The dugganites: A new potentially multiferroic Te6+-containing subclass of the langasites

Abstract

The langasites are a diverse group of materials with the Ca3Ga2Ge4O14 structure that contains four cationic sites that vary in both size and coordination possessing P321 symmetry. Paramagnetic transition metal ions can be placed onto the 3f site such that they form planar isolated equilateral trimers that stack normal to the plane. The geometrically frustrated nature of the trimer sublattice prevents the system from simultaneously satisfying all of its energetic obligations at low temperatures. Consequently, nature attempts to make energetic compromises that result in complex magnetism. The objective of this thesis is to explore these magnetic states such that they can be understood, and perhaps, even exploited for future engineering applications.

Four different langasites are presented in this work. Ba3NbFe3Si2O14 has Fe3+ ions occupying the trimer site. Below 26 K, the magnetic moments in this material order into a unique doubly chiral magnetic structure. The coupling of the magnetic structure to the lattice also results in ferroelectric polarization in this material. Pb3TeMn3P2O14, Pb3TeCo3P2O14, and Pb3TeCo3V2O14 have all been prepared phase pure and studied using x-ray diffraction, magnetization in fields up to 35 T, heat capacity, dielectric measurements, neutron diffraction, and inelastic neutron spectroscopy. All three Te6+-containing materials, known as the dugganite subclass of the langasite series, distort into large supercells away from P321 symmetry. Magnetic phase diagrams were constructed for each of these new systems using multiple experimental probes. Pb3TeMn3P2O14 has a complex magnetic structure believed to be quite similar to Ba3NbFe3Si2O14. On the other hand, Pb3TeCo3P2O14 shows very different magnetism: a unique two-tiered magnetic structure was solved using neutron scattering, Rietveld refinement, and representational analysis. Pb3TeCo3V2O14 behaves quite like Pb3TeCo3P2O14, except a second zero-field magnetic transition is observed, implying that orbital hybridization with diamagnetic P5+ and/or V5+ is very important in these materials.