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.
The kinetics of a bcc–hcp structural phase transition on the melting curve of He4 near the triple points (bcc–hcp–He II and bcc–hcp–He I) in the temperature range 1.25–2.0 K is investigated. The capillary blocking method is used to make the samples, and high-precision pressure measurement is used to investigate the kinetics. It is established that far from the triple points exponential functions describe the pressure change accompanying a temperature change on the melting curve. Anomalous behavior of the pressure and temperature of the sample is observed at a transition through the triple points. This attests to remelting of the crystal at a structural phase transition.
The existing methods of measuring pressure in a crystallization thermometer used for measuring ultralow temperatures are analyzed. It is shown that a method based on measuring the resonance frequency of a resonance circuit, which includes a capacitive pressure gauge, can be used to increase measurement sensitivity and accuracy. A low-temperature FET oscillator is described. This oscillator makes it possible to increase the sensitivity and accuracy of temperature measurements in the range 0.9 mK–1 K by more than an order of magnitude.
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