Universität Osnabrück, Graduiertenkolleg Mikrostruktur oxidischer

GRADUIERTENKOLLEG MIKROSTRUKTUR OXIDISCHER KRISTALLE 11
Zone-center lattice dynamics in LiNbO3 and LiTaO3
Beginn des Projekts: 01.12.1997
Ende des Projekts: 30.11.2000
Dipl.-Phys. Vasile Caciuc
Betreuer: Prof. Dr. G. Borstel
Zusammenfassung
In the framework of the density functional theory, the equilibrium ground-state structure of LiNbO3 in
its both paraelectric and ferroelectric phases is fully optimized using the full-potential linearized plane wave
method. The exchange-correlation energy functional is evaluated within the local density approximation. With
this option the unit cell, c/a ratio and all internal geometry parameters are found to be in good agreement with
the experimental ones.
The Γ-TO phonon frequencies calculated with the frozen phonon method are in good agreement with
the data available for the A1, A2 and E modes in nearly stoichiometric samples. It was shown that the anharmonic
effects play an important role for the softest and the hardest of the A1 modes. The phonon frequencies are
predicted for the Raman and infrared silent A2 modes. The theoretical analysis of the experimentally measured E
modes is performed on the basis of their different behaviour with respect to the 6 Li isotope shift. In addition, the
displacement patterns of all Γ-TO modes are available from the calculated eigenvectors.
The zone-center TO phonons in LiTaO3 were also analyzed within the frozen-phonon approach. The
experimentally observed shift of the TO3 frequency to the higer values compared to LiNbO3 is explained by a
substantial contribution of Li movement in the corresponding eigenvector of LiTaO3, while it is absent in
LiNbO3.
Because the properties of LiNbO3 depend significantly on the presence of both intrinsic and extrinsic
defects, the doped LiNbO3 with trivalent ions was intensively studied over the last years. The ground-state properties
for a 40 atom supercell spanned by [-111], [111] and [111] with Fe on Nb site were investigated. Further
calculations based on a charge compensated mechanism for Cr impurities in a 2x2x2 supercell are now in progress.
Stand der Forschung
The ferroelectric material LiNbO3 has been extensively studied over recent several years due to its
various applications in electro-optics and non-liner optics [1]. Particularly, its ferroelectric transition temperature
of 1480 K is among the highest known to date. For the structurely related ferroelectric system LiTaO3, this parato
ferroelectric transition temperature drops down to 950 K. The Raman spectra recorded for the A1 modes also
exhibit an interesting behaviour. Because the atomic mass of Ta is almost twice as those of Nb, one should expect
that the Raman frequencies in LiTaO3 will be shifted to a lower value compared to LiNbO3. This is indeed
experimentally observed, except for the TO3 mode.
The early theoretical descriptions of the ferroelectric phase transition in these compounds were based
on the effective Hamiltonian model proposed by Lines [2,3]. When applied to LiTaO3, for instance, the pattern
of the para- to ferroelectric phase transition was a mixture of displacive and order-disorder characteristics, being
driven by Li ions movements in a triple potential well. A phenomenological temperature-dependent triple potential
well was also used by Bakker et. al [4] in a quantum mechanical calculation to assign a previously observed
resonance at 32 cm -1 [5] in LiTaO3 to the tunneling of the Li ions through the central and the lowest well. Another
description of the ferroelectric phase transition in LiTaO3 was proposed by Birnie [6] in which Li atoms
were modeled as Frenkel defects. In this case, a double potential well describe the displacements of Li ions between
their centrosymmetric positions and the normally vacant octahedral sites.
Using the full-potential linearized augmented plane wave method (FLAPW), Inbar and Cohen [7,8,9]
investigated from first principles the ferroelectric instability in LiNbO3 and LiTaO3. The total-energy calculations
performed for the ferroelectric distortion simulated as a uniformly scaled experimental soft mode coordinate
revealed that the Li displacements are described only by a single anharmonic potential well. Since a deep
double well was found for a coupled displacements of oxygen and lithium atoms, it was concluded that the lowering
of the total energy associated with the para- to ferroelectric phase transition in these materials is mainly
due to the shift (with respect to Nb) and distortion of the oxygen octahedra. Also, the zone-center potential-