Soft Report - Dipartimento di Fisica - Sapienza

Scientific Report – Non Equilibrium Dynamics and ComplexityDroplets of Liquid Gallium Under High Temperature and HighPressure ConditionsIn the last few years the study of liquidpolymorphism has received a great attention. Theexistence of two possible distinct liquids, with thesame chemical composition but with differentphysical properties is quite consistent withexperimental data and theoretical predictions.Liquid-liquid phase transition has been related to thesoft-core interaction potential characterized by thecoexistence of metallic and covalent behaviour in thebonding (for example as liquid Sn, Bi, Ga or Ge). Inprinciple Gallium is one of the best candidate forshowing liquid polymorphism. This is due to twoprincipal reason: one its ice-type phase diagram witha negative temperature dependence and the secondone because its display extended polymorphism inthe solid phase. The samples we investigatedconsist of gallium droplets (~ 4% weight) inserted inan inert matrix of mixed epoxy resin and LiF. Weperform on our sample different spectroscopictechniques: X-ray absorption on Ga K-edge level(XAS) , single x-ray absorption detection (SEXAD)and energy x-ray diffraction (ESXD). Ourmeasurements were performed at the BM29 of theESRF facility using a Paris-Edimburgh large volumecell in a range of pressure up to 6.7 Gpa and atemperature range of 298-440 K. In the next figurewe show the behaviour of the x-ray diffractionspectra as function of the pressure where it ispossible to observe clearly the appearance of Ga(III)crystalline phase Bragg peaks above 4.2 GPa(channel 2) whereas no Ga(III) (013) peak isdetected at 2.3 GPa (channel 3). It is important tonote the appearance of Ga(III) in a region wherethat Ga(II) has a stable crystalline phase. In thesame figure on the right side temperature scans(SEXAD) recorded at different pressure are reported,and it is possible to see the discontinuities relatedwith the solid-liquid phase transition. The increasingdiscontinuities observed as function of pressureFig. 2.confirms the occurrence of a partial crystallization ofthe gallium droplets.In the following picture the XAS measurements in anenergy interval close the Ga-edge for two differentthermal histories are reported. We can observe aobvious change in the absorption in the liquid phaseobtained between 1.6 GPa and 0 GPa and this mayindicate the occurrence of a possible transformationin the microscopic structure or/and in the electronicstate.Moreover the shape of the difference signal (blackcurve) it is quite different from the one obtainedbetween the 5.8 and 2.7 GPa spectra and alsodifferent from a typical Da(E) curve of twoequilibrium liquid Ga XAS spectra (right panel). Thisresult may infer the existence of a different liquidphase above 1.6 Gpa.So considering both ESXD and SEXAD spectra wecan observe that the quantity of crystallized galliumdroplets increases as function of pressure while noevidence of crystallization is observed up to 2.6 Gpa,well under the melting point line at roomtemperature.In conclusion the interesting hypothesis of a liquidliquidphase transition occurring between 0 and 1.6GPa is compatible with our results and it spursfurther investigation on this systems.References[1] R. Poloni, S. De Panfilis, A. Di Cicco, G. Pratesi,E. Principi, A. Trapananti, and A. Filipponi "Liquidgallium in confined droplets under high-temperatureand high-pressure conditions", Phys. Rev. B 71,184111 (2005).AuthorsR. Poloni, S. De Panfilis, A. Di Cicco, G. Pratesi, E.Principi, A. Trapananti, and A. FilipponiFig. 1.SOFT Scientific Report 2004-0676