Structural aspects of the metal--insulator transitions in Cr- and Nb-doped VO$sub 2$

ub x/O/sub 2/ with x = 0.05 transform to the rutile phase through an intermediate orthorhombic phase, while V/sub 0.95/Cr/sub 0.05/O/sub 2/ is orthorhombic at room temperature indicating the importance of antiferroelectric displacements of V/sup 4+/ ions in the metal--insulator transitions. (auth)