Gallium

Gallium — Material Page

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Gallium (Ga, atomic number 31) is a soft, silvery post-transition metal in Group 13 of the periodic table that melts at just 29.76 °C — barely above room temperature — yet has an extraordinarily wide liquid range, remaining liquid all the way to 2,403 °C, one of the broadest liquid temperature ranges of any element. In solid form gallium adopts an unusual orthorhombic crystal structure (α-Ga) with a quasi-molecular arrangement of Ga₂ dimers, rather than a close-packed metallic lattice, contributing to its very low melting point and the anomalous property — shared with water, bismuth, and antimony — of expanding on solidification (~3.1% volume increase). Liquid gallium has a negligible vapor pressure at room temperature, low viscosity, and excellent wetting of most surfaces, making it practical to handle in open laboratory settings without evaporation hazard. Gallium has two stable isotopes (⁶⁹Ga, 60.1%; ⁷¹Ga, 39.9%) and is classified by the EU as a critical raw material, produced almost entirely as a byproduct of aluminum (bauxite processing) and zinc smelting — primary gallium mine production is negligible and global supply (~300–400 tonnes/year) is entirely secondary.

Gallium's overwhelming commercial and technological importance lies in its compound semiconductors, which enable the photonic and high-frequency electronic technologies at the core of the modern digital economy. Gallium nitride (GaN) is the material responsible for the blue and white LED revolution that has transformed global lighting — the 2014 Nobel Prize in Physics was awarded to Akasaki, Amano, and Nakamura for the invention of efficient blue InGaN LEDs. GaN's wide bandgap (3.4 eV), high critical electric field (3.3 MV/cm), and high electron mobility make it the dominant material for power switching devices (GaN-on-Si HEMTs) that are replacing silicon MOSFETs in smartphone chargers, EV on-board chargers, and 5G base station power supplies — enabling 2–5× higher switching frequency at equivalent efficiency and dramatic size reduction. Gallium arsenide (GaAs, Eg = 1.42 eV) has an electron mobility ~6× higher than silicon and a direct bandgap enabling efficient light emission — the basis of all III-V solar cells used on spacecraft and concentrator photovoltaic systems, and of the pseudomorphic HEMT (pHEMT) transistors in every mobile phone RF front-end and satellite receiver.

Metallic gallium and gallium-based alloys serve a distinct and growing set of applications exploiting gallium's near-room-temperature liquid phase, from high-precision temperature calibration to soft robotics and next-generation thermal interface materials. Gallium's melting point (29.7646 °C) is an ITS-90 secondary fixed point for thermometer calibration and defines the upper limit of the liquid gallium thermometry range. Galinstan (Ga-In-Sn eutectic alloy, mp –19 °C) is a non-toxic liquid metal that has replaced mercury in laboratory thermometers and is the focus of intense research as a stretchable conductor for soft electronics, microfluidic circuits, and reconfigurable antennas. Liquid gallium alloys (EGaIn, Galinstan) wet and conform to irregular surfaces at pressures that mercury would not, enabling liquid metal thermal interface materials with conductivities of 13–40 W/m·K — far exceeding any polymer-based TIM. Gallium's low toxicity relative to mercury, thallium, and other liquid-near-RT metals makes it significantly safer for these open-handling applications.

