Disorder–Order Structural Transformation in Electron-Poor Sr₃Au₈Sn₃ Driven by Chemical Bonding Optimization

Abstract

Sr3Au8Sn3 was synthesized through fusion of a stoichiometric amount of pure metals at 800 °C and annealing treatments at lower temperatures. Single-crystal X-ray diffraction analyses revealed that Sr3Au8Sn3 has a La3Al11-type Immm structure (a = 4.6767(8) Å, b = 9.646(2) Å, c = 14.170(2) Å, Z = 2) if annealed at 550 °C and above but a Ca3Au8Ge3-type structure (Pnnm, a = 9.6082(8) Å, b = 14.171(1) Å, c = 4.6719(4) Å, Z = 2) if annealed at 400 °C. The transition occurs at about 454 °C according to DTA data. Both structures feature columns of Sr-centered pentagonal and hexagonal prisms of Au and Sn stacked along the respective longest axial directions, but different "colorings" of the polyhedra are evident. In the high-temperature phase (Immm) all sites shared between the two prisms adopt 50:50 mixtures of Au/Sn atoms, whereas in the low-temperature phase (Pnnm) Au or Sn are completely ordered. A Klassengleiche group-subgroup relationship was established between these two structures. LMTO-ASA calculations reveal that ΔE for the disorder-to-order transformation on cooling is driven mainly by optimization of the Au-Au and Au-Sn bond populations around the former mixed Au/Sn sites, particularly those with extremely short bonds at the higher temperature. These gains also overcome the smaller effect of ordering on the entropy decrease.

The Crystal Orbital Hamilton Population (COHP) Method as a Tool to Visualize and Analyze Chemical Bonding in Intermetallic Compounds

“…These cages enclose the active-metals that are stringed along, or slightly displaced from, the axes running through the one-dimensional tunnels parallel to the given crystallographic paths such that the cages and the encased active-metals may show certain shifts from perfect linear chains for some of the representatives. For instance, the crystal structure of Sr 3 Au 8 Sn 3 [101] contains one-dimensional tunnels of pentagonal and hexagonal prisms that are assembled by the gold and tin atoms and surround the strontium atoms (Figure 4). A chemical-bonding analysis based on the COHP curves for this material indicates that a structural transformation from a high-temperature to a low-temperature polymorph of this compound is influenced by the trend to optimize the Au−Au and Au−Sn bonding interactions.…”

On the Structure and Stability of BaAl4‐Type Ordered Derivatives in the Sr–Au–Sn System for the 600 °C Section

“…After all, temperature‐dependent structural transitions between the ThCr 2 Si 2 ‐ and CaBe 2 Ge 2 ‐type structures have been reported for many systems such as RE Ir 2 Si 2 ( RE = Y, La, Nd, Gd, Dy, Ho),20 in which the ThCr 2 Si 2 ‐type was found as low‐temperature and the CaBe 2 Ge 2 ‐type as high‐temperature polymorphs. In a different region of the Sr‐Au‐Sn system, two temperature‐dependent polymorphs were also found for Sr 3 Au 8 Sn 3 : a La 3 Al 11 ‐type structure (space group Immm ) at 550 °C and higher temperatures; and a Ca 3 Au 8 Ge 3 ‐type structure ( Pnnm ) at 400 °C 21. In addition, first principles calculations on SrAu 2 Sn 2 indicated that the CaBe 2 Ge 2 ‐type structure had lower total energy by 270 meV/f.u.…”

Compounds with Polar Metallic Bonding

“…It was decided to test the ROTAX [28] algorithm implemented in WinGx [29] and check the possibility of interpreting our crystal as a non-merohedral twin. In fact, a two-fold rotation along the [101] direction To check this hypothesis and refine the collected data as accurately as possible, the initially selected batch of ca. 1000 reflections (comprising those of weak intensity considered as super reflections) was separated into two groups with the help of the CELL_NOW [10] program, suggesting the same twin law for the two monoclinic domains.…”