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A polycrystalline sample of the Sn20.5□3.5As22I8 clathrate was synthesised by a standard ampoule method from elements. Temperature dependencies of the heat capacity Cp(T) and unit cell parameter a(...

In the realm of intermetallic compounds, there is a small group of phases that demonstrate the properties of non-metals. These are intermetallics formed by elements from different blocks of the periodic table, viz., transition metals on the one hand and p-block metals (or metalloids) on the other hand. The interaction between transition metal d-orbitals and p-element s- and p-orbitals leads to the opening of a gap in the band structure, and the Fermi level may fall into the band gap at a certain valence electron concentration, thus providing the non-metallic properties. Despite the lack of a harmonious theory describing the electronic structure of the intermetallics in question, there are approaches that allow one to chemically modify them and thus to design novel thermoelectric materials through the interplay between metals and semiconductors. The review concerns the main families of intermetallic compounds with non-metallic properties. Specific features of their crystal and electronic structures, the reasons for the onset of non-metallic (semiconducting) state, and the prospects for application of these compounds as thermoelectric materials are analyzed.

Intermetallic compounds with semiconducting properties are rare, but they give rise to advanced materials for energy conversion and saving applications. Here, we present ReGa2Ge, a new electron-pre...

We report synthesis, crystal and electronic structure, and transport properties of new intermetallic compound ReGa0.4Ge0.6, which was obtained by two-step ampule method from the elements. ReGa0.4Ge0.6 crystallizes in its own structure type (space group I4/mmm, a = 2.89222(3) A, c = 15.1663(3) A, and Z = 4) which can be described as a sequential alternation of blocks of rhenium atoms and blocks of gallium and germanium atoms. Chemical bonding analysis reveals pronounced covalency of Re–Re, Re–E, and E–E (E = Ga and Ge) interactions and an interesting bonding pattern that includes many variations of localized bonding within a single compound, including pairwise homo- and heterometallic bonding, three-centered homometallic and four-centered bonding, and possibly even more delocalized bonding, which is not often encountered in such a simple intermetallic compound. Metallic behavior is confirmed by electronic structure calculations and by measurements of electrical resistivity.

ReGaGe2 is a new member of the family of intermetallic compounds with non-metallic properties. It displays highly localized covalent bonding patterns. Its electronic structure is governed by mixing of Re d orbitals with the s and p orbitals of Ga and Ge and features the Fermi level falling into the opened band gap, ensuring experimentally confirmed semiconducting properties.

We have explored the crystal structure and magnetic properties of two intermetallic solid solutions of the IrIn3 structure type. Although Fe0.95Co0.05Ga3 and Fe0.975Ni0.025Ga3 are isomorphous and possess exactly the same concentration of valence electrons, they display noticeable differences in their crystal structure associated with a different nature of the Fe Co and Fe Ni dumbbells. The former contains an odd number of electrons compared to an even number of the latter dumbbell. Responding to localized spins of the Fe Co dumbbells, the 69Ga NQR relaxation rate in Fe0.95Co0.05Ga3 is governed by temperature independent spin-lattice relaxation typical for diluted paramagnetic centers at low temperatures. In Fe0.975Ni0.025Ga3 with zero magnetic moment of the dumbbells the 69Ga spin-lattice relaxation rate is strongly reduced and in-gap states in the energy gap show up as a broad maximum near 70 K.

We performed an experimental study on thermal properties of the Sn18In6As21.5I8 clathrate by measuring temperature dependencies of its heat capacity (2-300 K) and thermal expansion (5-300 K). By comparing the results with those published previously for Sn-based clathrates Sn24P19.2I8, Sn20Zn4P20.8I8, and Sn17Zn7P22I8, we established that partial replacement of tin and phosphorus by heavier indium and arsenic, respectively, leads to lowering vibration frequencies in both host and guest substructures. Deviation of the observed thermal properties at low temperatures from those predicted by the Einstein-Debye model is caused by the Schottky-like contribution of two-level systems to heat capacity and thermal expansion. These systems form owing to transitions of guest atoms in non-spherical 24-vertex cages between stationary states with close energies.

