The Study On Local Structure Evolution Of Liquid Gallium Under Pressure

The study of the gallium under extreme conditions is of fundamental interest.Gallium,as a highly polymorphic metal,exhibits uncommon physical properties arising from the coaction of metallic and covalent bonding.The stable Ga I phase is a rare example of a quasi-molecular metal,while Ga-II and Ga-III,the high-pressure solid phases,have common metallic properties.An additional interest in this system stems from liquid gallium.The liquid gallium can be also subject to a undercooling.In this state,the density at ambient pressure exceeds of about 3% that of the stable Ga-I phase.Furthermore,liquid-liquid transition has been reported as a possible phenomenon.Although there have been a number of experimental and theoretical studies of liquid Ga,some nature remains unclear yet.For instance,the determination of density under pressure in liquid state is a challenging work due to lake of long-range order.The aim of this work is to shed light on some ambiguous properties including the density,local structure and liquid-liquid transition under extreme conditions.The results of the study are summarized as follows.The relatively volume of liquid and solid gallium as a function of pressure and temperature has been studied using synchrotron x-ray microtomography combined with energy dispersive x-ray diffraction techniques up to 3.02 GPa at 300 K and up to 3.63 GPa at 330 K.Both sets of P-V relation were directly determined from 3D tomography reconstruction data,and the corresponding isothermal bulk moduli were determined as 23.6(0.5)GPa and 24.6(0.4)GPa,respectively.Furthermore,a potential liquid-liquid phase transition region is proposed based on the abnormal compressibility of Ga melt at about 2.44 GPa and 330 K conditions.Based on measured density which a prerequisite for meaningful modeling of the pair distribution function(PDF)using the reverse Monte Carlo(RMC)approach,local structure of liquid gallium under pressure up to 1.9 GPa was characterized by In situ high energy X-ray PDF measurements and RMC simulations.The pressure range in this work includes the well-known solid-solid phase transition from Ga-I to Ga-II at low temperature.In term of previous research,the local structure of liquid gallium within this domain was suggested a mixture of two local structures,Ga-I and Ga-II,based on fitting experimental PDF to known crystal structure,with a controversy.However,our result shows a distinctly different result that the local structure of liquid gallium resembles the atomic arrangement of both gallium phase II and III(the high pressure crystalline phase).A melting mechanism is proposed for Ga,in which the atomic structure of phase ? breaks up at the onset of melting,providing sufficient free volume for atoms to rearrange,to form the melt.Since both PDF and density have been determined already,the fractal feature can be studied in terms of scaling power law.Generally,a single scaling exponent,D_f,can characterize the fractal structures of metallic glasses.However,when the scaling power law is applied to liquid gallium,the results show a multiple scaling exponent and the values are beyond 3 within the first four coordination spheres,indicating that this power law fails to describe the fractal features of liquid gallium.In fact,the increase in the first coordination number with pressure leads to that first coordination spheres at different pressures are not similar to each other in geometrical sense;in other words,the fractal dimensionality varies with pressure.As the pressure increases,this multiple scaling power behavior is confined within a correlation length of 11.65 to 11.38 ? based on a percolation model.Beyond this length the liquid gallium system is homogeneous,as indicated by the fractal dimensionality,D_f,which is close to 3 beyond the first four coordination spheres.We also extend our high pressure microtomography study to X-ray scattering study on gallium at 330 K up to 3.7 GPa.This P-T range covers a liquid and a solid region.The crystallized liquid gallium presents a twinning mixture crystal structure consisted of Ga-II and Ga-III at pressure range from 3.4 GPa to 3.7 GPa,which deviates from the commonly accepted phase diagram of gallium and hence suggests a metastable region.In terms of RMC simulation,Ga melt,Ga-II and Ga-III share similar structural motifs,making it possible to form both Ga-II and Ga-III.Furthermore,according to classical nucleation theory,this metstability behavior relates to probability of formation of Ga-II nuclei comparable with that of Ga-III.Besides the observed solid metastable region,a rare liquid metastable region around 2.4 GPa-330 K also presents in gallium.Within this metastable region,two states of liquid gallium phase were displayed in different experiments depending on the thermal and pressure history.In fact,both solid and liquid metastable regions have more to do with P-T path.In summary,this work firstly provides the information on densities by direct measurement using X-ray microtomography and suggests a pressure-induced potential liquid-liquid phase transition at 330 K.Based on the local sturcuture of liquid gallium,a melting model was proposed.Furthrmore,a multiple scaling power behavior was observed.At 330 K a solid state metastalbe region and a liquid state metastalbe region are discovered.These findings will trigger further investigations of the structure and physical properties of gallium to help us understand more about the nature of this fascinating element.