COMPARATIVE ANALYSIS OF THE INFLUENCE OF DOPING IONS OF YTTRIUM AND BORON FORMATION OF DEFECTS IN LITHIUM NIOBATE CRYSTALS

Кристалл ниобата лития, являющийся широко используемым и весьма востребованным в настоящее время нелинейно-оптическим материалом, примечателен своей способностью к варьирования широкого спектра сегнетоэлектрических и нелинейно-оптических характеристик в зависимости от типа и концентрации примесного иона, а также от соотношения Li / Nb . На основе разработанного нами подхода к моделированию кластеров в кристалле ниобата лития, в котором рост кластера идёт не элементарными ячейками, а кислородными октаэдрами, проведён сравнительный анализ особенностей внедрения в кристалл примесных ионов иттрия и бора с одинаковым зарядом +3 . Показано, что встраивание, вследствие различного ионного радиуса, идёт по двум механизмам: если для металла иттрия действует обычный механизм, когда примесной ион локализуется внутри кислородного октаэдра, то ион неметаллического элемента бора встраивается в тетраэдрические пустоты структуры, а именно в кислородные плоскости, образующие октаэдр. При этом влияние данных примесных ионов на одну из важнейших характеристик ниобата лития оказывается диаметрально противоположным: иттрий усиливает фоторефрактивный эффект, бор -понижает, что необходимо учитывать при направлении целевого использования кристаллов ниобата лития. Lithium niobate crystals, which are a widely used and highly demanded as nonlinear optical material, are remarkable for their ability to vary a wide range of ferroelectric and nonlinear optical characteristics depending on the type and concentration of the impurity ion, as well as on the Li / Nb ratio. Based on our approach to modeling clusters in the lithium niobate crystal, in which the cluster grows not by unit cells, but by oxygen octahedra, we have carried out a comparative analysis of the features of the incorporation of impurity ions of yttrium and boron with the same charge +3 into the crystal. It is shown that due to the different ionic radii the incorporation proceeds by to two mechanisms. If for yttrium the usual mechanism operates, when the impurity ion is localized inside the oxygen octahedron. The ion of boron, i.e. a nonmetallic element, is incorporated into the tetrahedral voids of the structure, namely, into the oxygen planes forming the octahedron. In this case, the influence of these impurity ions on one of the most important characteristics of lithium niobate turns out to be diametrically opposite: yttrium enhances the photorefractive effect, boron decreases it, which must be taken into account in the direction of targeted use of lithium niobate crystals.

Studies on impurity seeding and transport in edge and SOL of tokamak plasma

We present numerical simulation studies on impurity seeding using Nitrogen, Neon, and Argon gases. These impurity gases are ionized by the electron impact ionization. These ions can be at multiply ionized states, recombine again with the plasma electrons, and radiate energy. The radiation losses are estimated using a non-coronal equilibrium model. A set of 2D model equations to describe their self-consistent evolution are derived using interchange plasma turbulence in the edge and SOL regions and solved using BOUT++. It is found that impurity ions (with single or double-positive charges) move in the inward direction with a velocity ∼ 0.02cs so that these fluxes are negative. These fluxes are analyzed for different strengths of an effective gravity that help to understand the impurity ion dynamics. Increased gravity shows an accumulation of certain charged species in the edge region. The radiation loss is seen to have a fluctuation in time with frequency 5-20 kHz that closely follows the behavior of the interchange plasma turbulence. The simulation results on the radiated power and its frequency spectrum compare favourably with observations on the Aditya-U tokamak. The negative fluxes of the impurity ions, their dynamics in the edge region, and the fluctuating nature of the radiation loss are the most important results of this work.

MHD simulations of cold bubble formation from 2/1 tearing mode during massive gas injection in a tokamak

Massive gas injection (MGI) experiments have been carried out in many tokamaks to study disruption dynamics and mitigation schemes. Two events often observed in those experiments are the excitation of the m = 2, n = 1 magnetohydrodynamic (MHD) mode, and the formation of cold bubble structure in the temperature distribution before the thermal quench (TQ). Here m is the poloidal mode number, n the toroidal mode number. The physics mechanisms underlying those phenomena, however, have not been entirely clear. In this work, our recent NIMROD simulations of the MGI process in a tokamak have reproduced the main features of both events, which has allowed us to examine and establish the causal relation between them. In these simulations, the 3/1 and 2/1 islands are found to form successively after the arrival of impurity ion cold front at the corresponding q = 3 and q = 2 rational surfaces. At the interface between impurity and plasma, a local thin current sheet forms due to an enhanced local pressure gradient and moves inward following the gas cold front, this may contribute to the formation of a dominant 2/1 mode. Following the growth of the 2/1 tearing mode, the impurity penetration into the core region inside the q = 2 surface gives rise to the formation of the cold bubble temperature structure and initiates the final TQ. A subdominant 1/1 mode developed earlier near the q = 1 surface alone does not cause such a cold bubble formation, however, the exact manner of the preceding impurity penetration depends on the nature of the 1/1 mode: kink-tearing or quasi-interchange.

