Electronic and Structural Properties of MPtxB6-2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework.

Two new ternary platinum borides, YPtxB6-2x and YbPtxB6-2x, were obtained by argon-arc melting of the elements followed by annealing at 780 °C (750 °C). The structures of these compounds combine the fragments of CaB6- and AuCu3-type structures [space group Pm3̅m; x = 1.15, a = 4.0550(4) Å and x = 1.34, a = 4.0449(2) Å for YPtxB6-2x and YbPtxB6-2x, respectively; single-crystal X-ray diffraction]. Two possible variants of B/Pt ordering (space group P4/mmm) were created via a group-subgroup approach targeting the derived stoichiometry. The architecture of the type-I YPtxB6-2x structure model (a' = a, b' = b, c' = c) combines the 4.82 boron nets alternating with the layers of Y and Pt; the type-II YPtxB6-2x structure model (a' = 2a, b' = 2b, c' = c) exhibits columns of linked [B24] truncated cubes filled with Y running along the c axis. The striking features of both structural models are [B4Pt2] octahedra. The structural similarities with hitherto reported structures (YB2C2, M2Ni21B20, MNi21B20, and ErNiB4) were drawn supporting the verity of these models. A chemical bonding analysis for type-I and type-II YPtxB6-2x based on electron localization function distribution revealed a two-center interaction forming the 4.82 boron nets for type-I YPtxB6-2x and a covalent bonding within [B4Pt2] octahedra as well as a two-center interaction for B-B intraoctahedral bonds for type-II YPtxB6-2x. Analysis of Bader charges revealed the cationic character of the yttrium atoms. The interactions for nondistorted areas of the structures agree well with the bonding picture calculated for constituent building structures, YB6 and YPt3. Electronic structure calculations predict YPtxB6-2x to be a metal with the density of states of around N(EF) = 1 states eV-1 f.u.-1. The exploration of the Y-Pt-B system in the relevant concentration range elucidated the homogeneity field of YPtxB6-2x (0.90 ≤ x ≤ 1.40) and revealed the existence of three more ternary phases at 780 °C: YPt2B (space group P6222), YPt3B (space group P4mm), and YPt5B2 (space group C2/m).

La2Pd3Ge5 and Nd2Pd3Ge5 Compounds: Chemical Bonding and Physical Properties.

The two La2Pd3Ge5 and Nd2Pd3Ge5 compounds, crystallizing in the oI40-U2Co3Ge5 crystal structure, were targeted for analysis of their chemical bonding and physical properties. The compounds of interest were obtained by arc melting and characterized by differential thermal analysis, scanning electron microscopy, and X-ray diffraction both on powder and on a single crystal (for the La analogue), to ensure the high quality of the samples and accurate crystallographic data. Chemical bonding was studied by analyzing the electronic structure and effective QTAIM charges of La2Pd3Ge5. A significant charge transfer mainly occurs from La to Pd so that Ge species assume tiny negative charges. This result, together with the -(I)COHP analysis, suggests that, in addition to the expected homopolar Ge bonds within zigzag chains, heteropolar interactions between Ge and the surrounding La and Pd occur with multicenter character. Covalent La-Pd interactions increase the complexity of chemical bonding, which could not be adequately described by the simplified, formally obeyed, Zintl-Klemm scheme. Electric resistivity, specific heat, magnetization, and magnetic susceptibility as a function of temperature indicate for both compounds a metallic-like behavior. For Nd2Pd3Ge5, two low-temperature phase transitions are detected, leading to an antiferromagnetic ground state.

Th7 Fe3 -Type Related Structures in Pd(Pt)-Cu-B Systems: Pd6 CuB3 -A New Structure Type for Borides.

