High pressure induced phase transformation of SiO2 and GeO2: difference and similarity

We report high pressure polymorphism of SiO₂ and GeO₂ at room and high temperatures. It was found that kinetics has a large effect on pressure induced phase transitions of SiO₂ and GeO₂. The high pressure behavior of SiO₂ and GeO₂ polymorphs depends on the starting material and pressure–temperature history. Our studies show that SiO₂ and GeO₂ have a common sequence of high pressure, high temperature structural transformations when the same type of starting material (amorphous or α-quartz) was used: (amorphous or α-quartz)⇒d-NiAs⇒rutile⇒CaCl₂⇒α-PbO₂⇒pyrite (Pa-3) types. In the case of cristobalite as starting material, the α-PbO₂-type phase can be synthesized directly from the high pressure room temperature phase omitting rutile- or CaCl₂-type structure. The crystallization of the d-NiAs phase in a narrow temperature interval 1000–1300 K can be used as an indicator of presence of a cation disordered network in both SiO₂ and GeO₂ materials at high pressure before heating.     

Elastic properties of Se and As2Se3 glasses under pressure and temperature

The sound velocities and their pressure and temperature variations of Se and As₂Se₃ glasses have been determined by means of a pulse superpositions method, and other elastic constants and their pressure and temperature derivatives were calculated from these data. The bulk modulus was found to be 94·6 kbar for Se glass and 143·7 kbar As₂Se₃ glass, both of which are higher than the values calculated from the previous compression data. No anomaly was observed in any of the pressure and temperature dependence of elastic behavior of these glassses. Furthermore, the comparison of the pressure and temperature derivatives of the bulk modulus indicates that the thermodynamic self-consistency is satisfied on these materials. The bulk moduli of these glasses and crystalline As and Se were used to obtain an empirical bulk modulus-volume relationship for compounds in the As?Se system. The acoustic Grüneisen parameter was calculated and compared with the thermal Grüneisen parameter.     

相变及空位缺陷对高压下GeO_2光学性质的影响

本文采用第一性原理方法,在100 GPa的压力范围内,计算了GeO_2理想晶体和含锗、氧空位点缺陷晶体的光学性质.吸收谱数据表明,压力诱导的三个结构相变对GeO_2晶体的吸收谱均有影响:第一个相变将导致其吸收边蓝移,而第二和第三相变将使得其吸收边红移.锗和氧空位点缺陷的存在将导致GeO_2的吸收边红移,但氧空位点缺陷引起的红移更明显.尽管如此,分析发现,在100 GPa的压力范围内,压力、相变以及空位点缺陷等因素都不会导致GeO_2晶体在可见光区出现光吸收现象(是透明的).波长在532 nm处的折射率数据显示,在GeO_2的四个相区,其折射率均随压力增加而降低;而且,GeO_2的三个结构相变以及锗、氧空位点缺陷都会导致其折射率有所增大.本文预测,GeO_2有成为冲击光学窗口材料的可能.     

Network rigidity in GeSe2 glass at high pressure

Acoustic measurements using synchrotron radiation have been performed on glassy GeSe2 up to pressures of 9.6 GPa. A minimum observed in the shear-wave velocity, associated anomalous behavior in Poisson's ratio, and discontinuities in elastic moduli at 4 GPa are indicative of a gradual structural transition in the glass. This is attributed to a network rigidity minimum originating from a competition between two densification mechanisms. At pressures up to 3 GPa, a conversion from edge- to corner-sharing tetrahedra results in a more flexible network. This is contrasted by a gradual increase in coordination number with pressure, which leads to an overall stiffening of the glass.     

掺Bi的GeO2-Al2O3-Na2O玻璃的热学和光学性质

用高温熔融法制备了96GeO-(3-χ)Al2O3-χNa2O-1NaBiO3 （将χ分别为0,0.5,1.5的玻璃命名为A1,A2,A3）和 96GeO-(3.5-ψ)Al2O3-ψχNa2O-0.5Bi2O3（将ψ分别为0,0.5,2的玻璃命名为B1,B2,B3）玻璃。观察到样品在1 220 nm处（800 nm 激光二极管激发）的超宽带发光特性（半高宽约250 nm）。结果表明,以NaBiO3形式引入Bi5+到玻璃原料中比以Bi2O3形式引入Bi3+到原料中得到的玻璃在1 220 nm处的发光强度大4.6倍,且荧光寿命和荧光半高宽也分别从280 μs和195 nm增加到了434 μs和275 nm。从A3,A2和A1的吸收边带的红移可初步推断出A3,A2及A1玻璃中Bi5+的含量逐步增加。总结吸收光谱与发射光谱的变化规律,认为Bi离子近红外高发射强度和宽荧光半高宽是由Bi5+的发光引起的。在两组玻璃中,热稳定性以及荧光发射截面积与荧光寿命的乘积值和荧光发射截面积与荧光半高宽的乘积值随着Na2O含量的增加而增加。     

