High Pressure Synthesis And Physical Properties Of A-and B-site Multiply Ordered Perovskites

In this thesis,several A-and B-site multiply ordered perovskites with chemical formula AA’₃B₂B’₂O₁₂ have been synthesized for the first time using high-pressure and high-temperature conditions.The crystal structures,charge configurations and comprehensive physical properties are systematically studied.Meanwhile,related physical mechanisms are discussed based on first pricinple calculations.The main results are shown as follows:（1）By using high pressure and high temperature conditions,LaMn₃Ni₂Mn₂O₁₂ has been prepared as the first cubic Pn-3 both A-and B-site multiply ordered perovskite with the A’-site occupied by Jahn-Teller Mn³⁺ions.By means of magnetic measurement,neutron diffraction and X-ray absorption spectrum,we have confirmed the LaMn₃³⁺Ni₂²⁺Mn₂⁴⁺O₁₂2 charge configuration,in which all the transition metal ions are in high spin states and participate in magnetic exchange interaction.Magnetic measurement,specific heat measurement and neutron diffraction show that the A’-site Mn³⁺-and the B/B’-site Ni²⁺/Mn⁴⁺-sublattice,respectively,order into a G-type collinear antiferromagnetic structure at T_N?46 K and an orthogonal magnetic structure at T_C?34 K,thereby inducing a net magnetic moment.First principle calculations show that,in LaMn₃Ni₂Mn₂O₁₂,the A’-site Mn³⁺spins play a crucial role in the formation of B/B’-site Ni²⁺/Mn⁴⁺orthogonal spin structure.（2）A series of multiply ordered perovskite solid-solution compounds PrMn₃Ni_xMn_4-xO₁₂（x=0,1,2）have been synthesized for the first time under high pressure and high temperature conditions.X-ray diffraction and selected area electron diffraction reveal that,with increasing Ni content,the crystal structure changes from monoclinic I2/m or rhombohedral R-3 symmetry with x=0 into cubic Im-3 symmetry with x=1,and then to cubic Pn-3 symmetry with x=2.The variation of synthesis pressure and crystal symmetry probably lead to the transformation of the B-site Mn³⁺between high spin state and low spin state.The magnetic ground states of the monoclinic-and rhombohedral-PrMn₇O₁₂ are,respectively,an orbital ordered canted antiferromagnetic and a collinear antiferromagnetic.PrMn₃NiMn₃O₁₂ probably orders into a highly canted antiferromagnetic ground state.The spins of A’-site Mn³⁺and B/B’-site Ni²⁺/Mn⁴⁺in PrMn₃Ni₂Mn₂O₁₂2 should also order into the same ground state magnetic structure as that of LaMn₃Ni₂Mn₂O₁₂,which possessing a net ferromagnetic component.（3）Multiply ordered perovskite CaCu₃Ni₂Os₂O₁₂（space group:Pn-3）,containing both3d and 5d magnetic elements,has been synthesized under high pressure and high temperature conditions.By using magnetic measurement,X-ray absorption spectra,X-ray magnetic circular dichroism and first principle calculations,we have determined the charge configuration of CaCu₃²⁺Ni₂²⁺Os₂⁶⁺O₁₂2 and the collinear ferrimagnetic ground state with spin alignment of Cu²⁺（↑）-Ni²⁺（↑）-Os⁶⁺（↓）.X-ray magnetic circular dichroism shows that not only the 5d element Os⁶⁺but also the 3d elements Cu²⁺and Ni²⁺have nonnegligible orbital contribution.Because of the heavily octahedral distortion as well as the strong electronic correlation effect and spin-orbit coupling,CaCu₃Ni₂Os₂O₁₂2 displays insulating behavior.Besides,the strong antiferromagnetic interaction between A′-site Cu²⁺and B’-site Os⁶⁺leads to a high temperature ferrimagnetic transition with T_C≈393 K.