I-VII Compounds Research Articles

A Method for Predicting the Melting Temperature of Ionic Compounds.

Predicting the melting temperature of materials has always been a topic of great concern. This article proposes an alternative model for determining the melting temperature of materials based on the main idea of the Lindemann melting criterion combined with the first-principles calculations of density functional theory. To verify the accuracy of the melting model, this article selected typical ionic crystals of MgO and 10 alkali metal halides as the validation objects. The calculation results indicate that the melting temperature of the MgO crystals and I-VII compounds is in good agreement with the experimental results.

Density functional and Boltzmann transport investigation on mechanical, thermoelectric and optical properties of bilayer I-VII compounds

Density functional and Boltzmann transport investigation on mechanical, thermoelectric and optical properties of bilayer I-VII compounds

Anionic character of the conduction band of sodium chloride

The alkali halides are ionic compounds. Each alkali atom donates an electron to a halogen atom, leading to ions with full shells. The valence band is mainly located on halogen atoms, while, in a traditional picture, the conduction band is mainly located on alkali atoms. Scanning tunnelling microscopy of NaCl at 4 K actually shows that the conduction band is located on Cl− because the strong Madelung potential reverses the order of the Na+ 3s and Cl− 4s levels. We verify this reversal is true for both atomically thin and bulk NaCl, and discuss implications for II-VI and I-VII compounds.

Structural, vibrational, electronic, elastic and thermoelectric properties of monolayer alkali halide compounds from first principles investigation

A detailed analysis of structural, vibrational, electronic, elastic and thermoelectric properties for sixteen hexagonal monolayers which belongs to family of group I-VII compounds (alkali halides) are performed with first-principles calculations using density functional theory (DFT) and semi-classical-Boltzmann transport equations (BTE). The thermoelectric properties of monolayer alkali halides is reported for the first time. The frequency dependent phonon dispersion relations indicate the dynamical stability of all the considered group I-VII hexagonal monolayers, namely, MX (M=Li, Na, K, Rb and XF, Cl, Br, I) in their planner structure. Along with phonon dispersion relations, we also investigated their mechanical stability by calculating elastic properties. The obtained elastic parameters and polar diagrams of Young’s modulus and Poisson’s ratio confirms mechanical stability of all the considered systems. All the monolayers alkali halides are found to possess wide indirect energy bandgaps ranging between 3.91 and 6.74 eV, which indicates that these compounds are active in the UV region of electromagnetic spectra. In order to gain more insight on I-VII MX compounds, we also obtained their effective mass, deformation potential, relaxation time and mobility for electron and holes along armchair and zigzag directions. Finally, we computed the thermoelectric properties for all considered hexagonal monolayers (MX) with respect to temperature. The investigated thermoelectric properties include electrical conductivity (σ), thermal conductivity (k), Seebeck coefficients (S) and figure of merit (ZT). Among all the MX hexagonal monolayers, LiI, KBr, KI and RbCl observed to exhibits outstanding thermoelectric properties for a wide range of temperature (200–800 K). The present investigation certainly reports the merits of I-VII MX monolayers as possible materials for future thermoelectric devices.

Application of time-dependent density-functional theory to the dielectric function of various nonmetallic crystals

The dielectric function of a range of nonmetallic crystals of various lattice types is studied by means of a real-space and full-potential time-dependent density-functional method within the adiabatic local-density approximation. Results for the dielectric constant e ‘ ~at optical frequencies! are given for crystals in the sodium chloride, the fluoride, the wurtzite, the diamond, and the zinc-blende lattice structure. The frequency-dependent dielectric function e(v) for the crystals in the diamond and zinc-blende lattice structure are also presented. We compare our calculated results with experimental data and other theoretical investigations. Our results for the dielectric constants e ‘ and the dielectric functions e(v) are in good agreement with the experimental values. The accuracy of the results is comparable to the one which is commonly found for time-dependent densityfunctional theory calculations on molecular systems. On average we find a deviation of 4‐5 % from experiment for the group IV and III-V compounds in the wurtzite, zinc-blende and diamond lattice structure, 8‐9 % for the II-VI and I-VII compounds in the zinc-blende and sodium chloride lattice structure, and up to 14% deviation for the fluoride lattice structure. The spectral features of the dielectric functions e(v) appear in the

Transverse Effective Charges of ANB8-NCompounds with Zincblende Structure

The transverse effective charge Z T of A N B 8- N compounds is expressed as a function of the Phillips ionicity. The chemical trend of Z T for zincblende A ∣ N B ∣8- N compounds is calculated on the basis of a lattice-distortion model where atoms are assumed to be slightly displaced. The calculated Z T is compared with the experimental Z T for III-V, II-VI, and I-VII compounds. It is found that the theoretical Z T well reproduces the observed chemical trend of Z T for these compounds.

