铁基磁性化合物与合金的结构和热力学性质从头计算研究

发布时间：2018-06-27 09:48

本文选题：第一性原理计算 + FeMnP_(1-x)T_x(T=Si　；参考：《内蒙古师范大学》2017年硕士论文

【摘要】：本文主要研究了两个不同的磁性体系的相结构、力学性质、磁有序性和热力学性质.论文主要内容分为两个部分:第一部分是Fe_2P型FeMnP_(1-x)T_x(T=Si,Ga,Ge)化合物的相结构、力学性质和热力学性质的理论研究;第二部分是稀释掺杂的Fe_(14)Tm_2(Tm=Cr,Mn,Co,Ni)合金的结构与磁性的关联、力学性质和间隙(C和N)原子对合金四方相稳定性的影响.作为磁制冷工质材料,FeMnP_(1-x)Si_x化合物在反复磁化和热加载下工作,该化合物在有限温度下应具备良好的机械性能和力学稳定性.实验发现FeMnP_(1-x)Si_x化合物的机械性能较差,然而到目前为止,针对此类化合物的机械性能、韧脆性的研究较少.本文从理论上预测了Ga原子替代P而形成的Fe_2P型FeMnP_(1-x)Ga_x化合物可能具有良好的机械性能和较大的磁卡效应.以密度泛函理论、线弹性理论、经典统计理论为基础,采用VASP、Phonopy、Gibbs等软件包计算研究了Fe MnP_(1-x)T_x(T=Si,Ga,Ge)系列化合物的总能量、磁矩、形成焓、电子结构、力学性质和热力学性质.首先,对比研究了Fe_2P型FeMnP_(1-x)Ga_x化合物可能形成的六方相和体心正交相的稳定性.计算结果表明化合物的六方相和体心正交相的总能量、磁矩和形成焓非常接近且形成焓均为负值.弹性常数的计算结果表明,两个相均为力学稳定相,而且弹性模量的计算可知铁磁(FM)态六方相的呈韧性,体心正交相呈脆性.计算两相的声子谱发现,六方相是动力学稳定的,而体心正交相的声子色散关系中存在明显的虚频,动力学不稳定.进一步研究了六方相FeMn P1-xGax化合物的力学性能,并与FeMnP_(1-x)T_x(T=Si,Ge)化合物进行了对比.为了模拟顺磁态化合物的力学性能,我们采用反铁磁(AFM)模型计算了化合物体系的弹性常数.计算结果表明六方相FeMnP_(1-x)T_x(T=Si,Ga,Ge)化合物在力学上稳定,FM态Fe MnP_(0.67)T_(0.33)(T=Si,Ga,Ge)化合物呈现韧性,FeMnP_(0.67)Ga_(0.33)的韧性最好,FeMnP0.67Si0.33的韧性相对较差.AFM态FeMnP0.33T0.67(T=Si,Ga,Ge)化合物呈现韧性,FeMnP0.33Ge0.67的韧性最好,FeMnP_(0.33)Ga_(0.67)的韧性相对较差.FM态FeMnP0.33T0.67(T=Si,Ga,Ge)和AFM态FeMnP_(0.67)T_(0.33)(T=Si,Ga,Ge)化合物处在韧脆性的临界状态.原子占位的无序性可能会改善化合物的韧性.弹性常数随不同掺杂原子的变化规律可通过电子结构分析和Force theorem解释.最后,通过对比铁磁态和顺磁态的Gibbs自由能G(P,T)确定了FeMnP_(1-x)Ga_x化合物的居里温度,计算了体系经历铁磁顺磁相变时的热容、熵变以及热膨胀系数,并与FeMnP_(1-x)Ge_x化合物进行了对比.化合物的热容在相变时产生跃变,体系经历了一级相变.FeMnP_(0.67)Ga_(0.33)化合物的居里温度为Tc=500K,此时的熵变为(35)S=69.34 J K~(-1) kg~(-1),而FeMn P0.67Ge0.33在Tc=590K时的熵变为(35)S=66.69 J K~(-1) kg~(-1).具有高Ga组份的FeMnP_(0.33)Ga_(0.67)化合物的Tc=770K,体系的熵变为(35)S=89.26 J K~(-1) kg~(-1).总之,FeMnP_(1-x)Ga_x化合物的六方相为稳定相,且各项性能都非常接近巨磁卡FeMnP_(1-x)Ge_x化合物,因此FeMnP_(1-x)Ga_x化合物是有巨磁卡效应的室温磁制冷材料.金属铁及其合金中存在显著的磁性-结构关联效应,即体系的晶格结构决定磁有序性.因此通过调控合金的结构可达到调控磁性状态的目的.另外,四方相稀释Fe合金的磁晶各向异性与其结构四方性(c/a)有关.因此,关于不同结构与磁有序性的稀释Fe合金的结构稳定性和力学性质的研究显得尤为重要.我们研究了Fe和Fe_(14)Tm_2(Tm=Cr,Mn,Co,Ni)二元合金处于不同结构(bcc,fcc,bct)和磁有序性(FM,AFM)时的相对稳定性和力学性质.计算结果表明:Fe和Fe_(14)Tm_2(Tm=Cr,Mn,Co,Ni)二元合金的bcc相FM态比fcc相FM态、AFM态更稳定,fcc相的FM态具有高自旋(HS)和低自旋(LS)的特性,fcc相AFM态要比FM态稳定.进一步研究了四方结构的Fe_(14)Tm_2合金,发现Fe14Cr2、Fe_(14)Mn_2的AFM态要比FM态稳定,Fe14Co2、Fe14Ni2的FM态要比AFM态稳定.弹性常数的计算表明,仅fcc相FM-HS态的Fe_(14)Tm_2(Tm=Cr,Mn,Co,Ni)二元合金,fcc相FM-LS态的Fe14Ni2合金,四方相FM-HS态金属Fe不满足力学稳定性条件,其它相均在力学上稳定.说明合金化使得体系的HS态在力学上稳定.将C或N原子填充在四方相Fe14Co2二元合金的八面体间隙时,其LS态是比bcc相更稳定的结构,同时含间隙原子的Fe和Fe14Co2合金均满足四方结构力学稳定性条件,即间隙原子的填充使体系四方相更加稳定,电子结构的定性分析表明含有间隙原子的合金可能具有更大的磁晶各向异性。
[Abstract]:This paper mainly studies the phase structure, mechanical properties, magnetic ordering and thermodynamic properties of two different magnetic systems. The main contents of this paper are divided into two parts: the first part is the phase structure of Fe_2P type FeMnP_ (1-x) T_x (T=Si, Ga, Ge) compounds, the theoretical study of the mechanical properties and thermodynamic properties; the second part is the dilution doped Fe_ (14) Tm_. The relationship between the