The control of materials elaboration is a strength of the SPMCE group. The extensive study of chemical composition variations in most materials (substitution, non-stoichiometry etc.) requires the optimization of synthesis, which is indeed a key step for accessing new properties. Both powder and crystal synthesis are actively developed in our group. The different techniques curently used are listed here :
These standard procedures are widely used, with the help of ball-mills and a wide range of tube and muffle furnaces operating up to 1800°C. Sample environement can be air, flowing oxygen or argon, as well as silica ampoules sealed under vacuum.
High pressure (up to 7 GPa) and high temperature (HP-HT) syntheses are used to stabilize specific phases that are inaccessible at ambient pressure. The systematic exploration of P,T phase diagrams allows to stabilize compounds with unusual valence states or coordination, leading to interesting physical properties. Our activity focuses on perovskite-type oxides with potential multiferroic or magnetoelectric properties, new superconductors (iron-based pnictides and chalcogenides) and hydrides of intermetallic compounds. These syntheses take advantage of the development of large-volume HP-HT cells at Institut Néel (“Instrumentation” group).
Sol-gel and chimie douce (ion exchange, intercalation) syntheses are currently used for various material synteses. Intercalation can be controlled electrochemically for lithium intercalation properties (battery materials) or chemically in alkali element-containing pnictides and chalcogenides investigated for superconductivity. Hydrothermal synthesis has been extensively used to produce materials with magnetic frustration properties (atacamite family), including as small single crystals.
Intermetallic compounds of particular interest for applications such as permanent magnets, magnetostrictive materials and magnetocaloric materials are synthesized by induction melting of the high-purity elements in inert atmosphere. Phases that are difficult to obtain by induction melting such as p-element compounds (carbides, borides etc) can now be prepared using a two-arc melting set-up. In order to stabilize specific crystal structures, post-melting annealing treatments and/or quenching can be carried out in evacuated silica ampoules.
Solid-gas reactions are used in order to introduce interstitial elements (hydrogen or eventually nitrogen) in the crystal structure of intermetallic compounds. Thermal activation of the reaction of hydrogen gaz with intermetallic ingots or powders is achieved under high pressure (up to 7 GPa). These studies aim to (1) search for new hydride compounds by exploring phase diagrams, (2) modify the structural and magnetic properties of the host intermetallic compounds leading to interesting physical properties.
Crystals of complex oxides are currently grown using several techniques, such as floating zone, high-temperature flux or chemical vapour transport. These allowed us to obtain crystals of numerous compounds with magnetic frustration properties, such as langasites, rare-earth pyrochlores, spinels and lamellar oxides. A self-flux technique in sealed silica tubes is used for the growth of superconducting iron chalcogenides and arsenides crystals. The flux technique is also extended to HP-HT conditions to grow single crystals of high pressure oxides (distorted perovskites, Cu-Ge spinel) or compounds for which pressure increases the component solubility in the liquid phase (iron-based arsenides). Single crystals of intermetallic compounds are also grown under argon by the Bridgman and Czochralski techniques.
Figure : Single crystal of SrFe2As2 iron-based arsenide
All these studies strongly benefit from the technical expertise and scientific support of the “Instrumentation”, “cristaux massifs" and “Epitaxie et couches minces" technological groups of Institut Néel.
Broyage mécanique par billes Déformation plastique sévère : laminage à froid et ECAP