Structures and phase transitions of CePd _3+xGa _8-x: New variants of the BaHg ₁₁ structure type

New distorted variants of the cubic BaHg ₁₁ structure type have been synthesized in Ga flux. Multiple phases of CePd _3+xGa _8-x, which include an orthorhombic Pmmn structure (x = 3.21(2)), a rhombohedral R3̄m structure (x = 3.13(4)), and a cubic Fm3̄m superstructure (x = 2.69(6)), form preferentially depending on reaction cooling rate and isolation temperature. Differential thermal analysis and in situ temperature-dependent powder X-ray diffraction patterns show a reversible phase transition at approximately 640 °C between the low temperature orthorhombic and rhombohedral structures and the high temperature cubic superstructure. Single crystal X-ray diffraction experiments indicate that the general structure of BaHg ₁₁, including the intersecting planes of a kagomé-type arrangement of Ce atoms, is only slightly distorted in the low temperature phases. A combination of Kondo, crystal electric field, and magnetic frustration effects may be present, resulting in low temperature anomalies in magnetic susceptibility, electrical resistivity, and heat capacity measurements. In addition to CePd _3+xGa _8-x, the rare earth analogues REPd _3+xGa _8-x, RE = La, Nd, Sm, Tm, and Yb, were successfully synthesized and also crystallize in one of the lower symmetry space groups.