On-line Gas Chromatographie Studies of Chlorides of Rutherfordium and Homologs Zr and Hf

The influence of physical parameters on the in-situ metal carbonyl complex formation studied with the Fast On-line Reaction Apparatus (FORA)

The Fast On-line Reaction Apparatus (FORA) was used to investigate the influence of various reaction parameters onto the formation and transport of metal carbonyl complexes (MCCs) under single-atom chemistry conditions. FORA is based on a 252Cf-source producing short-lived Mo, Tc, Ru and Rh isotopes. Those are recoiling from the spontaneous fission source into a reaction chamber flushed with a gas-mixture containing CO. Upon contact with CO, fission products form volatile MCCs which are further transported by the gas stream to the detection setup, consisting of a charcoal trap mounted in front of a HPGe γ-detector. Depending on the reaction conditions, MCCs are formed and transported with different efficiencies. Using this setup, the impact of varying physical parameters like gas flow, gas pressure, kinetic energy of fission products upon entering the reaction chamber and temperature of the reaction chamber on the formation and transport yields of MCCs was investigated. Using a setup similar to FORA called Miss Piggy, various gas mixtures of CO with a selection of noble gases, as well as N2 and H2, were investigated with respect to their effect onto MCC formation and transport. Based on this measurements, optimized reaction conditions to maximize the synthesis and transport of MCCs are suggested. Explanations for the observed results supported by simulations are suggested as well.

Evolution of the periodic table through the synthesis of new elements

This brief introduction to the synthesis and chemistry of elements discovered since 1940 is focused primarily on Z=93–118. The goal of this work is not to simply catalogue the nuclear fusion reactions needed to prepare new elements, but rather to focus on the chemical and physical properties that these elements possess. These elements share a single common feature in that they all have large Z values, and thus have electronic structures that are significantly altered by both scalar relativistic effects and spin-orbit coupling. These effects scale nonlinearly with increasing Z and create unexpected deviations both across series and down groups of elements. The magnitude of these effects is large enough that orbital energies rearrange and mix in ways that complicate incomplete depictions of electronic structure that are based solely on electron repulsion. Thus, the primary aim of this review is to document the impact of relativistic effects on electronic structure and how this has altered not just our understanding of the chemistry of heavy elements, but has even created in the need to rearrange the Periodic Table itself.

Theoretical predictions of properties and volatility of chlorides and oxychlorides of group-4 elements. II. Adsorption of tetrachlorides and oxydichlorides of Zr, Hf, and Rf on neutral and modified surfaces

With the aim to interpret results of gas-phase chromatography experiments on volatility of group-4 tetrachlorides and oxychlorides including those of Rf, adsorption enthalpies of these species on neutral, and modified quartz surfaces were estimated on the basis of relativistic, two-component Density Functional Theory calculations of MCl4, MOCl2, MCl6(-), and MOCl4(2) with the use of adsorption models. Several mechanisms of adsorption were considered. In the case of physisorption of MCl4, the trend in the adsorption energy in the group should be Zr > Hf > Rf, so that the volatility should change in the opposite direction. The latter trend complies with the one in the sublimation enthalpies, ΔH(sub), of the Zr and Hf tetrachlorides, i.e., Zr < Hf. On the basis of a correlation between these quantities, ΔH(sub)(RfCl4) was predicted as 104.2 kJ/mol. The energy of physisorption of MOCl2 on quartz should increase in the group, Zr < Hf < Rf, as defined by increasing dipole moments of these molecules along the series. In the case of adsorption of MCl4 on quartz by chemical forces, formation of the MOCl2 or MOCl4(2-) complexes on the surface can take place, so that the sequence in the adsorption energy should be Zr > Hf > Rf, as defined by the complex formation energies. In the case of adsorption of MCl4 on a chlorinated quartz surface, formation of the MCl6(2-) surface complexes can occur, so that the trend in the adsorption strength should be Zr ≤ Hf < Rf. All the predicted sequences, showing a smooth change of the adsorption energy in the group, are in disagreement with the reversed trend Zr ≈ Rf < Hf, observed in the "one-atom-at-a-time" gas-phase chromatography experiments. Thus, currently no theoretical explanation can be found for the experimental observations.

