Indium

Indium — Material Page

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Indium (In, atomic number 49) is a soft, silvery-white post-transition metal in Group 13 of the periodic table — one of the rarest stable elements in Earth's crust (~0.25 ppm), yet one of the most technologically critical, consumed primarily in the transparent conductive oxide indium tin oxide (ITO) that coats virtually every touchscreen and flat-panel display manufactured today. With a melting point of 156.6 °C, a face-centered tetragonal crystal structure, and Mohs hardness of just ~1.2, indium is exceptionally soft and malleable — it can be cut with a knife and emits a characteristic "indium cry" (a crackling sound) when bent, caused by twinning of the crystal lattice. Its elastic modulus of only ~11 GPa places it among the most compliant of all metals, and its Poisson's ratio of ~0.45 (close to the theoretical incompressibility limit of 0.5) reflects its nearly incompressible plastic deformation behavior. Indium is produced almost entirely as a byproduct of zinc smelting, with global primary production of ~800–900 tonnes/year concentrated in China, South Korea, Japan, and Canada; it is classified as a critical raw material by both the EU and US due to supply concentration and lack of substitution for ITO applications.

Indium tin oxide (ITO, typically In₂O₃ with 10 wt% SnO₂) dominates global indium consumption (~75–80%) because it uniquely combines electrical conductivity (sheet resistance 10–100 Ω/sq) with optical transparency (>85% in the visible spectrum) in a thin (~100–200 nm) sputtered film that can be patterned by photolithography. ITO is the transparent electrode in every LCD, OLED, and plasma display panel, every capacitive touchscreen (smartphone, tablet, laptop, industrial HMI), every thin-film solar cell with a transparent front contact, and every electrochromic smart glass panel. The ITO market consumes approximately 60% of primary indium production and is the primary driver of indium price volatility — the global shift from LCD to OLED and OLED to micro-LED displays is gradually reducing ITO intensity per display unit but increasing display area per device, keeping demand broadly stable. Research into ITO alternatives (graphene, silver nanowires, conducting polymers, AZO) has been ongoing for two decades but none has displaced ITO in high-performance applications due to ITO's unique combination of conductivity, transparency, and etch-patterning compatibility with established display manufacturing infrastructure.

Beyond ITO, indium's low melting point, extraordinary malleability, excellent wetting of most metal and ceramic surfaces, and high purity availability (to 6N8, 99.99998%) give it critical roles in UHV sealing, cryogenic bonding, low-temperature soldering, and III-V compound semiconductor growth. Indium metal wire and foil gaskets are the standard face-seal material for ultra-high vacuum flanges that must be bakeable to ~150 °C — the indium cold-welds under compression to form a hermetic, vibration-resistant seal that cannot be achieved with elastomeric O-rings at elevated temperature or under radiation. Indium-based solders (In-Sn, In-Bi, In-Pb, In-Ag) with melting points from –50 °C (In-Bi-Sn eutectic) to ~200 °C provide the lowest-temperature joining options for heat-sensitive assemblies including cryogenic detector bonding, infrared focal plane array hybridization (indium bump bonding), and glass-to-metal sealing in optical sensors. In compound semiconductors, InGaN covers 370–1,100 nm for LEDs and laser diodes; InGaAs and InP are the standard materials for 1.3 and 1.55 µm telecom lasers and photodetectors; and CIGS (Cu-In-Ga-Se) thin-film solar cells hold the thin-film efficiency record at ~23%.

