Qing Feng,Chngzhong Lio,Cheng Dong,Kimin Shih,Minhu Su

a College of Physics and Engineering,Chengdu Normal University,Chengdu,Sichuan 611130,China

b Department of Civil Engineering,The University of Hong Kong,Hong Kong,China

c National Laboratory for Superconductivity,Institute of Physics and Beijing National Laboratory for Condensed Matter Physics,Chinese Academy of Sciences,Beijing 100190,China

d Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and Resources,School of Environmental Science and Engineering,Guangzhou University,Guangzhou 510006,China

Abstract A new ternary Mg1.4Co21.6B6 compound in the Mg-Co-B system was synthesized via a conventional solid-state reaction method and the effect of Ni-substitution on its crystal structure,thermal stability,solid solubility and physical properties were systematically investigated.The crystal structure of the Mg1.4Co21.6B6 compound was fully determined by the X-ray diffraction technique with Rietveld refinement method.It is found that Mg1.4Co21.6B6 crystallizes in the form of C6Cr23 structure type(space group:Fm-3m(No.225),a=10.5617(2)?A,Z=4).The results showed that the 4a sites have been occupied completely by Co atoms in present compound which with M2-xNi21+xB6 form belonging to the W2Cr21C6-type.When Mg1.4Co21.6B6 is repeatedly sintered at elevated temperatures,it becomes unstable and decomposes into Co3B and Mg.The lattice parameters of the Mg1.4Co21.6B6 solid solution alters dramtically with increasing Ni substitution,with no regular trend being observed.The electrical and magnetic performances of the 3.6Mg:3Co:17Ni:6B and 3.6Mg:3Co:18Ni:6B(nominal compositions)samples suggest that both samples are typical ferromagnetic materials.The temperature in the maximum drop of the ρ(T)curve decreases as a function of the Ni content.Base on the correlation between the critical temperature and Ni content,a linear fitting equation is obtained and the critical temperature of Mg1.4Co21.6B6 calculated utilizing the linear fitting equation.The findings in this work may provide certain reference values for material science on electrical magnetic properties and other references for researching the material further.

Keywords:Mg-Co-Bsystem;Crystal structure;Stability;Ni-substituting effect;Rietveld refinement.

1.Introduction

The compounds in the Mg-Ni-B system are of great interest for material scientists due to their excellent hydrogen storage property and superconductivity Mg-Ni alloys have also been considered as potential candidates in the field of hydrogen storage materials[1–3].Boron element is usually regarded as metalloid which has properties in between those of metals and nonmetals.Boron could act as nonmetal for the formation of Mg-B-based metastable phases in the Mgbased system[4].Furthermore,it has been demonstrated that the nonmetal elements with high electronegativity have better improvement on hydrogen storage of the Mg-based system comparing to most of the reported transition metal(TM)systems[4].MgB2not only has the highest superconducting transition temperature(39 K)among binary compounds but also is highly cost-effective for applications in magnetic storage[5,6].However,due to the electron-deficient property of boron,few of versatile Mg-B based structures are gained for metal borides[7,8].For multi-electron materials with a potential high capacity,it is mainly attributed to the fact that TM borides can provide more electrons through specific electrochemical reactions[9–12].In the past decades,some TM borides have been proved to possess excellent electrochemical performance for alkaline batteries and some of them have been adopted as anode materials for alkaline batteries[12,13].Therefore,the rational design and synthesis of a novel and economical ternary Mg-TM-B alloy(TM=Co,Ni)would provide the property and structure of these battery materials for better performance and broaden its applications.However,few work has been reported on the electromagnetic properties of Mg-TM-B alloy(TM=Co,Ni).The purpose of this paper is also to expand the research on electromagnetic properties in present work.

The electron-deficient boron requires valence saturation so that numerous metal borides with particular structures can be formed[7,8].However,the issue corresponding to the occupy positions of atomics and the stability of the boridesτphase has been widely discussed in the literature[14,15].It is reported that the boridesτphase crystallizes in a cubic unit cell with Cr23C6structure type,or in ternary derivatives of its prototype,such as W2Cr21C6-type and Mg3Ni20B6-type.Such B-hosting compounds have been developed into over a hundred of ternary intermetallic compounds[16].Taking only Ni borides into account,many different M atoms,such as Mg,have been reported for theτphase.It has been demonstrated that rare earth(RE)metal-containing ternary borides in the formulae of RE2Ni21B6can be formed,and the RE(RE=Lu and Ho)atomics occupies the 8c position[14].Systemic changes on lattice parameters in multiphase samples are observed,which can manifest a conceivable homogenous range.

