Copper sulphides, metal atom clusters & intermetallics: Energy conversion

Solid state synthesis, luminescence, thermoelectrics,…

Solid state synthesis at high temperature and multiscale characterizations of copper sulphides, cluster based compounds and intermetallics is a renowned activity of CSM. Two sub-project are developed:

• Research of new compounds using innovative syntheses.
• Deep studies of physical properties of targeted compounds anteriorly found in the team in particular optical and thermoelectric.

Copper sulphides with complex structures derived from mineral phases are promising semiconductor materials for thermoelectric applications at intermediate temperatures. Within the framework of the ANR MASSCOTE in partnership with the CRISMAT laboratory of Caen (Dr. E. Guilmeau and his team) and the IJL of Nancy (Pr. B. Malaman, Dr. C. Candolfi), an important part has concerned the design, the study of crystal structure and thermoelectric properties relations of several sphalerite derivatives copper sulphides. Very high figure of merit (ZT at 675K = 0.93 for colusite Cu₂₆V₂Sn₆S₃₂), record power factors (1.94 mW m^-1 K^-2 at 700 K for colusite Cu₂₆Cr₂Ge₆S₃₂) or very low thermal conductivities (~ 0.8 W m^-1 K^-1 at 700 K for Cu_22-xZn_xFe₈Ge₄S₃₂ with 1.2 ≤ x ≤ 2) were obtained / explained by our in-depth crystal chemistry characterizations coupling X-rays (including synchrotron), neutron diffraction and Mössbauer spectroscopy, highlighting structural defects, low distortions of the sub - “Cu₂₆S₃₂” conductive network for colusites which influence the electronic properties and / or structural transitions (eg cubic germanite to tetragonal renierite) which affect thermal conductivity.

J. Am. Chem. Soc., 2018, 140, 2186; Adv. Energy Mater., 2019, 9, 1803249;
Angew. Chem. Int. Ed., 2019, 58, 15455; Inorg. Chem., 2017, 56, 13376;
ACS Appl. Energy Mater., 2019, 2, 7679; J. Mater. Chem. C, 2016, 4, 7455;
J. Phys. Chem. C, 2017, 121, 16454; Phys. Rev. Mater., 2020, 4, 025404;
J. Alloys Compd., 2020, 831, 154767; J. Mater. Chem. C, 2021, 9, 773.

Exploratory research of new thermoelectric chalcogenides and new properties in cluster compounds.
Molybdenum condensed cluster chalcogenides constitute a large class of materials with complex crystal structures. In close collaboration with the Inorganic Theoretical Chemistry (CTI) team of ISCR, a part of this research focuses on establishing the correlations between the structural and electronic properties of these phases with their physical properties. This represents the second axis of ANR MASSCOTE in partnership with the IJL of Nancy (Pr. B. Lenoir & Dr. C. Candolfi). The structuring of cluster chalcogenides by the formation of chalcogen bridges between clusters promotes a semi-metallic type behavior favorable to relatively high power factors while cationic rattling favors a very low thermal conductivity. The combination of these properties is favorable to the discovery of promising thermoelectric materials for high temperature applications. During the period, a comprehensive study focused on the influence of the substitution of selenium atoms by sulfur or tellurium atoms on the thermoelectric properties of condensed cluster compounds especially in the Ag_3.8Mo₉Se₁₁ phase.

Inorg. Chem., 2019, 58, 5533-5542
J. Alloys Compd., 2018, 739, 360-367

Tailoring Heterometallic Cluster Functional Building Blocks.
In the frame of IRP CLUSPOM's activities with Russia (Nikolaev Institute of Inorganic Chemistry / Novossibirsk), various exploratory research has been carried out jointly. They involve the solid state chemistry at high temperature and chemistry in solution. For example, it has been shown in the chemistry of selenides that the mixture of metal atoms of different natures (Re and Mo) within the same metal cluster {Re6-xMox} makes it possible to achieve new optical and redox properties which could not be obtained in homonuclear clusters {Re6} or {Mo6}. It is thus possible to stabilize clusters in higher degrees of oxidation. A remarkable result is the ability of these clusters, depending on their chemical environment, to exhibit several reversible electrochemical transitions within a narrow potential window, accompanied by a change in their absorption properties. Such a property now opens perspectives for the use of heterometallic clusters in chemical sensors.

