Einsteinium

Characteristics

Physical

Einsteinium is a synthetic, silvery, radioactive metal. In the periodic table, it is located to the right of the actinide californium, to the left of the actinide fermium and below the lanthanide holmium with which it shares many similarities in physical and chemical properties. Its density of 8.84 g/cm³ is lower than that of californium (15.1 g/cm³) and is nearly the same as that of holmium (8.79 g/cm³), despite einsteinium being much heavier per atom than holmium. Einsteinium's melting point (860 °C) is also relatively low—below californium (900 °C), fermium (1,527 °C) and holmium (1,461 °C). Einsteinium is a soft metal, with a bulk modulus of only 15 GPa, one of the lowest among non-alkali metals.

Unlike the lighter actinides californium, berkelium, curium and americium, which crystallize in a double hexagonal structure at ambient conditions; einsteinium is believed to have a face-centered cubic (fcc) symmetry with the space group Fm3m and the lattice constant a = 575 pm. However, there is a report of room-temperature hexagonal einsteinium metal with a = 398 pm and c = 650 pm, which converted to the fcc phase upon heating to 300 °C.

The self-damage induced by the radioactivity of einsteinium is so strong that it rapidly destroys the crystal lattice, and the energy release during this process, 1000 watts per gram of ²⁵³Es, induces a visible glow. These processes may contribute to the relatively low density and melting point of einsteinium. Further, due to the small size of available samples, the melting point of einsteinium was often deduced by observing the sample being heated inside an electron microscope. Thus, surface effects in small samples could reduce the melting point.

The metal is trivalent and has a noticeably high volatility. In order to reduce the self-radiation damage, most measurements of solid einsteinium and its compounds are performed right after thermal annealing. Also, some compounds are studied under the atmosphere of the reductant gas, for example H₂O+HCl for EsOCl so that the sample is partly regrown during its decomposition.

Apart from the self-destruction of solid einsteinium and its compounds, other intrinsic difficulties in studying this element include scarcity—the most common ²⁵³Es isotope is available only once or twice a year in sub-milligram amounts—and self-contamination due to rapid conversion of einsteinium to berkelium and then to californium at a rate of about 3.3% per day:

${\displaystyle {}_{99}{}^{253}\text{Es}\underset{20\text{d}}{\overset{\alpha }{\to }}{}_{97}{}^{249}\text{Bk}\underset{314\text{d}}{\overset{{\beta }^{-}}{\to }}{}_{98}{}^{249}\text{Cf}}$

Thus, most einsteinium samples are contaminated, and their intrinsic properties are often deduced by extrapolating back experimental data accumulated over time. Other experimental techniques to circumvent the contamination problem include selective optical excitation of einsteinium ions by a tunable laser, such as in studying its luminescence properties.

Magnetic properties have been studied for einsteinium metal, its oxide and fluoride. All three materials showed Curie–Weiss paramagnetic behavior from liquid helium to room temperature. The effective magnetic moments were deduced as 10.4±0.3 μ_B for Es₂O₃ and 11.4±0.3 μ_B for the EsF₃, which are the highest values among actinides, and the corresponding Curie temperatures are 53 and 37 K.

Chemical

Like all actinides, einsteinium is rather reactive. Its trivalent oxidation state is most stable in solids and aqueous solution where it induces a pale pink color. The existence of divalent einsteinium is firmly established, especially in the solid phase; such +2 state is not observed in many other actinides, including protactinium, uranium, neptunium, plutonium, curium and berkelium. Einsteinium(II) compounds can be obtained, for example, by reducing einsteinium(III) with samarium(II) chloride.

Isotopes

Eighteen isotopes and four nuclear isomers are known for einsteinium, with mass numbers 240–257. All are radioactive; the most stable one, ²⁵²Es, has half-life 471.7 days. The next most stable isotopes are ²⁵⁴Es (half-life 275.7 days), ²⁵⁵Es (39.8 days), and ²⁵³Es (20.47 days). All the other isotopes have half-lives shorter than 40 hours, most shorter than 30 minutes. Of the five isomers, the most stable is ^254mEs with a half-life of 39.3 hours.

