Semiconductors have been fundamental to various devices that are typically operated with electric field, such as transistors, memories, sensors, and resistive switches. There is growing interest in the development of novel inorganic materials for use in transistors and semiconductor switches, which can be operated with a temperature gradient. Here, we show that a crystalline semiconducting noble metal sulfide, AgCuS, exhibits a sharp temperature dependent reversible p-n-p type conduction switching, along with a colossal change in the thermopower (ΔS of ~1757 μV K(-1)) at the superionic phase transition (T of ~364 K). In addition, its thermal conductivity is ultralow in 300-550 K range giving AgCuS the ability to maintain temperature gradients. We have developed fundamental understanding of the phase transition and p-n-p type conduction switching in AgCuS through temperature dependent synchrotron powder X-ray diffraction, heat capacity, Raman spectroscopy, and positron annihilation spectroscopy measurements. Using first-principles calculations, we show that this rare combination of properties originates from an effective decoupling of electrical conduction and phonon transport associated with electronic states of the rigid sulfur sublattice and soft vibrations of the disordered cation sublattices, respectively. Temperature dependent p-n-p type conduction switching makes AgCuS an ideal material for diode or transistor devices that operate reversibly on temperature or voltage changes near room temperature.

We use a combination of first-principles density functional theoretical analysis and experimental characterization to understand the lattice dynamics, dielectric and ferroelectric properties of lead-free relaxor ferroelectric Na0.5Bi0.5TiO3 (NBT) system. Vibrational spectrum determined through our calculations agrees well with the observed Raman spectrum, and allows assignment of symmetry labels to modes. The calculated Born effective charges reveal (a) two distinct types of Ti ions at the B-site with anomalous dynamical charges differing by up 1.6e, and (b) Na and Bi ions at the A-site exhibit disparate dynamical charges of about 1 and 5.5e, respectively. Thus, there exist hetero-polar activity at both A and B-sites in NBT, and disorder associated with these hetero-polar ions is responsible for its relaxor behaviour. Large dielectric response of NBT arises primarily from phonons, and specifically the modes involving Bi-O (109 cm−1) and Ti-O (246, 276 cm−1) vibrations, respectively.

Crystalline solids with intrinsically low lattice thermal conductivity (κ ) are crucial to realizing high-performance thermoelectric (TE) materials. Herein, we show an ultralow κ of 0.35 Wm K in AgCuTe, which has a remarkable TE figure-of-merit, zT of 1.6 at 670 K when alloyed with 10 mol % Se. First-principles DFT calculation reveals several soft phonon modes in its room-temperature hexagonal phase, which are also evident from low-temperature heat-capacity measurement. These phonon modes, dominated by Ag vibrations, soften further with temperature giving a dynamic cation disorder and driving the superionic transition. Intrinsic factors cause an ultralow κ in the room-temperature hexagonal phase, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high-temperature rocksalt phase. Despite the cation disorder at elevated temperatures, the crystalline conduits of the rigid anion sublattice give a high power factor.

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Crystalline solids with intrinsically low lattice thermal conductivity (k L )a re crucial to realizing high-performance thermoelectric (TE) materials.H erein, we show an ultralow k L of 0.35 Wm À1 K À1 in AgCuTe, which has aremarkable TE figure-of-merit, zT of 1.6 at 670 Kw hen alloyed with 10 mol %S e. First-principles DFT calculation reveals several soft phonon modes in its room-temperature hexagonal phase, which are also evident from low-temperature heat-capacity measurement. These phonon modes,d ominated by Ag vibrations,soften further with temperature giving adynamic cation disorder and driving the superionic transition. Intrinsic factors cause an ultralow k L in the room-temperature hexagonal phase, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high-temperature rocksalt phase.D espite the cation disorder at elevated temperatures,t he crystalline conduits of the rigid anion sublattice give ah igh power factor.

The interface engineering strategy has been an emerging field in terms of material improvisation that not only alters the electronic band structure of a material but also induces beneficial effects on electrochemical performances. Particularly, it is of immense importance for the environmentally benign electrochemical nitrogen reduction reaction (NRR), which is potentially impeded by the competing hydrogen evolution reaction (HER). The main problem lies in the attainment of the desired current density at a negotiable potential where the NRR would dominate over the HER, which in turn hampers the Faradaic efficiency for the NRR. To circumvent this issue, catalyst development becomes necessary, which would display a weak affinity for H-adsorption suppressing the HER at the catalyst surface. Herein, we have adopted the interfacial engineering strategy to synthesize our electrocatalyst NPG@SnS2, which not only suppressed the HER on the active site but yielded 49.3% F.E. for the NRR. Extensive experimental work and DFT calculations regarded that due to the charge redistribution, the Mott–Schottky effect, and the band bending of SnS2 across the contact layer at the interface of NPG, the d-band center for the surface Sn atoms in NPG@SnS2 lowered, which resulted in favored adsorption of N2 on the Sn active site. This phenomenon was driven even forward by the upshift of the Fermi level, and eventually, a decrease was seen in the work function of the heterostructure that increased the conductivity of the material as compared to pristine SnS2. This strategy thus provides a field to methodically suppress the HER in the realm of improving the Faradaic efficiency for the NRR.

Electrochemical reduction of nitrogen to ammonia can potentially replace the existing centralized fossil fuel-based Haber-Bosch process with small, decentralized units relying on electrical energy from renewable sources, thus supporting a...

Ammonia is the feedstock for nitrogen fertilizers and a potential carbon-free energy carrier, but the current production emits more CO₂ than any other chemical producing reaction in the world. The demand for decarbonizing the ammonia industry by using renewable energy has renewed research interests into catalyst development for effective N₂ reduction under mild conditions, a grand scientific challenge. Conventional heterogeneous catalysts based on metallic Fe or Ru mediate dinitrogen dissociation and hydrogenation through a relatively energy-costing pathway. The ternary ruthenium complex hydrides Li₄RuH₆ and Ba₂RuH₆ reported in this work, on the other hand, represent an entirely new class of compound catalysts, which are composed of the electron- and H-rich [RuH₆] anionic centers for non-dissociative dinitrogen reduction, where hydridic H transports electron and proton between the centers, and the Li(Ba) cations for stabilizing N_xH_y (x: 0 to 2, y: 0 to 3) intermediates. The dynamic and synergistic involvement of all the components of the ternary complex hydrides facilitates a novel reaction mechanism with a narrow energy span and perfectly balanced kinetic barriers for the multi-step process, leading to ammonia production from N₂+H₂ with superior kinetics under mild conditions.