Rocksalt Crystals Research Articles

Metavalent Bonding in 2D Chalcogenides: Structural Origin and Chemical Mechanisms.

An unusual set of anomalous functional properties of rocksalt crystals of Group IV chalcogenides were recently linked to a kind of bonding termed as metavalent bonding (MVB) which involves violation of the 8-N rule. Precise mechanisms of MVB and the relevance of lone pair of Group IV cations are still debated. With restrictions of low dimensionality on the possible atomic coordination, 2D materials provide a rich platform for exploration of MVB. Here, we present first-principles theoretical analysis of the nature of bonding in five distinct 2D lattices of Group IV chalcogenides MX (M: Sn, Pb, Ge and X: S, Se, Te), in which the natural out-of-plane expression of the lone pair versus in-plane bonding can be systematically explored. While their honeycomb lattices respecting the 8-N rule are shown to exhibit covalent bonding, their square and orthorhombic structures exhibit MVB only in-plane, with cationic lone pair activating the out-of-plane structural puckering that controls their relative stability. Anomalies in Born-effective charges, dielectric constants, Grüneisen parameters occur only in their in-plane behaviour, confirming MVB is confined strictly to 2D and originates from p-p orbital interactions. Our work opens up directions for chemical design of MVB based 2D materials and their heterostructures.

Metavalent Bonding in 2D Chalcogenides: Structural Origin and Chemical Mechanisms

AbstractAn unusual set of anomalous functional properties of rocksalt crystals of Group IV chalcogenides were recently linked to a kind of bonding termed as metavalent bonding (MVB) which involves violation of the 8‐N rule. Precise mechanisms of MVB and the relevance of lone pair of Group IV cations are still debated. With restrictions of low dimensionality on the possible atomic coordination, 2D materials provide a rich platform for exploration of MVB. Here, we present first‐principles theoretical analysis of the nature of bonding in five distinct 2D lattices of Group IV chalcogenides MX (M: Sn, Pb, Ge and X: S, Se, Te), in which the natural out‐of‐plane expression of the lone pair versus in‐plane bonding can be systematically explored. While their honeycomb lattices respecting the 8‐N rule are shown to exhibit covalent bonding, their square and orthorhombic structures exhibit MVB only in‐plane, with cationic lone pair activating the out‐of‐plane structural puckering that controls their relative stability. Anomalies in Born‐effective charges, dielectric constants, Grüneisen parameters occur only in their in‐plane behaviour, confirming MVB is confined strictly to 2D and originates from p‐p orbital interactions. Our work opens up directions for chemical design of MVB based 2D materials and their heterostructures.

A Novel Strategy for Comprehensive Estimation of Lattice Energy, Bulk Modulus, Chemical Hardness and Electronic Polarizability of ANB8-N Binary Inorganic Crystals

How to search for a convenient method without a complicated calculation process to predict the physicochemical properties of inorganic crystals through a simple micro-parameter is a greatly important issue in the field of materials science. Herein, this paper presents a new and facile technique for the comprehensive estimation of lattice energy (U), bulk modulus (B), chemical hardness (ƞ), and electronic polarizability (α), just by using a simple mathematic fitting formula with a few structure parameters, such as the systems of rock salt crystals (group I–VII, II–VI) and tetrahedral coordinated crystals (group II–VI, III–V). For the typical binary ANB8-N crystal systems, our present conclusions suggest that a good quantitative correlation between U, B, ƞ, α and chemical bond length (d) is observed, the normal mathematical expression is P = a·db (P represents these physicochemical parameters), constants a and b depend on the type of crystals, and the relevant squares of the correlation coefficient (R2) are larger than 0.9. The results indicate that lattice energy, bulk modulus, and chemical hardness decrease with increases in chemical bond length, but electronic polarizability increases with an increase in chemical bond length. Meanwhile, the new data on the lattice energy, bulk modulus, chemical hardness, and electronic polarizability values of binary ANB8-N crystal systems considered in the present study are calculated via the obtained curve fitting equations without any complex calculation process. We find that there is a very good linear trend in our calculated results along with the values reported in the literature. The present study will be important in solid-state chemistry, which may give researchers useful guidance in searching for relevant data for predicting the properties of new materials or synthetic routes based on a simple mathematic empirical model.

