Karlin's bioinorganic research focuses on coordination chemistry relevant to biological and environmental processes, involving copper or heme porphyrin-iron complexes. Karlin's main approach involves synthetic modeling, i. He is the winner of the prestigous F.
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Series: Advances in Inorganic Chemistry
His work, however, triggered an effervescence of works in all aspects of chemistry. Balard, to whom we owe the discovery of bromine. Nevertheless, the fact remains that during this same period the discipline of solid-state chemistry was still lagging behind, despite the work of a few pioneers who laid the foundations of crystallography. However this discipline lacked analytical techniques, which limited knowledge of the structure of matter. The second was the work of Max Von Laue who, in , illuminated a nickel crystal with an X-ray to obtain the first spotted diffraction pattern.
For the first time, this revealed the presence of order and symmetry in the group of atoms that makes up crystal. The X-ray diffraction technique thus saw the light of day. It was soon exploited by the Bragg father and son, who not only established its fundamental rules, but also solved the structure of hundreds of compounds, showing how atoms are organized within compounds familiar to us, such as rock salt NaC1 and many others. For example, and to provide an order of magnitude, a 1mm 3 grain of rock salt contains as many atoms as there are stars in the universe.
The answer was found by extending the quantum approach adopted at the molecular level to crystal, through judicious approximations. The final result was the formation of a continuum of energy levels called bands , capable of hosting the electron and establishing an electronic structure for each compound. New compounds could be designed on the basis of structural affiliation by following a deductive reasoning. The relations between structures and properties allowed for predictive models to be built, and to confer specific electric, magnetic or electrochemical properties to a material.
Solid-state chemistry thus enjoyed a euphoric revival in the fifties, despite the remaining unknowns. In the mid-twentieth century, solid-state chemistry thus became a science founded on the properties-structures relationship, allowing for the development of new materials with specific functionalities. The four key advances that revived solid-state chemistry. The periodic table Fig. They prepare new compositions based on its content, though certain elements cannot be used because of their non-reactivity, limited abundance, cost and toxicity.
Just as a slide rule could be used to perform operations, so this periodic table allows chemists to make an informed choice of elements to combine, provided they understand the information it hides. Electronegativity is of crucial importance, since it governs the ionic-covalent nature of the metal-ligand bond, on which the physical properties magnetic, electrochemical, etc. With this information, chemists can control the chemical bond and thereby carefully choose the appropriate elements to design new compounds.
It indicates whether the cations, depending on their size, will prefer tetrahedral or octahedral sites, with large cations preferring octahedral sites. Hence, 3D metals generally present higher oxidation states in oxides than in sulphides, as sulphur is larger than oxygen. The respective size of the anions and cations, as well as their interactions, govern the stacking of the polyhedra, which may share vertexes, edges or sides.
The final arrangement produces structures with a symmetry that makes them similar to works of art. Under the effect of pressure, in other words, as it moves towards the centre of the Earth, the pyroxene structure containing SiO 4 tetrahedra turns into a perovskite structure containing SiO 6 octahedra connected by vertexes, and then into a post-perovskite structure, where SiO 6 octrahedra are linked by edges and vertexes.
Let us take a very obvious example: zirconia ZrO 2 as a substitute for yttrium. This doping breaks the local symmetry of the crystal, which translates into a different ionicity for the Cr-O bond, and of course modifies the optical spectrum. With this example, note that the defect often becomes dominant in steering the optical, electrical or magnetic properties of a compound, thus affording chemists a degree of additional creative freedom.
Take for example an insulating compound for which the d and p bands are far apart. If the structure is maintained in the case of a rigid model , the replacement of the sulphur with less electronegative elements, such as selenium Se and tellurium Te , results in the p band penetrating the d band. Moving right or down in the periodic table, the reverse occurs: the d band penetrates the p band. The two processes cause the material to shift from a semi-conducting behaviour to a metallic or even superconducting behaviour.
In solid form, reactivity is governed by mass transport; atoms must move to combine. Similarly, pressure encourages contact between grains, affording greater reactivity and therefore a smaller reaction time. Ultimately, the laws of thermodynamics prevail and allow chemists to produce the most stable material. It was ultimately a societal issue, once again, namely the oil crisis, which spurred its diversification by leading solid-state chemists to address the need to find a solution to the energy cost of materials.
