Search

The formation and crystal structure of the binary Cu3As phase have been re-investigated. Some physical properties were then measured on both single crystal and polycrystalline bulk. Cu3As melts congruently at 835 °C. At room temperature (RT), this compound has been found to crystallize in the hexagonal Cu3P prototype (hP24, P63cm) with lattice parameters: a = 7.1393(1) Å and c = 7.3113(1) Å, rather than in the anti HoH3-type (hP24, P–3c1) as indicated in literature. A small compositional range of 74.0–75.5 at.% Cu (26.0–24.5 at.% As) was found for samples synthesized at 300 and 400 °C; a corresponding slight understoichiometry is found in one out of the four Cu atomic sites, leading to the final refined composition Cu2.882(1)As. The present results disprove a change in the crystal structure above RT actually reported in the phase diagram (from γ' to γ on heating). Instead, below RT, at T = 243 K (−30 °C), a first-order structural transition to a trigonal low-temperature superstructure, LT-Cu3−xAs (hP72, P–3c1) has been found. The LT polymorph is metrically related to the RT one, having the c lattice parameter three times larger: a = 7.110(2) Å and c = 21.879(4) Å. Both the high- and low-temperature polymorphs are characterized by the presence of a tridimensional (3D) uncommon and rigid Cu sublattice of the lonsdaleite type (Cu atoms tetrahedrally bonded), which remains almost unaffected by the structural change(s), and characteristic layers of triangular 'Cu3As'-units (each hosting one As atom at the center, interconnected each other by sharing the three vertices). The first-order transition is then followed by an additional structural change when lowering the temperature, which induces doubling of also the lattice parameter a. Differential scanning calorimetry nicely detects the first low-temperature structural change occurring at T = 243 K, with an associated enthalpy difference, ΔH(TR), of approximately 2 J/g (0.53 kJ/mol). Low-temperature electrical resistivity shows a typical metallic behavior; clear anomalies are detected in correspondence to the solid-state transformations. The Seebeck coefficient, measured as a function of temperature, highlights a conduction of n-type. The temperature dependence of the magnetic susceptibility displays an overall constant diamagnetic response. [ABSTRACT FROM AUTHOR]

The crystallographic and transport properties of thin films fabricated by pulsed laser deposition and belonging to the Sm y (Fe x Ni1- x )4Sb12 filled skutterudite system were studied with the aim to unveil the effect exerted by temperature and duration of thermal treatments on structural and thermoelectric features. The importance of annealing treatments in Ar atmosphere up to 523 K was recognized, and the thermal treatment performed at 473 K for 3 h was selected as the most effective in improving the material properties. With respect to the corresponding bulk compositions, a significant enhancement in phase purity, as well as an increase in electrical conductivity and a drop in room temperature thermal conductivity, were observed in annealed films. The low thermal conductivity, in particular, can be deemed as deriving from the reduced dimensionality and the consequent substrate/film interfacial stress, coupled with the nanometric grain size. [ABSTRACT FROM AUTHOR]

The central part of the La–Ni–Si system has been investigated at 800 °C by means of single crystal X-ray diffraction, microscopy and analytical microprobe techniques. The result led to the identification of four new metal-rich silicides: LaNi2Si (R3¯m, a = 4.0263(3) Å, c = 15.066(2) Å, Z = 3), La2Ni3Si2 (P21/c, a = 6.8789(7) Å, b = 6.2167(3) Å, c = 12.214(1) Å, β = 90.92(1), Z = 4), La3Ni3Si2 (Pnma, a = 7.501(2) Å, b = 14.316(4) Å, c = 6.149(2) Å, Z = 4), La6Ni7Si4 (Pbcm, a = 6.066(1) Å, b = 7.488(1) Å, c = 29.682(5) Å, Z = 4). LaNi2Si belongs to the SrCu2Ga structure type, which is not represented among silicides, while La2Ni3Si2 crystallizes in its own structure type. Both compounds feature layered polyanionic motifs consisting of Ni and Si, which are separated by La. Instead, La6Ni7Si4 and La3Ni3Si2 are characterized by polyanionic networks. The former compound belongs to the Pr6Ni7Si4 structure type, with only two other representatives (Ce and Nd); the latter has been observed only with Rh and Ir. The two structures reveal close structural relationships having multiple identical slabs. Tight-binding electronic structure calculations by linear muffin-tin-orbital methods were performed for LaNi2Si, La2Ni3Si2 and La3Ni3Si2 to gain insights into their structure–bonding relationships. Their band structures suggest a metallic character for all compounds. The overall crystal orbital Hamilton populations are dominated by polar Ni–Si bonds, though homoatomic Ni–Ni and La–Ni(Si) bond contributions are not negligible. The variety of bonding patterns may serve as a logical explanation for the number of discovered compounds in this system as well as for the diversity of the observed structures. [ABSTRACT FROM AUTHOR]

