HOMOLOGOUS SERIES OF 15 RF (R=La–Lu) IN THE CATEGORICAL CRYSTALLOCHEMICAL TETRAHEDRON: POLYMORPHISM – ISOMORPHISM – TRUE MORPHOTROPY – STRUCTURAL HOMOLOGY

The homologous series of 15 RF (R = La–Lu) is based on the atomic number (Z) of the chemical elements Z. It is a T–x–quasi–system (QS) LaF–LuF of 14 partial QS–N systems (N 1 to 14): LaF–CeF, CeF–PrF, and further to YbF–LuF (ΔZ = 1). The crystal–chemical category triad isomorphism, polymorphism, and morphotropy are the homologous parameters of the RF series. The series was first proposed as the vertex of the structural homology (SH) of the crystal–chemical tetrahedron. The SH vertex is associated with the vertices of isomorphism by five and polymorphism by nine QS–N. On the edge of SH – true morphotropy there are QS–8 (GdF–TbF with peritectic) and QS–11 (HoF–ErF with eutectic) phase reactions of absolute true morphotropic transformation.