Structural instability in CePd2(Al,Ga)2 and LaPd2(Al,Ga)2

The present study is focused on the structural phase transition in (La,Ce)Pd2Al2−xGax investigated by low temperature X-ray powder diffraction. All parent compounds crystallize in the tetragonal CaBe2Ge2 structure type (P4/nmm, 129) at room temperature and undergo structural phase transitions at low temperatures. Rietveld analysis of the measured patterns shows that the low temperature structures of (La,Ce)Pd2Al2 and (La,Ce)Pd2Ga2 have the same space groups and Wyckoff positions, but exhibit different magnitude of distortion and different characteristics of the structural transition. A small continuously appearing distortion is present in the Al parent compounds while the Ga ones exhibit a large discontinuously appearing distortion. These low temperature structures can be described using the Cmme, 67 space group which is translationengleiche subgroup of the tetragonal P4/nmm, 129 space group. The concentration dependence of the temperature where the distortion occurs has a minimum or is almost suppressed at a concentration of x∼0.4 (cerium series) and x∼1.6 (lanthanum series). To support our experimental findings we also present density function theory calculations which confirm the structural stability as experimentally observed. We discuss the impact of structural properties in the studied series on the previously observed development of physical properties such as superconductivity and magnetic excitations in neutron scattering spectra. [Display omitted] •(Ce,La)Pd2(Al,Ga)2 were investigated by low temperature X-ray powder diffraction.•The structural instability of room temperature tetragonal structure was found.•The low temperature structure was determined as an orthorhombic structure (Cmme).•The phase diagram was constructed in dependence of temperature and Ga content.