Search

The influence of the cobalt substitution for manganese ions in the mixed valence perovskites La0.8Na0.2Mn1−xCoxO3 (0⩽x⩽0.2) was investigated by X-ray, electric transport and magnetic measurements. The study carried out on sintered polycrystalline samples revealed the rhombohedral ( R 3 ¯ c ) structure and the insulator–metal transition connected with a ferromagnetic arrangement in the whole concentration range. Increasing concentration of cobalt ions leads to a gradual decrease of PM-FM and I–M transition temperatures. An influence of the cobalt ions on the observed behavior is attributed to charge compensation Mn3+→Mn4+ leading to the formation of stable couples Mn4+–Co2+. Therefore the double-exchange interactions Mn3+–O2−–Mn4+ partly vanish and they are replaced by positive superexchange interactions Mn4+–O2−–Co2+, but of a semiconducting character.

An electon and X-ray diffraction investigation of powder and single crystals shows that U 3 Co 4 Ge 7 crystallizes in a tetragonal structure with the space group I 4/ mmm ,: a = 410.87(7) pm, c = 2747.7(9) pm, V = 0.4638(2) nm 3 , Z = 2, R = 0.039 for the Rietveld refinement, and R = 0.050 for the single-crystal study (373 F values and 21 variables). Its structure shows some similarities with that of Eu 2 Pt 7 AlP 4- x and can be interpreted as an intergrowth of CaBe 2 Ge 2 - and Cu 3 Au-type blocks. U 3 Co 4 Ge 7 orders ferro- or ferrimagnetically at 21.5(5) K and exhibits another magnetic transition below 20.5(3) K. Its properties are compared to those observed for other ternary germanides such as UCo 2 Ge 2 and UCoGe.

"U 2 Fe 17 " does not exist in thermodynamic equilibrium; however, alloying uranium with electron donor elements such as Al, Si, or Ge yields compound formation with crystal structures isotypic with the Th 2 Ni 17 -type or derivative structures such as U 2 Fe 12.8 Si 4.2 . The octahedral voids can be partially filled with carbon atoms without exceeding one carbon atom per formula unit. Powder X-ray diffraction of the carbon-containing compounds in all cases revealed the Th 2 Ni 17 -type of structure with a small increase in volume by ∼0.0075 nm 3 per interstitial carbon atom per formula unit. There is only a small influence of the carbon-induced volume increase on the ferromagnetic ordering temperature of the alloys.

CeRhSi 2 and CeIrSi 2 crystallize in the orthorhombic CeNiSi 2 -type structure. Magnetic susceptibility and electrical resistivity measurements reveal that cerium in each of these compounds is in an intermediate valence state, but also that the spin fluctuation temperature is much higher for CeIrSi 2 than that observed for CeRhSi 2 . The magnetic behavior of the ternary silicides such as Ce M 2 Si 2 , Ce 2 M 3 Si 5 , Ce M Si 3 , and Ce M Si 2 with M = Rh or Ir depends on the average distance Ce- M which determine the strength of the 4ƒ(Ce)-4 d or -5 d ( M ) hybridization.

After melting and quenching, the ternary silicides U 2 Fe 17- y Si y exist far 3.3 ≤ y ≤ 4.5 but they partially decompose after annealing at 850-900°C. Their crystal structure, determined by X-ray diffractometry on both single crystals and powder, derives from the hexagonal Th 2 Ni 17 -type but depends strongly on the silicon content. For y = 3.7 some uranium sites are partially replaced by pairs of iron atoms and conversely. This structure shows some similarities to that observed for the binary compound Ho 2 Fe 17 . On the other hand, for y = 4.2, all the uranium atoms and pairs of iron atoms are statistically distributed. In contrast, the ternary silicide U 2 Co 15 Si 2 , which is obtained as single phase after annealing at 850°C, adopts the Th 2 Ni 17 -type structure. In all compounds, iron or cobalt atoms of the pair are never substituted by silicon atoms.

The compounds exhibiting the tetragonal BaAl 4 structure and its modifications represent one of the families of intermetallic systems that have turned out to be particularly numerous and interesting. Their general formula is RT 2 X 2 , where R=f elements (4 f or 5 f ), T =transition metals (3 d , 4 d , 5 d ), and X =silicon or germanium. All these compounds have metallic behavior, but different situations can occur according to the strength of the hybridization of the f states with the valence-band states: Pauli paramagnetism, mixed and intermediate valence states, spin fluctuations, Kondo state, heavy fermion state, and local moment magnetism. The general features of the rare earths and actinides are reviewed so as to illustrate these several physical properties, and a comparison between the rare earth and uranium systems is made.

