Vapor phase transport and stoichiometry control of cadmium sulfo-selenide

Cadmium sulfo-selenide Cd_1+y(S_xSe_1?x) single crystals were grown by the sublimation method under controlled partial pressures of sulfur and selenium. The partial pressure control was achieved by amounts of sulfur and selenium introduced in a closed growth tube. To control the deviation from stoichiometry y for the specified compositions x, the sum of chalcogen partial pressures Pps(=p_S2+P_Se2) was regulated, holding the partial pressure ratio () constant. The comp osition x of single crystals, grown with appreciable growth rate, was found to be the same one as the source specimen but the electrical conductivity changed extremely from 1 to 10^-12 (ω cm)^-1 with increasing Pps. Hall effect and thermally stimulated current (TSC) were also measured on the high and low conductivity specimens respectively and found to depend upon the controlled Pps. From these experimental results, it was confirmed that the deviation from stoichiometry y was effectively controlled. The transport rate was also precisely measured as a function of controlled Pps.     

Neutron scattering from vitreous silica I. The total cross-section

The paper is the first in a series of five reporting a comprehensive set of neutron diffraction experiments on vitreous silica. A theoretical foundation is presented for the scattering of neutrons by amorphous solids and a measurement of the total cross-section of vitreous silica is described in the energy range 1 to 1660 meV. The data are consistent with a very small incoherent scattering cross-sectio( (δ^inc_SiO2 < 0.1 barn) and have been Fourier transformed to yield the real space radial density function R(r). Values obtained for the $\text{Si-O}\text{[(1.62 ± 0.01)}\text{A}\text{?}\text{]}$ and $\text{O—O [(2.64±0.01}\text{A}\text{?}\text{]}$ distances are in good agreement with the results of conventional diffraction experiments. The results demonstrate the usefulness of total cross-section structural measurements, which could prove particularly valuable for kinetic studies or in the case of hostile sample environments (e.g. very high temperatures and pressures).     

A comparison of the structures of vapour-deposited and bulk arsenic sulphide glasses

Accurate comparative measurements have been made of the X-ray diffraction pattern of a bulk arsenic sulphide glass prepared from the melt and a 250 μm film prepared by vapour deposition of the evaporated bulk. By using Ag Kα radiation, measurements have been made up to $\text{Q = 21}\text{A}\text{?}{}^{\mathrm{?1}}\text{(Q = 4π}\text{sin}\text{θ/λ)}$ so as to give good spatial resolution. Significant differences are found between bulk and film specimens, and in particular a shoulder is found on the high r side of the first peak in the distribution function for the film. Also, the film is richer in arsenic than the parent bulk. The results are compared in detail with a number of models, all of which involve covalent bonding requirements being fully satisfied, and it is concluded that there are more AsAs bonds in the film than the minimum required by stoichiometry and that a significant fraction ($\text{≈}\text{2}\text{3}$) of these are unusually long ($\text{≈2.6}\text{A}\text{?}$ compared to the more usual 2.5 Å). The most probable explanation is that the vapour contains a significant fraction of As₄S₄ species (in which the AsAs distances are $\text{≈2.6}\text{A}\text{?}$) and that the film contains a mixture of molecular species which have undergone considerable polymerization. The extent to which this results in a sheet-like structure similar to that proposed for the bulk glass remains unclear.     