A new member of the series of Th7 Fe3 -type derivative structures, h-(Pd0.86 Cu0.14 )7 B3 (≡Pd6.02 Cu0.98 B3 , unique structure type Pd6 CuB3 , space group P63 cm, a=12.9426(9) Å, c=4.8697(4) Å, single-crystal X-ray diffraction (XRD) data) was obtained from as cast alloys and alloys annealed at 600-650 °C. Further substitution of Cu by Pd led to formation of a Mn7 C3 -type structure, o-(Pd0.93 Cu0.07 )7 B3 (≡Pd6.51 Cu0.49 B3 , space group Pnma, a=4.8971(2) Å, b=7.5353(3) Å, c=12.9743(6) Å, single-crystal XRD). Isotypic LT h-(Pt0.70 Cu0.30 )7 B3 (≡Pt4.90 Cu2.10 B3 ) was observed in the Pt-Cu-B system as a low-temperature (LT) phase (T≤600 °C) (powder XRD), whereas the Th7 Fe3 -type (high-temperature (HT) h-(Pt0.73 Cu0.27 )7 B3 ≡Pt5.11 Cu1.89 B3 , space group P63 mc, a=7.4671(1) Å, c=4.9039(1) Å, powder XRD) proved to be stable at high temperature. The three structures are built of columns of face connected metal octahedra and columns of metal tetrahedra alternatingly fused by common faces and vertices. Boron atoms are found in trigonal prisms formed by metal atoms. The volumes of the three new Th7 Fe3 -type derivative borides relate as 1:2:3. Superconductivity was discovered for Pt4.9 Cu2.1 B3 (Pd6 CuB3 -type) and Pt5.1 Cu1.9 B3 (Th7 Fe3 -type) below 0.67 and 0.66 K, respectively. Despite the close value of the transition temperature the values of the upper critical field at 0 K differ as 0.37 T and 0.27 T for the two compounds.

Yb9+xCuMg4-x (x = 0.034): A κ-Phase Formed by Lanthanoids.

Atom order in the crystal structures of Yb2Cu2-xMg (x = 0.17; Mo2FeB2-type; P4/mbm; a = 0.75592(2) nm; c = 0.40282(1) nm) and Yb9+xCuMg4-x (x = 0.034; Hf9Mo4B-type; P63/mmc; a = 1.0169(5) nm; c = 1.0290(5) nm) was determined from powder and X-ray single-crystal counter data analyses supported by electron probe microanalyses. Among the group of the so-called κ-phases, Yb9+xCuMg4-x is the first representative formed by a lanthanoid element. The structure of this κ-phase can be viewed as a typical network of corner-connected empty Yb6-octahedra, which encompass Yb6Mg6-icosahedra (filled by a mix of Mg/Yb atoms) and Yb6-trigonal prisms centered by Cu atoms to complete the three-dimensional metal framework. From another point of view, the same structure is considered as built from infinite polyicosahedral columns of Yb9Mg4 composition with Cu atoms located in trigonal prismatic interstices, highlighting similarities with other Yb-rich Yb-Cu-Mg phases. Density functional theory (DFT) calculations classify Yb9CuMg4 as a polar intermetallic. Metallic-like behavior is inferred from the Sommerfeld constant, γ = 49.2 mJ/mol·K(2), derived from the electronic density of states, calculated at the Fermi level. DFT integration of the f-density of states indicates almost completely filled f-states, revealing 13.6 and 13.7 electrons in the valence band for Yb1 and Yb2 atoms, respectively, close to the Yb(2+) ground state ((1)S0) for both Yb atoms. Magnetic susceptibility data recorded on the same compound are consistent with a nonmagnetic divalent Yb(2+) state. Temperature-dependent heat capacity data display a metallic behavior characterized by a small Sommerfeld constant γ = 64.8 mJ/mol·K(2) and a rather low Debye temperature ΘD = 140 K as typical for soft materials.

Thermoelectric properties of a Mn substituted synthetic tetrahedrite.

