Lattice dynamics study of cubic Tb₂O₃

Abstract

We report a joint experimental and theoretical study of the lattice dynamics of cubic Tb2O3. Up to 16 optical Raman‐active modes have been observed with polarized and unpolarized Raman scattering measurements on a high‐quality Tb2O3 single crystal. The measured wavenumbers have been compared with those of other rare‐earth (RE) and related sesquioxides with cubic (C‐type or bixbyite) structure. First‐principles calculations have allowed us to assign the symmetry of the experimentally observed Raman‐active modes. Additional lattice‐dynamical calculations on the related cubic RE sesquioxides Dy2O3, Gd2O3, Eu2O3, and Sm2O3 indicate that the phonon wavenumbers of the Raman‐active modes in these compounds are monotonically reduced with increasing the lattice parameter along the Dy2O3‐Tb2O3‐Gd2O3‐Eu2O3‐Sm2O3 series, thus prompting for a revision of the experimental Raman spectra of some of these compounds (mainly Eu2O3 but also Gd2O3).

Structural and Lattice-Dynamical Properties of Tb2O3 under Compression: A Comparative Study with Rare Earth and Related Sesquioxides

“…In contrast, PBEsol seems to be in better agreement for the high-wavenumber modes (>250 cm –1 ). While PBEsol clearly underestimates the unit-cell volume of both the C-, B-, and A-type phases of Tb 2 O 3 , the reduced interatomic distances predicted in this case seem to yield increased spring constants and therefore improved results in the case of the high-wavenumber modes (with mainly O vibrations as shown in ref ( 89 )). From now on, we rely on the GGA-PBE vibrational results as this functional better reproduces both the compression behavior of the unit-cell volume and the low-wavenumber modes (with mainly Tb vibrations).…”

“…We have recently characterized the Raman-active modes of C-type Tb 2 O 3 at room pressure. 89 In the present work, we have performed unpolarized RS measurements and observed up to 16 out of 22 Raman-active modes of the cubic phase as a function of pressure (see Figure 11 ). It can be observed that the cubic phase is stable from 0 to 6.8 GPa and has completely disappeared at 7.5 GPa.…”

“…In regard to the lattice dynamics of C-type SOs, several works have recently addressed the comparison of Raman-mode wavenumbers in C-type SOs. 89 , 122 In this work, we will concentrate on the comparison of the pressure coefficients provided in Tables S3 and S4 . In particular, the large difference in mass of the Ln cations and O anions has allowed us to distinguish two regions: the low-wavenumber region (below 250 cm –1 ) dominated by pure Ln vibrations and the high-wavenumber region (above 250 cm –1 ) dominated by pure oxygen vibrations.…”

“… 88 Details of the growth method and the structural and vibrational properties of the Tb 2 O 3 single crystals at room conditions can be found elsewhere. 88 , 89 The single crystals were ground for powder XRD experiments, which were first performed at room conditions with a PANalytical X’Pert Pro MPD diffractometer (Cu Kα1 Kα2 radiation). Le Bail fits confirmed the cubic Ia 3̅ symmetry and allowed one to extract a lattice parameter, a = 10.7323(3) Å, and volume, V = 1236.17 Å 3 , in good agreement with previous structural characterizations.…”

“…Using such high quality material, the room-pressure lattice-dynamics properties of cubic Tb 2 O 3 have been recently investigated. 89 …”