Dedicated to Professor Rudolf Hoppe on the occasion of his 90th birthdayThe prediction and identification of stable and particularly of metastable compounds is important in achieving innovative materials. [1] With this objective in mind, approaches containing both theoretical examinations of phase stabilities and concepts of a rational synthesis [2] are gaining increasing importance. Thus, an investigation of energy landscapes [3] provides important information about local and global minima to predict the existence of new compounds and structures.Herein, we introduce a combination of quantum-chemical calculations and thermodynamic considerations to realize target-oriented planning and optimization of chemical synthesis. The analysis of phase formation is acquired with an in situ method for monitoring gas-phase reactions. Using the system P-As, we investigated a textbook example of monotropic phase transitions, which features a variety of known and postulated allotropes. [4] To estimate the relative stabilities of the compounds under discussion, structure allotropes of N, P, and As were modeled by means of DFT calculations. The calculated electronic energies (normalized to one atom Pn) for molecular Pn 2 and Pn 4 , the black/orthorhombic (o-Pn), gray/ trigonal (t-Pn), and the simple-cubic (c-Pn) allotrope as well as the tubular polymeric forms of Hittorf (H-Pn), [5] Ruck (R-Pn), [6] and Pfitzner (P-Pn), [7] which are known for phosphorus, are shown in Figure 1. The high stability of N 2 can be observed as well as the preference for solid-state structures for P and As. Figure 1 b emphasizes the results for the t and o forms of P and As. The known stability of o-P compared to the high-pressure modifications t-P and c-P is correctly predicted as well as the stability of gray t-As compared to the known high-pressure modification c-As, hypothetical tubular As allotropes, and predicted o-As. Does this mean that o-As can be synthesized as a metastable compound? [8a] The calculated values of the total electronic energies correctly express the higher stability of o-P compared with the high-pressure phase t-P and also t-As compared to o-As. From the computed values DE el = E el (o)ÀE el (t) of À4 kJ mol À1 for P and + 2.5 kJ mol À1 for As, a transition from the o-to the tstructure in the ideal solution As x P 1Àx can be estimated to be about x = 0.6 ( Figure 2). Taking thermodynamic energy terms into consideration (for details, see the Supporting Information), a distinctive stabilization of the o phase is calculated and the o-t transition shifts to a higher As content (x = 0.9). According to the calculated values, very low energy differences decide whether or not o-As can actually be synthesized. [8b]
The application areas of rechargeable Li-ion batteries continue to grow, hence improvement in their energy density, rate capability and cycle life is necessary. A typical cathode contains usually redox active transition metal oxides as active materials, conductive additives to ensure electronic conductivity and binder supporting matrix. In this work we report the behavior and properties of carbon black free LiFePO 4 composite electrode, where poly (3,4-ethylene dioxythiophene): poly (styrene sulfonate) (PEDOT:PSS) is accomplishing a dual role of binder and conducting additive. The effect of the polymer amount on the morphometric properties of the electrodes was studied using SEM, mercury porosimetry and high resolution X-ray computed tomography. The electrochemical performance and the cycling stability of the composite electrodes were compared to the behavior of conventional cathodes with carbon additives and PVDF binder. With increasing PEDOT:PSS content a decrease in the overvoltage and correspondingly an improvement in the rate capability is observed. Composite cathodes containing 8% PEDOT:PSS show comparable electrode capacity and better cyclic stability than conventional composite cathode.
Thermoelectric and phase analytical measurements were performed to investigate the physical properties of trimorphic Ag5Te2Cl. The material is a mixed electron/silver ion conductor featuring drastic property changes during a silver order/disorder phase transition at 334 K. The transition is characterized by a jump in the total electric conductivity by 2 orders of magnitude directly affecting the electric and thermoelectric properties. Silver ions are arranged in well-defined strands along the crystallographic c-axis characterized by a set of not fully occupied sites. Heat capacity measurements show a large effect, whereas the thermopower and thermal diffusivity drop significantly at the temperature of transition. Right after the transition, an attractive d10−d10 interaction within the disordered silver substructure occurs affecting the c-lattice parameter upon heating. Due to this interaction a modulation of the electronic structure and the thermoelectric properties can be observed which have been investigated in detail. While the thermopower stays low with increasing temperature the thermal diffusivity relaxes fast to values before the transition. At 355 K, the thermopower starts rising again, which is consistent with a small effect in the heat capacity and a reduction of the c-lattice parameter upon heating. Further heating leads to a reduction of the d10−d10 interactions and a drastic increase in the thermopower. The observed phenomenon must be regarded as a new example of a compound following the recently discovered concept of low-dimensional partially covalent-bonded structure units that can positively influence thermoelectric properties in bulk materials. Ag5Te2Cl is the first example where mobile d10 ions interact to create low-dimensional partially covalent-bonded subunits in a solid, which then leads to a switching of thermoelectric and electronic properties. The system shows very low thermal conductivities between 0.19 W m−1 K−1 and 0.60 W m−1 K−1 in the temperature range 298 to 500 K, reaching a maximal ZT value of 0.033 at high temperatures.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.