Herein,a new ternary compound Mg1.4Co21.6B6derived from the Mg-Co-B system has been successfully synthesized via a facile solid-state reaction method.The as-prepared boride composite is demonstrated to be an excellent alternative to noble compounds in the sense that it consists of low-cost,earth-abundant,non-noble and non-toxic elements.The crystal structure of Mg1.4Co21.6B6is determined using powder X-ray diffraction combining with Rietveld refinement method,since it is a well known technique to effectively determine crystal structures and microstructures.The lattice parameters,phase stability and solid solubility are also described.Detailed investigation is devoted to the temperature dependency on both magnetic and electric resistivity properties of the nominal Mg3.6Co20-xNixB6compositions.

2.Experimental

2.1.Sample preparation

The new compound Mg1.4Co21.6B6was prepared by a solid-state reaction from elements with a nominal composition of 3.6Mg:20Co:6B.Since loss of Mg via evaporation might happen under high-temperature heating,excessive Mg is required to obtain high-quality samples[17].Pure metals of Mg(in form of chips,99.95%),Co(325 mesh,99.70%),Ni(325 mesh,99.5%)and metalloid B(amorphous powder,99.90%)were used as starting materials and mixed by grinding in an agate mortar.The well-mixed precursors were pelletized into aΦ10 mm pellet.To avoid contact and reaction with the stainless-steel base of the furnace tube at high temperature,the pellet was wrapped by Ta metal foil.The wrapped pellet was then sealed in stainless-steel tube in an electric arc furnace under Ar atmosphere.The sample was first heated at 873 K for 5 h and at 1273 K for 10 h thereafter.Finally,the sintered sample was cooled to room temperature in the furnace.For the Ni-substituting samples,the nominal composition is 3.6Mg:20-xCo:xNi:6B(x=0,2,4,6,8,10,12,13,14,15,16,17,18,20).Each Ni-substituting sample was wrapped in Ta metal foil and placed in an independent stainless-steel tube,since the vaporized Mg might contaminate all samples if more than one sample was placed in the same stainlesssteel tube.The heating procedure of Ni-substituting samples was identical to that for the un-substituted one.

2.2.X-ray diffraction characterization

The crystal structure of the prepared products was examined by an M18X-AHF rotating anode X-ray diffractometer with Cu-Kαradiation(λKα1=1.54056?A,λKα2=1.54439?A,50 kV/200 mA)at room temperature.For the sample with a nominal composition of 3.6Mg:20Co:6B,the 2θscanning range was from 10° to 120° with a step size of 0.02° and counting time of 3 seconds.For the Ni-substituting samples with composition of 3.6Mg:20-xCo:xNi:6B,the 2θscanning range was from 20° to 100° with a step size of 0.02° and counting time of 1 second.PowderX[18]and TOPAS V4.0(BrukerAXS,Karlsruhe,Germany)programs were used for the indexing of the X-ray diffraction pattern and the Rietveld refinement of the crystal structures,respectively.

2.3.Physical property measurement

The temperature dependence of electrical resistivity measurement was carried out by standard four-probe method in the temperature range of 5-300 K.The magnetization in the temperature range of 2-300 K was operated at a superconducting quantum interference device magnetometer(MPMSXL,Quantum Design).Data was collected under zero-fieldcooled(ZFC)conditions in an applied field of 100 Oe.As to ZFC measurement,magnetization was measured with increasing temperature in the range of 2-300 K.