Small polaron conduction in V2O5P2O5 glasses

Dc conductivity measurements have been made between 90 and 520 K on three bulk samples of V₂O₅P₂O₅ glass. Heat treatment is found to result in a reduction of the activation energy at a given temperature and this is most noticeable at low temperatures. The behaviour at low temperatures can be described using Mott's variable range hopping arguments, and at high temperatures by non-adiabatic small polaron hopping between nearest neighbours. At intermediate temperatures a simple model is used in which excitations by optical and acoustic phonons are considered to make independent contributions to the jump frequency. Mott's theory is extended to the polaron case for $\text{T>}\text{1}\text{4}\text{?}$ and is shown to be in good agreement with results. Values for $\text{r}{}_{\mathrm{<mtext>p</mtext>}}\text{(～2.8}\text{A}\text{?}\text{)}$ the polaron radius and $\text{α(～3.5}\text{A}\text{?}{}^{\mathrm{?1}}\text{)}$ the electron decay constant are shown to be consistent with the model for small polarons. A method is suggested for obtaining α and N(E_F) from the ac conductivity and the slope of 1nσ versus $\text{1}\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ at low temperatures. Values of N(E) are obtained which correlate with those obtained by the previous analysis. This implies that the disorder energy separating adjacent sites Δ₀ is large (～0.4 eV) in these materials.     

Temperature variation of the mobility gap in non-polar amorphous semiconductors

For a non-polar amorphous semiconductor such as a-Si, we derive an explixit formula for (?E_g/?T)_V, the derivative with temperature of the mobility gap E_g at constant volume V. Within the framework of second-order perturbation theory for the electron-phonon (eφ) interaction, many of our physical assumptions are fundamentally different from those that apply to the crystal phase. The principal ingredients of our model are: (1) the random-phase-model (RPM); (2) the principle of non-conservation of particle momentum in the eφ interaction; and (3) the deformation potential approximation. Narrowing of E_g is found with increasing values of the temperature T. At very low T, we have $\text{(?E}{}_{\mathrm{<mtext>g</mtext>}}\text{/?T)}{}_{\mathrm{<mtext>V</mtext>}}\text{≌ ? ¢A · c}{}_{\mathrm{V}}\text{(T)}$, where c_V(T) is the average lattice specific heat per mode at constant volume and ¢A is a positive dimensionless quantity in the model. By contrast with low-temperature behavior of the crystal, this result implies that the mobility gap at constant volume dynamically responds to the phonomic “gas” of the disordered lattice. The high-T limit yields behavior quite similar to that of the crystal phase. We find $\text{(?E}{}_{\mathrm{<mtext>g</mtext>}}\text{/?T)}{}_{\mathrm{V}}\text{≌ ? x ¢A · k}{}_{\mathrm{<mtext>B</mtext>}}$, where k_B is Boltzmann's constant and the parameter x, expected to be confined to the interval $\text{1}\text{2}\text{? x ? 1}$, measures the admixture of the optical-phonon and acoustical-phonon coupling strengths.     

Retarded elasticity in B2O3GeO2 glasses

The retarded elasticity was investigated for B₂O₃GeO₂ glasses having network structure in the glass transition range by using a compressive method. The compliance (J_∞) determined at the final stage of each measurement displayed a maximum for roughly constant viscosity (η ? 10¹⁴ P) in all the B₂O₃GeO₂ glasses and was simulated by the same equation applied for AsS glasses reported in a previous paper [1].

$\text{J}{}_{\mathrm{\infty }}\text{=(1?k}{}_2\text{keta;}{}_{\mathrm{G}}\text{keta;}\text{)[?}{}_1\text{(}\text{k}{}_1\text{keta;}\text{)+?}{}_2\text{(}\text{nk}{}_1\text{keta;}\text{)]}$

, where $\text{K}{}_1\text{, k}{}_2\text{, ?}{}_1\text{, ?}{}_2\text{and}\text{n}$ are parameters and η_G is the viscosity related to the retarded elasticity. The terms (k₁/η) and (nk₁/η) are assumed to be equal to one for all their values exceeding one. For B₂O₃GeO₂ glasses, the deformation due to the retarded elasticity could be alloted to two structural elements: the first element related to the term $\text{?}{}_1\text{(k}{}_1\text{/η)}$ and the second element related to the term $\text{?}{}_2\text{(nk}{}_1\text{/η)}$. The values of $\text{?}{}_1$ showed almost no variance with the glass composition, but $\text{?}{}_2$ had a minimum at the composition of 50 mol% GeO₂. These data suggest that the contribution of the second element is the smallest at B₂O₃/GeO₂ = 1. The values of k₂ were close to that of As₂S₃ glass having the network structure. k₁ and n (or nk₁) were almost constant regardless of the composition, respectively. These data suggest that the inhibition due to the viscosity starts at an approximately constant viscosity in B₂O₃GeO₂ glasses.     

