Universal Polar Instability in Highly Orthorhombic Perovskites.

The design of novel multiferroic ABO perovskites is complicated by the presence of necessary magnetic cations and ubiquitous antiferrodistortive modes, both of which suppress polar distortions. Using first-principles simulations, we observe that the existence of quadlinear and trilinear invariants in the free energy, coupling tilts, and antipolar motions of A and B sites to the polar mode drives an avalanche-like transition to a non-centrosymmetric 2 symmetry in a wide range of magnetic perovskites with small tolerance factors, overcoming the above restrictions. We find that the 2 phase is especially favored with tensile epitaxial strain, leading to an unexpected but technologically useful out-of-plane polarization.

The Interplay of Electronic Configuration and Anion Ordering on the Magnetic Behavior of Hydroxyfluoride Diaspores.

We report a new nickel hydroxyfluoride diaspore Ni(OH)F prepared using hydrothermal synthesis from NiCl·6HO and NaF. Magnetic characterization reveals that, contrary to other reported transition-metal hydroxyfluoride diaspores, Ni(OH)F displays weak ferromagnetism below the magnetic ordering temperature. To understand this difference, neutron diffraction is used to determine the long-range magnetic structure.