24 BORON 3 (1), 24 - 32, 20178 ABSTRACT Ti 2 AlB 0.5 C 0.5 and Ti 2 AlN 0.5 C 0.5 compounds which are called M 2 AX type MAX phases referred to as 211 and have hexagonal crystal structure with conform to P6 3 /mmc space group, have been examined by using Generalized Gradient Approximation (GGA) in the Density Functional Theory (DFT) as implemented in CASTEP soft- ware package. In this study, the electronic, elastic, and lattice dynamical proper- ties of these compounds have been investigated within the ab initio study. These compounds show metallic behavior since there is no band gap in the calculated electronic band structures. The estimated elastic constants of these compounds indicate that they are mechanically stable and their bonding nature is ionic and also, Ti 2 AlN 0.5 C 0.5 compund has anisotropic character in mechanically whereas the behavior of Ti 2 AlB 0.5 C 0.5 compound is nearly isotropic. Moreover, both of our compounds are brittle materials. Also, these compounds are dynamically stable since there are no soft modes in their plotted phonon dispersion curves. BOR ISSN: 2149-9020 JOURNAL OF BORON DERGİSİ ULUSAL BOR ARAŞTIRMA ENSTİTÜSÜ NATIONAL BORON RESEARCH INSTITUTE 18 20 01 03 The first principles investigation of structural, electronic, mechanical and lattice dynamical properties of the B and N doped M 2 AX type MAX phases Ti 2 AlB 0.5 C 0.5 and Ti 2 AlN 0.5 C 0.5 compounds Gökhan Sürücü 1,2,3 , Aytaç Erkişi 4 1 Department of Electric and Energy, Ahi Evran University, Kirsehir, 40100, Turkey, ORCID ID orcd.org/0000-0002-3910-8575 2 Department of Physics, Middle East Technical University, 06800, Ankara, Turkey 3 Photonics Application and Research Center, Gazi University, Ankara, 06500, Turkey 4 Department of Physics Engineering, Hacettepe University, Beytepe, Ankara 06800, Turkey, ORCID ID orcd.org/ 0000-0001-7995-7590 BOR DERGİSİ JOURNAL OF BORON *Correspondig author: thesurucu@gmail.com http://dergipark.gov.tr/boron ARTICLE INFO Article history: Received 10 August 2017 Accepted 04 January 2018 Available online 26 March 2018 Research Article DOI: 10.30728/boron.333855 Keywords: MAX phases, Ab initio calculations, Elastic constant; Electronic band structure; Phonon 1. Introduction Among the nanolaminated materials MAX phases which have hexagonal crystal structure and comform to P6 3 /mmc space group, are determined by the gen- eral chemical formula M n+1 AX n (n = 1,2, and 3) which are often referred to as 211 (n = 1) or 312 (n = 2), herein M is an early transition metal, A is an A-group element and X is either carbon (C) or nitrogen (N) [1,2]. The MAX phases are thermodynamically stable materials and candidate for many industrial applica- tions such as high-temperature structural, electrical, and tribological applications [3-5], due to have unique physical, chemical, electrical, and mechanical prop- erties some of which are good thermal and electrical conductivity [6,7], strength and stifness mechanically [8,9], readily machinable [6,10], relatively soft [6,11], and high thermal shock resistance and good damage tolerance [1,6] properties. Therefore, since the discov- ery of metallic and ceramic behaviors of MAX phases by Barsoum and El-Raghy [12], these materials have attracted the attention of many scientists up to now [13-20] since they have. Although there are a lot of theoretical and experi- mental study about the discovery physical properties of MAX phases in the literature until now, to the best of our knowledge, there is no study on the mechani- cal and dynamic stability, electronic band structure of Ti 2 AlB 0.5 C 0.5 compound. Therefore, the focus of this study is about the structural, mechanical, electronic, and lattice dynamic properties of Ti and Al based M 2 AX type Ti 2 AlB 0.5 C 0.5 and Ti 2 AlN 0.5 C 0.5 compounds while B or N element is substitutied with C element in our com- positions. In the present work, we have investigated the struc- tural, electronic, mechanical, and vibrational proper- ties of Ti 2 AlB 0.5 C 0.5 and Ti 2 AlN 0.5 C 0.5 compounds which have hexagonal crystal structure (P6 3 /mmc) in detail. The computational methods are given in the second section. The discussion about the calculated some structural parameters, estimated mechanical proper- ties and electronic and lattice dynamical behaviors of these compounds is given in the third section. Finally in the last section, the obtained results are summa- rized. To the extend we know in the literature, espe- cially for Ti 2 AlB 0.5 C 0.5 compound, there is no such a detailed study before in theoretically.