高压下SiO_2的第一性原理计算

基于密度泛函理论(DFT)的第一性原理,采用Hartree-Fork(HF)方法,分别计算了Si O2的α-石英结构、金红石结构以及氯化钙结构的总能量随体积的变化关系。利用Murnaghan状态方程,通过能量和体积拟合,得到了3种结构的体变模量及其对压强的一阶导数。计算结果表明,随着压强的增加,Si O2会从α-石英结构转变为金红石结构,与实验结果和其它理论结果一致;金红石结构与氯化钙结构之间不存在相变,可以共存。此外,对具有α-石英结构的Si O2的晶格常数、电子态密度和带隙随压强的变化关系进行了计算和分析,结果表明:加压作用下,能带向高能方向移动,Si─O键缩短,电子数转移增加,带隙展宽,电荷发生重新分布。     

Zeeman effect in α-quartz

The helical crystal structure in α-quartz acts as the natural micro-solenoids for an electromagnetic wave passing through them, producing a longitudinal magnetic field in the direction of the optical axis. The longitudinal magnetic field further induces the Larmor frequency for the rotation of the bound electrons. The calculated Larmor frequency was experimentally confirmed by monitoring a line splitting of the infrared OH-band in the transmission spectra of α-quartz. A shift in the resonance frequency of the OH-band is equal to the Larmor frequency induced by the natural Zeeman effect.     

α-石英晶体中旋光性对Raman散射的影响

讨论了旋光性对Raman散射的影响。在α-石英晶体中获得了消除旋光影响的纯Raman谱,在此基础上,讨论了α-石英晶体(单轴晶体)的Raman谱中,x轴和y轴的非等价性问题,以及与这种非等价性相应的振动模的对称性。 关键词：     

高压下α-石英和柯石英的相变行为

利用金刚石压腔和同步辐射X射线衍射技术,对α-石英和柯石英在常温高压下的相变行为进行了研究。实验结果表明:α-石英在约23 GPa开始发生结构相变,在约44 GPa相变完成,直至59 GPa仍能观察到结晶态;柯石英在约22 GPa转变为柯石英-Ⅱ相,高于36 GPa时,继续发生结构转变,直至59 GPa仍有结晶态;氖气和氩气所提供的不同静水压条件对α-石英和柯石英的高压相变行为影响不大。实验结果为进一步厘清二氧化硅物相的压致相变行为和相变机制提供了实验支撑。     

α-β石英相变的应变参数计算及其地质意义

 利用已有的α和β石英压缩性、热膨胀性、弹性及相变温度压力资料，计算了α-β石英相转变时，α和β石英的晶胞参数。依据虎克定律以及高压下β石英的弹性参数，估算了α-β石英相转变时的应变、应力和应变能。结果表明，在0~1.1 GPa条件下，随压力升高，α-β石英相变的线应变介于-0.006~0.005之间，体应变介于-0.016~0.012之间，应力介于-0.46~0.14 GPa之间；应变能介于965~2 760 kJ/m³之间。压力为0.5 GPa左右时，α-β石英相变的应变、应力和应变能均达到最小值。在此基础上，讨论了壳内大规模酸性岩浆活动引起的α-β石英相变对壳内岩石的作用。     

Dispersion curves of photon-induced surface phonon-polaritons and of Brewster modes

The reflectivity for angular scans at constant frequency is calculated in a frequency range which covers a dispersion curve of photon-induced surface phonon-polaritons on α-quartz. Additionally the dispersion of a backbent branch and of Brewster modes is deduced.     

Structural ordering in a silica glass matrix under Mn ion implantation

Mn(+)-implanted, amorphous SiO(2) samples were synthesized using pulsed-ion implantation without thermal annealing. The crystal and electronic structures have been studied using x-ray diffraction and synchrotron-based soft x-ray absorption and emission spectroscopy at the Si and Mn L(2,3) edges. We find a combination of small MnO clusters and Si crystallites at shallow depths while tetrahedral Mn coordination is found deeper in the host target. Through a combination of techniques, we find that the implantation process simultaneously decreases the long-range order in the near-surface region and increases order deeper in the SiO(2) host. Our results suggest Mn substitution into Si sites at deep levels catalyzes the formation of α-quartz, providing insight into the complex interactions that determine the local structure around the impurities as well as the overall changes to the crystallinity of implanted SiO(2).     

Electron states in α-quartz (SiO2)

Valence electron states of α-quartz SiO₂ are calculated self-consistently using the pseudopotential method. Excellent agreement is found with photoemission and UV-absorption data. X-ray emission spectra are calculated in an OPW scheme and compared to experiments. While the Si- and O - K spectra agree well with experiment, the Si L_2,3 spectrum shows substantial differences. An explanation is offered base on the formation of amorphous Si in SiO₂ during electron irradiation.     

Temperature-induced phase transition in quartz nanocrystals dispersed in pseudotachylite

The size and concentration of α-quartz nanocrystals dispersed in samples of pseudotachylite and the internal stresses in these nanocrystals have been determined using infrared spectroscopy in the temperature range 300–800 K. Pseudotachylite is a product of intense crushing of granite that undergoes in the Earth’s crust faults. It has been found that the size of the nanocrystals is ～20 nm and does not depend on temperature. As the temperature increases, their concentration decreases monotonically and tends to zero at ～650 K. This process is paralleled by a growth of the concentration of β-quartz nanocrystals. The α-quartz nanocrystal concentration regains its initial level with decreasing temperature. Thus, the α → β phase transition in quartz nanocrystals in pseudotachylite starts at temperatures lower by ～500 K than that in the bulk of the macrocrystal (846 K), and is stretched by ～350 K. At room temperature, the unit cell of nanocrystals is compressed by surface tension forces. These forces retard the α → β phase transition. The thermal expansion coefficient of nanocrystals is larger than that of macrocrystals, which entails a decrease of compression and a monotonic decrease of the concentration of α-quartz nanocrystals with increasing temperature.     

