Pressure-volume-temperature relations in liquid and glassy selenium

The PVT properties of amorphous selenium are studied experimentally and theoretically in the temperature range 0–70°C and for pressures up to 200 MPa. PVT surfaces are determined for the metastable liquid and for a glass formed by a pressurization and cooling procedure. Its liquid—glass intersection line $\text{T}{}_{\mathrm{<mtext>g</mtext>}}{}^2\text{(P)}$ is compared with the glass transition line T_g (P), here obtained by pressurizing the liquid isothermally at a rate of 2 MPa/min. Analytical expressions based on the PVT data are compared with the predictions of the Simha-Somcynsky hole theory originally formulated for open chainmolecular fluids. The agreement for the liquid, although satisfactory, is not as good as for amorphous organic polymers thus far studied, possibly because selenium contains both open-chain and Se₈ ring molecules. The theoretical scaling parameters for the best fit to experiment are compared with those obtained for polymers. A very high characteristic pressure and thus cohesive energy density are noted. The theoretical hole fraction is found to be nearly constant along the $\text{T}{}_{\mathrm{<mtext>g</mtext>}}{}^2\text{(P)}$ line for low pressures. For the glass, a theoretical equation of state obtained from that for the liquid by freezing the hole fraction, compares favorably with experiment. The Prigogine-Defray ratio ΔκΔC_p/TV(Δα)² at atmospheric pressure, calculated using literature values of ΔC_p, is found to be about 2.0.     

Doping behavior of sulfur during growth of GaAs from the vapor phase

A study has been made of sulfur doping from H₂S in the vapor growth of GaAs using the Ga-AsCl₃-H₂ system. In order to separate the effects of the gaseous components the AsH₃-HC1-Ga-H₂ system was also investigated. A linear relationship was found to exist between the measured electron concentrations and PH₂S over a wide range. The $\text{P}{}^{\mathrm{<mtext>-1</mtext><mtext>2</mtext>}}{}_{\mathrm{<mtext>AS</mtext>}}{}_4$ and P^-2_GaCl (or: P^-2_HCl or P^-1_GaCl or P^-1_HCl) dependence of the rate of sulfur uptake combined with the pronounced increase in this uptake upon approaching the exact <100 > orientation suggests that the process is kinetically limited. Some model considerations are presented, based on the hypothesis that the sulphur is competing for adsorption on As sites with the reactive components in the system.     

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.     

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.     

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.     

Ageing and crystallization of non-crystalline Se thin films: A tentative structural model

The crystallization of various types of amorphous selenium films prepared under rigorous control was studied by DTA, optical microscopy, X-ray diffraction and magnetic measurements. The results, particularly the observation of ageing during several years, made it possible to correlate the preparation of the films, the nature of the condensed species and the evolution below and above T_g: all the results are consistent with the assumption that the constituent species are essentially cyclic $\text{Se}{}_{\mathrm{n}}\text{(n ? 8)}$ with a distribution of n depending on the preparation.     

Electrical conductivity and photoconductivity of As2S3PbS glasses

Measurements of dc electrical conductivity and photoconductivity of various glassy compositions (x = 0.1?0.625) in (As₂S₃)_1?x(PbS)_x have been made. Experimental results of the temperature dependence of dc conductivity from room temperature to 200°C (which includes the glass transition temperature) are reported. All the compositions exhibit intrinsic conduction in the measured temperature range. Thermal activation energy, glass transition temperature and σ₀ for the compositions studied, were determined from the experimental data. The low value of $\text{σ}{}_0\text{(10?10}{}^{\mathrm{?2}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$) in these semiconducting glasses is attributed to the greater participation of localized states in the conduction process.In the measurements of photoconductivity, the variation of photocurrent with temperature, photon energy, light intensity and electric field is observed. The recombination model has been involved to explain the results of photoconductivity. Both electrical and photoconductivity data support the presence of higher density of localized states in the x = 0.1 composition than in others.     

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.     

