THE STRUCTURAL, ELECTRONIC AND MAGNETIC

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(A=Sr, Ba) compounds with space group I4/mmm (139) are studied by ... In the formulation of AT2Pn2 phase, A generally corresponds to alkaline earth metal, T ... ARu2Sb2 (A=Sr,Ba) belongs to the space group 139(I4/mmm) and atomic ...
Advanced Materials Research Vol. 584 (2012) pp 187-191 Online available since 2012/Oct/22 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.584.187

THE STRUCTURAL, ELECTRONIC AND MAGNETIC PROPERTIES OF ARu2Sb2 (A=Sr, Ba) D.S.JAYALAKSHMI and M.SUNDARESWARI* Sathyabama University, Jeppiaar Nagar, Old Mamallapuram Road, Chennai-600 119. e-mail: *[email protected], [email protected] Keywords: Intermetallic compounds, inter-atomic bonding, pnictogen, ThCr2Si2-type, ab-initio calculations.

Abstract. The structural, electronic and magnetic properties of ThCr2Si2-type namely, ARu2Sb2 (A=Sr, Ba) compounds with space group I4/mmm (139) are studied by means of Full Potential – Linearized Augmented Plane Wave Method (FP-LAPW) method by using WIEN2K code. The necessary input parameters to perform the ab-initio calculation of ARu2Sb2 (A=Sr, Ba) compounds are taken from ARu2As2 (A=Sr, Ba) compounds. To the best of our knowledge, the properties of these compounds have not yet been investigated. From this work the optimized structural parameters, bulk modulus, electronic specific heat coefficient, Fermi energy, magnetic moments are estimated. Besides, Density of States (DOS) histograms and electron density plots are plotted for the above mentioned compounds in order to analyze the bonding nature between the atoms in these compounds. Introduction The electronic structures of ThCr2Si2-type compounds were studied by means of experimental methods as well as by theoretical methods. In 1965, Ban and Sirika reported that more than 700 compounds have been found to adopt the ThCr2Si2 (s.g.I4/mmm;139) type structure. This is the highest number of representatives for any crystal structure. From the literature it is understood that ThCr2Si2 type pnictides have led to renewed interest in the area of superconductivity and magnetism. In the formulation of AT2Pn2 phase, A generally corresponds to alkaline earth metal, T is transition metal and Pn are pnictogens (N, P, As, Sb, Bi) [1-3]. Within the broad family of “122” phase, AFe2As2 (A=Ca, Sr, Ba) are located on the border of magnetic instability and commonly exhibit a temperature-dependent structural and magnetic phase transitions with the formation of antiferromagnetic spin ordering. Secondly, superconductivity emerges either by hole or electron doping of the parent compounds with the transition temperature (Tc) nearly 38K. Interestingly, the same result may be achieved by partial substitution of some non-magnetic 4d, 5d metals (Ru, Ir etc.) for magnetic 3d metal (Fe) [4-7]. Series of compounds of ARu2As2 (A=Sr, Ba and Eu) were previously synthesized in single crystal form and their crystal structure, electronic properties were reported [8, 9]. These compounds can be potential parent compounds for high-temperature superconductors. It is of interest to look for other materials with related structures and investigate their properties. To the best of our knowledge, the compounds ARu2Sb2 (A=Sr, Ba) have not been investigated before. The crystal structures of ARu2Sb2 (A=Sr, Ba) are isostructural with ARu2As2 (A=Sr, Ba). The structure is built up of Ru-Sb layers alternated by A layers stacked along the crystallographic c-axis. Within the Ru-Sb layers, the Sb atoms lie outside the plane, which is formed by the Ru atoms. In this study, we report the structural, electronic, magnetic and inter-atomic bonding properties of the predicted ARu2Sb2 (A=Sr,Ba) compounds by means of FLAPW-GGA method with the purpose to evaluate the systematic trends of the above properties as a function of the alkaline earth metal type (Sr versus Ba) in comparison with ARu2As2 (A=Sr,Ba) as a parent phase.

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Computational Aspects Our calculations were carried out by means of Full Potential Linearized Augmented Plane Wave (FP-LAPW) method implemented in the WIEN2K suite of programs [10]. The Generalised Gradient Approximation (GGA) to exchange correlation potential in the PBE form was applied. [11]. The plane wave expansion was taken to be RMT x KMAX equal to 7 and K sampling with 10x10x10 k-points in the Brillouin zone was used. The self consistent calculations were considered to be converged when the difference in the total energy of the crystal did not exceed 0.1mRy and the difference in the total electronic charge did not exceed 0.001e as calculated at consecutive steps. During this process, the structure is fully relaxed until the forces on the atom become smaller than 0.1mRy/a.u. ARu2Sb2 (A=Sr,Ba) belongs to the space group 139(I4/mmm) and atomic positions for alkaline- earth metals 2a(0,0,0), for Ru (0,0.5,0.25) and the pnictogens 4e(0,0,ZPn) ,where ZPn are the so-called internal coordinates governing the Ni-Pn distances. All the lattice parameters including ZBi are optimized in the present calculation. Results and Discussion The equilibrium structural parameters of the ARu2Sb2 (A=Sr, Ba) compounds are computed and are presented in the Table 1. From the literature it is noted that, all the Fe based compounds have the anti ferromagnetic ground states, Co based compounds have the ferromagnetic ground states and the Ni and Ru based compounds have the non magnetic ground states. Table 1 The optimized lattice parameters, internal coordinates (ZPn), c/a ratio, bulk modulus, electronic specific heat coefficient, Fermi energy and magnetic moment for SrRu2Sb2 and BaRu2Sb2 compounds. .

