= Holmium =

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= Holmium =
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Introduction
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Holmium is a chemical element; it has symbol Ho and atomic number 67.
It is a rare-earth element and the eleventh member of the lanthanide
series. It is a relatively soft, silvery, fairly corrosion-resistant
and malleable metal. Like many other lanthanides, holmium is too
reactive to be found in native form, as pure holmium slowly forms a
yellowish oxide coating when exposed to air. When isolated, holmium is
relatively stable in dry air at room temperature. However, it reacts
with water and corrodes readily, and also burns in air when heated.

In nature, holmium occurs together with the other rare-earth metals
(like thulium). It is a relatively rare lanthanide, making up 1.4
parts per million of the Earth's crust, an abundance similar to
tungsten. Holmium was discovered through isolation by Swedish chemist
Per Theodor Cleve. It was also independently discovered by
Jacques-Louis Soret and Marc Delafontaine, who together observed it
spectroscopically in 1878. Its oxide was first isolated from
rare-earth ores by Cleve in 1878. The element's name comes from
'Holmia', the Latin name for the city of Stockholm.

Like many other lanthanides, holmium is found in the minerals monazite
and gadolinite and is usually commercially extracted from monazite
using ion-exchange techniques. Its compounds in nature and in nearly
all of its laboratory chemistry are trivalently oxidized, containing
Ho(III) ions. Trivalent holmium ions have fluorescent properties
similar to many other rare-earth ions (while yielding their own set of
unique emission light lines), and thus are used in the same way as
some other rare earths in certain laser and glass-colorant
applications.

Holmium has the highest magnetic permeability and magnetic saturation
of any element and is thus used for the pole pieces of the strongest
static magnets. Because holmium strongly absorbs neutrons, it is also
used as a burnable poison in nuclear reactors.

Properties
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Holmium is the eleventh member of the lanthanide series. In the
periodic table, it appears in period 6, between the lanthanides
dysprosium to its left and erbium to its right, and above the actinide
einsteinium.

Physical properties
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With a boiling point of 3000 K, holmium is the sixth most volatile
lanthanide after ytterbium, europium, samarium, thulium and
dysprosium. At standard temperature and pressure, holmium, like many
of the second half of the lanthanides, normally assumes a hexagonally
close-packed (hcp) structure. Its 67 electrons are arranged in the
configuration [Xe] 4f11 6s2, so that it has thirteen valence electrons
filling the 4f and 6s subshells.

Holmium, like all of the lanthanides, is paramagnetic at standard
temperature and pressure. However, holmium is ferromagnetic at
temperatures below 19 K. It has the highest magnetic moment () of any
naturally occurring element and possesses other unusual magnetic
properties. When combined with yttrium, it forms highly magnetic
compounds.

Chemical properties
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Holmium metal tarnishes slowly in air, forming a yellowish oxide layer
that has an appearance similar to that of iron rust. It burns readily
to form holmium(III) oxide:

:4 Ho + 3 O2 → 2 Ho2O3

It is a relatively soft and malleable element that is fairly
corrosion-resistant and chemically stable in dry air at standard
temperature and pressure. In moist air and at higher temperatures,
however, it quickly oxidizes, forming a yellowish oxide. In pure form,
holmium possesses a metallic, bright silvery luster.

Holmium is quite electropositive: on the Pauling electronegativity
scale, it has an electronegativity of 1.23. It is generally trivalent.
It reacts slowly with cold water and quickly with hot water to form
holmium(III) hydroxide:
:2 Ho (s) + 6 H2O (l) → 2 Ho(OH)3 (aq) + 3 H2 (g)

Holmium metal reacts with all the stable halogens:
:2 Ho (s) + 3 F2 (g) → 2 HoF3 (s) [pink]

:2 Ho (s) + 3 Cl2 (g) → 2 HoCl3 (s) [yellow]

:2 Ho (s) + 3 Br2 (g) → 2 HoBr3 (s) [yellow]

:2 Ho (s) + 3 I2 (g) → 2 HoI3 (s) [yellow]

Holmium dissolves readily in dilute sulfuric acid to form solutions
containing the yellow Ho(III) ions, which exist as a [Ho(OH2)9]3+
complexes:

:2 Ho (s) + 3 H2SO4 (aq) → 2 Ho3+ (aq) + 3 (aq) + 3 H2 (g)

Oxidation states
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As with many lanthanides, holmium is usually found in the +3 oxidation
state, forming compounds such as holmium(III) fluoride (HoF3) and
holmium(III) chloride (HoCl3). Holmium in solution is in the form of
Ho3+ surrounded by nine molecules of water. Holmium dissolves in
acids. However, holmium is also found to exist in +2, +1 and 0
oxidation states.

