First principles study of structural and vibrational properties of Sr 2 BWO 6 (B ¼ Mg, Zn) in cubic and tetragonal phases Karandeep, H.C. Gupta * , S. Kumar Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India article info Article history: Received 10 June 2013 Accepted 15 June 2013 Available online 26 June 2013 Keywords: Double perovskite Phase transition Phonons DFT abstract The structural and vibrational properties of Sr 2 MgWO 6 and Sr 2 ZnWO 6 double perovskites in the space group Fm 3m (cubic) and I4/m (tetragonal) have been investigated using the pseudopotential plane wave method based on density functional theory (DFT) under local density approximation (LDA) and gener- alized gradient approximation (GGA). The structural parameters along with the Raman and infrared wavenumbers at zone center are calculated. The calculated values of lattice constants and Raman wavenumbers are in very good agreement with the experimental values for both cubic and tetragonal phases. Ó 2013 Elsevier Masson SAS. All rights reserved. 1. Introduction The ideal structure of double perovskites A 2 BB 0 O 6 consists of corner sharing octahedral BO 6 and B 0 O 6 , with 12-coordinated interstitial spaces between octahedra occupied by the A-site cat- ions. This structure has cubic symmetry in Fm 3m space group. However, the size mismatch between the octahedral network and A-site cations leads to the octahedral tilting distortions, which lowers the cubic symmetry of double perovskites. Such materials possess two or more phase transitions at different temperatures. Double perovskites containing tungsten have been extensibly studied because of their magnetic and dielectric properties [1e4]. Gateshki et al. [5,6] have studied the crystal structure of Sr 2 BWO 6 (B ¼ Mg, Co, Ni, Cu, Zn, Ca) series by X-ray powder diffraction and concluded that all materials in this series undergo temperature induced phase transitions. Sr 2 MgWO 6 has tetragonal I4/m structure at room temperature and it undergoes phase transition to cubic Fm 3m structure at 570 K, whereas a monoclinic P2 1 /n structure was found for Sr 2 ZnWO 6 at room temperature which first undergoes a phase transitions to tetragonal I4/m at 340 K and then to cubic Fm 3m structure at 770 K. Patwe et al. [7] have studied the thermal expansion coefficients in Sr 2 MgWO 6 . They also found a phase transition from tetragonal to cubic structure at 770 K. Recently, Manoun et al. [8e10] have studied these phase transition in Sr 2 MgWO 6 and Sr 2 ZnWO 6 by Raman spectroscopy and concluded similar results. Guo et al. [11] have studied the electronic properties and phase transition mechanism in Sr 2 MgWO 6 by first principles method. To the best of our knowledge no theoretical studies showing the behavior of vibrational modes during this phase transition have been performed. A theoretical study of discussing the behavior of vibrational modes during this phase transition is necessary in order to take full advantage of such materials for extensive technological applications. First-principles calculations offer one of the most powerful tools for theoretical studies of these properties. Hence in this paper an attempt has been made to investigate the structural and vibrational properties of Sr 2 BWO 6 (B ¼ Mg, Zn) belonging to Fm 3m and I4/m space groups by pseudopotential plane wave method within the framework of density functional theory (DFT) under the local density approximation (LDA) and generalized gradient approximation (GGA). 2. Computational details All calculations are performed by using Quantum ESPRESSO distribution which is based on density functional theory with plane-wave pseudopotential method [12]. The local density approximation (LDA) of Perdew and Zunger (PZ), generalized gradient approximation (GGA) of Perdew, Burke and Ernzerhof (PBE) and its improved version for solids (PBEsol) along with the ultrasoft pseudopotentials are used in these calculations [13e16]. The Ultrasoft pseudopotentials for Sr, Mg, Zn, W and O are gener- ated by taking 4s 4p 5s, 2s 2p 3s, 3d 4s, 5s 5p 5d 6s and 2s 2p as valence states, respectively. All pseudopotentials except oxygen include non-linear core correction to account for large overlap * Corresponding author. Tel.: þ91 9810100123. E-mail address: hcgupta@physics.iitd.ac.in (H.C. Gupta). Contents lists available at SciVerse ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie 1293-2558/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2013.06.006 Solid State Sciences 23 (2013) 72e78