FeMnP_(1-x)T_x(T=Si,Ga,Ge)系列化合物机械性能的第一性原理研究

以密度泛函理论为基础,使用投影缀加波方法、VASP程序包研究了FeMnP_(1-x)T_x(T=Si,Ga,Ge)化合物的力学性质,结果表明FeMnP_(1-x)Ga_x化合物的晶格参数、弹性常数和电子结构与FeMnP_(1-x)Ge_x化合物比较接近,同时该化合物在力学上稳定,是预期具有较大的磁熵变和高磁热效应的材料.依据Pugh判据,FeMnP_(0.67)T_(0.33)(T=Si,Ga,Ge)化合物具有良好的延展性,三者之中FeMnP_(0.67)Ga_(0.33)韧性最好,FeMnP_(0.67)Si_(0.33)韧性相对较差,说明Ga替代P可改善此类化合物的机械性能.最后从化合物体系电子总态密度随不同掺杂T原子的演化规律解释了自洽计算得到的弹性常数的变化规律.

First principles study of mechanical properties of FeMnP1-xTx (T=Si,Ga, Ge) compounds

Magnetic refrigeration technology is considered as a better alternative to traditional steam compression scheme, since it has many advantages such as environment friendly characteristic, more compact solid refrigerant, low cost, etc. The mechanical stability is of essential importance for serving as magnetic refrigerant materials which work under repeatedly thermal and magnetic cycles. Recent experiment reveals that the polycrystalline FeMnP_1-xSi_x compounds are brittle, and even fracture of samples during post heat treatment is observed. Therefore, the improvement of the ductility of Fe₂P-Type FeMn-based magnetocaloric materials becomes an important issue in practical application. So far, there are few studies of the mechanical properties of these compounds. Alloying is an effective method to improve the mechanical properties of single phase materials, and Ga or Ge could be a better choice to replace the Si element. In this paper, we study the mechanical properties of giant magnetocaloric FeMnP_1-xT_x (T=Si, Ga, Ge) compounds by the projector augmented wave method as implemented in VASP (Vienna ab initio simulation package) in the framework of density functional theory. It is found that the lattice parameter, total energy, magnetic moment, elastic constant and the electronic structure of FeMnP_1-xGa_x compounds are similar to those of FeMnP_1-xGe_x compounds, therefore, it is believed that the FeMnP_1-xGa_x compounds are candidate refrigerant for room temperature magnetic refrigeration. The relatively large single crystalline elastic constants of FeMnP_1-xT_x (T=Si, Ga, Ge) compounds show that this family of compounds is mechanically stable. This ensures the long-term applicability of FeMnP_1-xT_x compounds in magnetic refrigeration facilities. For polycrystalline compounds, we calculate their shear moduli and bulk moduli by Hill averaging scheme. And according to Pugh criterion, the ductility or brittleness characteristics of FeMnP_1-xT_x (T=Si, Ga, Ge) compounds are discussed. All the FeMnP_0.67T_0.33 (T=Si, Ga, Ge) compounds are ductile, among them, FeMnP_0.67Ga_0.33 compound shows the best ductility, whereas the ductility of FeMnP_0.67Si_0.33 compound is the weakest. This result proves that substituting P with Ga could improve the ductility of this class of compound. The mechanical properties of polycrystalline FeMnP_0.33T_0.67 compounds are close to the ductile/brittle critical point. For FeMnP_0.33T_0.67 compounds, the T atoms just occupy the 2c sites of metalloid atom in Fe₂P-type structure, therefore it is expected that the occupation disorders of P and T atoms at high T concentration could improve the ductility of the compounds according to the result of FeMnP_0.67Ga_0.33 compound. Finally, the self-consistent elastic constants of different compounds are understood from the calculated electronic density of states and force theorem.