LPE growth kinetics of CaGe-Substituted EuTm2Fe5O12 garnet films

The diffusion-reaction growth melts fluxed with PbO-B₂O₃-Fe₂O₃, has been extended to include a thermally-activated diffusion term. The fluxed melt had a lower PbO : B₂O₃ ratio (11 : 1) than that generally used (≈16 : 1) for garnet LPE films. Measured growth rates for LPE films, produced from melts with four different concentrations of CaGe-Substituted EuTm₂Fe₅O₂ garnet, fit the model with: diffusivity, $\text{D (cm}{}^2\text{/s) = 0.06 exp(}\text{-1.0 eV}\text{kT}\text{)}$; reactivity, $\text{K (cm/s) = 8000 exp(}\text{-1.61 eV}\text{kT}\text{)}$; viscosity, $\text{v (}\text{cm}{}^2\text{s}\text{) = 0.01}$; and concentration, $\text{c}{}_{\mathrm{e}}\text{(}\text{g}\text{cm}{}^3\text{) = 4220 exp ?}\text{1.0678 eV}\text{kT}$ for rotation rates ω (rpm) = 36 to 196 in the growth temperature range 890 to 965°C. It is shown that the garnet melt concentration terms are the most critical ones in determining the growth rate, hence the activation energy (heat of solution) for the equilibruim concentration is reported to five significant figures. Evaluating the D and the K effects by both a numerical (results given above) and an analytic method demonstrates that these parameters are least critical in determining the growth rate in this case.     

Editorial

NMR techniques are employed to determine the relative fractions of boron atoms in Na₂OB₂O₃SiO₂ glasses of high soda content. The data show that if enough Na₂O is added, four-coordinated borons are destroyed and borons with one or two non-bridging oxygens are created, but that both the beginning point and the rate of these processes depend strongly on the amount of silica present. These findings are shown to be quantitatively inconsistent with structural models previously suggested in the literature. Utilizing the concept of proportionate atomic sharing of the additional Na₂O, a new structural model is proposed for K K = mol% SiO₂/mol% B₂O₃) which is consistent with all the data including previously reported data for glasses in the region of relatively low soda content. Using R = mol% Na₂O/mol% B₂O₃, the new model states that for , all the additional Na₂O is employed in forming non-bridging oxygens on the silica tetrahedra; then for R_D1 R R_D3 = 2 + K, the fraction $\text{(K +}\text{1}\text{4}\text{K)/(2 + K)}$ of the additional Na₂O destroys reedmernerite groups and forms pyroborate units plus silica tetrahedra with two non-bridging oxygens per Si atom, while the fraction $\text{(2 ?}\text{1}\text{4}\text{K)/(2 + K)}$ of the additional Na₂O destroys diborate groups and forms additional pyroborate units.     

Description of chemical ordering in amorphous alloys

Partial coordination numbers $\text{Z}{}_{\mathrm{ij}}{}^1$ for a chemically disordered binary amorphous alloy A_XAB_XB are obtained in terms of total coordination numbers Z_i and average coordination numbers 〈Z〉 $\text{Z}{}_{\mathrm{ij}}{}^1\text{=x}{}_{\mathrm{j}}\text{Z}{}_{\mathrm{i}}\text{Z}{}_{\mathrm{j}}\text{/〈Z〉}$. Departures from complete chemical disorder are characterized by a short-range order coefficient with η_AB^max=x_BZ_B/x_AZ_A for X_BZ_B<x_AZ_A and η_AB^max=x_AZ_A/x_BZ_B for x_BZ_B>x_AZ_A.For complete chemical disorder ν_AB⁰ = 0; for complete chemical order, i.e. with chemical preference for AB nearest neighbor pairs, ν_AB⁰ = 1. For departures from chemical disorder associated with clustering, i.e. chemical preference against AB nearest neighbor pairs, ν_AB⁰ < 0. Experimental results for several amorphous alloys are used to demonstrate the usefulness of these results. A more precise calculation of η_AB^max is necessary to compare the degree of ordering in the metal-metalloid and metal-metal alloys.     

