In recent years, many soft-chemical reactions of layered perovskites have been reported, and they can be classified into sets of similar reactions. Simple ion-exchange and intercalation reactions replace or modify the interlayer cations of layered perovskites, and more complex metathesis reactions replace interlayer cations with cationic structural units. Topochemical condensation reactions that involve dehydration and reduction provide access to a variety of metastable structural features in three-dimensional perovskites, and similar reactions can be used to convert among higher order layered perovskite homologues. Other techniques, such as high pressure and anion intercalation/deintercalation, also yield interesting metastable phases. When combined, the individual reactions complement each other, and a powerful toolbox of solid-state reactions emerges. By using layered perovskites as templates, it is possible to retrosynthetically design new product perovskites that retain the structural features of the...

Perovskites by Design: A Toolbox of Solid-State Reactions

A new multistep approach was developed to synthesize atomically ordered intermetallic nanocrystals, using AuCu and AuCu(3) as model systems. Bimetallic nanoparticle aggregates are used as precursors to atomically ordered nanocrystals, both to precisely define the stoichiometry of the final product and to ensure that atomic-scale diffusion distances lower the reaction temperatures to prevent sintering. In a typical synthesis, PVP-stabilized Au-Cu nanoparticle aggregates synthesized by borohydride reduction are collected by centrifugation and annealed in powder form. At temperatures below 175 degrees C, diffusion of Cu into Au occurs, and the atomically disordered solid solution Cu(x)Au(1)(-)(x) exists. For AuCu, nucleation occurs by 200 degrees C, and atomically ordered AuCu exists between 200 and 400 degrees C. For AuCu(3), an AuCu intermediate nucleates at 200 degrees C, and further diffusion of Cu into the AuCu intermediate at 300 degrees C nucleates AuCu(3). Atomically ordered AuCu and AuCu(3) nanocrystals can be redispersed as discrete colloids in solution after annealing between 200 and 300 degrees C.

Synthesis of Atomically Ordered AuCu and AuCu3 Nanocrystals from Bimetallic Nanoparticle Precursors

Hexanuclear and higher nuclearity clusters of the Groups 4–7 metals with stabilizing π-donor ligands

A topochemical route to nondefect, three-dimensional perovskites from lamellar Dion−Jacobson and Ruddlesden−Popper precursors was demonstrated. The method involves reduction of one of the ions (in this case Eu3+) in the precursor phase and concomitant loss of oxygen. CsEu2Ti2NbO10, a three-layer Dion−Jacobson compound, was ion-exchanged to AEu2Ti2NbO10 (A = Na, Li) and reduced in hydrogen to form the SrTiO3-type perovskites AEu2Ti2NbO9. Similarly, K2Eu2Ti3O10, a three-layer Ruddlesden−Popper compound, underwent divalent ion exchange to form the Dion−Jacobson compounds AIIEu2Ti3O10 (AII = Ca, Sr) and MIIEu2Ti3O10 (MII = Ni, Cu, Zn), which were reduced in hydrogen to perovskite-type AIIEu2Ti3O9 and MIIEu2Ti3O9, respectively. The AII and Eu2+ ions of AIIEu2Ti3O9 remain ordered, while A-site disordering occurs in the other perovskites. In all cases, the anisotropic texture of the layered precursors is retained in the product perovskite phase.

Topochemical Synthesis of Three-Dimensional Perovskites from Lamellar Precursors

The synthesis, structure, and selected physical properties of a new class of layered intergrowth materials, referred to as ferecrystals, are reviewed. In these layered intergrowths, size and composition can be controlled at atomic length scales by utilizing structural interfaces between component compounds that lack an epitaxial relationship. Opportunities to observe and control size-dependent phenomena in intergrowths of technologically important compound semiconductors are discussed.

Ferecrystals: non-epitaxial layered intergrowths

In this article we compare and contrast the evolution of ternary modulated reactants of the form M−Mo−Se (M = Ni, Zn, Sn, In, and Cu) with each other and with the binary Mo−Se system The binary elementally modulated reactants interfacially nucleate MoSe2 over a broad composition range surrounding that of the binary compound Mo6Se8 Increasing the concentration of any of the studied ternary elements except nickel above a critical value in the initial reactant suppressed interfacial nucleation of the diselenide The nickel-containing reactants all interfacially nucleated NixMoSe2 at low temperatures The subsequent nucleation and growth of crystalline compounds from the amorphous intermediates obtained in the other four systems was found to depend on both the identity and concentration of the ternary element In the Sn−Mo−Se system, a layered dichalcogenide was the first compound nucleated In the Zn−Mo−Se system, the dichalcogenide and the ternary compound ZnxMo6Se8 were observed to nucleate at approximat

The use of ternary cations to control nucleation : avoiding binary compounds as reaction intermediates

The kinetics of compound formation in the molybdenum−selenium system has been investigated using elementally modulated reactants to control overall composition and diffusion length. We observed the facile formation of MoSe2 at low temperatures when the composition was above 50 atom % selenium. No evidence was found for the low-temperature formation of the other known stable molybdenum selenide, the cluster compound Mo6Se8. When the composition of the initially modulated reactant was close to 25% selenium, a previously unreported compound was observed to form. This new compound, Mo3Se, has the A-15 crystal structure. The superconducting transition temperature appears to be very sensitive to composition, with a sharp resistive transition at 7 K in one sample and a sharp diamagnetic transition observed in a second sample at 2.2 K. The kinetics of phase formation in this system is analyzed in terms of nucleation kinetics.

Kinetics of Formation of Molybdenum Selenides from Modulated Reactants and Structure of the New Compound Mo3Se

Low temperature synthesis using modulated elemental reactants: a new metastable ternary compound, NixMoSe2

Kinetics of Formation of Molybdenum Selenides from Modulated Reactants and Structure of the New Compound Mo3Se.

Thermoelectric materials with a high Seebeck coefficient and a large power factor are provided. The materials are impact resistant and resistant to heat-distortion. Such materials include a rare earth element, Bi, and Te and have a rhombohedral crystal structure. In some examples, the rare earth element is selected from the group consisting of Ce, Sm and Yb. Such materials can be formed as films with a thickness of from 0.01 to 500 μm on a resin substrate. Production methods may include laminating different types of layers of thickness of 20 nm or less and heat-treating the resultant composition-modulated composite. The material may be separated from a substrate for sintering.

Robert Schneidmiller

Papers

The use of ternary cations to control nucleation : avoiding binary compounds as reaction intermediates

Kinetics of Formation of Molybdenum Selenides from Modulated Reactants and Structure of the New Compound Mo3Se

Low temperature synthesis using modulated elemental reactants: a new metastable ternary compound, NixMoSe2

Kinetics of Formation of Molybdenum Selenides from Modulated Reactants and Structure of the New Compound Mo3Se.

Thermoelectric material having a rhombohedral crystal structure