Volume 62, Issue 7

The new compounds A 2 MnV 2 O 7 (A = K, Rb) with structures related to the melilite-type have been synthesized by a solid state reaction route. The crystal structures of K 2 MnV 2 O 7 , Rb 2 MnV 2 O 7 and KRbMnV 2 O 7 have been determined using X-ray single crystal diffraction data. The compound K 2 MnV 2 O 7 crystallizes with a melilite-type structure with tetragonal unit cell parameters a = 8.609(3), c = 5.538(4) Å and space group P4̅2 1 m. The structures of Rb 2 MnV 2 O 7 and KRbMnV 2 O 7 are derived from the melilite-type structure with space group P4̅2/mnm and unit cell parameters a = 8.577(6), c = 11.809(6) Å , and a = 8.530(6), c = 11.466(5) Å , respectively. The three structures consist of [MnV 2 O 7 ] 2 − layers perpendicular to the c axis separated by A + layers. The [MnV 2 O 7 ] 2 − layers feature corner-sharing MnO 4 tetrahedra and V 2 O 7 pyrovanadate units, the linkage leading to rings of five tetrahedra. The doubling of the c parameter for Rb 2 MnV 2 O 7 or RbKMnV 2 O 7 is explained by the existence of a mirror plane perpendicular to the [001] direction between two [MnV 2 O 7 ] 2 − layers. The A + alkali cations occupy distorted square antiprisms of oxygen atoms in K 2 MnV 2 O 7 and distorted square prisms of oxygen atoms in Rb 2 MnV 2 O 7 and RbKMnV 2 O 7 .

Magnetic properties and bonding analyses of perovskite structure-derived TFe 3 N (T = Ru, Os) nitrides have been investigated within density functional theory using both pseudo potential and all electron methods. At equilibrium, spin degenerate non-magnetic (NM) and ferromagnetic (FM) calculations of energy versus volume show that the ground state of the two compounds is ferromagnetic. Magnetic moments of Ru/Os and Fe, respectively, being situated at two different crystallographic sites are studied over a wide range of the cubic lattice parameter. The volume expansion indicates that iron atoms show itinerant magnetism while Ru and Os exhibit a localized behavior. Important magnetovolume effects are observed, with saturation of the magnetic moment reached in RuFe 3 N but not in OsFe 3 N. The electronic structure is visualized for the different binding characters Fe-N versus Ru/Os-N with the help of electron localization plots. The density of states of the ferromagnetic ground state is interpreted on the basis of a covalent magnetic model which goes beyond the Stoner rigid band model. An Invar-like behavior is predicted for the two nitrides.

The intermetallic compound Ce 2 Ni 1.88 Cd (tetragonal Mo 2 FeB 2 -type structure) has been investigated by magnetization (down to 1.8 K), electrical resistivity (ρ) and heat capacity (down to 0.5 K) measurements as a function of temperature (T). On the basis of these data, it is concluded that this compound is a heavy-fermion with a Kondo temperature in the range 235 - 250 K. Fermi-liquid behaviour expected for Kondo lattices is obeyed for the low temperature ρ data, but the ratio of the coefficient of the T 2 term in the electrical resistivity to the square of the linear coefficient of the heat capacity is apparently low compared to the ‘universal ratio’ known for other cerium compounds, as though there is a breakdown of the Kadowaki-Woods relation in this compound. The value of this ratio falls in the range known for many intermediate-valent ytterbium compounds.

The structure of CeB 4 has been determined by single crystal X-ray diffraction. The compound crystallizes in the ThB 4 structure type (space group P4/mbm, a = 7.2034(8), c = 4.1006(5) Å; 270 reflections with F o ≥ 4σ (F o ), R1 = 0.023, wR2 = 0.052). The results of the magnetic and electrical resistivity measurements indicate a strong f-d hybridization of the 4 f electrons of the cerium atom

The cerium plumbides CeCuPb, CePdPb, CeAgPb, and CeAuPb have been synthesized from the elements in sealed tantalum tubes in a high-frequency furnace followed by annealing. The copper, silver and gold compounds crystallize with the NdPtSb-type structure, space group P6 3 mc: a = 466.1(2), c = 778.1(3) pm for CeCuPb, a = 484.8(2), c = 773.0(3) pm for CeAgPb, and a = 480.24(9), c = 772.9(2) pm for CeAuPb. CePdPb crystallizes with the ZrNiAl-type structure: P6̅2m, a = 775.0(2), c = 413.32(8) pm. The dc susceptibility measurements show trivalent cerium for all four plumbides with experimental magnetic moments of 2.53(2), 2.71(2), 2.63(2), and 2.58(2) μB/Ce atom for CeCuPb, CePdPb, CeAgPb, and CeAuPb, respectively. The compounds CeCuPb, CeAgPb, and CeAuPb, studied by dc susceptibility and specific heat measurements, order antiferromagnetically at N´eel temperatures of 8.2(1), 7.4(1), and 3.7(1) K, respectively.

