Pillars of Discovery: Two Novel Divalent Transition Metal Germanates as Potential Multiferroics

The pursuit of novel magnetic materials has driven inorganic crystal chemistry towards many exciting discoveries, from rare earth magnets to hexaferrites to elusive quantum materials such as spin liquids. One material class of interest are multiferroics, which combine the desirable properties of ferromagnetism and ferroelectricity, the latter of which requires crystallization in a polar space group. High-quality single crystals of inorganic oxides can be grown <i>via</i> hydrothermal synthetic methods using magnetic ions such as Co²⁺ and Cu²⁺, with non-magnetic Ge⁴⁺as a building block. Two such compounds were hydrothermally synthesized, Ba₇Rb₂Co₆Ge₁₈O₅₀ (<b>1</b>) and Sr₇K₂Cu₆Ge_17.3O_47.2(OH)_2.8 (<b>2</b>), which crystallize in the acentric, polar space group <i>P</i>6₃<i>cm</i>. The polar feature of the unit cell is the ‘pillar’ propagating along each vertex on the <i>c</i>-axis. These pillars contain alternating trimers of TM²⁺ (TM = Co, Cu) in tetrahedral and trigonal bipyramidal geometries, and are separated by a central germanate cluster. With the conditions for ferroelectricity met, initial magnetic studies were performed, and their results are presented here. The Co-germanate shows two magnetic transitions in the susceptibility measurements, both suggesting magnetic short-range orders. The compound does not have structural disorder, therefore the magnetic short-range order likely originates from magnetic frustration. Two successive transitions possibly represent dimensionality cross-over transitions. The neutron scattering studies are underway to uncover the nature of magnetic transitions and phases in Ba₇Rb₂Co₆Ge₁₈O₅₀. <br/> <br/>*The research was supported by the U.S. Department of Energy (DOE), Early Career Research Program Award KC0402020 and used resources at the HFIR, DOE Office of Science User Facilities operated by ORNL.