High area-capacity Mg batteries enabled by sulfur/copper integrated cathode design

Rechargeable Mg batteries potentially display lower cost and competitive energy density compared with their Li-ion counterparts. However, the practical implementation of high area-capacity cathodes still remains a formidably challenging task. This work presents the sulfur/copper integrated cathodes fabricated by the conventional blade-coating process and slurry-dipping method. The sulfur/copper foil integrated cathodes deliver a high area-capacity of 2.6 mAh cm−2 after 40 cycles, while the sulfur/copper-foam integrated cathode exhibits an ultrahigh area-capacity of 35.4 mAh cm−2, corresponding to 743.1 Wh L−1 at the electrode level (1.5 times higher than the LiCoO2-graphite system). The in-situ formed copper sulfide intermediates with sufficient cation defects can act as functional intermediates to regulate the sulfur electrochemistry during the first discharge process. The subsequent cycles are operated by the reversible displacement reaction between Mg-ions and copper sulfide active substances. In particular, the copper ions prefer to extrude along the [001] direction in copper sulfides lattice and simultaneously the rock-salt MgS crystals are generated. Besides, the nonuniform surface topography of the cycled Mg-metal anode, caused by the spatial inhomogeneity in current distribution, is demonstrated to lead to the battery performance degradation for high area-capacity Mg batteries.

Front Cover: Cobalt Oxide Nanowires with Controllable Diameters and Crystal Structures for the Oxygen Evolution Reaction (Eur. J. Inorg. Chem. 18/2022)

The Front Cover shows an illustration of replication of mesostructured cobalt oxide and its utilization as electrocatalyst for water electrolysis. The impact of the nanowire diameters of the two-dimensional ordered mesoporous spinel (Co3O4) and rock-salt (CoO) cobalt oxides on their electrochemical oxygen evolution reaction (OER) activity has been systematically studied. The magnifying glass reveals an operando Raman spectroscopy technique that is used to monitor the alteration of the electrocatalyst during the OER. A phase transformation from rocksalt (CoO) to a distorted spinel (Co3O4) phase is observed, which was found to be responsible for a high OER activity in the starting rocksalt crystal phase. More information can be found in the Research Article by E. Budiyanto and H. Tüysüz.

Dielectric response of rock-salt crystals at finite temperatures from first principles

We combine ab initio simulations and Raman scattering measurements to demonstrate explicit anharmonic effects in the temperature-dependent dielectric response of a NaCl single crystal. We measure the temperature evolution of its Raman spectrum and compare it to both a quasiharmonic and anharmonic model. Results demonstrate the necessity of including anharmonic lattice dynamics to explain the dielectric response of NaCl, as it is manifested in Raman scattering. Our model fully captures the linear dielectric response of a crystal at finite temperatures and may therefore be used to calculate the temperature dependence of other material properties governed by it.

Classification of thermoluminescence features of the natural halite with machine learning

Radiation dosimeters are used to measure the absorbed radiation dose of any living organism during the time intervals. They include defective crystals that store radiation until they are stimulated. Thermoluminescence (TL) is a way to see the absorbed dose of the dosimeters. The irradiated crystal is heated up to 500°C to reveal the absorbed dose as a luminescence light. The TL dosimetric properties of natural halite (rock-salt) crystals extracted from Meke crater lake in Konya, Turkey, were investigated in this study. Support Vector Machine (SVM), Artificial Neural Network (ANN) and K-Nearest Neighbor (K-NN) were also examined utilizing machine learning for categorization of TL characteristics. According to the experimental output, the TL glow curve has two main peaks located at 100 and 270°C with good dosimetric properties. In the three classifiers, SVM has the biggest accuracy and precision. High training-low testing and results from normalized data give the best accuracy, precision, sensitivity and F-score.

