  We have studied the interplay between orbital ordering, Jahn-Teller and
GdFeO3-type lattice distortions in perovskite-type transition-metal oxides
using model Hartree-Fock calculations. It has been found that the covalency
between A-site cations and oxygens causes interaction between the Jahn-Teller
and GdFeO3-type distortions. The present calculations explain why the d-type
Jahn-Teller distortion and orbital ordering compatible with it are realized in
LaMnO3, YVO3 and YTiO3
