Cation ordering in ABO3 perovskites adds to their chemical variety and can lead to properties such as ferrimagnetism and magnetoresistance in Sr2 FeMoO6 . Through high-pressure and high-temperature synthesis, a new type of "double double perovskite" structure has been discovered in the family MnRMnSbO6 (R=La, Pr, Nd, Sm). This tetragonal structure has a 1:1 order of cations on both A and B sites, with A-site Mn(2+) and R(3+) cations ordered in columns and Mn(2+) and Sb(5+) having rock salt order on the B sites. The MnRMnSbO6 double double perovskites are ferrimagnetic at low temperatures with additional spin-reorientation transitions. The ordering direction of ferrimagnetic Mn spins in MnNdMnSbO6 changes from parallel to [001] below TC =76 K to perpendicular below the reorientation transition at 42 K at which Nd moments also order. Smaller rare earths lead to conventional monoclinic double perovskites (MnR)MnSbO6 for Eu and Gd.

The perovskite polymorph of Mn(2)CrSbO(6) compound has been synthesized at 8 GPa and 1473 K. It crystallizes in the monoclinic P21/n space group with cell parameters a = 5.2180 (2) Å, b = 5.3710(2) Å, c = 7.5874(1) Å and β = 90.36(1)°. Magnetic susceptibility and magnetization measurements show the simultaneous antiferromagnetic ordering of Mn(2+) and Cr(3+) sublattices below TN = 55 K with a small canting. Low temperature powder neutron diffraction reveals a commensurate magnetic structure with spins confined to the ac-plane and a propagation vector κ = [1/2 0 1/2]. The thermal treatment of this compound induces an irreversible phase transition to the ilmenite polymorph, which has been isolated at 973 K and crystallizes in R3[combining macron] space group with cell parameters a = 5.2084 (4) Å and c = 14.4000 (11) Å. Magnetic susceptibility, magnetization and powder neutron diffraction data confirm the antiferromagnetic helical ordering of spins in an incommensurate magnetic structure with κ = [00 0.46] below 60 K, and the temperature dependence of the propagation vector up to κ = [00 0.54] at about 10 K.

The perovskite "PbCrO(3)" was synthesized at high pressure and high temperature. Its magnetic properties have been investigated by means of magnetization, specific heat, and resistivity measurements. Earlier workers had concluded it to have a G-type antiferromagnetic structure. However, our measurements suggest a rather more complex situation: first, a weak ferromagnetic transition of the Cr(IV) spins occurs at 245 K; this is followed by a temperature-driven spin reorientation starting at 185 K and ending at 62 K. Since zero-magnetic-field spin reorientation in the "PbCrO(3)" perovskite should not be expected, an intrinsic "magnetoelectric effect", associated with the lone-pair Pb electrons, seems to be responsible for the observed smooth rotation of the Cr-spins.

Two new cation-ordered polymorphs of Mn2ScSbO6 have been synthesised at high-pressure. At 5.5 GPa and 1523 K Mn2ScSbO6 crystallizes in the Ni3TeO6-type structure with the polar R3 space group and cell parameters a = 5.3419 (5) Å and c = 14.0603 (2) Å. Below TC = 42.0 K it exhibits ferrimagnetic order with a net magnetization of 0.6μB arising from unusual site-selective Mn/Sc disorder and is thus a potential multiferroic material. A double perovskite phase obtained at 12 GPa and 1473 K crystallizes in the non-polar P21/n monoclinic space group with cell parameters a = 5.2909 (3) Å, b = 5.4698 (3) Å, c = 7.7349 (5) Å and β = 90.165 (6) °. Magnetization and neutron diffraction experiments reveal antiferromagnetic order below TN = 22.3 K with the spins lying in the ac plane.