General Properties

Property	Value	Notes
Atomic Number	49	Group 13 (boron group), Period 5; post-transition metal; between cadmium (48) and tin (50); one of the rarest stable elements (~0.25 ppm crustal abundance), produced exclusively as a byproduct of zinc smelting
Atomic Mass	114.818 u	Two naturally occurring isotopes: ¹¹³In (4.29%) stable and ¹¹⁵In (95.71%) technically radioactive (t½ = 4.4 × 10¹⁴ yr, β⁻) but effectively stable on any practical timescale; ¹¹⁵In is the dominant isotope by a large margin
Density (20 °C)	7.31 g/cm³	Intermediate between cadmium (8.65) and tin (7.26); denser than most soft metals; in liquid form (above 156.6 °C) density is 7.02 g/cm³ — indium contracts on solidification (unlike gallium which expands), a useful property for casting and bump bonding applications
Melting Point	156.5985 °C (429.7485 K)	An ITS-90 fixed point — the indium melting point is one of the defining calibration points of the International Temperature Scale of 1990, used for calibrating platinum resistance thermometers in the range 0–420 °C; the very precisely known value (±0.001 °C reproducibility in sealed cells) bridges the water triple point (0.01 °C) and zinc point (419.527 °C)
Boiling Point	2,072 °C (2,345 K)	Wide liquid range (~1,915 °C); indium vapor pressure is negligible below ~600 °C enabling open-air handling without evaporation concerns up to soldering temperatures; above ~1,000 °C, indium vapor pressure becomes significant — relevant to MBE effusion cell temperature control
Thermal Conductivity	81.6 W/m·K	Good thermal conductivity — much higher than indium alloys or thermal greases, enabling indium foil and wire to function as efficient thermal interface materials at cryogenic stages in dilution refrigerators and between detector arrays and cold plates in space telescopes
Electrical Resistivity	83.7 nΩ·m (20 °C)	Moderate resistivity for a soft metal; superconducting below Tc = 3.41 K (type I superconductor); indium's superconducting transition is used as a thermometry reference and in research on type I superconductor vortex physics
Crystal Structure	Face-centered tetragonal (fct); a = 3.252 Å, c = 4.946 Å	Slightly distorted FCC structure (c/a = 1.521 vs. ideal FCC c/a = 1.414); the tetragonal distortion arises from the partly filled 5p shell and is responsible for indium's characteristic "cry" (twinning sound) when bent; no allotropic transitions under ambient conditions

Mechanical Properties

Property	Value	Notes
Hardness	Mohs ~1.2; ~9 HV Vickers	Extremely soft — softer than tin (Mohs 1.5) and only slightly harder than thallium; can be scratched by a fingernail; the low hardness is the basis of indium's use as a gasket and sealing material — it cold-flows under modest compression force to conform perfectly to mating surfaces without requiring high bolt loads
Elastic (Young's) Modulus	~11 GPa	One of the lowest elastic moduli of any pure metal — comparable to soft polymers; reflects the weak metallic bonding of the large In atom with its partially filled 5p orbital; enables indium bump bonds in infrared focal plane array hybridization to accommodate differential thermal expansion between detector and readout IC substrates without delamination during thermal cycling
Poisson's Ratio	~0.45	Extremely high Poisson's ratio — approaching the theoretical limit of 0.5 for an incompressible material; indium deforms plastically with near-zero volume change, making it ideal as a hydrostatic pressure medium and for cold-welding applications where gap filling without volume change is required
Malleability & Cold Welding	Exceptional — cold-welds under pressure at RT	Indium cold-welds to itself and to many metals (Cu, Au, Ag, Ni) under modest contact pressure at room temperature — the atomic-scale oxide film is disrupted by plastic deformation, exposing bare metal surfaces that bond without requiring flux, elevated temperature, or vacuum; exploited in UHV flange sealing, cryogenic joints, and indium bump bonding of detector arrays

