Transition Intermetallic Compounds Research Articles

Comprehensive first-principles analysis of phonon thermal conductivity and electron-phonon coupling in different metals

Separating electron and phonon thermal conductivity components is imperative for understanding the principle thermal transport mechanisms in metals and highly desirable in many applications. In this work, we predict the mode-dependent electron and phonon thermal conductivities of 18 different metals at room-temperature from first-principles. Our first-principles predictions, in general, agree well with experimental data. We find that the phonon thermal conductivity is in the range of 2 - 18 $W/mK$, which accounts for 1% - 40% of the total thermal conductivity. It is also found that the phonon thermal conductivities in transition metals and transition-intermetallic-compounds (TICs) are non-negligible compared to noble metals due to their high phonon group velocities. Besides, the electron-phonon coupling effect on phonon thermal conductivity in transition metals and intermetallic compounds is stronger than that of nobles, which is attributed to the larger electron-phonon coupling constant with a high electron density of state within Fermi window and high phonon frequency. The noble metals have higher electron thermal conductivities compared to transition metals and TICs, which is mainly due to the weak electron-phonon coupling in noble metals. It is also shown that the Lorenz ratios of transition metals and transition-intermetallic-compounds hold larger deviations from the Sommerfeld value $L_0=2.44 \times 10^{-8} W \Omega K^{-2}$. We also find the mean free paths (MFPs) for phonon (within 10 nm) are smaller than those of electron (5 - 25 nm). The electrical conductivity and electron thermal conductivity are strongly related to the MFPs of the electron.

Pseudogap opening in single-crystal YMn4Al8 investigated through Mn55 and Al27 NMR measurements

$^{55}\mathrm{Mn}$ and $^{27}\mathrm{Al}$ NMR measurements are performed on a single crystal of ${\mathrm{YMn}}_{4}{\mathrm{Al}}_{8}$, which is known as one of the rare examples of pseudogap opening in transition intermetallic compounds, to investigate the formation of this pseudogap. The NMR spectrum, Knight shift, linewidth, and spin-lattice relaxation rate $1/{T}_{1}$ are measured as functions of the temperature down to 4 K for the $c$ axis, which is parallel and perpendicular to a magnetic field of 8 T. The temperature dependencies of the Knight shifts and $1/({T}_{1}T)$ for both NMRs as well as the magnetic susceptibility show significant decrease as the temperature decreases below room temperature, indicating the opening of a pseudogap down to \ensuremath{\sim}100 K, where these data become nearly constant, confirming that the formation of the spin pseudogap is incomplete and a finite density of states remains below. By analyzing $^{55}K(T)$ and $^{27}K(T)$ thoroughly, we decompose the orbital and spin shifts for both Mn and Al sites, from which the anisotropies of the Korringa ratios are extracted. The Korringa ratios at the Mn sites are temperature dependent and highly anisotropic above 100 K, and then decrease approaching unity and become isotropic and constant below 100 K. From the temperature dependencies of $1/({T}_{1}T)$ for both NMRs, the pseudogap temperature, ${T}^{*}$ is found to be approximately 270 K. From the anisotropy of $^{55}1/({T}_{1}T)$ during the opening of the pseudogap, we observed that the antiferromagnetic spin fluctuation is anisotropic and stronger by up to $\ensuremath{\sim}4.2$ times in the basal plane perpendicular to the quasi-one-dimensional (1D) manganese chain. This significant anisotropy must originate from the 1D nature of the Mn magnetic interaction.

Onset of ferromagnetism in 3d-substituted FeAl alloys. II: Mn and Co substitutions

For pt.I see ibid., vol.15, p.681 (1985). Magnetic and structural studies using X-ray, neutron diffraction, magnetisation and small-angle neutron scattering (SANS) techniques are reported for FeAl1-xTx alloys in which partial substitution of Al by Mn or Co has been effected. The results of the neutron diffraction studies are consistent with a B2 model in which the Co atoms exercise a site preference for Fe sites, whereas Mn atoms appear to enter both Fe and Al sites. Consequently, for both systems displacement of Fe atoms into Al sites creates 'antistructure' (AS) Fe atoms. The susceptibilities of both alloy systems were observed to increase with the concentration of the transition metal solute. An apparent onset of spontaneous magnetisation was observed beyond certain 'critical' concentrations: x=0.3 and x=0.2 for Mn and Co substitutions respectively. However, it was not until a Co concentration with x=0.3 was reached that a Curie temperature could be established in the Arrott plots. No TC could be established at all in similar plots for the Mn series having x<or=0.4. Furthermore, the SANS measurements revealed no sharp peaking in the intensity contours of the critical magnetic scattering at temperatures where Arrott plots (based on high-field magnetic data only) had indicated a Curie temperature. Possible mechanisms for these observations are discussed in terms of current theories of the onset of magnetic order in magnetic transition intermetallic compounds.

Onset of ferromagnetism in 3d-substituted FeAl alloys. I. Ti, V and Cr substitutions

For pt.I see ibid., vol.15, p.681 (1985). Magnetic and structural studies using X-ray, neutron diffraction, magnetisation and small-angle neutron scattering (SANS) techniques are reported for FeAl1-xTx alloys in which partial substitution of Al by Mn or Co has been effected. The results of the neutron diffraction studies are consistent with a B2 model in which the Co atoms exercise a site preference for Fe sites, whereas Mn atoms appear to enter both Fe and Al sites. Consequently, for both systems displacement of Fe atoms into Al sites creates 'antistructure' (AS) Fe atoms. The susceptibilities of both alloy systems were observed to increase with the concentration of the transition metal solute. An apparent onset of spontaneous magnetisation was observed beyond certain 'critical' concentrations: x=0.3 and x=0.2 for Mn and Co substitutions respectively. However, it was not until a Co concentration with x=0.3 was reached that a Curie temperature could be established in the Arrott plots. No TC could be established at all in similar plots for the Mn series having x<or=0.4. Furthermore, the SANS measurements revealed no sharp peaking in the intensity contours of the critical magnetic scattering at temperatures where Arrott plots (based on high-field magnetic data only) had indicated a Curie temperature. Possible mechanisms for these observations are discussed in terms of current theories of the onset of magnetic order in magnetic transition intermetallic compounds.

Charge Transfer and Energy of Formation in Binary Alloys: CsCl Intermetallic Compounds

AbstractThe density functional obtained through Löwdin's α‐expansion method is utilized to study equi‐volume charge transfer and energy of formation of equiatomic ordered transition intermetallic compounds having CsCl structure. Electronic charge is transferred, by and large, in the direction as expected from the Pauling electronegativity series. The computed formation energies predict the stability of compounds satisfactorily. It is concluded that the α‐expansion method may be applied with confidence to study ground state properties of metals and alloys.

Charge transfer and heat of formation in CsCl intermetallic compounds

Equivolume charge transfer in transition intermetallic compounds with the CsCl structure was computed using a local density approximation of energy density functional theory. When the two components are in the same period, electronic charge is transferred from the element to the left of chromium to the element at the right, the charge transfer increasing in general with increasing separation in the Periodic Table. The influence of charge transfer on compound formation was studied. The formation energy was computed as the sum of elastic, electrochemical and Madelung contributions. The results provide a satisfactory description of compound stability. Theoretical Wigner-Seitz radii were also computed, which agreed satisfactorily with experiment.

Hydrogen absorption of rare-earth (3d) transition intermetallic compounds

Hydrogen absorption of rare-earth (3d) transition intermetallic compounds