Electron Spin Coherence of Phosphorus Donors in Isotopically Purified 29Si

We investigate spin coherence time of electrons bound to phosphorus donors in silicon single crystals, employing a pulsed electron paramagnetic resonance technique. The samples were isotopically controlled so that they may possess different concentrations (about 5% and 100%) of ²⁹Si, which is the only non-zero-spin (spin-1/2) stable isotope of Si. Both ²⁹Si-concentration dependence and orientation dependence of the electron spin coherence time demonstrate that the decoherence is caused by spectral diffusion due to mutual flip-flops of the environmental nuclear spins. The detail analysis of spin echo decay curves enables the unique assignment of the host sites responsible for electron spin echo envelope modulation.     

Preparation of single-crystal <Superscript>29</Superscript>Si

A process has been developed for the preparation of single-crystal ²⁹Si from ²⁹Si-enriched silane. A silicon single crystal has been grown with a ²⁹Si content over 99.9 at %. The oxygen and carbon concentrations in the crystal are under 1 × 10¹⁶ cm⁻³, and its resistivity exceeds 1 kΩ cm.     

High-purity silicon isotopes <Superscript>28</Superscript>Si, <Superscript>29</Superscript>Si,and <Superscript>30</Superscript>Si

The current status of the problem of obtaining high-purity silicon isotopes ²⁸Si, ²⁹Si, and ³⁰Si is analyzed. The scheme of obtaining monoisotopic silicon includes the stages of isotope separation in the form SiF₄, synthesis and deep purification of isotopically enriched silane, obtaining polycrystalline silicon-28,-29, and-30, and growing monocrystals. The basic problems and methods of their solution in the synthesis and deep purification of silane and obtaining poly-and monocrystals of isotopically enriched silicon are discussed. Data characterizing the achieved level of chemical and isotopic purity of high-purity monocrystals of silicon-28 with a main isotope content of more than 99.99% and silicon-29 and silicon-30 with isotopic purity higher than 99% are presented. In monocrystalline ²⁸Si, the boron content was 4.5 × 10¹³, the phosphorus content was 5 × 10¹¹, the carbon and oxygen contents were <1 × 10¹⁶ at/cm³, and the specific resistance was 800 Ω cm. The results of investigation of heat capacity, heat conduction, photoluminescence, and electron paramagnetic resonance spectra for monoisotopic silicon-28 are presented. The heat conduction of monoisotopic silicon is increased considerably owing to the reduced photon scattering on isotopic inhomogeneities. In the region of 20–30 K, the heat conduction of silicon-28 with an isotopic purity of 99.98% is higher by a factor of 8 than the heat conduction of natural silicon. Investigations of photoluminescence spectra in the magnetic field in the low-temperature region demonstrated the capability of optical detection of nuclear spin states of a phosphorus admixture in high-purity silicon-28. p ]Topical questions for further investigations and possible fields of practical application of high-purity isotopically enriched silicon are discussed.     

Structural studies of geopolymers by <Superscript>29</Superscript>Si and <Superscript>27</Superscript>Al MAS-NMR

A systematic study of geopolymers by ²⁹Si and ²⁷Al MAS NMR has been carried out in an attempt to understand polymer structural details. ²⁷Al MAS NMR data shows that transient aluminium species are formed during the reaction of metakaolin with NaOH. Interaction of silicate anions with the aluminium sites of metakaolin was evident during the synthesis of geopolymers as observed from low field shift of ²⁹Si MAS NMR resonance lines of silicate centres. As the reaction progresses, the coordination of aluminium (IV, V and VI) in metakaolin changes almost completely to IV. ²⁹Si MAS NMR of selected compositions of the ternary system of sodium silicate, metakaolin and aqueous alkali reveals that geopolymerisation occurs in a distinct compositional region. At high alkalinity [> 30% (mol/mol) overall Na²O content], connectivity of silicate anions is reduced, consistent with poor polymerisation. At low alkalinity [<10% (mol/mol) overall Na²O content], a clear ²⁹Si NMR resonance line due to unconverted metakaolin is observed. NMR spectra were recorded from a series of samples with a fixed Na²O content (20 mol%) and varied SiO²/Al²O³ ratio to observe aluminium substitution in the cross-linked silicon tetrahedra of polymer network. Aluminium insertion into the silicate network is confirmed from the observed ²⁹Si NMR shift as a function of Si/Al ratio. The identification of the presence or absence of metakaolin in the cured geopolymer product is not possible even by ²⁹Si NMR as the signal from metakaolin is indistinguishable from a broad ²⁹Si NMR peak consisting of many resonance lines from the network of cross-linked silicon/aluminium tetrahedra. In an attempt to identify metakaolin signal, we prepared geopolymers with higher SiO²/Al²O³ molar ratios. Since aluminium substitutions in the silicate tetrahedral network are decreased, this results in better-resolved ²⁹Si NMR lines. The ²⁹Si NMR signal due to metakaolin is then distinguishable in the spectra of cured products in a series of samples with 3 to 11 mol% metakaolin. These results indicate that a geopolymer structure is a network of silicon/aluminium tetrahedra with some presence of unreacted metakaolin. The silicon/aluminium tetrahedra might have connectivity ranging from 1 to 4.     

