In this article, we have studied the mechanical, electronic, and optical features of ThCr2Si2- type compound SrCo2Si2. The investigation has been done by using the first-principles method depend on the density functional theory (DFT) and the calculations were completed with the Cambridge Serial Total Energy Package (CASTEP) code. The optimized lattice parameters are well in accord with the existing synthesized values. The investigated elastic constants for this compound are positive which ensured the mechanical stability of this phase. The calculated values of Pughs ratio and Poissons ratio ensure the brittle character of SrCo2Si2. The universal anisotropic constant AU ensures the anisotropic behavior of SrCo2Si2.The softness nature of SrCo2Si2 is confirmed by the bulk modulus calculations. The overlapping of the valence band and conduction band near the Fermi level indicates the metallic nature of SrCo2Si2. At the Fermi level, the major contribution comes from Co-3d and Si-3p states. The large reflectivity in the high-energy region indicates that this compound might be useful as coating materials for reducing solar heating. The photoconductivity and absorption begin with zero photon energy which also ensures the metallic nature of SrCo2Si2.
ThCr2Si2 type ternary intermetallic materials usually hold a superconducting ground position. The rare-earth AM2X2 structural materials have received great interest of researchers because of their many rich characteristics. Recently AM2X2 (where, A is a lanthanide element or any alkaline earth element; M is any transition metal; X = P, Ge, Si or As) type compounds have achieved great interest having their many interesting features such as mixed valency, superconductivity at both high and low temperature, valence fluctuation and heavy fermions behavior (Stewart, 2001). These types of transition metal with AM2X2 type structure confirm extremely good-looking and wealthy physics in view of the fact that of their close energies relating to the spin, charge and orbital motion (Imada et al., 1998). There are more than two thousand classes of ternary inter met-allic compounds (Villars and Genzual, 2007) which are essentially obtained from the BaAl4 type stru-ctures. Among these classes ThCr2Si2 type com-pounds were first discovered and illustrated in 1965 by Ban and Sikirica, (1965). A complete and accept-able geometric assessment of about six hundred phases of ThCr2Si2- type structures are represented by Just and Paufler, (1996).
In recent times ThCr2Si2- type structure has gained massive consideration of researchers after disco-vering a new superconductor (Ba0.6K0.4) Fe2As2 be-longs to the “122” family of iron-arsenides with ThCr2Si2 type structure exhibits high transition tem-perature 38K (Rotter et al., 2008). On the other hand, Pt, Ni and Pd-based ThCr2Si2 type boro-carbides (Nagarajan et al., 1994; Cava et al., 1994; Batlogg et al., 1994) have been discovered in the recent years with the transition temperature up to 23K which raises the hope to constitute a family of new high temperature superconductors. In iron-based compounds, the ternary intermetallic (122-type compounds) in the company of ThCr2Si2 type structure as example AFe2As2 (A = Sr, Ca, Ba, etc.) are free from oxygen having metallic nature (Rotter et al., 2008; Sasmal et al., 2008; Torikachvili et al., 2008). These types of compounds have been com-prehensively studied for interpreting their super-conducting mechanism (Johnston 2010; Stewart 2011). These types of compounds have high chemi-cal flexibility and abundance of substitution possi-bilities. In recent times concentration was given to a series of the iron based and arsenic-free AeT2X2 compounds (Ae = alkaline earth metal, T = Ni, Pd; X = P, Ge) with very low transition temperatures (Tc ~ 0.3 - 3.0 K) (Mine et al., 2008). Undoped RT2Si2 (R = La, Y, Th; T = Ir, Pt) type superconductors are predicted to CaBe2Ge2-type structures (Yang et al., 2011). However the compounds with alkaline earth metals having ThCr2Si2- type structure are inade-quate (Ronning et al., 2009; Fujii and Sato, 2009; Shelton et al., 1984; Doerrscheidt et al., 1976; Rieger and Parthé, 1969; Bodak and Gladyshevskii, 1968; Palenzona et al., 1987).
In this work we have studied a new phase with alka-line earth metal on the A-site crystallizing in the ThCr2Si2- type structure are inadequate. The com-pound SrCo2Si2 signifies the third ternary SrT2Si2 compound (T = 3d-block transition metals) besides SrCu2Si2 (Kranenberg et al., 2002) and SrZn2Si2 (May and Schäfer, 1972). For SrT2Si2 the isostru-ctural compounds with T = Pd, Ag have been chara-cterized and reported (Eisenmann et al., 1970). The compound SrCo2Si2 is isoelectronic to the parent Fe-pnictide superconductors AeFe2As2 at the X site, contrast to their electronic bonding situation will be of special interest. Here we have studied the detailed physical properties of Co-based material SrCo2Si2 by using the DFT based calculations implemented in CASTEP code.
