Lan(n+1)+xNin(n+5)+ySi(n+1)(n+2)-z: A Symmetric Mirror Homologous Series in the La-Ni-Si System.

A series of four homologous silicides have been discovered during systematic explorations in the central part of the La-Ni-Si system at 1000 °C. All compounds La12.5Ni28.0Si18.3 (n = 3; a = 28.8686(8), c = 4.0737(2) Å, Z = 3), La22.1Ni39.0Si27.8 (n = 4; a = 20.9340(6), c = 4.1245(2) Å, Z = 1), La32.9Ni49.8Si39.3 (n = 5; a = 24.946(1), c = 4.1471(5) Å, Z = 1), and La44.8Ni66.1Si53.4 (n = 6; a = 28.995(5), c = 4.158(1) Å, Z = 1) crystallize in the hexagonal space group P63/m and can be generalized according to Lan(n+1)+xNin(n+5)+ySi(n+1)(n+2)-z with n = 3-6. Their crystal structures are based on AlB2-type building blocks, fused La-centered Ni6Si6 hexagonal prisms, yielding larger oligomeric equilateral domains with the edge size equal to n. The domains extend along the c axis and show checkered ordering of the cationic and anionic parts, while all their atoms are located on mirror planes. Lan(n+1)+xNin(n+5)+ySi(n+1)(n+2)-z can be considered as a mirror series to the La-rich La(n+1)(n+2)Nin(n-1)+2Sin(n+1), where an exchange of the formal cationic and anionic sites, i.e., La and Si, occurs. The La-Ni-Si system is the first system where two such analogous series have been observed.

Cu3As: Uncommon Crystallographic Features, Low-Temperature Phase Transitions, Thermodynamic and Physical Properties.

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.

Effect of the annealing treatment on structural and transport properties of thermoelectric Smy(FexNi1-x)4Sb12thin films.

The crystallographic and transport properties of thin films fabricated by pulsed laser deposition and belonging to the Smy(FexNi1-x)4Sb12filled 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.

Investigation on the Power Factor of Skutterudite Smy(FexNi1-x)4Sb12 Thin Films: Effects of Deposition and Annealing Temperature.

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.

Crystal and Magnetic Structures of the Ternary Ho2Ni0.8Si1.2 and Ho2Ni0.8Ge1.2 Compounds: An Example of Intermetallics Crystallizing with the Zr2Ni1-xP Prototype.

We report two new rare-earth (R) ternary intermetallic compounds-Ho2Ni0.8T1.2 with T = Si and Ge-that correspond to the R5Ni2T3 phase earlier reported to form in Dy-Ni-T and Ho-Ni-T ternary systems. The compounds crystallize in a filled version of the orthorhombic Zr2Ni1-xP-type structure with x = 0.52; their stoichiometry, determined from both single-crystal and powder X-ray diffraction data, is centered on Ho2Ni0.8T1.2 with a narrow solid solubility range for the silicide, while the germanide appears to be a line phase. In addition to R = Dy and Ho, R2Ni0.8T1.2 compounds also form for R = Y and Tb, representing the first examples of rare-earth-based compounds adopting the Zr2Ni1-xP structural prototype. Bulk magnetization data reveal the main transitions of the ferrimagnetic or ferromagnetic type at TC = 38 K for Ho2Ni0.8Si1.2 and TC = 37 K for Ho2Ni0.8Ge1.2, which are followed by subsequent magnetic reordering at lower temperatures. Neutron diffraction shows complex magnetic structures below TC with both ferromagnetic and antiferromagnetic components and magnetic propagation vector κ1 = [0, 0, 0]. Below TN ≅ 24 K (22 K) for the silicide (germanide), an additional antiferromagnetic coupling following an incommensurate magnetic propagation vector κ2 = [κx, 0, 0] appears to coexist with the first magnetic structure.

Stability, Crystal Chemistry, and Magnetism of U2+xNi21-xB6 and Nb3-yNi20+yB6 and the Role of Uranium in the Formation of the Quaternary U2-zNbzNi21B6 and UδNb3-δNi20B6 Systems.

We investigated the U-Ni-B and Nb-Ni-B systems to search for possible new heavy fermion compounds and superconducting materials. The formation, crystal chemistry, and physical properties of U2Ni21B6 and Nb3-yNi20+yB6 [ternary derivatives of the cubic Cr23C6-type (cF116, Fm3̅m)] have been studied; the formation of the hypothetical "U3Ni20B6" and "Nb2Ni21B6" has been disproved. U2Ni21B6 [a = 10.6701(2) Å] crystallizes in the ordered W2Cr21C6-type, whereas Nb3-yNi20+yB6 [a = 10.5842(1) Å] adopts the Mg3Ni20B6-type. Ni in U2Ni21B6 can be substituted by U, leading to the solid solution U2+xNi21-xB6 (0 ≤ x ≤ 0.3); oppositely, Nb in Nb3Ni20B6 is partially replaced by Ni, forming the solution Nb3-yNi20+yB6 (0 ≤ y ≤ 0.5), none of them reaching the limit corresponding to the hypothetically ordered "U3Ni20B6" and "Nb2Ni21B6". These results prompted us to investigate quaternary compounds U2-zNbzNi21B6 and UδNb3-δNi20B6: strong competition in the occupancy of the 4a and 8c sites by U, Nb, and Ni atoms has been observed, with the 4a site occupied by U/Ni atoms only and the 8c site filled by U/Nb atoms only. U2Ni21B6, U2.3Ni20.7B6, and Nb3Ni20B6 are Pauli paramagnets. Interestingly, Nb2.5Ni20.5B6 shows ferromagnetism with TC ≈ 11 K; the Curie-Weiss fit gives an effective magnetic moment of 2.78 µB/Ni, suggesting that all Ni atoms in the formula unit contribute to the total magnetic moment. The M(H) data at 2 K further corroborate the ferromagnetic behavior with a saturation moment of 10 µB/fu (≈0.49 µB/Ni). The magnetic moment of Ni at the 4a site induces a moment in all of the Ni atoms of the whole unit cell (32f and 48h sites), with all atoms ordering ferromagnetically at 11 K. Density functional theory (DFT) shows that the formation of U2Ni21B6 and Nb3Ni20B6 is energetically preferred. The various electronic states generating ferromagnetism on Nb2.5Ni20.5B6 and Pauli paramagnetism on U2Ni21B6 and Nb3Ni20B6 have been identified.

