Compositional evolution of the NaZn₁₃ structure motif in the systems La-Ni-Ga and Ce-Ni-Ga

Phase relationship and structural behaviour in the substitutional series LaNi_13-xGa_x and CeNi_13-xGa_x have been studied by a combination of X-ray powder diffraction measurements, differential scanning calorimetry, electron diffraction tomography and metallographic analyses. The sequence of morphotropic phase transformations has been found in the series LaNi_13-xGa_x resulting in five varieties of the NaZn₁₃ structure: the cubic phase with aristotype structure at x = 2 (space group Fm3[combining macron]c, Pearson symbol cF112), two tetragonal phases at x = 2.5-4.25 (space group I4/mcm, Pearson symbol tI56-I) and 7-7.5 (space group I4/mcm, Pearson symbol tI56-II), both with an atomic arrangement of the CeNi_8.5Si_4.5 type and two orthorhombic phases at x = 4.5-5.75 (LaNi₇In₆ structure type, space group Ibam, Pearson symbol oI56) and x = 6.37-6.87 (a new derivative of the NaZn₁₃, prototype structure, space group Fmmm, Pearson symbol oF112). The related series CeNi_13-xGa_x shows similar behaviour. The corresponding tI56-I ↔oI56 ↔oF112 ↔tI56-II phases are formed at x = 4-4.25, 4.5-6, 6.37-6.87 and 7-7.37, respectively. In contrast to the lanthanum analogues, the phase with cubic symmetry was not found for this system. Complex twinned and multiple twinned (twinning of twins) domain structures which are revealed for the tetragonal and both orthorhombic phases clearly indicate temperature-induced polymorphic phase transitions during the formation of these phases. LaNi_13-xGa_x samples show paramagnetic behavior, whereas the CeNi_13-xGa_x series exhibits Curie-Weiss paramagnetism.