Be–Se double-phonon behavior in Zn1–x–yMgyBexSe alloy

International audiencehe longitudinal optical (LO) and transverse optical (TO) phonons of zincblende Zn1–x–yMgyBexSe with x,y~0.15 are identified by Raman scattering. The Be–Se region exhibits a two-mode behavior similar to that observed with the random Zn1–xBex(Se,Te) systems. This indicates that the percolation picture set for the latter mixed crystals with contrasted bond stiffness extends to Be-based multinaries. Accordingly the low- and high-frequency modes refer to Be–Se bonds within the Be-rich hard-like region and the (Zn,Mg)-rich soft-like one, respectively. Besides, we derive the (x,y) values together with the fraction p of Be atoms in the hard-like region from combined contour modeling of the TO and LO Raman line shapes. We find p~x, as in the ZnBe(Se,Te) references. We deduce that the Be incorporation is random in ZnMgBeSe

A percolation context evidenced by thermostimulated currents in oil-resin art media

Degradation over time of home-made thixotropic oil/resin mixtures, similar to those used as natural media by the art painter J.M. William Turner (1775–1851) is investigated by using a combination of dielectric methods, i.e., low-frequency dielectric spectroscopy (LFDS) and the technique of thermostimulated currents (TSC). These address the direct conductivity (d.c.), i.e., the circulation of free charges, and the anelastic orientation of permanent dipoles, respectively. The oil/resin mixtures can be regarded as conducting/insulating-like composite systems with respect to the two conductivities. This provides the opportunity to follow selectively the behavior of the oil, at any composition. In fresh mixtures, LFDS measurements give evidence for three d.c. regimes, delimited by the critical resin-contents (R) R 1∼30% and R 2∼60%. This parallels observation with the naked eye of phase separation at RR 2 in four-year-old naturally-aged samples. Such a stable composition-dependent context with two marked thresholds encourages a discussion of the time-behavior in terms of percolation, i.e., relative spatial organization of the parent substances, rather than by using a microscopic approach at the molecular scale. At R>R 1 the surveillance of the TSC signal indicates that the oil clusters tend to coalesce so as to form bigger clusters with aging. The coalescence process appears to be more efficient when the resin forms a continuum (R>R 2) than a dispersion (R<R 2)

Long-wave phonons in ZnSe-BeSe mixed crystals: Raman scattering and percolation model

International audienc