Magnesium metal is under extensive study for its high hydrogen absorption capacity. The main results obtained by the authors, going from the effects of ‘reactive mechanical grinding’ to the addition of nano oxides or to the deposition of nanoparticles are reported and discussed. The absorption properties are compared with the results of other research groups. An improvement of kinetics has been achieved and the mechanisms of the hydrogenation reaction is almost fully understood, but the effects of catalysts are still subject to hypotheses. Recent developments of MgH 2 tanks are also presented.

The hydrogen absorption-desorption characteristics of composites consisting of 90 wt.% MgH 2 - 10 wt.% Mg 2 Ni 0.8 Co 0.2 prepared by ball milling for 30 and 180 min under argon have been investigated. The results obtained have been compared with those for the 90 wt.% Mg-10 wt.% Mg 2 Ni 0.8 Co 0.2 composite synthesized in the same medium after 30 min of milling. The presence of the Mg 2 Ni 0.8 Co 0.2 phase has been found to improve the hydriding kinetics of magnesium, the absorption capacity remaining high at temperatures below 573 K. The use of magnesium hydride instead of magnesium has proved to have a favorable effect on the properties of the composites after prolonged activation in an inert medium. It has been established that the absorption-desorption characteristics of the composite 90 wt.% MgH 2 -10 wt.% Mg 2 Ni 0.8 Co 0.2 activated mechanically for 180 min are comparable with those of the composite 90 wt.% Mg-10 wt.% Mg 2 Ni 0.8 Co 0.2 after mechanical activation for only 30 min. The favorable absorption-desorption characteristics of the composites have been explained with the catalytic effect of the additive, the presence of magnesium hydride and the duration of ball milling.

Crystallographic superstructures in the deuterides CeCuSiD 1.64(5) and CeCuGeH 1.15(5) have been characterized by neutron diffraction. Deuterium atoms are inserted in [Ce 3 Cu] and [Ce 3 Si] or [Ce 3 Ge] tetrahedral sites available in the hexagonal ZrBeSi-related structure of these deuterides. A partial but ordered occupancy of these tetrahedra by D atoms induces the occurrence of superstructures along the c axis, and the CeCuXD y structures can be described with a c parameter multiplied by 3 and 7 for X = Ge and Si, respectively, in comparison with the c parameters of the ZrBeSi-type subcells.

The ternary indium palladium intermetallics In 3 Pd 5 , InPd 2 , and InPd 3 have been synthesized by iodine-catalyzed reactions from the elements. Rietveld refinements on X-ray powder diffraction patterns provide the first accurate crystal structure data for In 3 Pd 5 (Pbam, No. 55, a = 1104.20(2), b = 561.346(8), c = 424.263(6) pm, Rh 5 Ge 3 -type) and InPd 2 (Pnma, No. 62, a = 561.676(6), b = 421.710(4), c = 822.78(8) pm, Co 2 Si-type). X-Ray powder diffraction apparently confirms the TiAl 3 structure type proposed in the literature for InPd 3 . However, Rietveld refinement on neutron powder diffraction data reveals an In/Pd distributional disorder. Therefore, we describe the crystal structure of InPd 3 in a AuCu-type model instead (P4/mmm, No. 123, a = 287.224(4), c = 380.079(7) pm), with mixed occupancy of one crystallographic site by 50% In and 50% Pd. In contrast to In 3 Pd 5 and InPd 2 , which can be considered to be line compounds, InPd 3 shows a non-negligible homogeneity range with unit cell volumes ranging from 0.126132(5) nm 3 for the indium-rich to 0.125474(8) nm 3 for the palladium-rich In 1 + x Pd 3 − x phases. Mean In-Pd distances in these indium palladium intermetallics range from 272.3 pm (In1 in In 3 Pd 5 ) with coordination number 8 for indium to 281.2 pm for 12-coordinated In in InPd 3 .

The binary compound Yb 5 Bi 3 was synthesized by reaction of the elements in a sealed Ta container. Its crystal structure was determined from single-crystal X-ray diffraction data: β -Yb 5 Sb 3 -type, space group Pnma, Pearson code oP32, a = 12.6375(6), b = 9.7243(4), c = 8.4117(5) Å, V = 1033.72(9) Å 3 , Z = 4, R gt (F) = 0.028, wR ref (F 2 ) = 0.069, T = 290 K. Band structure calculations and analysis of the chemical bonding suggest mainly ionic interactions in the crystal structure and a possible presence of ytterbium in two valence states Yb 2+ and Yb 3+ . The magnetization measurements showed that at low temperatures Yb 5 Bi 3 contains ytterbium exclusively in the 4f 14 configuration without fluctuations to the Yb 4f 13 configuration up to 400 K. From the Yb-L III X-ray absorption spectroscopy data the effective valence of ytterbium was found to be 2.11 (89% of Yb in 4f 14 configuration).

