O. Uemura

Structural and electrical properties of AgI dispersed As-chalcogenide glasses

Abstract X-ray diffraction, EXAFS and Raman spectroscopic studies, together with transport phenomena measurements, have been carried out for (AgI) x (As 2 Se 3 ) 1− x glasses in order to investigate the ionic conduction mechanism in AgI doped non-oxide glasses. The addition of AgI into As 2 Se 3 glass is responsible for a pronounced increase in the electrical conductivity. Particularly, the ionic component of the electrical conductivity is dominant in highly doped glasses. Results of Raman spectra, EXAFS measurements and least squares fitting analysis for observed X-ray structural functions suggest that the local structure of the present glasses can be described as a pseudo-binary mixture of As–Se networks and fourfold coordinated AgI clusters.

TOF-neutron diffraction study of liquid Tl-As2X3 systems (X: Se, Te)

Abstract Time-of-flight neutron diffraction has been measured on both liquid Tl-As2Te3 and Tl-As2Se3 systems, which exhibit the metal-non-metal transition at the Tl-rich composition. Liquid As2Te3 contains direct As-As bonds in the first coordination shell as well as As-Te and perhaps Te-Te bonds, while liquid As2Se3 contains only As-Se bonds in the first coordination shell. The coordination number of component atoms in both liquids was determined to be three for As, and two for Te and Se, through a least-squares fit to the observed intensity function. The first sharp diffraction peak in S(Q) in liquid As2X3 (X: Te or Se), relating to -As-X-As- linkages, rapidly decreases upon the addition of Tl and disappears completely in the transition composition range in both liquid Tl-As2X3 systems. In the liquid Tl-As2Te3 system, Ass-Te bonds are gradually replaced by partially ionic Tl-Te bonds with increasing Tl content, leading to the sharp metal-non-metal transition at Tl6As2Te3. On the other hand, As-Se bonds in the liquid selenide system remain even in the vicinity of the transition composition, strongly influencing the electronic properties over a wide composition range.

X‐Ray Diffraction Study of Amorphous Ge(Se1—xTex)2 Alloys

X-ray diffraction and Raman scattering measurements have been carried out on amorphous Ge(Se 1-x Te x ) 2 alloys with x = 0 to 0.70. The first peak area of the distribution function indicates that the coordination number of Ge is nearly four at any of the compositions covered, suggesting that tetrahedral units having Ge at the central site are formed in the system. Further, Raman vibrational modes corresponding to five anion-mixed GeX (4-n)/2 Y n/2 tetrahedral units were observed in this alloy. The Ge-Te bond length, 0.261 nm, in the anion-mixed tetrahedra, obtained by the least squares fitting procedure for X-ray intensity functions is closer to the sp 3 -orbital covalent distance, similar to the Ge-Se bond length, which also evidences the existence of the tetrahedral units in the system.

Short-Range Structure in Amorphous Ge3(Se1—xSx)4 and Ge3(Se1—xTex)4 Alloys

The short-range structure in amorphous Ge 3 (Se 1-x S x ) 4 alloys with = 0 to 1, and Ge 3 (Se 1-x Te x ) 4 alloys with =0 to 0.25 has been investigated through X-ray diffraction and Raman scattering measurements. Raman spectra in both amorphous alloys have revealed that structural units containing a direct Ge-Ge bond, such as X 3/2 Ge-(GeX 2/2 ) n -GeX 3/2 or Ge(Ge (4-m)/4 X m/2 ), are formed in the alloy, in addition to the remaining GeX 4/2 tetrahedral units (X = Se, S and Te). The structural analysis by the X-ray diffraction method has indicated that the bonding distances of Ge-S, Ge-Se, Ge-Te and Ge-Ge pairs, which are involved in the first coordination shell, are all composition-independent and very close to the correponding covalent values, and the coordination numbers of the constituting atoms are nearly four for Ge and two for X. Any indication on a direct X-X bond has not been observed in the present measurements, which suggests that every X atom in the alloy preferentially coordinates to Ge atoms. The local order corresponding to crystalline GeX has not been found out in both amorphous alloys.

Neutron diffraction study of liquid (Tl1/2Pb1/2)−X(X:Te, Se) systems

Abstract Neutron diffraction measurements have been carried out on liquid (Tl1/2 Pb1/2)1−y Xy (X:Te,Se) systems which exhibit a typical compound-forming behaviour at y = 3 7 . Radial distribution analysis at y = 3 7 has indicated that the bonding nature of Tl X and Pb X is more or less ionic, as expected from the previous result of electronic properties. In order to investigate the difference in electronic behaviour between these two liquid systems in the chalcogen rich region, we derive the X—X partial distribution function gX−X(r, assuming that each partial function at y > 3 7 is independent of the alloy composition. gSe−Se(r) has a well-resolved first peak at r ∼ 0.230 nm, suggesting that covalent Se Se bonds appear in the Se rich region. On the other hand, gTe−Te(r) is somewhat structureless, although the first peak is included in the covalent region. The whole functional form of both gX−X(r) is roughly similar to that of g (r) in pure liquid Te or Se.