Stabilization by Si Substitution of the Pseudobinary Compound Gd₂(Co_3–<i>x</i>Si_<i>x</i>) with Magnetocaloric Properties around Room Temperature

We report the discovery of a new solid solution Gd₂(Co_3–<i>x</i>Si_<i>x</i>) with 0.29 < <i>x</i> < 0.50 in the Gd–Co–Si ternary system. Members of this solid solution crystallize with the La₂Ni₃-type structure and correspond to the stabilization of “Gd₂Co₃” through silicon substitution. The structure of the member Gd₂(Co_2.53(3)Si_0.47) was determined by X-ray diffraction on a single crystal. It crystallizes with the space group <i>Cmce</i> and cell parameters <i>a</i> = 5.3833(4), <i>b</i> = 9.5535(6), and <i>c</i> = 7.1233(5) Å. Co/Si mixing is observed on two crystallographic positions. All compounds studied in the solid solution present a ferrimagnetic order with a strong composition-dependent Curie temperature <i>T</i>_C with 280 K < <i>T</i>_C < 338 K. The magnetocaloric effect, which amounts to around 1.7 J K^–1 kg^–1 for Δ<i>H</i> = 2 T, is interestingly tunable around room temperature over a temperature span of 60 K through only 4–5% of composition change

Cu₂ZnGeSe₄ Nanocrystals: Synthesis and Thermoelectric Properties

A synthetic route for producing Cu₂ZnGeSe₄ nanocrystals with narrow size distributions and controlled composition is presented. These nanocrystals were used to produce densely packed nanomaterials by hot-pressing. From the characterization of the thermoelectric properties of these nanomaterials, Cu₂ZnGeSe₄ is demonstrated to show excellent thermoelectric properties. A very preliminary adjustment of the nanocrystal composition has already resulted in a figure of merit of up to 0.55 at 450 °C

Crystallographic Control at the Nanoscale To Enhance Functionality: Polytypic Cu₂GeSe₃ Nanoparticles as Thermoelectric Materials

The potential to control the composition and crystal phase at the nanometer scale enable the production of nanocrystalline materials with enhanced functionalities and new applications. In the present work, we detail a novel colloidal synthesis route to prepare nanoparticles of the ternary semiconductor Cu₂GeSe₃ (CGSe) with nanometer-scale control over their crystal phases. We also demonstrate the structural effect on the thermoelectric properties of bottom-up-prepared CGSe nanomaterials. By careful adjustment of the nucleation and growth temperatures, pure orthorhombic CGSe nanoparticles with cationic order or polytypic CGSe nanoparticles with disordered cation positions can be produced. In this second type of nanoparticle, a high density of twins can be created to periodically change the atomic plane stacking, forming a hexagonal wurtzite CGSe phase. The high yield of the synthetic routes reported here allows the production of single-phase and multiphase CGSe nanoparticles in the gram scale, which permits characterization of the thermoelectric properties of these materials. Reduced thermal conductivities and a related 2.5-fold increase of the thermoelectric figure of merit for multiphase nanomaterials compared to pure-phase CGSe are systematically obtained. These results are discussed in terms of the density and efficiency of phonon scattering centers in both types of materials