Octet Compounds Research Articles

Stability of the sc16 polymorph of GaAs

The occurrence of the high-pressure simple cubic sc16 polymorph in GaAs has been experimentally confirmed, but whether it is a stable polymorph of the octet compound remains controversial. Using a laser-heated diamond anvil cell and x-ray microdiffraction techniques, we document the emergence of sc16-GaAs at 13.6 GPa upon heating the ambient-pressure zincblende phase. The single-crystal structural refinement of a selected micro-grain provides a detailed description of the coordination geometry in sc16-GaAs. The transition from zincblende to sc16 is associated with a change from ideal tetrahedral coordination to distorted tetrahedral coordination and a slight increase in bond lengths but similar tetrahedral volume. The 7.6% volume decrease is accomplished by a more compact arrangement of the tetrahedra, rather than an increase in coordination number, while relaxing the bond length. The results of first-principles calculations complement the structural experimental study and confirm the stability of the sc16 phase. The distortion in the tetrahedral coordination present in the high-pressure sc16 phase with respect to the ideal tetrahedral coordination of the low-pressure zincblende phase is not associated with a change in the bonding character as shown by our ELF analysis.

Peculiar bond characters of fivefold coordinated octet compound crystals.

The present work exemplifies complementary perspectives offered by the band and bond pictures of solids, with an emphasis on the chemical intuition pertaining to the latter, especially in the presence of interfaces. The modern computational method of constructing a unique set of maximally localized Wannier functions from delocalized band states imparts new interpretations to the familiar concept of chemical bonds in the context of crystalline solids. By bridging the band and bond pictures using advanced computational tools, we reveal for the first time the unusual bond characters of a long-predicted fivefold coordinated structure of binary octet compounds ANB8−N consisting of AA′ stacked planar AB honeycombs. While the isolated monolayer retains the familiar pz–π bonding in a honeycomb framework as in graphene and hexagonal boron nitride, the bulk foregoes in-plane π bonding and embraces out-of-plane ⋯A–B–A–B⋯ chain bonding via overlapping pz orbitals. Not only does the chemical intuition gained by invoking the bond picture clarify the chemical nature of the fivefold coordination, but it also facilely explains a salient discrepancy in theoretical predictions in otherwise sound ample experimental evidence in the form of epitaxial thin films, paving the way towards rational synthesis of such thin films for optoelectronic applications. On the other hand, we show that the conduction band minimum, important in determining the electrical and optical properties, is a distinctly extended state that can only be properly described within the band picture.

On aspects of electron-LO phonon interaction in magnetized semiconductors in the presence of a hybrid pump field

Most realistic propagation of an intense hybrid pump wave in a magnetized semiconductor plasma has been considered to study some aspects of electron-LO phonon interactions. Hydrodynamic model for one component plasma along with coupled mode theory has been used to study parametric amplification due to polaron mode. Expressions for parametric gain coefficient arising due to parametric instability and threshold field required to incite parametric amplification has been derived. The compound semiconductors of group III-V and II-VI are unique within the universe of simple octet compounds, enable them to dominate higher performance electronics and optoelectronics. Present study aims to compare materials for which favourable magnitudes of parametric gain and threshold value could be obtained with suitable values of external parameters. Numerical estimations were carried out using the data of two different group compound semiconductors namely ZnSe and GaAs. Both the gain coefficients and threshold pump field are found to be strongly dependent on the carrier concentration of the medium. Resonance between plasma frequency and collective excitation frequency affects the process of amplification in both cases. Higher gain is achieved for GaAs which has smaller coupling coefficient as compared to ZnSe. Hybrid pump propagation is found to strengthen the electron-LO phonon coupling.

Classification of octet AB-type binary compounds using dynamical charges: A materials informatics perspective

The role of dynamical (or Born effective) charges in classification of octet AB-type binary compounds between four-fold (zincblende/wurtzite crystal structures) and six-fold (rocksalt crystal structure) coordinated systems is discussed. We show that the difference in the dynamical charges of the fourfold and sixfold coordinated structures, in combination with Harrison’s polarity, serves as an excellent feature to classify the coordination of 82 sp–bonded binary octet compounds. We use a support vector machine classifier to estimate the average classification accuracy and the associated variance in our model where a decision boundary is learned in a supervised manner. Finally, we compare the out-of-sample classification accuracy achieved by our feature pair with those reported previously.

Thermal conductivity of zinc blende and wurtzite CdSe nanostructures.

