Rutile structure:

The radius radio is in the range 0.73 to 0.41. in rutile (TiO²) each Ti⁴+ ion is octahedrally surrounded by six O²- ions whereas each other O²- ion is surrounded by only three Ti⁴+ ions arranged at the tree corners of a plane triangle. The Co-ordination number of Ti⁴+ and O²+ ions are 6 and 3, respectively. It is not exactly a close packed structure. Ti⁴+ ions in rutile may however, be considered as forming a sufficiently distorted body centred cubic lattice. 

         Example  compounds  having radius ratio below 0.41 are SiO² and BeF² but these are only a few. The Co-ordination number of Si⁴+ and Be²+ is four and that of O²- is two. However, these are appreciably co-valent.