Gallium Properties: Physical, Chemical, Thermal, and Semiconductor Data

Gallium at a Glance

What Are the Basic Physical Properties of Gallium?

Gallium is a soft, silvery-white metal that melts at 29.7646°C. Solid gallium has density 5.91 g/cm³ at 25°C; liquid gallium at 6.095 g/cm³ is denser. Solid gallium is orthorhombic (Cmce), consisting of covalently bonded Ga₂ dimers. Mohs hardness is 1.5 - it can be cut with a fingernail.

Gallium sits between zinc (Zn, element 30) and germanium (Ge, element 32) in Period 4. In Group 13, it falls between aluminum (Al) and indium (In). Unlike aluminum, gallium does not form a protective oxide that resists base attack indefinitely - gallium oxide (Ga₂O₃) dissolves in both strong acids and strong bases, classifying gallium as amphoteric. Unlike indium, gallium forms robust compound semiconductors with arsenic (GaAs) and nitrogen (GaN) that drive modern electronics.

The orthorhombic crystal structure of solid α-gallium is unusual. Each atom bonds to one nearest neighbor at 2.44 Å (a covalent Ga₂ dimer) and six next-nearest neighbors at 2.70-2.79 Å. This structure, with its directional covalent bonding inside a metallic lattice, produces the anisotropic electrical conductivity described below. The Ga₂ dimer persists at least partially into the liquid phase, contributing to gallium's anomalously high viscosity and surface tension compared to simple liquid metals.

What Is Gallium's Melting Point and Why Is It a Temperature Calibration Standard?

Gallium melts at 29.7646°C (85.5763°F, 302.9146 K). The gallium triple point at 302.9166 K (29.7666°C) serves as an official fixed point in the International Temperature Scale of 1990 (ITS-90). NIST Standard Reference Material 1751 - high-purity gallium (99.9999956%) - is the physical artifact used to calibrate platinum resistance thermometers between 14 K and 505 K.

The 29.76°C melting point is close enough to typical ambient temperatures that storage conditions matter for commercial gallium. Tropical ambient temperatures of 30°C, heated shipping containers, or summer warehouse conditions all exceed the melting point. Gallium ingots stored above this threshold partially or fully liquefy - creating no chemical hazard but risking container breaches and product loss if the packaging is rigid. Temperature-controlled storage at 20-25°C is standard for 4N and above grades.

At 99.9999% purity, the melting point is reproducible to ±0.00007°C, which is why NIST SRM 1751 uses it as a metrology standard. The practical consequence for buyers: gallium purchased in solid ingot form at 4N purity will arrive in solid form in cold climates but may arrive partially melted in summer months. Both states are commercially normal. The metal is not degraded by the phase transition. See gallium metal forms and packaging for container specifications for liquid-phase gallium.

What Are Gallium's Thermal Properties?

Gallium has a heat of fusion of 5.59 kJ/mol (80.4 J/g) and heat of vaporization of 258.7 kJ/mol. Thermal conductivity in solid form is 29.4 W/(m·K). The liquid range of ~2,199°C (from 29.76°C to ~2,229°C boiling point) is one of the widest of any element - exceeded only by rhenium among common metals.

The wide liquid range of ~2,199°C has practical uses: gallium and its alloys are stable-phase liquid coolants across a broad temperature range in thermal management research. Galinstan (68.5% Ga / 21.5% In / 10% Sn, melting point -19°C) extends the low end of this range well below room temperature, creating a non-toxic liquid metal coolant from -19°C to over 1,300°C - a ~1,319°C liquid window with no electrical insulation requirement.

The thermal expansion anisotropy of solid gallium reflects its orthorhombic structure: expansion along the a-axis is 31.9 × 10⁻⁶/K, b-axis 16.2 × 10⁻⁶/K, c-axis 13.3 × 10⁻⁶/K. This directional variation is relevant in thin-film gallium deposition and solid-state gallium-containing components where thermal cycling generates anisotropic stresses.

What Are Gallium's Electrical and Electronic Properties?

Gallium metal has electrical resistivity of approximately 270 nΩ·m in solid form, with strong directional anisotropy: conductivity along the c-axis is 1x, a-axis is 3.2x, and b-axis is 7x the c-axis value. First ionization energy is 5.999 eV (578.8 kJ/mol). Electron affinity is 0.301 eV. Gallium transitions from diamagnetic solid to paramagnetic liquid at the melting point.

Gallium metal is not used as an electrical conductor in commercial electronics. Its conductivity is several orders of magnitude below copper. Its electrical interest lies entirely in its compound semiconductor forms (GaAs, GaN, GaP) and its liquid metal behavior in flexible electronics and thermal interface materials. In liquid metal form, gallium and its alloys (EGaIn, Galinstan) are used as soft electrical conductors in stretchable circuits, where the combination of metallic conductivity and mechanical compliance is not achievable with solid wire or printed conductive inks.