Many binary octet compounds including CdSe can be grown in either the wurtzite (WZ) or zinc blende (ZB) phase, which has aroused great interest among the research community in understanding the phase dependence of the thermal transport properties of these compounds. So far, it has been debatable whether the ZB phase possesses higher thermal conductivity than the WZ phase. In this work, we report on thermal conductivity measurements of CdSe nanowires/nanoribbons with both WZ and ZB phases via a suspended device method. At room temperature, the thermal conductivity of all the ZB CdSe nanostructures measured in this work is higher than the bulk thermal conductivity of the WZ CdSe reported in the literature, suggesting that the bulk thermal conductivity of the ZB CdSe is higher than that of the WZ phase. Our result is different from previous experimental results in the literature for InAs nanowires which suggest similar thermal conductivity values for the bulk ZB and WZ InAs crystals. The higher thermal conductivity of the ZB CdSe can be explained by its lower anharmonicity and a smaller number of atoms per unit cell compared to the WZ phase.

Extending the applicability of the Tkatchenko-Scheffler dispersion correction via iterative Hirshfeld partitioning.

Recently we have demonstrated that the applicability of the Tkatchenko-Scheffler (TS) method for calculating dispersion corrections to density-functional theory can be extended to ionic systems if the Hirshfeld method for estimating effective volumes and charges of atoms in molecules or solids (AIM's) is replaced by its iterative variant [T. Bučko, S. Lebègue, J. Hafner, and J. Ángyán, J. Chem. Theory Comput. 9, 4293 (2013)]. The standard Hirshfeld method uses neutral atoms as a reference, whereas in the iterative Hirshfeld (HI) scheme the fractionally charged atomic reference states are determined self-consistently. We show that the HI method predicts more realistic AIM charges and that the TS/HI approach leads to polarizabilities and C6 dispersion coefficients in ionic or partially ionic systems which are, as expected, larger for anions than for cations (in contrast to the conventional TS method). For crystalline materials, the new algorithm predicts polarizabilities per unit cell in better agreement with the values derived from the Clausius-Mosotti equation. The applicability of the TS/HI method has been tested for a wide variety of molecular and solid-state systems. It is demonstrated that for systems dominated by covalent interactions and/or dispersion forces the TS/HI method leads to the same results as the conventional TS approach. The difference between the TS/HI and TS approaches increases with increasing ionicity. A detailed comparison is presented for isoelectronic series of octet compounds, layered crystals, complex intermetallic compounds, and hydrides, and for crystals built of molecules or containing molecular anions. It is demonstrated that only the TS/HI method leads to accurate results for systems where both electrostatic and dispersion interactions are important, as illustrated for Li-intercalated graphite and for molecular adsorption on the surfaces in ionic solids and in the cavities of zeolites.

Coherence of the Even-Odd Rule with an Effective-Valence Isoelectronicity Rule for Chemical Structural Formulas: Application to Known and Unknown Single-Covalent-Bonded Compounds

Ions or molecules are said to be isoelectronic if they are composed of different elements but have the same number of electrons, the same number of covalent bonds and the same structure. This criterion is unfortunately not sufficient to ensure that a chemical structure is a valid chemical compound. In a previous article, a procedure has been described to draw 2D valid structural formulas: the even-odd rule. This rule has been applied first to single-bonded molecules then to single-charged single-bonded ions. It covers hypovalent, hypervalent or classic Lewis’ octet compounds. The funding principle of the even-odd rule is that each atom of the compound possesses an outer-shell filled only with pairs of electrons. The application of this rule guarantees validity of any single-covalent-bond chemical structure. In the present paper, this even-odd rule and its electron-pair criterion are checked for coherence with an effective-valence isoelectronic rule using numerous known compounds having single-covalent-bond connections. The test addresses Lewis’ octet ions or molecules as well as hypovalent and hypervalent compounds. The article concludes that the even-odd rule and the effective-valence isoelectronicity rule are coherent for known single-covalent-bond chemical compounds.

