A concentration-saturated helium mixture at the melting pressure consists of two liquid phases and one or two solid phases. The equilibrium system is univariant, whose properties depend uniquely on temperature. Four coexisting phases can exist on singular points, which are called quadruple points. As a univariant system, the melting pressure could be used as a thermometric standard. It would provide some advantages compared to the current reference, namely pure \(^3\)He, especially at the lowest temperatures below 1 mK. We have extended the melting pressure measurements of the concentration-saturated helium mixture from 10 to 460 mK. The density of the dilute liquid phase was also recorded. The effect of the equilibrium crystal structure changing from hcp to bcc was clearly seen at \(T=294\) mK at the melting pressure \(P=2.638\) MPa. We observed the existence of metastable solid phases around this point. No evidence was found for the presence of another, disputed, quadruple point at around 400 mK. The experimental results agree well with our previous calculations at low temperatures, but deviate above 200 mK.
Melting pressure thermometry Quadruple point Solid nucleation Nucleation overpressure
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This work has been supported in part by the EU 7th Framework Programme (FP7/2007-2013, Grant No. 228464 Microkelvin) and by the Academy of Finland through its LTQ CoE grant (Project No. 250280). We also acknowledge the National Doctoral Programme in Materials Physics for financial support. We thank A. Sebedash and I. Todoshchenko for useful discussions.
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