Einsteinium compounds

Einsteinium compounds are compounds that contain the element einsteinium (Es). These compounds largely have einsteinium in the +3 oxidation state, or in some cases in the +2 and +4 oxidation states. Although einsteinium is relatively stable, with half-lives ranging from 20 days upwards, these compounds have not been studied in great detail.

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.

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 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.

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. 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:

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.