Structural Stability of Metastable TbCu7-Type SmFe Phase with Nb Substitution
Sm-Fe-N based magnets with theoretical magnetic properties comparable to NdFeB materials are expected to become new-type permanent magnetic materials with great potential.Among the Sm-Fe-N based materials,TbCu7-type SmFe alloy has attracted con-siderable attention due to its excellent magnetic performance.However,structure of TbCu7-type SmFe compounds is metastable and therefore thermodynamically unstable,which is experimentally stabilized by adding the third element.In recent years,the experimen-tal research of TbCu7-type SmFe material has been very mature,but its theoretical research is not deep enough.In this paper,Nb-doped TbCu7-type SmFe alloy as an example,using the first principle calculation method combined with experimental data to explore the occupation of Nb elements in the crystal structure,and its influence on structural stability and magnetic performance.First-princi-ples calculation was performed by using Vienna Ab-initio Simulation Package(VASP).Generalized gradient approximation(GGA)proposed by Perdew-Burke-Ernzerhof(PBE)was used to describe exchange-correlation effects.Cut-off energy for the plane wave basis was set to 450 eV.The Sm-f electrons were treated as open-core states.For experimental method,raw materials with purity of Sm,Fe and Nb 99.95％(mass fraction)were used to prepare SmFe9.5-xNbx(x=0,0.1,0.2,0.3)alloys.Those alloys made by arc-melting were crumbled into fragments,and subsequently melt spun in a molybdenum wheel at the speed of 40 m·s-1.Then the obtained ribbons were wrapped in tantalum foil and annealed at the temperature of 600～900 ℃ for 1 h in flowing Ar atmosphere.After that the annealed Sm-Fe-Nb ribbons were pulverized to about 106 µm.The obtained magnetic powders were nitrided at 450 ℃ for 24 h in pure nitrogen atmosphere.The entering of Nb element into TbCu7-type SmFe mainly replaced the Fe atomic crystal sites of its crystal structure,where the Fe atomic sites mainly occupied in the site of 2e,3g and 6l.The preferred site of Nb element was obtained by calculation of the formation energy and displacement energy of Nb elements in the TbCu7 structure.Data analysis suggested that formation energy and substitution energy reached the minimum when Nb occupied Fe-2e crystal sites rather than Fe-3g and Fe-6l sites.The result indicated that Nb atoms occupied Fe-2e sites preferentially with the most stable structure,which was consistent with the results obtained from structural refinement based on experimental results.Temperature was also a key factor affecting the change of site occupancy rate.Since first-principles calculations were conducted under the temperature of 0 K conditions,the site occupancy at room temperature and high temperature could not be generalized.Therefore,it was necessary to study the occupancy situation at room temperature and even high temperature conditions.The occupancy probability of Nb atoms in 2e sites began to decline while the probability of Nb atoms in 3g sites and 6l sites gradually increased when the temperature exceeded 870 ℃.XRD patterns showed that TbCu7-type SmFe phase started to transform to Th2Zn17-type SmFe phase at the temperature of around 850 ℃.The simulation results and the experimental re-sults were anastomosed,which identified the rationality of first-principles calculation.The influence of Nb content on stability of TbCu7 structure was obtained from both simulated and experimental results.As the Nb content increased,the formation energy and displace-ment energy of the compound gradually decreased,which indicating that Nb doping could effectively stabilize the TbCu7-type struc-ture.Since the atomic radius of Nb element is larger than the atomic radius of Fe,the addition of Nb atoms expanded the cell volume.With the increase of Nb content,the cell volume increases continuously,which were in good agreement with the experimental results.Super-cell structure of TbCu7-type SmFe was constructed.Cell parameters c/a,occupancy of Nb element,occupancy probability of Nb at high temperature and influence of Nb content on stability of TbCu7 structure were obtained from calculation results.First principles calculation was applied to TbCu7-type SmFe for the first time,and established a suitable model for calculation,providing theoretical reference for the theoretical research of TbCu7-type SmFe.The specific conclusions were as follows:Nb atoms entered the SmFe crystal cell and preferentially occupied the Fe-2e crystal site,which was the reason for Nb atoms could effectively stabilize the TbCu,-type SmFe structure.When the temperature was around 870 ℃,the probability of Nb atoms occupying the 2e crystal site began to decrease.At this time,due to structural instability,a transition from TbCu7-type SmFe to Th2Zn17-type SmFe occurred,and the calculated results are consistent with the XRD test results.With the increase of Nb doping content,the cell volume expanded and the crystal parameters c/a began to increase.The findings illustrated from the other side that Nb addition could effectively stabilize the TbCu7-type structure.
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