New Ternary superconducting compound LaRu2As2 ...

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New Ternary superconducting compound LaRu2As2: DFT calculations of some physical properties M. A. Hadi1*, M. S. Ali2, S. H. Naqib1 and A. K. M. A. Islam1,3 1 Department of Physics, University of Rajshahi, Rajshahi-6205, Bangladesh Department of Physics, Pabna University of Science and Technology, Pabna-6600, Bangladesh 3 International Islamic University Chittagong, Chittagong-4203, Bangladesh

2

Abstract Here we have represented the DFT calculations performed within the first-principles pseudopotential method to realize the aspects of some physical properties including elastic and electronic properties of newly discovered superconductor LaRu2As2. Thus, the optimized structural parameters, independent elastic constants, bulk elastic properties, Debye temperature, electronic energy band structure, total and partial densities of states, inter-atomic bonding nature, Vickers hardness, electron charge density and Fermi surface of LaRu2As2 are obtained for the first time and analyzed in comparison with those for the related ThCr2Si2 type compounds. Keywords: New superconductor LaRu2As2; DFT calculations; Mechanical properties; Electronic features

1. Introduction A great variety of compounds with RT2X2 stoichiometry take on the body-centered tetragonal ThCr2Si2 type structure (space group I4 /mmm, No. 139) [1]. In this stoichiometry, R is symbolized for a rare earth, alkaline earth or alkali element, T refers a transition metal and X represents p-metal atoms, namely B, P, Si, As or Ge. This large family includes more than 700 members of 122-type intermetallics (so-called 122 phases) and exhibits an outstanding collection of physical and chemical properties [2]. ThCr2Si2 type structure forms an innate multilayer system in which the planes of the rare earth ions are taken apart from the transition metal layers (T) by the sheets of p-metal atoms (X), comprising of R–X–T–X–R stacking of individual basal planes along the c-axis. This layered structure often causes a strong uniaxial anisotropy in these ternary compounds with c-axis being an anisotropy axis. In other words, this structure is naturally characterized by the layers of edge sharing TX4 tetrahedron parallel to the x–y plane, separated by the basal planes of metal atoms (R). In this structure the [T2X2] blocks are separated by R atomic sheets. In turn, inside [T2X2] blocks, T ions form a square lattice sandwiched between two X sheets shifted so that each T is surrounded by a distorted X tetrahedron TX4. The interlayer distances between the X atoms exhibit a wide range of variation if R or T is changed for a given p-metal element X [2,3]. A lot of interest in this group of materials have paid attention in recent years by the discovery of high-temperature superconductivity in iron arsenide AFe2As2 (A = K, Ca, Sr, Ba, etc.) [4,5]. Since Ru lies in the same group together with Fe in the periodic table, the superconductivity in the level of iron arsenide is expected for Ru-based compounds, and several studies have been devoted to these materials [6-10]. Motivated by this, Guo et al. [11] have recently synthesized the polycrystalline samples of ThCr2Si2-type LaRu2As2 and reported the bulk superconductivity in LaRu2As2 insuring by resistivity and magnetization measurements with an onset TC of 7.8 K. In this paper, the study of LaRu2As2 has been carried out via the ground state electronic structure calculations using the plane-wave pseudopotential approach within the density functional formalism as applicable to the CASTEP code. The aim of the present paper is to investigate the ground state properties including structural, elastic, electronic and bonding characters of the new 122 compound by calculating its total energy at optimized volume. To our best knowledge, this is the first ab initio calculations completed on this newly discovered superconductor LaRu2As2. The rest of this paper is divided into three sections. In Section 2, a brief description of the method of computation used in this study has been presented. The results obtained for the structural, elastic, electronic, and bonding properties of LaRu2As2 are analyzed in Section 3. Finally, Section 4 recaps the main conclusions of the present work.

*

Corresponding author:[email protected]