Chemistry of superheavy elements

The chemistry of superheavy elements - or transactinides from their position in the Periodic Table - is summarized. After giving an overview over historical developments, nuclear aspects about synthesis of neutron-rich isotopes of these elements, produced in hot-fusion reactions, and their nuclear decay properties are briefly mentioned. Specific requirements to cope with the one-atom-at-a-time situation in automated chemical separations and recent developments in aqueous-phase and gas-phase chemistry are presented. Exciting, current developments, first applications, and future prospects of chemical separations behind physical recoil separators (“pre-separator”) are discussed in detail. The status of our current knowledge about the chemistry of rutherfordium (Rf, element 104), dubnium (Db, element 105), seaborgium (Sg, element 106), bohrium (Bh, element 107), hassium (Hs, element 108), copernicium (Cn, element 112), and element 114 is discussed from an experimental point of view. Recent results are emphasized and compared with empirical extrapolations and with fully-relativistic theoretical calculations, especially also under the aspect of the architecture of the Periodic Table.

Sulfate complexation of element 104, Rf, in H2SO4/HNO3 mixed solution

The cation-exchange behavior of 261Rf (T 1/2= 78 s) produced in the 248Cm(18O, 5n) reaction was studied on a “one-atom-at-a-time” scale in 0.15–0.69 M H2SO4/HNO3 mixed solutions ([H+]=1.0 M) using an automated ion-exchange separation apparatus coupled with the detection system for alpha-spectroscopy (AIDA). It was found that adsorption probabilities ( decrease with an increase of [HSO4 −], showing a successive formation of Rf sulfate complexes. Rf exhibits a weaker complex formation tendency compared to the lighter homologues Zr and Hf. This is in good agreement with theoretical predictions including relativistic effects.

Relativistic electronic structure studies on the heaviest elements

Spectacular developments in relativistic quantum theory and computational algorithms in the last two decades allowed for accurate predictions of properties of the heaviest elements and their experimental behaviour. The most recent works in this area of investigations are overviewed. Preference is given to those related to experimental research. The role of relativistic effects is elucidated.

Aqueous-phase chemistry of the transactinides

The experimental techniques developed to perform rapid chemical separations of the heaviest elements in the aqueous phase are presented. In general, these include transport of the nuclear reaction products to a separation device by the gas-jet technique and dissolution in an aqueous solution containing inorganic ligands for complex formation. The complexes are chemically characterized by a partition method which can be liquid–liquid extraction, ion-exchange- or reversed-phase extraction chromatography. The separated fractions are quickly evaporated to dryness for the preparation of samples for α-particle spectroscopy. Comments are given on the special situation in which chemistry has to be studied with single atoms. Theoretical predictions of chemical properties are compared to the presently known chemical behaviour of rutherfordium, Rf (element 104), dubnium, Db (element 105), seaborgium, Sg (element 106), and hassium, Hs (element 108) and to that of their lighter homologs in the Periodic Table in order to assess the role of relativistic effects in the chemistry of the heaviest elements.

Gas chromatography of indium in macroscopic and carrier-free amounts using quartz and gold as stationary phases

The chemical investigation of E113 is likely to become soon feasible. The determination of chemical properties of carrier-free amounts of the lighter homologues of element 113, especially indium and thallium, allows designing experimental set-ups and selecting experimental conditions suitable for performing these studies. Here, we present investigations of the interaction of indium species with quartz and gold surfaces. Deposition temperatures as well as enthalpies of adsorption were determined for indium T dep=739±20 °C (−ΔH ads(In)=227±10 kJ mol−1) and for indium hydroxide T dep=250±20 °C (−ΔH ads(InOH)= 124±10 kJ mol−1) respectively, on quartz. In case of adsorption of indium on a gold surface only a lower limit of the deposition temperature was established T dep>980 °C (−ΔH ads(In)≥315±10 kJ mol−1). Investigations of macroscopic amounts of indium in thermosublimation experiments at similar experimental conditions were instrumental to establish a tentative speciation of the observed indium species.

On-line gas chromatographic studies of Rf, Zr, and Hf bromides

The Heavy Element Volatility Instrument (HEVI), an on-line isothermal gas chromatography system, has been used to separate the volatile bromide compounds of the group 4 elements Zr and Hf and the transactinide Rf according to their volatilities, and to provide data on the gas phase chemical properties of very short-lived isotopes in amounts as low as a few atoms. For these studies261Rf was produced via the248Cm(18O, 5n) reaction.165-167Hf was produced via the reactionnatEu(19F, xn), and85Zr was produced via thenatCu(28Si, 3p3n) reaction. The half-life for261Rf was measured to be 75±7 seconds.

