Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 147–149, 2002 147 The Ionic Radius of No3+ A. Bilewicz∗ Department of Radiochemistry, Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland Received: November 13, 2001; In Final Form: April 30, 2002 259 248 18 5+ Nobelium ( No, T1/2 = 58 min) was produced in bombardment of Cm target with O ions. Next, it was oxi- dised by H5IO6 and loaded together with lanthanide tracers on a chromatographic column filled with cryptomelane 2+ MnO2. In the HNO3 elution curve two peaks are observed. First one is related to the nonoxidized No and the second corresponds to No3+ close to positions of elution peaks of Ho3+ and Y3+. The ionic radius deduced from the elution position of No3+ is to be 89.4 ± 0.7 pm. 1. Introduction to possess interesting ion exchange properties. The cryptome- lane MnO2 phase has a well-defined 2 × 2 tunnel-framed struc- Studies of the heaviest actinides and comparison with the ture with exchangeable hydrogen, alkali, or alkali earth cations.6 properties of lanthanides are interesting because they help to The presence of exchangeable ions in the tunnels requires that understand the influence of relativistic effects on the chemical some of the manganese atoms must be present in oxidation properties. The relativistic effects influence the contraction of states lower than 4. The formulae could be represented by the 3+ lanthanides and actinides ionic radii by two ways: split- IV III 7 HMn7 Mn O16 as postulated by Tsuji and Tamaura. The av- ting of outermost p orbitals and stabilization of p1/2 orbitals, and erage tunnel diameter is 0.28 nm so the selectivity of the sor- expanding the f5/2 and f7/2 orbitals which results in less effective bent is related to the steric effect in the tunnel structure.8 Cryp- shielding of the nuclear charge. For the heaviest actinides these tomelane MnO2 has been demonstrated to show excellent ion- might result in ionic radii different from non-relativistic extrap- exchange selectivity towards cations with crystal ionic radii of 3+ olations. For example, in the case of Fm the relativistic effect 130–150 pm, e.g. K+,NH+,Ba2+, and Ra2+ (Ref. 6–8). For 3+ is responsible for 8 pm contraction of the ionic radius, while for 4 3+ 3+ lanthanides and actinides the distribution coefficients are much Er only 2.5 pm. The determination of the ionic radius of No lower, but strongly depend on the radius of cations. Addition- 3+ ion is of key interest, particularly since the ionic radius of Lr ally, in acidic solutions the sorbent exhibits both ion exchange 1 has been determined. This measurement would conclude the and strong oxidizing properties, so No3+ should be stabilized in ionic radius determinations of the 3+ actinide ions. cryptomelane MnO2 phase. In the present work, cryptomelane- The ground state electronic configuration of nobelium is 3+ type MnO2 is used for the determination of No ionic radius by 14 2 − [Rn]5 f 7s . Energy of 5 f7/2 orbitals in nobelium ( 15.43 eV) chromatographic methods. is more negative than 4 f7/2 orbitals in ytterbium (−14.67 eV) (homologue in lanthanide series) which causes that nobelium 2. Experimental as the only one among f electron cations is very stable in the oxidation state 2+. The redox potential of the No2+/No3+ cou- 2.1. Synthesis of Cryptomelane Manganese Dioxide. ple was determined to be 1.4 ± 0.2 V.2 This aspect of nobelium Manganese dioxide samples were synthesized according to the chemistry provides special problems for the determination of the procedure used by Tsuji and Abe.9 The precipitates were ob- 3+ 2+ 3 No ionic radius. In the determination experiment first No tained by adding 250 cm of 0.5 M KMnO4 in 1 M H2SO4 3+ 3 must be oxidized to the No . This can be accomplished with to 250 cm of 1 M MnSO4 solution containing 1 M of H2SO4 ◦ the use of strong oxidizing agent such as KMnO4 (1.5 V) or at 60 C. The precipitates were then aged overnight, filtered, ◦ H5IO6 (1.7 V). The second problem is to stabilize in the 3+ ox- and washed with water. The product was dried at ca 70 C for idation state during the determination experiment. three days, and then ground and sieved. The samples were trans- + Ionic radii are usually obtained from X-ray diffraction data formed to the H form by percolating 6 M HNO3 solution. for oxides or fluorides. However, these techniques require 2.2. Radioactive Tracers. The radionuclides of 89Sr, 46Ca, weighable quantities of the compound. Unfortunately, elements 90Y, 177Lu, 169Yb, 170Tm, 166mHo, 152Eu, and 241Am were pur- heavier than fermium are produced in quantities of few atoms chased from Isotope Products in Swierk, Poland. All tracers at a time, so that experimental structural data for these elements were stored in 6 M HNO3 solutions to prevent them from hy- are not available. The values of ionic radii for Md2+,Md3+, drolysis. 