High-pressure synthesis, crystal chemistry and physics of perovskites with small cations at the A site.

ABO3 perovskites with small cations at the A site (A = Sc(3+), In(3+) and Mn(2+) and B = Al(3+) and transition metals) are reviewed. They extend the corresponding families of perovskites with A(3+) = Y, La-Lu, and Bi and A(2+) = Cd, Ca, Sr and Ba and exhibit the largest structural distortions. As a result of these large distortions, they show, in many cases, distinct structural and magnetic properties. These are manifested in: B-site-ordered monoclinic structures of ScMnO3 and 'InMnO3'; an unusual superstructure of ScRhO3 and InRhO3; antiferromagnetic ground states and multiferroic properties of Sc2NiMnO6 and In2NiMnO6; two magnetic transitions in ScCrO3 and InCrO3 with very close transition temperatures; a Pnma-to-P-1 structural transition and k = (½, 0, ½) magnetic ordering in ScVO3; and incommensurate magnetic ordering of Mn(2+) spins in metallic MnVO3. A large number of simple ScBO3, InBO3 and MnBO3 perovskites has not been synthesized yet, and the number of experimental and theoretical works on each known ScBO3, InBO3 and MnBO3 perovskites counts to only one or two (except for ScAlO3). The synthesis, crystal chemistry and physics of perovskites with small cations at the A site is an emerging field in perovskite science.