The effects of As and Te on the crystallization and optical gaps of selenium

Glasses with compositions Te_xSe_100−x (0x25 at.%), As_ySe_100−y (0y40 at.%) and As₂Te_zSe_98−z (0z25 at.%) were prepared. The effects of the additives As and Te on the crystallization behaviour and optical gaps of selenium were studied. The relationship between the above properties and the chemical bond nature of the glass constituents is discussed. Materials with long lifetime and high sensitivity can be obtained by the addition of certain amounts of As and Te into Se.     

Effect of Yb content on purity and crystal growth of Ba2NaNb5O15

Growth by the micro-pulling-down technique (μ-PD) of homogeneous and crack-free fiber single crystals with composition Ba₂Na_1−xYb_xNb₅O₁₅ (0<x<0.08) is reported. The effect of Yb³⁺ addition to barium sodium niobate (BNN, x=0), having the tungsten bronze-type structure, was examined by room temperature X-ray powder diffraction and differential thermal analysis coupled to thermogravimetry. In the region of the monophased field the structure is tetragonal from x=0.02 to 0.16. Volume change is mainly by variation of the a-axis length. Addition of Yb³⁺ to BNN could be effective for the production of high optical quality, crack-free, bulk crystals by the Czochralski technique.     

Crystal growth of Na(V,Ti)2O5

Large single crystals of quasi-one-dimensional antiferromagnetic spin system NaV_2−zTi_zO₅ (0z0.06) have been successfully grown by a flux method. We present growth conditions together with a characterization of the single crystals by means of X-ray powder diffraction, energy dispersive X-ray (EDX) analysis and magnetic susceptibility measurements.     

Effects of phosphorous beam equivalent pressure on GaInAsP/GaAs grown by solid source molecular beam epitaxy with a valve phosphorous cracker cell

We report the growth and characterization of GaInAsP films on GaAs substrates by solid source molecular beam epitaxy (SSMBE) using a valve phosphorous cracker cell at varied white phosphorous beam equivalent pressure (BEP). It is found that the GaInAsP/GaAs can be easily grown with the solid sources, and the incorporated phosphorous composition as a function of the beam equivalent pressure ratio, R=f_P/(f_P+f_As), can be well described by a parabolic relationship. With the increase of the incorporated phosphorous composition, the GaP-, InP-, InAs- and GaAs-like phonon modes shift towards opposite directions and their emission intensities also change. The first three modes shift to larger wave numbers while the last one shifts to smaller wave number. The lattice mismatch, Δa/a, of the materials grown with varied phosphorous BEP follows a linear relationship. Photoluminescence (PL) measurements reveal that as the phosphorous BEP ratio increases, the peak position or energy band gap of the material shifts towards higher energy; the full-width at half-maximum (FWHM) becomes narrower, and the luminescence intensity becomes higher. In addition, the materials also show smooth surfaces that do not change significantly with phosphorous beam equivalent pressure.     

Electronic conduction in vitreous semiconductors in the pseudo-binary system (As2S3)1-x(PbS)x

AC conductivity and dielectric relaxation measurements of the bulk amorphous compositions in the pseudo-binary system (As₂S₃)_1-x(PbS)_x (x = 0, 0.1, 0.4 and 0.5) in the frequency range 500 Hz-10 kHz and in the temperature span 180–450 K are reported. The temperature dependence of the ac conductivity, σ_ac(ω), has a broad structure at all frequencies in compositions with x = 0.1, 0.4 and 0.5 whose peak position is not thermally activated. A similar structure was also observed in the data on the dielectric constant, ε₁, which peaked at a frequency of 1 kHz in the composition with x = 0.5. Analysis of the results using the correlated barrier hopping model revealed that the electronic conduction takes place by single polaron and bipolaron hopping processes at high and low temperatures, respectively, in compositions containing Pb. The microstructure and phase-separation in the glasses containing Pb influence the electrical transport and dielectric dispersion. This study has revealed the possible presence of a phase transformation at around 300 K at a frequency of 1 kHz in the dielectric dispersion behaviour of composition with x = 0.5.     

