Results 1 - 10 of 10991
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[en] Exchange and crystal fields (CF) acting on R3+ ions in R2Fe14B were investigated by means of inelastic magnetic neutron scattering. Experimental data for R=Pr, Nd, Gd, Tb, Dy, Ho, Er, and Tm are reviewed. Molecular fields H acting on R ions are calculated using neutron-scattering data for the CF models proposed earlier for these compounds based on single-crystal magnetization measurements. For Gd2Fe14B the molecular field acting on the Gd3+ ions was directly determined by inelastic neutron scattering as 2μBHm/kB = 435 K
[en] For a group of s-, p-, and d-metal diborides (MB2, where M = Mg, Al, Sc, Ti, V, Y, Zr, Nb, or Ta), variations in the cohesive energy and the formation energy in the presence of cation vacancies (MB2 → M0.75B2), as well as the formation energy of cation-site vacancies, are analyzed using the FLMTO-GGA full-potential approach. The cationic nonstoichiometry for MB2 phases is achievable only when they are prepared under nonequilibrium conditions
[ru]Для группы диборидов s-, p-, d-металлов (МВ2, где М = Mg, Al, Sc, Ti, V, Y, Zr, Nb и Та) в рамках полнопотенциального метода FLMTO-GGA выполнен анализ изменений их энергий когезии и энергий формирования в присутствии вакансий по катионной подрешетке (МВ2 → М0.75B2), а также энергий образования катионных вакансий. Установлено, что катионная нестехиометрия для МВ2-фаз может быть достигнута лишь при неравновесных условиях их получения
[en] This paper describes a method for the formation of complex boride whiskers within a final metal matrix. The complex boride comprises boron and at least two of the elements selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W
[en] Magnetic and structural behaviour and phase relationships of materials of composition R3Ni7B2 (R = Nd-Lu) were investigated. Detailed X-ray analysis yields that two hexagonal structures are encountered. For the heavy rare earth (Gd-Lu) the compounds crystallize in the CeNi3 structure. The space group is P63/mmc and each unit cell contains two formula units. The R3Ni7B2 where R = Nd-Sm (including Yb3Ni7B2) crystallize in the CeCo4B structure. The space group is P6/mmm and each unit cell contains one formula unit. The detailed crystal structures are discussed. The magnetic measurements show that Yb3Ni7B2 and Lu3Ni7B2 are Pauli paramagnetic. Sm3Ni7B2 is ferromagnetically ordered with a huge intrinsic magnetic hardness. The magnetization at the coercive field at low temperatures is extremely time dependent. The R3Ni7B2 which crystallize in CeNi3 structure are antiferromagnetic at low temperatures. All Mossbauer and magnetization experimental results can be explained assuming an antiferromagnetic exchange interaction in both 2(c) and 4(f) crystallographic sites and a ferromagnetic interaction between these sites. (author)
[en] An investigation of the interplay between long-range magnetic order and superconductivity in pseudoternary Ho(Rh/sub 1-x/Ir/sub x/)4B4 and Dy(Rh/sub 1-x/Ir/sub x/)4B4 systems was carried out by means of low temperature specific heat measurements. The properties of the intermediate valent compounds CeT3B2 and UT3B2 (T = CO, Ru, Rh, and Ir), as well as the behavior of superconductivity and ferromagnetism in pseudoternary Ce(Ru/sub 1-x/Rh/sub x/)3B2 compounds, were characterized by low temperature specific heat, electrical resistivity, and magnetic measurements. A low temperature phase diagram vs alloy composition was determined for Ho(Rh/sub 1-x/Ir/sub x/)4B4. The specific heat features associated with long-range ordering of the Ho+3 magnetic moments suggest mean field theory ferromagnetism for x less than or equal to 0.2. Antiferromagnetic ordering was observed for x greater than or equal to 0.2 and antiferromagnetic spin wave behavior was found for the ordered Ho+3 moments for x greater than or equal to 0.5. Unusual magnetic ordering occurs for 0.275 less than or equal to x less than or equal to 0.4 including two distinct magnetic transitions. The compound CeRh3B2 exhibits ferromagnetic ordering for T less than or equal to 113 K with a saturation magnetic moment 0.37 μ/sub B/ per formula unit, while CeRu3B2 exhibits superconductivity below 0.68 K. Values of the electronic specific heat coefficient γ for the compounds range from 9.7 mJ/mol-K2 for CeCo3B2 to 64 mJ/mol-K2 for UIr3B2. A phase diagram of the transition temperatures vs alloy composition was determined for Ce(Ru/sub 1-x/Rh/sub x/)3B2. Superconductivity of CeRu3B2 disappears for Rh substitution x greater than or equal to 0.38
[en] The present work deals with the high-temperature thermoelectric properties of transition metal [eg V, Cr, Mn, Ni, Cu] and lanthanide [e.g. Sc, Y, Gd, Er, Dy]-borides. In particular, intercalation compounds of beta-rhombohedral boron, compounds of the type MB_6_6, dodecaborides and hexaborides were examined. In the case of intercalation compounds of beta-rhombohedral boron it was found that the incorporation of metals such as Sc, Mn or Cu result in favorable thermoelectric properties. The reason is most likely the preferred occupation of the metal position M2 instead of M4. Composites, for example, DyB_6_6-DyB_1_2 show high electrical conductivities, high Seebeck effects at high temperatures due to the presence of DyB_1_2 and low thermal conductivities as a result of the DyB_6_6 matrix. At 1100K the composite DyB_6_6-DyB_1_2 shows a ZT value of 0.55, thus exceeding the ZT of boron carbide (B_1_3C_2) at this temperature which is considered the best p-type boride material. A composite of ErB_1_2-ErB_4-ErB_2 has negative Seebeck coefficients and shows a ZT value of 0.5 at 840K. Furthermore, the structure of tetragonal Scandiumdodecaboride ScB_1_2 was solved on the basis of synchrotron data from a crystalline powder, after it has been debated for decades but never fully resolved.
[en] The bond ionicity in biborides MB2 (M=Mg, Ti, V, Cr, Mn, Zr, Hf, Ta, Al and Y) has been studied by using the complex chemical bond theory based on a generalization of the Phillips-Van Vechten-Levine scheme. The ionicity of M-B bonds decreases in the following order: Mg, Al, Mn, Y, Cr, Zr, Hf, Nb, Ta, V and Ti. The Mg-B bond in MgB2 has the largest value of ionicity of 96.8% among these examined diborides. Our calculations support the recent results of band structure calculations that Mg atoms are fully ionized so that they can donate valence electrons to the system. The observed superconductivity loss in the solid solution Mg1-xAlxB2 with x>0.1 can be understood in terms of the ionicity and the number of valence electrons. (author). Letter-to-the-editor
[en] We have performed accurate ab initio total energy calculations using the full-potential linearized augmented plane-wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange-correlation potential to systematically investigate elastic properties of 18 stable, metastable and hypothetical hexagonal (AlB2-like) metal diborides MB2, where M = Na, Be, Mg, Ca, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ag and Au. For monocrystalline MB2, the optimized lattice parameters, independent elastic constants (Cij), bulk moduli (B) and shear moduli (G) are obtained and analyzed in comparison with the available theoretical and experimental data. For the first time, numerical estimates of a set of elastic parameters of the polycrystalline MB2 ceramics (in the framework of the Voigt-Reuss-Hill approximation), namely bulk and shear moduli, compressibility (β), Young's modulus (Y), Poisson's ratio (ν) and Lame coefficients (μ, λ), are performed.