It relies on the principle of topotactic reactions, in other words, reactions that preserve the structural skeleton of the initial precursor and therefore do not break chemical bonds, which explains the low energy input needed to trigger these reactions. Similar to millefeuilles , they consist in inserting lithium ions between the layers of the host structure Fig. Through grafting reactions, these host structures can also fix organic monomers, in order to obtain thicker millefeuilles with applications in the domain of catalysis or as flame retardant or even intumescent materials.
Insertion reaction governing the functioning of lithium-ion accumulators: analogy with the millefeuille. High-temperature solid-state chemistry was taking a major turn towards soft chemistry processes, as the second oil crisis intensified the need to reduce the temperature of materials preparation. These first include superconducting materials with no resistance below a certain temperature, called critical temperature T c , which gives them the advantage of transporting electricity without loss due to the Joule effect in this temperature domain.
These materials also include electrode materials designed to store energy in chemical form and convert it into electricity through a system that we call a battery. The choice of these types of materials is not random; it is informed by the synergy between them, as the same compound can serve as both an electrode material and a superconductor, hence the name bi-functional materials. Moreover, their properties are governed by redox reactions enlisting electron transfers.
This is the question I will now try and answer, drawing on three examples from my research activities, namely:. These are compounds built from Mo 6 S 8 clusters, which delimit tunnels within which copper atoms sit, and which lend themselves to a varied soft chemistry involving either the cations M , or the molecular entities Mo 6 X 6 , or both. Copper Cu can for example be extracted from this compound through chemical oxidation in the presence of an oxidizing agent I 2 , to produce an open frame called a host material for: i the electrochemical insertion of various cations and the development of Li, Na or Mg ion accumulators, and ii the absorption through reaction in a gaseous state of highly volatile metals to obtain lead and tin phases and even thallium Tl phases, which had never been prepared and proved to be the superconductor with the highest critical temperature T c.
This attests to the bi-functional nature of these compounds, which can be used as both electrode materials and superconductors. But these reactions still needed to be totally reversible, which was not the case. It was only eight years later, following my collaboration with Jean Galy and Patrick Rozier, that we found the ideal material. This is the lamellar phase Cu 2.
The electrochemical reduction of this crystallized compound by lithium led to a massive displacement of the copper, as attested to by the appearance of particles looking like an octopus, with the dendrites of the metallic copper Cu for tentacles. Most spectacular, however, was the total reabsorption of these dendrites during the oxidation stage to return a perfectly crystallized compound, reflecting the total reversibility of the displacement reaction. Thus was born a new reactional concept for the elaboration of electrode materials.
The last of the series is the mono-dimensional compound M 2 Mo 6 Se 6 , formed of chains of Mo 6 Se 6. While this Lego chemistry was widely practised in inorganic synthesis, its feasibility at low temperatures remained to be proven. We reached this next stage when we understood that the phases consisting of negatively charged linear chains strongly shielded by cations could be conducive to exfoliation.
Among these new phases, the compound Li 2 Mo 6 Se 6 proved to be an excellent electrode material due to its amphoteric nature. In it allowed for the first demonstration of the concept of symmetric lithium-ion technology in other words using the same material for the two electrodes. It can also involve anions, as is shown by the example of the superconducting cuprate family, the different phases of which share the common characteristic of having an oxygen non-stoichiometry. For the insertion of 0. This is an elegant approach showing how, through electrochemical insertion, chemists can precisely control the number of electrons injected and thus minutely and continuously follow the effect that the injection of anions or cations can have on the electric, magnetic, and other properties of various materials.
In my opinion, this is an opportunity that physicists have not sufficiently explored. Most encouraging of all, after thirty years in existence this chemistry still offers vast and original prospects for the synthesis of eco-compatible materials. This is the LiCoO 2 lamellar phase, used in the first commercial lithium-ion accumulators, with a layered structure as pointed out earlier.
Although these developments strongly contributed to the development of Li-ion technology, they nevertheless raised many fundamental questions, first of all regarding the possibility of removing all the Li from the initial LiCoO 2 phase. Chemical oxidation using strong oxidizing agents particularly Br 2 was unsuccessful, as it proved to be incomplete.