Nuclear fission

Einsteinium has a high rate of nuclear fission that results in a low critical mass. This mass is 9.89 kilograms for a bare sphere of ²⁵⁴Es, and can be lowered to 2.9 kg by adding a 30-centimeter-thick steel neutron reflector, or even to 2.26 kg with a 20-cm-thick reflector made of water. However, even this small critical mass far exceeds the total amount of einsteinium isolated so far, especially of the rare ²⁵⁴Es.

Natural occurrence

Due to the short half-life of all isotopes of einsteinium, any primordial einsteinium—that is, einsteinium that could have been present on Earth at its formation—has long since decayed. Synthesis of einsteinium from naturally occurring uranium and thorium in the Earth's crust requires multiple neutron capture, an extremely unlikely event. Therefore, all einsteinium on Earth is produced in laboratories, high-power nuclear reactors, or nuclear testing, and exists only within a few years from the time of the synthesis.

The transuranic elements up to fermium, including einsteinium, should have been present in the natural nuclear fission reactor at Oklo, but any quantities produced then would have long since decayed away.

The absorption spectrum of einsteinium has been detected in Przybylski's Star, along with other actinide elements.

Synthesis and extraction

Einsteinium is produced in minute quantities by bombarding lighter actinides with neutrons in dedicated high-flux nuclear reactors. The world's major irradiation sources are the 85-megawatt High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL), Tennessee, U.S., and the SM-2 loop reactor at the Research Institute of Atomic Reactors (NIIAR) in Dimitrovgrad, Russia, which are both dedicated to the production of transcurium (Z>96) elements. These facilities have similar power and flux levels, and are expected to have comparable production capacities for transcurium elements, though the quantities produced at NIIAR are not widely reported. In a "typical processing campaign" at ORNL, tens of grams of curium are irradiated to produce decigram quantities of californium, milligrams of berkelium (²⁴⁹Bk) and einsteinium and picograms of fermium.

The first microscopic sample of ²⁵³Es weighing about 10pr nanograms was prepared in 1961 at HFIR. A special magnetic balance was designed to estimate its weight. Larger batches were produced later starting from several kilograms of plutonium with the einsteinium yields (mostly ²⁵³Es) of 0.48 milligram in 1967–1970, 3.2 milligrams in 1971–1973, followed by steady production of about 3 milligrams per year between 1974 and 1978. These quantities however refer to the integral amount in the target right after irradiation. Subsequent separation procedures reduced the amount of isotopically pure einsteinium roughly tenfold.

Laboratory synthesis

Heavy neutron irradiation of plutonium results in four major isotopes of einsteinium: ²⁵³Es (α-emitter; half-life 20.47 days, spontaneous fission half-life 7×10⁵ years); ^254mEs (β-emitter, half-life 39.3 hours), ²⁵⁴Es (α-emitter, half-life 276 days) and ²⁵⁵Es (β-emitter, half-life 39.8 days). An alternative route involves bombardment of uranium-238 with high-intensity nitrogen or oxygen ion beams.

²⁴⁷Es (half-life 4.55 min) was produced by irradiating ²⁴¹Am with carbon or ²³⁸U with nitrogen ions. The latter reaction was first realized in 1967 in Dubna, Russia, and the involved scientists were awarded the Lenin Komsomol Prize.

²⁴⁸Es was produced by irradiating ²⁴⁹Cf with deuterium ions. It mainly β-decays to ²⁴⁸Cf with a half-life of 25±5 minutes, but also releases 6.87-MeV α-particles; the ratio of β's to α-particles is about 400.

${\displaystyle {}_{98}{}^{249}\text{Cf}+{}_1{}^2\text{H}⟶{}_{99}{}^{248}\text{Es}+3{}_0^1\text{n}\left({}_{99}{}^{248}\text{Es}\underset{27\text{min}}{\overset{ϵ}{\to }}{}_{98}{}^{248}\text{Cf}\right)}$

^{249, 250, 251, 252}Es were obtained by bombarding ²⁴⁹Bk with α-particles. One to four neutrons are released, so four different isotopes are formed in one reaction.

