In 1969, it was suggested that atomic vacancies could form a Bose condensate in solid helium-4, which would cause the solid to have superfluid properties. This scenario has still not been observed, because it seems that vacancy activation energy is too high. However, this energy lowers as the molar volume of the solid increases, but this lowering is limited by the melting line. The work presented in this thesis, aims to produce a metastable state relative to melting of solid helium, to extend the lowering of vacancy energy and tend to the realization of the scenario. To this end, we developped a technique allowing us to focus an ultrasonic sound wave inside a helium monocrystal, despite its anisotropy. Pressure oscillations causes by the sound wave bring transiently the solid below its melting pressure, close to 25 bar. An interferometric measurement of the acoustic field, made through the cryostat viewports, is used to determine the pressure of the metastable sample produced. Our results show that one can obtain metastable states down to 21 bar, i.e. 4,5 bar below the melting pressure. Beyond this threshold, the crystal undergoes an unexpected instability, much before the predicted spinodal limit. The instability analysis shows that this could be a nucleation phenomenom of the liquid phase, although the pressure threshold is incompatible with the actual model.