Structural Characterization of the Delithiated Noncrystalline Phase in a Li-Rich Li2VO2F Cathode Material

As a cathode material for lithium-ion batteries, the disordered rock-salt-type Li2VO2F has a high theoretical capacity of 462 mA h/g; however, this capacity fades rapidly upon cycling. Here, we performed X-ray total scattering measurements of the material to gain insights into the battery properties of Li2VO2F. The atomic pair distribution function obtained from these measurements indicates that the disordered rock-salt crystal partially transforms to an amorphous phase on delithiation. Structural information of the amorphous phase was extracted by precisely evaluating the structural parameters of the disordered rock-salt crystal. Using the structural information, we modeled an atomic three-dimensional structure by the reverse Monte Carlo method. The structural model reproduced the tetrahedral or pyramidal structure with the anions at the vertices and a V ion at the center. These polyhedral structures are likely to involve the anion transfer between the crystalline and amorphous phases. It induces the concentration fluctuation of anions in the crystalline phase and might be a factor for capacity fading of the Li2VO2F cathode material.

Hyperbolic Metamaterials with Extreme Mechanical Hardness

AbstractHyperbolic metamaterials (HMMs) are highly anisotropic optical materials that behave as metals or as dielectrics depending on the direction of propagation of light. They are becoming essential for a plethora of applications, ranging from aerospace to automotive, from wireless to medical and IoT. These applications often work in harsh environments or may sustain remarkable external stresses. This calls for materials that show enhanced optical properties as well as tailorable mechanical properties. Depending on their specific use, both hard and ultrasoft materials can be required, although the combination with optical hyperbolic response is rarely addressed. Here, the possibility to combine optical hyperbolicity and tunable mechanical properties in the same (meta)material is demonstrated, focusing on the case of extreme mechanical hardness. Using high‐throughput calculations from first principles and effective medium theory, a large class of layered materials with hyperbolic optical activity in the near‐IR and visible range is explored, and a reduced number of ultrasoft and hard HMMs is identified among more than 1800 combinations of transition metal rocksalt crystals. Once validated by the experiments, this new class of metamaterials may foster previously unexplored optical/mechanical applications.

First-principles study of the order-disorder transition and its effects on the optoelectronic property of ABiS2 ( A=Na , K)

Using first-principles methods, we have studied structural and electronic properties of ternary sulfide $A\mathrm{Bi}{\mathrm{S}}_{2}$ ($A=\mathrm{Na},\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) compounds for low cost and environmentally friendly optoelectronic applications. We find that the most stable crystal structure of these compounds at low temperature forms a 1 \ifmmode\times\else\texttimes\fi{} 1 rocksalt based ${\mathrm{Bi}}^{3+}$ and ${A}^{+}$ superlattice along the [111] direction. The ordered phase will transform to the disordered phase at high temperature, with the calculated order-disorder transition temperature for $\mathrm{K}\mathrm{Bi}{\mathrm{S}}_{2}$ (${T}_{c}=\ensuremath{\sim}1200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), higher than that for $\mathrm{Na}\mathrm{Bi}{\mathrm{S}}_{2}$ (${T}_{c}=\ensuremath{\sim}900\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) due to the larger size mismatch between K and Bi than between Na and Bi. The disordered phase has higher optical absorption than the ordered one with the light absorption threshold at 1.33 and 1.57 eV, respectively, for $\mathrm{Na}\mathrm{Bi}{\mathrm{S}}_{2}$ and $\mathrm{K}\mathrm{Bi}{\mathrm{S}}_{2}$. The good agreement between our calculated properties such as x-ray diffraction and light absorption of the partially disordered phase and the experimental observations suggests that the partially disordered phase, instead of the ordered phase, was formed in experimental samples. Our study provides theoretical insights into the order-disorder transition and the corresponding variation of the crystal and electronic structures of rocksalt based $A\mathrm{Bi}{\mathrm{S}}_{2}$ ($A=\mathrm{Na},\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) compounds, which is helpful for future synthesis and applications of these materials.