2. Method of calculations The methodology for electronic structure calculations as implemented in the Cambridge Serial Total Energy Package (CASTEP) code [12] is as follows: a set of Kohn-Sham equations are solved by means of the plane-wave pseudopotential method [13] within the strong electron ensemble Density Functional Theory (DFT) [14]. Exchange and correlation energies are described by a nonlocal correction for LDA in the form of GGA [15]. The wavefunctions are expanded in a plane wave basis set defined by use of periodic boundary conditions and Bloch‟s Theorem [16]. The electron-ion potential is treated by means of first-principles pseudopotentials within Vanderbilt-type ultrasoft formulations [17]. BFGS energy minimization technique [18] best for crystalline materials is used to find, self-consistently, the electronic wavefunctions and consequent charge density. In addition, the density mixing [19, 20] scheme is employed. In implement the special k-points sampling integration over the first Brillouin, the Monkhorst–Pack scheme [21] is used. The parameters are chosen for present calculations are as follows: energy cutoff 500 eV, k-point mesh 16 16 7, the difference in total energy per atom within 5 × 10-6 eV, maximum ionic Hellmann-Feynman force within 0.01 eV/Å, maximum stress within 0.02 GPa, and maximum ionic displacement within 5 × 10-4 Å. 3. Physical properties In this section the investigated physical properties of the 122 phase superconductor LaRu2As2 are analyzed; among them are the structural and elastic properties, Debye temperature, Band structure and DOS, and then Mulliken population and Vickers hardness and the electron charge density as well as Fermi surface of LaRu2As2 under zero pressure. 3.1 Structural properties The body centered tetragonal LaRu2As2 structure shown in Fig. 1 crystallizes with space group I4/mmm (No. 139). This structure retains only one crystallographic position for each atomic species and has one variable parameter zX, which settles the relative position of As inside the unit cell. Each unit cell consists of two formula units. The La, Ru and As atoms reside in the 2a, 4d and 4e sites, respectively. This structure arranges negatively charged [Ru2As2] blocks and positively charged La layers by turns along the c-direction. The optimized lattice constants and zX parameter of LaRu2As2 are given in Table 1 along with those obtained for some other 122 phase compounds. The computed lattice parameters show reasonable agreement with the measured values [11]. Moreover, the calculated structural parameters are slightly larger (a ~ 0.23%, c ~ 2.94% and V ~ 3.41%) than the experimental values, which is the general trend inherent to GGA calculations.

Fig. 1. Crystal structure of new 122 phase superconductor LaRu2As2. (Finally, one is chosen)

Table 1. Structural properties of LaRu2As2 in comparison with those for the related ThCr2Si2 type compounds. Compounds

a (Å)

c (Å)

V (Å3)

c/a

zX

LaRu2As2

4.1923 4.1826 4.1925 4.1525 4.2068 4.1713

10.9014 10.5903 12.3136 12.2504 11.2903 11.1845

191.59 185.27 216.44 211.24 199.81 194.61

2.6003 2.5320 2.9371 2.9501 2.6838 2.6813

0.3641 -0.3510 0.3527 0.3591 0.3612

BaRu2As2 SrRu2As2

B (GPa)

B

References [This] [11] [22] [23] [22] [23]

3.2 Elastic properties The elastic constants of a material related to its response to an applied stress describe the mechanical behavior of solids. The finite strain theory assigned in CASTEP code calculates the elastic constants successfully of a range of materials including metallic systems [24]. In this method, a given uniform deformation εj is applied and then the resulting stress σi is calculated. By wise choosing the functional deformation, elastic constants are then evaluated by solving the linear equation, σi = Cij εj. Using the calculated Cij, the polycrystalline bulk elastic properties namely, bulk modulus B and shear modulus G are calculated from the well-established Voight-Reuss-Hill (VRH) approximation, which is validated in many metallic and insulating materials [25-27]. Further, the equations, Y = (9GB)/(3B + G) and v = (3B – 2G) /(6B + 2G) are employed to determine the Young‟s modulus Y and Poisson‟s ratio v, respectively. The single crystal elastic constants (C11, C12, C13, C33, C44 and C66) and bulk elastic properties (B, G, Y, B/G and v) of tetragonal crystal system LaRu2As2 are evaluated and presented in Table 2 along with the results obtained in literature for La-based isostructural compounds [28]. The newly synthesized 122 phase obeys the mechanical stability criteria for tetragonal crystal [29]: C11 > 0, C33 > 0, C44 > 0, C66 > 0, C11 – C12 > 0, C11 + C33 – 2 C13 > 0, 2(C11 + C12) + C33 + 4 C13 > 0. The elastic constants C11 and C33 explain the linear compression resistance along the directions a and c, respectively. It is evident that the obtained elastic constants C11 and C33 are very large in comparison to others, implying that the LaRu2As2 crystal should be so much incompressible under uniaxial stress along the directions a and c. Again, the elastic constant C11 is much larger than C33, meaning that the incompressibility along the direction a should be much higher than that along c. In fact, the bonds aligned to the a-axis contribute a dominating effect on C11 making it much larger than C33. Since C11+C12 > C33, the bonding in the (001) plane should exhibit more elastic rigidity than that along the c-axis and the elastic tensile modulus should be higher on the (001) plane than that along the c-axis. The elastic constant C44 indicates the ability to resist the shear distortion in (100) plane, whereas the elastic constant C66 correlates to the resistance to shear in the direction [30]. Since C66 > C44, the new compound should be more capable to resist the shear distortion in the direction than in the (100) plane. The shear anisotropy factor A, defined as A = 2C66 /(C11 - C12) is 1.24, signifying that the shear elastic properties of the (001) plane in LaRu2As2 depend on the shear directions significantly. Cauchy pressure and Pugh‟s as well as Poisson‟s ratio are considered as powerful tools for measuring the failure mode, i.e., ductile or brittle nature, of a material. A material which easily changes its volume is brittle and a material which can be easily distorted is ductile. The Cauchy pressure (C12 – C44) serves as an indicator of ductility or brittleness of a material. When the pressure is positive (negative), the material is prone to be ductile (brittle). The Pugh‟s ratio (B/G) is also used as an index of the ductility/brittleness of materials. In reference to the Pugh‟s ratio, the high (low) B/G ratio is linked to the ductile (brittle) nature of the materials. To separate the ductile materials from brittle ones, the threshold value is 1.75. Frantsevich et al. [31] also take apart the ductile materials from brittle materials in terms of Poisson‟s ratio. Frantsevich rule proposes v ~ 0.26 as the border line that separates the brittle and ductile materials. A material having the Poisson‟s ratio greater than 0.26 will be ductile otherwise the material will be brittle. The Cauchy pressure (C12 – C44) of LaRu2As2 is positive, its Pugh‟s ratio B/G is 1.79 which is greater than1.75 and its Poisson‟s ratio v is 0.265 which is greater than 0.26. As a result, the compound LaRu2As2 should behave as a ductile mater.