structure and magnetic properties of 2 (Tm=Cr, Mn, Co, Ni) alloys, the mechanical properties and the effects of the interstitial (C and N) atoms on the stability of the tetragonal phase of the alloy. As a magnetic refrigerant material, FeMnP_ (1-x) Si_x compound works under repeated magnetization and thermal loading. The compound should have good mechanical and mechanical stability at a limited temperature. Experimental hair The mechanical properties of the present FeMnP_ (1-x) Si_x compounds are poor. However, to date, there are few studies on the mechanical properties of such compounds. This paper predicts that Fe_2P type FeMnP_ (1-x) Ga_x compounds formed by Ga atoms instead of P may have good mechanical properties and larger magnetic card effects. On the basis of linear elasticity theory and classical statistical theory, the total energy, magnetic moment, formation enthalpy, electronic structure, mechanical properties and thermodynamic properties of Fe MnP_ (1-x) T_x (T=Si, Ga, Ge) series compounds are calculated by using VASP, Phonopy, Gibbs and other software packages. First, the possible six square and body centers of Fe_2P type FeMnP_ are compared and studied. The calculation results show that the total energy of the six square phase and the orthogonal phase of the body center is very close and the formation enthalpy is negative. The calculation results of the elastic constants show that the two phases are all mechanical stable phase, and the calculation of the modulus of elasticity shows the toughness of the ferromagnetic (FM) state six square phase and the orthogonal phase of the body center. Brittleness. The calculation of the phonon spectra of the two phase shows that the six phase phase is dynamic and stable, while the phonon dispersion relation of the body center quadrature phase has obvious virtual frequency and dynamic instability. The mechanical properties of the six square phase FeMn P1-xGax compound are further studied and compared with the FeMnP_ (1-x) T_x (T=Si, Ge) compound. The elastic constants of the compound system are calculated by the anti ferromagnetic (AFM) model. The results show that the six square phase FeMnP_ (1-x) T_x (T=Si, Ga, Ge) compounds are stable in mechanics, and the FM state Fe MnP_ (0.67) T_ (0.33) compounds exhibit toughness, and (0.67) the toughness is the best. The poor.AFM state FeMnP0.33T0.67 (T=Si, Ga, Ge) compounds exhibit toughness, and the toughness of FeMnP0.33Ge0.67 is the best. The toughness of FeMnP_ (0.33) Ga_ (0.67) is relatively poor.FM state FeMnP0.33T0.67 (T=Si, Ga, 0.67) is at the critical state of the toughness. The disorder of atomic occupying potential may improve the compound The variation of elastic constants with different doping atoms can be explained by electronic structural analysis and Force theorem. Finally, the Curie temperature of FeMnP_ (1-x) Ga_x compounds is determined