Study on the Effect and Mechanism of Impurity Aluminum on the Solvent Extraction of Rare Earth Elements (Nd, Pr, La) by P204-P350 in Chloride Solution

Solvent extraction is the most widely used method for separation and purification of rare earth elements, and organic extractants such as di(2-ethylhexyl) phosphoric acid (P204) and di(1-methyl-heptyl) methyl phosphonate (P350) are most commonly used for industrial applications. However, the presence of impurity ions in the feed liquid during extraction can easily emulsify the extractant and affect the quality of rare earth products. Aluminum ion is the most common impurity ion in the feed liquid, and it is an important cause of emulsification of the extractant. In this study, the influence of aluminum ion was investigated on the extraction of light rare earth elements by the P204-P350 system in hydrochloric acid medium. The results show that Al3+ competes with light rare earths in the extraction process, reducing the overall extraction rate. In addition, the Al3+ stripping rate is low and there is continuous accumulation of Al3+ in the organic phase during the stripping process, affecting the extraction efficiency and even causing emulsification. The slope method and infrared detection were utilized to explore the formation of an extraction compound of Al3+ and the extractant P204-P350 that entered the organic phase as AlCl[(HA)2]2P350(o).

Фононный спектр и упругие свойства Y-=SUB=-2-=/SUB=-Sn-=SUB=-2-=/SUB=-O-=SUB=-7-=/SUB=-

In the framework of a unified ab initio approach, the phonon spectrum of yttrium stannate Y2Sn2O7 was calculated, the frequencies and types of IR and Raman modes were determined. From an analysis of the displacement vectors obtained in ab initio calculations, the degree of participation of ions in phonon modes is determined. The elastic constants and hardness of Y2Sn2O7 are calculated. The distance "impurity ion-ligand" in impurity centers Y2Sn2O7: Yb3 +, Y2Sn2O7: Eu3 +, Lu2Sn2O7: Yb3 +, Nd2Sn2O7: Gd3 +, Nd2Sn2O7: Tb3 + is calculated.

Global calculation of neoclassical impurity transport including the variation of electrostatic potential

Recently, the validity range of the approximations commonly used in neoclassical calculation has been reconsidered. One of the primary motivations behind this trend is observation of an impurity hole in LHD (Large Helical Device), i.e. the formation of an extremely hollow density profile of an impurity ion species, such as carbon $\text{C}^{6+}$ , in the plasma core region where a negative radial electric field ( $E_{r}$ ) is expected to exist. Recent studies have shown that the variation of electrostatic potential on the flux surface, $\unicode[STIX]{x1D6F7}_{1}$ , has significant impact on neoclassical impurity transport. Nevertheless, the effect of $\unicode[STIX]{x1D6F7}_{1}$ has been studied with radially local codes and the necessity of global calculation has been suggested. Thus, we have extended a global neoclassical code, FORTEC-3D, to simulate impurity transport in an impurity hole plasma including $\unicode[STIX]{x1D6F7}_{1}$ globally. Independently of the $\unicode[STIX]{x1D6F7}_{1}$ effect, an electron root of the ambipolar condition for the impurity hole plasma has been found by global simulation. Hence, we have considered two different cases, each with a positive (global) and a negative (local) solution of the ambipolar condition, respectively. Our result provides another support that $\unicode[STIX]{x1D6F7}_{1}$ has non-negligible impact on impurity transport. However, for the ion-root case, the radial $\text{C}^{6+}$ flux is driven further inwardly by $\unicode[STIX]{x1D6F7}_{1}$ . For the electron-root case, on the other hand, the radial particle $\text{C}^{6+}$ flux is outwardly enhanced by $\unicode[STIX]{x1D6F7}_{1}$ . These results indicate that how $\unicode[STIX]{x1D6F7}_{1}$ affects the radial particle transport crucially depends on the profile of the ambipolar- $E_{r}$ , which is found to be susceptible to $\unicode[STIX]{x1D6F7}_{1}$ itself and the global effects.