Band filling and disorder effects on the normal state thermoelectric behavior in MgB2.

The goal of this work is providing a comprehensive interpretation framework for the wide and varied experimental phenomenology of the Seebeck effect in MgB2 samples with different levels of doping in either π or σ bands and different levels of disorder, using a combined experimental and theoretical approach. We calculate the temperature dependent diffusive Seebeck coefficient S diff(T) with the Boltzmann equation resolved in relaxation time approximation, taking into account the scattering with phonons and impurities, the effect of renormalization and the effect doping in a rigid band approximation. We show that selective disorder has a sizeable effect on the S diff magnitude, as it tunes the relative contributions of σ and π bands. Disorder also affects the S diff temperature dependences, eventually yielding a linear S diff(T) behavior in the dirty limit. We also show that band filling has opposite effects on S, depending on which band dominates transport. In parallel, we carry out the Seebeck effect measurements on neutron-irradiated Mg11B2, and on two series of doped samples Mg1-x Al x B2 and Mg(B1-x C x )2. From comparison of calculated S diff(T) and experimental S(T) curves, we demonstrate that diffusive and phonon drag terms give comparable contributions in clean samples, but the phonon drag term is progressively suppressed with increasing disorder. In C and Al doped samples we observe very different experimental behaviors in terms of sign, magnitude and temperature dependence. Indeed, notwithstanding the similar electron doping introduced by both substitutions, C or Al doping yields disorder which mainly affects either σ or π bands, respectively. With the help of our theoretical approach, we are able to disentangle the various effects and prove that the Seebeck coefficient is a very sensitive probe of this kind of disorder.

R3Au9Pn (R = Y, Gd-Tm; Pn = Sb, Bi): A Link between Cu10Sn3 and Gd14Ag51.

A new series of intermetallic compounds R3Au9Pn (R = Y, Gd-Tm; Pn = Sb, Bi) has been discovered during the explorations of the Au-rich parts of rare-earth-containing ternary systems with p-block elements. The existence of the series is strongly restricted by both geometric and electronic factors. R3Au9Pn compounds crystallize in the hexagonal crystal system with space group P63/m (a = 8.08-8.24 Å, c = 8.98-9.08 Å). All compounds feature Au-Pn, formally anionic, networks built up by layers of alternating edge-sharing Au@Au6 and Sb@Au6 trigonal antiprisms of overall composition Au6/2Pn connected through additional Au atoms and separated by a triangular cationic substructure formed by R atoms. From a first look, the series appears to be isostructural with recently reported R3Au7Sn3 (a ternary ordered derivative of the Cu10Sn3-structure type), but no example of R3Au9M is known when M is a triel or tetrel element. R3Au9Pn also contains Au@Au6Au2R3 fully capped trigonal prisms, which are found to be isostructural with those found in the well-researched R14Au51 series. This structural motif, not present in R3Au7Sn3, represents a previously unrecognized link between Cu10Sn3 and Gd14Ag51 parent structure types. Magnetic property measurements carried out for Ho3Au9Sb reveal a complex magnetic structure characterized by antiferromagnetic interactions at low temperature (TN = 10 K). Two metamagnetic transitions occur at high field with a change from antiferromagnetic toward ferromagnetic ordering. Density functional theory based computations were performed to understand the materials' properties and to shed some light on the stability ranges. This allowed a better understanding of the bonding pattern, especially of the Au-containing substructure, and elucidation of the role of the third element in the stability of the structure type.

Crystal Chemistry of the New Families of Interstitial Compounds R6Mg23C (R = La, Ce, Pr, Nd, Sm, or Gd) and Ce6Mg23Z (Z = C, Si, Ge, Sn, Pb, P, As, or Sb).

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 Fm3̅m, 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.

Synthesis, crystal and electronic structure of the quaternary magnetic EuTAl4Si2 (T = Rh and Ir) compounds.

Single crystals of the quaternary europium compounds EuRhAl4Si2 and EuIrAl4Si2 were synthesized by using the Al-Si binary eutectic as a flux. The structure of the two quaternary compounds has been refined by single crystal X-ray diffraction. Both compounds are stoichiometric and adopt an ordered derivative of the ternary KCu4S3 structure type (tetragonal tP8, P4/mmm). The two compounds reported here represent the first example of a quaternary and truly stoichiometric 1:1:4:2 phase crystallizing with this structure type. In light of our present results, the structure of the BaMg4Si3 compound given in literature as representing a new prototype is actually isotypic with the KCu4S3 structure. Local spin density approximation including the Hubbard U parameter (LSDA + U) calculations show that Eu ions are in the divalent state, with a significant hybridization between the Eu 5d, Rh (Ir) 4d (5d), Si 3p and Al 3p states. Magnetic susceptibility measured along the [001] direction confirms the divalent nature of the Eu ions in EuRhAl4Si2 and EuIrAl4Si2, which order magnetically near ∼11 and ∼15 K, respectively.