Valence fluctuating thermoelectric high-borides with YCrB4 type of structure
Solid State and Surface Chemistry, Material Synthesis
Final Report Abstract
We performed detailed studies of the Y1-xCexCrB4 and Y1-xYbxCrB4 series. As planned in the project samples with x ranging from 0 to 1 were synthesized by arc-melting procedures as well as powder metallurgical methods with further heat treatments. All obtained specimens were characterized by powder X-ray diffraction (PXRD), energy- and wavelengths-dispersive spectroscopies (EDX and WDX, respectively). Unexpectedly, these characterizations revealed a very limited solubilities of Ce in YCrB4 and Y in CeCrB4. Looking further for strong electronic correlations in the quaternary Y1-xCexCrB4 (x = 0.05 and 0.95) borides we performed extensive studies of their magnetic and transport properties. And indeed, an intermediate (i.e. temperature dependent) valence state of Ce-ions was detected in Y0.05Ce0.95CrB4. However, the Kondo temperature TK = 1107 K deduced from magnetic susceptibility measurements by applying the Coqblin-Schriffer model was by a factor of ~4 larger than those reported for such famous strongly correlated system as CePd3, which is, despite being a metal, characterized by atypically good thermoelectric (TE) efficiency (ZT ≈ 0.2 at RT). This finding was in agreement with a simple metallic behaviour temperature dependence of electrical resitivity, indicated no Kondo correlations in Y0.05Ce0.95CrB4 and thus, rather poor TE performance (ZT ≈ 0.06 at 800 K). Here it should be also noted, that the whole Y1-xCexCrB4 series is characterized by extremely good thermal conductivity, which can be related to the strong hardness of these materials. One of the routes allowing an improvement of TE efficiency of Y1-xCexCrB4 would be a reduction of grain size [i.e. powdering of the samples with further densification applying spark plasma sintering (SPS)]. The synthesis of the ternary YbCrB4 boride was very challenging and the best specimen which could be synthesized contained small admixtures of CrB and Yb2O3 impurities. Measurements of magnetic susceptibility indicated Yb-ions to be in a valence state close to 3+. And thus, one could expect YbCrB4 to reveal semiconducting properties similarly as YCrB4. However, our attempts to obtain a bar of the YbCrB4 sample which would be suitable for transport properties measurements by applying SPS failed. As deduced from PXRD- and EDX-analyses no solubility of Yb in YCrB4 or Y in YbCrB4 is observed at ambient pressure. We also searched for boron-rich compounds in the isoelectronic RE-T-B (RE = Ce, Nd, Pr; T = Mo, W) systems. These investigations resulted in the discovery of the new CeMo2B5 structure type and its isostructural REMo2B5 (RE = Pr, Nd) and CeW2B5 compounds. However, further measurements of magnetic and transport properties combined with theoretical DFT calculations and analysis of the chemical bonding situation indicated these borides to be metals and to posses rather poor TE performance. We also succeeded in preparation and crystal structure determination of the new Hf2Ir5+δB2-2δ type. Since analysis of chemical bonding situation in this compound indicated the same features as those observed in such prominent thermoelectrics as intermetallic clathrates we studied its transport properties. However, Hf2Ir5+δB2-2δ is characterized by small Seebeck coefficients and good phonon mediated thermal conductivity typical for metals, which indicate it to reveal very poor TE efficiency.
Publications
- CeMo2B5: A New Type of Arrangement of Puckered Boron Hexagonal Rings. In: European Journal of Inorganic Chemistry, Volume 2019 (2019), 3572-3580
Sever Flipo, Miroslav Kohout, Friedrich Roth, Tina Weigel, Walter Schnelle, Matej Bobnar, Alim Ormeci, Ulrich Burkhardt, Christoph Hennig, Tilman Leisegang, Dirk-Carl Meyer, Andreas Leithe-Jasper and Roman Gumeniuk
(See online at https://doi.org/10.1002/ejic.201900411) - Crystal Structure and Physical Properties of the Cage Compound Hf2B2−2δIr5+δ. In: Inorganic Chemistry, Volume 59 (2020), 14280-14289
Olga Sichevych, Sever Flipo, Alim Ormeci, Matej Bobnar, Lev Akselrud, Yurii Prots, U Ulrich Burkhardt, Roman Gumeniuk, Andreas Leithe-Jasper, and Yuri Grin
(See online at https://doi.org/10.1021/acs.inorgchem.0c02073)