The magnetic phase change at the melting point - from diamagnetic solid to paramagnetic liquid - reflects a change in the electronic band structure when the directional covalent bonds of the orthorhombic solid dissolve into the liquid phase.

What Makes Gallium's Liquid State Physically Unusual?

Liquid gallium at 30°C has viscosity of 1.99 mPa·s - close to water (1.00 mPa·s) - despite metallic density of 6.095 g/cm³. Surface tension is ~710 mN/m, the highest of any room-temperature liquid metal. Gallium can remain liquid well below 29.76°C through supercooling - nanoparticles have remained liquid at -184°C, over 213°C below the bulk melting point.

Gallium's high surface tension creates an important practical issue: gallium does not spontaneously wet most surfaces, including glass. What appears as wetting on glass is actually gallium oxidizing against the glass surface, leaving a solid Ga₂O₃ layer on which the liquid gallium wets its own oxide, not the substrate. This oxide skin behavior - where a thin, solid oxide layer forms instantly on any exposed gallium surface in air - changes the effective surface energy of gallium dramatically, from ~710 mN/m for clean liquid gallium to ~350 mN/m for the oxide-coated surface.

This oxide skin has engineering consequences in stretchable electronics and soft robotics, where liquid metal gallium alloys are used as wires and contacts. The oxide layer provides a degree of self-contained shape retention that pure liquids lack - EGaIn (75.5% Ga / 24.5% In) is used specifically because the oxide skin holds channel shapes during fabrication. Removing the oxide with HCl restores the low-surface-tension liquid behavior and allows the alloy to flow freely.

Supercooling is a consistent behavior in gallium. At bulk scale, gallium with a native oxide layer supercools to approximately -15.6°C before crystallizing. Without the oxide layer, nucleation occurs at about +6.9°C. At the nanoparticle scale, confinement effects extend supercooling to -184°C. This behavior makes liquid gallium usable below 29.76°C in research applications where spontaneous crystallization would be a failure mode.

What Are the Properties of Gallium-Based Compound Semiconductors?

GaAs has a direct band gap of 1.42 eV and electron mobility of 8,500 cm²/(V·s). GaN has a direct band gap of 3.4 eV and breakdown field of 4 × 10⁶ V/cm, 8 times higher than GaAs. InGaN is band-gap tunable from 0.7 eV (pure InN) to 3.4 eV (pure GaN) by adjusting the indium-to-gallium ratio, enabling the full visible spectrum of LED emission.

The electron mobility of GaAs (8,500 cm²/(V·s)) is the property that drove GaAs into satellite communications, radar front ends, and mobile handsets. Electrons in GaAs reach saturation velocity at lower electric fields than in silicon, which translates to faster switching at lower supply voltages - a direct power-per-performance advantage in RF applications. For high-frequency applications above 10 GHz, GaAs-based pHEMTs (pseudomorphic high-electron-mobility transistors) remain the dominant device type.

GaN's combination of a 3.4 eV band gap (300x lower intrinsic carrier concentration than silicon), a breakdown field of 4 MV/cm, and saturation velocity of 2.7 × 10⁷ cm/s makes it the material of choice for 5G base station power amplifiers and EV charging power stages. For an equivalent on-resistance, a GaN transistor occupies roughly 10x less die area than a silicon device. This chip-area reduction is the economic driver behind GaN power device adoption. See gallium in 5G infrastructure for base station deployment data.

Ga₂O₃ (gallium oxide, beta phase) has a band gap of 4.8 eV and theoretical breakdown field of ~8 MV/cm - approximately twice GaN's values. These properties place it in the "ultra-wide bandgap" category alongside diamond and cubic boron nitride. Commercial Ga₂O₃ wafers are in early production (2-inch substrates available from Tamura Corporation and Novel Crystal Technology in Japan), and first power device demonstrations show specific on-resistance below 10 mΩ·cm² at 1,700 V breakdown.

The direct band gap of GaAs and GaN is the optical property that makes gallium compounds suitable for light emission. Silicon's indirect band gap means photon emission requires phonon assistance - inefficient and not commercially viable for LEDs or lasers. GaAs emits photons at ~870 nm (near-infrared) and is used in IR LEDs, laser diodes, and photovoltaic cells. GaN and InGaN emit at 365-550 nm depending on indium content, covering near-UV through green. See gallium in LEDs and displays for the InGaN wavelength tuning table.

How Does Gallium React Chemically?