We have investigated the formation and stability of the binary rare-earth phases, previously reported in the literature as “RPd2”. Although these phases apparently showed a simple stoichiometry, their structure was not that of a Laves phase. After more than half of a century their crystal structure and true stoichiometry have been finally established. The exact stoichiometry of the “RPd2” phases is R10Pd21. The formation of the R10Pd21 compounds has been ascertained for R = Y, Pr, Nd, Sm, Gd to Lu, including Yb; Sc does not form the 10 : 21 phase. Pr10Pd21 forms only as a high temperature phase. These compounds crystallize in the monoclinic Sm10Pd21-type (mS124-C2/m, No. 12, Z = 4); the lattice parameters and unit cell volume decrease linearly on going from Pr to Lu in agreement with the lanthanide contraction. The crystal structure has been determined by X-ray powder diffraction and refined by Rietveld method for the Y, Nd, Tb and Lu compounds. These phases form by a peritectic reaction; the formation temperature, Tf, of some of them has also been measured and compared with the values reported for the “RPd2” phases for which the literature data are available. The increase of both the volume contraction and Tf on going from Pr10Pd21 to Lu10Pd21 suggests an increase of the thermodynamic stability along the series from the lighter to the heavier R. Physical properties (heat capacity, magnetic susceptibility and magnetization, electrical resistivity and magnetoresistance) have been measured for some members of the series (Nd10Pd21, Gd10Pd21, Tb10Pd21, Dy10Pd21, Ho10Pd21, Lu10Pd21 and Y10Pd21). Antiferromagnetic ordering is found for compounds with magnetic R ions at low temperature; the highest Néel temperature, TN, is 10 K for Tb10Pd21. The present experimental work represents a valuable contribution towards a more complete knowledge of the R–Pd systems. In the light of these results, all the binary R–Pd phase diagrams have to be reinvestigated (mainly those based on data assessed by the CALPHAD technique, using optimized thermodynamic parameters) and updated to furnish the material scientists consistent information to challenge the study of more complex systems. [ABSTRACT FROM AUTHOR]