2. Computational details
The CASTEP code (Segall et al., 2002) written by FORTRAN 95 language is used to investigate the physical properties of SrCo2Si2. The calculations were done by DFT theory within GGA with the PBE exchange-correlation function (Clark et al., 2005; Materials Studio CASTEP, 2010; Hohenberg and Khon, 1964; Perdew et al., 2008). The pseudo atomic calculations were done for Sr-4s2 4p6 5s2, Si-3s2 3p2 and Co-3d7 4s2 valence electrons. The plane wave cut-off energy was set to 500 eV. The special k-point sampling of the Brillouin zone (BZ) was employed by using the Monkhorst-Pack method (Monkhorst and Pack, 1976) with special 10×10×10 grid points in the primitive cell of SrCo2Si2. The crystal structure of SrCo2Si2 was optimized by the Broyden-Fletcher-Goldfarb-Shanno (BFGS) minimi-zation technique (Pfrommer et al., 1997). For this optimization the criteria of convergence were set to 1.0×10-5 eV/atom for energy, 0.03 eV/Å for force, 0.05 Gpa for stress and 0.001 Å for ionic displacement. The elastic stiffness constants of SrCo2Si2 were obtained by the stress-strain method (Fan et al., 2006). Then the bulk properties were obtained by the elastic constant data of SrCo2Si2. In that case the criteria of convergence tolerance were set to 2.0×10-6 eV/atom for energy, 2.0×10-4 Å for maximum ionic displacement, 6.0×10-3 eV/Å for maximum ionic force and 0.1 GPA for maximum stress component. The maximum strain amplitude was set to be 0.003 in the present calculation of SrCo2Si2.
3.1 Structural properties - At normal temperature and pressure, SrCo2Si2 possesses a tetragonal crystal structure with the space group of I4/mmm (no.139) (Hoffmann et al., 2012). The conventional and opti-mized crystal structures of SrCo2Si2 are shown in Fig 1. The unit cell contains two formula units (Z=2) with ten atoms that means one formula unit for each primitive cell with five atoms. The atomic position of Sr, Co, and Si in the unit cell of SrCo2Si2 tetra-gonal crystal are 2a (0 0 0), 4d (0 0.5, 0.25) and 4e (0 0 0.3606) respectively.
Table 1: The calculated equilibrium lattice parameters, unit cell volume, bulk modulus of ThCr2Si2-type SrCo2Si2 compound in comparison with available experimental data.
The unit cell dimensions including equilibrium lat-tice parameters for tetragonal phase a0 and c0, bulk modulus B0 and the equilibrium cell volume V0 of SrCo2Si2 intermetallics at ambient temperature are charted in Table 1 with the experimentally evaluated values. From Table 1 it is obvious that the calcu-lated lattice parameters are exceedingly close to the experimental data which ensure the dependability of the DFT- based investigations. From Table 1 we have seen that, our calculated lattice parameters are slightly deviated from the experimental results. The motive is due to the temperature dependence of the lattice parameters and GGA route (Zhu et al., 2016).
3.2 Elastic properties - Elastic constants are very vital parameters which help us by providing the in-formation about the nature of force present in a solid material. A proper explanation about the mechanical and dynamical behavior of crystalline solid is pro-vided by the analysis of elastic constants. These pro-perties also ensure the mechanical stability, rigidity and ductile/brittle nature of a solid material (Gole-sorkhtabar et al., 2013; Koç et al., 2012). Different important properties of solid materials such as ducti-lity, anisotropy, stiffness, brittleness and stability can be derived from the elastic constant data (Rahaman et al., 2016). Hence in this article a thorough investi-gation into the mechanical nature of SrCo2Si2 has been done with accurate discussion and composition.
The elastic constants were achieved from a linear fit of the calculated stress-strain function according to Hooks law (Nye, 1961). A crystal with the tetra-gonal phase belongs to six independent elastic con-stants (C11, C12, C13, C33, C44 and C66).The estimated elastic constants of SrCo2Si2 are listed in Table 2. According to the stability criteria (Pokunov et al., 2004) of tetragonal phase (Eq. 1) the com-pounds under consideration have good stability in nature.