Ultrathin High-Quality SnTe Nanoplates for Fabricating Flexible Near-Infrared Photodetectors.

This work demonstrates a controlled van der Waals growth of two-dimensional SnTe nanoplates on mica substrates and their applications in flexible near-infrared photodetectors. The growth of nonlayered rock-salt structured SnTe crystals into two-dimensional SnTe nanoplate structures is mainly caused by the two-dimensional nature of the mica surface, which also results in the ultrathin nanoplates obtained (3.6 nm, equivalent to 6 monolayers). Furthermore, it is found that the shape of the SnTe nanoplates can be well engineered by changing their growth temperature due to the competition between the surface energy of the {100} crystallographic plane and that of the {111} plane. As a result of the favorable physical properties of topological crystalline insulators such as metallic surface (high electron mobility) and narrow bandgap, near-infrared photodetectors based on single SnTe nanoplate with the thickness of 3.6 nm present excellent device performance with a responsivity of 698 mA/W, a specific detectivity of 3.89 × 108 jones, and an external quantum efficiency of 88.5% under the illumination of a 980 nm laser at room temperature (300 K) without applying a gate voltage (Vg). Upon increasing the gate voltage from -30 to 30 V, the detector responsivity increases from 2.96 to 723 mA/W and the detector detectivity increases from 2.4 × 106 to 5.3 × 108 jones. Furthermore, upon increasing the thickness of SnTe nanoplate from 3.6 to 35 nm, the detector responsivity increases from 0.698 to 1.468 A/W. The device performance measured after bending for 300 times as well as after bending with different radii presents no obvious degradation, which exhibits the excellent flexibility of the SnTe nanoplate detectors. These results not only contribute to a deep understanding of the mechanisms of the van der Waals growth of nonlayered materials into two-dimensional structure but also demonstrate the immense potential of SnTe nanoplates to be used in flexible near-infrared detectors.

Comment on “Tuning topological surface states by cleavage angle in topological crystalline insulators”

The results of the commented paper seem to disagree with the prediction of the paper by Safaei et al. [Phys. Rev. B 88, 045305 (2013)]. Some of the definitions of surface unit cells used in the commented article do not lead to the first surface Brillouin zone, defining instead a folded Brillouin zone. This is the source of the seeming discrepancies. To dispel the misunderstandings we present general rules for Brillouin zone shapes and $L$ points projections for $(ppq)$ surfaces of the rock salt crystals. We show that using the first Brillouin zones allows a simple, straightforward prediction of the topological properties of the surface states.

Fully strained epitaxial Ti1−xMgxN(001) layers

Ti1−xMgxN(001) layers with 0.00 ≤ x ≤ 0.49 are deposited on MgO(001) by reactive magnetron co-sputtering from titanium and magnesium targets in 5 mTorr pure N2 at 600 °C. X-ray diffraction ω-2θ scans, ω-rocking curves, φ-scans, and high resolution reciprocal space maps show that the Ti1−xMgxN layers are rock-salt structure single crystals with a cube-on-cube epitaxial relationship with the substrates: (001)TiMgN║(001)MgO and [100]TiMgN║[100]MgO. Layers with thickness d = 35–58 nm are fully strained, with an in-plane lattice parameter a|| = 4.212 ±0.001 Å matching that of the MgO substrate, while the out-of-plane lattice parameter a⊥ increases with x from 4.251 Å for TiN(001) to 4.289 Å for Ti0.51Mg0.49N(001). This yields a relaxed lattice parameter for Ti1−xMgxN(001) of ao = (1-x)aTiN + xaMgN – bx(1-x), where aTiN = 4.239 Å, aMgN = 4.345 Å, and the bowing parameter b = 0.113 Å. In contrast, thicker Ti1−xMgxN(001) layers with d = 110–275 nm are partially (pure TiN) or fully (x = 0.37 and 0.49) relaxed, indicating a critical thickness for relaxation of 50–100 nm. The in-plane x-ray coherence length is large (100–400 nm) for fully strained layers with 0.00 ≤ x ≤ 0.45 but drops by an order of magnitude for x = 0.49 as the composition approaches the phase stability limit. It is also an order of magnitude smaller for thicker (d ≥ 110 nm) layers, which is attributed to strain relaxation through the nucleation and growth of misfit dislocations facilitated by glide of threading dislocations.

