Solids, Volume 3, Issue 1 (March 2022) – 11 articles

Thermoelectric materials convert waste heat to electricity and are part of the package of technologies needed to limit global warming. The tin chalcogenides SnS and SnSe are promising candidate thermoelectrics, with orthorhombic SnSe showing some of the highest figures of merit $ZT$ reported to date. As for other Group IV chalcogenides, SnS and SnSe can form rocksalt phases under certain conditions, but the thermoelectric properties of these phases are largely unexplored. We have applied a fully ab initio modelling protocol to compare the $ZT$ of the orthorhombic and rocksalt phases of SnS and SnSe. Electronic structures from hybrid density-functional theory were used to calculate the three electrical transport properties, including approximate models for the electron relaxation times, and lattice dynamics calculations were performed to model the phonon spectra and lattice thermal conductivities. We obtained good estimates of the $ZT$ of the well-studied orthorhombic phases. The rocksalt phases were predicted to show larger electrical conductivities and similar Seebeck coefficients to the orthorhombic phases, resulting in higher thermoelectric power factors, but these were offset by larger thermal conductivities. These results therefore motivate further investigation of the recently discovered “$\pi$-cubic” phases of SnS and SnSe, which are based on distorted rocksalt supercells, to establish their thermoelectric performance. Full article

Alkylsilane-derived hybrid films exhibiting excellent dynamic dewetting behaviors toward various liquids are promising, since they are smooth, highly transparent, and a low environmental burden. However, the detailed mechanism of their unique dynamic dewetting behaviors and its relation to the surface segregation of alkylsilanes during the film formation have not yet been clearly identified. In this study, we prepared various hybrid films by varying the mixing ratios of tetraethoxysilane (TEOS) and n-dodecyltriethoxysilane (C12TES) and investigated the changes in the s-CH₂/s-CH₃ peak strength ratios of the resulting hybrid films under dry and wet conditions by sum-frequency generation (SFG) spectroscopy. When the static/dynamic water contact angles significantly changed, it was clearly observed that the s-CH₂/s-CH₃ ratio of each hybrid film under dry and wet condition also changed markedly. With increasing TEOS concentration, the static contact angles became smaller, while the contact angle hysteresis tended to increase because of the increase in gauche defects at the air interface and hydrogen bonds. This finding suggests that the concentration and conformation of the alkyl chains derived from surface-segregated C12TES molecules play an important role in determining the final dewetting behaviors of the hybrid films to water. Full article

The phenomenon of total external reflection of X-rays at a sliding angle of incidence of a beam of incident X-rays is investigated. For metals, a quantitative law of direct dependence of the refractive index decrement on the interplane distance is obtained. The excitation of surface plasmons by X-rays that have experienced complete external reflection is detected. For surface plasmons, a dimensional effect was observed, expressed in an increase in the energy of plasmons and the concentration of conduction electrons with an increase in the depth of the output of surface plasmons. By the method of dispersion of surface plasmons, internal mechanical micro-stresses and spontaneous polarization of the surface layers of glassy dielectrics and in thin layers of vanadium dioxide were determined. The absence of micro-stresses in the lithium fluoride ionic single crystal was found out, and the polarization observed in it is due to the large dipole moment of the molecules of this crystal. In thin films of vanadium dioxide, the dependence of micro-stresses on the stresses in the substrates was found. Full article

Three-dimensional hybrid organic–inorganic lead halide perovskites are promising photovoltaic and light-emitting materials. A key phenomenon relevant for their optoelectronic applications is electron–phonon coupling. Since it can be strongly modified by structural deformation and changes in the dynamics of molecular cations, it is of great importance to study the temperature dependence of phonon properties of hybrid perovskites. In this work, temperature-dependent Raman scattering studies of FAPbBr₃ and MAPbBr₃ single crystals are reported in the 1800–22 cm⁻¹ and 300–90 K ranges. The Raman data of MAPbBr₃ showed clear anomalies near 236, 155 and 148 K, which were attributed to $Pm\overline{3}m$→I4/mcm→P4/mmm (or Imma)→Pnma phase transitions. They also provided strong evidence that crystal structure of the phase stable in the 155–148 K range is very similar to structure of the I4/mcm phase, not structure of the lowest-temperature Pmna phase, as suggested in some reports. Therefore, the symmetry of this phase seems to be more likely P4/mmm rather than Imma. An analysis of the temperature evolution of MAPbBr₃ Raman modes revealed that the phase transitions near 236 and 155 K are associated with weak distortion of the inorganic subnetwork and changes in the dynamics of MA⁺ ions. Very pronounced changes in the lattice modes region and a narrowing of bands below 148 K indicated that the phase transition to the Pnma phase is triggered by a freezing of MA⁺ motions, which in turn leads to strong distortion of the inorganic subnetwork. Raman studies of FAPbBr₃ showed that this material behaves in a very different way than MAPbBr₃. First of all, the molecular dynamics of FA⁺ cations are not frozen even in the lowest-temperature Pnma phase. Moreover, the distortion of the inorganic subnetwork is small in the Pnma phase. The observation of weak anomalies in the lattice modes region confirmed, however, that the two crystallographically resolved phase transitions ($Pm\overline{3}m$→P4/mbm near 260 K and P4/mbm→Pnma near 150 K) lead to weak distortion of the inorganic subnetwork. On the other hand, an analysis of FA⁺ internal modes indicated that these transitions, as well as two other crystallographically unresolved transitions near 120 and 180 K, are triggered by a change of FA⁺ dynamics. Full article

