Isostructural transition in RTAl compounds at high temperatures

Based on previously discovered forbidden values of the c/a ratio, we studied anomalies in the temperature evolution of the lattice parameters (a, c) of several hexagonal RTAl compounds at high temperatures (above room temperature). The compound ErNi0.45Cu0.56Al, that seemed to be impossible to be prepared in single-phase form, reaches the single-phase state at temperatures above 493 K (220 degrees C). On the other hand, the compound TbNi0.7Cu0.3Al - that was expected to undergo a first-order structural phase transition at high temperatures - exhibits a second-order type transition when approaching the critical values of the c/a ratio. These results are in agreement with idea of a forbidden range of c/a values in these compounds.