Table 2. Single crystal elastic constants Cij, polycrystalline bulk modulus B, shear modulus G, and Young‟s modulus Y, in GPa, Pugh‟s ratio G/B, and Poisson‟s ratio v of LaRu2As2. Compounds LaRu2As2 LaNi2Ge2 LaNi2P2

C11 244 152 202

C12 86 87 116

C13 82 40 15

C33 113 126 102

C44 62 87 116

C66 98 90 97

B 113 62 62

G 63 74 93

Y 159 159 186

B/G 1.79 0.84 0.67

v 0.265 0.073 0.002

A 1.24 2.77 2.26

Ref. [This] [28] [28]

Bulk and shear moduli are two important parameters related to materials‟ elastic behavior. Bulk modulus B estimates the resistance to fracture and shear modulus G assesses the resistance to plastic deformation of polycrystalline materials. The bulk modulus has a little connection with hardness, as is well known from dislocation theory [32]. On the other hand, a good relationship exists between hardness and shear modulus. Indeed, the hardness is more susceptible to the shear modulus than the bulk modulus. Therefore, the hardness of LaRu2As2 is expected to be small and LaRu2As2 should be soft and easily mechinable. Young‟s modulus Y estimates the resistance in opposition to longitudinal tension. Additionally, the Young‟s modulus influences the thermal shock resistance of a solid, as the critical thermal shock coefficient R is inversely proportional to the Young‟s modulus Y [33]. The larger the R value, the better the thermal shock resistance. The thermal shock resistance is a fundamental parameter for thermal barrier coating (TBC) materials selection. The small Y value signifies that LaRu2As2 should have reasonable resistance to thermal shock. 3.3 Debye temperature Debye temperature D is a characteristic temperature of crystals at which the highest-frequency mode (and hence all modes) of vibration are excited. It is related to many physical properties of materials and is used to make a division between high- and low-temperature regions for a solid material. The Debye temperature in addition, defines a border line between quantum and classical behavior of phonons. When the temperature T of a solid is elevated over D, every mode of vibrations is expected to be associated with an energy equal to kBT. At T < D, all high-frequency modes seems to be sleep. The vibrational excitations at low temperature arise only from acoustic vibrations. The Debye temperature, D, is determined by using one of the standards methods depending on the elastic moduli [34]. This method guides us to calculate the Debye temperature from the average sound velocity by the following equation: , where h stands for Planck‟s constant, kB represents Boltzmann‟s constant, n is the number of atoms per formula unit, M is symbolized for molar mass, NA is the Avogadro‟s number and denotes the mass density of the polycrystalline solid. The average sound velocity within a polycrystalline material is given by . Here vl and vt indicate the longitudinal and transverse sound velocities in crystalline solids and are expressed as and . The Debye temperature in cooperation with longitudinal, transverse and average sound velocities calculated within the present formalism is listed in Table 3. The values of vl, vt, vm, and D found in literature [28] are calculated by using elastic moduli B and G within the Voigt approximation. Here we converted these values into aggregate values using Hill approximations. As a rule of thumb, a lower Debye temperature involves in a lower phonon thermal conductivity. The examined phase LaRu2As2 has the lower Debye temperature in comparison with other two isostructural compounds and hence has a lower thermal conductivity. Accordingly, LaRu2As2 should have opportunity to use as a thermal barrier coating (TBC) material. Table 3. Calculated density ( in gm/cm3), longitudinal, transverse and average sound velocities (vl, vt, and vm in km/s) and Debye temperature ( D in K) of LaRu2As2. Compounds vl vt vm Refs. D LaRu2As2 8.799 4.731 2.676 2.976 335 [This] LaNi2Ge2 7.685 4.572 3.103 3.384 389 [28] LaNi2P2 6.853 5.210 3.684 3.994 478 [28]

