Lattice parameter and optical absorption spectra were measured on heavily nitrogen-doped bulk crystals and epilayers of 4H-SiC. The rate of c-lattice parameter change versus doping concentration in high quality material was less than -3.6 x 10(-25) Angstrom cm(3). In crystals containing high density of double stacking faults, the (0 0 0 8) X-ray reflection shifted toward the (2 2 2) reflection in 3C SiC. Optical absorption measurements in such crystals detected peak at 3.1 eV similar to that reported in 3C-SiC with concomitant decrease of free carrier peak characteristic of 4H-SiC polytype. (C) 2003 Elsevier B.V. All rights reserved.