Optical properties, lattice dynamics, and structural phase transition in hexagonal 2H-BaMnO3 single crystals

Optical properties and lattice dynamics of hexagonal 2H-BaMnO3 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 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-BaMnO3 structure belonging to the polar P6(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 +/- 5 K, which is accompanied by a lattice symmetry change from P6(3)mc to P6(3)cm. Simultaneously, at temperatures below TC 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.