Low-temperature specific heats of porous silica xerogels of low densities

Low-temperature specific heat measurements have been performed in porous silica xerogels with densities varying from 670 to 1730 kg m⁻³ to study the low-energy vibrational dynamics. The specific heat, C_p, shows a bump in the temperature range above 4 K, when reported in a plot of C_p/T³ against the temperature, T. The bump is almost independent of the sample density and is close to the boson peak observed in melt-quenched amorphous silica (a-SiO₂). At temperatures <4 K, an additional contribution to that predicted by the Debye theory is observed. It follows an approximately linear temperature dependence (C_exc=aT^1+v, v being equal to about 0.25). In the xerogel with the largest density, specific heat of about a factor 5 larger than that of a-SiO₂ is measured, which increases with decreasing sample density. By comparison with the corresponding properties of a-SiO₂, we conclude that the disorder introduced by the presence of pores does not measurably affect the excess density of vibrational states in a frequency range of the boson peak (BP), but increases the density of the two-level systems (TLS).     

Low-temperature growth of epitaxial layers of ZnSe1−xTex solid solutions

The layers of ZnSe_1−xTe_x (0 < x < 1.0) solid solutions have been grown by liquid-phase epitaxy in a closed tube at 620–680 °C. Zinc chloride served as a solvent. ZnTe and ZnSe crystals were used as sources and substrates with orienting surfaces (110) and (111) for ZnSe and (110) for ZnTe. The composition of the grown layer was specified by the relative content of the ZnSe and the ZnTe in the solution and was controlled by X-ray analysis. The position of the exciton bands in the photoluminescence spectra of ZnSe_1−xTe_x over the interval 0.3 < x < 1.0 is in agreement with the free exciton energies calculated for these compositions. Relatively low-ohmic (of about 10² Ω cm) epitaxial layers of ZnSe_1−xTe_x solid solutions were grown.     

铁离子掺杂对铜铬黑颜料呈色性能的影响

为改善CuCr₂O₄黑色颜料呈色性能,将Fe³⁺掺杂进入CuCr₂O₄晶体中,采用共沉淀法制备CuCr_2-xFe_xO₄(x=0,0.04,0.05,0.06,0.07),并对所制备样品进行TG-DTA、XRD、SEM、Raman、XPS、UV-Vis吸收光谱和色度值的测试与表征。结果表明,Fe以三价态固溶进入Cr³⁺位置,未出现杂质相,得到的CuCr_2-xFe_xO₄晶粒细小、分散均匀。Fe³⁺掺杂可减小CuCr₂O₄的禁带宽度,从1.25 eV减小到1.08 eV,禁带宽度的减小是由于2p(O^2-)→3d(Fe³⁺)和Fe³⁺的d-d(⁶A_1g→⁴T_1g和 ⁶A_1g→⁴T_2g)电荷跃迁引起的。禁带宽度的减小使得黑色颜料对可见光(380~780 nm)的吸收率提高,L^*值减小,呈现出良好的黑度。得到的最佳黑色颜料为CuCr_1.95Fe_0.05O₄,L^*=17.63,a^*=-0.77,b^*=-1.61。     

Non-linear As(P) incorporation in GaAs1−yPy on GaAs and InAs1−yPy on InP

We find anion incorporation into both GaAs_1−yP_y on GaAs and InAs_1−yP_y on InP during gas source molecular beam epitaxy (GSMBE) is not linearly related to the hydride gas fluxes within our reactor. The deviation from an ideal model, i.e., y = ƒ_P/(ƒ_P+ƒ_As), where As(P) is the flux of AsH₃ (PH₃), can be as large as a factor of two. GaAsP exhibits the largest degree of nonlinearity and incorporation is found to depend quadratically on the fluxes over a wide range of flux ratio. Significant deviations are also found for InAsP; however, anion incorporation can be modeled with y = ƒ_P/(ƒ_P+βƒ_As).     

Relationship between T0, Tg and their pressure dependence for supercooled liquids

We show that simple expressions can be derived from the Vogel–Fulcher–Tammann (VFT) law relating the glass transition temperature T_g, the VFT temperature T₀, their pressure derivatives, the steepness index of the ‘Angell plot’ and the strength parameter D of the VFT equation, in good agreement with experimental data. In the same way one can describe the dependence of the dT_g/dP on the relaxation time τ_g chosen to define the temperature T_g. Thus, this procedure allows a consistent rescaling and comparison of pressure dependent parameters obtained from different experiments and simulations.     

