======================================================================
=                               Indium                               =
======================================================================

                            Introduction
======================================================================
Indium is a chemical element; it has symbol In and atomic number 49.
It is a silvery-white post-transition metal and one of the softest
elements. Chemically, indium is similar to gallium and thallium, and
its properties are largely intermediate between the two. It was
discovered in 1863 by Ferdinand Reich and Hieronymous Theodor Richter
by spectroscopic methods and named for the indigo blue line in its
spectrum.

Indium is used primarily in the production of flat-panel displays as
indium tin oxide (ITO), a transparent and conductive coating applied
to glass. It is also used in the semiconductor industry, in
low-melting-point metal alloys such as solders and soft-metal
high-vacuum seals. It is used in the manufacture of blue and white LED
circuits, mainly to produce Indium gallium nitride p-type
semiconductor substrates. It is produced exclusively as a by-product
during the processing of the ores of other metals, chiefly from
sphalerite and other zinc sulfide ores.

Indium has no biological role and its compounds are toxic when inhaled
or injected into the bloodstream, although they are poorly absorbed
following ingestion.


                             Etymology
======================================================================
The name comes from the Latin word 'indicum' meaning violet or indigo.
The word 'indicum' means "Indian", as the naturally based dye indigo
was originally exported to Europe from India.


Physical
==========
Indium is a shiny silvery-white, highly ductile post-transition metal
with a bright luster. It is so soft (Mohs hardness 1.2) that it can be
cut with a knife and leaves a visible line like a pencil when rubbed
on paper. It is a member of group 13 on the periodic table and its
properties are mostly intermediate between its vertical neighbors
gallium and thallium. As with tin, a high-pitched cry is heard when
indium is bent - a crackling sound due to crystal twinning. Like
gallium, indium is able to wet glass. Like both, indium has a low
melting point, 156.60 °C (313.88 °F); higher than its lighter
homologue, gallium, but lower than its heavier homologue, thallium,
and lower than tin. The boiling point is 2072 °C (3762 °F), higher
than that of thallium, but lower than gallium, conversely to the
general trend of melting points, but similarly to the trends down the
other post-transition metal groups because of the weakness of the
metallic bonding with few electrons delocalized.

The density of indium, 7.31 g/cm3, is also greater than gallium, but
lower than thallium. Below the critical temperature, 3.41 K, indium
becomes a superconductor. Indium crystallizes in the body-centered
tetragonal crystal system in the space group 'I'4/'mmm' (lattice
parameters: 'a' = 325 pm, 'c' = 495 pm): this is a slightly distorted
face-centered cubic structure, where each indium atom has four
neighbours at 324 pm distance and eight neighbours slightly further
(336 pm). Indium has greater solubility in liquid mercury than any
other metal (more than 50 mass percent of indium at 0 °C). Indium
displays a ductile viscoplastic response, found to be size-independent
in tension and compression. However it does have a size effect in
bending and indentation, associated to a length-scale of order 50-100
μm, significantly large when compared with other metals.


Isotopes
==========
Indium has 39 known isotopes, ranging in mass number from 97 to 135.
Only two isotopes occur naturally as primordial nuclides: indium-113,
the only stable isotope, and indium-115, which has a half-life of 4.41
years, four orders of magnitude greater than the age of the Universe
and nearly 30,000 times greater than half-life of thorium-232. The
half-life of 115In is very long because the beta decay to 115Sn is
spin-forbidden. Indium-115 makes up 95.7% of all indium. Indium is one
of three known elements (the others being tellurium and rhenium) of
which the stable isotope is less abundant in nature than the
long-lived primordial radioisotopes.

The stablest artificial isotope is indium-111, with a half-life of
approximately 2.8 days. All other isotopes have half-lives shorter
than 5 hours. Indium also has 47 meta states, among which indium-114m1
(half-life about 49.51 days) is the most stable, more stable than the
ground state of any indium isotope other than the primordial. All
decay by isomeric transition. The indium isotopes lighter than 113In
predominantly decay through electron capture or positron emission to
form cadmium isotopes, while the indium isotopes heavier than 113In
predominantly decay through beta-minus decay to form tin isotopes.


