Structure phase transitions of polymorphic compounds with layered crystal structures: The REIr2Si2 case

REIr₂Si₂ (RE = La, Ce, Pr and Nd) were found to be polymorphic compounds which can crystallize in two different tetragonal types of crystallographic structures (β-phase: CaBe₂Ge₂-type; α-phase: ThCr₂Si₂-type). Here we study the structural phase transitions (α- to β- and β- to α-) using thermal expansion, differential thermal analysis and neutron diffraction. Both phase transitions occur at temperatures lower than 1300 °C, have a hysteresis up to 300 °C, do not show any intermediate steps and are accompanied with the absorption/release of the latent heat. We conclude that these structural phase transitions are of the first order.