${\displaystyle {}_{97}{}^{249}\text{Bk}\stackrel{+\alpha }{\to }{}_{99}{}^{249,250,251,252}\text{Es}}$

²⁵³Es was produced by irradiating a 0.1–0.2 milligram ²⁵²Cf target with a thermal neutron flux of (2–5)×10¹⁴ neutrons/(cm²·s) for 500–900 hours:

${\displaystyle {}_{98}{}^{252}\text{Cf}\stackrel{(\text{n},\gamma )}{\to }{}_{98}{}^{253}\text{Cf}\underset{17.81\text{d}}{\overset{{\beta }^{-}}{\to }}{}_{99}{}^{253}\text{Es}}$

In 2020, scientists at ORNL created about 200 nanograms of ²⁵⁴Es; allowing some chemical properties of the element to be studied for the first time.

Synthesis in nuclear explosions

The analysis of the debris at the 10-megaton Ivy Mike nuclear test was a part of long-term project. One of the goals was studying the efficiency of production of transuranic elements in high-power nuclear explosions. The motive for these experiments was that synthesis of such elements from uranium requires multiple neutron capture. The probability of such events increases with the neutron flux, and nuclear explosions are the most powerful man-made neutron sources, providing densities of the order 10²³ neutrons/cm² within a microsecond, or about 10²⁹ neutrons/(cm²·s). In comparison, the flux of HFIR is 5×10¹⁵ neutrons/(cm²·s). A dedicated laboratory was set up right at Enewetak Atoll for preliminary analysis of debris, as some isotopes could have decayed by the time the debris samples reached the mainland U.S. The laboratory was receiving samples for analysis as soon as possible, from airplanes equipped with paper filters which flew over the atoll after the tests. Whereas it was hoped to discover new chemical elements heavier than fermium, none of these were found even after a series of megaton explosions conducted between 1954 and 1956 at the atoll.

The atmospheric results were supplemented by the underground test data accumulated in the 1960s at the Nevada Test Site, as it was hoped that powerful explosions in a confined space might give improved yields and heavier isotopes. Apart from traditional uranium charges, combinations of uranium with americium and thorium have been tried, as well as a mixed plutonium-neptunium charge, but they were less successful in terms of yield and was attributed to stronger losses of heavy isotopes due to enhanced fission rates in heavy-element charges. Product isolation was problematic as the explosions were spreading debris through melting and vaporizing the surrounding rocks at depths of 300–600 meters. Drilling to such depths to extract the products was both slow and inefficient in terms of collected volumes.

Of the nine underground tests between 1962 and 1969, the last one was the most powerful and had the highest yield of transuranics. Milligrams of einsteinium that would normally take a year of irradiation in a high-power reactor, were produced within a microsecond. However, the major practical problem of the entire proposal was collecting the radioactive debris dispersed by the powerful blast. Aircraft filters adsorbed only ~4×10⁻¹⁴ of the total amount, and collection of tons of corals at Enewetak Atoll increased this fraction by only two orders of magnitude. Extraction of about 500 kilograms of underground rocks 60 days after the Hutch explosion recovered only ~1×10⁻⁷ of the total charge. The amount of transuranic elements in this 500 kg batch was only 30 times higher than in a 0.4-kg rock picked up 7 days after the test which showed the highly non-linear dependence of the transuranics yield on the amount of retrieved radioactive rock. Shafts were drilled at the site before the test in order to accelerate sample collection after explosion, so that explosion would expel radioactive material from the epicenter through the shafts and to collecting volumes near the surface. This method was tried in two tests and instantly provided hundreds of kilograms of material, but with actinide concentration three times lower than in samples obtained after drilling. Whereas such method could have been efficient in scientific studies of short-lived isotopes, it could not improve the overall collection efficiency of the produced actinides.

Though no new elements (except einsteinium and fermium) could be detected in the nuclear test debris, and the total yields of transuranics were disappointingly low, these tests did provide significantly higher amounts of rare heavy isotopes than previously available in laboratories.

