Non-Doniach-type phase diagram of CeNiGe2

The heavy-fermion antiferromagnet CeNiGe2 was investigated under pressure by resistivity and ac calorimetry up to 4 GPa and down to 40 mK. The two magnetic transitions found in both resistivity and specific heat at 0.1 GPa at TN1=3.95 and TN2=3.21 K are replaced by a single one at 0.7 GPa and 2.81 K. Increasing pressure initially reduces this further, however, at about 1.7 GPa, a new transition appears, accompanied by a marked change in the pressure dependence of the ordering temperatures, the temperature dependence of the resistivity, and the residual resistivity; the latter indicates a marked change in the quantum fluctuations present. There are signs that this new transition has some first-order character. The phase diagram of CeNiGe2 bears little resemblance to the Doniach phase diagram widely used to classify heavy-fermion compounds.