Dynamic displacement anisotropy of atoms, streamer discharges and phonon focusing in BeO and CdSe crystals and I-VII compounds

Streamer discharges at 77 K and crytallographically oriented surface spark breakdown at 295 K have been obtained in BeO crystals. An inverted temperature dependence has been found for the breakdown anisotropy compared with that in CdSe crystals. It is shown that the temperature dependences of the excitation thresholds of the incomplete electric breakdown at different crystallographic orientations in dielectrics as well as those of the streamer discharges in hexagonal semiconductors are controlled by the anisotroply of ionic dynamic displacements. The directions of maximum phonon focusing have been calculated in the localoisation planes of the incomplete breakdown in NaCl, KBr and LiF crystals at room temperature in accordance with well-known breakdown model along phonon streams. It is shown that the alkali halide breakdown data do not agree with this model. An anisotropy is predicted of the dynamic displacement of atoms in alkali halide crystals as well as in the basis plane and mirrorsymmetric directions of planes containing the crystallographic c-axis in hexagonal II–VI compounds, which allows us to explain the observable particularities of the discharge anisotropy in both cubic and hexagonal crystals.

High-pressure polymorphism of the copper(I) halides: A neutron-diffraction study to ~10 GPa.

The structural changes within the copper(I) halides induced by hydrostatic pressure have been investigated using the powder neutron-diffraction technique. The expected transition from the ambient zinc-blende structure with tetrahedral coordination to the octahedrally coordinated rocksalt structure is observed in both CuCl and CuBr. No rocksalt phase is observed in CuI, presumably due to the limited maximum pressure of our apparatus (\ensuremath{\sim}10 GPa). The lower symmetry phases which occur as intermediate structures between the zinc-blende and rocksalt phases have been studied in detail. CuCl and CuI undergo abrupt transitions whilst CuBr has three sluggish transitions, involving extensive coexistence of neighboring phases. There is one intermediate phase in CuCl (CuCl-IV), two in CuBr (CuBr-IV and CuBr-V) and at least two in CuI (CuI-IV and CuI-V). Three different structure types have been identified, in space groups Pa3\ifmmode\bar\else\textasciimacron\fi{} (CuCl-IV and CuBr-V), P4/nmm (CuBr-IV and CuI-V) and R3\ifmmode\bar\else\textasciimacron\fi{}m (CuI-IV). The intermediate structures all retain the tetrahedral cation coordination of the ${\mathrm{Cu}}^{+}$, though the environment is rather more distorted in CuCl and CuBr than in CuI. The evolution of the structures of these I-VII compounds is very different from that observed in the more covalent II-VI and III-V systems.

Heteroepitaxy of I-VII materials on III-V substrates

We predict, on the basis of ab initio total energy calculations, that epitaxial growth of I-VII compounds on III-V substrates can be accomplished. We suggest specific combinations of I-VII materials and III-V substrates that minimize lattice mismatch and structural energy cost and show that the interface dipole can be minimal. This makes zinc-blende I-VII materials potential candidates for passivating layers, solid-state laser applications, III-V window material, and hole traps.

An Insulating Overlayer for GaAs

We predict, on the basis of ab-initio total energy calculations, that epitaxial growth of I-VII compounds on III-V substrates can be accomplished (see Fig. 1). We suggest specific combinations of I-VII materials and III-V substrates that minimize lattice mismatch and structural energy cost and show that the interface dipole can be minimal (see Table I). This makes zinc-blende I-VII materials potential candidates for passivating layers, solid state laser applications, III-V window material and hole traps. The work is published in Reference [1].