Thermal & Environmental Properties

Property	Value	Notes
Stability in Air	Stable at RT; forms In₂O₃ slowly; no significant tarnishing below ~200 °C	Indium oxidizes very slowly at room temperature — bulk pieces retain their metallic luster for months in air; above ~200 °C oxidation accelerates forming In₂O₃; indium foil retains good solderability and wetting behavior without flux at temperatures just above its melting point, unlike tin which requires flux to remove surface oxide
Corrosion Resistance	Good — resistant to alkalis; dissolves in acids	Resistant to NaOH and other alkalis; dissolves readily in HCl, H₂SO₄, HNO₃, and other mineral acids; resistant to most organic acids at room temperature; electroplated indium coatings protect aluminum alloy engine bearings from corrosion by sulfuric acid in engine oil breakdown products (a historically important bearing application)
Oxidation States	+3 (primary, In₂O₃, InCl₃); +1 (InCl, InI)	In³⁺ is strongly dominant — forms the stable oxide In₂O₃ (the basis of ITO); In⁺ is a metastable state found in some halide compounds; the In³⁺/In⁰ standard electrode potential is –0.338 V vs. SHE; In³⁺ forms octahedral and tetrahedral complexes in aqueous solution
Superconductivity	Type I superconductor; Tc = 3.41 K	Indium undergoes a type I superconducting transition at 3.41 K — one of the classic type I superconductors studied in BCS theory development; used as a superconducting reference in low-temperature physics and as solder for electrical connections in dilution refrigerator wiring that must remain superconducting below 4 K to minimize heat dissipation

Chemical Properties

Property	Value / Behavior	Notes
Surface Oxide	In₂O₃ (cubic bixbyite structure); ITO = In₂O₃:SnO₂ (10 wt%)	In₂O₃ is an n-type degenerate semiconductor (Eg ~2.9 eV direct, ~0.9 eV indirect) — transparent in the visible, conductive when degenerately doped; ITO (In₂O₃ with ~10 wt% SnO₂) achieves sheet resistance of 10–100 Ω/sq with >85% visible transmission in 100–200 nm films sputtered from ceramic targets; the dominant transparent conductive oxide in displays, touchscreens, and thin-film solar cells
Wetting & Soldering	Wets Cu, Au, Ag, Ni, Ge, GaAs, glass without flux above mp	Liquid indium wets most metal and semiconductor surfaces spontaneously above its melting point without requiring flux — a key advantage over tin-based solders in sensitive optical, detector, and semiconductor applications where flux residues would cause problems; enables direct die attach, crystal bonding, and detector hybridization without chemical cleaning steps
Alloy Systems	In-Sn (mp 118 °C at eutectic), In-Bi (mp –50 to 72 °C), In-Ag, In-Pb	Indium forms eutectic alloys with tin (52In-48Sn eutectic at 118 °C), bismuth (In-Bi-Sn ternary eutectic at –50 °C), and other metals — providing the lowest-melting practical solder alloys available; Field's metal (32.5% Bi, 51% In, 16.5% Sn, mp 62 °C) and Cerrolow-117 are used in optical lens edging fixturing where workpieces must be released without heat damage
III-V Compound Semiconductors	InP (Eg 1.35 eV), InAs (0.36 eV), InSb (0.17 eV); alloys InGaAs, InGaN, CIGS	Indium-containing III-V compounds cover the bandgap range from InSb (0.17 eV, LWIR detector) through InAs (0.36 eV, MWIR), InP (1.35 eV, telecom laser/PD), InGaN (~0.7–3.4 eV, blue/green LED), to InGaP (~1.9 eV, LED, HBT); InP is the substrate for 1.55 µm telecom DFB lasers, avalanche photodiodes, and InGaAs/InP HEMTs used in 5G millimeter-wave front-ends

Identifier	Value
Symbol	In
Atomic Number	49
CAS Number	7440-74-6
UN Number	Not classified (bulk metal)
EINECS Number	231-180-0

