Hysteresis of the bcc–hcp transition in a solid mixture of He3 in He4

Abstract

Hysteresis of the bcc–hcp transition is observed by precision barometry in samples of a solid mixture of 1% He3 in He4 grown by the capillary blocking method. It is shown that the lines of equilibrium bcc–hcp and hcp–bcc transitions on the P–T diagram on cooling and heating do not coincide (the cooling line corresponds to higher pressure). In the process of thermocycling in the two-phase region the system executes a closed thermodynamic cycle, two branches of which correspond to the “normal” bcc–hcp transformation, with a slope of dP∕dT=6–12bar∕K, and the other two, to an “anomalous” transformation with a slope of dP∕dT=2.5–7bar∕K (the slope increases with decreasing molar volume). This effect is not observed in crystals of pure He4, and it can therefore be attributed to properties of the He3 impurity subsystem. A hierarchy of relaxation times for the pressure in the system is established which indicates that the mechanisms of transformation on different branches of the cycle are different. An interpretation of the observed effects is proposed in the framework of a model of a heterophase polydomain superstructure consisting of second-phase domains localized at He3 impurities in the He4 host matrix.