The half Heusler system Ti1+xFe1.33-xSb-TiCoSb with Sb/Sn substitution: phase relations, crystal structures and thermoelectric properties.

Investigations of phase relations in the ternary system Ti-Fe-Sb show that the single-phase region of the Heusler phase is significantly shifted from stoichiometric TiFeSb (reported previously in the literature) to the Fe-rich composition TiFe1.33Sb. This compound also exhibits Fe/Ti substitution according to Ti1+xFe1.33-xSb (-0.17 ≤ x ≤ 0.25 at 800 °C). Its stability, crystal symmetry and site preference were established by using X-ray powder techniques and were backed by DFT calculations. The ab initio modeling revealed TiFe1.375Sb to be the most stable composition and established the mechanisms behind Fe/Ti substitution for the region Ti1+xFe1.33-xSb, and of the Fe/Co substitution within the isopleth TiFe1.33Sb-TiCoSb. The calculated residual resistivity of Ti1+xFe1.33-xSb, as well as of the isopleths TiFe1.33Sb-TiCoSb, TiFe0.665Co0.5Sb-TiCoSb0.75Sn0.25 and TiFe0.33Co0.75Sb-TiCoSb0.75Sn0.25, are in a good correlation with the experimental data. From magnetic measurements and 57Fe Mössbauer spectrometry, a paramagnetic behavior down to 4.2 K was observed for TiFe1.33Sb, with a paramagnetic Curie-Weiss temperature of -8 K and an effective moment of 1.11µB per Fe. Thermoelectric (TE) properties were obtained for the four isopleths Ti1+xFe1.33-xSb, TiFe1.33Sb-TiCoSb, TiFe0.665Co0.5Sb-TiCoSb0.75Sn0.25 and TiFe0.29Co0.78Sb-TiCoSb0.75Sn0.25 by measurements of electrical resistivity (ρ), Seebeck coefficient (S) and thermal conductivity (λ) at temperatures from 300 K to 823 K allowing the calculation of the dimensionless figure of merit (ZT). Although p-type Ti1+xFe1.33-xSb indicates a semi-conducting behavior for the Fe rich composition (x = -0.133), the conductivity changes to a metallic type with increasing Ti content. The highest ZT = 0.3 at 800 K was found for the composition TiFe1.33Sb. The TE performance also increases with Fe/Co substitution and reaches ZT = 0.42 for TiCo0.5Fe0.665Sb. No further increase of the TE performance was observed for the Sb/Sn substituted compounds within the sections TiFe0.665Co0.5Sb-TiCoSb0.75Sn0.25 and TiFe0.33Co0.75Sb-TiCoSb0.75Sn0.25. However, ZT-values could be enhanced by about 12% via the optimization of the preparation route (ball-mill conditions and heat treatments).

The effect of nanocrystalline silicon host on magnetic properties of encapsulated iron oxide nanoparticles.

The purpose of this work is a detailed comparison of the fundamental magnetic properties of nanocomposite systems consisting of Fe3O4 nanoparticle-loaded porous silicon as well as silicon nanotubes. Such composite structures are of potential merit in the area of magnetically guided drug delivery. For magnetic systems to be utilized in biomedical applications, there are certain magnetic properties that must be fulfilled. Therefore magnetic properties of embedded Fe3O4-nanoparticles in these nanostructured silicon host matrices, porous silicon and silicon nanotubes, are investigated. Temperature-dependent magnetic investigations have been carried out for four types of iron oxide particle sizes (4, 5, 8 and 10 nm). The silicon host, in interplay with the iron oxide nanoparticle size, plays a sensitive role. It is shown that Fe3O4 loaded porous silicon and SiNTs differ significantly in their magnetic behavior, especially the transition between superparamagnetic behavior and blocked state, due to host morphology-dependent magnetic interactions. Importantly, it is found that all investigated samples meet the magnetic precondition of possible biomedical applications of exhibiting a negligible magnetic remanence at room temperature.

Mössbauer investigations of crystalline and quasicrystalline Al3(Mn, Fe) compounds.

(57)Fe Mössbauer and magnetic measurements are reported on Taylor phase compounds T-Al(3)(Mn,Fe), which can be seen as complex metallic alloys. The orthorhombic unit cell contains 156 atoms. These investigations are further extended to Al(71)Mn(19)Fe(10) crystallizing as a decagonal quasicrystal. Common to both structures are layers, which are oriented perpendicular to the b axis in the crystalline or to the periodic axis in the quasicrystalline compound, allowing the formation of characteristic building blocks. At low temperatures for all samples spin glass behavior is observed with freezing temperatures T(f) increasing with Fe content. Above T(f) the (57)Fe Mössbauer spectra were analyzed by a superposition of two subspectra with intensity ratio around 75:25, which can be allocated to Fe substituted on Mn sites surrounded either by Al and Mn or solely by Al with Mn only at the edge of the nearest neighbor shell. For both subspectra quadrupole splitting and center shift do not change significantly with Fe content and with structure. Below T(f) broad hyperfine field distributions with non-vanishing contributions at zero hyperfine field are present. Magnetic and electrostatic hyperfine interactions are governed by length scales which are determined by the very similar interatomic configurations forming the T-Al(3)Mn phase as well as the decagonal compound. The presence of long range atomic order is of less importance.