Isotope	Type	Notes
¹¹³In	Stable	4.29% natural abundance; I = 9/2, NMR-active; less commonly used than ¹¹⁵In for NMR due to lower natural abundance; enriched ¹¹³In is used as an IDMS spike for high-precision indium quantification in environmental and geological samples by MC-ICP-MS; ¹¹³In (from ¹¹³Sn generator) is used in nuclear medicine for SPECT imaging (113mIn, t½ = 99.5 min, 392 keV gamma)
¹¹⁵In	Radioactive*	95.71% natural abundance; t½ = 4.41 × 10¹⁴ yr (β⁻, decay to ¹¹⁵Sn) — effectively stable on any practical timescale; I = 9/2, NMR-active; ¹¹⁵In NMR is the standard nucleus for indium NMR spectroscopy, used to characterize indium coordination in ITO films, III-V semiconductor alloys, organometallic catalysts, and indium-based ionic liquids; ¹¹⁵In/¹¹³In isotope ratios (δ¹¹⁵In) are emerging as a geochemical tracer for hydrothermal ore-forming processes and indium cycling in marine sediments
¹¹¹In	Radioactive	t½ = 2.80 days (electron capture); emits 171 and 245 keV gamma rays; produced by proton bombardment of ¹¹²Cd or ¹¹¹Cd in cyclotrons; the standard SPECT radiolabeling agent for antibodies, peptides, and nanoparticles — ¹¹¹In-DTPA-octreotide (OctreoScan) was the first FDA-approved radiolabeled peptide imaging agent for neuroendocrine tumors; ¹¹¹In-capromab pendetide (ProstaScint) for prostate cancer; widely used for white blood cell labeling to detect infection and inflammation
¹¹³ᵐIn	Radioactive	t½ = 99.5 min (IT, isomeric transition); 392 keV gamma emitter; produced from ¹¹³Sn/¹¹³ᵐIn generator (t½ ¹¹³Sn = 115 days); historically one of the first generator-produced SPECT radionuclides used clinically for blood pool and liver imaging before Tc-99m generators became widely available; still used in some markets without Tc-99m generator access

Scientific & Research Applications

Use Case	Form Typically Used	Description
UHV Gasket Sealing	Indium wire (0.5–3 mm dia.), indium foil strip	Indium wire and foil gaskets are the standard face-seal for ultra-high vacuum (UHV) flanges that must be bakeable to 150 °C, operate under radiation, or mate with non-standard flange geometries incompatible with copper ConFlat gaskets. Indium cold-welds under bolt compression (typically 5–20 MPa contact pressure) to form hermetic seals with leak rates <10⁻¹⁰ mbar·L/s. Used on cryogenic detector vessels, X-ray beamline optical chambers, particle physics detector vacuum envelopes, and synchrotron insertion device gaps.
Cryogenic Detector Hybridization (Bump Bonding)	Indium bumps (evaporated or electroplated, 10–50 µm height)	Indium bump bonds interconnect infrared focal plane arrays (HgCdTe, InSb, QWIP detectors) to silicon readout ICs (ROICs) by cold-welding indium bumps at room temperature under compressive force — no elevated temperature required. Indium's low modulus (11 GPa) absorbs differential thermal contraction between the detector (cooled to 30–77 K) and ROIC substrates without delaminating. Used in all professional IR astronomy (James Webb Space Telescope NIRCam, MIRI) and high-end FLIR thermal cameras.
ITS-90 Temperature Calibration	High-purity In metal (≥5N) in sealed calibration cells	The indium melting point (156.5985 °C) is an ITS-90 fixed point used for calibrating standard platinum resistance thermometers (SPRTs) in the temperature range 0–420 °C. Sealed indium fixed-point cells (NIST, NPL) provide reproducible melting plateaus with uncertainties of ±0.1 mK. Bridges the temperature range between the water triple point (0.01 °C) and the zinc fixed point (419.527 °C) for pharmaceutical cold chain, precision calorimetry, and metrology applications.
Thin-Film Deposition Research (ITO)	In₂O₃:SnO₂ sputtering targets (90/10 wt%), In metal evaporation source	ITO sputtering targets (ceramic In₂O₃:SnO₂ 90/10 wt%) are deposited by DC or RF magnetron sputtering to produce transparent conductive films for display and solar cell research. Research focuses on ITO alternative materials (AZO, GZO, IZTO, graphene, silver nanowires) to reduce indium dependence; developing flexible ITO on polymer substrates for roll-to-roll processing; and optimizing ITO/perovskite interfaces for high-efficiency tandem solar cells.
Superconductivity Research	High-purity indium wire, In thin films	Indium is a classic type I superconductor (Tc = 3.41 K, Hc = 28.2 mT at 0 K) used as a reference material in low-temperature physics, as a soft solder for making superconducting joints in dilution refrigerator wiring (below Tc, indium joints have zero resistance and negligible heat dissipation), and in studies of proximity effects and vortex physics in hybrid normal-metal/superconductor nanostructures. Indium's type I behavior (complete Meissner effect, no vortex phase) is studied in contrast with type II superconductors.
Crystal Growth Flux & Substrate Bonding	Indium metal (5N–6N8), indium solder paste	Indium metal is used as a flux solvent for growing high-quality III-V and oxide single crystals by the flux method — the low melting point and ability to dissolve constituent elements at moderate temperatures prevents thermal decomposition of volatile compounds (GaAs, GaP, InP) during crystal growth. Indium is also used to bond semiconductor wafers and optical crystals to metal sample holders for polishing and ion beam thinning (TEM sample preparation) because the bond releases cleanly by warming above 156 °C without mechanical stress or chemical etching.