                             Chemistry
======================================================================
Indium has 49 electrons, with an electronic configuration of
[Kr]4d(10)5s(2)5p(1). In compounds, indium most commonly donates the
three outermost electrons to become indium(III), In(3+). In some
cases, the pair of 5s-electrons are not donated, resulting in
indium(I), In(+). The stabilization of the monovalent state is
attributed to the inert pair effect, in which relativistic effects
lowers the energy of the 5s-orbital, observed in heavier elements.
Thallium (indium's heavier homolog) shows an even stronger effect,
manifested by the pervasiveness of thallium(I) vs thallium(III),
Gallium (indium's lighter homolog) is only rarely observed in the +1
oxidation state. Thus, although thallium(III) is a moderately strong
oxidizing agent, indium(III) is not, and many indium(I) compounds are
powerful reducing agents. While the energy required to include the
s-electrons in chemical bonding is lowest for indium among the group
13 metals, bond energies decrease down the group so that by indium,
the energy released in forming two additional bonds and attaining the
+3 state is not always enough to outweigh the energy needed to involve
the 5s-electrons. Indium(I) oxide and hydroxide are more basic and
indium(III) oxide and hydroxide are more acidic.

A number of standard electrode potentials, depending on the reaction
under study, are reported for indium, reflecting the decreased
stability of the +3 oxidation state:
: In2+ + e−    ⇌ In+         E0 = −0.40 V
In3+ + e−      ⇌ In2+        E0 = −0.49 V
In3+ + 2 e−    ⇌ In+         E0 = −0.443 V
In3+ + 3 e−    ⇌ In          E0 = −0.3382 V
In+ + e−       ⇌ In          E0 = −0.14 V

Indium metal does not react with water, but it is oxidized by stronger
oxidizing agents such as halogens to give indium(III) compounds. It
does not form a boride, silicide, or carbide.
Indium is rather basic in aqueous solution, showing only slight
amphoteric characteristics, and unlike its lighter homologs aluminium
and gallium, it is insoluble in aqueous alkaline solutions.


Hydrides and halides
======================
The hydride InH3 has at best a transitory existence in ethereal
solutions at low temperatures. It polymerizes in the absence of bases.
Lewis bases stabilize a rich collection of indium hydrides of the
formula LInH3 (L = tertiary phosphine and N-Heterocyclic carbenes).

Chlorination, bromination, and iodination of In produce colorless
InCl3, InBr3, and yellow InI3. The compounds are Lewis acids, somewhat
akin to the better known aluminium trihalides. Again like the related
aluminium compound, InF3 is polymeric.

Indium halides dissolves in water to give aquo complexes such as
[Ir(H2O)6]3+ and [IrCl2(H2O)4]+.  Similar complexes can be prepared
from nitrates and acetates.  Overall, the pattern is similar to that
for aluminium(III).


Chalcogenides and pnictides
=============================
Indium derivatives of chalcogenides (O, S, Se, Te) are well developed.
Indium(III) oxide, In2O3, forms when indium metal is burned in air or
when the hydroxide or nitrate is heated. The analogous
sesqui-chalcogenides with sulfur, selenium, and tellurium are also
known.

The chemistry of indium pnictides (N, P, As, Sb) is also well known,
motivated by their relevance to semiconductor technology. Direct
reaction of indium metal with the pnictogens For applications in
microelectronics, the P, As, and Sb derivatives are made by reactions
of trimethylindium:
: (E = P, As, Sb)
Many of these derivatives are prone to hydrolysis.


Indium(I) compounds
=====================
Indium(I) compounds are not common. The chloride, bromide, and iodide
are deeply colored, unlike the parent trihalides from which they are
prepared. The fluoride is known only as an unstable gas. Indium(I)
oxide black powder is produced when indium(III) oxide decomposes upon
heating to 700 °C.


Compounds in other oxidation states
=====================================
Less frequently, indium forms compounds in oxidation state +2 and even
fractional oxidation states. Usually such materials feature In-In
bonding, most notably in the halides In2X4 and [In2X6]2−, and various
subchalcogenides such as In4Se3. Several other compounds are known to
combine indium(I) and indium(III), such as InI6(InIIICl6)Cl3,
InI5(InIIIBr4)2(InIIIBr6), and InIInIIIBr4.


Organoindium compounds
========================
Organoindium compounds feature In-C bonds. Most are In(III)
derivatives, but cyclopentadienylindium(I) is an exception. It was the
first known organoindium(I) compound, and is polymeric, consisting of
zigzag chains of alternating indium atoms and cyclopentadienyl
complexes. Perhaps the best-known organoindium compound is
trimethylindium, In(CH3)3, used to prepare certain semiconducting
materials.