Crystalline intermetallic compounds in the K-Te system: The Zintl-Klemm principle revisited

We have investigated the crystal structure, chemical bonding, and electronic properties of all known intermetallic compounds in the K-Te system, employing a first-principles local-density-functional approach including generalized gradient corrections. The structural parameters, such as equilibrium volume, lattice constants, and internal parameters are in very good agreement with experiment. The evaluation of the total and angular-momentum decomposed densities of states (DOS) and the partial charge densities gives further insight into the bonding properties of these solids. Our results confirm the validity of the Zintl-Klemm principle for the saltlike compound ${\mathrm{K}}_{2}\mathrm{Te}$ and for the two equiatomic phases $\ensuremath{\alpha}\ensuremath{-}{\mathrm{K}}_{2}{\mathrm{Te}}_{2}$ and $\ensuremath{\beta}\ensuremath{-}{\mathrm{K}}_{2}{\mathrm{Te}}_{2}$ consisting of isolated K ions and covalently bounded ${\mathrm{Te}}_{2}$ dimers. In the compound ${\mathrm{K}}_{5}{\mathrm{Te}}_{3}$ ionic regions consisting of ${\mathrm{K}}^{+}$ and ${\mathrm{Te}}^{2\ensuremath{-}}$ ions with closed octet shells and polyanionic regions with an atomic arrangement dominated by ${\mathrm{Te}}_{2}^{2\ensuremath{-}}$ dianions coexist in the crystal lattice. In the electron-deficient-compound ${\mathrm{K}}_{2}{\mathrm{Te}}_{3},$ larger polyanions---${\mathrm{Te}}_{3}^{2\ensuremath{-}}$ trimers---have to be formed to achieve saturated covalent bonds. Our results illustrate the validity and astonishing flexibility of the Zintl-Klemm principle, which allows us to interpret all crystal structures in the K-Te system, from the saltlike octet compound to the Te-rich phases with extended polyanionic superstructures, on a common footing.

Structural and electronic properties of liquid K–Te alloys

Ab initio MD studies of the structural and electronic properties of K–Te alloys at three different stoichiometric compositions are presented. The tellurium-rich mixture K 0.12Te 0.88 is entirely determined by the Te atoms, with a structure and electronic properties that are very similar to those of pure liquid Te. The properties of equiatomic KTe at 770 K are determined by the strong iono-covalent bonding, but the order of the crystalline phase, consisting of Te 2 2− dumbbells in a matrix of K cations, is blurred to a very high degree. The strong coupling of the potassium cations to the Te system is also shown by the diffusion constant. In the octet compound K 2Te at 1300 K we found that the ionic character of the solid salt is preserved in the liquid. The electronic properties of both liquid mixtures, although modified by thermal disorder, show some similarities to the crystalline phases, especially a deep minimum at the Fermi edge, confirming the low conductivity values that are experimentally measured.

Structure and electronic states of liquid Na–Pb alloys by an ab initio molecular-dynamics simulation

We have carried out an ab initio molecular-dynamics simulation for the liquid Na 0.8Pb 0.2 and Na 0.5Pb 0.5 alloys to investigate the ionic structure, the electronic states and especially the existence of the so-called `Zintl ion' or `octet compound'. The existence of these chemical units is not confirmed, but rather the intermediate-range ordering of Pb ions is seen for the liquid Na 0.8Pb 0.2 alloy. It is found that this ordering of Pb ions leads to the first sharp diffraction peak of the observed structure factor. The composition dependence of the electronic states is explained on the basis of the ionic configuration.

Polyanionic and octet phases in the K-Sb system. I. Crystalline intermetallic compounds

The crystal structure, chemical bonding, and electronic properties of intermetallic compounds in the K-Sb system have been investigated using first-principle--local-density-functional calculations including generalized gradient corrections. It is shown that the chemical bonding obeys a generalized Zintl principle, i.e., a formally complete electron transfer from K to Sb. The stable crystal structure is determined for the stoichiometric octet compound ${\mathrm{K}}_{3}$Sb by the formation of an ionic lattice and at the equiatomic composition by the formation of covalently bonded ${\mathrm{Sb}}_{\ensuremath{\infty}}^{\ensuremath{-}}$ helices in close analogy to the isoelectronic chalcogen elements. At intermediate compositions the Sb atoms cluster together to form chainlike polyanion radicals, the electrons provided by the excess alkali metal serving to partially saturate the dangling bonds at the chain ends. It is demonstrated that density-functional theory describes the crystal structure of all compounds with high accuracy. The overbinding characteristic of the local-density approximation is most pronounced in the alkali-rich limit, but merely causes a scaling of all interatomic distances without distorting the structure. Gradient corrections substantially improve the prediction at large K content, but tend to overshoot in the Sb-rich range.

Zinc-blende-wurtzite polytypism in semiconductors.

