Published: May 23, 2011 r2011 American Chemical Society 12604 dx.doi.org/10.1021/jp2017083 | J. Phys. Chem. C 2011, 115, 12604–12610 ARTICLE pubs.acs.org/JPCC Electric Polarization Field of Phonon Modes Induced by Pressure and Maximally-Localized Wannier Functions in Beryllium Chalcogenides: Theoretical Study S. Laref †,‡, * and A. Laref ‡ † Universit  e de Lyon, Institut de Chimie de Lyon, Laboratoire de Chimie, Ecole Normale Sup erieure de Lyon, 46 All  ee d’Italie, F69364 Lyon Cedex 07, France ‡ Physics Department, Science Faculty, University of Sidi Bel Abbes, Sidi Bel Abbes 22000, Algeria b S Supporting Information 1. INTRODUCTION The invention of new materials is critical for many potential innovative and clean energy technologies. The beryllium chalco- genides BeTe, BeSe, and BeS are IIVI compounds that crystal- lize at ambient pressure in the four-fold coordinated zinc-blend structure. Apart from BeO, which has a hexagonal structure, all of the other group-IIa chalcogenides adopt the rocksalt geometry. A unique feature in the beryllium compounds is that the Be ions are extremely small as compared to the anions except BeO. 1 This leads to an excess of the critical ratio of ionic radii, 4.45, for the zinc-blende structures in all three Be compounds; BeS, BeSe, and BeTe. Thus, unlike other IIaVI compounds which are ionic, Be chalcogenides exhibit a high degree of covalent bonding with the Philips ionicities ranging from 0.169 in BeTe to 0.312 in BeS. 2 This class of semiconductors has indirect fundamental band gaps, which are associated with ΓX transitions. Also, BeS has high hardness 3 while BeTe is a small gap semiconductor. 4 These interesting properties make them potentially useful for techno- logical applications. Therefore, there is renewed interest in these compounds, which could be used in building green and blue light emitting electro-optical devices. 5 These compounds are of fundamental interest and possibly technological important due to their unusual or extreme properties. Although extensive improvement has been made in theoretical explanation of the structural and electronic of IIVI beryllium chalcogenides compounds, many of their dynamical quantities are still not well-known. Understanding the effect of pressure on the phonon dispersion is the relative key for several fundamental and application studies of materials. Its knowledge allows one to correlate the results from an atomistic scale with macroscopic thermodynamic parameters. Experimentally, the phonon disper- sion can be determined by many techniques such as electron energy loss spectroscopy, Raman spectroscopy, neutron scatter- ing, IR absorption, and so forth. Moreover, the behavior of lattice vibrations under pressure provides useful information concern- ing bonding properties, phase transformations, structural in- stability, phononelectron interactions, and carrier transport properties. It can be also used to study many thermodynamic phenomena, such as neutron diffraction spectra, Raman spec- troscopy, thermal expansion, specific heats, and heat conduction. To date, probably as a result of their very high toxic nature, which makes it difficult to obtain them as single crystals or epitaxial layers, only few experimental studies 57 have been performed on these compounds. Our results have been partially used to derive those works published 20 years ago for the photoemission spectrum of BeTe. 4 In order to comply with the renewed interest and to support the experiments with a theoretical database, we have performed a state-of-the-art first Received: February 21, 2011 Revised: May 14, 2011 ABSTRACT: We present a theoretical study of polaron properties associated to the optical phonon modes induced by pressure on the beryllium chalcogenides. The calculations are performed using ab initio pseudopotential approach based on the density functional perturbation theory combined with maximal- lylocalized Wannier functions. Features such as phonon frequencies, dielectric constants, effective polar field, polaron effective mass, Fr€ ohlich coupling constant, Debye temperature, deformation potential, polaron diffusion constant, and maximallylocalized Wannier functions have been determined. Good agreement is found between our simulated results and available data. In another case, our calculated values are totally predictive. We show that the pressure dependence of those physicochemical considerations on the electric polariza- tion field is found to vary monotonously. These studies form the basis for further development of models to describe polaron transport in the monocrystalline phase of BeX (X = S, Se, and Te) compounds such as bulk crystals.