The temperature dependencies of heat capacity, CP(T), and cubic unit cell parameter, a(T), were experimentally obtained in the range of 2-300 K for the compounds Sn24P19.2I8, Sn20Zn4P20.8I8, and Sn17Zn7P22I8, which belong to a family of type-I clathrates. The experimental data were analyzed in the frames of the Debye-Einstein approximation, further accounting for the contributions of positional disorder in the clathrate frameworks as well as those of defect modes arising from the distribution of guest atoms over unequal in energy but close in space positions inside the framework cages. By fitting the experimental data, the Debye and Einstein characteristic temperatures describing the dynamics of the framework and guest atoms, respectively, were obtained. Their analysis revealed peculiar dependencies of the characteristic temperatures upon the number of substituted zinc atoms and the concentration of vacancies in the framework, which are discussed in this paper.

Temperature changes of the heat capacity and unit cell parameters of Sn24P19.4Br8 clathrate were experimentally determined in the temperature range of 2 to 300 K. The data obtained were analyzed using Debye–Einstein approximation and taking into account the impact of both disorder in the host matrix and the presence of vacancies in the framework. Anomalous negative contribution to the thermal expansion was revealed and related to the defect mode influence on the clathrate thermal properties as a result of vibrations of two-level systems (TLS). The guest atoms that have the opportunity to occupy spatially close yet energetically non-equivalent positions in the asymmetric environment of the host matrix atoms play a principal role in the TLS formation. The results are compared with those previously obtained for semiclathrate Ge31P15Se8.

ReGaGe2 is a new member of the family of intermetallic compounds with non-metallic properties. It displays highly localized covalent bonding patterns. Its electronic structure is governed by mixing of Re d orbitals with the s and p orbitals of Ga and Ge and features the Fermi level falling into the opened band gap, ensuring experimentally confirmed semiconducting properties.

International audience; We present a new Fe1–xRexGa3 solid solution, in which a 5d-metal––rhenium––partially substitutes for iron to the limiting composition of x = 0.10. The crystal structure refined for the composition Fe0.91Re0.09Ga3 shows the expected increase in the unit cell parameters compared to the parent FeGa3 compound, however the M–M (M = Fe, Re) distance decreases within the M–M dumbbell, indicating an increased M–M bonding density. Therein, investigation of the local structure by means of 69,71Ga NQR spectroscopy revealed the formation of homonuclear Fe–Fe and Re–Re dumbbells. Transport and thermoelectric properties have been investigated for the Re-substituted FeGa3. Electrical transport measurements showed preservation of the nonmetallic conductivity of Fe1–xRexGa3 despite the decrease of the valence electron concentration from 17 to 16.9 electrons per formula. At low temperatures, Fe1–xRexGa3 is a p-type semiconductor with the band gap of 0.4 eV, but with increasing temperature the sign of the dominant charge carriers changes. Owing to the alloying effect, Fe1–xRexGa3 displays 1.5 times lower thermal conductivity than FeGa3, which increases at high temperatures because of the growing contribution of the electronic term. © 2019

Nowotny chimney ladder (NCL) phases are intermetallic compounds formed by transition metals and metals of groups 13 and 14. This family can be expanded by combining two p-elements from different groups with those transition metals, for which the corresponding binary NCL phases are unknown. In this paper, we present three new compounds in the V-Al-Ge, Nb-Al-Ge, and Nb-Ga-Ge systems related to the TiSi2 structure type (Sp. Gr. Fddd) obtained with the standard ampule technique. The crystal structures of the new compounds were determined using synchrotron powder X-ray diffraction data. A transition to the CrSi2 structure type was detected upon changing the composition from VAl0.72(2)Ge1.28(2) to VAl1.534(3)Ge0.466(3). According to the 18&ndash, n rule, all the compounds are metallic conductors, which was supported by the electronic structure calculations. It was shown that the expected energy gap located above the Fermi level in the vanadium-based NCL compound collapsed into a pseudogap upon the replacement of V by Nb.

Unlimited solid solution Fe1� xCoxGa3 was prepared from Ga flux. Its crystal structure was refined for Fe0.5Co0.5Ga3 (P42/mnm, a¼6.2436(9), c ¼ 6.4654(13), Z¼ 4) and showed no ordering of the metal atoms. A combination of the electronic band structure calculations within the density functional theory (DFT) approach and 69,71 Ga nuclear quadrupole resonance (NQR) spectroscopy clearly shows that the Fe–Fe and Co–Co dumbbells are preferred to the Fe–Co dumbbells in the crystals structure. The band structure features a band gap of about 0.4 eV, with the Fermi level crossing peaks of a substantial density of electronic states above the gap for x40. The solid solution is metallic for x40.025. The study of the nuclear spin–lattice relaxation shows that the rate of the relaxation, 1/T1, is very sensitive to the Co concentration and correlates well with the square of the density of states at the Fermi level, N 2 (EF).