Phase formation and ground state properties of CeCo9Si4.

The phase relations of the CeCo(9 + δ)Si(4 - δ) system have been studied by means of scanning electron microscopy, electron microprobe analysis and x-ray diffraction. Essentially single phase samples CeCo(9 + δ)Si(4 - δ) (structure-type LaFe(9)Si(4) with space group I4/mcm) are formed in a narrow composition range - 0.3 ≤ δ < 0.1, where stoichiometric CeCo(9)Si(4) exhibits full structural order in space group I4/mcm. The evolution of the ground state of correlated 3d and 4f electrons in the solid solution CeCo(9 + δ)Si(4 - δ) has been investigated by dc susceptibility, magnetization, specific heat and resistivity measurements. Stoichiometric CeCo(9)Si(4) exhibits paramagnetic Kondo lattice behaviour with a largely reduced Co 3d contribution to the magnetic susceptibility as compared to nearly ferromagnetic LaCo(9)Si(4). Nonetheless, very similar to the solid solution LaCo(9 + δ)Si(4 - δ), weak ferromagnetism is observed in CeCo(9 + δ)Si(4 - δ) for δ > 0 and is attributed to substitutional disorder at the Si-sublattice.

The formation, structure and physical properties of M(2)Pd(14+x)B(5-y) compounds, with M = La, Ce, Pr, Nd, Sm, Eu, Gd, Lu and Th.

Novel ternary compounds, M(2)Pd(14+x)B(5-y) (M =  La, Ce, Pr, Nd, Sm, Eu, Gd, Lu, Th; x∼0.9, y∼0.1), have been synthesized by arc melting. The crystal structures of Nd(2)Pd(14+x)B(5-y) and Th(2)Pd(14+x)B(5-y) were determined from x-ray single-crystal data and both are closely related to the structure type of Sc(4)Ni(29)B(10). All compounds were characterized by Rietveld analyses and found to be isotypic with the Nd(2)Pd(14+x)B(5-y) type. Measurements of the temperature dependent susceptibility and specific heat as well as the temperature and field dependent resistivity were employed to derive basic information on bulk properties of these compounds. The electrical resistivity of M(2)Pd(14+x)B(5-y), in general, is characterized by small RRR (residual resistance ratio) values originating from defects inherent to the crystal structure. Whereas the compounds based on Ce, Nd, Sm and Gd exhibit magnetic order, those based on Pr and Eu seem to be non-magnetic, at least down to 400 mK. While the non-magnetic ground state of the Pr based compound is a consequence of crystalline electric field effects in the context of the non-Kramers ion Pr, the lack of magnetic order in the case of the Eu based compound results from an intermediate valence state of the Eu ion.

Electronic structure of a σ-FeCr compound.

The electronic structure of a σ-FeCr compound in a paramagnetic state was calculated for the first time in terms of isomer shifts and quadrupole splittings. The former were calculated using the charge self-consistent Korringa-Kohn-Rostoker (KKR) Green's function technique, while the latter were estimated from an extended point charge model. The calculated quantities combined with recently measured site occupancies were successfully used to analyze a Mössbauer spectrum recorded at room temperature using only five fitting parameters namely background, total intensity, linewidth, IS0 (necessary to adjust the refined spectrum to the used Mössbauer source) and the QS proportionality factor. Theoretically determined changes of the isomer shift for the σ-FeCr sample were found to be in line with the corresponding ones measured on a α-FeCr sample.

Focal dermal hypoplasia. Current concepts and differential diagnosis.

Focal dermal hypoplasia syndrome (FDH) is a congenital disorder of mesodermal derivation with some ectodermal participation. The disorder, first described in 1962, is characterized by tenacious defects consisting of thinning of the skin; herniations of adipose tissue; abnormal skin pigmentation and other skeletal, dental, ocular, hair, and nail abnormalities. A rare instance of focal dermal hypoplasia syndrome is reported as characterized by papillary gingival hyperplasia. Differential diagnostic guidelines that separate focal dermal hypoplasia from a host of other skin and mucous membrane disorders are discussed along with therapeutic modalities and the possible relationship of human papillomavirus to focal dermal hypoplasia syndrome.