Partial correlations in Ni60Nb40 metallic glass

Combined X-ray and neutron diffraction experiments were performed on the Ni₆₀Nb₄₀ metallic glass samples prepared by rapid quenching from the melt with natural Ni^nat and isotope ⁵⁸Ni, respectively. The partial structure factors were separated for the three kinds of atomic pairs: NiNi, NiNb and NbNb. The partial distribution functions were calculated by means of Fourier transformation and the following atomic distances were obtained: $\text{r}{}_{\mathrm{<mtext>NiNi</mtext>}}\text{=2.52}\text{A}\text{?}\text{, r}{}_{\mathrm{<mtext>NiNb</mtext>}}\text{=2.72}\text{A}\text{?}$ and $\text{r}{}_{\mathrm{<mtext>NbNb</mtext>}}\text{=2.70}\text{A}\text{?}$. The values n_NiNi=7.3, n_NiNb=4.5, n_NbNi=6.8 and n_NbNb=5.4 were obtained for the number of nearest neighbours.     

Spinodal decomposition during continuous cooling of a PbOB2O3Al2O3 glass

Spinodal decomposition during continuous cooling of the PbOB₂O₃Al₂O₃ quasi-binary glass system was analysed by numerical integration of Cook's differential equation (which includes the contribution of random density fluctuations) for small angle X-ray scattering (SAXS) intensity. The SAXS curves derived from the calculations have a wide range of k-Fourier components (0 < k < k_c) for which a positive amplification factor occurs and they show a “crossover” point at $\text{k}{}_{\mathrm{c}}\text{= 0.155}\text{A}\text{?}{}^{\mathrm{?1}}$. The wavenumber which receives maximal amplification, k_m, increases with the cooling rate, Q, as $\text{Q}{}^{\mathrm{<mtext>1</mtext><mtext>n</mtext>}}$, with n = 10.9. This Q dependence of k_m is similar to that predicted by Huston et al., however our results show a higher value of n. The dependence on Q and k_m of the SAXS intensity I(k_m) was also deduced. The measurements of SAXS curves were performed on glass samples prepared by the splat-cooling technique. Because of the difficulties which arise in the determination of the cooling rate of the samples, the only experimental results that could be compared with the theory are the k_m dependence of I and the value of k_c. These results are satisfactorily understood in terms of the present analysis.     

Observation of chemical reaction and liquid structure of SGe by Raman spectroscopy

The measurements of the Raman spectra on liquid Ge_XS_1?X (0≤X≤0.1) were carried out. The A₁ mode of $\text{GeS}{}_{\mathrm{<mtext>4</mtext><mtext>2</mtext>}}$ (ca. 340 cm^?1) was observed in the liquids. The comparison with the spectra of liquid S suggests that Ge reacts on the polymeric species. During the chemical reaction of solid Ge and liquid S below ca. 500°C, the L.T.GeS₂ crystal growth was observed. The spectral features of the L.T.GeS₂ surprisingly resemble those of GeS glasses. The glass forming region was extended to X = 0.03 by careful rapid quenching. The overall compositional dependence of the Raman spectra of Ge_XS_1?X (0.03≤×≤0.341) was analysed in comparison with the spectra of the L.T. and the H.T. crystal and can be explained by the model that with increase of X(>ca. 0.2) the H.T. GeS₂-like spectra emerge abruptly from the L.T.GeS₂-like spectra in lower X. Standing on the model, the glassy structures in this system are discussed.     

