First-principles calculations to investigate structural, electronic, half-metallic and thermodynamic properties of hexagonal UX2O6 (X=Cr,V) compounds

SaadiBerri^ab

^a

Laboratory for Developing New Materials and Their Characterizations, University of Setif 1, Algeria

^b

Department of Physics, Faculty of Science, University of M'sila, Algeria

Received 12 April 2019, Revised 18 May 2019, Accepted 26 May 2019, Available online 3 June 2019.

Abstract

Full potential linearized augmented plane wave plus local orbital's (FP-LAPW + LO) method within density functional theory (DFT) is used to investigate the structural, electronic and half-metallic properties of hexagonal UX₂O₆ (X = Cr,V). Features such as the lattice constant (a and c), bulk modulus and its pressure derivative are reported. The calculated lattice parameters are in good agreement with available experimental results. Band structure and overall densities of states have proved UV₂O₆ as an indirect half-metallic material with a band gap of 2.88 eV and UCr₂O₆ as a magnetic semiconductor. The results obtained, make the hexagonal UX₂O₆ a candidate material for future spintronic applications. Based on the quasi-harmonic Debye model, the thermodynamic properties of the material in question have been predicted taking into account of the lattice vibrations. The variation of the lattice constant, bulk modulus and heat capacity as a function of pressure in the range 0–40 GPa and temperatures of 0–1500 K is computed. Our findings show that external effects are highly effective in tuning some of the macroscopic properties of the compounds under study.

2. Method of calculations

As mentioned already, we have considered the experimental crystal parameters as reported by Kovba [36], Hoekstra and Siegel [37]. UX₂O₆ (X = V, Cr) compounds crystallize in the hexagonal space group P-31m (No.162), Z = 1. The crystal structures of UX₂O₆ (X = V, Cr) compounds is shown in Fig. 1. The present computations are performed through the FP-LAPW + LO method using DFT as implemented in WIEN2K code [38]. In the study of structural properties, the exchange correlation energy is treated within the GGA as parameterized by Perdew, Burk and Emzerhop Perdew (PBE)-GGA method [39]. The threshold energy between valence and core states is set to be −6.0 Ry. Here, the Kohn–Sham equations are solved by expanding the wave functions in the spherical harmonics form inside the atom spheres. A plane wave expansion has been used in the interstitial regions of atoms inside the unit cell. We have used lmax = 10 for angular momentum expansion and R_MTK_max = 8 as a plane wave cut-off with 1400 k points for hexagonal phase. Here R_MT is the average muffin-tin (MT) radius and K_max is the wave function cut-off. The radii R_MT of the muffin tins (MT) are chosen to be approximately proportional to the corresponding ionic radii. The energy between successive iterations is converged to 0.0001 Ry and forces are minimized to 1 mRy Bohr⁻¹. The 5f(U) and 3d (V and Cr) was treated using the GGA + U approach [40]. The GGA + U calculations used an effective parameter U_eff = U + J, where U is the Hubbard parameter and J is the exchange parameter. As a matter of fact, the use of the Hubbard parameter (GGA + U) approaches so as to treat the exchange-correlation potential is very efficient for studying strongly correlated electrons where the energy band gap of the material of interest can be evaluated more accurately. In these cases, the core electrons are taken to be relativistic whereas the valence electrons are considered to be as semi-relativistic. This is probably best suited for our system and for a full potential method. The U_eff is taken to be 5.01 eV and 4.97 eV for U(5f) and X(3d) atoms similarly to Refs [41], [42], respectively.

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Fig. 1. Crystal structure for UX₂O₆.


https://www.sciencedirect.com/science/article/pii/S2468217919301273