Structural Phase Transition and a Mutation of Electron Mobility in ZnxCd1−xO Alloys**Supported by the National Natural Science Foundation of China under Grant Nos 11474273 and 11634003, and the Youth Innovation Promotion Association of Chinese Academy of Sciences under Grant No 2017154.

We investigate the electronic structures and phase stability of ZnO, CdO and the related alloys in rocksalt (B1) and wurzite (B4) crystal, using the first-principle density functional theory within the hybrid functional approximation. By varying the concentration of Zn components from 0% to 100%, we find that the ZnxCd1 − xO alloy undergoes a phase transition from octahedron to tetrahedron at x = 0.32, in agreement with the recent experimental findings. The phase transition leads to a mutation of the electron mobility originated from the changes of the effective mass. Our results qualify ZnO/CdO alloy as an attractive candidate for photo-electrochemical and solar cell power applications.

Computational predictions turning the isomers of alanine to generate distinct morphs of free-flowing salt crystals.

The morphology of salt crystals has been an active area of research for decades and remains largely empirical. This study reveals the computational modeling guides to arrive at the correct answer, even for complex problems, such as the morphology of salt crystals with additives. There is no report on the two different morphologies of rock-salt crystals with two isomers of a single additive. Alanine isomers (α- and β-) can induce octahedron and rhombic dodecahedron morphologies in salt crystals. The computational study demonstrated that α-alanine preferentially interacts with {111} and β-alanine with {110} planes to yield such morphologies. The change in morphology with alanine can greatly influence the problem of the free-flowing issue of salt crystals.

Thermal Properties of Consolidated Granular Salt as a Backfill Material

Granular salt has been proposed as backfill material in drifts and shafts of a nuclear waste disposal facility where it will serve to conduct heat away from the waste to the host rock. Creep closure of excavations in rock salt will consolidate (reduce the porosity of) the granular salt. This study involved measuring the thermal conductivity and specific heat of granular salt as a function of porosity and temperature to aid in understanding how thermal properties will change during granular salt consolidation accomplished at pressures and temperatures consistent with a nuclear waste disposal facility. Thermal properties of samples from laboratory-consolidated granular salt and in situ consolidated granular salt were measured using a transient plane source method at temperatures ranging from 50 to 250 °C. Additional measurements were taken on a single crystal of halite and dilated polycrystalline rock salt. Thermal conductivity of granular salt decreased with increases in temperature and porosity. Specific heat of granular salt at lower temperatures decreased with increasing porosity. At higher temperatures, porosity dependence was not apparent. The thermal conductivity and specific heat data were fit to empirical models and compared with results presented in the literature. At comparable densities, the thermal conductivities of granular salt samples consolidated hydrostatically in this study were greater than those measured previously on samples formed by quasi-static pressing. Petrographic studies of the consolidated salt indicate that the consolidation method influenced the nature of the porosity; these observations are used to explain the variation of measured thermal conductivities between the two consolidation methods. Thermal conductivity of dilated polycrystalline salt was lower than consolidated salt at comparable porosities. The pervasive crack network along grain boundaries in dilated salt impedes heat flow and results in a lower thermal conductivity compared to hydrostatically consolidated salt.