A Monte Carlo code was used to deduce the enthalpy of adsorption (ΔHa) from the observed volatility and parameters of the chromatography system. The resulting adsorption enthalpies for the Zr, Hf, and Rf tetrabromides are: -108±5 kJ · mol-1, -113±5 kJ · mol-1 and -87±7 kJ · mol-1, respectively.

Volatilities of the group 4 bromides support the conclusion from previous results for the group 4 chlorides that Rf deviates from the trend expected by simple extrapolation of the properties of its lighter homologs in the periodic table. The group 4 bromides are also observed to be less volatile than their respective chlorides, as predicted by relativistic calculations.

Evidence for the formation of sodium hassate(VIII)

Hassium, element 108, was produced in the fusion reaction between26Mg and248Cm. The hassium recoils were oxidizedin-situto a highly volatile oxide, presumably HsO4, and were transported in a mixture of He and O2to a deposition and detection system. The latter consisted of 16 silicon PIN-photodiodes facing a layer of NaOH, which served, in the presence of a certain partial pressure of water in the transport gas, as reactive surface for the deposition of the volatile tetroxides. Six correlated α-decay chains of Hs were detected in the first 5 detectors centred around detection position 3. In analogy to OsO4, which forms Na2[OsO4(OH)2], an osmate(VIII), with aqueous NaOH, HsO4presumably was deposited as Na2[HsO4(OH)2], a hassate(VIII).

Extraction of short-lived zirconium and hafnium isotopes using crown ethers: A model system for the study of rutherfordium

The extraction of zirconium and hafnium from hydrochloric acid media was studied using the crown ethers dibenzo-18-crown-6 (DB18C6), dicyclohexano-18-crown-6 (DC18C6) and dicyclohexano-24-crown-8 (DC24C8) as extractants. The goal was to find an extraction system that exhibits a high selectivity between the members of group 4 of the periodic table and is suitable for the study of rutherfordium. It was found that Zr and Hf are both extracted using DB18C6, DC18C6 and DC24C8. The extraction yield increases with increasing acid concentration and increasing concentration of crown ether. The extracted species most likely consists of an ion-association complex formed between a Zr or Hf chloro complex and a hydronium crown ether complex. Conditions can be found for each extractant that provide for the separation of Zr from Hf. This selective separation between Zr and Hf makes the extraction with crown ethers from HCl well suited to study the extraction behaviour of Rf and compare it to the behaviour of Zr and Hf. These extraction systems can be used to determine whether the extraction behaviour of Rf is similar to Zr, similar to Hf or follows the trend established by the lighter homologs. The extraction kinetics are fast enough for the study of the 78-s isotope 261mRf.

Startup of transactinide chemistry in JAERI

The transactinide nuclides261Rf and262Db have been successfully produced in the248Cm(18O,5n) reaction at 99 MeV and in the248Cm(19F,5n) reaction at 100, 103, and 106 MeV, respectively, at the JAERI tandem accelerator. The on-line ion exchange experiments with an automated fast and repetitive liquid chromatography separation system were performed in the HNO3/HF system using Rf homologues89mZr and167,165Hf produced in the89Y(p,n) and152Gd(18O,xn) reactions, respectively. The radiotracers88Zr,175Hf, and234Th were also prepared and the distribution coefficients on ion exchange resins were measured systematically in 1-11 M HCl and 1-14 M HNO3with the batch method. It was found that anion exchange experiments of Rf in 8 M HNO3and 9 M HCl provided information useful to extract the ionic radius of Rf and to verify the influence of relativistic effects.

Aqueous chemistry of transactinides

The aqueous chemistry of the first three transactinide elements is briefly reviewed with special emphasis given to recent experimental results. Short introductory remarks are discussing the atom-at-a-time situation of transactinide chemistry as a result of low production cross-sections and short half-lives. In general, on-line experimental techniques and, more specifically, the Automated Rapid Chemistry Apparatus, ARCA, are presented. Present and future developments of experimental techniques and resulting perspectives are outlined at the end. The central part is mainly focussing on hydrolysis and complex formation aspects of the superheavy group 4, 5, and 6 transition metals with F-and Cl-anions. Experimental results are compared with the behaviour of lighter homologuous elements and with relativistic calculations. It will be shown that the chemical behaviour of the first superheavy elements is already strongly influenced by relativistic effects. While it is justified to place rutherfordium, dubnium and seaborgium in the Periodic Table of the Elements into group 4, 5 and 6, respectively, it is no more possible to deduce from this position in detail the chemical properties of these transactinide or superheavy elements.