3.Results and discussion

3.1.Crystal structure of Mg1.4Co21.6B6

All Bragg diffraction reflections of the as-prepared 3.6Mg:20Co:6B in the XRD pattern were success fully indexed using the Powder X programon the basis of the C6Cr23structure type(space groupFm-3m)[18,19].The indexing results indicated that the 3.6Mg:20Co:6B sample was in cubic symmetry with lattice parameter a=10.5528(2)?A and space group Fm-3m(No.225).The crystal structure of this new ternary compound 3.6Mg:20Co:6B(nominal composition)has not been reported in the literature.Rietveld refinement of the new phase based on a known isostructural model is an effective mean to unravelthe crystal structure in detail.In this work,the procedures to obtain the crystal structural model for 3.6Mg:20Co:6B were given as follow:firstly,several possible isostructural compounds with the same space group,similar lattice parameters and approximate stoichiometry compared to 3.6Mg:20Co:6B were preliminarily selected from the Inorganic Crystal Structure Database[20],as shown in Table S1(Supplementary Materials).Among these isostructural compounds,we further identified certain potential candidates with the best match compared to the 3.6Mg:20Co:6B structure.To confirm the final acceptable structure model,the trial-anderror method was adopted to eventually gain the best fit between the observed and calculated patterns.Noted that analogous M2(Ni21-xMx)B6borides were reported to crystallize in a cubic unit cell which belongs to the ternary derivatives of Cr23C6structure type(e.g.,W2Cr21C6-type and Mg3Ni20B6-type)[17,21,22].Taking into account the volatility of Mg at high temperature and the similar atomic radius of Co to Ni,it can be concluded that the crystal structure of the asprepared 3.6Mg:20Co:6B is isostructural with the Mg3Ni20B6-type structure.Therefore,the Mg3Ni20B6isomorphism structure type was used as the starting model for the structure determination of the 3.6Mg:20Co:6B sample.

The crystal structure of Mg1.4Co21.6B6was determined by Rietveld refinement method using the structure of Mg3Ni20B6as the starting model using the TOPAS program using by a split pseudo-Voigt peak function.All of the major peaks of the experimental XRD pattern are in good agreement with the calculated one with Mg1.4Co21.6B6structure.Regarding with the uncalculated weak diffraction peaks,which can be attributed to the starting material of MgO phase.Then the MgO(PDF No.45-0946)with the space group is Fm-3m[225]as the initial model for the structure determination of the calculated peaks.Therefore,we carried out the Rietveld refinement with two-phase model(Mg1.4Co21.6B6and MgO).The final reliability indices obtained from the Rietveld refinement were Rwp=10.53%,Rp=7.68% and GOF=1.14,implying that the calculated profile fitted quite well with the experimental data.The observed and calculated XRD patterns and the difference curve are shown in Fig.1.The diffraction peaks of MgO are marked by arrows and the refinement results of Mg1.4Co21.6B6are presented in Table 1.It is worth noticing that the composition of Mg1.4Co21.6B6was calculated from the site occupancies in the Rietveld refinement resultslisted in Table 1.The atomic coordinates,site occupancies and thermal parameters of Mg1.4Co21.6B6are given in Table 2.

Fig.1.The observed(crosses),calculated(line)and difference(bottom curve)profiles derived from Rietveld refinement analysis of the X-ray diffraction data of Mg1.4Co21.6B6.The diffraction peaks of MgO as impurity in the sample are indicated by an arrow.

Table 1Refined structural parameters of Mg1.4Co21.6B6 compound.

Table 2Crystallographic information of Mg1.4Co21.6B6 derived from Rietveld refinement analysis.

Fig.2.The crystal structure of Mg1.4Co21.6B6(a)and the corresponding view of the structure along(1 1 1)(b).

To further understand the intrinsic characteristic of Mg1.4Co21.6B6,the lattice unit cell and the crystal structure viewed along(111)of the ternary compound are shownin Fig.2a and b,respectively.Atoms are located in five different positions:4a(0,0,0),8c(1/4,1/4,1/4),24e(0.2709,0,

Fig.3.XRD patterns of 2Mg:21Co:6B and 4Mg:20Co:6B.The diffraction peaks of MgO in the sample are indicated by arrows.Miller indices with different colors are used to indicate the indexing results of corresponding crystalline phases.

Fig.4.XRD patterns of 1st annealing and 2nd annealing 3.6Mg:20Co:6B sample at 1273 K.The diffraction peaks of MgO are indicated by arrows.Miller indices with different colors are used to indicate the indexing results of corresponding crystalline phases.

0),32f(0.3840,0.3840,0.3840)and 48h(0,0.1699,0.1699).Most of the Co atoms occupy position 4a,32f and 48h.B atoms are located inposition 24e,while all the Mg atoms and a few Co atoms occupy position 8c.The occupation of Mg atoms in the 8c Wyckoff site might attribute to the formation of a relative large size of crystal structure[17].The Borides related to the M3Ni20B6(M=Mg,Sn,Al)and M3Co20B6(M=Ga,In)crystalline structure ofτphase has been shown in Table S1.It can be obtained from the atomic distribution of the Mg3Ni20B6-type that Ni or Co is believed to occupy positions 32f and 48h,respectively,B is located in 24e,especially in the positions 8c and 4a are fully occupied by M atoms.It can be observed that the positions 4a is occupied by Co in the new phase Mg1.4Co21.6B6,which are different from Mg3Ni20B6-type.

