Linking Intermetallics and Zintl Compounds: An Investigation of Ternary Trielides (Al, Ga, In) Forming the NaZn(13) Structure Type

Synthesis, crystal and electronic structure of BaLixCd13–x (x ≈ 2)

A new ternary phase has been synthesized and structurally characterized. BaLi_xCd_13–x (x ≈ 2) adopts the cubic NaZn₁₃ structure type (space group Fm3¯c, Pearson symbol cF112) with unit cell parameter a = 13.5548 (10) Å. Structure refinements from single-crystal X-ray diffraction data demonstrate that the Li atoms are exclusively found at the centers of the Cd₁₂-icosahedra. Since a cubic BaCd₁₃ phase does not exist, and the tetragonal BaCd₁₁ is the most Cd-rich phase in the Ba–Cd system, BaLi_xCd_13–x (x ≈ 2) has to be considered as a true ternary compound. As opposed to the typical electron count of ca. 27e-per formula unit for many known compounds with the NaZn₁₃ structure type, BaLi_xCd_13–x (x ≈ 2) only has ca. 26e-, suggesting that both electronic and geometric factors are at play. Finally, the bonding characteristics of the cubic BaLi_xCd_13–x (x ≈ 2) and tetragonal BaCd₁₁ are investigated using the TB-LMTO-ASA method, showing metallic-like behavior.

From simple to complex crystal chemistry in the RE-Au-Tt systems (RE = La, Ce, Pr, Nd; Tt = Ge, Pb)

Polar intermetallics are an intriguing class of compounds with complex relationships between composition and structure that are not fully understood. This work reports a systematic study of the underexplored ternary composition space RE-Au-Tt (RE = La, Ce, Pr, Nd; Tt = Ge, Pb) to expand our knowledge of the intriguing chemistry and diversity achievable with these metallic constituents. These composition spaces are particularly interesting because of the potential to find Au-bearing, highly polar intermetallic compounds. The elements were first reacted through arc welding under an inert atmosphere, followed by annealing at 850 °C. X-ray diffraction of the products identified seven unreported compounds ranging from the simple NaTl-type compounds La_1.5Au₂Pb_0.5 and Nd_2-x Au₂Pb _x to the more structurally complex La₅AuPb₃ in the Hf₅CuSn₃-type structure and Pu₃Pd₄-type RE₃Au₃Ge (RE = La, Ce, Pr, Nd). First-principles electronic structure calculations investigate the combination of Fermi surface-Brillouin zone interactions, electrostatic interactions, and delocalized metallic bonding that contributes to the formation of these phases. These calculations show that a mixture of electrostatic and metallic bonding plays a dominant role in these phases. The RE-Au-Tt composition space remains full of potential for discovering materials with relevant magnetic and quantum properties, provided the crystal chemistry can be comprehended.

Compositional evolution of the NaZn13 structure motif in the systems La-Ni-Ga and Ce-Ni-Ga

Phase relationship and structural behaviour in the substitutional series LaNi_13-xGa_x and CeNi_13-xGa_x have been studied by a combination of X-ray powder diffraction measurements, differential scanning calorimetry, electron diffraction tomography and metallographic analyses. The sequence of morphotropic phase transformations has been found in the series LaNi_13-xGa_x resulting in five varieties of the NaZn₁₃ structure: the cubic phase with aristotype structure at x = 2 (space group Fm3[combining macron]c, Pearson symbol cF112), two tetragonal phases at x = 2.5-4.25 (space group I4/mcm, Pearson symbol tI56-I) and 7-7.5 (space group I4/mcm, Pearson symbol tI56-II), both with an atomic arrangement of the CeNi_8.5Si_4.5 type and two orthorhombic phases at x = 4.5-5.75 (LaNi₇In₆ structure type, space group Ibam, Pearson symbol oI56) and x = 6.37-6.87 (a new derivative of the NaZn₁₃, prototype structure, space group Fmmm, Pearson symbol oF112). The related series CeNi_13-xGa_x shows similar behaviour. The corresponding tI56-I ↔oI56 ↔oF112 ↔tI56-II phases are formed at x = 4-4.25, 4.5-6, 6.37-6.87 and 7-7.37, respectively. In contrast to the lanthanum analogues, the phase with cubic symmetry was not found for this system. Complex twinned and multiple twinned (twinning of twins) domain structures which are revealed for the tetragonal and both orthorhombic phases clearly indicate temperature-induced polymorphic phase transitions during the formation of these phases. LaNi_13-xGa_x samples show paramagnetic behavior, whereas the CeNi_13-xGa_x series exhibits Curie-Weiss paramagnetism.

