Specific heat measurements near the two irregular points in the phase diagram of U1-xThxBe13

The phase diagram (T versus x) of the superconducting heavy-fermion system U1-xThxBe13 has two irregular points which mark the endpoints of the concen tration range where in the specific heat a double maximum structure (indica ting two successive phase transitions) is observed. Thus at each irregular point at least three phase boundary lines meet and, if all phase transition s are of second order, a fourth second-order phase boundary line is expecte d. We present specific heat measurements carefully performed on samples clo se to the critical concentrations. As no indication of a fourth-phase bound ary line at the irregular points was found, we discuss the thermodynamic al ternatives to this scenario. (C) 1999 Elsevier Science B.V. All rights rese rved.