EPR study of coordination of Ag and Pb cations in BaB<Subscript>2</Subscript>O<Subscript>4</Subscript> crystals and barium borate glasses

It is shown the possibility to determine the coordination of paramagnetic ions in disordered solid structures, e.g., in barium borate glasses. For this purpose the electron paramagnetic resonance (EPR) method was used to study α-and β-BaB₂O₄ crystals and glasses of 45·BaO × 55·B₂O₃ and 40·BaO × 60·B₂O₃ (mol%) composition activated by Ag⁺ and Pb²⁺ ions. After the samples were exposed to X-rays at 77 K, different EPR centers were observed in them. In α-and β-BaB₂O₄ crystals and glasses the EPR centers Ag²⁺, Ag⁰, Pb⁺, Pb³⁺, and hole centers of O⁻ type were studied. The EPR parameters of these centers and their arrangement in crystal structure were determined. It is shown that Pb³⁺ ions in β-BaB₂O₄ crystals occupy Ba²⁺ position in an irregular polyhedron from the eight oxygen, whereas in α-BaB₂O₄ crystals they occupy Bа₂ position in a sixfold coordination. Pb⁺ ions in α-BaB₂O₄ crystals occupy Bа₁ position in a ninefold coordination from oxygen. In barium borate glasses, Pb³⁺ ions were studied in coordination polyhedron from six oxygen atoms and in a polyhedron from nine to ten oxygen atoms. It is assumed that the established difference in the structural position of Pb³⁺ ions in glasses is due to their previous incorporation in associative cation–anion complexes (AC) and “free” structure-forming cations (FC). Computer simulations have been performed to analyze the stability of specific associative complexes and to compare their bond lengths with experimental data.     

Palynological evidence for the astronomical origin of lignite-detritus sequence in the Middle Pleistocene Marathousa Member, Megalopolis, SW Greece

The Marathousa Member, Middle Pleistocene strata in the fluvio-lacustrine Megalopolis basin, southwest Greece, displays distinct but complicated lithological cycles comprising first-order alternation of lignites and detrital muds and second-order alternation expressed by frequent intercalation of organic layers. Palynological evidence indicates that the lithological cycles are driven by the Earth’s orbital forcing. All the lignite seams yield temperate oak forest whereas the detrital beds provide semi-arid steppe mainly of Artemisia. This means that the first-order lithological cycle represents the glacial/interglacial cycle (i.e., the 100-kyr eccentricity cycle), providing a timescale of at least 350 kyr to the Marathousa Member. Pollen also detects smaller-scale climate fluctuations in many of the subordinate organic layers, with the total number of fluctuations being five in a complete lignite-detritus couplet. This means that the second-order lithological cycle reflects the 21-kyr insolation cycle. A tentative phase relation between the lithological cycles and the astronomical cycles is shown based on palynostratigraphy and electron spin resonance dating. Lacustrine environments with increased water tables are implied for the glacial periods sedimentologically, in contrast to local swamp vegetation for the interglacial periods. The subordinate organic layers were formed under intermediate environments (climate, water depth, etc.) between full glacials and interglacials.     

Radiation-induced defects in quartz. II. Single-crystal W-band EPR study of a natural citrine quartz