The crystal structure, composition and physical properties of Gd3Ni2, which was earlier reported to exist in the Gd–Ni system without any details of its structure and properties, have been determined. This rare earth binary compound is a high-temperature phase: it forms via a peritectic reaction at 988 K (715 °C) and decomposes below ≈923 K (650 °C). The compound can be retained at room temperature as a metastable phase by quenching after high temperature annealing. Gd3Ni2 crystallizes in the monoclinic Dy3Ni2 structure type [mS20, C2/m (No. 12), Z = 4; with lattice parameters a = 13.418(3) Å, b = 3.720(1) Å, c = 9.640(2) Å, β = 106.250(3)°]. Ni can be substituted by Co up to 50% (i.e. up to and including Gd3CoNi) with no change in the structural prototype; the substitution of Co for Ni stabilizes the R3CoxNi2−x phases down to room temperature. The crystal structure, magnetic properties and magnetocaloric effect (MCE) have been investigated for both Gd3Ni2 and the related Gd3CoxNi2−x solid solution alloys (0 ≤x≤ 1). The crystal structure of the Gd3CoNi is a ternary ordered derivative of the monoclinic Dy3Ni2-type, where Co fully occupies only one of the two 4i Wyckoff sites available for the transition metal. To the best of our knowledge, this is the first example of an intermetallic phase showing ordered site occupations by the chemically quite similar elements Co and Ni. All compounds show long range ferromagnetic ordering, with TC progressively increasing from 147 K (for Gd3Ni2) to 176 K (for Gd3CoNi) as a cubic function of the Co content. Evidence of Co contributing to the magnetic interactions in these compounds has been found. First-principles total energy calculations predicted the ordered occupation of Co and Ni at the crystallographic sites of Gd3CoNi, which was later confirmed by single crystal X-ray diffraction. The increased conduction electronic state (3d) exchange splitting at the Fermi level supports the experimentally observed enhanced Curie temperature in Gd3CoNi compared to Gd3Ni2. [ABSTRACT FROM AUTHOR]

The crystal chemical features of the new series of compounds R6Mg23C with R = La-Sm or Gd and Ce6Mg23Z with Z = C, Si, Ge, Sn, Pb, P, As, or Sb have been studied by means of single-crystal and powder X-ray diffraction techniques. All phases crystallize with the cubic Zr6Zn23Si prototype (cF120, space group Fm3m, Z = 4), a filled variant of the Th6Mn23 structure. While no Th6Mn23-type binary rare earth-magnesium compound is known to exist, the addition of a third element Z (only 3 atom %), located into the octahedral cavity of the Th6Mn23 cell (Wyckoff site 4a), stabilizes this structural arrangement and makes possible the formation of the ternary R6Mg23Z compounds. The results of both structural and topological analyses as well as of LMTO electronic structure calculations show that the interstitial element plays a crucial role in the stability of these phases, forming a strongly bonded [R6Z] octahedral moiety spaced by zeolite cage-like [Mg45] clusters. Considering these two building units, the crystal structure of these apparently complex intermetallics can be simplified to the NaCl-type topology. Moreover, a structural relationship between RMg3 and R6Mg23C compounds has been unveiled; the latter can be described as substitutional derivatives of the former. The geometrical distortions and the consequent symmetry reduction that accompany this transformation are explicitly described by means of the Bärnighausen formalism within group theory. [ABSTRACT FROM AUTHOR]

Two new polar intermetallic compounds Y3Au7Sn3 (I) and Gd3Au7Sn3 (II) have been synthesized and their structures have been determined by single crystal X-ray diffraction (P63/m; Z = 2, a = 8.148(1)/8.185(3), and c = 9.394(2)/9.415(3) for I/II, respectively). They can formally be assigned to the Cu10Sn3 type and consist of parallel slabs of Sn centered, edge-sharing trigonal Au6 antiprisms connected through R3 (R = Y, Gd) triangles. Additional Au atoms reside in the centres of trigonal Au6 prisms forming Au@Au6 clusters with Au–Au distances of 2.906–2.960 Å, while the R–R contacts in the R3 groups are considerably larger than the sums of their metallic radii. These exclusive structural arrangements provide alluring systems to study the synergism between strongly correlated systems, particularly, those in the structure of (II), and extensive polar intermetallic contacts, which has been inspected by measurements of the magnetic properties, heat capacities and electrical conductivities of both compounds. Gd3Au7Sn3 shows an antiferromagnetic ordering at 13 K, while Y3Au7Sn3 is a Pauli paramagnet and a downward curvature in its electrical resistivity at about 1.9 K points to a superconducting transition. DFT-based band structure calculations on R3Au7Sn3 (R = Y, Gd) account for the results of the conductivity measurements and different spin ordering models of (II) provide conclusive hints about its magnetic structure. Chemical bonding analyses of both compounds indicate that the vast majority of bonding originates from the heteroatomic Au–Gd and Au–Sn interactions, while homoatomic Au–Au bonding is evident within the Au@Au6 clusters. [ABSTRACT FROM AUTHOR]