3.4 Band structure and density of states An appropriate description of electronic structures (band structure, DOS, etc) is important to explain many physical phenomena, for example optical spectra of materials. The full picture of energy bands and band gaps of a solid is known as electronic band structure or simply band structure. Actually, in solid-state and condensed matter physics, the band structure defines those ranges of energy that an electron within the solid may have, and ranges of energy that it may not have. The number of bands indicates the available number of atomic orbitals in the unit cell. The calculated energy band structure at equilibrium lattice parameters is drawn along the high symmetry directions in the first Brillouin zone [Fig. 2a]. The dashed line in the band structure corresponds to the energy of the highest filled state. This energy level is known as Fermi level, EF. Depending on the position of EF, several important distinctions regarding the expected electrical conductivity of a material can be made. The Fermi level of new 122 phase superconductor takes position just above the valence bands maximum near the point. The new compound should behave metallic in nature since its valence bands crosses the Fermi level and overlap noticeably with the conduction bands. In addition, no band gap is found in the Fermi level. The near-Fermi bands show a complicated „mixed‟ character, combining the quasiflat bands with a series of high-dispersive bands intersecting the Fermi level. The detailed features of band structure can be explained with the calculated total and partial densities of states. 4

EF

0 -2

Energy (eV)

-4 -6 -8 -10 -12 -14 -16 -18 -20

Z

A M

Z

High symmetry points

R X

Density of states (states per unit cell per eV)

2

20 16 12 8 4 0 20 16 12 8 4 0

Total

EF

La 6s La 5p La 5d

10 8 6 4 2 0

Ru 5s Ru 4p Ru 4d

10 8 6 4 2 0

As 4s As 4p

-20

-16

-12

-8

-4

0

4

Energy (eV)

Fig. 2. Electronic structures of LaRu2As2; (a) electronic band structure (left panel), (b) total and partial densities of states (right panel). The electron density of states (DOS) of a system describes the number of states per interval of energy at each energy level that are available to be occupied by electrons. In general, a DOS is an average over the space and time domains occupied by the system. The density of states for a given band n, Nn (E), is defined as:

where En(k) describes the dispersion of the given band and the integral is determined over the first Brillouin zone. The total density of states, N(E), is obtained by summation over all bands:

