The Properties Of Dissociated Dislocations In MgB₂and Elastic Anisotropy Of Mg（Cd）CNi₃under High Pressure

The discovery of MgB2with superconducting transition temperature Tc=39Kcaused the attention of the whole scientific community. So far, it has the highestcritical temperature in the simple metal compounds superconducting materials, andis the promising superconducting material to achieve practical application. At present,the research literature on superconducting material mainly focused on the physicalproperties, such as superconducting energy gap, thermodynamic properties, thetransmission performance and so on. It is worth noting that the studies have shownthat defects (dislocation, Mg vacancy) and lattice strain can dramatically influence thesuperconductivity properties and mechanical behavior of MgB2. The dislocationproperties and elastic anisotropy under high pressure are lake of study in the previousliteratures, although defects and elastic behavior under high pressure are crucial forunderstanding superconducting properties and mechanical behavior of MgB2andother superconducting material. In this paper, we chiefly study the properties ofdissociated dislocation for MgB2and the elastic anisotropy for MgCNi3and CdCNi3.The main work and results of this paper as follows:(1) The elastic properties, generalized stacking fault energy and dissociateddislocations in MgB2under different pressure.<11-20>{0001} is a important slip system of hexagonal structure, and we selectthe <11-20>{0001} of MgB2as the research object. The <11-20> perfect dislocationin MgB2is proposed to dissociate into two partial dislocations in an energy favorableway, namely <11-20>â†'1/2<11-20>+SF+1/2<11-20>. This dissociation style isa amendment of the previous dissociation <1000>â†'1/3<1-100>+SF+1/3<2100> which proposed by Zhu et al. to model the partial dislocations and thestacking fault observed by transmission electron microscopy. The latter dissociationleads to a maximal stacking fault energy but a minimal one according to thegeneralized stacking fault energy we calculated from the first-principles methods.Moreover, the elastic constants and anisotropy of MgB2under different pressure arealso investigated. The core structures of the <11-20>{0001} dissociated dislocationsare researched within the modified Peierls–Nabarro (P–N) dislocation theory. Thevariational method is utilized to solve the modified P–N dislocation equation and thePeierls stress is also predicted for MgB2under different pressure. The effects of highpressure on the elastic anisotropy, the core structure and the Peierls stress are also presented. In addition, we used the P–N dislocation equation to discuss the dislocationwidth, the core structure, and the Peierls stress for MgB2with Mg vacancy and Bvacancy.(2) The elastic constants and anisotropy of superconducting MgCNi3and CdCNi3under different pressure.The second-order elastic constants (SOECs) and third-order elastic constants(TOECs) of MgCNi3and CdCNi3are provided by using first-principles calculationwhich based on density functional theory (DFT) combined with homogeneousdeformation theory. The Voigt–Reuss–Hill (VRH) approximation are utilized to obtainthe bulk modulus B, shear modulus G, averaged Young’s modulus E and Poisson’sratio ν for polycrystalline materials. By comparing with the experimental data, theseeffective modulus are consistent well with the experimental values. The SOECs forMgCNi3and CdCNi3under different pressure are also calculated based on thepressure derivatives from TOECs. Furthermore, the Zener anisotropy factor A,Chung–Buessem anisotropy index AC, and the universal anisotropy index AUare usedto account for the anisotropy of MgCNi3and CdCNi3. The anisotropy of Young’smodulus of MgCNi3and CdCNi3under different pressure are also presented.