Characterization and growth of ZnSTe epilayers by hot-wall epitaxy

ZnS_1−xTe_x epilayers were grown on GaAs(1 0 0) substrates by hot-wall epitaxy in a wide range of Te composition. The Te composition was determined by Rutherford backscattering spectrometry and the lattice constant was measured by double-crystal rocking curve. It was found that the lattice of the epilayer matches well with that of the substrate at x=0.37 as expected by Vegard's rule, and the energy gap was also determined as a function of Te composition by spectrophotometer. It showed that a quadratic relation with the composition: E_g(x)=3.71−5.27x+3.83x². Photoluminescence characteristics were also studied.     

Elastic properties of GexAsySe100−x−y glasses

The elastic properties of Ge_xAs_ySe_100−x−y (0x30; 10y40) glasses have been studied. The results were analyzed in terms of the dependence on the theoretical mean coordination number (mean number of covalent bonds per atom) m (m=2+(2x+y)×0.01). Three ranges of m (2.1m2.51, 2.51<m2.78, 2.78<m3) were revealed, where different dependencies of elastic moduli (Young’s modulus, shear modulus) and Poisson’s ratio of glasses on m were observed.     

Indium content determination related with structural and optical properties of InGaN layers

We studied the structural and optical properties of a set of nominally undoped epitaxial single layers of In_xGa_1−xN (0<x0.2) grown by MOCVD on top of GaN/Al₂O₃ substrates. A comparison of composition values obtained for thin (tens of nanometers) and thick (≈0.5 μm) layers by different analytical methods was performed. It is shown that the indium mole fraction determined by X-ray diffraction, measuring only one lattice parameter strongly depend on the assumptions made about strain, usually full relaxation or pseudomorphic growth. The results attained under such approximations are compared with the value of indium content derived from Rutherford backscattering spectrometry (RBS). It is shown that significant inaccuracies may arise when strain in In_xGa_1−xN/GaN heterostructures is not properly taken into account. Interpretation of these findings, together with the different criteria used to define the optical bandgap of In_xGa_1−xN layers, may explain the wide dispersion of bowing parameters found in the literature. Our results indicate a linear, E_g(x)=3.42−3.86x eV (x0.2), “anomalous” dependence of the optical bandgap at room temperature with In content for In_xGa_1−xN single layers.     

B NMR investigations in xV2O5–B2O3 and xV2O5–B2O3–PbO glasses

11B (I=3/2) MAS NMR in the binary glass system xV₂O₅–B₂O₃ (x=0.053, 0.43) and the ternary glass system xV₂O₅–B₂O₃–PbO (0.1x1.5) has been investigated at room temperature. In the xV₂O₅–B₂O₃ glasses, one NMR line due to BO₃ unit was observed. Meanwhile in the xV₂O₅–B₂O₃–PbO, two NMR lines which arise from BO₃ and BO₄ units were detected, where the appearance of BO₄ units is produced by the presence of PbO. From the computer-simulation of the ¹¹B NMR central transition line (m=−1/2↔1/2), the quadrupole parameters (e²qQ/h and η) for BO₃ units in xV₂O₅–B₂O₃, and those for BO₃ and BO₄ units in xV₂O₅–B₂O₃–PbO were obtained as a function of x. As the V₂O₅ content increases in xV₂O₅–B₂O₃–PbO, the e²qQ/h and η values of the BO₃⁻ associated resonance are found to slightly decrease and increase, respectively. Meanwhile, the e²qQ/h and η values of BO₄⁻ associated resonance in xV₂O₅–B₂O₃–PbO are found to slightly increase and decrease, respectively. By comparing the intensities of the total transitions (m=−3/2↔−1/2,m=−1/2↔1/2, and 1/2↔3/2) for the ¹¹B NMR line of BO₃ and BO₄ units contained in xV₂O₅–B₂O₃–PbO with those of respective standard samples of 0.053V₂O₅–B₂O₃ and NaBH₄, the quantitative fractions of BO₃ and BO₄ in xV₂O₅–B₂O₃–PbO were obtained as a function of x.     