General Properties

Property	Value	Notes
Atomic Number	31	Group 13 (boron group), Period 4; post-transition metal; between zinc (30) and germanium (32); one of only five elements liquid near room temperature (with Hg, Cs, Rb, Fr)
Atomic Mass	69.723 u	Two stable isotopes: ⁶⁹Ga (60.11%) and ⁷¹Ga (39.89%); both NMR-active (I = 3/2); ⁷¹Ga NMR has higher sensitivity due to smaller quadrupole moment and is the preferred nucleus for Ga NMR spectroscopy of semiconductors and biological systems
Density (solid, 20 °C)	5.91 g/cm³	Solid gallium; liquid gallium is denser at 6.095 g/cm³ — gallium expands ~3.1% on solidification (anomalous like water, Bi, Sb); this expansion must be accounted for in ampoule and container design to prevent cracking on freezing
Melting Point	29.7646 °C (302.9146 K)	An ITS-90 secondary fixed point for thermometer calibration; one of the most precisely known melting points of any element; melts in the hand at body temperature; the narrow solid-liquid temperature window (just ~6 °C above typical lab ambient) requires temperature control during handling and storage
Boiling Point	2,403 °C (2,676 K)	Extraordinarily wide liquid range of ~2,373 °C — one of the broadest of any element; liquid gallium has negligible vapor pressure below ~1,000 °C enabling high-temperature thermometry and use as a heat transfer fluid without evaporation losses
Thermal Conductivity	40.6 W/m·K (solid); ~29 W/m·K (liquid)	Moderate for a metal; gallium-based alloys (Galinstan: ~16 W/m·K; EGaIn: ~26 W/m·K) still far exceed polymer TIMs (~1–5 W/m·K) and approach or exceed solder alloys (~50 W/m·K) while remaining liquid at operating temperatures
Electrical Resistivity	270 nΩ·m (solid, 20 °C); ~272 nΩ·m (liquid)	Moderately high resistivity for a metal; liquid gallium's resistivity changes very little on melting (unlike most metals where resistivity jumps ~2× on melting), reflecting the quasi-molecular solid structure; liquid Ga is used as a stretchable conductor in soft electronics research
Crystal Structure	Orthorhombic (α-Ga, Cmca space group)	Unique quasi-molecular structure with Ga₂ dimers (bond length 2.44 Å) within layers; very different from close-packed metallic structures; responsible for the anomalously low melting point; several high-pressure polymorphs (β, γ, δ-Ga) exist above 1.2 GPa

Mechanical Properties

Property	Value	Notes
Hardness	Mohs <1; ~60 MPa compressive strength	Extremely soft — softer than lead and tin; can be scratched by a fingernail; solid gallium is brittle at low temperatures due to the layered quasi-molecular structure; mechanical properties are rarely of practical interest as gallium is used primarily in liquid form or as a compound semiconductor precursor
Elastic Modulus	~9.8 GPa	Very low modulus — comparable to soft polymers; reflects the weak van der Waals interactions between Ga₂ dimer layers in the orthorhombic structure; highly anisotropic — modulus varies significantly with crystal direction
Expansion on Solidification	+3.1% volume increase	Critical practical property: gallium cannot be stored in sealed rigid containers when liquid, as it will crack the container on freezing; must be stored in flexible-walled ampoules or with expansion volume; in semiconductor crystal growth, the volume expansion requires careful management of sealed growth ampoules

Thermal & Environmental Properties

Property	Value	Notes
Stability in Air	Stable at RT; oxidizes slowly above ~100 °C	Liquid gallium is relatively stable in air at room temperature — a thin Ga₂O₃ skin forms almost immediately on the surface but does not prevent handling; above ~100 °C oxidation accelerates; gallium does not react with water at room temperature
Wetting Behavior	Wets most metals and many non-metals; does not wet glass without surface modification	Liquid gallium wets and penetrates grain boundaries of aluminum alloys catastrophically — "gallium embrittlement" reduces the tensile strength of aluminum to near-zero within minutes of contact; all aluminum contact must be avoided; gallium does not wet glass (contact angle ~135°) or PTFE, enabling containment in borosilicate glass and fluoropolymer vessels
Oxidation States	+3 (primary); +1 (Ga₂O, GaCl)	Ga³⁺ is strongly dominant — analogous to Al³⁺ and In³⁺; Ga³⁺ forms the stable oxide Ga₂O₃ (β-Ga₂O₃, ultra-wide bandgap 4.8 eV — an emerging power semiconductor material); Ga⁺ is metastable in some halide compounds
Toxicology	Low acute toxicity; elemental Ga and Ga³⁺ compounds generally considered low risk	Elemental gallium and most Ga³⁺ salts have low acute toxicity in humans; gallium nitrate (Ga(NO₃)₃) is used clinically as an anti-cancer agent (inhibits ribonucleotide reductase) and for treatment of hypercalcemia; gallium's chemical similarity to Fe³⁺ enables uptake in iron metabolism pathways — the basis of ⁶⁷Ga citrate tumor scintigraphy; GaAs and GaN dust are respiratory hazards and classified as possible carcinogens (IARC Group 2A for GaAs)

