In a mixture of He3-He4 , one expects to find two types of phase transitions, namely, the superfluid and the binary. This is shown in the phase diagram in Fig.3.19 with x3 indicating the molar fraction of He3.
The l-line, extending from ( , ) to the tricritical point at ( , ), denotes the 2nd order superfluid phase transition.
The tricritical point is also the critical point of the 1st order binary phase transition. Thus, below , the mixture can segregate into coexisting He3 rich and He4 rich regions. This can be observed using NMR technique.
The tricritical point is so called because it is actually the intersect of 3 lines of 2nd order transitions. [see R. B. Griffith, Phys. Rev. Lett. 24, 715 (1970)]