Fig.5.XRD patterns of 3.6Mg:(20-x)Co:xNi:6B solid solution(x=0,2,4,6,8,10,12,14),showing the solid solubility of Ni in the solid solution samples(a);Details of the XRD patterns among 40°≤2θ≤50° to indicate the peak shifting(b).

In present work,the Mg3Ni20B6isomorphism structure type was used as the starting model for the structure determination of the 3.6Mg:20Co:6B sample.However,the refined results show that the composition is Mg1.4Co21.6B6,which are different from the Mg3Ni20B6-type since the 4a sites have been occupied absolutely by Co atoms,the position 8c being the only site with mixed Mg/Co occupation.Part atoms Mg locates at the Wyckoff 8c site have been substituted by Co in Mg1.4Co21.6B6,leading to the formation of solid solution form M2-xNi21+xB6(x=0.6 in present),which are also different from the M2+xNi21-xB6(e.g.,Zr2.14Ni20.72B6,Hf2.17Ni20.73B6[15])form in the W2Cr21C6-type.

The coordination polyhedra surrounding the atoms in positions 4a and 8c are highlighted,since the phase stability and the compositional boundaries of theτ-phase depend heavily on the occupation of these two crystallographic sites[14,15].The definition of theτcompositional boundaries are decided by two aspects:the located M atoms positions(4a,8c)and the maximum and minimum values of the M occupancy factor in 4a.That is,the presence of M atoms in the both positions directly account for the structural differences.The 8c and 4a polyhedra are directly interconnected as they share a ligand,namely the atoms(in 48h).Then the phase stability and homogeneity range in connection with the interatomic distances within the 4a and 8c polyhedra and the volumes of the latter[15].The 8c polyhedron volume would cause a change when any variation in the M atoms(4a)-TM(48h)distance within the 4a polyhedron,which results from the position of the 8c polyhedron linked to four adjacent 4a polyhedra through their common triangular faces[15].The increasing of the M amount in the 4a position,i.e.,when the 4a site occupation could become from a lower value to full occupation by M atoms in the crystal structure,the ordered Mg3Ni20B6-type structure can be confirmed.It has been observed that from the borides with the M2+xNi21-xB6structure type reveals that the 8c site is fully occupied by M atoms only,while the remaining M atoms partially occupy the 4a site along with Ni atoms.Therefore,the M atomic content of the 8c polyhedron remains unchanged,the homogeneity range of M in the M2+xNi21-xB6phase depends on the expand or contract capability of the 8c polyhedron[15].However,the 4a sites in the refined composition Mg1.4Co21.6B6have been occupied completely by Co atoms in present work,suggesting this compound with M2-xNi21+xB6form should be classified as belonging to the W2Cr21C6-type.Compared with the Mg3Ni20B6type,not only all the Mg atoms of 4a site but also part Mg atoms inposition 8c are replaced by Co in the compound Mg1.4Co21.6B6,which with M2-xNi21+xB6(x=0.6)form may be related to the decreased the difference between the sizes of atoms Mg and Co(rCo=1.260?A,rNi=1.240?A,rMg=1.60?A).

Compounds with the Cr23C6-type crystal structure containing rare earth RE are known in the RE-Ni-B systems.In these compounds B atoms are occupying the 24e,while in three crystallographic sites(4a,32f and 48h)occupied by Ni atoms are situated.However,in all cases a partial occupancy of the 8c position by RE atoms and occupancy are less than 1.0.The reason can be attributed to the anisotropic elongation of the displacement ellipsoid of the Ni2(32f)atoms,being part of the coordination sphere of the RE atoms,in the direction towards the central atom is related to the vacancies[14].Therefore,part Mg atoms have been substituted by Co locates at the Wyckoff 8c in Mg1.4Co21.6B6,which also are different from the RE2-xNi21B6(RE=Er,Yb and Lu)compounds crystallize with the Cr23C6type of structure due to the full occupation of the site 8c.