Relation between electrical and optical gaps of amorphous semiconductors in the SiAsTe system

Electrical and optical properties of semiconducting SiAsTe glasses have been investigated. Compositional dependences of the properties in the Si_xAs_yTe_z system are examined as a function of atomic percentage x (or y, z) of one element with parameters of constant atomic ratio y/z (or x/z, x/y) of the other two elements. A pre-exponential factor σ₀ in the dc conductivity formula is estimated to be $\text{(2.1 ± 0.6) × 10}{}^4\text{(Ω ·}\text{cm}\text{)}{}^{\mathrm{?1}}$, inependently of the compositions. A systematic relationship between the compositional changes in the electrical gap E_g(el) and optical gap E_g(op) has been found. The energy gaps increase linearly with increasing Si content and decreasing Te content, but are almost independent of As content. The relation between E_g(el) and E_g(op) is expressed by E_g(el) = 1.60 E_g(op) ? 0.15 in eV. On the other hand, the optical absorption coefficient α(Ω) near the band edge follows the empirical formula, $\text{α(Ω) = α}{}_0$ exp ($\text{h}\text{?}\text{Ω/E}{}_{\mathrm{<mtext>s</mtext>}}$). The experimentally determined factor E_s increases linearly with E_g(op) and is closely related to the energy difference between the two gaps. A tentative model to explain these experimental results is proposed by taking into account of the effect of the potential fluctuations in such disordered materials.     

Kinetics of ion exchange in glasses

The kinetics of $\text{K}{}^{\mathrm{+}}\text{?}\text{Na}{}^{\mathrm{+}}$ exchange in two glass systems, 20Na₂O·(60?x)B₂O₃· (20 + x)Si₂ (where x = 0, 15, 30 and 45 mol%) and Na₂O·3SiO₂, were studied as a function of glass composition, salt bath composition, exchange temperature and time The distribution of K in the glass specimens after exchange in molten KNO₃ was determined with an electron probe. Stresses in these speciments were measured photoelastically. The interdiffusion coefficient $\text{D}$ for ion exchange was calculated as a function of local composition in the glass using the Boltzmann-Matano method. The strong variation of $\text{D}$ in any particular glass approximated that predicted by a mixed alkali model (as advanced by Lacharme), where the glass in the ion-exchanged region approximates a composite of stacked layers of mixed alkali glasses with a gradually varying alkali ratio. The small discrepancy between the experiment and the mixed alkali model was partly, but not fully, reconciled by considering the strains in the glasses. The observation which remained unexplained was that the calculated stress profiles did not show perfect agreement, both in magnitude and in shape, with the experimentally measured stress profiles. It appeared that the kinetics of ion exchange in the glasses were also influenced by a network relaxation process which may have occurred well below the glass transition temperature.     

Intensity parameters and laser analysis of Pr and Dy in oxide glasses

Intensity parameters ($\text{Ω}{}_{\mathrm{\Lambda }}$) of Pr³⁺ and Dy³⁺ have been obtained in tellurite, borate and phosphate glasses. It has been found that $\text{Ω}{}_{\mathrm{\Lambda }}$ calculated by exlusion of hypersensitive transitions, gives a better fit between measured and calculated lifetimes and branching ratios than those including hypersensitive transitions. Using these parameters and calculated matrix elements U^(Λ), radiative transition probabilities, branching ratios and integrated cross sections for stimulated emission were calculated for ³P₀, ³P₁ and ¹D₂ excited states of Pr³⁺ and ${}^4\text{F}\text{9}\text{2}$ excited state of Dy³⁺. Potential transitions are indicated.     

Subject index

The structures of phosphorus-selenium glasses P_xPe_1?x with x=0 (pure selenium), 0.05, 0.15, 0.25, 0.40 and 0.50 have been investigated with neutron diffraction, extented, X-ray absorption fine structure measurements, and Raman and infrared spectroscopy. The behavior of the near-neighbor coordination at $\text{r～2.3}\text{A}\text{?}$ as a function of x suggests a picture in which P₄Se_n molecular units are linked together by Se atoms or chains. The diffraction patterns exhibit anomalous peaks at Q ～ 1.2Å which rise dramatically as P is added to the Se glass; these peaks can also be understood on a molecular picture. The Raman spectra at the larger P concentrations are characterized by series of relatively sharp peaks which also suggest molecular units and for x = 0.5 bear a close correspondence to the spectrum of crystalline P₄Se₃.     