Second-order structural phase transitions, free energy curvature, and temperature-dependent anharmonic phonons in the self-consistent harmonic approximation: Theory and stochastic implementation

The self-consistent harmonic approximation is an effective harmonic theory to calculate the free energy of systems with strongly anharmonic atomic vibrations, and its stochastic implementation has proved to be an efficient method to study, from first-principles, the anharmonic properties of solids. The free energy as a function of average atomic positions (centroids) can be used to study quantum or thermal lattice instability. In particular the centroids are order parameters in second-order structural phase transitions such as, e.g., charge-density-waves or ferroelectric instabilities. According to Landau's theory, the knowledge of the second derivative of the free energy (i.e. the curvature) with respect to the centroids in a high-symmetry configuration allows the identification of the phase-transition and of the instability modes. In this work we derive the exact analytic formula for the second derivative of the free energy in the self-consistent harmonic approximation for a generic atomic configuration. The analytic derivative is expressed in terms of the atomic displacements and forces in a form that can be evaluated by a stochastic technique using importance sampling. Our approach is particularly suitable for applications based on first-principles density-functional-theory calculations, where the forces on atoms can be obtained with a negligible computational effort compared to total energy determination. Finally we propose a dynamical extension of the theory to calculate spectral properties of strongly anharmonic phonons, as probed by inelastic scattering processes. We illustrate our method with a numerical application on a toy model that mimics the ferroelectric transition in rock-salt crystals such as SnTe or GeTe.

Temperature and radiation effects at the fluorine K-edge in LiF

The fluorine K-edge of LiF is studied both experimentally and theoretically as a function of temperature. Instantaneous thermal fluctuations in atomic positions are shown in molecular dynamics simulations to increase in amplitude from 0.029 to 0.064nm in the temperature range from 40 to 298K. This is sufficient to cause instantaneous deviations from local octahedral atomic symmetry in this rock-salt crystal, resulting in altered electronic structure. The lowered symmetry of the lowest core-excited states of fluorine atoms is evident in X-ray absorption spectra at the F K-edge. In addition, sufficient radiation exposure produces a new X-ray absorption peak, below the F K-edge of LiF, which is assigned to defects in LiF based on both calculations and comparison to previous experiments.

Crystal structures of model lithium halides in bulk phase and in clusters

We employ lattice energy calculations and molecular dynamics simulations to compare the stability of wurtzite and rock salt crystal structures of four lithium halides (LiF, LiCl, LiBr, and LiI) modeled using the Tosi-Fumi and Joung-Cheatham potentials, which are models frequently used in simulation studies. Both infinite crystals and finite clusters are considered. For the Tosi-Fumi model, we find that all four salts prefer the wurtzite structure both at 0 K and at finite temperatures, in disagreement with experiments, where rock salt is the stable structure and wurtzite exists as a metastable state. For Joung-Cheatham potentials, rock salt is more stable for LiF and LiCl, but the wurtzite structure is preferred by LiBr and LiI. It is clear that the available lithium halide force fields need improvement to bring them into better accord with the experiment. Finite-size clusters that are more stable as rock salt in the bulk phase tend to solidify as small rock salt crystals. However, small clusters of salts that prefer the wurtzite structure as bulk crystals tend to form structures that have hexagonal motifs, but are not finite-size wurtzite crystals. We show that small wurtzite structures are unstable due to the presence of a dipole and rearrange into more stable, size-dependent structures. We also show that entropic contributions can act in favor of the wurtzite structure at higher temperatures. The possible relevance of our results for simulation studies of crystal nucleation from melts and/or aqueous solutions is discussed.

Nanorange mechanical and fractal properties of rock salt crystal surface and their effect on fracture toughness and wettability

The scope of the studies embraces statistical and mechanical properties of surface of different kind crystals of salt rocks. Fractal dimension, hardness and elasticity moduli of such crystals are determined. The article gives estimates of fracture toughness and wettability of salt rock crystals as function of fractal dimension of the crystal surface microrelief.