Quantum chemical modelling of the structure and properties of hypervalent defects in vitreous SiO2 and GeO2

A cluster approximation using a semiempirical MNDO-PM3 scheme is used to study the structure and properties of the defect structures which develop in vitreous SiO₂ and GeO₂ during the interaction of the previously discovered most probable defects (two-member cycles, fragments with double bonds O=A<(A=Si, Ge) and open chains) with valence-saturated segments of a fracture surface. During the interaction of open chains with such a surface, defects with a large dipole moment (up to 15–20 D) are formed, which may create anisotropic highly polarized regions in the glass. The bonds around hypervalent centers are weakened, and the characteristics of the newly formed and already existing bonds approach one another; that is, in a grouping of this sort, other decay paths may exist that change the direction of fracture. In structures formed by the interaction of O=A< defects and two-member cycles with the surface, hypervalent bonds are easily broken; that is, the hypervalent configuration is transformed into an ordinary one. In a number of cases, the potential surface contains two or three minima with similar energies, separated by low or moderate potential barriers. Fiz. Tverd. Tela (St. Petersburg) 41, 1419–1423 (August 1999)     

GeO2含量对掺铒锗碲酸盐玻璃物性和光谱特性的影响

本文研究了x GeO2-(70-x)TeO2-5K2O-5Na2O-10Nb2O5-10ZnO-0.2Er2O3(x=0,10,25,50,70(摩尔百分数))玻璃的物性和光谱特性,讨论GeO2含量对锗碲酸盐玻璃物性和光谱特性的影响.研究发现:GeO2的加入提高了碲酸盐玻璃热稳定性,并且使玻璃的最大声子能量略微增加;随GeO2的增加,掺Er3+锗碲酸盐玻璃的Judd-Ofelt强度参量Ω2和Ω6逐渐增大,但玻璃受激发射截面有减小的趋势;由McCumber理论,计算了掺铒锗碲酸盐玻璃在1.53μm处最大受激发射截面为9.92×10-21cm2,Er3+离子4I13//2→4I15/2发射谱的最大荧光半高宽为52nm,同时,实验发现,在977nm LD抽运下,掺铒锗碲酸盐玻璃存在较强的荧光上转换现象,随GeO2含量的增加,上转换荧光强度呈降低的趋势.     

触发电压对±100 kV多级多通道开关性能的影响

研制了一种用于快脉冲直线型脉冲变压器的±100 kV多级多通道火花开关,研究了触发电压极性、幅值对多级多通道火花开关触发性能的影响,分析了不同触发电压下开关各部分延时及抖动。研究结果表明:触发电压的极性对双极性多级多通道火花开关触发性能影响较小;开关的触发放电延时和抖动随着触发电压的增大而减小。进一步分析了多级多通道火花开关的触发击穿模式以及影响开关击穿延时和抖动的主要因素,提出了减小开关触发击穿延时及抖动的技术途径。     

Topological characteristics of bonds in SiO2 and GeO2 oxide systems upon a glass-liquid transition

Using the Angell model of broken bonds (configurons), configuron clustering in a topologically disordered lattice (network) of amorphous SiO₂ and GeO₂ upon a glass-liquid transition is considered. It is shown that the glass-liquid transition is accompanied by the formation of a macroscopic (percolation) configuron cluster penetrating the entire bulk of the material and possessing fractal geometry. The glass-liquid (overcooled liquid) percolation phase transition in the amorphous substance is accompanied by a change in the Hausdorff dimension of the bond network structure for configurons from the three-dimensional Euclidean dimension in the glassy state to a fractal dimension of 2.55 ± 0.05 in the liquidlike state. Contrary to the kinetic character of the liquid-glass transition, the glass-transition temperature is a thermodynamic parameter of the amorphous substance, depending parametrically on the cooling rate.     

Optical properties of GeO2 in the ultraviolet region

The optical constants of amorphous and crystalline GeO₂ were determined by measurements of reflectivity in the region of phonon energy 0·5–25 eV and subsequent use of Kramers-Kronig analysis. The differences between the spectra of GeO₂ and those of SiO₂ are shown and discussed.The author would like to express her gratitude to Prof. J. Tauc for his interest in this work, to Dr. Z. Trousil, J. Charvát, Dr. S. Koc, Mrs. V. Míková and Dr. Németh-Sallay for the preparation of the samples, to Dr. L. ervinka and Dr. M. Rozsival for the test of their structure and to Dr. M. Závtová and Dr. I. Gregora for kind cooperation in the index-of-refraction measurements. Very useful comments by Dr, J. Sak are gratefully acknowledged.It is also a pleasure to thank Dr. R. P. Madden for discussion on the later stage of this work.     

