Interference absorption coefficient of X-rays in crystals in the presence of temperature gradient

We investigate experimentally and theoretically the behavior of the interference coefficient of absorption of X-ray radiation of a quartz single crystal in Laue geometry in the presence of temperature gradient. The total intensity of transmitted X-ray radiation and that reflected from different families of reflecting atomic planes of the quartz single crystal has been recorded. It was shown that with the increase in temperature gradient the coefficient of absorption of X-ray radiation decreases abruptly reaching its minimal value at a certain value of the temperature gradient.     

Effect of the Temperature Gradient on the X-ray Diffraction Spectrum of a Quartz Crystal

The spectra of X-ray diffraction from the reflecting atomic plane (1 0 1? 1) of a quartz single crystal are studied in Laue geometry under the action of temperature gradient on a BDER-KI-11K spectrometer with a resolution of 300 eV on the Am241 line of 17.74 keV. The temperature gradient leads to an increase in the intensity of the diffracted beam depending on the heating temperature. It is shown that the intensity of X-ray diffraction in Laue geometry may increase at a temperature gradient of 250°C/cm by two orders of magnitude in comparison with the uniform temperature state of the crystal. The rocking curve of the reflected beam is obtained at a fixed observation angle of 6° and a specified temperature gradient. It is demonstrated that the intensity of the reflected beam increases with increasing temperature gradient (to a certain value), while the spectral width of the reflection line remains constant and is governed by the energy resolution of the spectrometer. A further growth in the temperature gradient leads to an increase in the spectral width of the reflection line with decreasing intensity of the reflected beam.     

Study of X-ray diffraction processes in ADP crystals under the influence of temperature gradient

The influence of temperature gradient on the intensity of X-ray beam reflected from atomic planes of antiferroelectric crystal of ammonium dihydro-phosphate (ADP) was studied. It was found that under the influence of temperature gradient on ADP crystal, the intensity of diffracted X-ray beam initially decreases at small values of gradient and then monotonously increases with increasing gradient. It is assumed that in the initial period of the action of temperature gradient the crystal domains have no time to orient in the same direction, and the intensity of beam decreases owing to the scattering of X-rays in different directions when reflected from the walls of boundaries of domains. After the alignment of domains, their arrangement in the same direction, separate areas of unidirectional domains are formed under the action of temperature gradient and the intensity of diffracted X-ray beam increases, as confirm the experimental data. The specified mechanism is supposed to occur also in other crystals having the domain structure.     

Dynamical diffraction of X-rays in ADP single crystal under influence of temperature gradient

The dependence of the integrated intensity of X-ray beams diffracted from a single crystal of ammonium dihydrogen phosphate (ADP) in the Laue geometry on the applied temperature gradient has been investigated. It is found that the integrated intensity of the X-ray beam reflected from a single crystal depends almost linearly on the magnitude of the temperature gradient, applied perpendicular to the reflecting atomic planes, up to saturation. For small values of the temperature gradient, in case of antiparallel vectors of diffraction and temperature gradient, integrated intensity of the reflected X-rays is initially reduced by about 10% and then increases monotonically with increasing value of the temperature gradient. Based on the theoretical analysis, an explanation of the observed phenomena is given.     

Hard X-ray diffraction in quartz single crystals in the presence of temperature gradient

The diffraction of hard X-rays in quartz single crystals is considered in the Laue geometry in the presence of temperature gradient. Spectral and angular characteristics of the reflected beam were experimentally studied versus the magnitude of temperature gradient. It is shown that as the temperature gradient applied perpendicular to reflecting (10ī1) atomic planes of quartz single crystal increases, the focus of reflected beam approaches the crystal, the angular and spectral widths increase, and intensity increases by orders of magnitude.     

Study of angular width of an X-ray beam fully pumped from quartz single crystal in the presence of temperature gradient

We investigate the cross section of transmitted X-ray beam diffracted by reflecting atomic planes (10–11) of a SiO₂ single crystal in Laue geometry for different values of temperature gradient applied to the crystal. It is shown that the angular width of a fully pumped X-ray beam depends on the distance between the source and the studied sample and decreases with the increase in this distance.     

Self-heating effects in electro-optic light modulators

The performance of an electro-optic light modulator under high average laser power is studied theoretically and experimentally. Absorption of laser radiation produces a thermal gradient in the electro-optic crystal which degrades the performance of the modulator. Measurements on modulators using KDP and KD^*P crystals support the analytical results.     

Effect of a preliminary magnetic treatment of KDP crystals and solutions on the equilibrium between them

Measurements of the temperature of equilibrium between a nonmagnetic KDP crystal and its aqueous solution demonstrated that the magnetization of the KDP crystal or its solution causes a change in this temperature. It was shown that the effect of magnetization has the maximum magnitude immediately after magnetization and then weakens monotonically, virtually disappearing within 5 h. The effect exhibits saturation at magnetic induction values within B = 0.02−2.0 T. The most pronounced decrease in the solubility was observed for magnetized KDP crystals doped with chromium; the decrease for magnetized KDP crystals without chromium was more moderate and yet smaller for magnetized KDP solutions. The lifetimes of the magneto-induced state for these three cases were 2.5, 2.8, and 2.6 h, respectively.     