Chemical Properties

Property	Value / Behavior	Notes
Surface Oxide	Ga₂O₃ (β-gallium oxide; monoclinic)	β-Ga₂O₃ has an ultra-wide bandgap of 4.8 eV and a critical electric field of ~8 MV/cm — larger than GaN (3.3 MV/cm) and SiC (2.5 MV/cm); emerging as the next-generation wide-bandgap power semiconductor for >1 kV switching applications; also used as a catalyst support and in phosphors
Amphoteric Behavior	Dissolves in both strong acids and alkalis	Gallium is amphoteric: dissolves in HCl forming GaCl₃, and dissolves in NaOH forming ⁻; the amphoteric behavior mirrors aluminum and is relevant to gallium recovery from Bayer process bauxite liquors where Ga co-precipitates with Al(OH)₃
Semiconductor Compounds	GaAs (Eg 1.42 eV direct), GaN (3.4 eV direct), GaP (2.26 eV indirect), InGaN, AlGaN	Ga-based III-V and III-N compounds span a direct bandgap range from ~0.7 eV (InAs) to ~6.2 eV (AlN) through alloy composition, enabling continuous tunability across the visible and near-IR spectrum for LEDs, lasers, and photodetectors; the InGaN alloy system covers 370–1,100 nm making it the basis of all commercial LED phosphor-converted white lighting
Liquid Metal Alloys	EGaIn (75.5% Ga, 24.5% In, mp 15.5 °C); Galinstan (68.5% Ga, 21.5% In, 10% Sn, mp –19 °C)	Gallium-based eutectic alloys remain liquid at and below room temperature; non-toxic alternatives to mercury; Galinstan is used in medical thermometers, laboratory manometers, and thermal interface applications; EGaIn is the primary research material for soft electronics, microfluidics, and reconfigurable antennas due to its oxide skin that enables shape retention

Identifier	Value
Symbol	Ga
Atomic Number	31
CAS Number	7440-55-3
UN Number	Not classified (bulk metal)
EINECS Number	231-163-8

Isotope	Type	Notes
⁶⁹Ga	Stable	60.11% natural abundance; I = 3/2, NMR-active; larger quadrupole moment than ⁷¹Ga resulting in broader NMR linewidths in asymmetric environments; used in solid-state ⁶⁹Ga NMR studies of GaN, GaAs, and gallate minerals to probe local structure and defects
⁷¹Ga	Stable	39.89% natural abundance; I = 3/2, NMR-active; smaller quadrupole moment than ⁶⁹Ga gives narrower linewidths and higher sensitivity in solution NMR — the preferred nucleus for gallium NMR spectroscopy of GaN epilayers, biological Ga³⁺ complexes, and gallium-based drug coordination chemistry
⁶⁷Ga	Radioactive	t½ = 3.26 days (electron capture); emits 93, 185, and 300 keV gamma rays; produced by proton bombardment of enriched ⁶⁸Zn targets in cyclotrons; the standard SPECT radiotracer for tumor scintigraphy and infection/inflammation imaging — ⁶⁷Ga citrate accumulates in rapidly dividing cells and sites of inflammation by mimicking Fe³⁺ in transferrin-mediated uptake; despite competition from ⁱ⁸F-FDG PET, ⁶⁷Ga scintigraphy remains used for lymphoma staging, fever of unknown origin, and sarcoidosis in centers without PET access
⁶⁸Ga	Radioactive	t½ = 67.7 min (β⁺, 89%); positron emitter for PET imaging; produced from ⁶⁸Ge/⁶⁸Ga generator systems (no on-site cyclotron required) or by cyclotron bombardment of ⁶⁸Zn; ⁶⁸Ga-DOTATATE and ⁶⁸Ga-PSMA are among the most clinically important PET tracers — ⁶⁸Ga-DOTATATE is FDA-approved for neuroendocrine tumor detection; ⁶⁸Ga-PSMA-11 is FDA-approved for prostate cancer staging and biochemical recurrence detection, rapidly replacing traditional bone scan protocols