C11>0, C33>0, C66>0, C44>0
C11+C33-2C13>0, C11-C12>0
2(C11+C12) +4C13+C33>0 (1)
Table 2: The evaluated elastic constants Cij (in GPa) of SrCo2Si2 with similar type of compounds
By utilizing the evaluated data of Cij, the most important mechanical features such as bulk modulus B, shear modulus G, Youngs modulus Y, anisotropy factor A and Poissons ratio ν of intermetallic SrCo2Si2 have been calculated by using the Voigt-Reuss-Hill (VRH) averaging scheme (Hill, 1952). Which are listed in Table 3. The Voigt and Reuss bounds of B and G for cubic systems can be represented by the following expressions.
B_v=(〖2C〗_11+〖2C〗_12+C_33+〖4C〗_13)/9 (2)
B_R=C^2⁄(M ) (3)
G_v=(M+〖3C〗_11-〖3C〗_12+〖12C〗_44+〖6C〗_66 )⁄30 (4)
G_R=15/([〖18B〗_v/C^2 + 6/((C_(11-) C_12 ) ) +6/〖 C〗_44 + 3/C_66 ]) (5)
Where, M and C2 can be written as,
M = C11+C12+2C33-4C13 and C2= (C11+C12) C33-2C132
The arithmetic mean value of the Voigt (BV, GV) and the Reuss (BR, GR) bounds which is used to calculate the polycrystalline modulus is given by in terms of Voigt-Reuss-Hill approximations:
BH = B= 1/2 (B_R+ B_v ) (6)
GH =G= 1/2 (G_v+ G_R ) (7)
Using the following expressions we have also cal-culated the Youngs modulus (Y) and Poissons ratio (ν),
Y= 9GB/(3B+G) (8)
ν= (3B-2G)/(2(3B+G)) (9)
The Youngs modulus is specified by the ratio of the tensile stress to tensile strain, which measure the stiffness for solid material. The larger value of Y point outs the more stiffness of a compound (Chen, et al., 2011). The higher value makes the solid better stiffer. The calculated Youngs modulus is shown in Table 3 along with available similar type of com-pounds. From Table 3, we can say that the value of Youngs modulus of SrCo2Si2 is larger than SrRu2As2 and SrRh2Ge2 compounds indicating that the compound SrCo2Si2 is stiffer than SrRu2As2 and SrRh2Ge2 compounds.
The Poissons ratio is another useful parameter to understand the nature of bonding force in a material (Cao et al., 2013). The smaller value of ν (ν = 0.1) indicates the covalent materials whereas for ionic crystal ν = 0.25. The larger value of Poissons ratio (ν>0.26) indicates that the compound will be ductile and the compound will be brittle when the value of Poissons ratio is (ν<0.26). From Table 3, we see that the value of ν is 0.18 which refers the brittle nature of SrCo2Si2. The ratio between bulk and shear modulus (B/G) is known as Pughs ratio which is applied to understand the brittleness and ductility manner of solid material (Pugh, 1954). According to Pughs criteria a material should be brittle if its B/G < 1.75, otherwise it should be ductile. From our calculations we see that B/G < 1.75, hence the material SrCo2Si2 shows brittle manner which is very similar to Poissons ratio.
Table 3: Evaluated polycrystalline bulk modulus B (GPa), shear modulus G (GPa), Youngs modulus E (GPa), B/G value, Poissons ratio v and elastic anisotropy AU of SrCo2Si2 with similar types of compounds.
The universal anisotropic factor of a solid material is specified by the subsequent relation (Ranganathan, et al., 2008).
A^U=〖5G〗_v/G_R +B_v/B_R -6 (10)
AU = 0 indicates completely isotropic crystal and the deviation from this value shows the degree of aniso-tropy in a material. Chung and Buessen suggests two new relations (Chung and Buessem, 1967) to deter-mine the anisotropy indexes of bulk modulus and shear modulus given as follows,
A_B=((B_v-B_R))/(〖(B〗_v+B_R)) (11)
A_G=((G_v-G_R))/(〖(G〗_v+G_R)) (12)
For an isotropic crystal the value of A is 1 and for anisotropic crystal the values of A are either smaller or greater than unity. From Table 3 we see that the value of A is less than unity which represents the anisotropic nature of this compound.