                              History
======================================================================
In 1863, German chemists Ferdinand Reich and Hieronymus Theodor
Richter were testing ores from the mines around Freiberg, Saxony. They
dissolved the minerals pyrite, arsenopyrite, galena and sphalerite in
hydrochloric acid and distilled raw zinc chloride. Reich, who was
color-blind, employed Richter as an assistant for detecting the
colored spectral lines. Knowing that ores from that region sometimes
contain thallium, they searched for the green thallium emission
spectrum lines. Instead, they found a bright blue line. Because that
blue line did not match any known element, they hypothesized a new
element was present in the minerals. They named the element indium,
from the indigo color seen in its spectrum, after the Latin 'indicum',
meaning 'of India'.

Richter went on to isolate the metal in 1864. An ingot of 0.5 kg was
presented at the World Fair 1867.  Reich and Richter later fell out
when the latter claimed to be the sole discoverer.


                             Occurrence
======================================================================
Indium is created by the long-lasting (up to thousands of years)
s-process (slow neutron capture) in low-to-medium-mass stars (range in
mass between 0.6 and 10 solar masses). When a silver-109 atom captures
a neutron, it transmutes into silver-110, which then undergoes beta
decay to become cadmium-110. Capturing further neutrons, it becomes
cadmium-115, which decays to indium-115 by another beta decay. This
explains why the radioactive isotope is more abundant than the stable
one. The stable indium isotope, indium-113, is one of the p-nuclei,
the origin of which is not fully understood; although indium-113 is
known to be made directly in the s- and r-processes (rapid neutron
capture), and also as the daughter of very long-lived cadmium-113,
which has a half-life of about eight quadrillion years, this cannot
account for all indium-113.

Indium is the 68th most abundant element in Earth's crust at
approximately 50 ppb. This is similar to the crustal abundance of
silver, bismuth and mercury. It very rarely forms its own minerals, or
occurs in elemental form. Fewer than 10 indium minerals such as
roquesite (CuInS2) are known, and none occur at sufficient
concentrations for economic extraction. Instead, indium is usually a
trace constituent of more common ore minerals, such as sphalerite and
chalcopyrite. From these, it can be extracted as a by-product during
smelting. While the enrichment of indium in these deposits is high
relative to its crustal abundance, it is insufficient, at current
prices, to support extraction of indium as the main product.

Different estimates exist of the amounts of indium contained within
the ores of other metals. However, these amounts are not extractable
without mining of the host materials (see Production and
availability). Thus, the availability of indium is fundamentally
determined by the 'rate' at which these ores are extracted, and not
their absolute amount. This is an aspect that is often forgotten in
the current debate, e.g. by the Graedel group at Yale in their
criticality assessments, explaining the paradoxically low depletion
times some studies cite.


                    Production and availability
======================================================================
Indium is produced exclusively as a by-product during the processing
of the ores of other metals. Its main source material are sulfidic
zinc ores, where it is mostly hosted by sphalerite. Minor amounts are
also extracted from sulfidic copper ores. During the
roast-leach-electrowinning process of zinc smelting, indium
accumulates in the iron-rich residues. From these, it can be extracted
in different ways. It may also be recovered directly from the process
solutions. Further purification is done by electrolysis. The exact
process varies with the mode of operation of the smelter.

Its by-product status means that indium production is constrained by
the amount of sulfidic zinc (and copper) ores extracted each year.
Therefore, its availability needs to be discussed in terms of supply
potential. The supply potential of a by-product is defined as that
amount which is economically extractable from its host materials 'per
year' under current market conditions (i.e. technology and price).
Reserves and resources are not relevant for by-products, since they
'cannot' be extracted independently from the main-products. Recent
estimates put the supply potential of indium at a minimum of 1,300
t/yr from sulfidic zinc ores and 20 t/yr from sulfidic copper ores.
These figures are significantly greater than current production (655 t
in 2016). Thus, major future increases in the by-product production of
indium will be possible without significant increases in production
costs or price. The average indium price in 2016 was 240/kg, down from
705/kg in 2014.

China is a leading producer of indium (290 tonnes in 2016), followed
by South Korea (195 t), Japan (70 t) and Canada (65 t). The Teck
Resources refinery in Trail, British Columbia, is a large
single-source indium producer, with an output of 32.5 tonnes in 2005,
41.8 tonnes in 2004 and 36.1 tonnes in 2003.

The primary consumption of indium worldwide is LCD production. Demand
rose rapidly from the late 1990s to 2010 with the popularity of LCD
computer monitors and television sets, which now account for 50% of
indium consumption. Increased manufacturing efficiency and recycling
(especially in Japan) maintain a balance between demand and supply.
According to the UNEP, indium's end-of-life recycling rate is less
than 1%.


