Impurity defects in Cr-doped Bi12SiO20 and Bi12TiO20 crystals

Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀ crystals doped with l0^-5 to 2 x 10^-2 wt % Cr were grown by the Czochralski technique. The Cr content of the crystals and the valence state of Cr ions were determined by a new analytical technique with the use of 1,5-diphenylcarbazide. Correlations were revealed between the segregation coefficient of the dopant, the charge state of Cr ions, and the absorption coefficient of the crystals. Doping with Cr was found to shift the intrinsic edge to longer wavelengths and to give rise to a new absorption band in the near-IR region. The results are interpreted as indicating that the Cr ions in the sillenite crystals studied are in at least two oxidation states and occupy inequivalent sites in the sillenite cell.     

光折变Bi12SiO20晶体中的波耦合性质

研究在光折变Bi12SiO20晶体中二波耦合扩散全息记录的信号光能量增益和偏振态改变。在3种制备可得的晶体切割面上给出任意光栅取向的能量和偏振态耦合量的取值范围,分析了信号光能量增益和偏振态改变量分别达到最大值时各向同性和各向异性耦合的作用,此外研究旋光效应，压电及弹光效应在3种晶体切割面上对矢量波耦合的影响。     

Photorefractive and photochromic properties of ruthenium-doped Bi12SiO20

Ruthenium can easily be incorporated into Bi₁₂SiO₂₀ (BSO) and an unusually high (1.3 cm⁻¹) photorefractive gain was measured in the diffusion regime with a krypton laser at 647 nm. One particular experiment demonstrated that electron and hole gratings could eventually be formed with different time constants, thus leading to a reduction of the gain. Several complementary spectroscopic techniques were used to characterise our crystals. Magnetic circular dichroism (MCD) demonstrated that Ru substitutes under several valences for Bi in the pseudo-octahedron formed by bismuth and oxygen atoms. This was confirmed by electron paramagnetic resonance detected either classically (EPR at 9 GHz) or optically (ODMR, 35 GHz). Photochromism was investigated via a series of absorption and MCD experiments on oxidized and reduced samples. Our results in the visible spectral range could be understood via a correlation with the behaviour of the Ru⁵⁺ and Ru³⁺ MCD features in the near-IR, under similar illuminations. The primary process with red light is the ionisation of electrons from the VB to the Ru^4+/3+ acceptor level and the subsequent capture of the left holes at the Ru^5+/4+ level. Under blue light, paramagnetic Bi_Si⁴⁺ is formed via the ionisation of Bi_Si³⁺. The charge transfer transitions of Ru⁵⁺ were assigned via additional experiments on Ru-doped lithium niobate and garnets. The build-up and decay of photochromism were investigated, three different behaviours being observed, depending upon the initial conditions. The three-valence-two-level model is not adequate to explain the bi-exponential temporal behaviour of photochromism. It is suggested that a third relatively shallow level, possibly associated to iron, plays an important role.     

Growth and optical properties of synthetic opal filled with Bi12SiO20 and Bi12GeO20 nanocrystals

We report a technique for producing Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ nanocrystals in pores of synthetic opal. Bragg reflection and Raman spectroscopies are used to assess the degree of filling of opal pores and diagnose the state of the substance in the pores. The substance in the pores is shown to be in a crystalline state. The measured Raman spectra of the nanocrystals are compared to those of powders and single crystals. The spectra of the photonic crystals obtained contain new lines. We observe intensity redistribution at low frequencies and changes in the frequency of several Raman lines compared to the spectra of powders and single crystals.     

Optical properties of Bi12SiO20 (BSO) and Bi12TiO20 (BTO) obtained by mechanical alloying

Mechanical alloying has been used successfully to produce nanocrystalline powders of BTO and BSO. The milled BTO and BSO were studied by x-ray powder diffraction, DTA, infrared and Raman scattering spectroscopy. After 7 hours of milling the formation of BTO and BSO was confirmed by x-ray powder diffraction. The infrared and Raman scattering spectroscopy results suggest that the increase of the milling time lead to the formation of ferroelectric BTO and BSO, as seen by x-ray diffraction analysis. These materials are attractive for various electro-optical devices, including optical data processing. They present a number of attractive features as reversible recording materials for real-time holography and image processing applications. This milling process presents the advantage, that melting is not necessary, and the powder obtained is nanocrystalline with extraordinary mechanical properties. The material, can be compacted and transformed in solid piezoelectric ceramic samples. The high efficiency of the process, opens a way to produce commercial amount of nanocrystalline piezoelectric powders. Due to the nanocrystalline character of this powder, their mechanical properties have changed and for this reason a pressure of 1 GPa is enough to shape the sample into any geometry.     

