Crystal structures and thermodynamic stabilities of two new CaGe2 polymorphs
| Autor: | Ritsuko Yaokawa, Kazuhiko Mukai, Akihiro Nagoya, Hideyuki Nakano |
|---|---|
| Rok vydání: | 2018 |
| Předmět: |
Materials science
Polymers and Plastics Metals and Alloys Stacking Ionic bonding 02 engineering and technology Covalent Interaction Crystal structure 010402 general chemistry 021001 nanoscience & nanotechnology 01 natural sciences Dark field microscopy 0104 chemical sciences Electronic Optical and Magnetic Materials Crystallography Scanning transmission electron microscopy Ceramics and Composites Density functional theory 0210 nano-technology Stoichiometry |
| Zdroj: | Acta Materialia. 151:347-355 |
| ISSN: | 1359-6454 |
| DOI: | 10.1016/j.actamat.2018.03.063 |
| Popis: | A stoichiometric CaGe2 sample was synthesized using high-purity Ca and Ge pieces, and its crystal structure was characterized by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and X-ray diffraction. HAADF-STEM clearly indicated the presence of three different polymorphs of CaGe2, viz., 3R-, 4H-, and 6R-CaGe2. In particular, the 4H structure appeared in Zintl phases. 3R-CaGe2 has a trigonal rhombohedral crystal structure, which belongs to the R 3 ¯ m space group and consists of an ABC stacking sequence. 4H-CaGe2 has a hexagonal crystal structure with the P 3 ¯ m1 space group, in which Ge layers follow an A’ABC stacking sequence. Density functional theory calculations suggest that the CaGe2 polymorphs have similar electronic structures, with a covalent interaction between the Ca d and Ge p states, and with an ionic interaction between the Ca s, p states and Ge s, p states. Thermodynamic calculations indicate that the most stable phases are 3R-CaGe2 and 6R-CaGe2, below and above 450 K, respectively. |
| Databáze: | OpenAIRE |
| Externí odkaz: |
|
Full Text from ScienceDirect