Bringing order to large-scale disordered complex metal alloys: Gd2Au15−xSbxand BaAuxGa12−x

New complex metallic alloys, BaAu_xGa_12−xand Gd₂Au_15−xSb_x, display entire planes of disordered atom sites, forming a set planar conformations.

An Icosahedral Quasicrystal and Its 1/0 Crystalline Approximant in the Ca-Au-Al System

A new icosahedral quasicrystalline phase, CaAu_4.5-xAl_1.5+x [0.11 ≤ x ≤ 0.40(6); CaAu_4.4Al_1.6, a_QC = 5.383(4) Å, and Pm3̅ 5̅], and its lowest-order 1/0 cubic crystalline approximant phase, CaAu_3+xAl_1-x [0 ≤ x ≤ 0.31(1); a = 9.0766(5)-9.1261(8) Å, Pa3̅ (No. 205), and Pearson symbol cP40], have been discovered in the Ca-poor region of the Ca-Au-Al system. In the crystalline approximant, eight [Au_3-xAl_1+x] tetrahedra fill the unit cell, and each tetrahedron is surrounded by four Ca atoms, thus forming a three-dimensional network of {Ca_4/4[Au_3-xAl_1+x]} tetrahedral stars. A computational study of Au and Al site preferences concurs with the experimental results, which indicate a preference for near-neighbor Au-Al interactions over Au-Au and Al-Al interactions. Analysis of the electronic density of states and the associated crystal orbital Hamilton population curves was used to rationalize the descriptions of CaAu_4.5-xAl_1.5+x [0.11 ≤ x ≤ 0.46(6)] and CaAu_3+xAl_1-x [0 ≤ x ≤ 0.31(1)] as polar intermetallic species, whereby Ca atoms engage in polar covalent bonding with the electronegative, electron-deficient [Au_3-xAl_1+x] tetrahedral clusters and the observed phase width of the crystalline approximant.

Hidden electronic rule in the "cluster-plus-glue-atom" model

Electrons and their interactions are intrinsic factors to affect the structure and properties of materials. Based on the “cluster-cluster-plus-glue-atom” model, an electron counting rule for complex metallic alloys (CMAs) has been revealed in this work (i. e. the CPGAMEC rule). Our results on the cluster structure and electron concentration of CMAs with apparent cluster features, indicate that the valence electrons’ number per unit cluster formula for these CMAs are specific constants of eight-multiples and twelve-multiples. It is thus termed as specific electrons cluster formula. This CPGAMEC rule has been demonstrated as a useful guidance to direct the design of CMAs with desired properties, while its practical applications and underlying mechanism have been illustrated on the basis of CMAs’ cluster structural features. Our investigation provides an aggregate picture with intriguing electronic rule and atomic structural features of CMAs.