Polycrystalline PrCu 2 , which has a quadrupolar transition at 7.7 K, has been investigated using electrical resistivity, magnetization and dilatometry techniques. To study dilution effects, two solid solutions of PrCu 2 , (Pr 0.8 La 0.2 )Cu 2 , and (Pr 0.8 Y 0.2 )Cu 2 , were also studied. The quadrupolar transition decreases in temperature with doping, while it increases slightly with the magnetic field. In resistivity and thermal expansion, the magnetic contributions show a clear evidence of crystal field excitations. The analysis of both properties provided benchmark values of the Debye temperature and Grüneisen parameters.

Synthesis routes to rutile-type oxides with 4d and 5d transition elements are summarized. Trends in electronic structure have been established through an analysis in the framework of density functional theory presenting the band structure, the density of states and the properties of chemical bonding. The metal-oxygen bond is found to play the major role in bonding of the system in the valence band. Throughout the series 4d → 5d (RuO 2 , RhO 2 , PdO 2 and PtO 2 ) the crystal field analysis of the band structure shows a lowering of e g towards t 2 g manifolds and a broadening of the overall density of states. In the vicinity of the Fermi level the role of the antibonding metal-oxygen character is investigated in the context of instability towards possible magnetic polarization, especially for RuO 2 .

X-ray powder and EDX phase analyses, thermal analyses, impedance spectroscopic measurements, and crystal structure determinations of selected phases are reported to highlight the structural and electric properties of the solid solutions Ag 10 Te 4 Br 3 − x Cl x and Ag 10 Te 4 Br 3 − y I y . Bromide can be partially substituted by chloride or iodide up to x = 1.6 and y = 0.2, respectively, without changes in the structural properties of the tetramorphic, ion-conducting Ag 10 Te 4 Br 3 . Results from X-ray powder phase analyses have been complemented by data of single crystal structure determinations of selected samples. At r. t. the fully ordered γ -Ag 10 Te 4 Br 3 structure type is realized in all chloride containing phases, but a partial iodide substitution leads to the stabilization of the disordered β -Ag 10 Te 4 Br 3 structure type. Impedance spectroscopic measurements reveal a significant increase by one order of magnitude of the r. t. silver ion conductivity of Ag 10 Te 4 Br 2.8 I 0.2 as compared with Ag 10 Te 4 Br 3 .

The specific heat and thermal conductivity of HoCo 2 and Ho(Co 0.95 Si 0.05 ) 2 were measured as functions of temperature in several constant magnetic fields up to 8 T. From a specific-heat data analysis the isothermal entropy change and the magnetocaloric effect (MCE) have been evaluated in a wide temperature range for several values of the applied magnetic field. The considerable values of the magnetocaloric effect in the vicinity of the magnetic ordering transition are qualifying both compounds as suitable for magnetic refrigeration purposes. The magnetic phase transition temperature (TC) increases from 77 K for HoCo 2 to 103 K for Ho(Co 0.95 Si 0.05 ) 2 while the large MCE in the vicinity of TC is maintained, which demonstrates ways of tuning the operating temperatures of the magnetic refrigerant.

The structural arrangements of the graphite intercalates LnC 6 (Ln = La, Ce, Nd and Yb) were investigated using Density Functional Theory (DFT) within the Generalized Gradient Approximation (GGA). The EuC 6 -type structure (AαAβ AαAβ AαA stacking) is slightly energetically preferred for La and Ce, whereas with the other rare earth metals almost the same cohesive energies are found for the three different atomic arrangements AαAαAαAαAαA. . ., AαAβ AαAβ AαA. . ., and AαAβ AγAαAβ A. . . A rather important charge transfer occurs from the metals to the carbon sheets, with the electrons partially occupying the bottom of the carbon π* band. As a consequence, a lengthening of the C-C bond lengths of ca. 0.02 Å is computed with respect to the C-C bonds in graphite. Two-dimensional metallic character is expected for LaC 6 according to its band structure.

U 2 Co 2 InH 1.9 , synthesized by high-pressure hydrogenation of U 2 Co 2 In, crystallizes in the tetragonal structure similar to the parent compound, expanded by 8.4 %. Although U 2 Co 2 In is a weak paramagnet, its hydride shows properties suggesting a proximity to the magnetic order. Its magnetic susceptibility exhibits a maximum at T = 2.4 K, ascribed to spin fluctuations. Magnetization at low temperatures goes through a metamagnetic transition between 2 - 3 T. The specific heat characteristics, with a pronounced upturn of Cp/T vs. T at low temperatures which can be fitted using an additional −T 1/2 term, resemble the behaviour of U 2 Co 2 Sn. The γ coefficient of the electronic specific heat, reaching 244 mJ mol −1 K −2 , is gradually suppressed by high magnetic fields.