Double-hole aluminum center in α-quartz

EPR studies at ca. 25 K of a single crystal of α-quartz, which had been X-irradiated at 77 K, have disclosed the presence of a new type of impurity center. This is thought to consist of a substitutional aluminum 3+ ion with electron holes at two neighboring oxygen ions, forming a triplet system. The two primary ²⁷Al hyperfine multiples as well as the half-field spectrum have been observed, and have led to an approximate spin-Hamiltonian.     

The hydrothermal crystal growth of quartz: New developments

Abstract

α-quartz is an important piezoelectric material used in particular for its high frequency stability.

The increase of the power of high frequency oscillators implies α-quartz single crystal with a low concentration of defects.

Due to the low-temperature transition α → β quartz, the hydrothermal synthesis was selected to grow a-quartz single crystals.

On the basis of an analysis of the different factors able to induce physical and chemical defects in this lattice, new solvents and specific experimental (P, T) conditions have been investigated.     

Luminescence of polymorphous SiO2

The luminescence of self-trapped exciton (STE) was found and systematically studied in tetrahedron structured silica crystals (α-quartz, coesite, cristobalite) and glass. In octahedron structured stishovite only host material defect luminescence was observed. It strongly resembles luminescence of oxygen deficient silica glass and γ or neutron irradiated α-quartz. The energetic yield of STE luminescence for α-quartz and coesite is about 20% of absorbed energy and about 5(7)% for cristobalite. Two types of STE were found in α-quartz. Two overlapping bands of STEs are located at 2.5–2.7 eV. The model of STE is proposed as Si–O bond rupture, relaxation of created non-bridging oxygen (NBO) with foundation of a bond with bridging oxygen (BO) on opposite side of c or x,y channel. The strength of this bond is responsible for thermal stability of STE. Similar model of STE was ascribed for coesite and cristobalite with difference related to different structure. STE of Silica glass is strongly affected by disordered structure.     

The separation of magnetic dipole and electric quadrupole contributions to Raman optical activity via the Raman-induced Kerr effect

A procedure is presented for the complete separation of magnetic dipole and electric quadrupole contributions to Raman optical activity using the Raman-induced Kerr effect. The method involves carrying out a series of co/counter-propagating and frequency interchange experiments. Feasibility is discussed as well as application to some recent experimental observations in α-quartz.     

The low-temperature solubility of helium in quartz

Technical Physics - The solubility of helium in α- and β-quartz single crystals, optical quartz crystals, and Pyrex glass has been studied by the method of thermal desorption of helium...     

Dynamics of twinning in natural α-quartz

Finite amplitude internal friction experiments in α-quartz are described and interpreted in terms of the theory of nonlinear anelasticity. The theory predicts a linear relationship between the driving force of the excitation and the period of the automodulation at its onset. This relation is substantiated by experiments performed with a natural α-quartz reed vibrating in flexure at resonance frequencies between 65 and 170 Hz in the temperature range of 162° and 224°C. The data suggests that Dauphiné twinning in α-quartz causes the automodulation governed by an activation energy of 92$\text{kJ}\text{mole}$. This activation energy characterizes short distance oxygen diffusion.     

The hydrothermal growth of α-quartz and the nature of the physico-chemical defects detected in the resulting crystal

Abstract

The development of α-quartz for high frequencies devices leads to a strong reduction of the thickness of the piezoelectric sheet. Such a thickness is comparable to the size of the defects. Consequently, the main objective for the improvement of the performances of α-quartz used as a piezoelectric material is to reduce the density of physico-chemical defects.

Our work has been focussed in two directions:

(i) the identification of different physico-chemical defects using the overlap of different techniques (X-ray topography, IR spectroscopy, Cathodoluminescence, Thermo-luminescence, EPR,…)

(ii) attempts at improving the correlations between the different parameters governing the hydrothermal crystal growth of a-quartz and the nature or the density of defects.     

Electron states in α-quartz (SiO2)

Valence electron states of α-quartz SiO₂ are calculated self-consistently using the pseudopotential method. Excellent agreement is found with photoemission and u.v.-absorption data. X-ray emission spectra are calculated in an OPW scheme and compared to experiments. While the Si- and O-K spectra agree well with experiment, the Si L_{2, 3} spectrum shows substantial differences. An explanation is offered based on the formation of amorphous Si in SiO₂ during electron irradiation.     

Optical interference in overtones and combination bands in α-quartz

Successive optical interference fringes were monitored for α-quartz at the frequencies of the combination and overtones of the fundamental lattice vibration bands. The interference fringes were explained to be formed by the dynamic gratings induced by coupling of the transverse waves of the incident light with the optic modes in α-quartz. The distance of the dynamic gratings was evaluated to be 32.895 μm.     

Effects of the substrate on graphone magnetism: A density functional theory study

The magnetism of graphone, a single-side-hydrogenated graphene derivative, has been related to the localized and unpaired p-electrons associated with the unhydrogenated carbon atoms. In the present density functional theory study, the effects the adhesion to either Cu(111) or α-quartz (0001) surface on the magnetic properties of graphone have been investigated. The total magnetization of the graphone adsorbed to copper and quartz surface is reduced by four and two times, respectively, with respect to the isolated graphone. We have shown there is electronic charge transfer from surface towards three-fold coordinated C atoms of graphone, but the main role in the partial magnetism quenching is played by bond formation and the consequent electron pairing of p-electrons. The critical temperature has been investigated on the basis of the mean field theory to evaluate the stability of the magnetism at ordinary temperature.