Single-crystal electron paramagnetic resonance (EPR) spectra of a natural citrine quartz without any artificial irradiation, measured at W-band frequencies (∼94 GHz) and temperatures of 77, 110 and 298 K, allow better characterization of three previously-reported Centers (#6, #7 and B) and discovery of three new defects (B′, C′ and G′). The W-band EPR spectra reveal that Centers #6 and #7 do not reside on twofold symmetry axes, contrary to results from a previous X-band EPR study. The W-band spectra also show that the previously reported Center B is a mixture of two defects (B and B′) with similar g matrices but different-sized ²⁷Al hyperfine structures. Center C′ has similar principal g values to the previously reported Center C but is distinct from the latter by a larger ²⁷Al hyperfine structure with splittings from 0.10 to 0.22 mT. Also, Center G′ has a similar g matrix to the previously reported Center G but a different ²⁷Al hyperfine structure with splittings from 0.41 to 0.53 mT. These spin-Hamiltonian parameters, together with observed thermal properties and microwave-power dependence, suggest that Centers #6 and #7 probably represent O₂³⁻ type defects. Centers B and B′ are probably superoxide radicals (O₂⁻) with the unpaired spin localized on the same pair of oxygen atoms around a missing Si atom but linked to a substitutional Al³⁺ ion each at different neighboring tetrahedral sites. Similarly, Centers G and G′ are most likely superoxide radicals with the unpaired spin localized on another pair of oxygen atoms around a missing Si atom and linked to a substitutional Al³⁺ ion each at different neighboring tetrahedral sites. Center C′ is probably an ozonide radical associated with a missing Si atom and linked to a substitutional Al³⁺ ion at the neighboring tetrahedral site. This study exemplifies the value of  high-frequency EPR for discrimination of  similar defect centers and determination of  small local structural distortions that are often difficult to resolve in conventional  X- and Q-band EPR studies.     

Coordination and valent state of nickel ions in beryl and chrysoberyl crystals

We have studied the polarized optical absorption and the EPR spectra of Ni-doped beryls grown by hydrothermal, flux and gas-transport methods, and chrysoberyl grown by the Czochralski and flux methods. In beryls, three groups of bands belonging to three various Ni centres were distinguished by analysis of the absorption band intensities. The first group, bands with maximums at 21740 (E ⊥ c), 17240 (E || c) and 9260 (E ⊥ + || c), 7140 (E || + ⊥ c) cm⁻¹, are due to Ni³⁺ in octahedral Al³⁺ site. The second group is bands at 25640 (E ⊥ c), 22220 (E || c) and 13520 (E || + ⊥ c), 13160 (E ⊥+ || c) cm⁻¹ and 8930 (E ⊥ + || c), 7460 (E || c) cm⁻¹, which are caused by Ni²⁺ in octahedral Al³⁺ site. Weak wide bands at 17540 (E ⊥ c), 15500 (E || c) cm⁻¹ and 6580 (E || + ⊥ c), 5950 (E || c) cm⁻¹ are related to Ni²⁺ in tetrahedral Be²⁺ site. The occurrence of Ni ions in Be²⁺ site is proved by the EPR spectra of ^1VNi⁺ in γ-irradiated samples. According to the spectra of optical absorption of Ni-doped chrysoberyl, two types of Ni centres have been established: Ni³⁺ and Ni²⁺ ions in octahedral Al³⁺ sites. From the EPR spectra of the X-ray irradiated crystals BeAl₂O₄: Ni, it follows that 68% of Ni⁺ ions occupy octahedral Al³⁺ sites with mirror symmetry and 32% are in Al³⁺ sites with inversion symmetry. In the approximation of trigonal field with regard to Trees correction, the energy levels of Ni³⁺ and Ni²⁺ have been calculated in octahedral and tetrahedral coordination. There is good agreement between the obtained experimental and calculated data. The polarization dependence of the optical absorption bands is well explained in terms of the spin–orbit interaction.     

氢气还原处理对天然金红石可见光吸收的影响机制

天然半导体矿物金红石因结构中含有类质同象替代杂质元素V和Fe,具有一定的可见光吸收和光催化活性。为改善金红石的日光光催化性能,在H2还原气氛下,对天然金红石粉末进行500～900℃不同温度的热处理改性研究。紫外-可见漫反射吸收光谱（UV-Vis diffuse reflectance absorption spectra）表明H2还原处理显著改善了金红石在可见光区460～750 nm波段的光吸收,其中900℃处理样品的光吸收提升最为明显。电子顺磁共振（EPR）和X射线光电子能谱（XPS）测试表明,随着还原温度升高,杂质元素V和Fe从高价态（V5＋,Fe3＋）向较低的价态（V4＋,V3＋,Fe2＋）转化,同时金红石表面的化学吸附水含量也有所增加。本文认为H2还原热处理引起的过渡金属元素价态的改变,尤其是较低氧化态V离子（V4＋和V3＋）的形成,可能是导致金红石样品可见光吸收显著增加的主要原因。     

