14 research outputs found
Measurements and ab initio Molecular Dynamics Simulations of the High Temperature Ferroelectric Transition in Hexagonal RMnO3
Measurements of the structure of hexagonal RMnO3 (R=rare earths (Ho) and Y)
for temperatures significantly above the ferroelectric transition temperature
(TFE) were conducted to determine the nature of the transition. The local and
long range structural measurements were complemented by ab initio molecular
dynamics simulations. With respect to the Mn sites in YMnO3 and HoMnO3, we find
no large atomic (bond distances or thermal factors), electronic structure
changes or rehybridization on crossing TFE from local structural methods. The
local symmetry about the Mn sites is preserved. With respect to the local
structure about the Ho sites, a reduction of the average Ho-O bond with
increased temperature is found. Ab initio molecular dynamics calculations on
HoMnO3 reveal the detailed motions of all ions. Above ~900 K there are large
displacements of the Ho, O3 and O4 ions along the z-axis which reduce the
buckling of the MnO3/O4 planes. The changes result in O3/O4 ions moving to
towards central points between pairs of Ho ions on the z-axis. These structural
changes make the coordination of Ho sites more symmetric thus extinguishing the
electric polarization. At significantly higher temperatures, rotation of the
MnO5 polyhedra occurs without a significant change in electric polarization.
The born effective charge tensor is found to be highly anisotropic at the O
sites but does not change appreciably at high temperatures
Experimental evidence for an intermediate phase in the multiferroic YMnO3
We have studied YMnO by high-temperature synchrotron X-ray powder
diffraction, and have carried out differential thermal analysis and dilatometry
on a single crystal sample. These experiments show two phase transitions at
about 1100K and 1350K, respectively. This demonstrates the existence of an
intermediate phase between the room temperature ferroelectric and the high
temperature centrosymmetric phase. This study identifies for the first time the
different high-temperature phase transitions in YMnO.Comment: 10 pages 5 figures. New version, Additional data, Journal of Physics:
Condensed Matter, in Pres