Structural Studies on First-Order Phase Transitions inRPd$_{2}$Ga$_{2}$

We have investigated the structure of RPd$_{2}$Ga$_{2}$ (R = La, Ce) compounds by neutron powder diffraction. For the first time, a first-order structural transition is observed at T$_{t}$ = 70 K and T$_{t}$ = 125 K for CePd$_{2}$Ga$_{2}$ and LaPd$_{2}$Ga$_{2}$, respectively. The high-temperature structure (T $>$ T$_{t})$ for both compounds is the tetragonal CaBe$_{2}$Ge$_{2}$ type with lattice parameters of a = 4.4791(4) {\AA} and c = 9.83732(17) {\AA} and a = 4.83185(19) {\AA} and c = 10.7548(5) {\AA} for LaPd2Ga2 and CePd2Ga2 at 305 K, respectively. Below T$_{t }$ the symmetry of both structures descends to an orthorhombic space group, \textit{Pmmn}. Lattice parameters at 2 K are $a $= 6.07032(3) {\AA}; $b $= 12.90921(6) {\AA}; $c $= 9.87899(5) {\AA} and $a$ = 6.3996(4), $b$ = 11.9508(8), $c$ = 9.9291(7) for LaPd$_{2}$Ga$_{2}$ and CePd$_{2}$Ga$_{2}$. In this talk, evidence for the order of the structural transition will be presented and the low and high temperature structures will be discussed.