Room-Temperature Synthesis and Low Thermal Conductivity in Nanocrystalline Ag3CuS2

Noble-metal-based chalcogenide materials recently gained massive attention in the field of thermoelectrics. In most cases, materials are synthesized using (i) high-temperature solid-state reactions or (ii) soft chemical methods where temperature requirements are lower than those of solid-state reactions (generally below 400 °C). Herein, we present a simple, surfactant-free, room-temperature, and energy-efficient synthesis of Ag3CuS2 nanocrystals. The present synthesis technique is scalable and capable of gram-scale production. A spark plasma sintering (SPS) pressed sample exhibits ultralow thermal conductivity (∼0.31 W/mK at room temperature). We found that Ag3CuS2 exhibits low sound velocity, as well as a non-Debye-like behavior based on a low-temperature heat capacity measurement. A high degree of anharmonicity of bonding, soft vibrations modes, and nanoscale grain boundary scattering in Ag3CuS2 lead to ultralow thermal conductivity, which can be important for thermoelectrics, optoelectronics, and thermal barrier coating applications.

Thermal Analysis and Optimization of the Phase Diagram of the Cu-Ag Sulfide System

Thermal stabilities of selected ternary phases of industrial interest in the Ag-Cu-S system have been studied by the calorimetric and electromotive force techniques. The ternary compounds Ag1.2Cu0.8S (mineral mackinstryite) and AgCuS (mineral stromeyerite) were equilibrated through high-temperature reaction of the pure Cu2S and Ag2S in an inert atmosphere. The synthesized single solid sample constituting the two ternary phases was ground into fine powders and lightly pressed into pellets before calorimetric measurements. An electrochemical cell incorporating the two equilibrated phase and additional CuS as a cathode material was employed. The measurement results obtained with both techniques were analyzed and thermodynamic properties in the system have been determined and compared with the available literature values. Enthalpy of fusion data of the Ag-richer solid solution (Ag,Cu)2S have also been determined directly from the experimental data for the first time. The thermodynamic quantities determined in this work can be used to calculate thermal energy of processes involving the Ag-Cu-S-ternary phases. By applying the obtained results and the critically evaluated literature data, we have developed a thermodynamic database. The self-developed database was combined with the latest pure substances database of the FactSage software package to model the phase diagram of the Ag2S-Cu2S system.

Topological Quantum Materials from the Viewpoint of Chemistry

Topology, a mathematical concept, has recently become a popular and truly transdisciplinary topic encompassing condensed matter physics, solid state chemistry, and materials science. Since there is a direct connection between real space, namely atoms, valence electrons, bonds, and orbitals, and reciprocal space, namely bands and Fermi surfaces, via symmetry and topology, classifying topological materials within a single-particle picture is possible. Currently, most materials are classified as trivial insulators, semimetals, and metals or as topological insulators, Dirac and Weyl nodal-line semimetals, and topological metals. The key ingredients for topology are certain symmetries, the inert pair effect of the outer electrons leading to inversion of the conduction and valence bands, and spin-orbit coupling. This review presents the topological concepts related to solids from the viewpoint of a solid-state chemist, summarizes techniques for growing single crystals, and describes basic physical property measurement techniques to characterize topological materials beyond their structure and provide examples of such materials. Finally, a brief outlook on the impact of topology in other areas of chemistry is provided at the end of the article.

Temperature-driven n-p conduction type switching without structural transition in a Cu-rich chalcogenide, NaCu5S3

We report for the first time the discovery of reversible n-p conduction type switching in a chalcogenide, NaCu5S3, without structural transition. AC impedance and first-principles simulations of the ionic migration confirmed the local melting trends of the hexagonal copper lattice at high temperatures, which could result in superionic conductivity within NaCu5S3.

Controlling the thermoelectric power of silicon–germanium alloys in different crystalline phases by applying high pressure

Si–Ge crystals are promising materials for use in various stress-controlled electronic junctions for next-generation nanoelectronic devices.

Pressure-Driven Reversible Switching between n- and p-Type Conduction in Chalcopyrite CuFeS2

Temperature-dependent switching between p- and n-type conduction is a newly observed phenomenon in very few Ag-based semiconductors, which may promote fascinating applications in modern electronics. Pressure, as an efficient external stimulus that has driven collective phenomena such as spin-crossover and Mott transition, is also expected to initialize a conduction-type switching in transition metal-based semiconductors. Herein, we report the observation of a pressure-driven dramatic switching between p- and n-type conduction in chalcopyrite CuFeS₂ associated with a structural phase transition. Under compression around 8 GPa, CuFeS₂ undergoes a phase transition with symmetry breakdown from space group I-42 d to space group I-4 accompanying with a remarkable volume shrinkage of the FeS₄ tetrahedra. A high-to-low spin-crossover of Fe²⁺ ( S = 2 to S = 0) is manifested along with this phase transition. Instead of pressure-driven metallization, a surprising semiconductor-to-semiconductor transition is observed associated with the structural and electronic transformations. Significantly, both photocurrent and Hall coefficient measurements confirm that CuFeS₂ undergoes a reversible pressure-driven p- n conduction type switching accompanying with the structural phase transition. The absence of cationic charge transfer between copper and iron during the phase transition is confirmed by both X-ray absorption near-edge spectra (Cu/Fe, K-edge) and total-fluorescence-yield X-ray absorption spectra (Fe, K-edge) results, and the valence distribution maintains Cu²⁺Fe²⁺S₂ in the high-pressure phase. The observation of an abrupt pressure-driven p- n conduction type switching in a transition metal-based semiconductor paves the way to novel pressure-responsive switching devices.