2+ 3+ 259 259 No , and Lr were determined only by chromatographic meth- 2.3. Production of No. The No (T1/2 = 58 min) atoms ods.1,3,4 The ionic radii of Md3+ and Lr3+ were determined by were obtained by 18O irradiation of 248Cm target in reaction: comparing their elution positions with the positions of rare earth tracers and of actinides of known ionic radii. The ionic radii of 248Cm(18O, α3n)259No. (1) Md2+ and No2+ were determined by comparison with crystallo- graphic radii of Ca2+,Sr2+, and Ba2+ (Ref. 3,4). The experiments were carried out at Philips cyclotron at the For the determination of ionic radius of No3+, Henderson5 Paul Scherrer Institute, Villigen (Switzerland). Target of · −2 248 18 5+ used ion exchange chromatography with KMnO4 and H5IO6 as 0.730 mg cm Cm was irradiated for 2.5 h with O ions. oxidizing agents and α-HIB as eluent. Unfortunately, these The beam energy on target was 99 MeV, and the beam intensity strong oxidants decomposed both the α-HIB and the ion ex- was 1 ×1012 particles per second. Under these conditions, 259No change resin and, therefore, the determination of No3+ ionic ra- is produced with the cross section of about 30 nb. The target , dius could not be accomplished using this approach. contained 10% of Gd thus producing simultaneously 165 167Yb 165,167 In our laboratory, samples of cryptomelane-type MnO2 inor- and Lu. The reaction products recoiling out from the target ganic ion exchanger were synthesized. This sorbent is known were stopped in He gas and transported to the chemistry collec- tion site by a KCl aerosol-seeded gas-jet system. The aerosol ∗E-mail: [email protected]. FAX: ++48 228111532. with reaction products was collected on a glass filter. c 2002 The Japan Society of Nuclear and Radiochemical Sciences Published on Web 6/30/2002 148 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002 Bilewicz 0.8 8 Am 7 6 0.6 Eu 5 logK d 4 Y events 0.4 3 Er Tm 2 Yb 1 0.2 Lu Sr Lu Yb,Ca Tm Er Y, Ho Eu 0 0.8 1.2 1.6 2.0 2.4 2.8 3.2Kd 3.6 259 0 Figure 3. Sum of the elution curves from the experiments where No 0.01 0.0105 0.011 0.0115 0.012 was detected. r-1 ( pm-1) chromatographic system is defined as: Figure 1. Dependence of logarithm of distribution coefficient on re- ciprocal crystal ionic radius for lanthanides, yttrium, and americium. Vm −Vf Kd = (2) Vs 2.4. Chromatographic Procedure. A small glass column where Vm and Vs are the volumes of the mobile and the station- ary phase, respectively, and V f is the free column volume. (3 mm × 5 cm) was filled with 0.5 g cryptomelane MnO2 (0.01– 0.05 mm) and preconditioned with solution of HNO . The lan- 3 3. Results thanides and americium tracers were dissolved in 0.5 M HNO3, evaporated, and dissolved in 20 µL of 0.5 M HNO3 and 0.05 M The selectivity for the cations on cryptomelane MnO2 H5IO6. Next, the solution was placed on the top of the column. strongly depends on the ionic radii of the exchanged cations. The radionuclides were subsequently eluted from the column Because the diameter of the tunnel in the MnO2 phase (280 pm) 3 −1 with 0.5 M HNO3 solution at a flow rate of 0.1 cm min . Drop is similar to diameters of the metal cations, the selectivity of the fractions from the column were collected and measured by γ sorbent is associated with the steric effect. The hydrated 3+ spectrometry or by liquid scintillation counting (46Ca, 90Y). metal cations with diameter much larger than 280 pm must be dehydrated before entering the ion exchange phase. Therefore, The same chromatographic procedure was used in nobelium the selectivity of the cyptomelane MnO2 depends linearly on experiments. The aerosol, together with the reaction products, free energy of hydration,10 which is also linearly related to the 3 was dissolved in 2 cm of 0.5 M HNO3, evaporated, and dis- reciprocal of crystal ionic radius. The linearity of logKd on ionic 11 solved in 20 µL of 0.5 M HNO3 and 0.05 M H5IO6. The drops radii for f electron cations was also found by Tsuji et al. There from the column were collected on stainless steel disc and evap- are numerous data for ionic radii of the cations.