J. Nig. Soc. Phys. Sci. 5 (2023) 1029 Journal of the Nigerian Society of Physical Sciences First Principles calculation of Half metallic proprieties of QCrAs (Q=Hf, Ti and Zr) M. I. Babalola∗, B. E. Iyorzo, S. O. Ebuwa Department of Physics, University of Benin, Nigeria Abstract The structural, electrical, magnetic, mechanical, and thermodynamic properties of some novel half-Heusler alloys QCrAs(Q=Hf, Ti and Zr) are investigated using first principles calculations. The results show that the three half Heusler alloys are half metals and they can find application in spintronics industries. They possess magnetic moment of 3µB. The mechanical properties shows that they are mechanically stable. The B/G ratio of the three half-Heusler alloys show that they are ductile in nature and the Poisson’s ratio reveal that the plasticity of TiCrAs and ZrCrAs are higher than that of HfCrAs. The Debye temperature and average sound velocity of ZrCrAs is observed to be higher than the other two alloys. This implies that the thermal conductivity of ZrCrAs is the highest. DOI:10.46481/jnsps.2023.1029 Keywords: Half Heusler, Half-metallic gap, Electronic band structure, Mechanical properties Article History : Received: 03 September 2022 Received in revised form: 01 November 2022 Accepted for publication: 08 November 2022 Published: 21 January 2023 c© 2023 The Author(s). Published by the Nigerian Society of Physical Sciences under the terms of the Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Communicated by: K. Sakthipandi 1. Introduction For many years now, the search for functional materials has been on the increase in the scientific community. Examples of functional materials include thermoelectric materials [1], piezo- electric materials [2], optoelectric materials [3], spintronic ma- terials [4] etc. Of all materials that have been found and syn- thesized till date, the Heusler alloys and its family have been one of the most widely studied material [5-7], this is because they can be easily synthesized and have multifunctional capa- bility which are very useful in technological industries. The Heusler family with this multifunctional capabilities include the full Heusler alloy [8], quaternary Heusler alloy [9], half Heusler ∗Corresponding author tel. no: +234 8060797955 Email address: michael.babalola@uniben.edu (M. I. Babalola) alloy [10], binary Heusler alloy [11], and inverse Heusler al- loy [12]. Interestingly, different characteristics of these Heusler family can be predicted just by knowing their valence electron count [13]. Some years back, Gautier and coworkers predicted over 300 half Heusler alloys [14], ever since then, more half Heusler alloys have been investigated till date. Half Heusler alloys having valence electrons greater or less than 18 can be said to be ferromagnetic, hence such half Heusler allloys could possess half-metallic properties which is useful in spintronics industries. One of the properties sough after in spintronics is the half-metallic property. The half-metallic property is de- scribed as a situation whereby a material having metallic na- ture in one spin channel and having semiconducting proper- ties in another spin channel. This concept was discovered by De Groot and co-workers in 1983 [15]. Half-metals possess 1 Babalola et al. / J. Nig. Soc. Phys. Sci. 5 (2023) 1029 2 100% spin-polarization at the Fermi level, thereby making it possible for them to be used as a spin valve to enhance gi- ant magneto-resistance [16], spin injector electrode in tunnel magneto-resistance [17] and current perpendicular to plane gi- ant magneto-resistance (CPP-GMR) in spintronics [18]. They also find application in spin torque devices [19] and magnetic tunnel junction (MTJ) devices [20]. Half-Heusler alloys UVW(XYZ) with half-metallic character are usually described as having the U atomic position occupied by one of the fol- lowings: main group 1 or 2 element, the rare earth metals and a transition metal [13]. The V atomic position is occupied a trans- action metal which is less electropositive compared to U. The W atomic position is occupied by elements from a main group 3, 4 and 5. In this work, we use ab initio calculation to explore the half-metallic, mechanical and thermodynamic properties of the novel half Heusler alloys QCrAs (Q=Hf, Ti and Zr). The re- maining portion of this work is divided into the following sec- tions: Section 2 covers the computational specifics, Section 3 covers the discussion and results, and Section 4 concludes with a summary. 2. Computational details First principles spin-polarized density functional theory (SP- DFT) calculation has been used to perform the ground state properties of QCrAs(Q=Hf, Ti and Zr) alloys. A projected augmented wavefunction (PAW) type of the generalized gra- dient approximation (GGA) which is the choice of exchange correlation as implemented in the quantum espresso code[21] is used. The valence electron configurations of Hf(4f146s25d2), Ti(4s23d2),Cr(3d54s1), As(4d25s2) and Zr(5s24d2) are used. An optimized value of 70Ry for the plane-wave basis set of the ki- netic energy cutoff and a 9×9×9 k-point mesh are used to deter- mine the structural parameters for the three half Heusler alloys. Optimizaion was carried out for various values of k-point start- ing with 4X4X4, 5X5X5, . . . 15X15X15. At the end of the cal- culation, the difference between the last k-point and the others were observed. The difference between the energies of k-point 9X9X9 and the last k-point fell within the acceptable range of 0.1meV.Convergence threshold for all selfconsistency calcula- tion is set at 10−6 Ry/atom. Spin-polarised was taken care of by introducing nspin which activates the magnetism in the com- pounds. Magnetic spins were allocated to the atoms of the tran- sition metals using starting magnetization. Duriing band calcu- lation, we used spin component to distinguish between spin up and spin down components. The thermo pw package is used to compute the thermodynamic and mechanical properties [22]. 3. Results and discussion 3.1. Structural properties A cubic structure with MgAgAs type C1b structure forms during the crystallization of half Heusler alloys (UVW). They have space group of F-43m (No 216). The half Heusler stuc- ture is seen as a combination of zincblende and rocksalt struc- ture. Wyckoff locations 4b(1/2,1/2,1/2), 4c(1/4,1/4,1/4), and 4a Figure 1. Unit cell of QCrAs (Q=Ti, Hf and Zr) HH alloys (0,0,0) are occupied by the U, V, and W atoms, respectively. The unit cell of the half-Heusler alloy is shown in Fig. 1. The structural parameters such as the equilibrum lattice constant, bulk modulus and pressure derivative are computed by fitting the total energies versus lattice constant curve with the Mur- naghan equation of state with the results presented in Table 1. Fig. 2 shows the ferromagnetic state and non magnetic state of the three half-Heusler alloys, and from the graph, their fer- romagnetic state posses the lowest ground state energies. This implies that the three half-Heusler alloys are ferromagnetic in nature. The lattice constant of the three HH alloys satisfy the condition Ti0, C44 >0, C11 >C12 and C11+2C12 >0, C11- C12 >0. Once this criteria is satisfied then the material is said to be mechanically stable. According to the results in Table 2, ZrCrAs has the strongest resistance to linear and shear deforma- tion when compared to TiCrAs and HfCrAs. The B/G ratio of the three half-Heusler alloys are greater than the critical value of 1.75, it means the three alloys are ductile. The Poisson’s ratio determines a compound’s plasticity. The critical value for met- als and their alloys is 0