Hand Milling Induced Phase Transition for Marcasite-type Carbodiimide.

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• High-pressure studies traditionally focus on inorganic oxides and chalcogenides.
• Compounds with molecular anions, like carbodiimides, are less explored under pressure.
• Understanding phase transitions in these materials is crucial for novel applications.

Purpose of the Study:

• Investigate pressure-induced phase transitions in marcasite-type barium carbodiimides (Ba$_{0.9}$M$_{0.1}$NCN).
• Explore alternative methods like hand milling for inducing phase transitions.
• Elucidate the mechanism behind these structural transformations.

Main Methods:

• Hydrostatic pressure experiments on Ba$_{0.9}$M$_{0.1}$NCN (M = Ca, Sr).
• Hand milling as a mechanical stress method.
• Variable-cell nudged elastic band (VCNEB) calculations for mechanism analysis.

Main Results:

• Transformation to a CsCl-type tetragonal phase observed at low pressures (0.3–0.8 GPa).
• Metal coordination increases from 6 to 8 during the transition.
• Partial phase transitions induced by hand milling, distinct from high-energy ball milling.

Conclusions:

• Ba$_{0.9}$M$_{0.1}$NCN undergoes low-pressure phase transitions to a CsCl-type structure.
• Hand milling is an effective method for inducing phase transitions in molecular anion compounds.
• Structural transformation involves cation sliding and anion rotation, with stable linear NCN$^{2-}$ anions.