Elastic properties of quaternary oxypnictides LaOFeAs and LaOFeP as basic phases for new 26–52 K superconducting materials from first principles I.R. Shein and A.L. Ivanovskii * Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, Pervomaiskaya 91, 620041 Ekaterinburg, Russia Received 21 April 2008; revised 21 June 2008; accepted 17 July 2008 Available online 30 July 2008 The full-potential linearized augmented plane wave method with a generalized gradient approximation for the exchange-corre- lation potential has been applied to predict the elastic properties of quaternary oxypnictides LaOFePn (Pn = As and P) as the basic phases for the newly discovered 26–52 K superconductors. The optimized lattice parameters, independent elastic constants (C ij ), bulk moduli, compressibility and shear moduli are evaluated and discussed. Numerical estimates of the elastic parameters of the polycrystalline LaOFePn ceramics are performed for the first time. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Quaternary oxypnictides LaOFePn; Elastic behavior; Linear-augmented plane wave method (FLAPW) The elastic properties are of great importance for the materials science of superconductors (SCs); and elas- tic constants can be linked to such important physical parameters of SCs as the Debye temperature H D and the electron–phonon coupling constant k [1]. The elastic properties of various superconducting materials have been studied extensively over the years, and some corre- lations between the superconducting critical tempera- ture T C and mechanical parameters have been discussed. Hirsch [2] suggested that high T C s are associ- ated with low values of the bulk modulus B, i.e. high compressibility b. Many SCs with enhanced critical tem- peratures (such as YBCO, MgB 2 , MgCNi 3 , borocar- bides REM 2 B 2 C, carbide halides of the rare earth metals, RE 2 C 2 X 2 , etc.) are relatively soft: their bulk moduli do not exceed 200 GPa (b P 0.005 GPa 1 ) [1,3–7]. On the other hand, a superconducting transition (down to T C  11 K) has been found for such hard and incompressible material as boron-doped diamond [8,9]. As well as just scientific interest, mechanical proper- ties are also extremely important for technology and various advanced applications of SC materials [10]. In February 2008, a new layered superconductor, fluorine-doped quaternary oxyarsenide LaOFeAs (LaO 1x F x FeAs, x = 0.05–0.12), with a T C of about 26 K, was reported [11]. This finding has attracted a great deal of interest because it appears to be the first example of the long-sought after copper-free SCs with a combination of properties (high critical temperature, large upper critical field, bordering magnetic instability) suggesting unconventional superconductivity [12–17]. This discovery has also triggered much activity in the search for related materials among the family of quater- nary oxypnictides of rare earth and transition metals. A number of new SCs with enhanced T C s have been found, including La 1x Sr x OFeAs [18], GdO 1x F x FeAs [19], CeO 1x F x FeAs [20] and SmO 1x F x FeAs [21], with T C s in the range 25–43 K, and PrO 1x F x FeAs and NdO 1x F x FeAs [22,23], with T C s of about 50–52 K, and some of their properties (structural, magnetic and electronic) have been examined [11–23]. In this work, we focus on the elastic properties of quaternary oxypnictides LaOFePn (Pn = As and P) as the basic phases for the newly discovered 26–52 K SCs to predict their elastic constants (C ij ), bulk moduli B, compressibility b and shear moduli G. In addition, numerical estimates of the elastic parameters of the polycrystalline LaOFePn ceramics, important for future applications of these new superconducting materials, are performed for the first time. The quaternary oxypnictides LaOFePn adopt layered tetragonal ZrCuSiAs-type structures (space group P4/ 1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.07.028 * Corresponding author. Tel.: +7 343 3745331; fax: +7 343 3744495; e-mail: ivanovskii@ihim.uran.ru Available online at www.sciencedirect.com Scripta Materialia 59 (2008) 1099–1102 www.elsevier.com/locate/scriptamat