Isolation of <Superscript>58</Superscript>Co, <Superscript>54</Superscript>Mn,and <Superscript>86</Superscript>Rb from <Superscript>89</Superscript>Sr production wastes

Procedures for isolation of ⁵⁸Co, ⁵⁴Mn, and ⁸⁶Rb from ⁸⁹Sr production wastes are described. The production of ⁸⁹Sr was based on irradiation of natural Y in the form of oxide, placed in 1Cr18Ni10Ti stainless steel ampules, on a BR-10 fast reactor. ⁵⁸Co and ⁵⁴Mn were isolated from the casings of stainless steel ampules, and ⁸⁶Rb, from the tail solution after the isolation of ⁸⁹Sr.     

Eu<Superscript>2+</Superscript> and Gd<Superscript>3+</Superscript> Distributions in Fluorohafnate Glasses Studied by Electron Paramagnetic Resonance

We have studied electron paramagnetic resonance spectra of europium- and gadolinium-activated ZBLAN-type fluorohafnate glasses, using the composition 58HfF₄ · 20BaF₂ · 2LaF₃ · 3AlF₃ · 17NaF as an example. The ratio of the concentration of free Eu²⁺ and Gd³⁺ ions to that of ions in clusters has been quantitatively evaluated for the first time. The percentage of free ions has been shown to increase significantly with decreasing activator concentration. At activator concentrations of 1.25 mol % EuF₂ and 1 mol % GdF₃, the activator ions predominantly form clusters and only a small fraction of Eu²⁺ and Gd³⁺ are present as individual ions, whereas at 0.1 mol % EuF₂ the concentration of free ions is comparable to that of ions in the clusters.     

Distribution of <Superscript>137</Superscript>Cs, <Superscript>90</Superscript>Sr, <Superscript>239 + 240</Superscript>Pu, <Superscript>241</Superscript>Am,and <Superscript>244</Superscript>Cm among Components of Organic Matter of Soils in Near Exclusion Zone of the Chernobyl NPP

Distribution of Chernobyl-derived ¹³⁷Cs, ⁹⁰Sr, ^{239 + 240}Pu, ²⁴¹Am, and ²⁴⁴Cm among organic fractions of soddy-podzolic, sandy, soddy-meadow, and peat soils collected from the Chernobyl Exclusion Zone along the North-Western radioactive fallout track was determined. Regardless of the soil type, 80–85% of ¹³⁷Cs is tightly fixed on the mineral fraction of the soil. Depending on the soil type, 50–70% of ⁹⁰Sr and 15– 45% of ²⁴¹Am are associated with fulvic acid fractions. ²⁴¹Am and ²⁴⁴Cu are similarly distributed among the organic acid fractions. In all the soil types studied, ^{239 + 240}Pu is associated essentially with humic acid fractions. Natural ^{230, 232}Th and technogenic ^{239 + 240}Pu are similarly distributed among the organic fractions.__________Translated from Radiokhimiya, Vol. 47, No. 1, 2005, pp. 91–96.Original Russian Text Copyright © 2005 by Odintsov, Pazukhin, Sazhenyuk.     

Void formation and cracking of Zr<Superscript>41</Superscript>Ti<Superscript>14</Superscript>Cu<Superscript>12.5</Superscript>- Ni<Superscript>10</Superscript>Be<Superscript>22.5</Superscript> bulk metallic glass under planar shock compression

Planar shock compression effects on void formation and cracking in Zr⁴¹Ti¹⁴Cu^12.5Ni¹⁰Be^22.5 bulk metallic glass (BMG) are studied in this paper. Cracking was found to be a result of void linkage in some direction deviation from the maximum shear stress plane. Changing the state of the stress inside the BMG sample led to formation of different void distribution. Nucleation of the microvoids was possibly initiated by release of excess free volume under shock wave compression.     