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At this stage, it was important to know which redox pair we had activated within LiCoO 2 through the total removal of Li. Had we driven the Co 3 to its higher degree of oxidation, as is customary, or had we oxidized the anionic network, a rather unusual situation in chemistry? This was therefore quite a provocative scenario, in which copper Cu became more electronegative than oxygen Fig.
The band structure of layered oxides. The displacement of the bands leads the p band of the oxygen to overlap with and spill into the d band of the metal, causing holes to form on the oxygen band. Moreover, it was on the basis of this similarity that, in an article written in in memory of Jean Rouxel, I suggested that the layered Co phases could be superconductive.
From a fundamental point of view, the challenge was therefore twofold. For these materials to be commercially viable, we needed to understand not only the origin of this enhanced capacity, which exceeded theoretical capacity, but also the drop in potential. With only one redox Ru centre left, since tin Sn is electrochemically inactive, these phases are model materials for analytical studies seeking to understand their reactional mechanisms.
The identification by EPR not only of the presence of the peroxo group but also of its concentration allowed us to discover the reactional mechanism of Li insertion-extrusion in these Li-rich lamellar compounds, which amounts to a game of band structure overlapping. This compound also includes a redox element Nb in the fourth period, which was previously overlooked, as it was too heavy.
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We can thus see that this activity of the anionic network broadens the range of efficient battery materials. And so it is that often, in the world of research, new concepts benefit from the emulation created by internationalization, and open up new horizons when they reach maturity. To get further insights into these issues, we turned to high-resolution electronic microscopy, a technique to visualize atoms of a size and distance of the order of the angstrom, i.
Up to this point, I have been talking about the layers and the place of the atoms as they were deduced by X-ray diffraction. Now, thanks to high-resolution microscopy, they can be visualized directly, with both heavy atoms appearing as two white spots due to their significant electronic charge, whereas the lithium is not visible given its low atomic number. The question then was a matter of knowing what happened to this arrangement when lithium was inserted into and then extruded from the material during electrochemical cycling.
Unexpectedly, during the charge we observed a mass migration of the cations, which nevertheless returned roughly to their initial position upon the following discharge, thus confirming the reversibility of the system but providing no clues as to the drop in potential. For tin compounds Li 2 Ru 0. This contrasts with the Li-rich metal compounds in the third period Li 1. What are these sites? Looking at the crystallographic structure, we can see that they are tetrahedral sites. What happens, therefore, is that when the atoms migrate from the layer to the interlayer space during cycling, some of them stay trapped in the tetrahedral sites, which explains the drop in potential during cycling.
Now that we have found the origin of the problem, we are in the process of developing customized materials to bypass it. I think it would be wiser for our institutions to promote a complete synergy between science and technology, so as to answer rapidly the problems faced by society, rather than setting fundamental research in opposition to applied research, as is unfortunately often the case.
I will first point out that a compound, in itself, is useless. It is only the material, which should be seen as the assemblage of a chemical composition, of a means of development and of a function that can be useful. In the case of sustainable development, this implies that: i its composition must contain only abundant and non-toxic elements; ii its production must involve only low-energy-consumption processes; and iii its performance for the targeted application must be appealing for example, with respect to potential in the case of electrode materials.
There are two options. They can either use combinatorial experimental chemistry, a tedious approach with random results, or practise deductive chemistry. The second path is the one I chose and am describing to you now, which consists in: i proceeding by analogy; ii taking advantage of the strong structure-property coupling; and iii drawing on the understanding of reaction mechanisms to make an informed synthesis.
To illustrate this approach, I will take the electrode material currently most prized, LiFePO 4 , the aim being to increase its potential, which is only 3. Based on the established electrochemical property-structure relations, we know that the potential is especially high given that the Fe-O bond is ionic. Based on the periodic table, this therefore means the phosphate entity needs to be replaced with the sulphate entity, with the concomitant addition of fluorine, which is more electronegative than oxygen.
While the compound LiFeSO 4 F met our criteria, it still needed to be synthesized, using low-energy means. Even though the aqueous medium is ideal, sulphates are soluble therein, meaning that we needed to find an alternative. While the isolation of this new compound is certainly a progress, the most exciting was determining its formation mechanism, in other words, the key to the reaction. Once we had grasped this mechanism, we were able to generalize it, and pretty soon over 20 new Li and even Na and K fluorosulphates were obtained.