Industrial uses
=================
In 1924, indium was found to have a valued property of stabilizing
non-ferrous metals, and that became the first significant use for the
element. The first large-scale application for indium was coating
bearings in high-performance aircraft engines during World War II, to
protect against damage and corrosion; this is no longer a major use of
the element. New uses were found in fusible alloys, solders, and
electronics. In the 1950s, tiny beads of indium were used for the
emitters and collectors of PNP alloy-junction transistors. In the
middle and late 1980s, the development of indium phosphide
semiconductors and indium tin oxide thin films for liquid-crystal
displays (LCD) aroused much interest. By 1992, the thin-film
application had become the largest end use.

Indium(III) oxide and indium tin oxide (ITO) are used as a transparent
conductive coating on glass substrates in electroluminescent panels.
Indium tin oxide is used as an infrared radiation filter in
low-pressure sodium-vapor lamps. The infrared radiation is reflected
back into the lamp, which increases the temperature within the tube
and improves the performance of the lamp.

Indium has many semiconductor-related applications. Some indium
compounds, such as indium antimonide and indium phosphide, are
semiconductors with useful properties: one precursor is usually
trimethylindium (TMI), which is also used as the semiconductor dopant
in II-VI compound semiconductors. InAs and InSb are used for
low-temperature transistors and InP for high-temperature transistors.
The compound semiconductors InGaN and InGaP are used in light-emitting
diodes (LEDs) and laser diodes. Indium is used in photovoltaics as the
semiconductor copper indium gallium selenide (CIGS), also called CIGS
solar cells, a type of second-generation thin-film solar cell. Indium
is used in PNP bipolar junction transistors with germanium: when
soldered at low temperature, indium does not stress the germanium.

Indium wire is used as a vacuum seal and a thermal conductor in
cryogenics and ultra-high-vacuum applications, in such manufacturing
applications as gaskets that deform to fill gaps. Owing to its great
plasticity and adhesion to metals, Indium sheets are sometimes used
for cold-soldering in microwave circuits and waveguide joints, where
direct soldering is complicated. Indium is an ingredient in the
gallium-indium-tin alloy galinstan, which is liquid at room
temperature and replaces mercury in some thermometers. Other alloys of
indium with bismuth, cadmium, lead, and tin, which have higher but
still low melting points (between 50 and 100 °C), are used in fire
sprinkler systems and heat regulators.

Indium is one of many substitutes for mercury in alkaline batteries to
prevent the zinc from corroding and releasing hydrogen gas. Indium is
added to some dental amalgam alloys to decrease the surface tension of
the mercury and allow for less mercury and easier amalgamation.

Indium's high neutron-capture cross-section for thermal neutrons makes
it suitable for use in control rods for nuclear reactors, typically in
an alloy of 80% silver, 15% indium, and 5% cadmium. In nuclear
engineering, the (n,n') reactions of 113In and 115In are used to
determine magnitudes of neutron fluxes.

In 2009, Professor Mas Subramanian and former graduate student Andrew
Smith at Oregon State University discovered that indium can be
combined with yttrium and manganese to form an intensely blue,
non-toxic, inert, fade-resistant pigment, YInMn blue, the first new
inorganic blue pigment discovered in 200 years.


Medical applications
======================
Radioactive indium-111 (in very small amounts) is used in nuclear
medicine tests, as a radiotracer to follow the movement of labeled
proteins and white blood cells to diagnose different types of
infection. Indium compounds are mostly not absorbed upon ingestion and
are only moderately absorbed on inhalation; they tend to be stored
temporarily in the muscles, skin, and bones before being excreted, and
the biological half-life of indium is about two weeks in humans. It is
also tagged to growth hormone analogues like octreotide to find growth
hormone receptors in neuroendocrine tumors.


                  Biological role and precautions
======================================================================
{{Chembox
| container_only = yes
|Section7=
}}

Indium has no metabolic role in any organism.  According to one
overview "no evidence of any health hazard from industrial use of
indium."


                           External links
======================================================================
* [http://www.periodicvideos.com/videos/049.htm Indium]  at 'The
Periodic Table of Videos' (University of Nottingham)
*
[https://www.organic-chemistry.org/chemicals/reductions/indiumlowvalent.shtm
Reducing Agents > Indium low valent]
* [https://www.cdc.gov/niosh/npg/npgd0341.html NIOSH Pocket Guide to
Chemical Hazards]  (Centers for Disease Control and Prevention)
* usgs.gov (Mineral Commodity Summaries 2025):
[https://pubs.usgs.gov/periodicals/mcs2025/mcs2025.pdf#page=90 Indium]


License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL: http://creativecommons.org/licenses/by-sa/3.0/
Original Article: http://en.wikipedia.org/wiki/Indium

You are viewing proxied material from gopherpedia.com. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.