Zur Kenntnis von BaAlF5

BaAlF5 bildet drei Modifikationen. Die Tieftemperaturform (TTF, α‐BaAlF5) kristallisiert orthorhombisch mit a = 13,710; b = 5,604; c = 4,930 Å, Z = 4, Raumgruppe P212121, isotyp mit BaGaF5. Einkristalluntersuchungen an β‐BaAlF5 belegen eine monokline Struktur mit a = 5,148; b = 19,556; c = 7,553 Å, β = 92,43°, Z = 8. Die durch Abschrecken der Präparate erhaltene HTF γ‐BaAlF5 ist ebenfalls monoklin, a = 5,258; b = 9,724; c = 7,366 Å, β = 90,89°, Z = 4. Nach Temperversuchen und DTA‐Messungen wandelt sich α‐langsam irreversibel in β‐BaAlF5 um. Der Übergang β‐ in γ‐BaAlF5 erfolgt bei 789°C. On the Knowledge of BaAlF5 BaAlF5 occurs in three modifications. The hithertoo little characterized Low Temperature Form (LTF, α‐BaAlF5) crystallizes orthorhombic with a = 13.710; b = 5.604; c = 4.930 Å, Z = 4, space group P212121 isotypic with BaGaF5. Single crystal data show that β‐BaAlF5 has a monoclinic unit cell with a = 5.148; b = 19.556; c = 7.553 Å, β = 92.43°, Z = 8. The HTF γ‐BaAlF5 a = 5.258; b = 9.724; c = 7.366 Å, β = 90.89°, Z = 4, obtained by quenching. Investigations by is monoclinic with DTA and quenching prove that α‐ transforms slowly and irreversible to β‐BaAlF5 starting at 666°C. The reservible transformation β‐ to γ;BaAlF5 occurs at 789°C.