3.2.Effects of Mg content and thermal treatment of Mg1.4Co21.6B6

The volatilization of Mg metal under high temperature conditions might have a significant impact to the formation of Mg1.4Co21.6B6compound.Therefore,a two-step sintering process(873 K for 5 h followed by 1273 K for 10 h)is adopted to avoid the loss of Mg.Herein,the effects of excess Mg content(nominal composition 4Mg:20Co:6B)and deficient Mg content(nominal composition 2Mg:21Co:6B)on the formation of Mg1.4Co21.6B6were also studied.As shown in Fig.3,Mg1.4Co21.6B6can also be synthesised using the nominal composition 4Mg:20Co:6B sintered under the same thermal condition(873 K for 5 h and 1273 K for 10 h)with the presence of small amount of MgO.The MgO content may due to the oxidation of Mg metal during stainless-steel tube cutting and sample grinding.When the nominal composition 2Mg:21Co:6B was used with identical thermal treatment,Mg1.4Co21.6B6phase can also be obtained but the Co3B phase becomes the dominated phase(Fig.3).These results suggest that excess Mg contentis necessary to obtain high quality Mg1.4Co21.6B6compound.

To investigate the thermal stability of Mg1.4Co21.6B6,the as-prepared sample was grinded,pressed and sealed for heattreatment at 1273 K for 10 h and cooled to room temperature.Only Co3B was observed after heat-treatment at 1273 K(Fig.4),suggesting that Mg1.4Co21.6B6compound decomposed into Co3B and Mg(vaporized thus no diffraction signals in the XRD pattern).The stability of boridesτphase has been addressed in the literature[17].The thermal instability of powdered Mg1.4Co21.6B6may be attributed to the substitution of Co by Mg atoms,which lead to the expansion of the crystal lattice[17].In addition,the decompose of this phase after 10 h heat treatment might be due to the intensive grinding process which make the lattice distortion.It is also meaning that the decompose process is a kind of irreversible process.

3.3.Solid solubility of Ni in C6Cr23-type Mg-Co-B ternary compound

Since the substitution of Ni into Mg1.4Co21.6B6compound might have significant effects to its performance,the solid solubility of Ni was also investigated.Taking the vaporization of Mg metal during sample preparation into account,nominal compositions of 3.6Mg:(20-x)Co:xNi:6B(x=0,2,4,6,8,10,12,14,16,18,20)were used.Each sample was sealed in an independent stainless-steel tube and the preparation was identical to the aforementioned approach for Mg1.4Co21.6B6to avoid sample contaminations.Figs.5 and 6 show the XRD results of the 3.6Mg:(20-x)Co:xNi:6B solid solution.Nearly single phase of the new ternary compound was observed in the samples from x=0 to x=14 with small amount of MgO which is due to the oxidation of vaporized Mg metal.When x increases from 14 to 20,the new ternary compound is still the major phase,but a trace of MgNi2phase is present.The lattice parameters of the samples were calculated using TOPAS pro-gram and the results were present in Table 3 and Fig.7.When a low level of Ni is incorporated(0≤x≤6)(Phase I),the lattice parameter decreases with increasing Ni content.The same trend is also observed in phase(III)when the Ni contents(x)increases from 16 to 20.However,an opposite trend is observed with Ni content between x=6 to x=16(Phase II)-the lattice parameter increases with increasing Ni content.In addition,the lattice parameter does not obey the Vegard’s law,showing nonlinear variation.The change of lattice parameter with increasing Ni content might due to the complicated substitution of Ni,Co and Mg atoms in the crystalline phase.It is noted that the formula of the new phase is Mg1.4Co21.6B6and there is a reported phase with Mg3Ni20B6in the literature.The contents of Mg,Ni and Co vary in the solid solution between Mg1.4Co21.6B6and Mg3Ni20B6.The variation of the Mg,Ni and Co contents in the Mg3.6Co20-xNixB6solid solution may be the reason for the change of its lattice parameter.

3.4.Magnetic and electrical properties

Fig.8.The temperature dependence of magnetization(ZFC)and resistivity of sample 3.6Mg:3Co:17Ni:6B(a);The temperature dependence of magnetization(ZFC)and resistivity of sample 3.6Mg:2Co:18Ni:6B(b).The inset is the temperature derivative of magnetization(upper)and resistivity(lower),respectively.

Fig.8a and b shows the temperature dependence of magnetization(M(T),zero field cooling mode,ZFC)and electrical resistivity(ρ(T))of the 3.6Mg:3Co:17Ni:6B and 3.6Mg:2Co:18Ni:6B samples,respectively.From the temperature dependence of magnetization curves,it is evident that both 3.6Mg:3Co:17Ni:6B and 3.6Mg:2Co:18Ni:6B are typical ferromagnetic materials[23–26].The critical temperature(TC)for 3.6Mg:3Co:17Ni:6B and 3.6Mg:2Co:18Ni:6B samples are 62 K and 19 K,respectively.The temperature-dependent electrical resistivityρ(T)curves show a typical metallic character for both 3.6Mg:3Co:17Ni:6B and 3.6Mg:2Co:18Ni:6B samples.Interestingly,the critical temperature(TC)in magnetization curve is consistent with the temperature in the maximum drop ofρ(T)curve(dρ/dT)in both samples,as shown in the insets of Fig.8a and b.The above results can be explained by that the electron is scattered with complete disordered magnetic moment in the paramagnetic region(T>TC)while the spin starts ordering lead to a rapid decrease in electron scattering in conduction band in the ferromagnetic region(T

