Enhancing Strategy of the Small-Polaron Conductivity in LaCrO3: First-Principles Calculations and Experimental Validation.

LaCrO3 (LCO) has promising applications as a p-type conductive material in the fields of transparent conducting oxes, high-temperature sensors, and magnetohydrodynamic power generators. However, the easy volatility of the Cr element, along with the issues of low electrical conductivity caused by the small-polaron conduction mechanism and wide band gap, has hindered the widespread application of LCO. In this work, based on band engineering and defect engineering, we screened doping schemes through first-principles calculations that can reduce Cr volatility by enhancing the Cr-O bond energy. We also aimed to promote small-polaron hopping and improve the electrical conductivity by introducing impurity levels. Additionally, we conducted a thorough analysis of the small-polaron conductivity mechanism. Through the solid-state method, we successfully prepared codoped LCO with Ca and Zn. The Zn dopants effectively enhanced the Cr-O bond strength, suppressed the Cr volatility, and improved high-temperature stability. The Zn dopants introduced additional impurity energy levels within the band gap, significantly changing the mobility of small polarons. Through optimal doping concentration, the La0.7Ca0.3Cr0.95Zn0.05O3 sample demonstrated a significant enhancement in electrical conductivity compared to La0.7Ca0.3CrO3, increasing from 7 to 60 at 1000 K. Additionally, the impurity energy levels enhanced the asymmetry near the Fermi level, resulting in an increased Seebeck coefficient (S). This is beneficial for the production of high-temperature sensors. The output voltage of an LCO thermocouple module reaches up to 58 mV at 2170 K, indicating that the performance optimization strategy employed in this work has significant implications for the regulation and application of oxide electrical materials.

Modeling critical thermoelectric transports driven by band broadening and phonon softening.

Critical phenomena are one of the most captivating areas of modern physics, whereas the relevant experimental and theoretical studies are still very challenging. Particularly, the underlying mechanism behind the anomalous thermoelectric properties during critical phase transitions remains elusive, i.e., the current theoretical models for critical electrical transports are either qualitative or solely focused on a specific transport parameter. Herein, we develop a quantitative theory to model the electrical transports during critical phase transitions by incorporating both the band broadening effect and carrier-soft TO phonon interactions. It is found that the band-broadening effect contributes an additional term to Seebeck coefficient, while the carrier-soft TO phonon interactions greatly affects both electrical resistivity and Seebeck coefficient. The universality and validity of our model are well confirmed by experimental data. Furthermore, the features of critical phase transitions are effectively tuned. For example, alloying S in Cu2Se can reduce the phase transition temperature but increase the phase transition parameter b. The maximum thermoelectric figure of merit zT is pushed to a high value of 1.3 at the critical point (377 K), which is at least twice as large as those of normal static phases. This work not only provides a clear picture of the critical electrical transports but also presents new guidelines for future studies in this exciting area.

Dynamical approach to the atomic and electronic structures of the ductile semiconductor Ag2S.

Silver sulfide in monoclinic phase (α-Ag2S) has attracted significant attention owing to its metal-like ductility and promising thermoelectric properties near room temperature. However, first-principles studies on this material by density functional theory calculations have been challenging as both the symmetry and atomic structure of α-Ag2S predicted from such calculations are inconsistent with experimental findings. Here, we propose that a dynamical approach is imperative for correctly describing the structure of α-Ag2S. The approach is based on a combination of ab initio molecular dynamics simulation and deliberately chosen density functional considering both proper treatment of the van der Waals interaction and on-site Coulomb interaction. The obtained lattice parameters and atomic site occupations of α-Ag2S are in good agreement with experimental data. A stable phonon spectrum at room temperature can be obtained from this structure, which also yields a bandgap in accord with experimental measurements. The dynamical approach thus paves the way for studying this important ductile semiconductor in not only thermoelectric but also optoelectronic applications.

Covalently Assembled Black Phosphorus/Conductive C3N4 Hybrid Material for Flexible Supercapacitors Exhibiting a Superlong 30,000 Cycle Durability.

Black phosphorus (BP) has been demonstrated as a promising electrode material for supercapacitors. Currently, the main limitation of its practical application is the low electrical conductivity and poor structure stability. Hence, BP-based supercapacitors usually severely suffer from low capacitance and poor cycling stability. Herein, a chemically bridged BP/conductive g-C3N4 (BP/c-C3N4) hybrid is developed via a facile ball-milling method. Covalent P-C bonds are generated through the ball-milling process, effectively preventing the structural distortion of BP induced by ion transport and diffusion. In addition, the overall electrical conductivity is significantly enhanced owing to the sufficient coupling between BP and highly conductive c-C3N4. Moreover, the imbalanced charge distribution around the C atom can induce the generation of a local electric field, facilitating the charge transfer behavior of the electrode material. As a result, the BP/c-C3N4-20:1 flexible supercapacitor (FSC) exhibits an outstanding volumetric capacitance of 42.1 F/cm3 at 0.005 V/s, a high energy density of 5.85 mW h/cm3, and a maximum power density of 15.4 W/cm3. More importantly, the device delivers excellent cycling stability with no capacitive loss after 30,000 cycles.