A first-principles study of structure property correlation and the origin of ferrimagnetism is presented based on LSDA+U method. In particular, the results for the ground state structure, electronic band structure, de...A first-principles study of structure property correlation and the origin of ferrimagnetism is presented based on LSDA+U method. In particular, the results for the ground state structure, electronic band structure, density of states, Born effective charges, spontaneous polarization and cationic disorder are discussed. The calculations were done using Vienna ab-initio simulation package (VASP) with projector augmented wave method. We find that the ground state structure is orthorhombic and insulating having A-type antiferromagnetic spin configuration. The cationic disorder is found to play an important role. Although the cationic site disorder is not spontaneous in the ground state, interchange of octahedrally coordinated Ga2 and Fe2 sites is most favored. We find that ferrimagnetism? in gallium ferrite is primarily due to this exchange between Ga-Fe sites? such that Fe spins at Ga1 and Ga2 sites are antiferromagnetically aligned? while maintaining ferromagnetic coupling between Fe spins at Ga1 and Fe1? sites as well as between Fe spins at Ga2 and Fe2 sites. Further, the partial density of states shows noticeable hybridization of Fe 3d, Ga 4s,? Ga 4p and O 2p states indicating some covalent character of Ga/Fe-O bonds.? However, the charge density and electron localization functions show largely the ionic character of these bonds. Our calculation predicts spontaneous polarization of ~59 μC/cm2 along b-axis.展开更多
Bi_(2)Te_(3)-based thermoelectric(TE)materials have been demonstrated to be a potential candidate for mainly thermoelectric cooling/refrigeration applications.However,minority charge carriers excitation at high temper...Bi_(2)Te_(3)-based thermoelectric(TE)materials have been demonstrated to be a potential candidate for mainly thermoelectric cooling/refrigeration applications.However,minority charge carriers excitation at high temperature reduces thermopower which restricts these materials for the use in power generation.In present work,substitution of Ni on Sb site in Bi_(0.5)Sb_(1.5-x)NixTe_(3)(x=0,0.01,0.04 and 0.08)actuates the system to supress the intrinsic excitation leading to shift in highest ZT to higher temperature regime.The Density functional theory(DFT)calculations and experimental results reveal that Ni in Bi_(0.5)Sb_(1.5)Te3 provides the extra holes and slightly reduces the band gap Eg which enhances the s of Ni-doped Bi_(0.5)Sb_(1.5-x)NixTe_(3) samples and a at elevated temperature.Moreover,Ni-doping in Bi_(0.5)Sb_(1.5)Te_(3) also reduces kL which is attributed to the phonon scattering due to mass fluctuations and microstructural features such as grain boundary and strain field domain observed from HRTEM investigation.These favourable condition leads to maximum ZT~1.38 at 433K for Bi_(0.5)Sb_(1.46)Ni_(0.04)Te_(3) and ZT_(avg)~1.1 between 300K and 503K.Interestingly the calculated theoretical TE conversion device efficiency h of Bi_(0.5)Sb_(1.46)Ni_(0.04)Te3(η~5.5%)was achieved to be nearly twice than the efficiency of matrix Bi_(0.5)Sb_(1.5)Te3(h~3%).Experimental electronic transport is well corroborated with theoretically estimated DFT results.展开更多
文摘A first-principles study of structure property correlation and the origin of ferrimagnetism is presented based on LSDA+U method. In particular, the results for the ground state structure, electronic band structure, density of states, Born effective charges, spontaneous polarization and cationic disorder are discussed. The calculations were done using Vienna ab-initio simulation package (VASP) with projector augmented wave method. We find that the ground state structure is orthorhombic and insulating having A-type antiferromagnetic spin configuration. The cationic disorder is found to play an important role. Although the cationic site disorder is not spontaneous in the ground state, interchange of octahedrally coordinated Ga2 and Fe2 sites is most favored. We find that ferrimagnetism? in gallium ferrite is primarily due to this exchange between Ga-Fe sites? such that Fe spins at Ga1 and Ga2 sites are antiferromagnetically aligned? while maintaining ferromagnetic coupling between Fe spins at Ga1 and Fe1? sites as well as between Fe spins at Ga2 and Fe2 sites. Further, the partial density of states shows noticeable hybridization of Fe 3d, Ga 4s,? Ga 4p and O 2p states indicating some covalent character of Ga/Fe-O bonds.? However, the charge density and electron localization functions show largely the ionic character of these bonds. Our calculation predicts spontaneous polarization of ~59 μC/cm2 along b-axis.
文摘Bi_(2)Te_(3)-based thermoelectric(TE)materials have been demonstrated to be a potential candidate for mainly thermoelectric cooling/refrigeration applications.However,minority charge carriers excitation at high temperature reduces thermopower which restricts these materials for the use in power generation.In present work,substitution of Ni on Sb site in Bi_(0.5)Sb_(1.5-x)NixTe_(3)(x=0,0.01,0.04 and 0.08)actuates the system to supress the intrinsic excitation leading to shift in highest ZT to higher temperature regime.The Density functional theory(DFT)calculations and experimental results reveal that Ni in Bi_(0.5)Sb_(1.5)Te3 provides the extra holes and slightly reduces the band gap Eg which enhances the s of Ni-doped Bi_(0.5)Sb_(1.5-x)NixTe_(3) samples and a at elevated temperature.Moreover,Ni-doping in Bi_(0.5)Sb_(1.5)Te_(3) also reduces kL which is attributed to the phonon scattering due to mass fluctuations and microstructural features such as grain boundary and strain field domain observed from HRTEM investigation.These favourable condition leads to maximum ZT~1.38 at 433K for Bi_(0.5)Sb_(1.46)Ni_(0.04)Te_(3) and ZT_(avg)~1.1 between 300K and 503K.Interestingly the calculated theoretical TE conversion device efficiency h of Bi_(0.5)Sb_(1.46)Ni_(0.04)Te3(η~5.5%)was achieved to be nearly twice than the efficiency of matrix Bi_(0.5)Sb_(1.5)Te3(h~3%).Experimental electronic transport is well corroborated with theoretically estimated DFT results.
