We present results on surface freezing of Ga-based alloys, GaBi, GaPb and GaTl, above the
liquidus line between the Ga-rich eutectic and the monotectic point. Spectroscopic ellipsometry
(0.8 eV ≤hν≤4.2 eV)
and kinetic single wavelength ellipsometry (2.75 eV) have been employed to
probe the changes of the interfacial electronic structures on surface freezing.
To minimize thermal gradients across the sample a heatable cap that covers
the sample and crucible was developed. The surface freezing temperature,
TSF, for the spontaneous formation of a solid-like film on top of the Ga-rich liquid
on cooling the sample from the homogeneous phase region was found to be
independent of the temperature difference between the upper and lower furnace
(ΔT:
+10 to
−10 K) and only weakly dependent on the cooling rate (: 2.5–20 K h−1). In the case of GaPb the solid film consists of solid Pb with a thickness
h≥400 Å. Comparing with GaBi we draw analogous conclusions for GaPb and GaTl and suggest
that the surface freezing transition precedes the bulk phase transition along the liquidus
line as the alloy is cooled.
Single phase perovskite ceramic Pb(Mg 1/4 Cd 1/12 Nb 2/3 )O 3 (PMCN) was prepared by the high temperature solid-state reaction method. Dielectric constant and loss tangent were measured over a frequency range 1 kHz to 1 MHz in the temperature region −40 to 115 • C. Dielectric studies revealed the relaxor behaviour with diffuse phase transition. The frequency dependence of the temperature of the permittivity maximum (T m ) has been modeled using Vogel-Fulcher relation. The dielectric relaxation in PMCN is found to be analogous to the magnetic relaxation in spin-glass system.
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