John B. Wiley

Rapid solid-state precursor synthesis of materials.

Precursor reactions based on metathetical (exchange) pathways have been found to be an effective synthetic route for the preparation of a large number of materials. These solid-solid reactions are extremely rapid (typically less than 1 second) and often can be initiated at or near room temperature. They are potentially useful for controlling product particle size and for preparing highquality cationic or anionic solid solutions. The frequently self-propagating and sometimes explosive behavior exhibited by these reactions can be attributed to the large amount of heat they release. As a consequence, thermodynamic considerations can be used to help select the best set of precursors as judged from reaction enthalpies. The factors that influence these reactions are illustrated by a discussion of MoS2, ZrN, MoSi2, and GaAs, examples of the many compounds accessible by this synthetic route.

From Ceramics to Superconductors: Rapid Materials Synthesis by Solid-State Metathesis Reactions

The preparation of materials from gas and liquid phase precursor reactions is well documented. However, solid-state precursor routes have remained largely unexplored. This synthetic void led us to develop rapid ( where the Nal is simply washed away with water. Analogous reactions allow the preparation of carbides, suicides, chalcogenides (O, S, Se, and Te), and pnictides (N, P, As, and Sb), of main-group, transition-, and rare-earth metals. These methods can also be exploited to produce mixed-metal and mixed-nonmetal solid solutions. To gain insight into some of the mechanistic factors involved in these reactions, such as initiation, nucleation, and propagation, we have employed a variety of physical characterization methods. This paper will review the range of materials accessible by our synthetic approaches, as well as the control of product crystallinity, phase, and homogeneity through the optimization of reaction conditions.

Phases, kinetics and structure of alkali-C60 compounds : preparation of Rb3- and (Rb3-xKx)-C60 superconductors

Abstract System exploration of the compound-formation kinetics, phase composition and super-conductivity of Rb:C60 mixtures, as diagnosed by X-ray crystallography and shielding diamagnetism measurements, reveals a single superconducting phase: f.c.c. Rb3C60 compound (a = 14.43 A , T c = 29.6 K ) analogous to f.c.c. K3C60 (a = 14.26 A , T c = 19.2 K ) . The formation kinetics are interpreted in terms of staging, i.e., segregation and growth of several phase-separated compounds. The other prominent phase is b.c.c. Rb6C60, analogous to the A6C60 compounds (A =Cs, K) reported earlier, but there is also a b.c.t. Rb4C60 compound and indications of another phase intermediate between C60 and Rb3C60. In samples prepared from mixtures in the 2.2:1 to 2.8 range, the shielding reaches a maximum of >50%, a high value for powder samples, and the X-ray pattern is free of interfering phases. Also reported are indications of two mixed-alkali phases prepared in high yield and tentatively associated with Rb2K1C60 and Rb1K2C60 (Tc values near 28 and 22.5 K, respectively). In contrast to the always sharp onset for Rb3C60, these compounds exhibit the rounded onsets characteristic of residual inhomogeneity.

Solid-solution rubidium/potassium mixed-metal fullerides

Abstract Polycrystalline samples of mixed alkali-metal fullerides, Rb 3−x K x C 60 (x = 1, 2), were prepared by the direct reaction of C 60 with alkali metal vapor. Superconducting transition temperatures and lattice parameters, as determined by magnetic susceptibility and X-ray powder diffraction, are intermediate to the two endmembers, K 3 C 60 and Rb 3 C 60 . Compounds appear to be solid solutions that obey Vegards Law. No indication of size-difference-induced cation ordering on tetrahedral and octahedral sites is observed.