Behavior of Microamounts of <Superscript>22</Superscript>Na, <Superscript>85</Superscript>Sr, <Superscript>137</Superscript>Cs,and <Superscript>152</Superscript>Eu in Sorption and Coprecipitation on Mixed Potassium Neodymium Ferrocyanide from Solutions

Sorption of ⁸⁵Sr, ¹³⁷Cs, ²²Na, and ¹⁵²Eu on solid mixed potassium neodymium ferrocyanide KNd[Fe(CN)₆]·4H₂O from neutral, acidic, and alkaline media and also coprecipitation of these radionuclides with KNd[Fe(CN)₆]·4H₂O in its formation from a homogeneous solution were studied. It was found that ⁸⁵Sr and ²²Na do not noticeably coprecipitate with solid KNd[Fe(CN)₆]·4H₂O and are not sorbed by this substance. In aqueous medium, depending on the cesium concentration in solution, from 80 to 98% of ¹³⁷Cs coprecipitates with solid KNd[Fe(CN)₆]·4H₂O. In this case, the distribution coefficient K_d depends on both the cesium concentration in solution and solution pH. Within 30 min of contact of the solid and liquid phases, the degree of recovery of ¹³⁷Cs from aqueous solution with KNd[Fe(CN)₆]·4H₂O is approximately 95.0% of the initial amount. ¹⁵²Eu coprecipitates with solid KNd[Fe(CN)₆]·4H₂O during its formation from a homogeneous solution to 98–99.9%. The degree of recovery of ¹⁵²Eu from aqueous solution with KNd[Fe(CN)₆]·4H₂O precipitate within 60 min of contact of the solid and liquid phases is 70.3% of the initial amount.     

Fine purification of monoisotopic <Superscript>32</Superscript>S and <Superscript>34</Superscript>S

We have proposed and tested a combined process for ultrapurification of monoisotopic ³²S and ³⁴S sulfur, which comprises thermochemical treatment of sulfur vapor on silica and ceria packing, melting with aluminum, and distillation. The impurity composition of the purified sulfur has been determined by atomic emission and IR spectroscopy. We have obtained monoisotopic ³²S and ³⁴S sulfur samples comparable in chemical purity to high-purity sulfur of natural isotopic composition.     

Phase stabilization and structural studies of nanocrystalline La<Superscript>2</Superscript>O<Superscript>3</Superscript>-ZrO<Superscript>2</Superscript>

La²O³ doped nanocrystalline zirconia (ZrO²) has been prepared by chemical co-precipitation method for various dopant concentrations, varying from 3 to 30 mol%. Structural phases have been characterized by X-ray diffraction technique. All the as-synthesized samples were found to be in monoclinic phase. Annealing of the samples at different temperatures from 400 to 1000∘C stabilized ZrO² either partially or fully the tetragonal/cubic phases. When they were annealed at 1200∘C, the monoclinic phase appeared again with a new cubic pyrochlore structured La²Zr²O⁷ at the expense of stabilized tetragonal phase. Formability of the tetragonal/cubic phase has been influenced by the dopant concentration and the annealing temperature. Sample with 8 mol% La²O³ has been stabilized completely in tetragonal/cubic phase after annealing at 900∘C for 1 h. Smallness of the grain in these nanocrystalline materials may also have assisted in the formation of La²O³-ZrO² solid solution.     

Recombination luminescence of CaI<Subscript>2</Subscript> crystals activated with Eu<Superscript>2+</Superscript>, Gd<Superscript>2+</Superscript>, Tl<Superscript>+</Superscript>, Pb<Superscript>2+</Superscript>, and Mn<Superscript>2+</Superscript>

The spectral characteristics of thermostimulated luminescence, steady-state roentgenoluminescence and photostimulated luminescence (PSL) buildup and decay kinetics, and the effect of IR irradiation on the roentgenoluminescence yield and glow curves of CaI₂:Eu²⁺, CaI₂:Gd²⁺, CaI₂:Tl⁺, CaI₂:Pb²⁺, CaI₂:Mn²⁺, and CaI₂: Pb²⁺, Mn²⁺ crystals grown by the Bridgman-Stockbarger method have been studied in the temperature range 90–295 K. Coupled with earlier data, the present results on the influence of oxygen and hydrogen impurities on the spectral characteristics of CaI₂ indicate that the activation of calcium iodide with Eu²⁺, Gd²⁺, Tl⁺, Pb²⁺, and Mn²⁺ leads to the formation of cation impurity-native defect complexes, which act as carrier traps and are responsible for the thermostimulated luminescence in the range 150–295 K. IR exposure after 90-K x-ray excitation gives rise to flash PSL and influences the thermostimulated luminescence light sum. The nature of the emission and trapping centers involved and the mechanisms of recombination luminescence excitation in the crystals are discussed.     

Spectral parameters of Er<Superscript>3+</Superscript> ion in Yb<Superscript>3+</Superscript>/Er<Superscript>3+</Superscript>:KY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

The spectral parameters of Er³⁺ in Yb³⁺/Er³⁺:KY(WO₄)₂ crystal with space group C2/c have been investigated based on Judd-Ofelt theory. The spectral parameters were obtained: the intensity parameters are: ₂ = 6.33 × 10^–20 cm², ₄ = 1.35 × 10^–20 cm², ₆ = 1.90 × 10^–20 cm². The radiative lifetime and the fluorescence branch ratios were calculated. The emission cross section _e (at 1536 nm) is 2.0 × 10^–21 cm².     