Dynamics of muons in the ferroelectric material KDP

Muon spin relaxation in zero field and longitudinal field was measured in single crystal samples of KH₂PO₄ (KDP) and KD₂PO₄ (DKDP) over a temperature range of 5 K to 300 K. At low temperatures, diamagnetic muons and muon substituted radicals with nuclear hyperfine coupling can be observed. For both KDP and DKDP, a minor change was observed in the dynamics of the muon below 140 K. Above 140 K, the mobility of the muon appears to increase and the diffusion rate becomes faster with increasing temperature. Only a small increase in the relaxation rate is observed in KDP due to the presence of theH ⁺, suggesting that the relaxation effects probably originate from the³¹P.     

Investigation of bulk laser damage in KDP crystal as a function of laser irradiation direction, polarization, and wavelength

Bulk laser-induced damage in KDP crystal was measured using a single-shot 1-ns pulse Nd:YAG laser in a transverse and longitudinal single mode. It is found that the damage threshold of KDP single crystal depends on the laser irradiation direction, polarization direction and laser wavelength. The damage threshold in the direction of c axis is about two times higher than that of in the a(b) axis at 0.532 and 1.064 μm wavelength. This result is consistent with the mechanical strength tests for various directions of KDP crystal. Received: 19 February 1999 / Revised version: 3 September 1999 / Published online: 27 January 2000     

Temperature‐dependent Raman scattering of KDP:Mn (0.9% weight of Mn) crystal

Low temperature polarized Raman scattering measurements of KDP:Mn (0.9% weight of Mn) were performed at temperatures ranging from 14 to 300 K, over the spectral range 50–1250 cm⁻¹. In the present results we can see that the spectra of undoped and doped samples at room temperature are very different. Doped samples maintain the KDP structure as tetragonal, with the same factor group D_2d but with a different class of the space group, different from the original 12. The results show that the crystal undergoes a phase transition at temperature between 115 and 97 K, which is much lower than the phase transition temperature of undoped KDP that occurs at 122 K, where the crystal changes from the para‐electric to the ferroelectric phase. Further, at very low temperature (14 K) we can see that the spectra of KDP:Mn (0.9% weight of Mn) present a behavior very different from the behavior presented by the spectra of KDP doped with low Mn concentration. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of magnetic treatment of KDP and ADP crystals and solutions on their equilibrium temperature

The effect of magnetic treatment of KDP and ADP crystals and solutions on their equilibrium temperature has been revealed and studied. The change in the temperature reaches a maximum immediately after the magnetization. Then, the effect decreases monotonically, and the equilibrium temperature approaches the equilibrium value for 5 h. It has been found that the observed decrease in the solubility depends on the impurity concentration, and it is maximum in magnetically treated KDP crystals with a chromium impurity; then, it decreases in the series: KDP crystal, KDP-ADP crystals in KDP-ADP solution, KDP crystal in KDP-ADP solution, and KDP solution. For all five systems under study, the relaxation times have been determined. It has been revealed that the effect rapidly increases and reaches saturation at B = 0.02 T. The energy effects in the crystals and solutions have been estimated.     

The nonlinear refractive indices and nonlinear third-order susceptibilities of quadratic crystals

The third-order nonlinear susceptibilities and nonlinear refractive indices of KDP, BBO, and LiNbO₃ crystals at wavelengths of 1064 and 532 nm are measured using the Z-scan technique. The measurements are made for different energies of incident radiation and different focusing conditions and crystal lengths. It is found that, as the angle between the light beam and the principal optic axis of a KDP crystal increases, the nonlinearity drops, with its magnitude at a wavelength of 1064 nm being higher than at 532 nm. For a BBO crystal, the nonlinearity dispersion is normal. The mechanisms of nonlinear losses in KDP, BBO, and LiNbO₃ crystals are identified. The values of the nonlinear susceptibilities thus obtained are analyzed and compared with the results of calculations based on an empirical model and with the data of other authors.     

Effect of organic dyes on the dielectric properties of KH<Subscript>2</Subscript>PO<Subscript>4</Subscript> crystals

The effect of organic dyes on the dielectric properties of KH₂PO₄ (KDP) crystals is studied over a wide range of temperatures. The dielectric properties of KDP crystals doped with molecules of the Chicago Sky Blue and Amaranth organic dyes are investigated for the first time. The dye molecules can be incorporated into the crystal lattice of KDP and selectively paint the pyramidal growth sectors of the crystal. The influence of dye organic impurities on the domain contribution to the permittivity is analyzed with due regard for the sectoral crystal structure. It is demonstrated that, upon doping of KDP crystals with organic dyes, the blocking effect of background impurities on domain walls is substantially weakened in the prismatic growth sector of the crystal in the polar phase. This leads to a noticeable change in the dielectric properties, specifically to an increase in the domain contribution to the permittivity of the crystal.     