Separation

Separation procedure of einsteinium depends on the synthesis method. In the case of light-ion bombardment inside a cyclotron, the heavy ion target is attached to a thin foil, and the generated einsteinium is simply washed off the foil after the irradiation. However, the produced amounts in such experiments are relatively low. The yields are much higher for reactor irradiation, but there, the product is a mixture of various actinide isotopes, as well as lanthanides produced in the nuclear fission decays. In this case, isolation of einsteinium is a tedious procedure which involves several repeating steps of cation exchange, at elevated temperature and pressure, and chromatography. Separation from berkelium is important, because the most common einsteinium isotope produced in nuclear reactors, ²⁵³Es, decays with a half-life of only 20 days to ²⁴⁹Bk, which is fast on the timescale of most experiments. Such separation relies on the fact that berkelium easily oxidizes to the solid +4 state and precipitates, whereas other actinides, including einsteinium, remain in their +3 state in solutions.

Trivalent actinides can be separated from lanthanide fission products by a cation-exchange resin column using a 90% water/10% ethanol solution saturated with hydrochloric acid (HCl) as eluant. It is usually followed by anion-exchange chromatography using 6 molar HCl as eluant. A cation-exchange resin column (Dowex-50 exchange column) treated with ammonium salts is then used to separate fractions containing elements 99, 100 and 101. These elements can be then identified simply based on their elution position/time, using α-hydroxyisobutyrate solution (α-HIB), for example, as eluant.

The 3+ actinides can also be separated via solvent extraction chromatography, using bis-(2-ethylhexyl) phosphoric acid (abbreviated as HDEHP) as the stationary organic phase, and nitric acid as the mobile aqueous phase. The actinide elution sequence is reversed from that of the cation-exchange resin column. The einsteinium separated by this method has the advantage to be free of organic complexing agent, as compared to the separation using a resin column.

Preparation of the metal

Einsteinium is highly reactive, so strong reducing agents are required to obtain the pure metal from its compounds. This can be achieved by reduction of einsteinium(III) fluoride with metallic lithium:

EsF₃ + 3 Li → Es + 3 LiF

However, owing to its low melting point and high rate of self-radiation damage, einsteinium has a higher vapor pressure than lithium fluoride. This makes this reduction reaction rather inefficient. It was tried in the early preparation attempts and quickly abandoned in favor of reduction of einsteinium(III) oxide with lanthanum metal:

Es₂O₃ + 2 La → 2 Es + La₂O₃

Chemical compounds

Oxides

Einsteinium(III) oxide (Es₂O₃) was obtained by burning einsteinium(III) nitrate. It forms colorless cubic crystals, which were first characterized from microgram samples sized about 30 nanometers. Two other phases, monoclinic and hexagonal, are known for this oxide. The formation of a certain Es₂O₃ phase depends on the preparation technique and sample history, and there is no clear phase diagram. Interconversions between the three phases can occur spontaneously, as a result of self-irradiation or self-heating. The hexagonal phase is isotypic with lanthanum oxide where the Es³⁺ ion is surrounded by a 6-coordinated group of O²⁻ ions.

Halides

Einsteinium halides are known for the oxidation states +2 and +3. The most stable state is +3 for all halides from fluoride to iodide.

Einsteinium(III) fluoride (EsF₃) can be precipitated from Es(III) chloride solutions upon reaction with fluoride ions. An alternative preparation procedure is to exposure Es(III) oxide to chlorine trifluoride (ClF₃) or F₂ gas at a pressure of 1–2 atmospheres and temperature 300–400°C. The EsF₃ crystal structure is hexagonal, as in californium(III) fluoride (CfF₃) where the Es³⁺ ions are 8-fold coordinated by fluorine ions in a bicapped trigonal prism arrangement.

Es(III) chloride (EsCl₃) can be prepared by annealing Es(III) oxide in the atmosphere of dry hydrogen chloride vapors at about 500°C for some 20 minutes. It crystallizes upon cooling at about 425°C into an orange solid with a hexagonal structure of UCl₃ type, where einsteinium atoms are 9-fold coordinated by chlorine atoms in a tricapped trigonal prism geometry. Einsteinium(III) bromide (EsBr₃) is a pale-yellow solid with a monoclinic structure of AlCl₃ type, where the einsteinium atoms are octahedrally coordinated by bromine (coordination number 6).

