| As one of the new types of Weyl semimetal candidates,the ternary rare-earth aluminum germanide RAlGe(R=rare earth)exhibits many novel physical properties such as complex magnetic properties,excellent electronic transports and negative magnetoresistance effects due to its unique energy band structure and special atomic spatial arrangement,and has potential application in the future development of electronic devices with lower energy consumption,higher integration and enhanced functionality.At present,this material system is still at the basic research stage,and there are also some scientific issues to be studied,such as unclear crystal structure,lack of electromagnetic response data and unknown structure-property relationship.In this work,the polycrystalline samples of heavy rare-earth RAlGe(R=Gd,Tb,Dy,Ho,Er,Tm)are synthesized by using the non-consumable vacuum arc melting,and their crystal structures,microscopic morphologies,magnetic properties,electronic transports and heat capacity effects are systematically investigated to solve the issues in the crystal structures and physical properties by using X-ray diffractometer,scanning electron microscope(SEM)equipped with energy dispersive spectrometer(EDS),superconducting quantum interference device(SQUID)and physical property measurement system(PPMS).The main research conclusions are shown as follows:1.Among the heavy rare-earth RAlGe,the phase transition only exists in GdAlGe.When the temperature is below 1204 K,GdAlGe crystallizes in an orthorhombic YAlGe-type(space group:Cmcm)rather than DyAlGe-type(space group:Cmc21)structure,R,Al and Ge atoms are orderedly distributed at the 4c(0,0.31,0.25),4a(0,0,0)and 4c(0,0.61,0.25)sites,respectively.When the temperature increases above 1204 K,the orthorhombic structure transforms into a tetragonal LaPtSi-type(space group:I41 md)rather than α-ThSi2 type(space group:I41/amd)structure,Gd,Al and Ge atoms occupy the 4a(0,0,0.58),4a(0,0,0.17)and 4a(0,0,0)positions,respectively.In consistent with orthorhombic GdAlGe,TbAlGe,ErAlGe and TmAlGe also crystallize in the orthorhombic YAlGe-type structure,and lattice parameters decrease linearly with increasing the atomic numbers of heavy rare earth.DyAlGe and HoAlGe are special because they prefer to crystallize in a new type of disordered orthorhombic YAlGe-type structure,and the atoms in the unit cell are disorderedly distributed at the above three crystallographic sites,which are occupied by(RI,AlI)at 4c(0,0.31,0.25)site,(GeⅠ,RⅡ)at 4c(0,0.61,0.25)site and(AlⅡ,GeⅡ)at 4a(0,0,0)site,respectively.2.Heavy rare-earth RAlGe shows complex magnetic properties under the interaction between the magnetic isosceles triangles(R3 △).The tetragonal GdAlGe orders ferromagnetically with Cuire temperature Tc=21 K,which originates from the interaction of parallel-arranged Gd3△↑-Gd3△↑.The orthorhombic GdAlGe is antiferromagnetic with Neel temperature TN=34 K,which is due to the interaction of antiparallel-arranged Gd3 △↑-Gd3▽↓.Similar to the orthorhombic GdAlGe,the other heavy rare-earth RAlGe also order antiferromagnetically and TN gradually decreases with increasing the atomic numbers of heavy rare earth.In addition,another lowtemperature magnetic transition Tt is observed in TbAlGe,DyAlGe and HoAIGe,which is related to the magnetic structure transition caused by R-spin reorientation effects.When the temperature is below Tt,the arranged direction of magnetic moments is suggested to be antiparallel and perpendicular to the(0kl)basal plane.When the temperature gradually increases above Tt,the magnetic moments also rotate in the parallel direction of the(0kl)basal plane,the whole remains antiferromagnetic.3.The electronic transport properties of heavy rare-earth RAlGe show strong relation effects with magnetic properties.The resistivity transitions of all heavy rareearth RAlGe are consistent with their magnetic transitions,and the electron-magnon scattering and electron-phonon scattering play a major role in the resistivities below the transitions.With increasing the temperature above the transitions,the electron-magnon scattering gradually decreases and the electron-phonon scattering becomes the dominant scattering mechanism.In the whole testing temperature region,only GdAlGe presents the positive magnetoresistance effect,which originates from the positive contribution of conduction electrons under the Lorentz force and the increase of electron spin disorder scattering caused by strong antiferromagnetic interactions.Other heavy rare-earth RAlGe show a dynamic evolution from positive to negative magnetoresistances,which is related to the temperature-induced magnetic structure transitions and the decreases of electron spin disordered scattering caused by external magnetic fields.4.The heat capacities of heavy rare-earth RAlGe also exhibit strong relation with magnetic properties and electronic transports.The heat capacity transitions are consistent with magnetic and electronic transitions,and the Cm-T curves have the typical λ-type peaks near these transitions.The heat capacities of heavy rare-earth RAlGe originate from the contribution of free electron,lattice/phonon and magnon to the heat capacity,and the Cm/T of heavy rare-earth RAlGe varies linearly with T2 when the temperature is below the heat capacity transitions. |