Formation energies and electronic structures of native point defects in potassium dihydrogen phosphate

The formation energies and the equilibrium concentration of vacancies,interstitial H,K,P,O and antisite structural defects with P and K in KH 2 PO 4 (KDP) crystals are investigated by ab initio total-energy calculations.The formation energy of interstitial H is calculated to be about 2.06 eV and we suggest that it may be the dominant defect in KDP crystal.The formation energy of an O vacancy (5.25 eV) is much higher than that of interstitial O (0.60 eV).Optical absorption centres can be induced by defects of O vacancies,interstitial O and interstitial H.We suggest that these defects may be responsible for the lowering of the damage threshold of the KDP.A K vacancy defect may increase the ionic conductivity and therefore the laser-induced damage threshold decreases.     

On the Possibility of Two-Dimensional Focusing of Reflected X-Rays from Quartz Single Crystal in the Presence of External Temperature Gradient

The Laue reflection from a quartz single crystal was experimentally studied in the presence of temperature gradient applied perpendicularly to reflecting atomic planes (\(10\bar 11\)), so that a two-dimensional bending of these planes occurs. It is shown that as a result of such an application of temperature gradient to a crystal, the two-dimensional focusing of reflected X-rays is obtained. With increasing temperature gradient the focus of X-rays approaches the crystal more rapidly in the plane of reflection than in the perpendicular one.     

KDP晶体固-液界面吸附行为的分子模拟研究

通过构建晶体表面-KDP分子界面吸附结构模型, 采用分子动力学和密度泛函计算方法研究KDP分子在(001)和(010)面吸附的物理化学过程, 考察了温度对物理吸附行为的影响. 研究表明: KDP晶体表面的吸附过程和生长习性主要由化学吸附主导, 化学吸附能的计算表明[K-O₈]基元在(001)界面的结合能是(010)界面结合能的2.86倍; 在饱和温度附近, [H₂PO₄]^-阴离子在KDP界面的物理结合能随温度的变化呈现振荡特征, 溶液中有较多的离子团簇形成, 溶液变得很不稳定; 当温度从323 K降低至308 K时, 水分子在界面的结合能总体呈下降趋势, 而KDP分子在界面的吸附能总体呈上升趋势, 脱水过程是水分子和[H₂PO₄]^-阴离子在固液界面边界层竞争吸附的结果. 研究结果对确足晶体生长界面动力学过程发展和完善晶体生长理论有重要意义. 关键词： 分子动力学 双层结构模型 结合能     

Radiation effect on optical,morphological and magnetic properties of aluminum-substituted M phase barium hexaferrite

The effect of gamma irradiation on the features of aluminum-substituted barium hexagonal ferrite particles BaAl_xFe_12?xO₁₉ with 0?≤?x?≤?3.5 has been studied. Optical absorption measurements have been performed and the results reflected a great dependence of the fundamental absorption edge on the radiation dose. It is found that the calculated optical band gap (E_g) increases due to an increase in the homogeneity with an increase in the Al content. Increasing the radiation dose up to 1?MGy induces a direct transition and consequently decreases the energy gap. This behavior is associated with the generation of excess electronic localized states. Moreover, the characteristic features of the irradiated samples have been studied using a scanning electron microscope. Also, all samples were characterized using the X-ray diffraction technique, and the values of crystal size, microstrain and dislocation density were calculated. On the other hand, the magnetic behavior of the samples was studied using a vibrating sample magnetometer technique after each radiation dose. The saturation magnetization (M_s) and the magneton number (n_B) decrease with an increase in the Al³⁺ substitution and at the same time decrease with the radiation dose 250?kGy to 1?MGy.     

Single crystal diffraction with X-rays and neutrons: High quality at high pressure?

Abstract

Single crystal X-ray and neutron diffraction is essential for determining occupancies, positional as well as static and dynamic displacement parameters in crystalline matter by measuring Bragg, satellite or diffuse reflections. In our contribution a new low-temperature high pressure cell for neutron single crystal diffraction will be presented. It is designed to operate from a few K to ambient temperature in “orange” cryostats at pressures up to at least 3 GPa. We will present first neutron diffraction results obtained at E4/HMI Berlin and discuss the quality of the data and the significance of the results. A software package has been written (as a part of the PROMETHEUS system), which deals with the data reduction for both X-ray and neutron high pressure cell single crystal data. Likewise a data collection program has been developed for single crystal data collection on four-circle diffractometers using Merrill-Bassett cells. A series of single crystal experiments on H₂O and D₂O ice VI and KDP (KH₂PO₄) show that results of very high quality can be obtained routinely including even higher order terms in the atomic displacement parameters.     

High-temperature Raman spectra of KDP:Ni crystal

High-temperature Raman scattering measurements of KDP doped with Ni (1% weight) were performed over the spectral range 50–1200 cm^?1. The X-ray powder diffraction patterns taken at room temperature and analyzed by Rietveld refinement showed that doped samples of KDP:Ni have the same tetragonal structure of a pure KDP crystal, but with a contraction of the crystalline cell. The behavior of the high-temperature Raman spectra indicate that two phase transitions occur, one at 413 K and another one at 443 K. These phase transitions occur at temperatures below the thermal dehydratation point, 488 K. DTA and TGA measurements reveals that KDP:Ni and pure KDP have the same thermal dehydratation temperature.