Crystals

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791 KiB

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Bonding Schemes for Polar Intermetallics through Molecular Orbital Models: Ca-Supported Pt–Pt Bonds in Ca₁₀Pt₇Si₃

by Daniel C. Fredrickson, Isa Doverbratt, Siméon Ponou and Sven Lidin

Crystals 2013, 3(3), 504-516; https://doi.org/10.3390/cryst3030504 - 03 Sep 2013

Cited by 11 | Viewed by 8009

Exploratory synthesis in the area of polar intermetallics has yielded a rich variety of structures that offer clues into the transition in bonding between Zintl and Hume-Rothery phases. In this article, we present a bonding analysis of one such compound, Ca₁₀Pt [...] Read more.

Exploratory synthesis in the area of polar intermetallics has yielded a rich variety of structures that offer clues into the transition in bonding between Zintl and Hume-Rothery phases. In this article, we present a bonding analysis of one such compound, Ca₁₀Pt₇Si₃, whose large Ca content offers the potential for negative formal oxidation states on the Pt. The structure can be divided into a sublattice of Ca cations and a Pt–Si polyanionic network built from Pt₇Si₃ trefoil units linked through Pt–Pt contacts of 3.14 Å. DFT-calibrated Hückel models reveal that the compound adheres well to a Zintl-like electron counting scheme, in which the Pt–Si and Pt–Pt contacts are equated with two-center two-electron bonds. The experimental electron count is in excess of that predicted by 2%, a discrepancy which is attributed to the electron transfer from the Ca to the Pt–Si network being incomplete. For the Pt–Pt contacts, the occupancy of the bonding orbitals is dependent on the participation of the surrounding Ca atoms in bridging interactions. This use of multi-center interactions isolobal to classical two-center two-electron bonds may illustrate one path by which the bonds delocalize as one moves from the Zintl phases toward the Hume-Rothery domain. Full article

(This article belongs to the Special Issue New Trends in Intermetallics Development and Application)

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981 KiB

Open AccessArticle

Zintl Salts Ba₂P₇X (X = Cl, Br, and I): Synthesis, Crystal, and Electronic Structures

by Juli-Anna Dolyniuk and Kirill Kovnir

Crystals 2013, 3(3), 431-442; https://doi.org/10.3390/cryst3030431 - 26 Aug 2013

Cited by 14 | Viewed by 9893

Abstract

Two barium phosphide halides, Ba₂P₇Br and Ba₂P₇I, were synthesized and structurally characterized by single crystal X-ray diffraction. Both compounds crystallize in the monoclinic space group P2₁/m (No. 11) and are isostructural [...] Read more.

Two barium phosphide halides, Ba₂P₇Br and Ba₂P₇I, were synthesized and structurally characterized by single crystal X-ray diffraction. Both compounds crystallize in the monoclinic space group P2₁/m (No. 11) and are isostructural to Ba₂P₇Cl. The crystal structures of Ba₂P₇X (X = Cl, Br, I) feature the presence of heptaphosphanortricyclane P₇³⁻ clusters along with halogen anions and barium cations. According to the Zintl concept, Ba₂P₇X compounds are electron-balanced semiconductors. Quantum-chemical calculations together with UV-Visible spectroscopy confirm the title compounds are wide bandgap semiconductors. The bonding in the P₇³⁻ clusters was analyzed by means of electron localization function. The elemental compositions were confirmed using energy dispersive X-ray spectroscopy. Full article

(This article belongs to the Special Issue New Trends in Intermetallics Development and Application)

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364 KiB

Open AccessArticle

Electronic Origin of the Orthorhombic Cmca Structure in Compressed Elements and Binary Alloys

by Valentina F. Degtyareva

Crystals 2013, 3(3), 419-430; https://doi.org/10.3390/cryst3030419 - 19 Jul 2013

Cited by 9 | Viewed by 8742

Abstract

Formation of the complex structure with 16 atoms in the orthorhombic cell, space group Cmca (Pearson symbol oC16), was experimentally found under high pressure in the alkali elements (K, Rb, Cs) and polyvalent elements of groups IV (Si, Ge) and V (Bi). [...] Read more.

Formation of the complex structure with 16 atoms in the orthorhombic cell, space group Cmca (Pearson symbol oC16), was experimentally found under high pressure in the alkali elements (K, Rb, Cs) and polyvalent elements of groups IV (Si, Ge) and V (Bi). Intermetallic phases with this structure form under pressure in binary Bi-based alloys (Bi-Sn, Bi-In, Bi-Pb). Stability of the Cmca-oC16 structure is analyzed within the nearly free-electron model in the frame of Fermi sphere-Brillouin zone interaction. A Brillouin-Jones zone formed by a group of strong diffraction reflections close to the Fermi sphere is the reason for the reduction of crystal energy and stabilization of the structure. This zone corresponds well to the four valence electrons in Si and Ge, and leads to assume an spd-hybridization for Bi. To explain the stabilization of this structure within the same model in alkali metals, that are monovalents at ambient conditions, a possibility of an overlap of the core, and valence band electrons at strong compression, is considered. The assumption of the increase in the number of valence electrons helps to understand sequences of complex structures in compressed alkali elements and unusual changes in their physical properties, such as electrical resistance and superconductivity. Full article

(This article belongs to the Special Issue New Trends in Intermetallics Development and Application)

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751 KiB

Open AccessArticle

Corrosion Study and Intermetallics Formation in Gold and Copper Wire Bonding in Microelectronics Packaging

by Chwee Sim Goh, Wee Ling Eddy Chong, Teck Kheng Lee and Christopher Breach

Crystals 2013, 3(3), 391-404; https://doi.org/10.3390/cryst3030391 - 17 Jul 2013

Cited by 53 | Viewed by 15943

Abstract

A comparison study on the reliability of gold (Au) and copper (Cu) wire bonding is conducted to determine their corrosion and oxidation behavior in different environmental conditions. The corrosion and oxidation behaviors of Au and Cu wire bonding are determined through soaking in [...] Read more.

A comparison study on the reliability of gold (Au) and copper (Cu) wire bonding is conducted to determine their corrosion and oxidation behavior in different environmental conditions. The corrosion and oxidation behaviors of Au and Cu wire bonding are determined through soaking in sodium chloride (NaCl) solution and high temperature storage (HTS) at 175 °C, 200 °C and 225 °C. Galvanic corrosion is more intense in Cu wire bonding as compared to Au wire bonding in NaCl solution due to the minimal formation of intermetallics in the former. At all three HTS annealing temperatures, the rate of Cu-Al intermetallic formation is found to be three to five times slower than Au-Al intermetallics. The faster intermetallic growth rate and lower activation energy found in this work for both Au/Al and Cu/Al as compared to literature could be due to the thicker Al pad metallization which removed the rate-determining step in previous studies due to deficit in Al material. Full article

(This article belongs to the Special Issue New Trends in Intermetallics Development and Application)

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