T. Usuki

Local atomic arrangement in Ge-Te and Ge-S-Te glasses

Abstract X-ray diffraction and Raman spectroscopic studies have been made on GexTe1 − x (x = 0.16–0.20) and Ge(S1 −xTex)2 (x = 0–0.75) glasses. The position and width of the first peak in g(r) in Te-enriched Ge-Te glasses vary with composition, suggesting that the nearest neighbor shell in the glass contains more than one atomic correlation, namely Ge-Te and Te-Te. It was indicated by a least squares analysis of intensity functions that each Ge atom is surrounded by four Te atoms, implying that the chemically ordered 4-2 coordination model is favored in Ge-Te glasses. Results of both the Raman spectrum and diffraction intensity analysis on Ge(S1 − xTex)2 glasses show that basic structural units in the glasses are anion-mixed tetrahedra, Ge(S(4 − n)/2 Ten/2) type where n = 0 ~ 4, with a Ge atom in the central site. The results agree with the case in Ge(S1 − xSex)2 glasses. However, S-enriched anion-mixed units, for example, n = 1 or 2 type, are not formed in the present telluride glass, due to a considerable difference in atomic size between S and Te.

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.

Short-range order of amorphous and liquid As-Te-I system

Abstract Raman scattering and X-ray diffraction measurements have been carried out on the amorphous As 50 − x Te 50 I x system with x = 0–25 . The Raman band relating to the AsI 3 molecular unit, which has been observed in both amorphous As-S-I and As-Se-I systems, was not detected in the present system. Raman spectra also revealed that mixed-anion structural units, such as As(Te (3 − n)/2 I n ) type, are not present in the telluride system. It was made clear from the X-ray radial distribution analysis of the system that the direct As-As bond is included in the first coordination shell, together with As-Te and/or Te-Te bonds, and that I atoms introduced are preferentially coordinated to Te. The presence of the As-As bond was also confirmed by EXAFS measurement. Radial distribution functions in the liquid As 40 − x Te 60 I x system ( x = 0–35 ) obtained by neutron diffraction showed that the liquid short-range order is similar to that in the amorphous state, except for an increase in the coordination number around Te.

Ionic and electronic conductions in liquid metal–chalcogen systems

Abstract The measurements of electronic and ionic conductions in liquid In–Se and Cu–Se systems have been carried out separately as a function of concentration and temperature. The electronic conductivities of these systems have minima near the composition x =0.58 for the In 1− x Se x system and x =0.33 for the Cu 1− x Se x system. Both systems have maxima in the ionic conductivities at the same composition. In proportion as the electronic conductivity increases with changing composition, the ionic conductivity decreases. In Se-rich region of Cu–Se system, the change from negative to positive temperature dependence of the electronic conductivity and a discontinuous change in the ionic conductivity have been observed at a certain temperature.

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.

Mercury Sulfide Dimorphism in Thioarsenate Glasses

Crystalline mercury sulfide exists in two drastically different polymorphic forms in different domains of the P,T-diagram: red chain-like insulator α-HgS, stable below 344 °C, and black tetrahedral narrow-band semiconductor β-HgS, stable at higher temperatures. Using pulsed neutron and high-energy X-ray diffraction, we show that these two mercury bonding patterns are present simultaneously in mercury thioarsenate glasses HgS-As2S3. The population and interconnectivity of chain-like and tetrahedral dimorphous forms determine both the structural features and fundamental glass properties (thermal, electronic, etc.). DFT simulations of mercury species and RMC modeling of high-resolution diffraction data provide additional details on local Hg environment and connectivity implying the (HgS2/2)m oligomeric chains (1 ≤ m ≤ 6) are acting as a network former while the HgS4/4-related mixed agglomerated units behave as a modifier.

Zero-dimensional Cryogenic Glasses and Supercooled Liquids in the Se-Cl System

Raman spectroscopy and high-energy x-ray diffraction measurements of binary Se1−xClx glasses, supercooled and normal liquids have shown a gradual transformation of the chain-like one-dimensional (1D) structure of glassy selenium (x = 0) into a zero-D molecular structure of small non-linear and non-planar oligomeric molecules of SenCl2-type, where 2 ≤ n ≤ 5. Rapid cooling of the Se1−xClx liquids into liquid nitrogen yields transparent cryogenic glasses over the entire composition range, 0 ≤ x ≤ 0.5, whose glass transition decreases monotonically from 45°C to −70°C.

Inhomogeneity and glass-forming ability in the bulk metallic glass Pd{sub 42.5}Ni{sub 7.5}Cu{sub 30}P{sub 20} as seen via x-ray spectroscopies

Core-level photoemission spectroscopy and anomalous x-ray scattering (AXS) measurements were performed for the