Alkaline earth-gold-aluminides: synthesis and structure of SrAu3Al2, SrAu2.83Al2.17, BaAu2.89Al2.11 and BaAu7.09Al5.91

New alkaline earth-gold-aluminides were synthesized from the elements in sealed tantalum or quartz ampoules in muffle furnaces at maximum annealing temperatures of 1325 K. The structures were refined from single crystal X-ray diffractometer data. SrAu3Al2 crystallizes in an ordered version of the LT-SrZn5 structure: Pnma, a = 1315.9(3), b = 549.0(1), c = 684.5(3) pm, wR2 = 0.0232, 930 F 2 values, 35 variables. SrAu2.83Al2.17 (a = 1065.0(2), b = 845.0(2), c = 548.1(1) pm, wR2 = 0.0416, 452 F 2 values, 22 variables) and BaAu2.89Al2.11 (a = 1096.1(3), b = 835.7(3), c = 554.0(1) pm, wR2 = 0.0280, 501 F 2 values, 22 variables) both adopt the BaZn5 type, space group Cmcm with Au/Al mixing on the 4c site. The gold and aluminum atoms in both types form three-dimensional networks of condensed tetrahedra with the strontium and barium atoms in large cavities. BaAu7.09Al5.91 is a new member of the NaZn13 type: Fm3̅c, a = 1257.6(2) pm, wR2 = 0.0267, 168 F 2 values, 12 variables. Both the 96i and 8b sites show Au/Al mixing. The crystal chemical details are discussed.

Synthesis, structure, and physical properties of Ln(Cu,Al,Ga)(13-x) (Ln = La-Pr, and Eu) and Eu(Cu,Al)(13-x)

Ln(Cu,Al,Ga)(13-x) (Ln = La-Pr, and Eu; x ~ 0.2) were synthesized by a combined Al/Ga flux. Single crystal X-ray and neutron diffraction experiments revealed that these compounds crystallize in the NaZn(13) structure-type (space group Fm3[overline]c) with lattice parameters of a ~ 12 Å, V ~ 1600 Å, and Z ~ 8. Our final neutron models led us to conclude that Cu is occupationally disordered on the 8b Wyckoff site while Cu, Al, and Ga are substitutionally disordered on the 96i Wyckoff site of this well-known structure-type. The magnetic susceptibility data show that Ce(Cu,Al,Ga)(13-x) and Pr(Cu,Al,Ga)(13-x) exhibit paramagnetic behavior down to the lowest temperatures measured while Eu(Cu,Al,Ga)(13-x) displays ferromagnetic behavior below 6 K. Eu(Cu,Al)(13-x) was prepared via arc-melting and orders ferromagnetically below 8 K. The magnetocaloric properties of Eu(Cu,Al,Ga)(13-x) and Eu(Cu,Al)(13-x) were measured and compared. Additionally, an enhanced value of the Sommerfeld coefficient (γ = 356 mJ/mol-K(2)) was determined for Pr(Cu,Al,Ga)(13-x). Herein, we present the synthesis, structural refinement details, and physical properties of Ln(Cu,Al,Ga)(13-x) (Ln = La-Pr, and Eu) and Eu(Cu,Al)(13-x).

Ca4Au10In3: Synthesis, Structure, and Bonding Analysis. The Chemical and Electronic Transformations from the Isotypic Zr7Ni10 Intermetallic

The title compound, Ca(4)Au(10)In(3) (e/a = 1.59), was synthesized by conventional high-temperature solid-state reactions and structurally analyzed by single-crystal X-ray diffraction: space group Cmca, a = 13.729(4) A, b = 10.050(3) A, c = 10.160(3) A, Z = 4. The structure, isotypic with that of Zr(7)Ni(10), features a novel three-dimensional [Au(10)In(3)] polyanionic framework built from sinusoidal Au layers that are interconnected by significant Au-Au and Au-In interactions. A prominent electronic feature is the presence of a pseudogap and empty bonding states above the Fermi level according to LMTO calculations, reminiscent of the tunable electronic properties discovered for Mg(2)Zn(11)-type phases. The natures of the chemical and electronic redistributions from Zr(7)Ni(10) to Ca(4)Au(10)In(3) are considered. The Au backbone appears to be particularly important.

