The unusual phase diagram of CeNiGe2

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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. 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.