Gas phase chemistry of technetium and rhenium oxychlorides

The chloride and oxychloride chemistry of the group 7 elements Tc and Re was investigated in order to develop an experimental approach to a gas chemical characterisation of bohrium (Bh, element 107). In thermochromatography experiments with trace amounts of101,104Tc and183,184Re the formation of one volatile compound was observed in O2/HCl containing carrier gas, which was attributed to MO3Cl (M = Tc, Re). From the measured deposition temperatures the adsorption enthalpies on quartz surfaces ΔHads(TcO3Cl) = -51 ± 3 kJ/mol and ΔHads(ReO3Cl) = -62 ± 3 kJ/mol were evaluated. The sublimation enthalpies were derived using an empirical correlation between Δ Hadsand ΔHsubl: ΔHsubl(TcO3Cl) = 49±10 kJ/mol and ΔHsubl(ReO3Cl) = 67±10 kJ/mol. A fast gas chemical separation technique for highly volatile compounds of short-lived isotopes based on isothermal gas solid adsorption chromatography (OLGA-principle) was developed. With a modified OLGA device, model studies with the short-lived nuclides106,107,108Tc and169,170,174,176Re were carried out in preparation of an experimental gas chemical investigation of bohrium (Bh, element 107). Separation times of less than 3 s were achieved. A good separation of the oxychlorides of group 7 elements from chloride and oxychloride compounds of152-155Er,151-154Ho (as models for actinide elements),98-101Nb,99-102Zr (as models for light transactinide elements),218Po, and214Bi was accomplished in this chemical system.

Chemical studies on rutherfordium (Rf) at JAERI

Chemical studies on element 104, rutherfordium (Rf), at JAERI (Japan Atomic Energy Research Institute) are reviewed. The transactinide nuclide261Rf has been produced in the reaction248Cm(18O, 5n) at the JAERI tandem accelerator with the production cross section of about 13 nb. On-line anion-exchange experiments on Rf together with the lighter homologues, group-4 elements Zr and Hf, in acidic solutions have been conducted with a rapid ion-exchange separation apparatus. From the systematic study of the anion-exchange behavior of Rf, it has been found that the properties of Rf in HCl and HNO3solutions are quite similar to those of Zr and Hf, definitely confirming that Rf is a member of the group-4 elements. However, we have observed an unexpected chemical behavior of Rf in HF solutions; the fluoride complex formation of Rf is significantly different from those of the homologues. Prospects of extending chemical studies on transactinide elements in the near future at JAERI are briefly considered.

Doubly magic nucleus (108)(270)Hs162

Theoretical calculations predict 270Hs (Z=108, N=162) to be a doubly magic deformed nucleus, decaying mainly by alpha-particle emission. In this work, based on a rapid chemical isolation of Hs isotopes produced in the 26Mg+248Cm reaction, we observed 15 genetically linked nuclear decay chains. Four chains were attributed to the new nuclide 270Hs, which decays by alpha-particle emission with Qalpha=9.02+/-0.03 MeV to 266Sg which undergoes spontaneous fission with a half-life of 444(-148)(+444) ms. A production cross section of about 3 pb was measured for 270Hs. Thus, 270Hs is the first nucleus for which experimental nuclear decay properties have become available for comparison with theoretical predictions of the N=162 shell stability.

Chemistry of Superheavy Elements

The number of chemical elements has increased considerably in the last few decades. Most excitingly, these heaviest, man-made elements at the far-end of the Periodic Table are located in the area of the long-awaited superheavy elements. While physical techniques currently play a leading role in these discoveries, the chemistry of superheavy elements is now beginning to be developed. Advanced and very sensitive techniques allow the chemical properties of these elusive elements to be probed. Often, less than ten short-lived atoms, chemically separated one-atom-at-a-time, provide crucial information on basic chemical properties. These results place the architecture of the far-end of the Periodic Table on the test bench and probe the increasingly strong relativistic effects that influence the chemical properties there. This review is focused mainly on the experimental work on superheavy element chemistry. It contains a short contribution on relativistic theory, and some important historical and nuclear aspects.