A New Material with a Composite Crystal Structure Causing Ultralow Thermal Conductivity and Outstanding Thermoelectric Properties: Tl2Ag12Te7+δ

A new state-of-the-art thermoelectric material, Tl₂Ag₁₂Te_7+δ, which possesses an extremely low thermal conductivity of about 0.25 W m^-1 K^-1 and a high figure-of-merit of up to 1.1 at 525 K, was obtained using a conventional solid-state reaction approach. Its subcell is a variant of the Zr₂Fe₁₂P₇ type, but ultimately its structure was refined as a composite structure of a Tl₂Ag₁₂Te₆ framework and a linear Te atom chain running along the c axis. The super-space group of the framework was determined to be P6₃(00γ) s with a = b = 11.438(1) Å, c = 4.6256(5) Å, and that of the Te chain substructure has the same a and b axes, but c = 3.212(1) Å, space group P6(00γ) s. The modulation leads to the formation of Te₂ and Te₃ fragments in this chain and a refined formula of Tl₂Ag_11.5Te_7.4. The structure consists of a complex network of three-dimensionally connected AgTe₄ tetrahedra forming channels filled with the Tl atoms. The electronic structures of four different models comprising different Te chains, Tl₂Ag₁₂Te₇, Tl₂Ag₁₂Te_7.33, and 2× Tl₂Ag₁₂Te_7.5, were computed using the WIEN2k package. Depending on the Te content within the chain, the models are either semiconducting or metallic. Physical property measurements revealed semiconducting properties, with an ultralow thermal conductivity, and excellent thermoelectric properties at elevated temperatures.

Tuning of p-n-p-Type Conduction in AgCuS through Cation Vacancy: Thermopower and Positron Annihilation Spectroscopy Investigations

Understanding the complex phenomenon behind the structural transformations is a key requisite to developing important solid-state materials with better efficacy such as transistors, resistive switches, thermoelectrics, etc. AgCuS, a superionic semiconductor, exhibits temperature-dependent p-n-p-type conduction switching and a colossal jump in thermopower during an orthorhombic to hexagonal superionic transition. Tuning of p-n-p-type conduction switching in superionic compounds is fundamentally important to realize the correlation between electronic/phonon dispersion modulation with changes in the crystal structure and bonding, which might contribute to the design of better thermoelectric materials. Herein, we have created extrinsic Ag/Cu nonstoichiometry in AgCuS, which resulted in the vanishing of p-n-p-type conduction switching and improved its thermoelectric properties. We have performed the selective removal of cations and measured their temperature-dependent thermopower and Hall coefficient, which demonstrates only p-type conduction in the Ag_{1- x}CuS and AgCu_{1- x}S samples. The removal of Cu is much more efficient in arresting conduction switching, whereas in the case of Ag vacancy, p-n-p-type conduction switching vanishes at higher vacant concentrations. Positron annihilation spectroscopy measurements have been done to shed further light on the mechanisms behind this structural transition-dependent conduction switching. Cation (Ag⁺/Cu⁺) nonstoichiometry in AgCuS significantly increases the vacancy concentration, hence, the p-type carriers, which is confirmed by positron annihilation spectroscopy and Hall measurement. The Ag_{1- x}CuS and AgCu_{1- x}S samples exhibit ultralow thermal conductivity (∼0.3-0.5 W/m·K) in the 290-623 K temperature range because of the low-energy cationic sublattice vibration that arises as a result of the movement of loosely bound Ag/Cu within the stiff S sublattice.

Large change in thermopower with temperature driven p–n type conduction switching in environment friendly BaxSr2−xTi0.8Fe0.8Nb0.4O6 double perovskites

Temperature driven p–n type conduction switching in combination with colossal change in thermo-power in Ba_xSr_2−xTi_0.8Fe_0.8Nb_0.4O₆ (BSTFN) double perovskites.

The effect of order-disorder phase transitions and band gap evolution on the thermoelectric properties of AgCuS nanocrystals

Copper and silver based chalcogenides, chalco-halides, and halides form a unique class of semiconductors, as they display mixed ionic and electronic conduction in their superionic phase. These compounds are composed of softly coupled cationic and anionic substructures, and undergo a transition to a superionic phase displaying changes in the substructure of their mobile ions with varying temperature. Here, we demonstrate a facile, ambient and capping agent free solution based synthesis of AgCuS nanocrystals and their temperature dependent (300-550 K) thermoelectric properties. AgCuS is known to show fascinating p-n-p type conduction switching in its bulk polycrystalline form. Temperature dependent synchrotron powder X-ray diffraction, heat capacity and Raman spectroscopy measurements indicate the observation of two superionic phase transitions, from a room temperature ordered orthorhombic (β) to a partially disordered hexagonal (α) phase at ∼365 K and from the hexagonal (α) to a fully disordered cubic (δ) phase at ∼439 K, in nanocrystalline AgCuS. The size reduction to the nanoscale resulted in a large variation in the thermoelectric properties compared to its bulk counterpart. Temperature dependent Seebeck coefficient measurements indicate that the nanocrystalline AgCuS does not display the p-n-p type conduction switching property like its bulk form, but remains p-type throughout the measured temperature range due to the presence of excess localized Ag vacancies. Nanocrystalline AgCuS exhibits a wider electronic band gap (∼1.2 eV) compared to that of the bulk AgCuS (∼0.9 eV), which is not sufficient to close the band gap to form a semimetallic intermediate state during the orthorhombic to hexagonal superionic phase transition, thus AgCuS nanocrystals do not show conduction type switching properties like their bulk counterpart. The present study demonstrates that ambient solution phase synthesis and size reduction to the nanoscale can tailor the order-disorder phase transition, the band gap and the electronic conduction properties in superionic compounds, which will not only enrich solid-state inorganic chemistry but also open a new avenue to design multifunctional materials.