CrystEngComm, 2018, 20, 4164-4172
Chemistry-a European Journal, 2019, 25(66), 15040-15045

New multifunctional materials exhibiting protonic conductivity and luminescence.
The search for new multifunctional materials displaying proton conducting properties is of paramount necessity for the development of electrochromic devices and supercapacitors as well as for energy conversion and storage. Proton conductivity is reported for the first time in molybdenum cluster-based materials: the new (H)4[Mo6Br6S2(OH)6]-12H2O (1) and two previously reported compounds: (H)2[Mo6X8(OH)6]-12H2O (X = Br (2), Cl (3)), initially formulated as [Mo6X8(OH)4(H2O)2]-12H2O. The self-assembling of the luminescent [Mo6Li8(OH)a6]2-/4- cluster units leads to both luminescence and proton conductivity (=1.4 10-4 S.cm-1 in (H)2[Mo6Cl8(OH)6]-12H2O under wet conditions) in the three materials. The latter results from the strong hydrogen-bond network that develops in between the clusters and the water molecules and is magnified by the presence of protons that are statistically shared by apical hydroxyl groups in between adjacent clusters. Their role in the proton conduction is highlighted at the molecular scale by ab initio molecular dynamics simulations that demonstrate that a Grotthuss type proton transfer mechanism through the hydrogen-bond network occurs. An infra-red fingerprint of the mobile protons is proposed based on both theoretical and experimental proofs. These findings pave the way to the development of new multifunctional proton conducting materials displaying original and exciting properties.

Inorg. Chem., 2018, 57(16), 9814-9825

Concerning the origin of luminescence of octahedral metal cluster based compounds and the demonstration of cathodoluminescence in cluster compounds, we have carried out spectroscopic studies (absorption & emission) combining physicochemical measurements and quantum chemistry calculations in order to understand the electronic phenomena at the origin of luminescence in compounds with Mo6 clusters. The very wide emission window in the red-near infrared of the clusters is linked to several excited states of different geometries (elongation along the 4-fold axis, aperture of a Mo-Mo bond) and to geometric relaxation phenomena. The type of excitation (photons or electrons) influences the geometries of the excited states. The demonstration of cathodoluminescence was carried out within the framework of the collaboration with UMI LINK located at the NIMS in Tsukuba (Japan).

Physical Chemistry Chemical Physics, 2015, 17, 28574-28585
Science and Technology of Advanced Materials., 2017, 18, 458–466

Crystal engineering of luminescent materials by trapping molecular clusters in an aluminium-based host matrix. 
The solid state chemistry enables to play with metal atoms generating packings of polyhedrons each based on a central metal ions and a coordination sphere of ligands (e.g. O, chalcogens, halogens…). Instead of metal atoms, we pioneer a novel concept to design functional materials using preformed metal aggregates acting as complementary building blocks. Their programmed association within a 3D organization is insured by a natural, non-toxic and cheap supramolecular linker (γ-cyclodextrin), and the resulting arrangement mimics the crystal edifices found in oxide solid-state chemistry. Following the idea developed in solid state oxide chemistry to trap luminophores in an inert matrix, we show herein how highly luminescent metal [Mo6Xi8Cla6]2- cluster units (noted {Mo6}2-)) can be integrated in an inert hybrid matrix involving γ-cyclodextrin (noted γ-CD) moieties (cyclic oligosaccharides) and aluminium based polycations ([Al13O4(OH)24(H2O)12]7+ (noted {Al13}7+)) to form a tailor-made architecture related to that of the lithium alumina silicate LiAlSiO4. 
Taking benefit of the extensive library of soluble metallic aggregates, cyclodextrin driven self-assembling concept offers important perspectives in the field of crystal engineering and more generally in material science. Indeed, considering the design of a material with several desired properties (optical, catalytic, magnetic…), building blocks that will carry those specific properties could be selected among numerous types of soluble metallic aggregates such as transition metal atom clusters, polyoxometalates, Al3+ based polycations, polynuclear single molecule magnets (Mn12, Fe8..) or polynuclear d10 complexes.
Picture : Illustration of SiO4 and LiO4 pseudo-tetrahedrons observed in the crystal structure of β-eucryptite. b) View of SiO4 and LiO4 polyhedrons in the structure type LiAlSiO4 showing that those building blocks are connected by their vertex. Color code:  LiO4 are in grey and SiO4 are in red. c) Illustration of the pseudo super-tetrahedron {Al13}Cl4 and {Mo6}Cl4 which are drastically larger than the polyhedral observed in β-eucryptite. d) View of {Al13}Cl4 and {Mo6}Cl4 polyhedrons in {Al13}{[Mo6Xi8Cla6]@2CD}Cl5•60H2O (X =Br or I) showing the strong analogy with β-eucryptite structure. Color code: {Al13}Cl4 are in grey and {Mo6}Cl4 are in red.

