Thermal expansion behaviour and phase stability of AFe$_2$As$_2$ (A=Ca, Sr and Eu) using powder diffraction technique

摘要

The thermal expansibilities and phase stabilities of AFe$_2$As$_2$ (A = Ca, Sr and Eu) have been investigated by powder diffraction techniques in the temperature range 5a€“600 K.We found the anisotropic thermal expansivities with temperature for all the compounds. The lattice parameter in the tetragonal phase (AT) of CaFe$_2$As$_2$ contracts with increasing temperature, whereas CT expands. The rate of contraction in AT is lower than the rate of expansion in CT. Other compounds show normal thermal expansion behaviour along both a- and c-axes. In-plane expansion (i.e., along the a-axis) is found to be the smallest for EuFe$_2$As$_2$ and the highest for BaFe$_2$As$_2$. However, therate of change of thermal expansivities along out-of-plane (i.e., along the c-axis) is higher as we go from Ba, Sr, Eu and Ca, respectively. Above 600 K, we notice the appearance/disappearance of certain reflections which suggest that tetragonal phase is not stable above this temperature for these compounds.