XAS study of lanthanum coordination environments in glasses of the system K2O---SiO2---La2O3

The La L₁ and L₃ XANES and L₃ EXAFS have been investigated for the series of glasses 10K₂O---50SiO₂---x La₂O₃ (x = 1, 5, 10) and (10 − x)K₂O---40SiO₂−(x/3)La₂O₃ (x = 7.5, 5, 2.5) and model compounds La₂O₃, LaAlO₃, LaPO₄, La₂NiO₄, La₂CuO₄ and La(OH)₃. An edge resonance at 25 eV above the L₁ edge in the glass spectra is concentration-dependent, decreasing in intensity with increasing lanthanum concentration. The 2s → nd forbidden transition increases with La₂O₃ concentration, indicating a reduction in the ‘average’ site symmetry of the first coordination shell of La. Mapping _X(k) space, which is a new and promising technique, was employed to extract bond distance, coordination number and thermal parameters from the EXAFS. By this method, one calculates the complete _X(k) space a function of all physically reasonable values of the adjusted parameters in all possible combinations. The advantage in this method is the assurance of a global minimum. Bond lengths were comparable to those obtained by Fourier transforming the phase corrected EXAFS. The values are 2.42 Å (± 0.03 Å) for La---O. The coordination numbers (N ≤ 7 ± 1.5) were derived by mapping and comparison to the published structures for other La compounds. _X(k) mapping is compared with least-squares fitting the data, and the correlation between the Debye-Waller factor and coordination number is also discussed.     

金红石型Ti1-2xCrxMoxO2黑色色料制备及其性能研究

以TiO₂、Cr₂O₃和MoO₃为主要原料,采用固相反应法合成了金红石型Ti_1-2xCr_xMo_xO₂(x=0、0.05、0.10、0.15、0.20)色料。研究了Cr/Mo共掺杂量和煅烧温度对Ti_1-2xCr_xMo_xO₂色料的晶型结构、呈色性能和微观形貌等的影响。研究结果表明,5%~15%摩尔分数的Cr/Mo等量共掺杂都获得单相金红石型结构黑色色料,均可促进锐钛矿向金红石转变,且Mo掺杂有利于提高Cr在TiO₂晶格中的固溶度。随Cr/Mo共掺杂量增加,样品的黑度(L^*值)呈先减小后增大的趋势,而红色度(a^*值)和黄色度(b^*值)都表现出相反的趋势。煅烧温度在950~1 100 ℃时,样品的色度参数变化相对较小。在1 100 ℃煅烧得到的Ti_0.70Cr_0.15Mo_0.15O₂色料黑色呈色性能最佳,其L^*、a^*和b^*值分别为24.41、2.12和-2.53,且在陶瓷釉中表现出良好的呈色性能和化学稳定性。适量Cr/Mo共掺杂使TiO₂晶格产生畸变和电荷缺陷,从而吸收绝大部分可见光,是其呈现黑色的主要原因。     

Nuclear magnetic resonance studies of alkaline earth phosphosilicate and aluminoborosilicate glasses

The ¹¹B, ²⁷Al, ²⁹Si and ³¹P magic angle spinning (MAS) NMR spectra of MO–P₂O₅, MO–SiO₂–P₂O₅ and MO(M^′₂O)–SiO₂–Al₂O₃–B₂O₃ (M=Mg, Ca, Sr and Ba, M^′=Na) glasses were examined. In binary MO–P₂O₅ (M=Ca and Mg) glasses, the distributions of the phosphate sites, P(Q_n), can be expressed by a theoretical prediction that P₂O₅ reacts quantitatively with MO. In the ternary 0.30MO–0.05SiO₂–0.65P₂O₅ glasses, the 6-coordinated silicon sites were detected, whose population increases in the order of MgOxCaO–0.05SiO₂–(0.95−x)P₂O₅ glasses, its population increases with an increase in f (=([P₂O₅]−[MO]−[B₂O₃]−[Na₂O])/[SiO₂]) and has maximum at f=9. The signal due to the 5-coordinated silicon atoms is also observed when x is smaller than 0.45. When three network-forming oxides such as SiO₂, Al₂O₃ and B₂O₃ coexist, Al₂O₃ reacts preferably with MO. The populations of 4-coordinated boron atoms, N₄, are expressed well with r/(1−r), where r=([Na₂O]−[Al₂O₃])/([Na₂O]−[Al₂O₃]+[B₂O₃]). The correlation of the Raman signal at 1210 and 1350 cm⁻¹ with the NMR signal of Si(Q₆) at −215 ppm is also seen.     