Electronic Tuning of Mg2Cu6Ga5. A Route to Crystalline Approximant and Quasicrystalline Phases

Studies of Mg2Cu6Ga5 reveal that this compound contains incomplete Bergman clusters in its structure and shows a pseudogap and empty bonding states just above the Fermi energy according to band calculations. Under a rigid band assumption, such a compound may be tuned to approximant and quasicrystal phases in which the required number of electrons are attained. Here, we replace part of Mg in the isotypic Mg2Cu6Ga5 with Sc, and both 1/1 approximant and icosahedral quasicrystal phases are obtained after some fine-tuning. This method closely correlates the pseudogap and bonding with Hume-Rothery concepts, thus giving useful directions for future quasicrystal searches, especially when approximants are not known.

Rare Beryllium Icosahedra in the Intermediate Valence Compound CeBe13

Single-crystal X-ray diffraction experiments show that the Be atoms in CeBe13 form a Be12 icosahedra, which is a very unusual structural feature due, in part, to the remarkably low valence electron count of Be. Magnetization studies show that CeBe13 displays intermediate valence behavior, in which valence fluctuations between the Ce 4f0 and 4f1 states give rise to enhanced electronic specific heat and magnetic susceptibility. Calculations using ab initio theory were used to determine the electronic structure and bonding and to give insight into the relationship between the crystal structure, the bonding, and the intermediate valence behavior of CeBe13. The hybridization between the localized f electrons and the conduction electrons is responsible for the large values of the electronic specific heat coefficient (gamma approximately 100 mJ/mol K2) and magnetic susceptibility (chi approximately 1 x 10-3 emu/mol), which is in marked contrast to those of ordinary metals that have gamma approximately 1 mJ/mol K2 and chi approximately 1 x 10-5 emu/mol values. The magnetic susceptibility, chi = M/H versus T, of a single crystal of CeBe13 exhibits a broad maximum at Tmax approximately 130 K and is typical of intermediate valence systems with an unusually large energy scale (Kondo), TK approximately 500 K.

Li17Ag3Sn6: A Polar Intermetallic π-System with Carbonate-like [AgSn3]11- Anions and Trefoil Aromatic [Ag2Sn3]6- Layers

A new lithium silver stannide, Li17Ag3Sn6, was synthesized from high-temperature reactions of the pure elements in tantalum containers. Its crystal structure, in the space group, P31m, with a = 8.063(3) A, c = 8.509(4) A, Z = 1, features two distinct AgSn-based anionic layers. Defect graphitic layers of Ag2Sn3, with ordered vacancies at one-third of the Ag sites, are alternately stacked with Kagome-like nets of isolated trigonal planar AgSn3 units. Double layers of Li ions are sandwiched between the stacked AgSn-based layers. Theoretical calculations show unusual pi-interactions within both anionic layers, with the trigonal planar [AgSn3]11- units being isoelectronic with CO(3)2-. In addition, the chemical bonding of the layered [Ag2Sn3]6- pi-network features incompletely filled lone-pair Sn states involved in in-plane trefoil aromatic interactions. Transport and magnetic susceptibility measurements on Li17Ag3Sn6 indicate excellent metallic behavior and temperature-independent paramagnetism consistent with results from band structure calculations. The "trefoil" aromaticity, previously postulated for aromatic molecular systems, is finally observed, albeit in a polar intermetallic solid-state structure that lies at the border between metals and nonmetals.

Icosahedron oligomerization and condensation in intermetallic compounds. Bonding and electronic requirements

Icosahedron-based clustering has been found to be very common in intermetallics, particularly for group 13 and early p-block icosogen elements. Linking of the icosahedral building blocks depends on the valence electron concentrations. Vertex-, edge-, or face-sharing icosahedra occur as the structure compensates for electron deficiency. Some examples of icosahedron-based clusters have been selected for an analysis of the relationships between the structural features (icosahedron oligomerization, atomic defects, etc.) and the bonding and electronic requirements. The extended Hückel method has been used with either a molecular approach or an electronic band structure calculation to rationalize bonding in the intermetallic framework.