Scientific & Research Applications

Use Case	Form Typically Used	Description
GaN & GaAs Semiconductor Research	6N Ga metal (MOCVD/MBE precursor source)	High-purity gallium (6N, 99.9999%) is the gallium source for MOCVD (trimethylgallium, TMGa, derived from Ga metal) and MBE (elemental Ga effusion cells) growth of GaN, GaAs, InGaAs, AlGaN, and related III-V/III-N heterostructures. Research targets include GaN-on-diamond for ultra-high power density transistors, non-polar and semi-polar GaN for improved LED efficiency, and InGaN solar cells for efficiency beyond single-junction Si limits.
β-Ga₂O₃ Power Semiconductor Research	High-purity Ga metal, Ga₂O₃ powder, Ga₂O₃ single crystals	β-Ga₂O₃ (ultra-wide bandgap 4.8 eV, critical field ~8 MV/cm) is attracting intense research as a next-generation power semiconductor for >1 kV switching with theoretical Baliga figure of merit ~3,400× silicon. Bulk Ga₂O₃ single crystals (grown by edge-defined film-fed growth or floating zone) are now commercially available up to 4-inch diameter; MOSFET and Schottky diode research is ongoing at multiple universities and companies including Novel Crystal Technology and Flosfia.
ITS-90 Thermometric Calibration	High-purity Ga metal (≥99.9999%) in sealed cells	The gallium melting point (29.7646 °C) is an ITS-90 secondary fixed point used for calibrating platinum resistance thermometers (PRTs) and thermocouples below 100 °C. Sealed gallium fixed-point cells (NIST, NPL, PTB) provide reproducible melting plateaus with uncertainties of ±0.1 mK, bridging the gap between the ice point (0.01 °C water triple point) and the indium point (156.5985 °C). Essential for calibrating high-accuracy laboratory thermometers used in pharmaceutical storage, metrology, and precision calorimetry.
Soft Electronics & Microfluidics Research	EGaIn (Ga-In eutectic), Galinstan, liquid Ga metal	Gallium-based liquid metal alloys (EGaIn, Galinstan) are the primary research materials for stretchable conductors, soft robotic actuators, reconfigurable microwave antennas, and liquid metal microfluidic circuits. EGaIn forms a thin Ga₂O₃ skin that provides shape retention when injected into microchannels, enabling printing of complex 3D interconnects. Research groups at Harvard, Michigan, and MIT have demonstrated EGaIn-based stretchable circuits that maintain conductivity at 500% strain.
Solar Neutrino Detection (SAGE / GALLEX)	Metallic Ga (60–100 tonnes, radiochemical grade)	The SAGE (Soviet-American Gallium Experiment) and GALLEX experiments used gallium metal as a radiochemical neutrino detector — solar neutrinos convert ⁷¹Ga to ⁷¹Ge (t½ = 11.4 days) via νₑ + ⁷¹Ga → e⁻ + ⁷¹Ge; the ⁷¹Ge is extracted and counted by proportional counting. These experiments provided the first detection of pp solar neutrinos and confirmed the solar neutrino problem, contributing to the discovery that neutrinos have mass.
PET Imaging Research (⁶⁸Ga tracers)	⁶⁸Ge/⁶⁸Ga generator eluate, ⁶⁸GaCl₃ cyclotron production	⁶⁸Ga-labeled DOTA-conjugated peptides (DOTATATE, DOTATOC for somatostatin receptors; PSMA-11 for prostate-specific membrane antigen) are among the fastest-growing PET imaging agents. The ⁶⁸Ge/⁶⁸Ga generator (t½ ⁶⁸Ge = 270.9 days) provides ⁶⁸Ga on-demand without a cyclotron, enabling deployment in smaller hospitals. ⁶⁸Ga-DOTATATE (Netspot) and ⁶⁸Ga-PSMA-11 (Illuccix) are both FDA-approved; research is expanding to ⁶⁸Ga-FAPI (fibroblast activation protein) for pan-cancer imaging.