Industrial & Commercial Applications

Sector	Form / Compound Used	Description
Touchscreens & Flat Panel Displays (ITO)	ITO sputtering targets → 100–200 nm ITO film on glass or PET	ITO coatings on glass or PET substrates form the transparent electrodes of every LCD, OLED, plasma display, and projected capacitive touchscreen. In smartphone screens, ITO (or its patterned replacement in OLED: thin-film silver mesh or ITO/Ag/ITO stacks) carries the capacitive sensing signal and the OLED anode current. Global ITO demand consumes ~60–70% of primary indium production; ITO sputtering targets (~5–10 kg per target, 10–30 cm diameter) are replaced every 100–300 kWh of sputtering, recycled at 75–85% indium recovery efficiency.
CIGS Thin-Film Solar Cells	In metal (co-evaporated or sputtered with Cu, Ga, Se)	Copper indium gallium diselenide (CIGS, Cu(In,Ga)Se₂) thin-film solar cells hold the thin-film PV efficiency record at ~23.4% (EMPA, 2019) for small cells and ~17–19% for commercial modules (Solar Frontier, Avancis). CIGS absorbers are deposited by co-evaporation or sputtering of CuIn(Ga) precursors followed by selenization. CIGS module production consumes ~5–10% of primary indium; scale-up is constrained by indium supply — each GW of CIGS production requires ~40–80 tonnes of indium, creating supply chain tension with ITO demand.
Low-Temperature & Cryogenic Solders	In-Sn (52/48, mp 118 °C), In-Ag (97/3, mp 143 °C), In-Bi eutectic alloys	Indium-based solders provide the only practical joining solutions for heat-sensitive assemblies: In-Sn (mp 118 °C) for bonding optical crystals, CCD sensors, and glass optics without thermal damage; In-Bi-Sn ternary eutectics (mp 60–80 °C) for lens edging and fixturing; In metal (mp 156.6 °C) for bonding semiconductor wafers to carriers in ion implantation and sputtering processes. Indium solders are also used in cryogenic detector systems and infrared focal plane arrays where the bond must survive repeated thermal cycling to 77 K or below.
InP & InGaAs Telecom Semiconductors	InP wafers (2–4 inch), InGaAs/InP epilayers (MOCVD)	Indium phosphide (InP) is the substrate for virtually all 1.3 and 1.55 µm telecommunications lasers (DFB, VCSEL), electro-absorption modulators, and InGaAs avalanche photodetectors used in fiber optic networks — the backbone of the internet. InP HEMTs and HBTs (InGaAs/InAlAs/InP) achieve transit frequencies exceeding 600 GHz, used in millimeter-wave transceivers for 5G/6G, satellite communication, and radar. The InP-based photonic integrated circuit (PIC) market is growing rapidly for datacenter optical interconnects.
Bearing Overlay Coatings	Electroplated indium overlays (2–5 µm) on Pb-bronze or aluminum bearings	Electroplated indium overlays on engine bearings (historically on copper-lead tri-metal bearings, now on aluminum alloy bi-metal bearings) provide corrosion protection against acidic breakdown products of engine oil (H₂SO₄, organic acids) and improve run-in behavior. Indium diffuses into the bearing surface to form intermetallic compounds that resist acid attack; used in high-performance automotive, marine, and aircraft engine plain bearings. Indium-electroplating is also applied to silver contacts in high-reliability aerospace connectors to prevent silver sulfide tarnishing and maintain low contact resistance.
InGaN LEDs & Laser Diodes	InGaN/GaN quantum wells (MOCVD, In from TMIn precursor)	Indium incorporation into GaN quantum wells (InₓGa₁₋ₓN, x = 0.1–0.45) is the mechanism that shifts LED emission from UV (pure GaN, 365 nm) to blue (x ≈ 0.15, 450 nm), green (x ≈ 0.30, 530 nm), and amber (x ≈ 0.40, 590 nm). All white LED lamps use a blue InGaN chip (~450 nm) plus YAG:Ce phosphor. InGaN blue laser diodes (450 nm) are used in Blu-ray players and laser-based automotive headlights. Indium content uniformity across the wafer is the primary challenge in high-In InGaN growth for green and amber emission, where "V-pit" defects and composition fluctuations limit efficiency.