Brillouin and raman scattering from glasses under high pressure

Abstract

Brillouin and Raman Scattering Spectra in SiO₂ and GeO₂ glasses have been measured in a diamond anvil cell up to pressures of 14 GPa. The elastic properties and equation of state for each glass type were obtained from the Brillouin scattering measurements with respect to pressure. Both elastic constants and compressibility of SiO₂ and GeO₂ showed anomalous behavior with respect to pressure. This anomalous behavior is reconciled with a model based on the pressure dependent bending of the oxygen angles in both glass types. The Raman measurements corroborate the conclusions from the Brillouin scattering results, namely that the SiO₂ and GeO₂ bond angles are changing with pressure or the oxygen angle distribution is changed without bond breaking.     

Density and structural changes of silicate glasses under high pressure

ABSTRACT

Density and structural changes of matter in their liquid or amorphous form, such as silicates melts, molecular fluids, or glasses, are of extreme importance to model the interior of planetary bodies. However, measuring the evolution of amorphous materials under extreme conditions of pressure and temperature remains challenging mainly because of the sample dimensions and the weak interaction with X-ray probes. This contribution highlights recent developments to measure the density of amorphous material, mainly silicate glasses made of light elements, to high pressure conditions. In particular, the X-ray absorption method at synchrotron sources is discussed as a new opportunity for high pressure experiments on glasses, fluids, and melts. Recent achievements using X-ray Raman scattering spectroscopy to obtain data on the local electronic environment of the main constituents of silicate glasses at high pressure are also presented. Finally, perspectives of these recent developments are discussed as well as their potential for high pressure research in the next years.     

Enhanced photosensitivity in sol–gel derived 20GeO2:80SiO2 thin films

We investigate the photosensitivity of binary 20GeO₂:80SiO₂ (germanosilicate) inorganic films. The samples have been fabricated by the sol–gel spin-coating method and the densification has been performed by rapid thermal annealing at various temperatures ranging from 500 °C to 1000 °C. The –OH absorption bands in the Fourier-transform infrared (FTIR) spectra and the refractive-index data show that the films annealed below 900 °C are porous and the films annealed at 900 °C and above are dense. An ultraviolet (UV) KrF laser at 248 nm has been used to induce the change in the refractive index of the samples. We have achieved a large refractive-index change (Δn) of 0.0098 after UV illumination in excess of 1 min for our dense germanosilicate films. This UV-induced refractive-index change is attributed to the formation of GeE’/SiE’ centers from Ge–Ge/Si–Si (neutral oxygen monovacancy) and Ge²⁺ centers and to the creation of oxygen deficiency related defects. From our experiments, the oxygen deficiency related defects correspond to the absorption band at 620–740 cm^-1 in the FTIR spectra and these are the defects which make a large contribution to Δn. The attenuation coefficient of the as-deposited and UV-illuminated dense samples is about 0.42 dB/cm at 1550 nm. For porous samples, UV exposure has densified the samples to some extent. PACS 82.50.Hp; 71.23.Cq; 81.20.Fw     

Low-temperature photoluminescence of ion-implanted SiO2:Sn+ films and glasses

Low-temperature photoluminescence spectroscopy with pulsed synchrotron excitation is applied to study the regularities of excitation and relaxation of both point defects and nanoparticles formed by tin implantation into SiO₂ films and glasses. It has been found that tin implantation followed by air and nitrogen annealing yields the formation of α-Sn nanoclusters and nonstoichiometric SnO _x nanoparticles, while a stable phase of SnO₂ does not appear. Alternative channels of luminescence excitation are revealed for nanoclusters, including energy transfer from excitons and electron-hole pairs of the host SiO₂ matrix.     

The electronic structure of SiO2, GeO2 and intermediate SixGe1−xO2 compositions: experiment and theory

Theoretical calculations are reported for SiO₂, GeO₂ and the intermediate composition Si_.5Ge_.5O₂ which reproduce the main observed features and trends in experimental photoemission spectra. The agreement between the two establishes the importance of band theory in understanding the electronic structure of these materials, and demonstrates that detailed quantitative predictions are feasible for such complex materials in terms of the empirical tight-binding method. The calculations further establish that the structure in the valence bands is determined mainly by nearest- neighbor oxygen-oxygen interactions.