The calculated total and partial densities of states are shown in Fig. 2(b). The large value of DOS at the Fermi level EF reassures the metallic nature of new 122 phase superconductor LaRu2As2. The Fermi level lies in a region of relatively low DOS, indicating to a certain extent that the new compound should be stable in perspective of electronic structure as said by free electron model [35]. The atom resolved partial DOS gives more insights on the electronic structure. Electrons located at the levels above EP become delocalized and the material turns into metalized. Mainly Ru 4d states contribute to the DOS at the Fermi level along with little contribution from La 5d and As 2p states. The DOS at the Fermi level is seen to be 5.4 states/cell/eV. The large value of DOS is a sign of high metallic conductivity of LaRu2As2. The valence bands situated in the lowest energy range of -17.9 to -16.3 eV arises entirely from La 5p states. These valence bands are observed to be separated by a wide prohibited energy gap of ~3.3 eV form the next valence bands lying in the energy range from -13.0 eV to -10.6 eV. These valence bands are again separated by a forbidden energy gap of width 3.4 eV from highest valence bands. The highest valence bands consist of three distinct peaks and broaden in the energy range from -6.6 eV to Fermi level. It is observed that the left peak is formed almost entirely by As 4p and Ru 4d states. The middle and highest peak are due to equal contribution from Ru 4d and As 4p states. The first peak near to Fermi level consists of Ru 4d states. It is observable that within the range from -6.6 eV to EF, the covalent interactions occur between the comprising atoms. This is due to the reason that the states are truly degenerate regarding angular momentum and lattice site. Besides, some ionic character is expected simply for the difference in the value of electronegativity among the constituent elements. 3.5 Mulliken population and Vickers hardness Mulliken population analysis describes the allocation of electrons in several fractional methods among the various parts of bonds and overlap population provides relations with covalency or ionicity of bonding and bond strength. Population analysis within CASTEP module is accomplished by using a projection of the plane wave states onto a localized basis using a method proposed by SanchezPortal et al. [36]. Population analysis of the resulting projected states is then carried out by applying the Mulliken scheme [37]. This technique is extensively used in analysis of electronic structure calculations using Linear Combination of Atomic Orbitals (LCAO) basis sets. Mulliken scheme provides two fundamental quantities in relation to atomic bond population are the effective charge and the bond order values between a pair of bonding atoms using minimal basis within the Mulliken formalism [37,38] as follows:

where i, j denote the orbital quantum numbers, n indicates the band index, as well as are the eigenvector coefficients of the wave function and is the overlap matrix between atoms and . The effective valence charge and Mulliken atomic population are two important parameters to describe the bonding character in solids. The difference between the formal ionic charge and Mulliken charge on the anion species in the crystal defines the effective valence and measures the dominance of covalency and ionicity on chemical bonding. An ideal ionic bond occurs if zero effective valences exist. On the other hand, when the effective valence carries a value greater than zero the level of covalency is found to be increased. The effective valence given in the Table 4 would be a good sign of prominent covalency in bonding within the new ternary 122 phase LaRu2As2. Table 5 also lists the calculated bond overlap populations between nearest neighbors in this crystal. The overlap population with nearly zero values means that the interaction between the electronic populations of the two atoms is not worth mentioning. A bond with small Mulliken population is really weak and which makes a negligible contribution to materials hardness. The low overlap population indicates a high degree of ionicity, whereas the high value implies that the level of covalency is high in the chemical bond. Positive and negative bond overlap populations lead to bonding and antibonding states, respectively.

Table 4. Population analysis of LaRu2As2. Mulliken Atomic populations Species s p d Total As Ru La

1.57 2.52 1.92

3.31 6.89 6.01

0.00 7.36 1.77

4.88 16.76 9.71

Charge (e) 0.12 –0.76 1.29

Effective valence Charge (e) -3.76 1.71

Table. 5. Calculated bond and total Vickers hardness H v , Hv (in GPa) of LaRu2As2 along with bond number n , bond length d (Å), bond volume 3 vb (Å ) and bond and metallic populations P , P . Bond

n

d

P

P

vb

Hv

Hv

Ru–Ru Ru–La

2 8

2.96437 3.43822

0.98 1.47

0.000115 0.000115

13.23 20.64

9.8 7.0

7.5

Hardness assesses the ability of a material to refuse to accept the plastic deformation. The amount of force per unit area acted opposite to plastic deformation takes part in estimating the hardness of a material. A formula is developed for calculating the hardness of non-metallic materials with Mulliken population in first-principles method [39]. This method is not suitable for compounds having partial metallic bonding because metallic bonding is delocalized and no direct relation with hardness is observed [40]. Gou et al. [41] formulate an equation considering a correction for metallic bonding to calculate the bond hardness of mixed metallic crystals as follows:

where Mulliken overlap population of the -type bond is presented by , is a symbol for metallic population and is calculated by using the unit cell volume V and the number of free electrons in a cell as , and stands for the volume of a bond of -type, which can be calculated from the bond length of type and the number of bonds of type per unit volume through . For the complex multiband crystals the hardness can be calculated as a geometric average of all bond harnesses as follows [42,43]: H V = [ ( H v ) n ]1 /

