Synthesis of new high-pressure A-site manganite perovskites and investigation of their magnetic properties
Alharbi, Khalid Naif F
Perovskites, with the general formula, ABO3, have been extensively studied due to their large variety of intriguing electronic and magnetic properties, which can be achieved via a number of possible chemical compositions. The use of A and B site cations of various sizes and charges induce cation order, giving rise to A2BB’O6 and AA’B2O6 double perovskites (DPv), while, the combination of cation order into both sublattices in an AA’BB’O6 double double perovskite (DDPv) is uncommon. However, the well-known DDPv arrangement in A and B-sites is a layered arrangement of A and A’, which coexists with the rock-salt motif of B and B’. The A-site is usually occupied by a large cation with 12-fold coordination, but it can host smaller cations comparable in size to B-site cations as it becomes distorted (e.g. larger tilt angles). The smaller the ionic radius of A, the higher the distortion, and higher pressure is required to stabilise the structure. Employing the formula A= Mn2+ demonstrates some significant structural features and magnetic properties. An attempt has been made to synthesise new DPv and DDPv materials, which contain Mn2+, into the A-site to enhance the physical properties of the material. Three different compounds were successfully synthesised: Mn2NiReO6, CaMnCrSbO6 and CaMnMnWO6. Firstly, Mn2NiReO6 is a new member of the ´all transition metal’ (ATM) double perovskite family and is obtained under high pressure (8 GPa) and high temperature (1573 K) conditions. The crystal structure was confirmed by PXRD and NPD, with monoclinic P21/n symmetry, which shows fully ordered Ni2+ and Re6+ within the octahedral sites in a rock-salt motif and a large distortion, with the greatest tilt angles observed to date, in A-site manganite double perovskite oxides. Magnetic structure refinement indicates that all moments are ordered antiferromagnetically below TM1 = 80 K. However, the unusual continuous spin rotation of Mn spins occur down to a second transition (TM2 = 42 K). This effect has not been reported in any of the previous Mn2BB’O6 compounds. Secondly, CaMnCrSbO6 was synthesised under high pressure (10 GPa) and high temperature (1373 K) conditions. The compound crystallises via PXRD and NPD with a DDPv structure, which combines columnar order in the A-site cations (Ca and Mn) and rock-salt order of the B-site cations (Cr and Sb). The Mn in the A and A` sites show alternating square planar and tetrahedral coordination. CaMnCrSbO6 has a single magnetic transition at 49 K, where Mn2+ and Cr3+ spin order into antiparallel FM sublattices. Finally, CaMnMnWO6 crystallises via PXRD and NPD at 10 GPa with the monoclinic P21/n space group in a DPv structure with rock-salt order of the B sites at a high temperature (1573 K). While at a lower temperature (1273 K), a DDPv structure with the P42/n space group is observed with A-site cations ordered in the columnar order and B-site cations in the rock salt environment. The magnetic refinement reveals a spin glass behaviour for the DPv structure, whereas, the DDPv magnetic structure shows antiferromagnetic coupling between ferromagnetic sublattices at TC = 45 K.