The divalent compounds of einsteinium are obtained by reducing the trivalent halides with hydrogen:

2 EsX₃ + H₂ → 2 EsX₂ + 2 HX; X = F, Cl, Br, I

Einsteinium(II) chloride (EsCl₂), einsteinium(II) bromide (EsBr₂) and einsteinium(II) iodide (EsI₂) have been produced and characterized by optical absorption, with no structural information available yet.

Known oxyhalides of einsteinium include EsOCl, EsOBr and EsOI. These salts are synthesized by treating a trihalide with a vapor mixture of water and the corresponding hydrogen halide: for example, EsCl₃ + H₂O/HCl to obtain EsOCl.

Organoeinsteinium compounds

Einsteinium's high radioactivity has a potential use in radiation therapy, and organometallic complexes have been synthesized in order to deliver einsteinium to an appropriate organ in the body. Experiments have been performed on injecting einsteinium citrate (as well as fermium compounds) to dogs. Einsteinium(III) was also incorporated into β-diketone chelate complexes, since analogous complexes with lanthanides previously showed strongest UV-excited luminescence among metallorganic compounds. When preparing einsteinium complexes, the Es³⁺ ions were 1000 times diluted with Gd³⁺ ions. This allowed reducing the radiation damage so that the compounds did not disintegrate during the 20 minutes required for the measurements. The resulting luminescence from Es³⁺ was much too weak to be detected. This was explained by the unfavorable relative energies of the individual constituents of the compound that hindered efficient energy transfer from the chelate matrix to Es³⁺ ions. Similar conclusion was drawn for americium, berkelium and fermium.

Luminescence of Es³⁺ ions was however observed in inorganic hydrochloric acid solutions as well as in organic solution with di(2-ethylhexyl)orthophosphoric acid. It shows a broad peak at about 1064 nanometers (half-width about 100 nm) which can be resonantly excited by green light (ca. 495 nm wavelength). The luminescence has a lifetime of several microseconds and the quantum yield below 0.1%. The relatively high, compared to lanthanides, non-radiative decay rates in Es³⁺ were associated with the stronger interaction of f-electrons with the inner Es³⁺ electrons.

Applications

There is almost no use for any isotope of einsteinium outside basic scientific research aiming at production of higher transuranium elements and superheavy elements.

In 1955, mendelevium was synthesized by irradiating a target consisting of about 10⁹ atoms of ²⁵³Es in the 60-inch cyclotron at Berkeley Laboratory. The resulting ²⁵³Es(α,n)²⁵⁶Md reaction yielded 17 atoms of the new element with the atomic number of 101.

The rare isotope ²⁵⁴Es is favored for production of superheavy elements due to its large mass, relatively long half-life of 270 days, and availability in significant amounts of several micrograms. Hence ²⁵⁴Es was used as a target in the attempted synthesis of ununennium (element 119) in 1985 by bombarding it with calcium-48 ions at the superHILAC linear particle accelerator at Berkeley, California. No atoms were identified, setting an upper limit for the cross section of this reaction at 300 nanobarns.

${\displaystyle {}_{99}{}^{254}\text{Es}^{+}{}_{20}{}^{48}\text{Ca}⟶{}_{119}{}^{302}\text{Uue}^{\ast }⟶\text{no}\text{atoms}}$

²⁵⁴Es was used as the calibration marker in the chemical analysis spectrometer ("alpha-scattering surface analyzer") of the Surveyor 5 lunar probe. The large mass of this isotope reduced the spectral overlap between signals from the marker and the studied lighter elements of the lunar surface.

Safety

Most of the available einsteinium toxicity data is from research on animals. Upon ingestion by rats, only ~0.01% of it ends in the bloodstream. From there, about 65% goes to the bones, where it would remain for ~50 years if not for its radioactive decay, not to speak of the three-year maximum lifespan of rats, 25% to the lungs (biological half-life ~20 years, though this is again rendered irrelevant by the short half-life of einsteinium), 0.035% to the testicles or 0.01% to the ovaries—where einsteinium stays indefinitely. About 10% of the ingested amount is excreted. The distribution of einsteinium over bone surfaces is uniform and is similar to that of plutonium.