Purity	Main Use
99.8% (2N8)	General solder alloys and sealing applications — suitable for indium-based low-temperature solder formulations (In-Sn, In-Bi eutectic alloys), bearing overlay electroplating baths, and general industrial bonding where sub-0.2% metallic impurities are acceptable
99.995% (4N5)	Electronics, touchscreens, and ITO production — the standard purity for ITO ceramic sputtering target manufacture (In₂O₃:SnO₂), electroplated indium for aerospace bearing and connector applications, and cryogenic indium bump bonding of IR focal plane arrays where controlled impurity levels are required for consistent cold-welding behavior
99.999% (5N)	Semiconductors and high-reliability solders — used for InP and InAs substrate wafer growth (Czochralski or VGF), MOCVD source material (TMIn synthesis feedstock), UHV indium wire gaskets, and ITS-90 fixed-point calibration cells where sub-10 ppm metallic impurities are required for reproducible melting point behavior
99.99998% (6N8)	Crystal growth, advanced optics, and ultra-pure semiconductor processes — the highest purity grade for III-V single crystal growth by the flux method (InP, InAs, InSb research crystals), MBE effusion cell loading for InGaAs and InAlAs quantum well research, and fundamental low-temperature physics experiments (superconductivity, cryogenic detector development) requiring sub-ppb metallic impurity levels

Synonym / Alternative Name	Context
In	Chemical symbol
Indium metal	Standard commercial and regulatory designation for the elemental form; used in supply chain documentation, REACH/RoHS filings, and semiconductor industry procurement specifications
Elemental indium	Scientific term distinguishing the pure metal from indium compounds (In₂O₃, ITO, InP, InAs, InCl₃, TMIn, etc.) in materials and chemical literature
Indio	Spanish and Italian language equivalent; the name derives from the characteristic indigo-blue spectral emission lines (451.1 nm and 410.2 nm) by which Ferdinand Reich and Hieronymus Theodor Richter discovered the element in 1863 by flame spectroscopy of zinc ore residues from the Freiberg mines in Saxony
Indium (2N8 / 4N5 / 5N / 6N8)	Trade purity notation used across the indium supply chain: 2N8 = 99.8%, 4N5 = 99.995%, 5N = 99.999%, 6N8 = 99.99998%; the purity grade critically determines suitability for ITO production, solder formulation, semiconductor growth, and metrology applications
Indium (ITO-grade)	Informal trade designation for indium purified to ≥4N5 suitable for ITO sputtering target manufacture — the largest single application of indium globally, consuming ~60–70% of primary production

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