Electrical properties of glassy AsxGe10Te90−x alloys

This paper analyses the electrical properties of glassy alloys of As_xGe₁₀Te_90?x, while reporting the conductivity and dielectric constant of As₅Ge₁₀Te₈₅ and As₁₅Ge₁₀Te₇₅ compositions in the temperature range 77–383 K and the frequency range from dc to 5 MHz. The dc conductivity has been shown to be of the form

The ratio σ₀₁/σ₀₂ is of the order of 10⁶. ΔE₁, the higher temperature activation energy, is dependent on the composition, while ΔE₂, the lower temperature activation energy, is less dependent on the composition. The dielectric constant has been found to be independent of temperature and frequency up to about 253 K. However, at higher temperatures, it becomes activated and proportional to log ω.Some common features of As_xGe₁₀Te_90?x are a kink in dc conductivity, a ω^0.8 relationship for ac conductivity, no evidence of variable-range hopping at low temperatures, field-dependent conductivity and memory switching. The data can be interpreted in terms of the dangling-bond theory of Mott and his collaborators. A high density of states of the order of 10²⁰eV^?1 cm^?3 near the Fermi level may be expected.     

Change of short-range order with temperature and composition in liquid GexSe1−x as shown by density measurements

Density measurements were performed on melts of the binary chalcogenide system Ge_xSe_1?x (0 ? x ? 0.5) up to 1000°C. The isotherm of molar volumes V_m at 750°C shows a relative maximum near GeSe₂. Molar volumes of the system behave linearly between GeSe and Se at 1000°C. V_m's of melts between x = 0.30 and x = 0.35 decrease at high temperatures on heating. The anomalous density behaviour of the melts clearly shows a change of short-range order from a less to a more densely packed structure, caused by the development of a pσ-bonding system. Within the composition range $\text{0 ? x ?}\text{1}\text{3}$ the short-range order at lower temperatures is determined mainly by $\text{GeSe}{}_{\mathrm{<mtext>4</mtext><mtext>2</mtext>}}$ tetrahedra linked directly corner-to-corner or via Se atoms. At higher temperatures pσ bonds arise more and more, even in melts rich in selenium. Within the composition range $\text{1}\text{3}\text{? x ? 0.5}$ the short-range order is mainly determined by a distorted octahedral configuration, even at lower temperatures. From the short-range orders of melts and from crystalline structures of GeSe₂ and GeSe, the tendency of glass formation from the melt is discussed in detail.     

Non-radiative energy transfer from trivalent dysprosium to praseodymium in calibo glass

Non-radiative energy transfer from trivalent dysprosium to praseodymium in calibo glass has been observed by measuring the decay time and emission intensity of Dy³⁺ with varying Pr³⁺ concentration and found to occur from the ${}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ level of Dy³⁺ to various levels of Pr³⁺. The interaction mechanism of donor (Dy³⁺) and acceptor (Pr³⁺) ions is found to be electric dipole-quadrupole and due to this transfer is weak. Various important parameters such as the critical transfer distance R₀ transfer efficiency P_da, and transfer probability have been computed.     

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.     

Nucleation kinetics of sodium disilicate

The kinetics of crystal nucleation in Na₂O · 2SiO₂ have been determined over the range of undercoolings between 173 and 373°C. The plot of log(I_v?) versus $\text{1}\text{ΔT}{}^2{}_{\mathrm{r}}\text{T}{}^3{}_{\mathrm{r}}$ is a straight line of negative slope over some 13 orders of magnitude in I_v. The slope of this relation indicates a nucleation barrier of about 45 kT at ΔT_r = 0.2, and the intercept at $\text{1}\text{ΔT}{}^2{}_{\mathrm{r}}\text{T}{}^3{}_{\mathrm{r}}\text{= 0}$ is 10²⁶ cm^-3 sec^-1. poise. The results are in good agreement with predictions of the theory of homogeneous nucleation, even in the pre-exponential factor.     

Energy transfer in Eu3+-Yb3+ and Tb3+-Yb3+ system in glasses

The behavior of the phonon-assisted energy transfer between trivalent rare-earth ions in glasses was investigated. The ions Eu³⁺ and Tb³⁺ as energy donors and Yb³⁺ as acceptor were selected. The energy gap between the levels of the donor and acceptor was estimated on the basis of the energy diagram of each ion determined from absorption and emission spectra. The probability for the transfers of (Eu, ⁵D₀-⁷F₆): (Yb, ${}^2\text{F}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{-}{}^2\text{F}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$) and (Tb, ⁵D₄-⁷F₀): (Yb, ${}^2\text{F}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{-}{}^2\text{F}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$) in silicate, borosilicate, phosphate and germanate glasses was measured in the temperature range of liquid-nitrogen temperature - 650K. The probability of transfer was the smallest in phosphate glass and B₂O₃ had the effect of increasing it. In germanate glass the dependence of the probability of the energy gap was relatively weak. These results were correlated to the difference in the phonon energy and the strength of the electron-lattice coupling in each glass.     