n

where n is the number of bond of type composing the actual multiband crystal. The calculated Vickers hardness considering the positive as well as reasonable populations between nearest neighbors for the new compound LaRu2As2 is given in Table 5. The hardness value of 7.5 GPa for LaRu2As2 is very less than that of diamond (96 GPa), which is the hardest material known to date. Therefore, LaRu2As2 is a soft material and it should be easily mechinable. 3.6 Electron charge density and Fermi surface To visualize the nature of chemical bonding in LaRu2As2, the electron charge density distribution is calculated and the contour of electron charge density is presented in Fig. 3. In the adjacent scale, the blue and red colors signify the low and high electron density, respectively. Atom with large electronegativity (electronic charge) exerts a pull on electron density towards itself [44]. Because of large difference in electronegativity and radius of atoms, the electronic charges around Ru (2.20) and As (2.18) are greater than that around La (1.10). The higher electronegativity of Ru and As exhibits strong accumulation of electronic charge while relatively low charge density indicates weak charge accumulation of the other element. It will be suitable to start with a typical ionic picture depending on the standard oxidation numbers of atoms: La2+, Ru2+ and As3. After that, the charge states of the sheets/blocks are [La]2+ and [(Ru2+)2(As3-)2]2-; specifically, the charge transfer takes place from La2+ sheets to [Ru2As2]2- blocks. Moreover, within the [Ru2As2] blocks, ionic bonding occurs between RuAs atoms. The nature of covalent interaction, i.e. the formation of Ru-As bonds due to the

hybridization of Ru 4d-As 4p states, is also visible from atom resolved DOSs, Fig. 2. Besides, inside the [Ru2As2] blocks metallic-like Ru-Ru bonding arise owing to overlapping of the near- Fermi Ru 4d states [45]. Finally, it is noticeable that the As-As bonds do not exist between the adjacent two [Ru2As2] blocks - contrary to strong As-As interactions in (Ca,Ba)Fe2As2. It possibly will be clarified by the difference in the strength of Ru-As and Fe-As bonds, which controls the formation of As-As interactions [46]. Therefore, the intra-atomic bonding in LaRu2As2 can be described as a highanisotropic combination of ionic, covalent and metallic interactions. La

La Ru

La

La Ru

Ru As

As

As

As

As

As

As

Ru La

Ru

Ru La

c-axis

As

La

La

e/Å3

As

Ru

As

As

As Ru

La

Ru

As

As

As

As Ru

Ru La

Ru

La

La

8.590 7.642 6.694 5.745 4.797 3.849 2.900 1.952 1.004 0.055

Fig. 3. Charge density (left) and Fermi surface (right) of LaRu2As2.

The Fermi surface topology has been calculated, which is presented in Fig. 3 (right). As we observe, the near-Fermi surface band pictures have a complicated „mixed‟ character: simultaneously with quasi-flat bands and high-dispersive bands intersects the Fermi level. These features concede a multi-sheet Fermi surface consisting of two quasi-two-dimensional hole-like sheets in the corners of the Brillouin zone. The nearest sheet is also parallel to A-M direction and the distant sheet is half-fold and intersected at the centre of the sheet by the Γ-M line. Between these two sheets a concave sheet appears and the Γ-M line serves as the axis of this tree-dimensional sheet. The central electron-like sheet is very complicated and has four wings shaped of open half-tube cutting along axis. These four wings form a red-cross shape keeping each arm along Z-R direction. The existence of the multi-band nature suggests that the new compound LaRu2As2 should be a class of multiple-gap superconductor. 4. Conclusion DFT calculations performed within the first-principles pseudopotential method to calculate some physical properties of newly discovered superconductor LaRu2As2. The calculated lattice constants show sound agreement with the experimental results. The calculated single crystal elastic constants obey the mechanical stability conditions for the new tetragonal system. The crystal LaRu2As2 is predicted to be very much incompressible under uniaxial stress along the directions a than that along c. The newly synthesized compound should be more competent to resist the shear distortion in the direction than in the (100) plane. Cauchy pressure and Poisson‟s as well as Pugh‟s ratios propose that LaRu2As2 should be a ductile material. The young‟s modulus of small value indicates that LaRu2As2 should attain reasonable resistance to thermal shock. The low Debye temperature also suggests that this compound should be favorable for use as a thermal barrier coating material. The calculated electronic structures reveal that the intra-atomic bonding in new 122 phase compound LaRu2As2 may be explained as a mixture of ionic, covalent and metallic interactions. The hardness

value of 7.5 GPa implies that this material should be soft and easily mechinable. The obtained Fermi surface exhibits the multi-band nature, suggesting that the new ternary compound LaRu2As2 should be a kind of multiple-gap superconductor. Finally, it is expected that these findings will stimulate much activities in research for experimental elastic properties, Debye temperature and Vickers hardness for newly synthesized LaRu2As2.

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