In 1991 the layered organic compound $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$ with a triangular lattice was synthesized for the first time. Although, originally, the focus was on the superconducting properties under pressure, this frustrated Mott insulator has been the most promising quantum-spin-liquid candidate for almost two decades, widely believed to host gapless spin excitations down to $T\to 0$. The recent observation of a spin gap rules out a gapless spin liquid with itinerant spinons and puts severe constraints on the magnetic ground state. This review evaluates magnetic, thermal transport, and structural anomalies around $T^{\star }=6$ K. The opening of a spin gap yields a rapid drop of spin susceptibility, NMR Knight shift, spin-lattice relaxation rate, and $\mu$-SR spin fluctuation rate, but is often concealed by impurity spins. The concomitant structural transition at $T^{\star }$ manifests in thermal expansion, THz phonons and ${}^{63}$Cu NQR relaxation. Based on the field dependence of $T^{\star }$, a critical field of 30–60 T is estimated for the underlying spin-singlet state. Overall, the physical properties are remarkably similar to those of spin-Peierls compounds. Thus, a strong case is made that the ‘6K anomaly’ in $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$ is the transition to a valence-bond-solid state and it is suggested that such a scenario is rather the rule than the exception in materials with strong magnetic frustration. Full article

Pharmaceutical cocrystals are currently gaining interest among the scientific community, due to their great potential for providing novel crystalline forms with superior properties such as solubility, dissolution rate, bioavailability, and stability. Robust computational tools are valuable tools in the rationalization of cocrystal formation, by providing insight into the intermolecular interactions of multicomponent molecular solids. In this study, various computational techniques based on charge density analysis were implemented to assess structural and energetical perspectives of the interactions responsible for the formation and stability of entacapone-theophylline-water (ETP-THP-water, 1:1:1). Significant non-covalent interactions (NCIs) were identified and evaluated by Hirshfeld surface analysis and density functional theory (DFT) computations, and three-dimensional networks (energy vector diagrams, lattice energy frameworks) were constructed, outlining the crucial stabilizing role of water and the dominance of π-π stacking interactions in the cocrystal. Furthermore, thermal dehydration studies confirmed the strong binding of water molecules in the crystal lattice, as expressed by the high activation energy. Full article

The present article examined the influence of size and periodicity of simulated gold (Au) nanoparticles (NPs) embedded in Nickel Oxide (NiO) matrix on localized plasmonic resonances (LSPRs). The scope of this work is to comparatively study the theoretical outcomes exhibited against the experimental results delivered from previous works, including a significant number of simulations and testing of numerous NPs diameter values. A comparison between Au and NiO NPs over silver (Ag) and NiO NPs is also reported to investigate whether the nature of noble metal affects its behavior in terms of LSPRs. The computational results strongly support that the appearance and intensity of LSPRs is straightforward to the increase in the diameter of NPs. The simulation results are in a good agreement with the literature of small NPs, offering the opportunity to further understand the LSPR phenomenon and its more effective implementation to opto-electronic applications. Rigorous Coupled Wave Analysis (RCWA) is performed to stimulate the justification and knowledge of the theoretical conclusions. Full article

Metal-organic frameworks (MOFs) are promising materials for a myriad of applications because of their easy synthesis and large variability through the organic linker. For open-environment applications, the organic content can, however, give rise to fouling, that is, biofilm formation. Biofilms can destroy the MOF and reduce the sorption capacity. Therefore, it is necessary to formulate MOFs for open-environment application to avoid the growth of microorganisms. Chitosan is a polysaccharide biopolymer, obtained from chitin shells of shrimps by alkaline deacetylation, and has known fungistatic properties. Here, chitosan is used as a matrix for MOF@chitosan composites with different aluminum-based MOFs to implement the fungistatic effect of chitosan to MOFs. The obtained composites with the highest possible MOF loadings of up to 90% were tested according to DIN EN ISO 846 to examine the fungistatic material properties against the fungi Chaetomium globosum and Aspergillus falconensis. Full article

We used density functional theory (DFT) calculations to investigate the structural, electronic, magnetic, mechanical, and thermodynamic properties of CoYSb (Y = Cr, Mo and W) compounds. These are XYZ type half-Heusler alloys, which also exist in the face centred cubic MgAgAs-type structure and conform to $F\overline{4}3m$ space group. We computed these properties in three different atomic arrangements known as Type-I, Type-II, and Type-III phases. In all these phases, the alloys were found to be in the ferromagnetic state. Furthermore, the calculated electronic band structure and the total electronic density of states indicated a metallic behavior in CoWSb, nearly half-metallic behavior in CoMoSb, and half-metallic behavior in CoCrSb, with a minority-spin band gap of 0.81 eV. Furthermore, the calculated mechanical properties predicted an anisotropic behavior of these alloys in their stable phase. Finally, due to its high Debye temperature value, CoCrSb shows stronger covalent bonding than CoMoSb and CoWSb, respectively. Full article

A novel Ti-Ni-Hf-Cu-Nb shape memory alloy has been developed by a new combinatorial alloy synthesis method, the Suspended Droplet Alloying. The influence of alloying elements on the transformation temperature, the microstructure and the shape memory effect of this alloy have also been studied. It was found that Cu has a greater negative influence on the transformation temperature of Ti-Ni-Hf-Cu_X alloys (about −5 K/at.%) than on the Ti-Ni-Cu_X alloys (−0.67 K/at.%). In addition, the negative effect intensifies with increasing Hf content. The transformation temperature rapidly decreases with increasing Nb composition in the Ti-Ni-Cu-Nb and Ti-Ni-Hf-Cu-Nb alloys, with the solid solution of Nb in the matrix being 1 at.%. A Ti-Ni-Cu-Hf-Nb alloy with high thermal cycle stability has been developed, where the alloying elements affect the transformation behaviour via altering the slipping energy and forming different types of precipitations. Full article