Landolt‐Börnstein. Numerical data and functional relationships in science and technology. New Series. Group III: Crystal and Solid State Physics. Vol. 22: Semiconductors. Subvolume a: Intrinsic Properties of Group IV Elements and III‐V, II‐VI and I‐VII Compounds. Ed. by O, Madelung Springer‐Verlag Berlin‐Heidelberg‐New York‐London‐Paris‐Tokyo 1987. XII + 451 pp. Hard cover DM 1120.—, ISBN 3–540–16609–2

Crystal Research and TechnologyVolume 23, Issue 10-11 p. 1360-1360 Book Review Landolt-Börnstein. Numerical data and functional relationships in science and technology. New Series. Group III: Crystal and Solid State Physics. Vol. 22: Semiconductors. Subvolume a: Intrinsic Properties of Group IV Elements and III-V, II-VI and I-VII Compounds. Ed. by O, Madelung Springer-Verlag Berlin-Heidelberg-New York-London-Paris-Tokyo 1987. XII + 451 pp. Hard cover DM 1120.—, ISBN 3–540–16609–2 P. Paufler, P. PauflerSearch for more papers by this author P. Paufler, P. PauflerSearch for more papers by this author First published: October ‐ November 1988 https://doi.org/10.1002/crat.2170231029Citations: 12AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume23, Issue10-11October ‐ November 1988Pages 1360-1360 RelatedInformation

Terms linear in k in the band structure of zinc-blende-type semiconductors.

A calculation of the coefficient ${C}_{k}$ of the terms linear in $\mathrm{k}$ in the ${\ensuremath{\Gamma}}_{8}$ valence bands of several III-V semiconductors is presented. On the basis of the systematics of these results it is concluded that these terms arise from coupling by the linear momentum and the spin-orbit operators to the uppermost $d$ levels of the core. A simple analytic expression to calculate ${C}_{k}$ for all III-V, II-VI, and I-VII compounds with zinc-blende structure is proposed. The results are in good agreement with the few experimental data available.

Microscopic theory of covalent-ionic transition of amorphizability of nonmetallic solids

Under heavy-ion bombardment, predominantly covalent ${A}^{N}{B}^{8\ensuremath{-}N}$ crystals become amorphous while predominantly ionic crystals do not. The critical ionicity (Phillips scale) which separates these two classes in ${f}_{i}^{\mathrm{ac}}=0.41$, so that AIN (${f}_{i}=0.45$) is grouped with II-VI and I-VII compounds as nonamorphizable. Several thermomechanical defect models are discussed and are found to give values of ${f}_{i}^{\mathrm{ac}}$ which are too high. I conclude that nonamorphizability is the result of recombination-enhanced defect annealing. If this is the case, then InN is predicted to be amorphizable, even though its Phillips ionicity is high [${f}_{i}(\mathrm{InN})=0.58$].

Experimental gradient elastic tensors: Measurement in I-VII semiconductors and the ionic contribution in III-V and I-VII compounds

Nuclear-acoustic-resonance measurements have determined the magnitudes and relative signs of the two nonzero tensor components, ${S}_{11}$ and ${S}_{44}$, of the tensor $S$ which relates elastic strain to electric field gradients at a nuclear position of $^{63}\mathrm{Cu}$$^{35}\mathrm{Cl}$, $^{63}\mathrm{Cu}$$^{79}\mathrm{Br}$, and $^{63}\mathrm{Cu}$$^{127}\mathrm{I}$. The measured magnitudes of ${S}_{11}$ in these I-VII compounds and in seven III-V compounds are shown to agree with calculated values of an ion-charge model based on free-atom values. Such agreement is found if ion charges characteristic of covalent bonding are used for the III-V compounds and ion charges characteristic of ionic bonding are used for I-VII compounds.

Effect of Pressure on the Absorption Edges of Some III-V, II-VI, and I-VII Compounds

The effect of pressure to 160 kilobars was measured on the absorption edges of the III-V compounds AlSb, GaSb, InP, and InAs, the II-VI compounds CdS, CdSe, and CdTe, and the I-VII compounds CuCl, CuBr, and CuI. It is possible to discuss the band structure of the III-V compounds with reasonable assurance relative to known group IV and III-V structures. For the II-VI and I-VII compounds, ionic and other effects must be important. A number of new phase transitions were noted at high pressure. For CuCl and CuBr the $T\ensuremath{-}P$ curves of some of these transitions were established.