Tetrahedrite compounds Cu(12-x)Mn(x)Sb4S13 (0 ≤x≤ 1.8) were prepared by solid state synthesis. A detailed crystal structure analysis of Cu10.6Mn1.4Sb4S13 was performed by single crystal X-ray diffraction (XRD) at 100, 200 and 300 K confirming the noncentrosymmetric structure (space group I4[combining macron]3m) of a tetrahedrite. The large atomic displacement parameter of the Cu2 atoms was described by splitting the 12e site into a partially and randomly occupied 24g site (Cu22) in addition to the regular 12e site (Cu21), suggesting a mix of dynamic and static off-plane Cu2 atom disorder. Rietveld powder XRD pattern and electron probe microanalysis revealed that all the Mn substituted samples showed a single tetrahedrite phase. The electrical resistivity increased with increasing Mn due to substitution of Mn(2+) at the Cu(1+) site. The positive Seebeck coefficient for all samples indicates that the dominant carriers are holes. Even though the thermal conductivity decreased as a function of increasing Mn, the thermoelectric figure of merit ZT decreased, because the decrease of the power factor is stronger than the decrease of the thermal conductivity. The maximum ZT = 0.76 at 623 K is obtained for Cu12Sb4S13. The coefficient of thermal expansion 13.5 ± 0.1 × 10(-6) K(-1) is obtained in the temperature range from 460 K to 670 K for Cu10.2Mn1.8Sb4S13. The Debye temperature, Θ(D) = 244 K for Cu10.2Mn1.8Sb4S13, was estimated from an evaluation of the elastic properties. The effective paramagnetic moment 7.45 µB/f.u. for Cu10.2Mn1.8Sb4S13 is fairly consistent with a high spin 3d(5) ground state of Mn.

Structural and physical properties diversity of new CaCu5-type related europium platinum borides.

Three novel europium platinum borides have been synthesized by arc melting of constituent elements and subsequent annealing. They were characterized by X-ray powder and single-crystal diffraction: EuPt4B, CeCo4B type, P6/mmm, a = 0.56167(2) nm, c = 0.74399(3) nm; Eu3Pt7B2, Ca3Al7Cu2 type as an ordered variant of PuNi3, R3m, a = 0.55477(2) nm, c = 2.2896(1) nm; and Eu5Pt18B(6-x), a new unique structure type, Fmmm, a = 0.55813(3) nm, b = 0.95476(5) nm, c = 3.51578(2) nm. These compounds belong to the CaCu5 family of structures, revealing a stacking sequence of CaCu5-type slabs with different structural units: CaCu5 and CeCo3B2 type in EuPt4B; CeCo3B2 and Laves MgCu2 type in Eu3Pt7B2; and CaCu5-, CeCo3B2-, and site-exchange ThCr2Si2-type slabs in Eu5Pt18B(6-x). The striking motif in the Eu5Pt18B(6-x) structure is the boron-centered Pt tetrahedron [BPt4], which build chains running along the a axis and plays a decisive role in the structure arrangement by linking the terminal fragments of repeating blocks of fused Eu polyhedra. Physical properties of two compounds, EuPt4B and Eu3Pt7B2, were studied. Both compounds were found to order magnetically at 36 and 57 K, respectively. For EuPt4B a mixed-valence state of the Eu atom was confirmed via magnetic and specific heat measurements. Moreover, the Sommerfeld value of the specific heat of Eu3Pt7B2 was found to be extraordinarily large, on the order of 0.2 J/mol K(2).

Electronic and structural properties of Y6Pt13X4, site occupancy variants of the Ba6Na16N subnitride (X = Al, Ga).

The existence of new ternary compounds Y6Pt13X4 (X = Al, Ga) with the site occupancy variant of the subnitride Ba6Na16N (space group Im3̄m, no. 229) has been established for the first time by single crystal and powder X-ray diffraction from alloys annealed at 600 °C. The striking structural units in these compounds are platinum centered [PtY6] octahedra interconnected via Y-Y bonds and embedded in the XPt3 framework. The structural similarities with the Ce3Ni6Si2-type compounds are discussed. Electronic density of states calculated in the framework of DFT claims the compounds to be metals. The electronic band structures of both compounds resemble each other due to intrinsic similarities in the crystal structures. The analysis of bonding within and between structural fragments based on evaluation of electronic structures, Bader charges and ELF distributions in Y6Pt13X4 (X = Al, Ga) suggests the overall picture of these phases as polar intermetallics, containing a mixture of electrostatically driven interactions (such as those between complementary charged yttrium and platinum) and metallic bonding (such as Pt-Al (Ga), Pt-Pt and Y-Al (Ga)).

