Einsteinium

Glow due to the intense radiation from ~300 µg of ²⁵³Es.[25]

Einsteinium is a synthetic, silver, 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 atomic einsteinium being much heavier than holmium. The melting point of einsteinium (860 °C) is also relatively low – below californium (900 °C), fermium (1527 °C) and holmium (1461 °C).[4][26] Einsteinium is a soft metal, with the bulk modulus of only 15 GPa, which value is one of the lowest among non-alkali metals.[27]

Contrary to 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.[28]

The self-damage induced by the radioactivity of einsteinium is so strong that it rapidly destroys the crystal lattice,[29] and the energy release during this process, 1000 watts per gram of ²⁵³Es, induces a visible glow.[3] These processes may contribute to the relatively low density and melting point of einsteinium.[30] Further, owing to the small size of the available samples, the melting point of einsteinium was often deduced by observing the sample being heated inside an electron microscope.[31] Thus, the surface effects in small samples could reduce the melting point value.

The metal is trivalent and has a noticeably high volatility.[32] In order to reduce the self-radiation damage, most measurements of solid einsteinium and its compounds are performed right after thermal annealing.[33] 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.[34]

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:[35][36][37]

${\displaystyle {\ce {^{253}_{99}Es ->[\alpha][20 {\ce {d}}] ^{249}_{97}Bk ->[\beta^-][314 {\ce {d}}] ^{249}_{98}Cf}}}$

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.[38]

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.[39][40]

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.[41] 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.[42] The oxidation state +4 was postulated from vapor studies and is as yet uncertain.[43]

Nineteen isotopes and three nuclear isomers are known for einsteinium, with mass numbers ranging from 240 to 257. All are radioactive and the most stable nuclide, ²⁵²Es, has a half-life of 471.7 days.[44] The next most stable isotopes are ²⁵⁴Es (half-life 275.7 days),[45] ²⁵⁵Es (39.8 days), and ²⁵³Es (20.47 days). All of the remaining isotopes have half-lives shorter than 40 hours, most shorter than 30 minutes. Of the three nuclear isomers, the most stable is ^254mEs with a half-life of 39.3 hours.[46]

Einsteinium has a high rate of nuclear fission that results in a low critical mass for a sustained nuclear chain reaction. This mass is 9.89 kilograms for a bare sphere of ²⁵⁴Es isotope, and can be lowered to 2.9 kilograms by adding a 30-centimeter-thick steel neutron reflector, or even to 2.26 kilograms with a 20-cm-thick reflector made of water. However, even this small critical mass greatly exceeds the total amount of einsteinium isolated thus far, especially of the rare ²⁵⁴Es isotope.[47]

Because of the short half-life of all isotopes of einsteinium, any primordial einsteinium—that is, einsteinium that could have been present on the Earth at its formation—has long since decayed. Synthesis of einsteinium from naturally-occurring actinides uranium and thorium in the Earth's crust requires multiple neutron capture, which is an extremely unlikely event. Therefore, all terrestrial einsteinium is produced in scientific laboratories, high-power nuclear reactors, or in nuclear weapons tests, and exists only within a few years from the time of the synthesis.[7]

The transuranic elements from americium to fermium, including einsteinium, were once created in the natural nuclear fission reactor at Oklo, but no longer.[48]

Einsteinium was theoretically observed in the spectrum of Przybylski's Star.[49] However, the lead author of the studies finding einsteinium and other short-lived actinides in Przybylski's Star, Vera F. Gopka, admitted that "the position of lines of the radioactive elements under search were simply visualized in synthetic spectrum as vertical markers because there are not any atomic data for these lines except for their wavelengths (Sansonetti et al. 2004), enabling one to calculate their profiles with more or less real intensities."[50] The signature spectra of einsteinium's isotopes have since been comprehensively analyzed experimentally (in 2021),[51] though there is no published research confirming whether the theorized einsteinium signatures proposed to be found in the star's spectrum match the lab-determined results.

Heavy neutron irradiation of plutonium results in four major isotopes of einsteinium: ²⁵³Es (α-emitter with half-life of 20.47 days and with a spontaneous fission half-life of 7×10⁵ years); ^254mEs (β-emitter with half-life of 39.3 hours), ²⁵⁴Es (α-emitter with half-life of about 276 days) and ²⁵⁵Es (β-emitter with half-life of 39.8 days).[60][46] An alternative route involves bombardment of uranium-238 with high-intensity nitrogen or oxygen ion beams.[61]

Einsteinium-247 (half-life 4.55 minutes) was produced by irradiating americium-241 with carbon or uranium-238 with nitrogen ions.[62] The latter reaction was first realized in 1967 in Dubna, Russia, and the involved scientists were awarded the Lenin Komsomol Prize.[63]

The isotope ²⁴⁸Es was produced by irradiating ²⁴⁹Cf with deuterium ions. It mainly decays by emission of electrons to ²⁴⁸Cf with a half-life of 25±5 minutes, but also releases α-particles of 6.87 MeV energy, with the ratio of electrons to α-particles of about 400.[64]