Industrial & Commercial Applications

Sector	Form / Compound Used	Description
LED & Solid-State Lighting	InGaN/GaN epilayers (MOCVD on sapphire or Si substrates)	GaN-based LEDs consume the majority of global gallium production (~70%). InGaN/GaN quantum well LEDs on sapphire substrates now achieve wall-plug efficiencies exceeding 80% for blue LEDs (450 nm) and white LEDs combine blue InGaN chips with YAG:Ce phosphor. MicroLED displays (arrays of GaN LEDs <100 µm) are the next-generation display technology for AR/VR headsets, wearables, and high-brightness applications, with GaN epitaxy quality and transfer yield being the key manufacturing challenges.
GaN Power Electronics	GaN-on-Si and GaN-on-SiC HEMTs (MOCVD-grown)	GaN-on-Si high-electron-mobility transistors (HEMTs) are displacing silicon MOSFETs in power switching applications up to ~650 V — smartphone chargers, laptop adapters, EV on-board chargers (3.3–22 kW), and 5G base station power amplifiers. GaN enables switching frequencies 5–10× higher than Si at equivalent efficiency, shrinking passive component (inductor, capacitor) size dramatically. The GaN power device market is projected to exceed $2 billion by 2026, driven by rapid EV and renewable energy deployment.
GaAs High-Frequency Electronics	GaAs pHEMT, HBT ICs on 6-inch GaAs wafers	GaAs pseudomorphic HEMT (pHEMT) and heterojunction bipolar transistor (HBT) integrated circuits are in every mobile phone, tablet, and wireless LAN device — handling RF signal amplification in the frequency range 0.4–100 GHz. InGaAs/GaAs pHEMTs provide gain up to 100 GHz for satellite and automotive radar; GaAs HBTs are used in power amplifiers for LTE and 5G sub-6 GHz bands. Despite competition from GaN-on-SiC for high-power applications, GaAs remains dominant for small-signal RF and millimeter-wave applications.
III-V Multi-Junction Solar Cells	GaAs, InGaP, InGaAs (MOCVD on Ge substrates)	Triple-junction III-V solar cells (InGaP/GaAs/Ge or InGaP/InGaAs/Ge) achieve efficiencies of 30–38% under 1-sun AM0 illumination — used on essentially all commercial telecommunications and Earth observation satellites. Concentrator photovoltaic (CPV) cells using 4- and 6-junction III-V stacks achieve >47% efficiency under 500–1,000 suns concentration. The high cost of GaAs-based cells limits their use to space and CPV; research into thin-film GaAs-on-Si transfer and epitaxial lift-off aims to reduce cost for terrestrial use.
Thermal Interface Materials (Liquid Metal)	Galinstan, EGaIn liquid metal TIMs	Gallium-based liquid metal alloys (Galinstan, EGaIn) are used as thermal interface materials between high-power processors and heat sinks in extreme performance computing — gaming laptops, workstations, and overclocked CPUs. Thermal conductivity of 13–26 W/m·K far exceeds thermal greases (~4–12 W/m·K) and phase-change materials. Liquid metal TIMs require careful application to prevent spreading and shorting of adjacent electronics due to gallium's high wettability of copper and solder; used in commercial gaming laptops by Lenovo (Legion series) and Sony (PlayStation 5).
Medical Thermometers & Instrumentation	Galinstan (Ga-In-Sn eutectic, mp –19 °C)	Galinstan replaced mercury in clinical thermometers following the EU Mercury Thermometers Directive (2007/51/EC) restricting mercury-filled fever thermometers. Galinstan's low melting point (–19 °C), negligible vapor pressure, and low toxicity make it a safe mercury substitute; its higher surface tension requires glass-surface treatment (Ga₂O₃ coating) to prevent adhesion. Galinstan is also used in laboratory manometers, tiltmeters, and electrical switches where mercury was previously standard.

Purity	Main Use
99.99% (4N)	General laboratory use and materials research — suitable for soft electronics and liquid metal alloy preparation (EGaIn, Galinstan), wetting and surface science studies, thermometric calibration at reduced precision, and synthesis of gallium compounds where sub-100 ppm metallic impurities are acceptable
99.9999% (6N)	Semiconductor-grade applications and high-purity electronics — the standard feedstock for TMGa synthesis (MOCVD precursor for GaN and GaAs epitaxy), MBE effusion cell loading for III-V and III-N heterostructure growth, ITS-90 fixed-point calibration cells, and GaN substrate boule growth where sub-ppm impurity content is essential for controlling carrier concentration and optical properties

Synonym / Alternative Name	Context
Ga	Chemical symbol
Gallium metal	Standard commercial designation for the elemental form; supplied as pellets, ingots, or sealed ampoules under inert atmosphere to prevent oxidation of the freshly exposed surface
Elemental gallium	General scientific term distinguishing pure gallium metal from gallium compounds (GaAs, GaN, Ga₂O₃, TMGa, etc.)
Galli	Finnish and Estonian language equivalent; the element name derives from Gallia, the Latin name for France — gallium was discovered by Paul Émile Lecoq de Boisbaudran in 1875 by flame spectroscopy of a zinc ore; Lecoq named it after his homeland, and the element had been predicted by Mendeleev (as "eka-aluminum") four years earlier based on gaps in his periodic table
Gallium (liquid metal)	Common descriptor in soft robotics, microfluidics, and thermal management literature referring to gallium in its liquid state or as a constituent of room-temperature liquid metal alloys (EGaIn, Galinstan)
Gallium (4N / 6N)	Trade notation specifying purity: 4N = 99.99%, 6N = 99.9999%; the purity designation is critical in semiconductor and metrology applications where even sub-ppm impurities affect electrical and optical properties

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