Strong anharmonicity-assisted low lattice thermal conductivities and high thermoelectric performance in double-anion Mo_(2)AB_(2)(A=S,Se,Te;B=Cl,Br,I)semiconductors

The thermoelectric properties of layered Mo_(2)AB_(2)(A=S,Se,Te;B=Cl,Br,I)materials are systematically investigated by first-principles approach.Soft transverse acoustic modes and direct Mo d–Mo d couplings give rise to strong anharmonicities and low lattice thermal conductivities.The double anions with distinctly different electronegativities of Mo_(2)AB_(2)monolayers can reduce the correlation between electron transport and phonon scattering,and further benefit much to their good thermoelectric properties.Thermoelectric properties of these Mo_(2)AB_(2)monolayers exhibit obvious anisotropies due to the direction-dependent chemical bondings and transport properties.Furthermore,their thermoelectric properties strongly depend on carrier type(n-type or p-type),carrier concentration and temperature.It is found that n-type Mo_(2)AB_(2)monolayers can be excellent thermoelectric materials with high electric conductivity,σ,and figures of merit,ZT.Choosing the types of A and B anions of Mo_(2)AB_(2)is an effective strategy to optimize their thermoelectric performance.These results provide rigorous understanding on thermoelectric properties of double-anions compounds and important guidance for achieving high thermoelectric performance in multi-anion compounds.

Novel structures and mechanical properties of Zr_(2)N:Ab initio description under high pressures

With the formation of structural vacancies,zirconium nitrides(key materials for cutting coatings,super wearresistance,and thermal barrier coatings) display a variety of compositions and phases featuring both cation and nitrogen enrichment.This study presents a systematic exploration of the stable crystal structures of zirconium heminitride combining the evolutionary algorithm method and ab initio density functional theory calculations at pressures of 0 GPa,30 GPa,60 GPa,90 GPa,120 GPa,150 GPa,and 200 GPa.In addition to the previously proposed phases P42/mnm-,Pnnn-,and Cmcm-Zr2 N,five new high-pressure Zr_(2)N phases of PA/nmm,IA/mcm,P2_(1)/m,P3 m1,and C2/m are discovered.An enthalpy study of these candidate configurations reveals various structural phase transformations of Zr2 N under pressure.By calculating the elastic constants and phonon dispersion,the mechanical and dynamical stabilities of all predicted structures are examined at ambient and high pressures.To understand the structure-property relationships,the mechanical properties of all Zr_(2)N compounds are investigated,including the elastic moduli,Vickers hardness,and directional dependence of Young’s modulus.The Cmncm-Zr2 N phase is found to belong to the brittle materials and has the highest Vickers hardness(12.9 GPa) among all candidate phases,while the I4/mcm-Zr2 N phase is the most ductile and has the lowest Vickers hardness(2.1 GPa).Furthermore,the electronic mechanism underlying the diverse mechanical behaviors of Zr2 N structures is discussed by analyzing the partial density of states.

Boron at tera-Pascal pressures

The study of boron structure is fascinating because boron has various allotropes containing boron icosahedrons under pressure. Here, we propose a new boron structure(space group Fm3m) that is dynamically stable at 1.4 tera-Pascal(TPa)using density functional theory and an evolutionary algorithm. The unit cell of this structure can be viewed as a structure with a boron atom embedded in the icosahedron. This structure behaves as a metal, and cannot be stable under ambient pressure. Furthermore, we found electrons gather in lattice interstices, which is similar to that of the semiconductor Na or Ca_(2)N-Ⅱ under high pressure. The discovery of this new structure expands our comprehension of high-pressure condensed matter and contributes to the further development of high-pressure science.

Dependence of mechanical properties on the site occupancy of ternary alloying elements in γ'-Ni3Al: Ab initio description for shear and tensile deformation

The site occupancy behavior of ternary alloying elements inγ'-Ni3Al(a key strengthening phase of commercial Ni-based single-crystal superalloys)can change with temperature and alloy composition owing to the effect of entropy.Using a total-energy method based on density functional theory,the dependence of tensile and shear behaviors on the site preference of alloying elements inγ'-Ni3Al were investigated in detail.Our results demonstrate that Fe,Ru,and Ir can significantly improve the ideal tensile and shear strength of theγ'phase when occupying the Al site,with Ru resulting in the strongest enhancement.In contrast,elements with fully filled d orbitals(i.e.,Cu,Zn,Ag,and Cd)are expected to reduce the ideal tensile and shear strength.The calculated stress-strain relationships of Ni3Al alloys indicate that none of the alloying elements can simultaneously increase the ideal strength of theγ'phase for both Ni1-site and Ni2-site substitutions.In addition,the charge redistribution and the bond length of the alloying elements and host atoms during the tensile and shear processes are analyzed to unveil the underlying electronic mechanisms.

Ultra-broadband near infrared phosphor with wide spectral range and long peak wavelength achieved by double-site occupation

The miniaturization and intelligence of near infrared(NIR)devices urgently require an excellent broadband NIR phosphor.However,emission spectra of most NIR phosphors cover less than 1200 nm with a full width at half maximum(FWHM)less than 200 nm and a peak wavelength less than 900 nm.Here,we successfully developed ultra-broadband NIR phosphors ScNbO_(4):Cr^(3+)which can overcome the above shortcomings based on a double-site occupancy strategy.The phosphors achieve ultra-broadband NIR emission from 800 to 1400 nm with a peak wavelength at 980 nm.Its FWHM is up to 217 nm.The valence state of Cr ions in phosphors was analyzed.The double-site occupancy of Cr^(3+)ions in the host and the relationship between host structure and optical properties are discussed in detail.NIR light transmission experiments show that phosphors have potential application value in non-destructive analysis.This work develops an excellent NIR luminescent material and provides an efficient method to obtain ultra-broadband NIR phosphors with longer peak wavelength.

Broadening and enhancing emission of Cr^(3+) simultaneously by co-doping Yb^(3+) in Ga1.4ln0.6SnO5

Cr^(3+)doped broadband near-infrared(NIR) emitting phosphors are currently the focus of research.Researchers have developed a variety of strategies to achieve broad and strong NIR emission,However,it is a conundrum to simultaneously broaden and enhance the emission of Cr^(3+)with a single strategy,In this work,we solved this problem by co-doping Yb^(3+).Under 452 nm excitation,Ga_(1.4)In_(0.6)SnO_(5)(GISO):0.01Cr^(3+)shows ultrabroadband NIR emission covering 650-1300 nm with a peak of 884 nm,The full width half maximum(FWHM) of the emission is 215 nm and the internal quantum yield(IQY) is 25%.This indicates that the double sites occupation strategy is favorable to achieve ultra-broadband NIR emission.The co-doping of Yb^(3+)can effectively broaden and enhance the emission of Cr^(3+).The FWHM for GISO:0.01Cr^(3+),0.002Yb^(3+)extends to 245 nm,and the IQY increases to 28%.Further increasing the concentration of Yb^(3+)to 0.005,the IQY is lifted to 32%.Finally,a phosphor-co nverted light emitting diode(pc-LED) was prepared by integrating the GISO:0.01Cr^(3+),0.002Yb^(3+)with a blue light chip.Under the current drive of 40 mA,the maximum output power of pc-LED is 4.54 mW,and the photoelectric conversion efficiency is 4.12%.These results indicate that Yb^(3+)ions can simultaneously broaden the emission band and improve the emission efficiency.This work provides an effective strategy for the design of efficient broadband NIR phosphors in the future.