New ferromagnetics based on manganese-alloyed chalcopyrites A<Superscript>II</Superscript>B<Superscript>IV</Superscript>C<Stack><Subscript>2</Subscript><Superscript>V</Superscript></Stack>

This review describes the principles of semiconductor spintronics, represents the physicochemical properties of materials based on manganese-alloyed A^IIB^IVC₂^V compounds, considers the results from theoretical simulation of magnetic properties of A^IIB^IVC₂^V alloyed with 3d metals, summarizes the basic approaches to explanation of ferromagnetism with Curie points above room temperature arising in A^IIB^IVC₂^V:Mn, and indicates promising ways to synthesize and study magnetic semiconductors based on chalcopyrites A^IIB^IVC₂^V in order to produce a suitable material for spintronic devices.     

Synthesis and properties of laser-active liquids POCl<Subscript>3</Subscript>-SbCl<Subscript>5</Subscript>-<Superscript>235</Superscript>UO<Stack><Subscript>2</Subscript><Superscript>2+</Superscript></Stack>-Nd<Superscript>3+</Superscript>

The effect of the synthesis conditions on the properties of inorganic laser-active liquids POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ -Nd³⁺ is considered. The kinetic dependences of the U(IV) content and decay time of the Nd³⁺ luminescence in POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions for various synthesis procedures at 380 K have been obtained. In POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions, nonradiative energy transfer Nd³⁺ → U⁴⁺ is observed, and quenching of the Nd³⁺ luminescence is described by the Stern-Volmer law: k _q = (6.4 ± 0.6) × 10⁵ l mol^?1 s^?1. Laser liquids POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ -Nd³⁺ with neodymium concentration of up to 0.7 M, uranyl concentration of up to 0.1 M, and decay time of the Nd³⁺ luminescence of up to 220 μs have been prepared for the first time.     

Electrical Conductivity of Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript> Crystals Doped with Ca<Superscript>2+</Superscript>, Pb<Superscript>2+</Superscript>, Sr<Superscript>2+</Superscript>, and Ba<Superscript>2+</Superscript>

Single crystals of Bi₂WO₆ (a layered perovskite-like compound) doped with Ca²⁺, Pb²⁺, Sr²⁺, and Ba²⁺ on the Bi³⁺ site are grown, and their oxygen ionic conductivity is measured along the polar axis. The intrinsic conductivity of the doped crystals differs insignificantly from the conductivity of undoped Bi₂WO₆, indicating that the oxygen ions in the Bi₂O₂ layers contribute little to the oxygen ionic conductivity of the crystals. The sharp change in the activation energy for conduction at 600°C attests to a transition from one conduction mechanism to another in going from low to high temperatures.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 863–865.Original Russian Text Copyright © 2005 by Kharitonova, Voronkova, Yanovskii.     

Mössbauer spectroscopy of <Superscript>57</Superscript>Fe and <Superscript>57</Superscript>Co in β-MnO<Subscript>2</Subscript> matrix

Behavior of Fe and Co in the β-MnO₂ matrix was studied by absorption and emission Mössbauer spectroscopy. It was found that Fe and Co are stabilized in the matrix in oxidation states +3 and +4, respectively.     

<Superscript>99</Superscript>Tc NMR Spectroscopy

⁹⁹Tc NMR data for Tc compounds in various oxidation states are summarized.__________Translated from Radiokhimiya, Vol. 47, No. 4, 2005, pp. 291–304.Original Russian Text Copyright © 2005 by Mikhalev.     

Morphology and luminescent properties of Al<Superscript>3+</Superscript>/Mg<Superscript>2+</Superscript>-doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> phosphor

Al³⁺/Mg²⁺ doped Y₂O₃:Eu phosphor was synthesized by the glycine-nitrate solution combustion method. In contrast to Y₂O₃:Eu which showed an irregular shape of agglomerated particles (the mean particle size >10 μm), the morphology of Al³⁺/Mg²⁺ doped Y₂O₃:Eu crystals was quite regular. Al³⁺/Mg²⁺ substituting Y³⁺ in Y₂O₃:Eu resulted in an obvious decrease of the particle size. Meanwhile, higher the Al³⁺/Mg²⁺ concentration, smaller the particle size. In particular, the introduction of Al³⁺ ion into Y₂O₃ lattice induced a remarkable increase of PL and CL intensity. While, for Mg²⁺ doped Y₂O₃:Eu samples, their PL and CL intensities decreased. The reason that causes the variation of PL and CL properties for Al³⁺ and Mg²⁺ doped Y₂O₃:Eu crystals was concluded to be related to sites of Al³⁺ and Mg²⁺ ions inclined to take and the difference of ion charge.