The zinc-blende (ZB) and wurtzite (W) structures are the most common crystal forms of binary octet semiconductors. In this work we have developed a simple scaling that systematizes the T=0 energy difference \ensuremath{\Delta}${\mathit{E}}_{\mathrm{W}\mathrm{\ensuremath{-}}\mathrm{ZB}}$ between W and ZB for all simple binary semiconductors. We have first calculated the energy difference \ensuremath{\Delta}${\mathit{E}}_{\mathrm{W}\mathrm{\ensuremath{-}}\mathrm{ZB}}^{\mathrm{LDF}}$(AB) for AlN, GaN, InN, AlP, AlAs, GaP, GaAs, ZnS, ZnSe, ZnTe, CdS, C, and Si using a numerically precise implementation of the first-principles local-density formalism (LDF), including structural relaxations. We then find a linear scaling between \ensuremath{\Delta}${\mathit{E}}_{\mathrm{W}\mathrm{\ensuremath{-}}\mathrm{ZB}}^{\mathrm{LDF}}$(AB) and an atomistic orbital-radii coordinate R\ifmmode \tilde{}\else \~{}\fi{}(A,B) that depends only on the properties of the free atoms A and B making up the binary compound AB. Unlike classical structural coordinates (electronegativity, atomic sizes, electron count), R\ifmmode \tilde{}\else \~{}\fi{} is an orbital-dependent quantity; it is calculated from atomic pseudopotentials. The good linear fit found between \ensuremath{\Delta}${\mathit{E}}_{\mathrm{W}\mathrm{\ensuremath{-}}\mathrm{ZB}}$ and R\ifmmode \tilde{}\else \~{}\fi{} (rms error of \ensuremath{\sim}3 meV/atom) permits predictions of the W-ZB energy difference for many more AB compounds than the 13 used in establishing this fit. We use this model to identify chemical trends in \ensuremath{\Delta}${\mathit{E}}_{\mathrm{W}\mathrm{\ensuremath{-}}\mathrm{ZB}}$ in the IV-IV, III-V, II-VI, and I-VII octet compounds as either the anion or the cation are varied. We further find that the ground state of MgTe is the NiAs structure and that CdSe and HgSe are stable in the ZB form. These compounds were previously thought to be stable in the W structures.

Predictions and systematizations of the zinc-blende-wurtzite structural energies in binary octet compounds.

To systematize the wurtzite (W) versus zinc blende (ZB) structural preferences among the binary octet compounds, we have calculated the correponding energy difference \ensuremath{\Delta}${\mathit{E}}_{\mathit{W}\mathrm{\ensuremath{-}}\mathrm{ZB}}^{\mathrm{LDF}}$(AB) for thirteen AB compounds using the local-density formalism (LDF). We then uncovered a linear scaling betwteen \ensuremath{\Delta}${\mathit{E}}_{\mathit{W}\mathrm{\ensuremath{-}}\mathrm{ZB}}^{\mathrm{LDF}}$ and an atomistic orbital-radii coordinate R\ifmmode \tilde{}\else \~{}\fi{}(A,B) that can be simply calculated from the properties of the free A and B atoms. This permits predictions of W-ZB energy differences for all binary compounds and exposes simple chemical trends, including the stabilization of the ZB form in the sequence B=O\ensuremath{\rightarrow}S\ensuremath{\rightarrow}Se\ensuremath{\rightarrow}Te for ${\mathit{A}}^{\mathrm{II}}$${\mathit{B}}^{\mathrm{VI}}$ and A=Ga\ensuremath{\rightarrow}Al\ensuremath{\rightarrow}In for ${\mathit{A}}^{\mathrm{III}}$${\mathit{B}}^{\mathrm{V}}$'s. We propose new structural assignments for the low-temperature ground state of CdSe (ZB) and MgTe (NiAs type).

