The emergence of MAX borides as well as MAB phases attracted great attention because of the renewable developments of ternary ceramics and offering great opportunities in potential applications.However,the number of borides remains limited,and further fundamental descriptions and detailed investigations on various properties are still lacking.In this report,we employ an integrated computational scheme that combines density functional theory with the evolutional algorithm to search for the favorable structures of P-and S-glued ternary borides terminated by Nb metal.We discover that the structures of 212-type,as e.g.Nb2PB2 and Nb2SB2,belong to the P6m2 space group,while those of 211-type,as e.g.Nb2PB and Nb2SB,prefer to crystallize in the P63/mmc space group,and the corresponding carbides Nb2PC and Nb2SC are also considered for the sake of completeness and comparative analsys.The predicted Nb2PB2,Nb2PB,Nb2SB,Nb2PC and Nb2SC are energetically stable,as revealed by the negative formation energies and by the proposed reaction paths with respect to the most competing phases,as well as dynamically stable,as suggested by the non-imaginary phonon spectra.The thermal conductivities of the six materials show unusual behaviors,particularly for the acoustic and optical contributions,and are accompanied by a strong anisotropy.Most importantly,Nb2PB2 is found to be an excellent thermal conductor with a total thermal conductivity of～65 W/(m K),while Nb2SC is found to be an ultra-low thermal conductor,with a total thermal conductivity of～5 W/(m K).These values are clearly outside the currently reported range of thermal conductivities,which makes Nb2PB2 and Nb2SC extremely interesting for fundamental research as well as prospective applications with the aid of artificial tunings on the almost independent MB block and the A layer.The discovery of these novel materials is expected to contribute substantially to the rapid development of ternary ceramics and to accelerate attempts in the applicability of MAX phases for heat conduction.

Shaohan Li;Weiwei Sun;Yi Luo;Jin Yu;Litao Sun;Bao-Tian Wang;Ji-Xuan Liu;Guo-Jun Zhang;Igor Di Marco

School of Materials Science and Engineering,Southeast University,Nanjing 211189,China;Jiangsu Province Key Laboratory of Advanced Metallic Materials,Southeast University,Nanjing 211189,China;SEU-FEI Nano-Pico Center,Key Laboratory of MEMS of Ministry of Education,Southeast University,Nanjing 210096,China;Institute of High Energy Physics,Chinese Academy of Sciences(CAS),Beijing 100049,China;Collaborative Innovation Center of Extreme Optics,Shanxi University,Taiyuan 030006,China;State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,College of Materials Science and Engineering,Institute of Functional Materials,Donghua University,Shanghai 201620,China;Asia Pacific Center for Theoretical Physics,Pohang,Gyeonbuk 790-784,Republic of Korea;Department of Physics,POSTECH,Pohang,Gyeonbuk 790-784,Republic of Korea;Department of Physics and Astronomy,Uppsala University,Box 516,Uppsala SE-75120,Sweden

材料科学技术（英文版）

[1]Shaohan Li;Weiwei Sun;Yi Luo;Jin Yu;Litao Sun;Bao-Tian Wang;Ji-Xuan Liu;Guo-Jun Zhang;Igor Di Marco-.Pushing the limit of thermal conductivity of MAX borides and MABs)[J].材料科学技术（英文版）,2022(02):79-88

Nb2PB2,Nb2SB2,P6m2,Nb2PB,Nb2SB,Nb2PC,Nb2SC,analsys,tunings

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