高能球磨法制备Bi12SiO20粉体

以Bi2O3和SiO2为原料，采用高能球磨法制备了硅酸铋（Bi12SiO20）粉体。通过x射线衍射和扫描电子显微镜分析了合成粉体的相结构和形貌，研究了球磨时间对产物相组成的影响；使用红外光谱分析和拉曼散射光谱对球磨得到的样品进行了分析。结果显示：球磨8h后出现了Bi12SiO20的晶相结构；随着球磨时间的延长促进了Bi12SiO20的形成，从而提供了一种可以商业化大规模生产Bi12SiO20压电粉体的方法。     

Comparison of space- and ground-grown Ce: Bi12SiO20 single crystals

Ce doped Bi₁₂SiO₂₀ single crystals were grown either on board of the Chinese Spacecraft-Shenzhou No.3 (SZ-3) or on the ground at the same conditions with the exception of microgravity. The surface morphology of crystals clearly showed significant differences between the space- and ground-grown portions. The space- and ground-grown crystals have been measured by X-ray rocking curve, Ce concentration distribution in growth direction, dislocation density, absorption spectrums. These results show that the compositional homogeneity and structural perfection of Ce doped crystal grown in space are obviously improved.     

Experimental study of the temperature stability of magnetic field sensors based on Bi12SiO20 crystals

The temperature characteristics of the conversion coefficient of the sensitive element on a fiber-optic magnetic field probe based on a Bi₁₂SiO₂₀ crystal are studied experimentally. A laboratory model of the sensitive element of a magnetic field sensor is built, and the temperature drift of the conversion coefficient is found to be ∼0.15% over the temperature interval from +15 to +70 °C. Pis’ma Zh. Tekh. Fiz. 24, 26–33 (June 12, 1998)     

Growing high-quality Bi12SiO20 crystals of large diameter (85 mm)

Conditions that ensure reproducible growth of ??110??-oriented perfect Bi₁₂SiO₂₀ (BSO) crystals by the low-thermal-gradient Czochralski technique, whereby the entire crystallization front is occupied by the (110) crystal face, have been determined with the aid of numerical simulations. Using the established regime, BSO crystals have been obtained with a diameter of 85 mm, a length of 200 mm, and a mass of 10 kg. The density of dislocations in the crystals does not exceed 10 cm^?2, and the refractive index inhomogeneity is below 10^?3.     

Nonstationary photocurrent in a Bi12SiO20 crystal grown in an argon atmosphere

A detailed investigation of the nonstationary photocurrent in crystals with a sillenite structure grown in an oxygen-free (argon) atmosphere is performed. Basic parameters of the photoinduced charge carriers in the crystals investigated, such as the mean carrier lifetime and mobility, the mean photoconductivity, the carrier diffusion length, and the Debye screening length, are determined. Pis’ma Zh. Tekh. Fiz. 25, 63–69 (May 12, 1999)     

Growth crystallography of Bi12SiO20 single crystals solidified by a floating zone method

Relations between the growth directions of Bi₁₂SiO₂₀ (BSO) single-crystal rods solidified by a floating zone method and the pulling parameters were studied using an X-ray back-reflection Laue technique. It was found that when a platinum wire is used as a seed, the growth direction of the produced BSO single-crystal rod is related to the pulling rate. Statistically, the probability of the growth direction approaching 〈0 1 1〉, 〈1 1 2〉 or 〈0 0 1〉 appears to increase in this order with increase of the pulling rate. In addition when a BSO crystal is used as a seed, the growth direction of the produced BSO crystal rod has the same orientation as the seed crystal if the pulling rate is less than 30 mm h^-1. If the pulling rate is higher than 30 mm h^-1, the growth appears to incline mostly to 〈1 1 2〉 if the seed orientation is near 〈0 1 1〉 or 〈1 1 1〉 but far from 〈0 0 1〉, or to 〈0 0 1〉 if the seed orientation is near 〈0 0 1〉 but far from both 〈0 1 1〉 and 〈1 1 1〉. The angle of inclination increases with the pulling rate, and also with the difference in angle between the orientation of seed crystal and 〈1 1 2〉 or 〈0 0 1〉. The facet morphology of the BSO single-crystal rod is related to its growth direction. The cross-section of the BSO single-crystal rod grown along 〈0 0 1〉 is an octagon with tetrad-rotational symmetry, that grown along 〈0 1 1〉 is an ellipsoid with diad-rotational symmetry, and that along 〈1 1 1〉 is a hexagon with triad-rotational symmetry. The cross-sections of the BSO single-crystal rods grown in other directions are not regular, because there is no clear symmetry. This revised version was published online in November 2006 with corrections to the Cover Date.     