by comparing the Gibbs free energy G (P, T) of the ferromagnetic and paramagnetic states. The heat capacity, entropy change and thermal expansion coefficient of the system have been calculated when the system has undergone ferromagnetic paramagnetic phase transition. The heat capacity of the compound is compared with the FeMnP_ (1-x) Ge_x compound. The heat capacity of the compound produces a jump in the phase transition. The system experiences the first order phase transition of the.FeMnP_ (0.67) Ga_ (0.33) compound and the Curie temperature is Tc=500K, and the entropy becomes (35) S=69.34 J K~ (-1) kg~ (-1), and the entropy becomes (35). The entropy of the FeMnP_ (0.33) Ga_ (0.67) compound of the high Ga component changes to (35) S=89.26 J K~ (-1) kg~ (-1). In a word, the six square phase of the FeMnP_ (1-x) compound is a stable phase, and the properties are very close to the giant magnetocaloric compound. Therefore, the compound is a room temperature magnetic refrigeration material with giant magnetic card effect. There is a significant magnetic structure association effect in metal iron and its alloys, that is, the lattice structure of the system determines the magnetic ordering. Therefore, the purpose of regulating magnetic state can be achieved by regulating the structure of the alloy. In addition, the magnetocrystalline anisotropy of the tetragonal Fe alloy is related to the structure of the Quartet (c/a). Therefore, about the different structures and magnetic order The study of structural stability and mechanical properties of Fe alloys with sexual dilution is particularly important. We have studied the relative stability and mechanical properties of Fe and Fe_ (14) Tm_2 (Tm=Cr, Mn, Co, Ni) alloy in different structures (BCC, FCC, BCT) and magnetic ordering. The results show that the two yuan alloy of 14 The FM state of C phase is more stable than FCC phase FM state, AFM state is more stable, FM state of FCC phase has the characteristics of high spin (HS) and low spin (LS), FCC phase AFM state is more stable than FM state. It is clear that the Fe_ (14) Tm_2 (14) Tm_2 (Tm=Cr, Mn, Co, Ni) alloy, FCC phase FM-LS state Fe14Ni2 alloy, and the Quartet phase FM-HS state metal does not satisfy the mechanical stability conditions, and the other phases are stable in mechanics. In the gap, the LS state is a more stable structure than the BCC phase. Both Fe and Fe14Co2 alloy with interstitial atoms meet the mechanical stability conditions of the tetragonal structure. That is, the filling of the interstitial atoms makes the tetragonal phase more stable. The qualitative analysis of the electronic structure shows that the gold inclusion containing the interstitial atoms may have a larger magnetocrystalline anisotropy.
【学位授予单位】：内蒙古师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG132.2;O614.811

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