Raman spectroscopic study on the structure of ZnCl2---ZnX2 and ZnCl2---KX (X = Br, I) glasses

Raman spectra have been measured for ZnCl₂---ZnX₂ and ZnCl₂---KX (X = Br, I) glasses to investigate the structure of the glasses with varying composition. The assignment of each band was made, and the change of the spectra with composition was explained in terms of the bridging and non-bridging states of halide ions and the change of the tetrahedral units, ZnX_nCl_4−n²⁻ (n = 0–4), formed in the glasses. As the content of ZnX₂ in ZnCl₂---ZnX₂ glasses increases (20 → 80 mol%), the peak frequency of the Zn---Cl stretching mode increases (238 → 248 cm⁻¹ in X = I glasses, 238 → 259 cm⁻¹ in X = Br glasses) while the Zn---I and Zn---Br stretching frequencies decrease (173 → 120 cm⁻¹ for Zn---I, 196 → 157 cm⁻¹ for Zn---Br). The decrease of the Zn---I and Zn---Br band frequencies was attributed to the increase of the number n of the ZnX_nCl_4−n²⁻ tetrahedra. The increase of the Zn---Cl frequency suggests the existence of the bonding state of Cl⁻ ions which is intermediate between the bridging and the non-bridging states. In ZnCl₂---KX glasses, the Zn---Cl_non-bridging band at about 300 cm⁻¹ was observed in addition to the bands observed in ZnCl₂---ZnX₂ glasses. The addition of KX produces non-bridging anions while the tetrahedral units, ZnX_nCl_4−n²⁻ are also formed.     

Brillouin scattering and pockels coefficients in silicate glasses

The Brillouin spectra of binary and ternary silicate glasses were investigated and absolute values for the Pockets coefficients p₁₂ and p₄₄ and (p₁₁ – p₁₂) were calculated from the measured spectra. The results were interpreted as to the density and refractive index dependence of the Pockels coefficients in terms of the theoretical treatments by Carleton, Mueller and Sipe. From the measured p₁₂ and p₄₄ values the quantity (∂ε/∂p), the change of the dielectric constant with respect to density, was calculated and compared with (∂ε/∂) evaluated from the Lorentz-Lorenz equation, Drude equation and the Carleton model. The Lorentz-Lorenz and Drude equations overestimated the magnitude of (∂ε/∂), while the Carleton model with its two limiting cases set upper and lowe bounds to (∂ε/∂). From the (∂ε/∂) results the strain polarizability constant (λ₀) was determined and its physical significance discussed. In general, the alkali-oxide concentration dependence of the Pockels coefficients of various silicate glasses has been measured and qualitatively explained by several existing theoretical formulations.     

Physical properties of amorphous Ge20Sb25−xBixSe55 thin films

The influence of substitution of Sb atoms by Bi atoms on the electrical and optical properties of thin films of the Ge₂₀Sb_25−xBi_xSe₅₅ [0x15] system are reported. Results of dc conductivity and thermoelectric power measurements between 150 and 450 K show that the Ge---Sb---Se system is chemically modified by addition of large concentrations of Bi atoms between X=5 and X=10 at.%). A transition from p-type for Sb-doped to n-type for Bi-doped films and a decrease of resistivity is observed. The absorption edge shifts to shorter wavelength, thereby decreasing the optical band gap of the system. Compositional dependences of electrical conductivity, thermoelectric power, and the appearance of n-type conduction are discussed from the stand point of chemical bonds formed in the films and related to the defect states produced due to incorporation of Bi atoms in high concentrations. The coexistence of band and hopping conduction is proposed. The ac conductivity in 0.1–10.0 kHz frequency and 150–450 K temperature range was found to obey a power law σ(ω, T) = Aω^s. The results were interpreted in terms of Elliott's theory, which assumes correlated barrier hopping (CBH) between the charged defect centres. It was found that computed results from the CBH model and experimental one are qualitative agreement for the present materials.