New compounds Ce3Rh4Ge4 and Ce3Rh3IrGe4 as part of an extended solid-solution range of Ce3Rh4−x IrxGe4 have been synthesized by the arc melting technique starting from the pure elements. The crystal structures of Ce3Rh4Ge4 and Ce3Rh3IrGe4 have been refined from single crystal counter data. Both compounds were found to be isotypic with the U3Ni4Si4-type (space group Immm-D252h, No. 71; Z = 2). The following lattice parameters were obtained: a = 0.40915(5), b = 0.42400(19), c = 2.50673(82) nm for Ce3Rh4Ge4 and a = 0.40839(7), b = 0.42437(6), c = 2.50403(39) nm for isotypic Ce3Rh3IrGe4. For 721 (495) reflections (|Fo| > 3σ) the obtained reliability factors R = Σ |ΔF| Σ F o | were R = 0.08 for Ce3Rh4Ge4 and R = 0.07 for Ce3Rh3IrGe4. According to the topochemical description 2Ce(Rh0.5Ge0.5)2 + CeRh2Ge2 = Ce3Rh4Ge4 the structure type of U3Ni4Si4 is a combination of structural units of the AlB2-type and of the BaAl4-type (ThCr2Si2-type) with a complete statistical distribution of the Rh and Ir atoms. Both compounds, Ce3Rh4Ge4 and Ce3Rh3IrGe4, have been investigated at low temperatures by means of dc electrical resistivity and magnetic susceptibility measurements. No magnetic ordering has been detected above 2 K.

The structure of the ternary boride Nd 0.71 Rh 3.29 B 2 was determined by single-crystal X-ray diffraction. It has a hexagonal symmetry with space group P 6 2 m and cell parameters a = 5.595(2) and c = 2.855(1) A. Its structure is similar to that of the other CeCo 3 B 2 -type ternary borides. It is made up of two types of alternating layers, parallel to the (001) plane, formed by either (Nd,Rh) and B atoms or by Rh atoms alone.

The electric and magnetic properties of the La x Eu 1− x B 6 solid solutions have been investigated. A sharp evolution from semiconducting to metallic character is observed between x = 0 and x = 0.01. Evolution with x of the atomic volume, conductivity, Seebeck coefficient and magnetic properties are discussed in terms of a band model in which the conduction band is the 5 d or 6 s band of the rare-earth-atom sublattice. It is argued that chemical inhomogeneities produce a finite range of compositions over which the Seebeck coefficient changes from metallic character toward small-polaron character with decreasing x , whereas the metal-to-semiconductor transition appears more sharply defined.

Cubic MB6 borides (M = Th, La, Yb, Ca, Sr) have been prepared with a high purity either from the elements or by borothermic reduction of oxides. Magnetic susceptibility, thermoelectric power and specific heat measurements have been done to study the electronic structure of the compounds. All the specimens were sintered at high temperature except LaB6 samples which were zone refined. Hexabordies of elements having an oxidation state higher than two (M = Th, La) exhibit a metallic behavior. In YbB6 ytterbium is divalent. Hexaborides of elements having an oxidation state equal to two (M = Yb, Ca, Sr) are semiconductors. The electronic behavior of the compounds which have been studied agrees with the H. C. Longuet-Higgins and M. de V. Roberts' model.

A single crystal of the δ phase has been obtained in the boron-thorium system by slow cooling of a liquid corresponding to an atomic ratio B Th = 70 . The structure has been established, from two sets of 650 independent reflexions. Absorption correction was made necessary by the thorium atoms. From least-squares refinement, Fourier syntheses and electron density maps it appears that the δ phase is isotypic with the yttrium boride YB66. The crystal is cubic with a = 23.53 A ; the space group is Fm3c, Oh6. There are 1584 boron atoms and 24 thorium atoms in the unit cell. 1248 boron atoms are contained in 13 icosahedron units corresponding to the formula B12(B12)12. B-B bonds within icosahedra range from 1.749 to 1.911 A. The shortest intericosahedral bond is 1.598 A, the longest 1.837 A. The remaining boron atoms are statistically distributed in channels resulting from packing of the B12(B12)12 units and form nonicosahedral cages as in YB66. The thorium atoms are inserted in the channels; the thorium-boron bonds range from 2.71–2.84 A.

The magnitude of the magnetic hyperfine field at EuII in the cubic solid solutions (EuO)1−x(LnN)x (where Ln = Nd, Eu, Gd) has been determined at 4.2°K by 151Eu Mossbauer spectroscopy. The resultant effective hyperfine field is found to be almost independent of composition because of a number of compensating effects.