The structural phase transition in calcium-aluminum compound (CaAl4): a concerted application of Landau theory and energy band theory

摘要

CaAI4 undergoes a reversible, structural transformation at 170 °C according to high-temperature X-ray powder diffraction experiments. The high-temperature phase adopts the tetragonal BaAI4 structure (space group I4jmmm), while the low-temperature phase is monoclinic, C2jm, with lattice parameters, a = 6.1526 (15) A, b = 6.1730 (13) A, c = 6.3290 (14) A, (j = 118.026 (16)0 • The Landau theory of phase transitions correctly provided a structural model for the low-temperature phase, which could be subsequently refined. Also, electronic structure calculations on both forms of CaA14 allow rationalization of the transformation in terms of changes in local chemical bonding within the AI framework.