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

Abstract

Optical properties and lattice dynamics of hexagonal 2H-BaMnO 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 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 complimentary density functional theory lattice dynamics calculations are consistent with the 2H-BaMnO 3 structure belonging to the polar P6 3 mc space group at room temperature. All observed vibrational modes are assigned to the 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 the lattice symmetry change from the P6 3 mc to the P6 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 appearance of the electrical polarization along the c-axis and a previously known tripling of the primitive cell volume at low temperatures. PACS number(s): 63.20.dh, 75.47.Lx, 75.85.+t

Unusual ferroelectric and magnetic phases in multiferroic 2H−BaMnO3 ceramics

“…The only newly observed mode, activating below T C near 15 cm −1 and hardening to 26 cm −1 on cooling, could not be seen in Ref. 27 where the Raman spectra were measured only above 100 cm −1 . Fig.…”

“…The shape of the broad IR reflection bands reminds of spectra of crystalline glasses, 50 but our XRD did not reveal any traces of secondary amorphous phases in our ceramics. Although these heavily damped modes were not observed in IR-ellipsometric spectra of a 2H-BaMnO 3 single crystal 27 , it is worth noting that these ellipsometric spectra were taken mostly in the hexagonal plane (i.e., sensitive to E 1u or E 1 symmetry spectra). Thus, the broad modes are probably active in the E cpolarized spectra.…”

“…Some authors reported a non-polar space group P 6 3 /mmc (Z = 2) 21,24,25 but other structural and vibrational studies proposed a polar one, P 6 3 mc (Z = 2). 26,27 Also the low-temperature crystal structure was uncertain for a long time. Cussen and Battle, who had refined the crystal structure at 80 K, concluded that the space group is P 6 3 cm and Z = 6.…”

“…Since the diffraction experiments could not clearly distinguish between these two cases, older works 21,24,25 claimed a non-polar symmetry at room temperature. Some newer papers 26,27 , combining structural and lattice vibration studies, came to the conclusion that the high-temperature phase is polar, because two of the six observed phonons are both IR-and Raman-active.…”

Signatures of magnetostriction and spin-phonon coupling in magnetoelectric hexagonal15R−BaMnO3

“…Emergence of new modes in Raman spectra is possible below magnetic transition due to various reasons such as concurrent magnetic and structural phase transition [54], spinexcitations including one-magnon or two-magnon Raman processes [35,55], or magnetostriction [36,37]. Since phonons and magnetic excitations exhibit contrasting behaviour under external magnetic fields, we have investigated magnetic-field-dependence of the Raman spectrum to shed light on the origin of the new modes.…”