Design and synthesis of perovskites and corundum derivatives with unusual magnetic, electronic, and structural coupling
Creators: name: Frank, Corey Elizabeth
Publisher: name: No Publisher Supplied
Year: 2021

description: ABO3 perovskites and derivative structures are versatile materials, critical to the function of modern life, and the discovery of new perovskites is essential to developing the fields of photovoltaics, microelectronics, and telecommunication among others. The Goldschmidt tolerance factor (t=((r_A+r_x))/(√2 (r_B+r_x ) ) ) has been used for years to qualitatively predict stable perovskites; this formula predicts that with small A-cations (effective ionic radius &lt; 0.9 Å) and B transition metal B-cations the perovskite structure will not form. With careful synthesis techniques, and the application of high pressure, this limitation can be overcome, unlocking distorted materials with interesting electronic-magnetic-structural coupling behaviors. These materials include highly distorted transition metal only double perovskites, such as Mn2CoReO6; quadruple perovskites with a square-planar coordinated A’ cation, such as La3Mn4RhO12; and even double corundum derivatives which feature face-sharing dimers of octahedrally coordinated A and B cations, such as Fe3-xInSnxO6 (x = 0, 0.25, 0.5) and In2Mn1.1Sn0.9O6.Mn2CoReO6, the fourth known magnetic transition-metal-only double perovskite oxide (space group P21/n) was synthesized at high pressure and temperature (8 GPa, 1350 °C). Double perovskites in which A and B are close in size have been known to exhibit distinct magneto-structural coupling behaviours which are associated with large structural distortions. Single crystal X-ray diffraction and powder synchrotron diffraction confirm that structural distortions are induced by the small A-site Mn2+ cations, which are 8-fold coordinated in this structure, in contrast to the 12-fold coordination typical of undistorted perovskites. Mn2CoReO6 exhibits complex magnetic properties with exceptionally robust antiferromagnetic order (TN ≈ 94 K) involving all cation sublattices. When ¾ of the A cations in a perovskite are substituted by a square planar cation or Jahn-Teller distorted transition metal, a quadruple perovskite, such as LaMn3Rh4O12, may form. These phases typically require high pressure and temperature for their synthesis. LaMn3Rh4O12 was synthesized at 8 GPa, 1150⸰ C with an exceptionally long reaction time required for a high pressure synthesis (3 days). It is an antiferromagnetic (TN ≈ 41 K) 3-dimensional Mott variable-range hopping semiconductor, and the first quadruple perovskite with Mn on the A’ site and 4d Rh on the B site.When the t is very small (≤ 0.85) corundum and double corundum derivative structures are predicted to form. In this structure, the A and B form dimers of octahedrally coordinated cations which face share along the c axis. When cations of difference sizes and charges are inserted into these sites, they push each other away from the octahedral centroid at different strengths, due to Coulombic repulsion, which can lead to large polarization constants in polar structures. Additionally, cations with unpaired d electrons (magnetic cations) can be inserted into these sites without disrupting the mechanism of polarization, making these materials excellent candidates for potential multiferroics.In Chapter Five, three new double corundum derived compounds, Fe3-xInSnxO6, with magnetic transitions well above room temperature (TN ≈ 615 K, 550K, and 445 K for x = 0, 0.25, 0.5, respectively) are presented. Current data indicate that x = 0, and 0.25 crystalize as centrosymmetric, R3 ̅c, but increasing the concentration of closed-shell d10 Sn4+ to 12.5% causes x = 0.5 to crystallize as noncentrosymmetric, R3c, as confirmed by temperature dependent second harmonic generation measurements. Microprobe measurements indicate that for x = 0.25, 0.5, the inclusion of Sn4+ is not compensated for by vacancies, implying the presence of Fe2+, as corroborated by X-ray absorption near edge spectroscopy and single crystal structure refinements. Neutron powder diffraction experiments on x = 0.5 indicate that, like InFeO3, these compounds are canted A-type antiferromagnets with ferromagnetic layers that stack antiferromagnetically and with a single magnetic transition. Weak ferromagnetic interactions persist to very high temperatures. While investigations are ongoing and some properties need to be confirmed, these new compounds provide new information on tuning the magnetic and structural properties of corundum derived compounds in the search for new multiferroic materials.Finally, In2Mn1.1Sn0.9O6 is presented in Chapter 6. This double corundum derivative compound crystallizes in the R3 ̅c space group, and the cations are distributed statistically between the available sites. There is no observed magnetic transition down to 5 K, the so-far limit of measurement. However, the negative Weiss constant (θ = -87 K) indicates that short-range antiferromagnetic interactions dominate. Additionally, there appears to be a large Curie tail at low temperatures, which suggests the possibility of magnetic frustration, likely linked to disorder, which is known to lead to subtle and interesting magnetic phenomena. Future work on this system will include magnetization measurements to even lower temperatures and at a range of different fields, which should reveal spin freezing, or other subtle magnetic phenomena.