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The preparation of black single crystals of the two nonstoichiometric cerium coinage-metal oxysulfide compounds Ce[Cu.sub.x]OS and Ce[Ag.sub.x]OS (x approximately 0.8) by the reactions of [Ce.sub.2][S.sub.3] and CuO or [Ag.sub.2]O at 1223 or 1173 K, respectively is presented. Theoretical calculations with an on-site Coulomb repulsion parameter reveal that the Ce 4f states are fully spin-polarized and are not localized in CeCuOS, Ce[Cu.sub.0.75]OS, or CeAGOS.

Research has been conducted on the quaternary selenides. The synthesis of these compounds carried out via the use of high-temperature solid-state experimental methods is described and their structure, optical and magnetic properties have been investigated and are discussed.

The preparations of seven new isostructural uranium chalcogenides AMU[Q.sub.3] compounds (where A = alkali metal, M = Cu or Ag, Q = S or Se) via solid-state reactions are reported. The compounds are found to be semiconductors with optical band gaps smaller than 0.73 eV and density-of-state analysis revealed the M-Q orbital to overlap in the valence band and U-Q orbital to overlap in the conduction band.

A high-temperature sealed-tube reactions and spark plasma sintering were used to prepare the single crystal and bulk powder samples of the quaternary lanthanum copper oxysulfides [La.sub.5][Cu.sub.6.33][O.sub.4][S.sub.7] and [La.sub.5][Cu.sub.6][O.sub.4][S.sub.7]. The detailed study results provided insights into crystal structures, optical properties and transport properties, and magnetic properties of both the complexes.

The synthesis, crystal structures, magnetic and optical properties and electronic structure of a series of new quaternary semiconductor materials CsLnCd[Te.sub.3] (Ln = La, Pr, Nd, Sm, Gd-Tm, and Lu) are described. The studies have shown that CsLnCd[Te.sub.3] (Ln = Pr, Nd, Gd, Dy, Tm) materials have displayed temperature-dependent paramagnetism and have obeyed Curie-Weiss law, whereas CsSmCd[Te.sub.3] does not.

Research has been conducted on CsLnMSe (sub)3 compounds, where Ln is a rare-earth element Y, and M is Zn, Cd, Hg. The factors affecting optical properties of these compounds have been investigated via the use of theoretical calculations, and the details are reported.

Mn3Ta2O8, a stable targeted material with an unusual and complex cation topology in the complicated Mn-Ta-O phase space, has been grown as a ≈3-cm-long single crystal via the optical floating-zone technique. Single-crystal absorbance studies determine the band gap as 1.89 eV, which agrees with the value obtained from density functional theory electronic-band-structure calculations. The valence band consists of the hybridized Mn d-O p states, whereas the bottom of the conduction band is formed by the Ta d states. Furthermore, out of the three crystallographically distinct Mn atoms that are four-, seven-, or eight-coordinate, only the former two contribute their states near the top of the valence band and hence govern the electronic transitions across the band gap. [ABSTRACT FROM AUTHOR]

Single crystal and bulk powder samples of the quaternary lanthanum copper oxysulfides La5Cu6.33O4S7 and La5Cu6O4S7 have been prepared by means of high-temperature sealed-tube reactions and spark plasma sintering, respectively. In the structure of La5Cu6.33O4S7, Cu atoms tie together the fluorite-like ∞²[La5O4S5+] and antifluorite-like ∞²[Cu6S65-] layers of La5Cu6O4S7. The optical band gap, Eg, of 2.0 eV was deduced from both diffuse reflectance spectra on a bulk sample of La5Cu6O4S7 and for the (010) crystal face of a La5Cu6.33O4S7 single crystal. Transport measurements at 298 K on a bulk sample of La5Cu6O4S7 indicated p-type metallic electrical conduction with σelectrical = 2.18 S cm-1, whereas measurements on a La5Cu6.33O4S7 single crystal led to σelectrical = 4.5 10-3 S cm-1 along [100] and to semiconducting behavior. In going from La5Cu6O4S7 to La5Cu6.33O4S7, the disruption of the ∞²[Cu6S65-] layer and the decrease in the overall Cu2+ (3d9) concentration lead to a significant decrease in the electrical conductivity. [ABSTRACT FROM AUTHOR]