Ab initio full-potential study of mechanical properties and magnetic phase stability of rare earth diboride compounds

Abstract

We study the structures and magnetic phase stability of rare earth diboride compounds, RB2, using density functional simulations within the local density approximation. At zero pressure the hexagonal (P6/mmm) structure is energetically stable and at high pressure these materials prefer to keep the same structure. The five different elastic constants, bulk modulus, shear modulus, and hardness of all the hexagonal compounds have been calculated. The pressure dependence of the volumes is determined. The calculated large bulk modulus and high hardness reveal that they are incompressible and hard materials. The structural parameters and magnetic phase stability for RB2 (R = Tb, Dy, Ho, Er, Tm, and Yb) compare quite well with experimental results. For the others rare earth materials, our results are predictions.

Ab-initio studies of some rare-earth borides: CeB2, PrB2, NdB2, and PmB2

“…Ourr esults of PBESol calculations are higher (about 2%)than the PBE calculation. The calculated elastic constants for AlB 2 structure are lower/higher than the other theoretical results [14]. We attribute this discrepancy to the difference in the lattice parameters used in our calculations and given in Ref.…”

“…We attribute this discrepancy to the difference in the lattice parameters used in our calculations and given in Ref. [ 14]. It is, also, widely known that the main reasonfor adiscrepancy in the calculated lattice parameters, on the fundamental level, may arise from differentcalculation techniques, pseudopotential or exchange-correlation functional used in calculations.…”

“…The total energy was minimized by relaxing the structural parameters using ac onjugateg radient optimization method. The obtained lattice parameters, bond distances are listed in [14], the calculated lattice parameters for considered compounds with AlB 2 structure are higher (about 6-8%)t han the other theoretical results [14]. We notet hat the aniona toms are the same in these compounds, the cation sizes are different.…”

“…The calculated bulk modulus, shear modulus, Young's modulus, and Poisson's ratio are giveninTable 4t ogether with other theoretical values [14]. The values of the bulk moduliindicate that PmB 2 in the AlB 2 is the least compressible material.The shear modulus, ameasurement of resistance to shape change, is more pertinent to hardness and the larger shear modulus is mainly due to its larger C 44 .…”

“…They possess goodemissive properties and are used as cathode materials in emission electronics such as electron microscopes, Auger spectrometers, and microanalyzers [13] R B 2 ( R =Ce, Pr, Nd, Pm) compounds belong to the family of rare-earth borides and their physical properties are not sufficiently known so far. Zazoua et al [14] have investigated mechanical properties and magnetic phase stability of these compounds. The electronic structure and magnetic behaviorofhexagonal R B 2 have been studied using ab-initio density functional theory [15].…”