The Yttrium, Lanthanum, and Lanthanide Trifluorides: Internal Periodicity of Phase Transitions

Аннотация

Internal periodicity of phase transitions (fusion and polymorphism) has been revealed in trifluorides of Y, La, and 14 lanthanides. This periodicity is determined by the internal periodicity of filling of the 4f-electron subshell of rare-earth elements (REEs, R), in which cerium ⁵⁸Се–⁶⁴Gd and terbium ⁶⁵Тb–⁷¹Lu subsets can be selected. The lanthanide compression of ionic radii R³⁺ (15% towards the smaller one) induces the formation of three structure types: LaF₃, β-YF₃, and α-YF₃ (α-UO₃). The formula volumes V_form of REE trifluorides of the LaF₃ and β-YF₃ structure types only partially obey the internal periodicity of lanthanides with an increase in Z. The internal periodicity of the fusion and polymorphism of REE trifluorides (without ScF₃) divides RF₃ into four structural subgroups: A (R = La–Nd), B (R = Pm–Gd), C (R = Tb–Ho), and D (R = Er–Lu, Y), strictly specifying the percentage and elemental composition of each subgroup. The periodicity of the RF₃ polymorphism in the products manifests itself in the crystallization of the melts prone to supercooling in the form of a periodic change of single crystals with coarse- or fine-grained blocks of the low-temperature forms.