Fig.10.The critical temperature(TC)as a function of x of Ni content in the samples of 3.6Mg:(20-x)Co:xNi:6B samples(x=14,15,16,18,20)and the linear fitting equation.

The electrical resistivity as a function of temperature(ρ-T)for a series of 3.6Mg:(20-x)Co:xNi:6B(x=10,12,13,14,15,16,17,18,20)samples is presented in Fig.9.The temperature in the maximum drop of theρ(T)curve is derived from the derivative of resistivity(dρ/dT).As this temperature is consistent with the critical temperature(TC)in magnetization,it is also denoted as Tc here.As can be seen in Fig.9,all the measured samples show a character of metallic temperature dependence,and Tc can only be observed in the 3.6Mg:(20-x)Co:xNi:6B samples with x=14,15,16,17 and 18.With increasing Ni content in the samples,there is a significant decrease in Tc.Linear relationship between the value of x and Tc is found to be y=933.4-51x,as shown in Fig.10.Using this equation,we can demonstrate that the Tc for x=10,12,and 13 are 423.1 K,321.1 K and 270.1 K,respectively.For the 3.6Mg:3Co:17Ni:6B sample,the deduced Tc can be up to 933.4 K.A considerable residual resistivity value was observed in all samples,likely due to the MgO impurity and the lattice defects existing in crystals as well as the porosity in the measured material after solid state reaction.

Fig.9.The ρ-T curve of serial samples of 3.6Mg:(20-x)Co:xNi:6B when x=10,12,13,14,15,16,17,18,20 and the TC is indicated in the curves.

4.Conclusions

In this work,a new ternary compound Mg1.4Co21.6B6was successfully synthesized via a well-designed solid-state reaction.The crystal structure of the as-synthesized Mg1.4Co21.6B6is determined by XRD technique.Due to the isostructural similarity between Mg3Ni20B6and Mg1.4Co21.6B6,the crystal structure of Mg3Ni20B6was used as the starting model for the Rietveld refinement of Mg1.4Co21.6B6.The crystal structure of Mg1.4Co21.6B6is fully determined and we find that Mg1.4Co21.6B6crystallizes in C6Cr23-type with the space group of Fm-3m(No.225)and lattice parameters a=10.5617(2)(1)?A,cell volume V=1178.15(7)?A3,Z=4.The results indicated that part Mg atoms are substituted by Co locates at the Wyckoff 8c in Mg1.4Co21.6B6,however,which also are different from the RE2-xNi21B6compounds crystallize due to the full occupation of the site 8c.The compound Co3B co-exists in the repeatedly sintered Mg1.4Co21.6B6sample suggests that Mg1.4Co21.6B6become unstable at high temperature.The solid solubility of Ni in the Mg1.4Co21.6B6phase was determined by XRD analysis using the Ni incorporated Mg3.6Co20-xNixB6(0≤x≤20)samples.The lattice parameters of the Ni incorporated sample exhibit an irregular trend with increasing Ni content.It is likely because Ni atoms occupy different Wyckoff sites(4a,8c,32f,48h)which leads to the contraction/expansion the crystal lattice.The electrical and magnetic performance results suggest that both 3.6Mg:3Co:17Ni:6B and 3.6Mg:2Co:18Ni:6B are typical ferromagnetic materials.Electrical measurements show that the critical temperature decreases as a function of Ni content in the sample.Linear fitting equation y=933.4-51x is obtained on the basis ofthe correlation between mutations temperature and value x of the Ni content.The critical temperature 933.4 K of Mg1.4Co21.6B6is calculated using the linear fitting method.

Acknowledgements

This research work is supported by China’s Sichuan Science and Technology Program(2019YJ0441),Chengdu Normal University First-class Discipline Construction Major Scientific Research Projects(CS18ZDZ03),Chengdu Normal University Talent introduction scientificresearch special project(YJRC2015-3),The National Natural Science Foundation of China(11647095,51708143,22076034).

Supplementary material

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.jma.2021.03.025.