Seminars Archive

Study of liquid-liquid phase transitions of chalcogens and water using x-ray techniques

Abstract
Among a lot simple liquids, there are some anomalous liquids whose thermodynamic features are much different from other liquids. Liquid water near ambient condition is a representative example: temperature dependence of density shows maximum at 4 º C, speed of sound increases with increasing temperature and melting temperature decreases with increasing pressure. Although there are many attempts to explain these anomalies, liquid-liquid phase transition hypothesis[1] seems to be the most promising scenario. There exist two liquid structures, high-density and low-density ones, and the liquid-liquid phase transition between these two structures has a 2nd critical point (of course, 1st one is liquid-gas critical point) in the supercooled region. Water at ambient condition is located in the supercritical region of the transition and thus thermodynamic features are affected by the 2nd critical fluctuations and exhibit anomaly. We now try to verify and advance the hypothesis by studying other anomalous liquid systems. We adopt liquid chalcogen (Te, Se) systems. Liquid Te shows thermodynamic anomalies (density[2], sound velocity[3], melting curve[4]) similar to water. Liquid Se-Te mixtures are known to exhibit continuous liquid-liquid phase transition from 2-fold coordinated Se-like structure to 3-fold coordinated Te-like one[2,5] when the composition, temperature and/or pressure change. Our group has developed[6] unique technique for x-ray measurements (wide-angle diffraction, small-angle scattering=SAXS, inelastic scattering=IXS and Compton scattering) of liquids under highpressure(< 200MPa) and high-temperature(<1800ºC) conditions and carry out the experiments at the 3rd generation synchrotron radiation facility SPring-8 in Japan. In this seminar, I will show our recent results of SAXS[7,8] and IXS[7,9] measurements of liquid chalcogen systems and IXS of water[10]. SAXS proves that the density fluctuations show maximum and IXS shows that it becomes so-called fast-sound [11,12] state at liquid-liquid phase transition region. I will focus on the similarity/universality between liquid chalcogens and water and also discuss the anomaly of water recently observed by inelastic ultraviolet scattering method[11,13]. Finally, I will show my speculation about the universal concept of liquid-liquid phase transitions.

[1] P.H.Poole et al, Nature 360 (1992) 324; O.Mishima and H.E.Stanley, Nature 392 (1998) 326
[2] H.Thurn and J.Ruska, J.Non-Cryst.Sol.22 (1976) 331l
[3] M.B.Gitis et al, Sov. Phys. Acoust. 12 (1966) 14
[4] D.A.Young, Phase diagram of the elements §10.5, University of California Press.(1991)
[5] W.Hoyer et al, Z.Naturforsch.30a (1975) 1633 [6] K.Tamura and M.Inui, J.Phys:Condens.Matter 13 (2001) R337
[7] Y.Kajihara et al, 14th Liquid and Amorphous Metals conference, Rome in Italy, July 2010
[8] Y.Kajihara et al, J.Phys.:Conf.Ser.215 (2010) 012078
[9] Y.Kajihara et al, J.Phys.:Condens.Matter 20 (2008) 494244
[10] Yamaguchi et al, J.Phys.Chem.Sol. 66(2005)2246; Y.Kajihara et al, in preparation
[11] S.C.Santucci et al, Phys.Rev.Lett.97 (2006) 225701
[12] D.Ishikawa et al, Phys.Rev.Lett.93 (2004) 097801
[13] F.Bencivenga et al, J.Chem.Phys. 131 (2009) 144502