Lattice dynamics of rare-earth titanates with the structure of pyrochlore R 2Ti2O7 (R = Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu): Ab initio calculation

Abstract

The ab initio calculation has been performed for the crystal structure and the phonon spectrum of titanates with the structure of pyrochlore R 2 Ti 2 O 7 (R = Gd-Lu). The frequencies and types of fundamental vibrations have been found. For R = Tb, Tm, and Yb, this calculation has been carried out for the first time; furthermore, there is no available information on experimental studies of the phonon spectrum for Tm and Yb. The influence of hydrostatic pressure to 35 GPa on the structure, dynamics, and elastic properties of the Gd 2 Ti 2 O 7 lattice has been investigated. The dependence of the phonon frequencies on the pressure has been obtained. The calculations have predicted that the relative change in the pyrochlore structure volume during compression at pressures to 35 GPa is well described by the third order Birch-Murnaghan equation of states. The results of the calculations agree with the available experimental data. It has been shown that the struc tural, dynamic, and elastic properties of the R 2 Ti 2 O 7 crystal lattice can be adequately described in the case where the inner shells of the RE ion up to 4f are replaced by the pseudopotential.

First-principles calculation and experimental investigation of lattice dynamics in the rare-earth pyrochloresR2Ti2O7(R=Ruminy1, Valdez2, Wehinger3 et al. 2016Phys. Rev. B356480View full textAdd to dashboardCite

“…Second, using PBE parametrization tuned for densely packed solid structures (PBEsol), the lattice volume is underestimated by 0.8%; and thirdly, applying an additional isotropic pressure of 50 kbar, the lattice volume is underestimated by 1.6%, which is a typical value for a structural relaxation performed in the framework of the local density approximation (LDA). Overall, the calculated values for the lattice constants and x parameters for both Ho 2 Ti 2 O 7 and Tb 2 Ti 2 O 7 are in good agreement with both experimental values and density functional theory (DFT) calculations using comparable parametrizations [16,17].…”

“…By including the 4f electrons along with a Hubbard U parameter, or by the use of hybrid functionals, one may hope to describe magnetoelastic effects observed experimentally in rare-earth titanate pyrochlores, especially for R = Tb. Treating the 4f electrons explicitly in the DFT calculation may not lead to significant changes in the phonon dispersion calculated at T = 0 K [17], but could provide a qualitative description of magnetoelastic effects in the thermodynamic properties of the three compounds, especially in Tb 2 Ti 2 O 7 , which were observed experimentally [6]. The theoretical treatment of coupling of the lattice dynamics with the spin system may require an effective model Hamiltonian acting on a subset of the combined crystal field and phonon phase space, mapping out only the relevant crystal field states and normal modes of vibration.…”

“…The f electrons of the rare-earth ions were frozen into the core states, an approach which was used previously [16] and proven not to affect the results of phonon calculations [17]. The primitive reduced unit cell containing 22 atoms (see Fig.…”

“…For both applications, it was found that pyrochlore zirconates are generally favorable over titanates, hence more theoretical investigations on the lattice dynamics of zirconates have been carried out. In the pyrochlore titanates, available calculations of the phonon spectrum are limited to the point, where the energies and symmetries of phonons have previously been predicted [16,17]. Experimentally, the lattice dynamics of both titanate and zirconate pyrochlores were measured using the Q = 0 sensitive Raman scattering [18][19][20] and infrared absorption techniques [21], but neither could confirm the existence of the low-lying optical phonon modes that were thought to be responsible for the low thermal conductivity of pyrochlore materials at elevated temperatures [22].…”

Cтруктура и динамика решетки редкоземельных гранатов RE-=SUB=-3-=/SUB=-Al-=SUB=-5-=/SUB=-O-=SUB=-12-=/SUB=- (RE = Gd--Lu, Y): расчет ab initio

“…1). Такой подход позволил успешно описывать структуру и динамику решетки ряда редкоземельных пирохлоров и ферроборатов [21,22].…”