Cage compound Sc5Pt24B12: a Pt-stuffed variant of filled skutterudite structure. Electronic and structural properties.

The title compound was obtained from elements via arc melting and its crystal structure was determined from single-crystal X-ray diffraction data (space group Im3̄, a = 10.2042(6) Å). The refinement indicated the occupancy of icosahedral 2a and cubooctahedral 8c sites solely by Sc atoms which leads to the composition Sc5Pt24B12 in contrast to the previously reported ternary stannides of Gd3Ni8Sn16 type (RE5-xM12Sn24(+x) compounds). The compound is the first representative of borides crystallizing with a site exchange variant of this stannide structure type. The structural relationships of the boride structure and filled skutterudite LaFe4P12vs. the Remeika phase of Yb3Rh4Sn13-type are discussed. Analysis of chemical bonding classifies Sc5Pt24B12 as a cage compound exhibiting the ionic interaction of cationic scandium species in the cages of anionic framework, formed by covalently bonded B and Pt atoms. Electronic structure calculations show that the electronic states of atoms centered around the cubooctahedral 8c site, i.e. Sc2 3d-, Pt2 5d- and B 2p-states dominate the density of states (DOS) at the Fermi level EF. Strong effect of spin-orbit coupling on the band structure at the gamma point has been found from density functional theory calculations. Sc5Pt24B12 exhibits superconductivity with a transition temperature of TC = 2.45 K.

Anderson transition in stoichiometric Fe2VAl: high thermoelectric performance from impurity bands.

Discovered more than 200 years ago in 1821, thermoelectricity is nowadays of global interest as it enables direct interconversion of thermal and electrical energy via the Seebeck/Peltier effect. In their seminal work, Mahan and Sofo mathematically derived the conditions for 'the best thermoelectric'-a delta-distribution-shaped electronic transport function, where charge carriers contribute to transport only in an infinitely narrow energy interval. So far, however, only approximations to this concept were expected to exist in nature. Here, we propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario. An innovative approach of continuous disorder tuning allows us to drive the Anderson transition within a single sample: variable amounts of antisite defects are introduced in a controlled fashion by thermal quenching from high temperatures. Consequently, we obtain a significant enhancement and dramatic change of the thermoelectric properties from p-type to n-type in stoichiometric Fe2VAl, which we assign to a narrow region of delocalised electrons in the energy spectrum near the Fermi energy. Based on our electronic transport and magnetisation experiments, supported by Monte-Carlo and density functional theory calculations, we present a novel strategy to enhance the performance of thermoelectric materials.

Structure and properties of a novel boride: ThNi12B6.

Investigation of the system Th-Ni-B prompted a novel ternary compound ThNi12B6. X-ray structure analysis of single crystals obtained by the mechanical fragmentation of an as-cast sample revealed a fully ordered CeNi12B6-type structure (space group Cmc21, no. 36; a = 0.95638(1) nm, b = 0.73852(1) nm, c = 1.10195(1) nm; RF2 = 0.0305). Density functional theory (DFT) calculations have been performed comprising heat of formation, electronic band structure and density of states, Fermi surface via Wannier functions, phonon band structure and density of states, phonon and electronic contributions to specific heat and elastic constants Cij. Comparing the parameters evaluated from DFT with the experimental data, an overall satisfactory agreement has been achieved. Measurements of electrical resistivity, magnetic susceptibility and specific heat manifest a Pauli paramagnetic, metallic behaviour for ThNi12B6 without any anomalies, in close match with the isotypic homologue LaNi12B6. Static and dynamic hardness data show rather high values; Young's modulus is in the range of 240 GPa. The Debye temperature, θD = 490 K, gained via elastic constants, is slightly higher than the values extracted from specific heat or electrical resistivity data. A rather low coefficient of thermal expansion, α = 5.5 × 10-6 K-1, was derived from the temperature dependent length change.