Electronic structure of alkali-pnictide compounds

The authors present an investigation of the electronic structure and the crystal binding in a number of metallic and semiconducting alkali-pnictide compounds based on self-consistent linear-muffin-tin-orbital (LMTO) calculations. They show that at all compositions the electronic structure is dominated by the strong attractive potential of the pnictide anions. In compounds with the 'octet' composition A3B (A=alkali metal, B=Bi, Sb) they find a narrow gap separating the highest occupied anion band from the lowest empty cation band. The large difference in valence leads to a very large negative excess volume and a high electronic pressure on the alkali sites. This large electronic pressure leads to a lowering of the (n-1)d states relative to the ns states, especially for the heavy alkalis. As a consequence, the ionic gap in the octet compounds is very narrow and varies in a non-monotonic way in the series (Li, Na, K, Rb, Cs)3-(Sb, Bi). At the equiatomic composition, the alkali-Sb compounds contain infinite spiral Sb chains stabilized by strong (pp sigma ) interactions. The bands close to the Fermi level are formed by bonding, non-bonding, and antibonding Sb p states, with the Fermi level falling into the gap between the non-bonding and the antibonding band. The Bi-rich alkali-Bi compounds are metallic, but the electronic density of states still bears the signature of the strong Bi potential.

The electrical resistivities of liquid Li-Ge alloys

Resistivities of liquid Li-Ge alloys have been measured in the composition range from 17 to 70 at% Ge. Plotted as a function of composition the resistivity exhibits a sharp peak at 80 at% Li. This is considered as an indication for the formation of a liquid octet compound. Around that composition the values of the resistivity indicate a diffusive motion of electrons and the temperature derivative of the resistivity has a sharp negative minimum. This behaviour is compared with that of liquid Li-Si, Li-Sn and Li-Pb. The results are also related to earlier data.

Electrical resistivities of liquid Na-Bi and Rb-Bi alloys

The electrical resistivities p of liquid Na-Bi and Rb-Bi alloys have been measured as functions of composition and temperature. Plotted as a function of composition the resistivity of Na-Bi exhibits a maximum at the composition corresponding to Na3Bi while for Rb-Bi two maxima, at 25 and 40 at.% Bi, can be discerned. In both alloys, the maximum value of rho is far beyond the metallic range. In the composition ranges around the maxima the temperature dependence d rho /dT of the resistivity is negative and -d rho /dT attains extraordinarily large values. The results can be explained by assuming that for Na-Bi a strong octet compound is formed whereas for Rb-Bi both an octet and a clustered compound are formed. Comparison with the other alkali-Bi alloys shows that in the sequence from Li-Bi to Cs-Bi a transition from a simple octet configuration to a clustered configuration occurs.

Formation of zintl-ions in alkali-lead alloys

In the last years the liquid alkali-Pb alloys have been studied extensively. The measurements made include resistivity, structure factor, and the specific heat capacity. These alloys show anomalous behavior at either the octet composition with 80 at. % alkalimetal or at the equiatomic composition. From the resistivity measurements follows that a shift in stoichiometry occurs between Na-Pb and K-Pb. Whereas liquid Na-Ph, just as Li-Ph forms an octet compound, the K-Pb, Rb-Pb and Cs-Pb systems form only an equiatomic compound. In this paper we present the results of neutron diffraction and specific heat measurements. Together they constitute evidence for the formation of (Pb{sub 4}){sup 4-}-ions in liquid K-Pb, Rb-Pb and Cs-Pb.

Resistivity of liquid K-Bi and Cs-Bi alloys

The authors have measured the electrical resistivity of liquid K-Bi and Cs-Bi alloys as a function of composition and temperature. In both alloy systems strong indications for chemical compound formation were found: in a large composition range the resistivity attains values that are characteristic for Mott's 'regime of diffusive motion'. In K-Bi the octet compound K3Bi dominates. In Cs-Bi at 550 degrees C the resistivity reaches a maximum of more than 4000 mu Omega cm at a composition of 40 at.% Bi (T=550 degrees C). This is explained by assuming that the Bi ions form poly-anions (Zintl ions), quite possibly chain fragments Bin(n+2)- of variable length. The equi-atomic liquid alloy CsBi displays the unusual feature of a high resistivity (2000 mu Omega cm) combined with an almost negligible temperature dependence.

Hardness as a Factor Influencing the Coordination Number in Binary Octet Compounds of Nontransition Metals

Hardness as a Factor Influencing the Coordination Number in Binary Octet Compounds of Nontransition Metals

Electronegativity and hardness as coordinates in structure stability diagrams.

With electronegativity and hardness of an atom defined as 1/2(I + A) and 1/2(I - A), respectively, where I and A are the ionization potential and electron affinity, electronegativity difference and hardness sum are proposed as coordinates in structure stability diagrams. With these coordinates a successful topological classification of the crystal structures of octet and suboctet binary compounds is obtained, and a clear delineation of the structural classes portraying chemical periodicity is found.