Magnetic Properties of Es2O3 and EsF3

Abstract The first quantitative measurements of the magnetic properties of einsteinium have been made on Es 2 O 3 and EsF 3 . The data collected at temperature between 4.2 and 180 K while maintaining the samples in various applied magnetic fields. The paramagnetic moment per einsteinium atom was 10.4± 0.3 μ B for the Es 2 O 3 sample and 11.4± 0.3 μ B for the EsF 3 sample. These values may be compared to a theoretical value of 10.6 μ B for a 5f 10 (Es 3+ free ion calculated by using Hund's Rule and L−S coupling. Sample Preparation . The einsteinium used in the study was purified by ion exchange chromograph techniques and 30 μ g of a 253 Es− 245 Es isotope mixture were obtained as a chloride solution. A micro-precipitation technique using teflon molds [1] was employed to precipitate the einsteinium as an oxalate, and this material was then calcined up to 1000°C in air to form an off-white, Es 2 O 3 product. A similar approach was used to prepare EsF 3 . Electrolytic grade HF was added to the purified chloride solution of trivalent einsteinium to produce a micro-precipitate. The resulting washed precipitate was subsequently dried at 200°C under flowing argon. X-ray diffraction analysis of the Es 2 O 3 samples did not yield diffraction data, indicating the oxide was amorphous. However, from earlier work [2] it is known that einsteinium forms only a body centered cubic (bcc) sesquioxide up to 1000°C. Monoclinic, and hexagonal forms of Es 2 O 3 potentially exist but have not been reported. A radiation-induced transformation from bcc to the hexagonal form has been observed at room temperature. Efforts to prepare EsO 2 have been negative. It was also not possible to obtain X-ray diffraction data from the EsF 3 products. Due to a more transparent nature of the EsF 3 sample, it was possible to examine it by absorption spectrophotometry. These measurements showed that the einsteinium existed in the 3+ state. Experimental Approach . For the measurements of the molar magnetic susceptibility, X M , the total sample moment, X M nH (n = number of mol and H = applied field), produces the effect upon the experimental apparatus. Since n is such a limited quantity for einsteinium samples a very sensitive apparatus is required to make the measurements. We have constructed a SQUID instrument especially for such measurements [3] and a minimum measurable molar susceptibility of 1.9 × 10 −6 (emu/mol) has been achieved for a 1 microgram sample. Special care was used to avoid contaminating the sample with magnetic materials. The sample holder consisted of an 0.6 mm ID sapphire tube with 0.5 mm diameter gold inserts on each end which crush the sample in the middle of the sapphire tube for good thermal contact. A secondary tube of quartz was then placed around the sapphire holder. In the solid state compounds, Es 2 O 3 and EsF 3 , the 3+ ionic state of einsteinium would be assigned a 5f 10 configuration. If it sis assumed that the 5f electrons interact only slightly, the molar magnetic susceptibility, X M , should behave as: X M = N A μ 2 /3k(T + 0 p ) In this equation, it can be seen that X −1 M should be linear with temperature, T. In Fig. 1 we show a plot of X −1 M vs . sample holder temperature for our 4.08 μ g sample of Es 2 O 3 . Since little departure from linearity was observed (indicating negligible permanent moment alignment) with decreasing temperature, a linear least squares fit was made to the data to obtain μ= 10.4 ± 0.3 μ B per ‘heavy’ atom and θ p = 53 ± 5 K. At the time of the magnetic measurements on the Es 2 O 3 , the sample was 4 days old ( i.e 4 days after purification). Based on its half-life, 13% of the einsteinium should have decayed to berkelium. Although the 249 Bk daughter present in the sample at the time of measurement was produced from a 6 Mev α-decay, we have assumed that it existed as Bk 2 O 3 in our sample. This assumption is consistent with other studies which have followed the growth of daughter and grand-daughter products of einsteinium compounds [4]. The +3 free ion of berkelium should have a 5f 8 electronic configuration and a corresponding free-ion moment of 9.7. Bohr magneton per atom. Since our Es 2 O 3 sample was actually a mixture of einsteinium and berkelium oxides, the experimental moment should be corrected for the berkelium content. A simple correction can be made by assuming that each material behaves independently and that their moments are additive. Thus, if a mixture of atoms (87% einsteinium and 13% berkelium) gave a moment of 10.1 ± 0.3 Bohr magnetons per atom, then the einsteinium atoms should have a moment of 10.5 ± o.3 per atoms. This latter value is consistent with calculated value of 10.61 μ B for a 5f 10 (Es 3+ ion). A similar magnetic study was made for a 3.25 μ g sample of nominally EsF 3 ten days after its preparation, so that 31% of the einsteinium had decayed to berkelium. The data is also shown in Fig. 1 for an external field of 1615 gauss. A linear least squares fit to this data yields a moment per " ‘heavy’ atom of μ = 10.9 ± μ B and θ p = 37 ±5 K. A similar correction for the berkelium daughter gives a moment of 11.4 ± 0.3 μ B per einsteinium atom. Since the initial measurements, successive studies have been made on samples to follow the magnetic character as a function of the ingrowth of the berkelium daughter. No surprises have been encounter. The average moment per heavy atom follows the predicted value for a simple mixture of 5f 10 and a 5f 8 atomic configurations. Research sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contracts DE-ASO5-79ER10348 with The University of Tennessee (Knoxville) and W-7405-eng-26 with Union Carbide Corporation.