We have investigated the effects of the isovalent ruthenium substitution in LaFe [ABSTRACT FROM AUTHOR]

We present a systematic study of the superconducting properties of high quality V3Si thin films, grown by high vacuum pulsed laser deposition from a stoichiometric target, a technique never reported in literature for this compound. By changing both the substrate (crystal structure and orientation) and the deposition conditions (substrate temperature, target-substrate distance, laser frequency and pulse energy) the critical temperature and the resistivity values of the samples have been finely tuned. Best results (TC - 16.1 K and Residual Resistivity Ratio RRR = 8.5) are obtained for deposition temperatures higher than 12000 C on both LaAlO3 and MgO substrates. All the samples are polycrystalline, that however does not affect the good quality of the transport properties. The possibility of multiband superconductivity in this com- pound is investigated, probing as a function of temperature: (i) the critical field up to 28 T using voltamperometric measurements, and (ii) the superfluid density via magnetic penetration depth measurements with a single coil inductive technique operating in the MHz region. [ABSTRACT FROM AUTHOR]

Filled skutterudites are currently studied as promising thermoelectric materials due to their high power factor and low thermal conductivity. The latter property, in particular, can be enhanced by adding scattering centers, such as the ones deriving from low dimensionality and the presence of interfaces. This work reports on the synthesis and characterization of thin films belonging to the Smy(FexNi1−x)4Sb12-filled skutterudite system. Films were deposited under vacuum conditions by the pulsed laser deposition (PLD) method on fused silica substrates, and the deposition temperature was varied. The effect of the annealing process was studied by subjecting a set of films to a thermal treatment for 1 h at 423 K. Electrical conductivity σ and Seebeck coefficient S were acquired by the four-probe method using a ZEM-3 apparatus performing cycles in the 348–523 K temperature range, recording both heating and cooling processes. Films deposited at room temperature required three cycles up to 523 K before being stabilized, thus revealing the importance of a proper annealing process in order to obtain reliable physical data. XRD analyses confirm the previous result, as only annealed films present a highly crystalline skutterudite not accompanied by extra phases. The power factor of annealed films is shown to be lower than in the corresponding bulk samples due to the lower Seebeck coefficients occurring in films. Room temperature thermal conductivity, on the contrary, shows values comparable to the ones of doubly doped bulk samples, thus highlighting the positive effect of interfaces on the introduction of scattering centers, and therefore on the reduction of thermal conductivity. [ABSTRACT FROM AUTHOR]

We report the synthesis of new alloys of the composition RCu4Be derived by substituting Be for Cu in the well known parent binary RCu5 (R = rare earth) compounds. For R = Nd, Sm, Gd, Tb, Dy, Ho, and Y, the as cast RCu4Be compounds form in the hexagonal CaCu5 type structure. ErCu4Be and TmCu4Be adopt the cubic AuBe5 type symmetry like their parent compounds. The magnetic properties of RCu4Be compounds have been probed by magnetization, electrical resistivity, and heat capacity measured down to 1.8 K. With the exception of TmCu4Be and nonmagnetic YCu4Be the compounds order magnetically with the highest transition temperature of 56 K observed in GdCu4Be. The nature of the magnetic ordering is predominantly ferromagnetic though the corresponding RCu5 compounds are mostly antiferromagnetic with relatively low magnetic ordering temperatures. The thermal variation of the electrical resistivity is in conformity with their metallic nature and reflects the transition into the magnetically ordered state. The evidence for the bulk nature of the magnetic ordering is further elucidated from the heat capacity data which show anomalies at the magnetic transition. [ABSTRACT FROM AUTHOR]

The title compounds are prepared by melting mixtures of the elements twice at 1100 °C followed by annealing (Ta crucible, 700 °C, 14 d). [ABSTRACT FROM AUTHOR]