Porous Silicon Nanocomposites with Combined Hard and Soft Magnetic Properties.

Magnetic nanostructures of two ferromagnetic metals have been combined within porous silicon, and the magnetic switching behavior of the resulting porous silicon/metal nanocomposite has been modified by varying the arrangement. The two magnetic materials are Ni and Co, whereas Co is the magnetic harder one. These "hard/soft" magnetic nanocomposites have been achieved by two different routes. On the one hand, double-sided porous silicon has been used whereas one side has been filled with Ni nanostructures and the other one with Co nanostructures. On the other hand, Ni and Co have been deposited within one porous layer alternatingly. The filling of the pores has been carried out by electrodeposition with varying the deposition parameters. In systems which offer two distinct slopes of the hysteresis curves due to the different saturation behavior of the two types of deposited metal, magnetic exchange coupling is not present. For samples which show smooth hysteresis curves exchange, coupling between the Ni and Co nanostructures seems to be present. The aim is to control especially the structure size of the soft and the hard magnetic materials and the distance between them at the nanoscale to optimize exchange coupling resulting in a maximum energy product.

BaAl4 derivative phases in the sections {La,Ce}Ni2Si2-{La,Ce}Zn2Si2: phase relations, crystal structures and physical properties.

Phase relations and crystal structures have been evaluated within the sections LaNi2Si2-LaZn2Si2 and CeNi2Si2-CeZn2Si2 at 800 °C using electron microprobe analysis and X-ray powder and single crystal structure analyses. Although the systems La-Zn-Si and Ce-Zn-Si at 800 °C do not reveal compounds such as "LaZn2Si2" or "CeZn2Si2", solid solutions {La,Ce}(Ni1-xZnx)2Si2 exist with the Ni/Zn substitution starting from {La,Ce}Ni2Si2 (ThCr2Si2-type; I4/mmm) up to x = 0.18 for Ce(Ni1-xZnx)2Si2 and x = 0.125 for La(Ni1-xZnx)2Si2. For higher Zn-contents 0.25 ≤ x ≤ 0.55 the solutions adopt the CaBe2Ge2-type (P4/nmm). The investigations are backed by single crystal X-ray diffraction data for Ce(Ni0.61Zn0.39)2Si2 (P4/nmm; a = 0.41022(1) nm, c = 0.98146(4) nm; RF = 0.012) and by Rietveld refinement for La(Ni0.56Zn0.44)2Si2 (P4/nmm; a = 0.41680(6) nm, c = 0.99364(4) nm; RF = 0.043). Interestingly, the Ce-Zn-Si system contains a ternary phase CeZn2(Si1-xZnx)2 of the ThCr2Si2 structure type (0.25 ≤ x ≤ 0.30 at 600 °C), which forms peritectically at T = 695 °C but does not include the composition "CeZn2Si2". The primitive high temperature tetragonal phase with the CaBe2Ge2-type has also been observed for the first time in the Ce-Ni-Si system at CeNi2+xSi2-x, x = 0.33 (single crystal data, P4/nmm; a = 0.40150(2) nm, c = 0.95210(2) nm; RF = 0.0163). Physical properties (from 400 mK to 300 K) including specific heat, electrical resistivity and magnetic susceptibility have been elucidated for Ce(Ni0.61Zn0.39)2Si2 and La(Ni0.56Zn0.44)2Si2. Ce(Ni0.61Zn0.39)2Si2 exhibits a Kondo-type ground state. Low temperature specific heat data of La(Ni0.56Zn0.44)2Si2 suggest a spin fluctuation scenario with an enhanced value of the Sommerfeld constant.