Growth and morphology of 6H-SiC epitaxial layers by CVD

Single crystals of 6H-SiC were epitaxially grown on 6H-SiC substrates in the temperature range of 1500 to 1750°C with gas composition: H₂ ≈ 1 l/min, SiCl₄ ≈ 1 ml/min, C₃H₈ ≈ 0.05 ml/min. The grown layers were transparent and mirror-like. The morphology of the grown layer was strongly influenced by the polarity of the substrate surface. Aggregates of trapezoidal crystals were observed on the (000$\text{1}$)C surface and a mosaic pattern was observed on the (0001)Si surface. By observing the initial stage of the crystal growth, the growth mechanism of 6H-SiC is discussed. On (000$\text{1}$)C surfaces the vertical growth dominates, while on (0001)Si surfaces the lateral growth dominates.     

X-ray diffraction study of phase transitions in iron(II) trisnioximate hexadecylboronate clathrochelate complex

Crystals of the iron(II) nioximate hexadecylboronate clathrochelate complex—FeNx ₃(BHd)₂ [tris(μ-1,2-cyclohexanedionedioximato-O:O′)di-n-hexadecyldiborato(2?)-N,N′, N″,N?, N?,N″?]iron(II)—are investigated by differential scanning calorimetry and X-ray diffraction. Two structural phase transitions are revealed at T _cr1 = 290(3) K and T _cr2 = 190(3) K. The crystal structures of phases I, II, and III are determined by X-ray diffraction analysis at 303, 243, and 153 K, respectively. It is demonstrated that the I ? II phase transition is due to a change in the system of translations, and the II ? III phase transition is accompanied only by a jumpwise change in the unit cell parameters. The possible mechanisms of phase transitions are discussed in terms of geometry and molecular packing of FeNx ₃(BHd)₂ in all three phases.     

A new method of stirring for LPE growth

This paper describes a method of stirring a liquid during crystal growth in which each point on the growth face moves round a circle of radius b. If the face has a radius a and completes n revolutions per second, then the stirring is roughly equivalent to rotating the disc about a single axis at a rate $\text{2πc}{}^2\text{b}{}^2\text{n}\text{a}{}^2$ where c is a constant (c ∽ 2) but the system behaves as if only the liquid up to a distance $\text{(}\text{2v}\text{n}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ from the face is stirred. Uniform stirring occurs over the whole face except for an annulus of width 2b at the periphery. The necessary design criteria are discussed and a simple dynamically balanced apparatus and the results obtained with it whilst growing iron garnet films are described.     

Hydrogen permeation in the metallic glass Fe40Ni40P14B6

Hydrogen permeation from the gas phase through the metallic glass Fe₄₀Ni₄₀P₁₄B₆ (Metglas 2826) was measured with the electrochemical technique over 313 to 353K and 10⁵ to 10³ Pa. Specimen surfaces were coated with a thin layer of electrodeposited palladium to eleminate surface impedances to hydrogen entry and exit. Sievert's law behavior was observed over the temperature and pressure ranges studied. An excellent fit to the permeation transients as obtained from Fick's diffusion theory. Diffusivities derived from least squares fitting of permeation trasients were in good agreement with those obtained from time lag measurements. Within the concentration range studied, diffusivity was found to be independent of input hydrogen concentration. The expressions for the permeability coefficient, 2.24 × 10¹⁶ exp{[(48.99 ± 2.93) kJ/mol]/RT} atom H/ms $\text{Pa}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, the diffusivity 8.52 × 10^?7 exp{[(?47.07 ± 1.63) kJ/mol]/RT} m²/s, and the solubility, 2.62 × 10²²exp{[(?1.92 ± 4.56) kJ/mol]/RT} atom H/m³$\text{Pa}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, were determined.An annealing treatment at 573K for one hour decreases bot the permeability and diffusivity of hydrogen in the metallic glass.