Parameter

SrRu2Sb2

BaRu2Sb2

a [a.u.] c [a.u.] c/a ZSb Bulk modulus [GPa] Electronic specific heat coefficient[Gamma in mJ/(mol cell K**2)] Fermienergy [Rydberg] Magnetic moment

8.5364 21.7443 2.5472 0.3667 102 4.07

8.5119 23.8557 2.8026 0.3563 96 4.33

0.6347 -0.00001

0.6091 0.0000

Further, SrNi2As2 compound is reported as a bulk superconductor [12] and BaNi2Sb2 is a photon mediated superconductor with low Tc [13]. Here, the obtained magnetic moment, as reported in Table 1 reveals that SrRu2Sb2 and BaRu2Sb2 compounds exhibit diamagnetism. Hence it gives a lead for the possibility of these compounds to exist as superconductors. The existence of superconductivity in these compounds can be confirmed by performing the other calculations such as resistivity studies, etc.,

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SrRu2Sb2

189

BaRu2Sb2

Fig.1 Density Of States for ARu2Sb2 (A=Sr, Ba) compounds

SrRu2Sb2

BaRu2Sb2

Fig.2 Electron Density Plots for ARu2Sb2 (A=Sr, Ba) compounds From the Table 1 one can see that when we go from a smaller Sr atom to a larger atom Ba atom, the inter layer distance - the parameter ‘c’ grows appreciably and the parameter ‘a’ decreases, thus leading to anisotropic deformations of the crystal structure. From the bulk modulus values one can say that the compressible nature of both the compounds is more or less same. The electronic specific heat coefficient value is also of same range for both the compounds. Figure 1 shows the total and partial density of states of ARu2Sb2 (A=Sr, Ba) phases as calculated for equilibrium geometries. The metallic properties of these compounds can be confirmed from the band structure

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plot, where one can see several bands crossing the Fermi level and the same is also revealed from the DOS histograms as shown in Fig.1. From the DOS, it is clear that the near Fermi region is formed mainly by the states of Ru and Sb atoms in both the compounds and the contributions from the valence states of Sr/Ba atoms are negligible. To probe into the bonding properties of these compounds, the charge density plots along (100) plane is obtained and is given in Fig.2. From these plots one can infer that the electron density distribution contours around the atoms Ru and Sb are more of symmetrical and directional in nature, which suggests the possibility of existence of covalent bond between them. This is due to the interaction of Sb ‘p’ orbitals with Ru‘d’ orbitals in both the compounds. From the same plot one can also see that there is no Sb-Sb bonding i n these compounds and an ionic bonding exists between the [Ru2Sb2] blocks and Sr / Ba atom. CONCLUSIONS In summary by means of the FLAPW-GGA approach, we have systematically studied the structural, electronic and magnetic properties of the compounds ARu2Sb2 (A=Sr, Ba). Our results show that replacement of alkaline earth metal type lead to anisotropic deformations of the crystal structure. These compounds have almost zero magnetic moment and from the previous studies of similar type of compounds one can say that there is a possibility of these predicted compounds to exist as super conductors. Our analysis on the inter-atomic bonding of ARu2Sb2 phases reveals that inside the [Ru2Sb2] blocks mixed covalent-metallic bonds exist. The ionic bonding emerges between the [Ru2Sb2] blocks and A atom. Acknowledgments The authors express their gratitude to Dr.M.Rajagopalan, Emeritus Professor, Anna University, Chennai and Dr.N.V.Chandra Shekar, Condensed Matter Physics Division, Materials Science Group, IGCAR, Kalpakkam. REFERENCES [1] Z.Ban and M.Sirika, Acta Cryst. 18 (1965) 594. Reference to a book: [2] P.Villars and L.D. Calvert, “Pearson’s Handbook of Crystallographic Data for Intermetallic Phases,” second ed. ASM International, Materials Park, OH, 1991. [3] A.Mewis, Z.Naturforsch. B 35 (1980) 141. [4] A.L.Ivanovski, Physics – Uspekhi 51 (2008) 1229. [5] D.Kasinathan, A Ormeci, K.Koch,U.Burkhardt, W.Schnelle, A.Leithe-jasper ,H.Rosner,New J.PhyS.11 (2009) 025023. [6] F.Han, X.Zhu, P.Cheng, B.Shen, H.H.Wen, arxiv : 0903.1028. [7] S.Paulraj, S.Sharma, et al., arxiv: 0902.2728. [8] R.Nath,Y.Singh , D.C.Johnston, Phys.Rev.B 79 (2009) 174513. [9] I.R.Shein and A.L.Ivanovski, arxiv.org/pdf/ 0905.2255. Reference to a book: [10] P Blaha et al., WIEN2K, An Augmented Plane Wave plus Local Orbitals program for Calculating Crystal Properties,Vienna University of Technology, Vienna (2001)

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[11]. J P Perdew et al., Phys. Rev. Lett., 77 (1966) 386. [12] E.D.Bauer, F.Ronning et al., Phys.Rev.B 78 (2008) 172504. [13] Alaska Subedi and David J.Singh, Phys.Rev.B 78 (2008) 132511.

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The Structural, Electronic and Magnetic Properties of ARu2Sb2 (A=Sr, Ba) 10.4028/www.scientific.net/AMR.584.187