Isotopes
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The isotopes of holmium range from 140Ho to 175Ho. The primary decay
mode before the most abundant stable isotope, 165Ho, is positron
emission, and the primary mode after is beta minus decay. The primary
decay products before 165Ho are terbium and dysprosium isotopes, and
the primary products after are erbium isotopes.

Natural holmium consists of one primordial isotope, holmium-165; it is
the only isotope of holmium that is thought to be stable, although it
is predicted to undergo alpha decay to terbium-161 with a very long
half-life. Of the 35 synthetic radioactive isotopes that are known,
the most stable one is holmium-163 (163Ho), with a half-life of 4570
years. All other radioisotopes have ground-state half-lives not
greater than 1.117 days, with the longest, holmium-166 (166Ho) having
a half-life of 26.83 hours, and most have half-lives under 3 hours.

166m1Ho has a half-life of around 1200 years. The high excitation
energy, resulting in a particularly rich spectrum of decay gamma rays
produced when the metastable state de-excites, makes this isotope
useful as a means for calibrating gamma ray spectrometers.

Oxides and chalcogenides
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Holmium(III) oxide is the only oxide of holmium. It changes its color
depending on the lighting conditions. In daylight, it has a yellowish
color. Under trichromatic light, it appears orange red, almost
indistinguishable from the appearance of erbium oxide under the same
lighting conditions. The color change is related to the sharp emission
lines of trivalent holmium ions acting as red phosphors. Holmium(III)
oxide appears pink under a cold-cathode fluorescent lamp.

Other chalcogenides are known for holmium. Holmium(III) sulfide has
orange-yellow crystals in the monoclinic crystal system, with the
space group 'P'21/'m' (No. 11). Under high pressure, holmium(III)
sulfide can form in the cubic and orthorhombic crystal systems. It can
be obtained by the reaction of holmium(III) oxide and hydrogen sulfide
at 1598 K. Holmium(III) selenide is also known. It is
antiferromagnetic below 6 K.

Halides
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All four trihalides of holmium are known. Holmium(III) fluoride is a
yellowish powder that can be produced by reacting holmium(III) oxide
and ammonium fluoride, then crystallising it from the ammonium salt
formed in solution. Holmium(III) chloride can be prepared in a similar
way, with ammonium chloride instead of ammonium fluoride. It has the
YCl3 layer structure in the solid state. These compounds, as well as
holmium(III) bromide and holmium(III) iodide, can be obtained by the
direct reaction of the elements:

:2 Ho + 3 X2 → 2 HoX3

In addition, holmium(III) iodide can be obtained by the direct
reaction of holmium and mercury(II) iodide, then removing the mercury
by distillation.

Organoholmium compounds
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Organoholmium compounds are very similar to those of the other
lanthanides, as they all share an inability to undergo π backbonding.
They are thus mostly restricted to the mostly ionic cyclopentadienides
(isostructural with those of lanthanum) and the σ-bonded simple alkyls
and aryls, some of which may be polymeric.

History
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Holmium (, Latin name for Stockholm) was discovered by the Swiss
chemists Jacques-Louis Soret and Marc Delafontaine in 1878 who noticed
the aberrant spectrographic emission spectrum of the then-unknown
element (they called it "Element X").

The Swedish chemist Per Teodor Cleve also independently discovered the
element while he was working on erbia earth (erbium oxide). He was the
first to isolate impure oxide of the new element. Using the method
developed by the Swedish chemist Carl Gustaf Mosander, Cleve first
removed all of the known contaminants from erbia. The result of that
effort was two new materials, one brown and one green. He named the
brown substance 'holmia' (after the Latin name for Cleve's home town,
Stockholm) and the green one 'thulia'. 'Holmia' was later found to be
the holmium oxide, and 'thulia' was thulium oxide. The pure oxide was
only isolated in 1911 and the metal in 1939 by Heinrich Bommer.

In the English physicist Henry Moseley's classic paper on atomic
numbers, holmium was assigned the value 66. The holmium preparation he
had been given to investigate had been impure, dominated by
neighboring dysprosium. He would have seen x-ray emission lines for
both elements, but assumed that the dominant ones belonged to holmium,
instead of the dysprosium impurity.