${\displaystyle {\ce {^{249}_{98}Cf + ^{2}_{1}D -> ^{248}_{99}Es + 3^{1}_{0}n}}\quad \left({\ce {^{248}_{99}Es ->[\epsilon][27 {\ce {min}}] ^{248}_{98}Cf}}\right)}$

The heavier isotopes ²⁴⁹Es, ²⁵⁰Es, ²⁵¹Es and ²⁵²Es were obtained by bombarding ²⁴⁹Bk with α-particles. One to four neutrons are liberated in this process making possible the formation of four different isotopes in one reaction.[65]

${\displaystyle {\ce {^{249}_{97}Bk ->[+\alpha] ^{249,250,251,252}_{99}Es}}}$

Einsteinium-253 was produced by irradiating a 0.1–0.2 milligram ²⁵²Cf target with a thermal neutron flux of (2–5)×10¹⁴ neutrons·cm⁻²·s⁻¹ for 500–900 hours:[66]

${\displaystyle {\ce {^{252}_{98}Cf ->[{\ce {(n,\gamma)}}] ^{253}_{98}Cf ->[\beta^-][17.81 {\ce {d}}] ^{253}_{99}Es}}}$

In 2020, scientists at the Oak Ridge National Laboratory were able to create 233 nanograms of ²⁵⁴Es, a new world record. This allowed some chemical properties of the element to be studied for the first time.[67]

Estimated yield of transuranium elements in the U.S. nuclear tests Hutch and Cyclamen.[68]

The analysis of the debris at the 10-megaton Ivy Mike nuclear test was a part of long-term project. One of the goals of which was studying the efficiency of production of transuranium elements in high-power nuclear explosions. The motivation 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 the HFIR reactor 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.[6]

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 conducted in confined space might result in 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.[6][68]

Among the nine underground tests that were carried between 1962 and 1969,[69][70] the last one was the most powerful and had the highest yield of transuranium elements. Milligrams of einsteinium that would normally take a year of irradiation in a high-power reactor, were produced within a microsecond.[68] However, the major practical problem of the entire proposal was collecting the radioactive debris dispersed by the powerful blast. Aircraft filters adsorbed only about 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 about 1×10⁻⁷ of the total charge. The amount of transuranium 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 demonstrated the highly non-linear dependence of the transuranium elements yield on the amount of retrieved radioactive rock.[71] 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 kilograms of material, but with actinide concentration 3 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.[72]

Although no new elements (apart from einsteinium and fermium) could be detected in the nuclear test debris, and the total yields of transuranium elements were disappointingly low, these tests did provide significantly higher amounts of rare heavy isotopes than previously available in laboratories.[73]

Elution curves: chromatographic separation of Fm(100), Es(99), Cf, Bk, Cm and Am

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.[74] 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.[75]

Separation of trivalent actinides from lanthanide fission products can be done 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.[76]

Separation of the 3+ actinides can also be achieved by 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.[76]

Einsteinium is highly reactive and therefore strong reducing agents are required to obtain the pure metal from its compounds.[77] 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 high vapor pressure, which is higher than that of 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:[28][30][78]

Es₂O₃ + 2 La → 2 Es + La₂O₃

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.[29][36] 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.[87] The hexagonal phase is isotypic with lanthanum oxide where the Es³⁺ ion is surrounded by a 6-coordinated group of O²⁻ ions.[79][84]

Einsteinium(III) iodide glowing in the dark

Einsteinium halides are known for the oxidation states +2 and +3.[86][88] The most stable state is +3 for all halides from fluoride to iodide.

Einsteinium(III) fluoride (EsF₃) can be precipitated from einsteinium(III) chloride solutions upon reaction with fluoride ions. An alternative preparation procedure is to exposure einsteinium(III) oxide to chlorine trifluoride (ClF₃) or F₂ gas at a pressure of 1–2 atmospheres and a temperature between 300 and 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.[35][89][90]

Einsteinium(III) chloride (EsCl₃) can be prepared by annealing einsteinium(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.[82][89][91] 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).[85][89]

The divalent compounds of einsteinium are obtained by reducing the trivalent halides with hydrogen:[92]

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

Einsteinium(II) chloride (EsCl₂),[93] einsteinium(II) bromide (EsBr₂),[94] and einsteinium(II) iodide (EsI₂)[86] have been produced and characterized by optical absorption, with no structural information available yet.[85]

Known oxyhalides of einsteinium include EsOCl,[86] EsOBr[92] and EsOI.[86] 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.[34]

The high radioactivity of einsteinium has a potential use in radiation therapy, and organometallic complexes have been synthesized in order to deliver einsteinium atoms to an appropriate organ in the body. Experiments have been performed on injecting einsteinium citrate (as well as fermium compounds) to dogs.[3] Einsteinium(III) was also incorporated into beta-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 period of 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 other actinides americium, berkelium and fermium.[95]

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.[96]