P. Pecheur

Electronic structure of Al-Pd-Mn crystalline and quasicrystalline alloys

An experimental and theoretical analysis of the electronic structure of Al-Pd alloys is presented. The experimental study, based on soft x-ray emission and absorption spectroscopies, is further extended to the case of quasicrystalline Al-Pd-Mn alloys and their approximants. The electronic structure is dominated by the sp-d hybridization. The Al-sp band is strongly modified by the interaction with the Pd-d states in the middle of the valence band. Close to the Fermi level, hybridization with the Mn-d states is found to enhance the structure induced pseudo-gap in the Al partial density of states, both in the icosahedral and the decagonal quasicrystalline phases.

Electronic structure of hexagonal

We have investigated the electronic structure of hexagonal through theoretical as well as experimental means. Two different calculations have been carried out within the LMTO and tight-binding - LMTO methods. The experimental procedure involved soft-x-ray spectroscopy complemented by photoemission measurements. Co states have been found near the Fermi level, in interaction with Al states of hybridized p, s - d character. As a result, a pseudo-gap is generated at the Fermi level. Occupied Al states beyond 5 eV form the Fermi level are found to be almost pure s in character. The comparison between the experimental results and calculations indicates a rather good agreement with results from LMTO, consistent with the fact that this is a more elaborate band structure calculation. Agreement is only acceptable with results from the TB - LMTO calculations but these can be improved with a more careful treatment of the TB structure constants.

Study of Chevrel phases for thermoelectric applications: band structure calculations on compounds (M = metal)

Chevrel phases, , present large voids in their lattice, where cations can be inserted. These cations are weakly bound and constitute good scattering centres for phonons. The lattice thermal conductivity of these compounds must be very low, and these phases are then good candidates for thermoelectric application. But most of them are metallic and superconducting. Some band structure calculations have been performed in order to find a semiconducting one. and are found to be metallic and semiconducting.

Ferromagnetism of the chimney-ladder compound Cr11Ge19

Abstract Using a non-self-consistent tight-binding-LMTO method, we have obtained the densities of states for Cr 11 Ge 19 and Mn 11 Si 19 . The calculations show a deep minimum in the density of states at an energy corresponding to a valence electron concentration of 14 per Cr (Mn) atom. For Cr 11 Ge 19 , the Fermi level lies in a high density of states region, suggesting magnetism. Experimentally we have found that this compound is indeed ferromagnetic below 100 K.

The electronic structure of MxMo6Se8−δ Chevrel phases with defects

The electronic structure of defected MxMo6Se8−δ Chevrel phases (M = Ti, Sn and vacancy) is investigated with the KKR–CPA method. In all cases presented the Se vacancy does appear to behave as a double donor, filling some of the holes below the gap. From the computed densities of states the stability of the defects in these compounds can be understood through the presence of an energy gap near the Fermi level. Then it seems energetically favourable to fill the bonding holes at the valence band edge, which may tentatively explain why x>1 in SnxMo6Se7.5 (), and x<1 in TixMo6Se7.5 (). The low DOS at EF corresponds well to the semi-metallic properties of the Ti0.8Mo6Se7.5 alloy, previously observed in resistivity experiments.

Theoretical study of structural stability and magnetism in the Pd3Mn antiphase

Abstract Relative stability of L1 2 , DO 22 and DO 23 structures as well as magnetic properties are studied in the Pd 3 Mn antiferromagnet using the Korringa–Kohn–Rostoker band structure calculations within the local-spin-density approach. The DO 23 phase of Pd 3 Mn is found to be the most stable from the total energy analysis if accounting for the spin-polarisation. The equilibrium lattice constant in DO 23 lies between a eq of the L1 2 and DO 22 phases, being about 1% larger than the experimental value. Noteworthy, the total energies of the ferromagnetic DO 22 and DO 23 phases of Pd 3 Mn markedly approach, when going down to the experimental lattice constant. In ferromagnetic state of Pd 3 Mn the following magnetic moments (in μ B ) 4.08, 0.12, 0.16 and 0.15 are computed on Mn and 4 c , 4 d and 4 e sites of Pd, respectively. In antiferromagnetic state, albeit no magnetic moment on Pd (4 d ), the KKR values are close to the ferromagnetic results, namely (in μ B ) 3.99 on Mn, 0.18 on Pd (4 c ) and 0.16 on Pd (4 e ). The electronic structure and magnetic properties of Pd 3 V and Pd 3 Fe in the L1 2 , DO 22 phases are also reported. The KKR calculations show that the ground state of Pd 3 V is either magnetic ( L1 2 ) or non-magnetic ( DO 22 ). Conversely, Pd 3 Fe exhibits ferromagnetic properties in both crystal structures with the Fe and Pd magnetic moments (in μ B ) 3.28, 0.29 and 3.22, 0.22, 0.26 in L1 2 and DO 22 , respectively. Both structural and magnetic properties derived from the band structure KKR calculations generally correspond well to the experimental data.

Electronic structure of MnSi0.7Al1.3 and related transition metal alloys with the TiSi2 structure

Abstract The densities of states of some ternary alloys (NbSi 1.4 Al 0.6 , MoAl 1.3 Si 0.7 , MoGa 1.4 Ge 0.6 , MnAl 1.3 Si 0.7 ) with the TiSi 2 structure have been obtained with the tight binding-linear muffin tin orbital (TB-LMTO)-average T matrix approximation (ATA) method and have been found to be similar to those of the chimney-ladder structures, previously computed with the same method. This is in agreement with the similar behaviour of their valence electron concentration. MnAl 1.3 Si 0.7 has been found found to be magnetic and a more elaborate KKR-CPA calculation predicts a magnetic moment of 0, 4 μ B for Mn in this alloy.

Thermoelectric properties of Mo/sub 6/Se/sub 8/-based chevrel phase with semiconducting properties

Mo/sub 6/Se/sub 8/-based Chevrel phase compounds with inserted cations into cavities are interesting as a new candidate of PGEC (Phonon Glass Electron Crystal) thermoelectric materials. From band structure calculations, the compositions including TiMo/sub 6/Se/sub 8/ have been found to be semiconductor or semimetal, and therefore are expected to have good Seebeck coefficient besides low thermal conductivity. However it had been apparent that insertion of enough cations to the cavities in Mo/sub 6/Se/sub 8/ with simple process was difficult due to secondary phase and oxide appearance. We present in this paper an estimation of the effect of two inserted cations experimentally and theoretically in order to improve thermoelectric properties of Chevrel phases. The thermoelectric properties are significantly enhanced by Cu-addition, and the dimensionless figure of merit, ZT=0.22 is estimated for nominal Cu/sub 0.3/Ti/sub 1.0/Mo/sub 6/Se/sub 8/ sample at 800/spl deg/C. It is found from band structure calculations by the self-consistent Korringa-Kohn-Rostoker (KKR) method that the Fermi level tends to shift to upper state by Cu-addition. Cu-addition can compensate a loss of Ti and control carrier concentration instead of Ti.