Novel magnetic and ferroelectric materials from high pressure and high temperature syntheses

The use of high pressure – high temperature synthesis techniques allows for stabilizing new atomic arrangements that were not possible with the common temperature / catalysis approaches for synthesis used before. Therefore, novel materials are emerging, often offering interesting physical properties. We use neutron and synchrotron x-ray diffraction to study their crystal and magnetic structures, and correlate these to the observed physical properties.

In a long-known family of the garnet structures, a new Co₃Al₂Si₃O₁₂ compound has been synthesized under 5.5 GPa and 1250°C. We have studied its magnetic properties by a combination of magnetic susceptibility, neutron powder diffraction, and specific heat measurements [1]. Although the magnetic Co²⁺ ions form a highly geometrically frustrated hyper-Kagome lattice, we find that Co₃Al₂Si₃O₁₂ exhibits a long-range antiferromagnetic order below T_N = 11 K, almost ideally corresponding to the Curie-Weiss temperature, and thus without showing magnetic frustration. From our neutron diffraction experiments, we have determined its rather complex antiferromagnetic structure characterized by the coexistence of collinear and commensurate spiral magnetic ordering.

At yet higher pressure (8 GPa) and 1150°C, a new PbFeO₃ compound was synthesized, with an unusual 2a_p x 6a_p x 2a_p orthorhombic perovskite supercell with Cmcm symmetry. By combining the synchrotron x-ray and neutron diffraction [2], we unravel the reason for such an unusual superstructure to the basic cubic perovskite with a lattice parameter a_p, which is essentially a rather peculiar charge ordering of the Pb²⁺ and Pb⁴⁺ ions. The compound undergoes an antiferromagnetic order with weak ferromagnetic canting at ~600 K, which then switches to a collinear antiferromagnetic order at 418 K, accompanied also with a spin reorientation.