地开石-高岭石混层矿物的电子顺磁共振(EPR)研究

本文首次利用EPR研究了我国北方石炭-二叠纪煤系地层粘土岩中的粗晶(1～2mm)。与粉末状地开石、高岭石、地开石-高岭石混合样的Fe~(3+)EPR谱对比证明,这些粗晶是一种地开石-高岭石(D-K)的非规则混层矿物,其单晶的EPR表明,Fe~(3+)离子有两种位置:低对称位置,引起弱各向异性的Ⅰ线,λ～0.30;高对称位置,给出强各向异性的E线,λ～0.24。这对历来认为高岭石的Fe~(3+)谱中Ⅰ线为各向同性的看法是一个有益的修正。Ⅰ呈双线(Ⅰ_1及Ⅰ_2)是鉴定这种粗晶属于D-K混层矿物的一个特征标志。     

青田石“灯光冻”品种的宝石矿物学特征及其成因

采用X射线粉末衍射和电子顺磁共振等方法,对青田石中的珍贵品种＂灯光冻＂样品进行了研究。结果表明,＂灯光冻＂样品的矿物组成为叶蜡石,多型特征为2M型,含极少量的1TC型;其结构有序度高,化学成分表现为贫Al、富Si型,边缘部位具有石英化迹象。＂灯光冻＂样品中的Fe3＋呈八面体占位,与Ti4＋,Si-O--Al空穴心共同致色。＂灯光冻＂样品中的叶蜡石晶体呈不规则叶片状,大小较均一,呈弯曲片状集合体结构。     

蒙脱石热处理产物的电子顺磁共振研究

本文对广东和平蒙脱石及其热处理产物进行了化学分析、差热和热重分析、Ｘ射线粉末衍射分析、红外吸收光谱分析及电子顺磁共振研究。结果表明,在热处理过程中,蒙脱石的结构发生很大的变化。蒙脱石的电子顺磁共振谱中有两个主要的特征信号,其ｇ值分别为４１６和２．１,前者对应于蒙脱石八面体片中Ｆｅ３＋的结构信号,后者对应于蒙脱石晶体表面吸附的某种超微粒富铁相———无定形的氧化物和氢氧化物中的Ｆｅ３＋的结构信号。随着蒙脱石热处理温度的不同,两者的信号强度发生很大的变化。这种变化反映出蒙脱石结构中的Ｆｅ和吸附的Ｆｅ在相变过程中所发生的价态和结构变化及迁移。     

ESR测年研究中人工辐照吸收剂量的确定

在ESR测年研究中,被测样品的年代是通过样品的古剂量与样品所接受的年剂量的比值得到的。古剂量则是在实验室内经放射性源的人工辐照模拟自然界的ESR信号增长情况获得的。因此,人工辐照样品吸收剂量的确定成为影响测年可靠性的一个重要因素。目前一般有两种方法确定测年样品的人工辐照吸收剂量值一种是剂量率法,即在辐照前利用标准剂量体系对辐照位置的吸收剂量率进行校准,然后在同一位置辐照被测样品,通过校准得到的吸收剂量率和辐照时间确定样品的吸收剂量,通常所用的标准剂量体系为硫酸亚铁剂量计;另一种是伴随法,即在辐照前将参考剂量计与被测样品一起放到辐射场中辐照,参考剂量计一般采用经标准剂量体系(如硫酸亚铁剂量计或量热计等)校准过的丙氨酸剂量计,照后由参考剂量计直接给出被测样品的吸收剂量。文中就确定吸收剂量的两种方法的可靠性进行了探讨,并利用两种类型的丙氨酸剂量计通过伴随法测定了被测样品的吸收剂量。结果表明,根据目前ESR测年研究中地质样品人工辐照的实际情况,利用伴随法通过参考剂量计确定被测样品的吸收剂量更为实用与可靠