Occurrence and production
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Like all the other rare-earth elements, holmium is not naturally found
as a free element. It occurs combined with other elements in
gadolinite, monazite and other rare-earth minerals. No
holmium-dominant mineral has yet been found. The main mining areas are
China, United States, Brazil, India, Sri Lanka, and Australia with
reserves of holmium estimated as 400,000 tonnes. The annual production
of holmium metal is of about 10 tonnes per year.

Holmium makes up 1.3 parts per million of the Earth's crust by mass.
Holmium makes up 1 part per million of the soils, 400 parts per
quadrillion of seawater, and almost none of Earth's atmosphere, which
is very rare for a lanthanide. It makes up 500 parts per trillion of
the universe by mass.

Holmium is commercially extracted by ion exchange from monazite sand
(0.05% holmium), but is still difficult to separate from other rare
earths. The element has been isolated through the reduction of its
anhydrous chloride or fluoride with metallic calcium. Its estimated
abundance in the Earth's crust is 1.3 mg/kg. Holmium obeys the
Oddo-Harkins rule: as an odd-numbered element, it is less abundant
than both dysprosium and erbium. However, it is the most abundant of
the odd-numbered heavy lanthanides. Of the lanthanides, only
promethium, thulium, lutetium and terbium are less abundant on Earth.
The principal current source are some of the ion-adsorption clays of
southern China. Some of these have a rare-earth composition similar to
that found in xenotime or gadolinite. Yttrium makes up about
two-thirds of the total by mass; holmium is around 1.5%. Holmium is
relatively inexpensive for a rare-earth metal with the price about
1000 USD/kg.

Applications
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Glass containing holmium oxide and holmium oxide solutions (usually in
perchloric acid) have sharp optical absorption peaks in the spectral
range 200 to 900 nm. They are therefore used as a calibration standard
for optical spectrophotometers. The radioactive but long-lived 166m1Ho
is used in calibration of gamma-ray spectrometers.

Holmium is used to create the strongest artificially generated
magnetic fields, when placed within high-strength magnets as a
magnetic pole piece (also called a magnetic flux concentrator).
Holmium is also used in the manufacture of some permanent magnets.

Holmium can act as a sensitizer in sodium yttrium fluoride which takes
advantage of its absorption in the NIR-II window. Holmium allows for
lanthanide nanomaterials to have tunable emission and excitation in
the NIR-II. Under 1143 nm excitation the interfacial energy transfer
to other lanthanides allows a redshift in emission for biological
applications. This allows deeper penetration than typically used 980
nm and 808 nm lasers.

Holmium-doped yttrium iron garnet (YIG) and yttrium lithium fluoride
have applications in solid-state lasers, and Ho-YIG has applications
in optical isolators and in microwave equipment (e.g., YIG spheres).
Holmium lasers emit at 2.1 micrometres. They are used in medical,
dental, and fiber-optical applications. It is also being considered
for usage in the enucleation of the prostate.

Since holmium can absorb nuclear fission-bred neutrons, it is used as
a burnable poison to regulate nuclear reactors. It is used as a
colorant for cubic zirconia, providing pink coloring, and for glass,
providing yellow-orange coloring. In March 2017, IBM announced that
they had developed a technique to store one bit of data on a single
holmium atom set on a bed of magnesium oxide. With sufficient quantum
and classical control techniques, holmium may be a good candidate to
make quantum computers.

Holmium is used in the medical field, particularly in laser surgery
for procedures such as kidney stone removal and prostate treatment,
due to its precision and minimal tissue damage. Its isotope,
holmium-166, is applied in targeted cancer therapies, especially for
liver cancer, and it also enhances MRI imaging as a contrast agent.

Biological role and precautions
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Holmium plays no biological role in humans, but its salts are able to
stimulate metabolism. Humans typically consume about a milligram of
holmium a year. Plants do not readily take up holmium from the soil.
Some vegetables have had their holmium content measured, and it
amounted to 100 parts per trillion. Holmium and its soluble salts are
slightly toxic if ingested, but insoluble holmium salts are nontoxic.
Metallic holmium in dust form presents a fire and explosion hazard.
Large amounts of holmium salts can cause severe damage if inhaled,
consumed orally, or injected. The biological effects of holmium over a
long period of time are not known. Holmium has a low level of acute
toxicity.

See also
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*:Category:Holmium compounds
* Period 6 element

Further reading
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* R. J. Callow, 'The Industrial Chemistry of the Lanthanons, Yttrium,
Thorium, and Uranium', Pergamon Press, 1967.

External links
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* [http://www.periodicvideos.com/videos/067.htm Holmium] at 'The
Periodic Table of Videos' (University of Nottingham)

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Original Article: http://en.wikipedia.org/wiki/Holmium