Absorption and Circular Dichroism Spectra of Bi12SiO20<Nd> Crystals

The absorption and circular dichroism spectra of Nd-doped Bi₁₂SiO₂₀ crystals were measured in the range 10000 to 20000 cm^–1. The spectra showed electronic transitions related to only one type of Nd-related center: Nd substituting for Bi in position with symmetry C ₁. The bands due to the transitions from the first Stark component of the ground state to Stark components of excites states (⁴ I _9/2 ⁴ G _5/2, ⁴ I _9/2 ² G _7/2, and ⁴ I _9/2 ⁴ G _7/2) were analyzed in detail. The dipole strength D _0k , rotatory power R _0k , and anisotropy factor G _0k of these transitions were calculated. The intensities of transitions to Stark components were shown to vary by more than one order of magnitude within an excited-state multiplet. The anisotropy factors of the ⁴ I _9/2 ² G _7/2 and ⁴ I _9/2 ⁴ G _7/2 transitions, allowed in the magnetic dipole approximation, are, on the average, larger than that of the ⁴ I _9/2 ⁴ G _5/2 transition, which points to a significant contribution of the magnetic moment (<>²) to the total intensity of the transition.     

Randomly accessible, multilayered optical memory with a Bi(12)SiO(20) crystal

A read-and-write, randomly accessible, multilayered optical memory with a Bi(12) SiO(20) crystal as the medium is demonstrated. Data are recorded in the crystal as an absorption change that is due to the photochromic effect. These data are successfully recorded, read, and selectively erased in five layers in the crystal. The axial-separation distance between neighboring layers is 30 μm, and the lateral distance between bits is 5 μm. Selective bit erasure of the data is accomplished by illumination of the recorded bit datum with He-Ne laser light. To our knowledge, this is the first successful demonstration of the selective optical erasure of the photochromic effect in a BSO crystal.     

Spectroscopic Properties of Bi12SiO20and Bi12TiO20Crystals Doped with Mn, Mn + P, Cr + P, Cr + Ga, and Cr + Cu

The absorption and circular dichroism spectra of Bi₁₂SiO₂₀and Bi₁₂TiO₂₀crystals codoped with Mn or Cr and a penta- (P⁵⁺), tri- (Ga³⁺), or divalent (Cu²⁺) element were studied. The results demonstrate that the sillenite crystals contain Mn and Cr in two oxidation states, 5+ and 4+. The introduction of P⁵⁺increases the fractions of Mn⁴⁺and Cr⁴⁺, while Ga³⁺and Cu²⁺have the opposite effect. The bands observed in the spectra are assigned to electronic transitions of [MnO₄]^4–and [MnO₄]^3–.     

不同磨料对微晶玻璃化学机械抛光的影响

为了提高微晶玻璃化学机械抛光(CMP)的材料去除速率(MRR),降低其表面粗糙度,利用自制的抛光液对微晶玻璃进行化学机械抛光,研究了4种含不同磨料(Si O2、Al2O3、Fe2O3、Ce O2)的抛光液对微晶玻璃化学机械抛光MRR和表面粗糙度的影响.利用纳米粒度仪检测抛光液中磨料的粒径分布和Zeta电位,利用原子力显微镜观察微晶玻璃抛光前后的表面形貌.实验结果表明,在相同条件下,采用Ce O2作为磨料进行化学机械抛光时可以获得最好的表面质量,抛光后材料的表面粗糙度Ra=0.4 nm,MRR=100.4 nm/min.进一步研究了抛光液中不同质量分数的Ce O2磨料对微晶玻璃化学机械抛光的影响,结果表明,当抛光液中Ce O2质量分数为7%时,最高MRR达到185 nm/min,表面粗糙度Ra=1.9 nm;而当抛光液中Ce O2质量分数为5%时,MRR=100.4 nm/min,表面粗糙度最低Ra=0.4 nm.Ce O2磨料抛光后的微晶玻璃能获得较低表面粗糙度和较高MRR.