Investigation of Medium Range Order by Photoemission, Reflectivity, Laser-Induced Crystallization Raman Spectroscopy in Amorphous GeSe₂

Abstract

It is very difficult to determine the structure of amorphous materials, since the long range order is completely destroyed, where the conventional crystallographic diffraction method becomes less powerful. In many cases, however, infrared¹, Raman¹ and Mossbauer² spectroscopies, and electro-microscopy are considerably effective means to investigate the intermediate structure of glasses,^3,4 while the X ray or neutron diffraction methods give important information among near neighbor atoms and some suggestions about medium range structures. Recent theoretical considerations relate glass forming tendencies and global profile of glass structures to the mean coordination number m. Phillips predicted that glass formation is easiest when the coordination number m = 2.4. ^4,5 Thorpe⁶ discussed that the elastic percolation begins at near m = 2.4 in a covalent network glass, if we could sweep the mean coordination in some means. For several years, we^7–12 have investigated composition and pressure dependence of the vibrational spectra in g-(Ge,Sn)(Se,S). We paid attention to the characteristic spectra of molecular clusters such as Ge(Se, or S)_4/2, (Ge₁-_ySny)(Se, or S)_4/2, Ge₂Se_6/2, Seⁿ, S₈, and Sⁿ. The GeSe_4/2 clusters in g-GeSe₂ are highly strained even without external pressure. The strain is released with increasing selenium composition x, as is observed in the composition dependence of the A₁ peak, and is relaxed at near x_c=0.80, where m = 2.4. ^{10, 11}Such critical behaviors were investigated in many situations in the glass system and the stability and the formation of the clusters were discussed.^{12, 13}