H. J. van Daal

Small-polaron versus band conduction in some transition-metal oxides

Abstract In this paper an attempt is made to establish the nature of free charge carriers and of charge carriers bound to centres in p-type NiO, CoO and MnO and in n-type MnO and α-Fe2O3. For free charge carriers, d.c. conductivity, Seebeck coefficient and Hall effect are considered. Effects arising from inhomogeneous conduction and impurity conduction are discussed. Impurity conduction appears to have a strong influence on transport properties in the case of α-Fe2O3, less so in NiO, whereas no influence of this effect has been found in CoO and MnO. It is shown that NiO and CoO do not exhibit the features characteristic of small-polaron conductors but rather can be consistently conceived of as large-polaron band semiconductors. It is suggested that magnetic resistance due to exchange coupling between charge-carrier spin and cation spins plays an important role. The anomalous behaviour of the Hall effect in NiO and α-Fe2O3 is extensively discussed. In contradistinction to NiO, CoO and n-type MnO, free char...

The ambivalence of Yb in YbAl2 and YbAl3

This paper reports the results of a study of the intermetallic compounds YbAl 2 and YbAl 3 with regard to lattice constant, electronic specific heat, superconductivity, magnetic susceptibility and resistivity. The experimental data suggest that the Yb ions in YbAl 3 are close to trivalent and yet, at low temperatures, without a magnetic moment. The low temperature data of both compounds are interpreted, assuming a virtual bound 4 f -state just below the Fermi level. The number of holes in this state is high, and estimated to be about 0.9 for YbAl 3 and 0.5 for YbAl 2 . The high temperature data are discussed with a two-level model, the excited state being the normal magnetic trivalent state.

High electronic specific heat of some cubic UX3 intermetallic compounds

Abstract The low temperature specific heat of cubic U X 3 intermetallic compounds with X = Al, Ga, In, Si, Ge and Sn have been measured. High values for the coefficient of the electronic specific heat have been found, ranging from 14 to 169 mJ/mol K 2 .

Influence of s-f exchange interaction on electrical conduction in rare-earth dialuminides

Abstract The electrical resistivity has been measured between 4.2 and 300°K for RA1 2 compounds, where R denotes the rare-earth elements La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Yb and Y. From an analysis of the resistivity behaviour of the ferromagnetic compounds values are deduced for the effective s-f exchange coupling constants. These values agree reasonably well with data given in literature for the Knight shift of the A1 nuclear magnetic resonance. For CeA1 2 a resistivity minimum has been observed at 13°K.

On the electronic conduction of α-SiC crystals between 300 and 1500°K

The Hall constant and the conductivity of lamellar hexagonal SiC crystals were measured in a temperature range from 77 to 1500°K. The measurements were performed in a plane perpendicular to the hexagonal c-axis using van der Pauws method. Besides being experimentally advantageous this method also may reveal inhomogeneities. Most crystals were p-type conducting. They were grown by the Lely method and purposely doped during growth. The amount of impurities present was investigated by chemical and spectro-chemical procedures. Dominating impurities determined by these methods are respectively Al, B and Cl. As to Boron a check by paramagnetic resonance methods was made. The Hall data in the temperature range from 300 to 1300°K were analysed using a Schablone technique in terms of donor and acceptor concentrations. The latter results were compared with those from chemical analysis. An acceptor level of 0.27 eV above the valence band could be related to aluminium. Another level at 0.39 eV was found. This level could not be identified. A ratio of four was found for the multiplicity factors of the two levels, from which it was concluded that the unknown acceptor has paired electrons in the non-ionized state. It appeared that chlorine has no effect on the electric conduction. The ionization energy of the majority centres was found to be a function of the minority centre concentration. The density of states of the valence band could be determined, from which a provisional value for the density-of-states effective mass of 1.0 m0 could be deduced for the holes. An analysis of the dependence of the mobility of holes and electrons is given in terms of optical, acoustical, piezoelectric and impurity scattering. For the electrons it follows from the polar scattering that the effective mass equals 0.72 m0, from the acoustical scattering that the deformation potential of the conduction band amounts to 11.5 eV and from piezoelectric scattering that the piezoelectric-mobility-constant lies in between those of CdS and ZnS. A similar description of the hole mobility gives an effective mass of 3.5 m0 for the holes. This high value probably indicates that another scattering mechanism is present. For this a scattering of holes between two subbands of a split valence band by optical phonons is proposed. A rough analysis of the impurity scattering in the p-type samples has been made.

The seebeck coefficient of YbAl2 and YbAl3

Abstract The absolute Seebeck coefficient of YbAl2, YbAl3, LuAl2 and LuAl3 has been measured at temperatures between 4 and 300 K. The results of the Yb compounds are in conformity with the proposed virtual bound state character of the Yb ground state.

Investigations on the resistivity of the compound CeAl3

Abstract The electrical resistivity of the compound CeAl3 between 1.3 and 280°K has been observed to behave markedly anomalous. Further information concerning the nature of this anomaly has been obtained from data on the resistivity of compounds Ce1−xRxAl3, R denoting La or Th.

Polar optical-mode scattering of electrons in SnO2

Abstract From data for the Hall mobility of electrons in SnO 2 and for the optical reflection in the Reststrahl region, it is concluded that polar optical modes with an average characteristic temperature of 1050°K dominate lattice scattering. It is furthermore found that the static dielectric constant for directions | and ⊥ c-axis has values of about 9 and 15, respectively, in contradistinction to the values of 24.0 and 23.4 as commonly accepted in literature.

Comparison of Anomalies Observed in U‐ and Ce‐Intermetallics

The temperature dependence of the electrical resistivity and the magnetic susceptibility has been investigated for the compounds UAl2, UAl3 and the isostructural compounds UX3 (X=Ga, In, Si, Ge, Sn), and finally for UAl4 and UGa2. The influence of a variable U concentration has been studied for the pseudobinary compounds U1−xYxAl2 and U1−xYxGa2. The results are discussed in terms of a localized‐spin‐fluctuations model. The differences and the similarities between the electrical properties of these U compounds and Ce compounds, such as CeAl2 and CeAl3, are briefly discussed.

Kondo sideband effects in the Seebeck coefficient of Ce1−xLaxAlx compounds

Abstract This paper reports the occurence of very large positive peaks and smaller negative peaks in the absolute Seebeck coefficient of Ce 1− x La x Al 3 compounds ( x = 0.50, 0.90 and 0.99) in the temperature range of 4 to 300 K. These peaks are attributed to Kondo sidebands.