title: First-Order Phase Transition in BaNi2Ge2and the Influence of the Valence Electron Count on Distortion of the ThCr2Si2Structure Type.
abstract: Structural instability has a strong influence on the understanding of superconductivity in iron-containing 122 phases. Similar to the 122 iron-based high-temperature superconductors, the intermetallic compound BaNi2Ge2undergoes an orthorhombic-to-tetragonal structural phase transition. The compound was prepared by arc-melting mixtures of the elements under an argon atmosphere. Single crystals were obtained by a special heat treatment in a welded tantalum ampule. The crystal structure of the compound was investigated by powder and single-crystal X-ray diffraction. Differential thermal analysis of BaNi2Ge2showed a reversible phase transition at ca. 480 °C. In situ temperature-dependent synchrotron powder X-ray diffraction studies revealed that below 480 °C the crystal structure of BaNi2Ge2is orthorhombic [own structure type, space group Pnma, a = 8.3852(4) Å, b = 11.3174(8) Å, and c = 4.2902(9) Å at 30 °C] and the high-temperature phase above 510 °C belongs to the tetragonal ThCr2Si2-type structure [space group I4/mmm, a = 4.2664(1) Å, and c = 11.2537(3) Å at 510 °C]. The reversible first-order low-temperature ↔ high-temperature phase transition around 480 °C is associated with distortion of the [Ni2Ge2] layer of low-temperature modification. The anisotropy of thermal expansion of the unit cell in BaNi2Ge2was analyzed. The crystal chemistry and chemical bonding are discussed in terms of linear muffin-tin orbital band structure calculations and a topological analysis using the electron localization function. In related compounds, the level of distortion of the uncollapsed tetragonal ThCr2Si2-type structure depends on the valence electron count (VEC).
authors.affiliation: Department of Chemistry, Technische Universität München , Lichtenbergstrasse 4, 85747 Garching, Germany.
authors.forename: Viktor
authors.identifier: http://orcid.org/0000-0002-7533-2670
authors.initials: V
authors.lastname: Hlukhyy
authors.affiliation: Hamburger Synchrotronstrahlungslabor, DESY , Notkestrasse 85, 22607 Hamburg, Germany.
authors.forename: Dmytro
authors.initials: D
authors.lastname: Trots
authors.affiliation: Department of Chemistry, Technische Universität München , Lichtenbergstrasse 4, 85747 Garching, Germany.
authors.forename: Thomas F
authors.initials: TF
authors.lastname: Fässler