A structural investigation of calcium fluorosilicate glasses

The structure of x CaF₂(1?x)CaOSiO₂ glasses was investigated by X-ray photoelectron, F K_α emission X-ray and infrared absorption spectroscopies and ionic refraction of fluorine. A maximum in F_1s binding energy and a minimum in chemical shift of F K_α X-ray were found in the region of 7–10 mol% CaF₂, indicating that the state of fluorine changes. The ionic refraction of fluorine increased with increasing CaF₂ content up to 7 mol% CaF₂ and was nearly constant for compositions with more than 7 mol% CaF₂. There was practically no change in $\text{Ca}{}_{\mathrm{<mtext>2</mtext><mtext>p</mtext><mtext>(</mtext><mtext>3</mtext><mtext>2</mtext><mtext>)</mtext>}}$ binding energy for compositions with less than 7 mol% CaF₂. These data suggest that the fluorine ion bonds with a silicon predominantly for a CaF₂ content of less than 7 mol% and bonds with a calcium ion for increased CaF₂ content. Further, partial charges of fluorine, calcium and silicon, calculated by the modified Sanderson method could explain the chance of F_1s and $\text{Ca}{}_{\mathrm{<mtext>2</mtext><mtext>p</mtext><mtext>3</mtext><mtext>2</mtext>}}$ binding energies with composition. It is suggested that when the CaF₂ content is more than 7 mol%, the coordination number of fluorine increases and that of calcium decreases with increasing CaF₂ content.     

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.     

Prediction of glass-forming ability for metallic systems

The relative glass-forming ability (GFA) of metallic alloys is considered in terms of a parameter $\text{ΔT}{}^1\text{= (T}{}_{\mathrm{<mtext>liq</mtext>}}{}^{\mathrm{<mtext>mix</mtext>}}\text{? T}{}_{\mathrm{<mtext>liq</mtext>}}\text{)/T}{}_{\mathrm{<mtext>liq</mtext>}}{}^{\mathrm{mix}}$, which represents the departure of the alloy liquids temperature, T_liq, from that of the simple rule of mixtures liquids temperature, T_liq^mix. For values of $\text{ΔT}{}^1\text{> 0.20}$ a metallic system is likely to form a glass by melt-quenching in useful thicknesses (i.e. > 20 μm) at a cooling rate of 10⁵?10⁷ K s^?1. Hence, a rapid assessment of the GFA of novel compositions may in general be obtained simply from a knowledge of the melting points of the pure components and the liquidust emperatures of the alloys.     

Electrical,structural and optical properties of amorphous carbon

The planar and transverse electrical resistivity of amorphous carbon (a-C) films getter-sputtered at low temperature (77–95 K) is well-fitted by the expression $\text{? = ?}{}_0\text{exp}\text{(T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ The exponent T₀ being approximately the same in both cases (≈ 7 × 10⁷ K) suggests that the amorphous films are isotropic. Films thinner than 600 Å display a two-dimensional hopping conductivity from which one deduces a density of states N(E_F) at the Fermi level of 10¹⁸ eV^?1 cm^?3 and a radius of the localized wave functions (a) of 12 Å. Tunneling experiments and optical absorption measurements are consistent with a pseudogap of approximately 0.8 eV. Electron diffraction experiments indicate that a-C films consist of a mixture of diamond and graphite bonds; this fact taken in the light of the other experiments would suggest that the graphite bonds act as the localized conduction states.     

Crystal growth of SnO2 by chemical transport method

Single crystals of SnO₂ were grown by the chemical transport method from the powder prepared by heating hydrous stannic oxide. The transport reactions were carried out when iodine and sulfur were simultaneously used as transporting agent, and fine crystals of a few millimeters in dimensions were grown after a ten-day transport; the transport proceeded from 1100°C (charge) to 900°C part in a silica ampoule. The crystals were identified as SnO₂ (cassiterite) by the Weissenberg method. Thermodynamic calculations led to the equilibrium $\text{SnO}{}_2\text{(s) + I}{}_2\text{(g)}\text{+}\text{1}\text{2}\text{S}{}_2\text{(g)}\text{?}\text{SnI}{}_2\text{(g) + SO}{}_2\text{(g)}$ for this transport reaction.