Understanding reactional mechanisms is therefore crucial in chemical synthesis. In fact I will point out that if a reaction identical to the one described above is carried out without ionic liquid, the result is a compound with the same formula, LiFeSO 4 F, but with a very different structure: a polymorph with a triplite structure. Solid-state chemistry is therefore a science at the crossroads between the expected and the unexpected. While the expected is of course intellectually pleasing, the unexpected is just as interesting, as it always opens up new perspectives.
An extension of this work 17 , based on a controlled ionothermal synthesis and solid-state reactivity, allowed for a whole new class of electrode materials to be developed including, in addition to fluorosulphates: oxysulphates, hydroxosulphates and lithium-bearing sulphates, made of transition metals, which were unknown three or four years ago Fig.
Comparison of lamellar oxides and polyanionic compounds in terms of electrochemical performance. Oxide-based accumulators, because of their high energy density and therefore their high autonomy, are mainly used for portable electronics. By contrast, accumulators made with polyanionic compounds, because of their abundance and low cost, target larger volume applications electric vehicles and others.
Solid-state chemists generally turn to the chemistry of life to try and achieve this. The prime example relates to well-known unicellular algae, diatoms, which are able to concentrate the silicon contained in sea-water in order to create highly textured silica shells. To broaden the spectrum of biomineralizable materials, we turned to the use of other simpler microorganisms, such as bacteria, and even yeasts, which are unicellular fungi.
Let us look at two examples. Its role here was mainly owed to the fact that its enzyme, urease, can hydrolyse urea to produce the medium basicity needed to precipitate LiFePO 4.
Progress in Inorganic Chemistry, Volume 58
This was confirmed by high-resolution microscopy, showing that the bacterium is surrounded by a biofilm inside of which small fine needles can be observed, with its diffraction pattern indicating the presence of LiFePO 4. It took a long time for this approach, though elegant, to make it beyond laboratory curiosity, due to issues with reproducing and upscaling it.
The precipitated nanometric particles Fig. Thanks to this original alveolar structure induced by the bacterium, these textured hematite samples display interesting electrochemical properties in terms of potential behaviour when they are used as electrode materials. The biologically assisted synthesis of textured electrodes. These two aspects highlight the fact that solid-state chemistry is a highly adaptive science, which can meet societal demands in the framework of sustainable development.
In this context I can cite the recent work of a Korean group which successfully prepared lamellar oxide particles with a concentration gradient, by combining soft chemistry and high temperatures. This is another booming aspect of solid-state chemistry. The best evidence thereof is probably the emergence of the electric vehicle. What was long thought of as an elusive idea will play a major role in the car industry in coming years.
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I am convinced that technological challenges bring about new scientific problems, and that they can be resolved through fundamental research. The materials and systems we are currently designing must be more sophisticated, miniaturized, recyclable, environmentally friendly, energy saving, highly reliable, and cheap. Therefore, only with an interdisciplinary chemistry will we be able to advance in this quest for ideal materials for systems of varied complexities. The scope of solid-state chemistry is currently extending to new domains such as biology, with the development of materials produced using bio-inspired synthesis processes, and the chemistry of organic-inorganic hybrid materials.
I will therefore use this opportunity to delve further into the issue of energy.
Of course, the chemical bond, the common denominator of the society of atoms constituting crystal, will be the guiding theme. I will conclude by returning to the question of time. As I pointed out in my introduction, we need to double our energy production. Our hopes are riding on materials and we must be optimistic about our capacities to design better ones.
Yet, unlike past generations, we have neither thousands of years nor centuries, but only thirty to forty years to double our energy production, with the additional constraint of sustainable development. What are the odds? What can we hope for? Fortunately, we have a periodic table full of elements. This is certainly a great advantage, as we can design and sculpt new materials as we please, with properties exacerbated by eco-compatible approaches. But it can also rapidly turn into a nightmare: given the large number of possible combinations, it is difficult to find the winning composition.
US President Barack Obama has made this aim one of his five scientific priorities for the next decade. X-ray diffraction has allowed us to understand the arrangement of atoms, and microscopy has allowed us to see them. Why not dream and hope that we will one day be able to see these famous electrons? This may seem like an adventurous gamble. This is an ambitious dream, the realization of which would trigger a scientific revolution comparable to the observation of the atom through microscopy. It would radically change our material design and elaboration strategies.
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