science24.com - Ab initio calculation of the Raman spectrum of Li2SrTa2O7 using generalized forces and symmetry coordinates

Search for content and authors

Ab initio calculation of the Raman spectrum of Li₂SrTa₂O₇ using generalized forces and symmetry coordinates

Thierry Pagnier

LEPMI, 1130 rue de la Piscine, Saint Martin d'Hères 38402, France

Abstract

In spite of an increasing efficiency of the ab initio DFT codes, the calculation of Raman spectra of crystals is still uncommon. Whatever the method, e.g. frozen phonons for the VASP/Phonon couple, second derivatives calculation for Abinit, the calculation starts with the determination of the whole dynamical matrix and its diagonalization. If the dynamical matrix elements are expressed in the basis of the displacements of all atoms in the x, y, and z directions of a fixed coordinate system, it has no special symmetry and the dynamical matrix diagonalization can be very difficult. Moreover, if one in only interested in the Raman bands, the calculation of the whole dynamical matrix is not necessary.

A new method has been developed, using atom displacements along the symmetry coordinates bases of the Raman active irreducible representations. If S_i and S_j are two symmetry coordinates bases of the same irreducible representation, the dynamical matrix element D_ij=∂²E/(∂S_i∂S_j) can be calculated from the forces observed in the S_j direction when atoms are moved according to S_i. If S_i and S_j are bases of different irreducible representations, the corresponding D_ij is zero by definition. The method described here allows therefore the calculation of a block-diagonalized matrix. Band frequencies and normal mode displacements can then be calculated independently for each symmetry.

This technique has been applied to Li₂SrTa₂O₇, a Ruddlesden-Popper-type oxide which structure and Raman spectrum have been recently obtained (1). The DFT code was Abinit (2), in the Local Density Approximation (LDA) framework. Despite the known limitations of this technique, a good fit with the experimental data has been obtained.

(1) T. Pagnier, C. Galven, F. Le Berre, M.P. Crosnier-Lopez, ICORS 2008, Uxbridge, West London, UK 17-22 august 2008

(2) www.Abinit.org

Dr F. Le Berre and M.P. Crosnier Lopez are kindly acknowledged for having provided the Li₂SrTa₂O₇ samples.

Legal notice

Legal notice:

Copyright (c) Pielaszek Research, all rights reserved.
The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/16305 must be provided.

Ab initio calculation of the Raman spectrum of Li2SrTa2O7 using generalized forces and symmetry coordinates

Presentation: Oral at E-MRS Fall Meeting 2008, Symposium A, by Thierry Pagnier See On-line Journal of E-MRS Fall Meeting 2008 Submitted: 2008-06-11 11:37 Revised: 2008-06-13 16:23

Ab initio calculation of the Raman spectrum of Li₂SrTa₂O₇ using generalized forces and symmetry coordinates

Presentation: Oral at E-MRS Fall Meeting 2008, Symposium A, by Thierry Pagnier
See On-line Journal of E-MRS Fall Meeting 2008

Submitted: 2008-06-11 11:37 Revised: 2008-06-13 16:23