STRUCTURAL AND MAGNETIC-PROPERTIES OF UCO2GE2

We have studied the effect of annealing on the structural and magnetic properties of UCo2Ge2 and thereby find the existence of two different crystallographic structures. The as-cast, high-temperature (HT) phase adopts a crystal structure with symmetry lower than I4/mmm; after annealing at 750-degrees-C for 5 days the ThCr2Si2 (I4/mmm) structure is formed. Additionally, the length of the c axis in these tetragonal structures changes dramatically: whereas the HT phase has a c parameter of almost-equal-to 9.3 angstrom, the low-temperature (LT), annealed phase has a much larger c axis of almost-equal-to 9.9 angstrom. This expansion of the unit cell greatly influences the magnetic properties of the material: while the LT sample orders antiferromagnetically at 174 K, the HT one does not order magnetically down to 350 mK. The antiferromagnetic transition is clearly indicated by our magnetization, resistivity and specific heat measurements.