Ab initiocalculations of phonons inLiNbO3

Abstract

Using the ab initio method within the local-density approximation with ultrasoft pseudopotentials and plane-wave basis, calculating the Hellmann-Feynman forces and applying the direct method, the phonondispersion relations for ferroelectric and paraelectric rhombohedral phases of LiNbO 3 crystal have been derived. The calculated phonon frequencies for the ferroelectric phase at the ⌫ point are compared with Raman, infrared, and neutron-scattering measurements, and they agree very well. The phonon frequencies at the Z, L, and F high-symmetry points are also calculated. It is disclosed that the force constants involving lithium ions are an order of magnitude smaller than the force constants between niobium and oxygen ions. As a consequence, the lithium ions do not contribute to high-frequency optical vibrations. In the paraelectric phase the phonon-dispersion curves show a soft branch with the soft mode of symmetry A 2u at the ⌫ point. This soft mode leads to the observed paraelectric-ferroelectric phase transition. The LO/TO splitting is considered by assuming formal values of ionic charges, and by fitting the longitudinal infrared active modes to the experimental data.

Phonons and related crystal properties from density-functional perturbation theory

“…Lattice vibrations along some high-symmetry lines in cubic semiconductors have been calculated in this way using reasonably sized super-cells (Kunc and Martin 1983). More recently several authors (Wei and Chou 1992, Frank et al 1995, Ackland et al 1997, Parlinski et al 2000 have started to calculate phonon dispersions from the interatomic force constants determined in real space using the frozen-phonon approach.…”

Phonons calculated from first-principles

“…It occurs at ZrO 2 [6], GeO 2 [28], SnO 2 [29], LiNbO 3 [30], MgSiO 3 [31], CaT iO 3 [32]. Crystal with point defects can also be treated easily, like NiAl with Fe replacing Ni and Al [34].…”

Особенности получения и строения кристаллов LiNbO-=SUB=-3-=/SUB=- : Zn в области концентрационного порога 6.76 mol.% ZnО

“…Особой чувстви-тельностью параметры спектральных линий обладают к процессам разупорядочения катионной подрешетки кристалла LiNbO 3 , происходящим при изменении его состава [2]. Спектр КРС кристалла LiNbO 3 в литера-туре хорошо изучен экспериментально (обзор дан в работах [2,[25][26][27][28][29]) и надежно интерпретирован (расчеты ab initio и методом валентно-силового поля [30][31][32]). ческих фононов (100−150 cm −1 ), в области колебаний катионов, находящихся в кислородных октаэдрах ВО 6 (В -Nb, Li, легирующий катион) (200−300 cm −1 ), в области колебаний атомов кислорода кислородных ок-таэдров (500−900 cm −1 ).…”