and grazing incidence geometry, respectively. The in-plane lattice parameter a of layers grown in this composition regime coincides with that of(2/3,)a((sapphire)). Their rotational and tilting disorder shows a minimum as compared to layers grown outside this regime. (GaN/AlxGa1-xN) multi-quantum well structures on top of such buffer layers are fully pseudomorphic having lowest interface disorder and best surface morphology as evaluated by specular and diffuse X-ray reflectivity measurements. The findings are explained by the assumption of a 2D coincidence site lattice for the epitaxial growth of AlGaN on sapphire. The coincidence site lattice has hexagonal symmetry with the lattice parameter three times a(A(0.22)Ga(0.78)N) equals two times a(sapphire).