Materials Horizons, 2020, 7(9), 2399-2406
 

Fundamental properties of Uranium intermetallics
The dual localized/itinerant character of 5f electrons and their more or less strong correlations in U-intermetallics give birth to a wide variety of exotic physical behaviours. Our group discovers and characterizes new compounds to better understand the chemical composition – crystal structure – physical properties relationships in these systems.

Two main families of compounds are currently investigated:

• Uranium germanides, focusing on new compounds where U-atoms are forming zig-zag chains similar to those found in the ferromagnetic superconductors UGe₂, UCoGe and URhGe. Two series of compounds, namely UT_1-xGe₂ (T = Fe, Ru, Os, Co, Ni) and U₃T’Ge₅ (T’ = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W), possess such chains and exhibit a wide variety of magnetic properties, from paramagnetism to ferromagnetism. Recently, TEM experiments evidenced crystallographic superstructures in the UT_1-xGe₂ family, probably arising from an ordering of the vacancies on the transition metal site. This work is carried out in collaboration with the Institute of low temperature and Structure Research from Wroclaw (Poland). The U-Mn-Ge ternary system, where both U- and Mn-atoms can carry magnetic moments, possibly leading to cooperative phenomena, is being investigated in collaboration with Anne Vernière (Institut Jean Lamour, Nancy, France).


• Uranium aluminides are studied for their ability to form magnetically frustrated phases in 0-neighbour (isolated U-atoms with d_U-U > 5Å in e.g. UT₂Al₁₀ with T = Fe, Ru, Os), 1-neighbour (isolated [U₂] dumbbells with d_U-U» 3.5 Å in U₆T₄Al₄₃ with T = V, Nb, Ta, Cr, Mo,W) and 2-neighbour (isolated [U₃] triangles with d_U-U» 3.5 Å in U₃T₄Al₁₂ with T = Fe, Ru, Os,Co) networks, leading to spin-glass or complex magnetic behaviour.

J. Solid State Chem, 2019, 277, 260-270
Intermetallics, 2021, 131, 107112

Uranium intermetallics as nuclear materials
An increased uranium density in the materials is required to compensate for the strong reduction of ²³⁵U enrichment of nuclear fuels for research and test reactors or irradiation targets for the production of medical radioisotopes, imposed by international treaties. Our expertise in the synthesis of novel compounds, comprehension of their reactivity in working environment and sintering of the materials lead to fruitful collaborations with the main actors (Framatome, CEA) of the nuclear domain. For example, we recently report on the alternative spark plasma technology to produce U₃Si₂/Al fuel plates/irradiation targets with improved properties, developed together with CEA Cadarache.

Magnesiothermic synthesis of thermoelectric skutterudites and silicides
One of our goal is to produce thermoelectric materials by innovative low temperature syntheses. For example, we adapted the magnesiothermic reduction process to directly prepare, with a precise control of doping level, submicron powders of skutterudites (e.g. In_0.22Co₄Sb₁₂), HMS (V_xMn_1-xSi_g with g » 1.74), and b-Co_xFe_1-xSi₂. The method is a scalable and economical viable process to obtain materials with similar figure-of-merit to conventionally synthesized ones but uses lower temperatures and shorter annealing duration. These studies are carried out in collaboration with Dr. Éric Alleno (ICMPE, Thiais, France), Dr. David Berthebaud (UMI3629 Link, Tsukuba, Japan) and Prof. Takao Mori (NIMS-MANA, Tsukuba, Japan). The high crystallinity of the obtained powders enables advanced structural characterization (synchrotron XRD, EBSD, TEM), including modelling of the stacking faults in b-FeSi₂ or of the composite structure of MnSi_g. The latter contributes to understanding the aging phenomena at high temperature of this silicide. Sylvain Le tonquesse was rewarded the “Prix de Thèse - Chemistry” from the “Association Française de Cristallographie for this work.

J. Alloys Compd., 2019, 796, 176-184; ACS Appl. Energ. Mater., 2019, 2, 8525;
J. Alloys Compd., 2020, 816, 152577; Chem. Mater., 2020, 32, 10601