The structural, electronic, and magnetic properties of MnAs crystal are studied. The WIEN2k code which uses a full-potential LAPW program based on density functional theory with GGA is used for the calculations. At first, the total energy of a MnAs crystal in different lattices is calculated and the corresponding - diagram is drawn for two different structures of MnAs. The effect of pressuring this crystal is determined. The calculations confirm that, MnAs has the NiAs-type structure at ambient pressure but transforms into the zinc-blend structure of a specific pressure value. Also, the electric field gradient (EFG) and hyperfine field (HFF) at the nuclear site of Mn and As are calculated. Finally, the effect of pressure on EFG and HFF is studied. 1. Introduction Because of its structural, electronic, and magnetic properties, MnAs is a suitable compound for spin-electronic and magnetooptic applications. Recently its different properties were widely studied by the use of different experimental and computational methods [1–5]. Among the most important electronic properties of MnAs are its EFG and HFF which can be used to study the nonsymmetric part of the electron density around the nucleus and the core magnetic moment of the crystal. The effect of pressure on both EFG and HFF is studied which is useful in nuclear physics too. Nowadays, it is possible to study these two properties via computer simulation methods. In this paper, the effect of pressure on MnAs (in different crystal structures) is studied by density functional theory (DFT) calculations with the main emphasis on the EFG. The dependence of the EFG on pressure is related to change in electron charge density distribution [6]. The EFG is defined as the second derivative of the electrostatic potential at the nucleus site which is written as a traceless tensor ( ). This tensor can be obtained from an integral over the nonspherical charge density. The coupling of an EFG to the quadrupole moment ( ) of a nucleus causes a splitting of the nuclear energy levels which can be detected by the use of nuclear quadrupole resonance (NQR) measurements. The quadrupole resonance frequency ( ) is obtained from the NQR measurement. If and are known, Then the EFG can be calculated [6–8]: The nuclear quadrupole moments ( ) can be measured in the laboratory (which is very important parameter in nuclear physics) for determining the charge distribution at the nucleus. But this practice is expensive and it has not enough precision. Its measurement error can be about 25% and it is not applicable for some nuclei [8]. When
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