First Principles Calculations Of Metastable Phase In Binary Transition Metal Compounds

Transition-metal pnictides have been extensively investigated for their remarkable and intriguing mechanical,thermal,optical,electrical,and magnetic properties,as well as ex-hibit a unique number of synergistic effects.The theoretical and experimental studies,focus on binary transition metal compounds,have made important breakthroughs in various field-s such as semimetals,semiconductors,superconductors,and thermoelectric materials.In general,binary transition metal compounds crystallize in NiAs phase(B81)or MnP phase(B31).Previous researches have mainly focused on the properties of transition metal com-pounds in ground state,while little research focus on metastable states of binary transition metal compounds.In this paper,a metastable combination of Cr-As and Mn-Te systems and their magnetic and electronic properties are studied,using the first principle calculation method based on density functional theory.Firstly,we present results of structural,magnetic,and electronic properties of the stable and metastable CrAs under ambient and high pressure.With the help of structure prediction method,we investigate the double helimagnetic Pnma phase CrAs,furthermore,three new low-enthalpy phases with diverse magnetic order are identified as metastable states under ambient pressure.As the analytical results of electronic structure,three metastable structure P6m2 type-A,P6m2 type-B and P4/-/nmm are antiferromagnetic,nonmagnetic and ferromag-netic,respectively.In addition,we investigate the structural and magnetic phase transitions under high pressure.The system tends to be antiferromagnetic after double helimagnetic state near the optimal superconductivity regime without crystal phase transition,which sug-gests that antiferromagnetic correlations between neighboring exchange interactions may be essential for emergence of superconductivity in CrAs rather than a structural phase transition.Secondly,we propose a metastable two-dimensional semimetal CrAs2 monolayer,which hosts antiferromagnetic type-I and II nodal lines in the absence of spin-orbital coupling(SOC).It is worth mentioning that the essential type-I nodal line is firstly proposed in AFM two-dimensional materials,that is filling enforced by nonsymmorphic analogue symmetry(?),and cannot be gapped even in the presence of SOC.The type-II nodal lines,formed by band inversion,are protected by the mirror symmetry Mz in the absence of SOC.Furthermore,the topological nontriviality is verified by edge states on the boundary.From the calculated exfoliation energies and phonon dispersion,we show that CrAs2 monolayer can be readily obtained in experiment from the bulk material and exists as freestanding two-dimensional crystal.Our results reveal CrAs2 monolayer as a promising platform for exploring the inter-play between magnetism and exotic topological phases in two-dimensional system.Finally,we systematically investigate the structural,magnetic and electronic properties of the MnTe in six competing structures:rocksalt(RS),cesiun-chloride(CC),zinc-blende(ZB),wurtzite(WZ),iron-silicide(IS)and nickel-arsenide(NA).The ground state of MnTe is completely determined.And the structural parameters,magnetic properties,bulk modulus,phase transition pressure,and the density of states are studied,too.The density of states shows that MnTe in RS,CC and IS structures are antiferromagnetic conductors,and MnTe in WZ,ZB and NA are antiferromagnetic semiconductors.These results provide us the possibility to apply them to the spintronics of antiferromagnetic systems.