3.3 Electronic properties - The band structure and density of states (TDOS and PDOS) provide a clear concept about the electronic properties of a material. The electronic band structure provides vital infor-mation about a material to be metal, semiconductor or insulator. The bonding features of a material are obtained from the partial and total density of states calculations (Hu, et al., 2014). The full picture of energy bands and band gaps of a solid is known as electronic band structure or simply band structure. In solid-state and condensed matter physics, the band structure defines certain ranges of energy that are allowed for electrons within a solid, and the ranges of energy that are not allowed for any electrons. The investigated band structure for SrCo2Si2 has been illustrated in Fig 2 in the energy range -10 eV to 10 eV which is observed along the high symmetry directions in the first Brillouin zone. The horizontal solid line at 0 eV indicates the Fermi level. From band structure it has seen that the valence bands and conduction bands are overlapped at Fermi level and there is no band gap indicating that this compound shows metallic manner. The metallic nature of SrCo2Si2 signifies that this compound might be super-conductor. The partial and total density of states of SrCo2Si2 is plotted in Fig 3. From Fig 3 we have observed that the total density of states (TDOS) of SrCo2Si2 is composed of four main peaks.
The first peak in the valence band lies between -36.17 eV and -34.75 eV. In SrCo2Si2, Sr-5s states contribute the most to create the first peak. The second peak lies between -18.63 eV and -16.93 eV in SrCo2Si2. This peak is dominated by Co-3d and Si-3p states. The third peak lies between -11.42 eV and -7.46 eV. This peak is contributed by Si-3p states. The fourth peak lies from -6.32 eV to 8.65 eV. This peak is dominated by Si-3s and Si-3p states. We observe clear coincidence between the Co-3d and Si-3p states in SrCo2Si2, which suggests the covalent nature of Co-Si bonds in SrCo2Si2 (Rahman et al., 2016). This is a common feature of ThCr2Si2 type compounds (Jeitschko et al., 1987). The calculated DOS at EF is 3.43 states/ eV-unit cell.
3.4 Optical properties - The study of photon energy dependent optical function of a solid material is so essential due to the fact that it helps to get a clear conception concerning the electronic configuration of materials. The optical properties of SrCo2Si2 with different photon energies are calculated by the frequency dependent dielectric function, ε(ω)= ε_1 (ω)+〖iε〗_2 (ω), which is closely correlated to the electronic configurations. The imaginary part ε_2 (ω) of dielectric function is obtained from the momen-tum matrix elements between the filled and the unfilled electronic state by utilizing the subsequent relation (Materials Studio CASTEP, 2010);
shows promises as good was coating materials in the ultraviolet region.
3.4.2 Absorption Coefficient - The absorption co-
The different physical features such as mechanical, electronic and optical properties of intermetallic SrCo2Si2 have been successively investigated by DFT simulation. The investigated optimized struc-tural parameters are well accord with the available synthesized data. The calculated elastic constants have maintained the born stability criteria which en-sure the theoretical mechanical stability of SrCo2Si2. The calculated values of Pughs ratio (B/G) and Poissons ratio ensure the brittle nature of SrCo2Si2. The stiffer behavior of this phase is ensured by Youngs modulus calculation. The analysis of uni-versal anisotropic factor ensured the anisotropic nature of SrCo2Si2.The calculated band structure shows the metallic nature and major the part arrives from the Sr-4p states at Fermi level. High reflectivity is observed in the ultraviolet region energy site which ensure about the use of SrCo2Si2 as a good coating material at ultraviolet energy region. The absorption quality is good in the ultraviolet region and high refractive index in the infrared region. This result ensured that this compound is promising for absorbing materials in the UV region. The effective plasma frequency of SrCo2Si2 is found to 14.42 eV which ensures that this material becomes transparent when the frequency of the incident photon is higher than 14.42 eV.
Thanks to the Physics Department of Pabna Uni-versity of Science and Technology for giving me opportunity to complete my research work.
The authors declared that there is no conflict of interest in this article.
Academic Editor
Dr. Wiyanti Fransisca Simanullang Assistant Professor Department of Chemical Engineering Universitas Katolik Widya Mandala Surabaya East Java, Indonesia.
Assistant Professor, Departments of Physics, Pabna University of Science and Technology, Pabna-6600, Bangladesh.
Islam MS, Rahman MA, and Farjana N. (2021). The physical properties of ThCr2Si2- type co-based compound SrCo2Si2: an ab-initio study, Int. J. Mat. Math. Sci., 3(3), 50-59. https://doi.org/10.34104/ijmms.021.050059