Erbium

Erbium (symbol Er, atomic number 68) is a chemical element. When isolated artificially, it appears as a silvery-white solid metal, but in nature it always occurs bonded with other elements. It belongs to the lanthanide series, a group of rare-earth elements, and was first discovered in the gadolinite mine at Ytterby, Sweden—the place that gave the element its name.

The element’s main applications stem from its pink-colored Er³⁺ ions, which fluoresce optically and are especially valuable in laser technology. When these ions are optically pumped at wavelengths around 980 or 1480 nm, they emit light at 1530 nm through stimulated emission. This process creates a mechanically simple laser amplifier for fiber-optic signals. The 1550 nm wavelength is critical for optical communications because standard single-mode fibers experience minimal signal loss there.

Beyond fiber-optic amplifiers, erbium ions are used in many medical fields, including dermatology and dentistry. When pumped at a different wavelength, the Er³⁺ ion emits at 2940 nm (as in the Er:YAG laser). This wavelength is strongly absorbed by water in tissues, keeping its effect very shallow. Such superficial energy deposition is useful in laser surgery and for efficiently producing steam that ablates enamel in common dental lasers.

**Characteristics**

**Physical properties**

Pure erbium metal is trivalent, malleable, soft, and stable in air, oxidizing more slowly than many other rare-earth metals. Its salts are rose-colored, and the element shows sharp absorption bands in visible, ultraviolet, and near-infrared light. Otherwise, it resembles other rare earths. Its sesquioxide is called erbia. The element’s properties are partly influenced by the type and amount of impurities present. Erbium has no known biological role, though it is thought to stimulate metabolism.

Erbium is ferromagnetic below 19 K, antiferromagnetic between 19 and 80 K, and paramagnetic above 80 K. It can form propeller-shaped atomic clusters (Er₃N), with erbium atoms spaced 0.35 nm apart. These clusters can be isolated by encapsulating them inside fullerene molecules, as confirmed by transmission electron microscopy.

Like most rare-earth elements, erbium typically appears in the +3 oxidation state, but it can also exist in the 0, +1, and +2 states.

**Chemical properties**

Erbium metal keeps its luster in dry air but slowly tarnishes in moist air. It burns readily to form erbium(III) oxide: 4 Er + 3 O₂ → 2 Er₂O₃. It is quite electropositive and reacts slowly with cold water, and quickly with hot water, to form erbium hydroxide: 2 Er(s) + 6 H₂O(l) → 2 Er(OH)₃(aq) + 3 H₂(g). Erbium metal reacts with all halogens, producing pink erbium(III) fluoride and violet erbium(III) chloride, bromide, and iodide. It dissolves easily in dilute sulfuric acid, forming rose-red hydrated Er(III) ions: 2 Er(s) + 3 H₂SO₄(aq) → 2 Er³⁺(aq) + 3 SO₄²⁻(aq) + 3 H₂(g).

**Isotopes**

Naturally occurring erbium consists of six stable isotopes: ¹⁶²Er, ¹⁶⁴Er, ¹⁶⁶Er, ¹⁶⁷Er, ¹⁶⁸Er, and ¹⁷⁰Er. The most abundant is ¹⁶⁶Er, at 33.503% natural abundance. Among artificial radioisotopes, the most stable are ¹⁶⁹Er (half-life 9.39 days), ¹⁷²Er (49.3 hours), and ¹⁶⁰Er (28.58 hours). All other radioactive isotopes have half-lives under 11 hours, most under 4 minutes. There are also 26 meta states, the most stable being ¹⁴⁹ᵐ¹Er with a half-life of 8.9 seconds.

Known erbium isotopes range from ¹⁴³Er to ¹⁸⁰Er. Before the most abundant stable isotope (¹⁶⁶Er), the primary decay mode is electron capture; after it, beta decay dominates. Decay products before ¹⁶⁶Er are isotopes of holmium (element 67), and after it, isotopes of thulium (element 69).

¹⁶⁵Er is useful in Auger therapy because it decays via electron capture and emits no gamma radiation. It can also serve as a radioactive tracer to label antibodies and peptides, though it cannot be detected by imaging for studying biological distribution. This isotope can be produced by bombarding ¹⁶⁵Ho with protons or deuterium—a convenient method because ¹⁶⁵Ho is monoisotopic and relatively inexpensive.

**Compounds**

**Oxides**

Erbium(III) oxide, also called erbia, is the only known oxide of erbium. It was first isolated by Carl Gustaf Mosander in 1843 and obtained in pure form in 1905 by Georges Urbain and Charles James. It has a cubic structure similar to bixbyite, with octahedral Er³⁺ centers. Erbium oxide is made by burning erbium metal, erbium oxalate, or other oxyacid salts of erbium. It is insoluble in water and slightly soluble in heated mineral acids. This pink compound is used as a phosphor activator and to produce infrared-absorbing glass.

**Halides**

Erbium(III) fluoride is a pinkish powder made by reacting erbium(III) nitrate with ammonium fluoride. It is used to create infrared-transmitting materials and up-converting luminescent materials, and it serves as an intermediate in producing erbium metal before reduction with calcium. Erbium(III) chloride is a violet compound formed by first heating erbium(III) oxide and ammonium chloride to produce the ammonium salt of the pentachloride ((NH₄)₂ErCl₅), then heating it in a vacuum at 350–400 °C. It forms crystals with an aluminium-like structure.