Optical properties, lattice dynamics, and structural phase transition in hexagonal2H−BaMnO3single crystals

Abstract

Optical properties and lattice dynamics of hexagonal $2H\ensuremath{-}\mathrm{BaMn}{\mathrm{O}}_{3}$ single crystals are studied experimentally in a wide temperature range by means of rotating analyzer ellipsometry and Raman scattering. The magnitude of the direct electronic band gap is found to be ${E}_{g}=3.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. At room temperature the far-infrared (IR) ellipsometry spectra reveal six IR-active phonons; two of them are polarized along the $c$ axis and four are polarized within the a-b plane. Seven phonon modes are identified in the Raman scattering experiments. Group theoretical mode analysis and complementary density functional theory lattice dynamics calculations are consistent with the $2H\ensuremath{-}\mathrm{BaMn}{\mathrm{O}}_{3}$ structure belonging to the polar $P{6}_{3}mc$ space group at room temperature. All observed vibrational modes are assigned to specific eigenmodes of the lattice. The neutron diffraction measurements reveal a structural phase transition upon cooling below ${T}_{C}=130\ifmmode\pm\else\textpm\fi{}5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is accompanied by a lattice symmetry change from $P{6}_{3}mc$ to $P{6}_{3}cm$. Simultaneously, at temperatures below ${T}_{C}$ several additional IR- and Raman-active modes are detected in experimental spectra. This confirms the occurrence of a structural transition, which is possibly associated with the appearance of electrical polarization along the $c$ axis and a previously known tripling of the primitive cell volume at low temperatures.