Gallium's dominant oxidation state is +3. It reacts slowly with hydrochloric acid and dissolves in hot sulfuric acid, but forms a passivating Ga₂O₃ film in cold nitric acid. Gallium dissolves in strong bases (NaOH, KOH) via gallate ion formation: 2Ga + 6NaOH + 6H₂O → 2Na₃[Ga(OH)₆] + 3H₂. Gallium does not react with water at temperatures up to 100°C.

Gallium's attack on aluminum is commercially and physically important. Gallium diffuses into aluminum grain boundaries at room temperature, disrupting the cohesion of the crystal lattice. A small quantity of liquid gallium applied to a polished aluminum bar causes it to fracture under bending stress that uncontaminated aluminum would withstand easily. This is not a simple surface reaction - gallium penetrates along grain boundaries by liquid metal embrittlement (LME), a mechanism involving wetting of grain boundary surfaces and reduction of grain boundary cohesion energy. Gallium-contaminated aluminum cannot be reused as structural material. This property explains why aluminum containers are prohibited for gallium storage and why HDPE (high-density polyethylene) is the standard material. See gallium metal forms and packaging for approved container specifications.

Gallium oxide (Ga₂O₃) forms at the metal surface in ambient air. The beta phase (β-Ga₂O₃) is thermally stable to ~1,900°C and is the most commercially relevant polymorph for semiconductor device development. Ga₂O₃ formation from GaN oxidation in air begins below 500°C and accelerates significantly above 800°C - a thermal stability consideration in GaN power device packaging.

Gallium trichloride (GaCl₃) melts at 77.9°C and is used as a gallium source in chemical vapor deposition processes for gallium nitride. Trimethylgallium (TMGa, Ga(CH₃)₃) is the pyrophoric organometallic precursor used in MOCVD (metal-organic chemical vapor deposition) for GaAs, GaN, and InGaN epitaxial layer growth. TMGa decomposes at ~550°C under H₂ carrier gas, releasing gallium atoms that incorporate into the growing crystal layer.

What Are Gallium's Isotopes and Nuclear Properties?

Gallium has two stable isotopes: Ga-69 (60.108% natural abundance, atomic mass 68.925573 u) and Ga-71 (39.892%, 70.924702 u). Both have nuclear spin I = 3/2 and are NMR-active. Ga-67 (radioactive, 3.26-day half-life) is used clinically as a gamma-emitting radiotracer for tumor and infection imaging.

Both Ga-69 and Ga-71 are NMR-active, enabling ⁷¹Ga-NMR and ⁶⁹Ga-NMR spectroscopy. ⁷¹Ga-NMR is more commonly used due to its slightly higher receptivity. The two isotopes show measurable isotope shifts in chemical shift, useful for studying gallium coordination chemistry in solution - particularly relevant in research on gallium-based pharmaceuticals and gallium coordination complexes.

What Is Gallium's Toxicity Profile and Medical Use?

Gallium is classified as relatively non-toxic. Gallium nitrate (Ga(NO₃)₃) is FDA-approved for treatment of cancer-related hypercalcemia at 200 mg/m²/day for 5 days by continuous IV infusion. Gallium-67 citrate is used in nuclear medicine imaging of tumors and infections. The primary clinical concern at therapeutic doses is renal toxicity in approximately 10-12% of patients.

Gallium metal itself does not have an established LD50 in humans, and occupational exposure limits have not been formally set by OSHA for elemental gallium. Gallium compounds vary in toxicity: gallium arsenide (GaAs) dust and fines carry arsenic-related hazards (IARC Group 1 carcinogen) at occupational exposure levels. Handling bulk GaAs wafers with skin contact is not acutely hazardous, but machining, grinding, or polishing GaAs generates inhalable particles that carry arsenic inhalation risk. Workplace controls for GaAs include respiratory protection during machining and wet processing to suppress dust generation.

Compared to heavy metals used in semiconductor manufacturing - arsenic, cadmium, mercury, lead - gallium's toxicity profile is favorable. It does not bioaccumulate in food chains, does not form volatile toxic compounds at room temperature, and does not present an acute environmental hazard in the form used commercially (4N-7N metal in HDPE containers).

How Do Gallium's Mechanical Properties Compare to Other Metals?

Gallium is a soft metal with Mohs hardness of 1.5 and Brinell hardness of approximately 60 MPa. Young's modulus of the bulk metal is approximately 8 MPa - comparable to a soft elastomer - due to the weak metallic bonding between covalent Ga₂ dimers. Bulk modulus is 44 GPa. These mechanical properties are irrelevant to most industrial uses of gallium, which consumes the metal in compound semiconductor or liquid metal form.

Solid gallium fractures with a conchoidal (shell-shaped, glass-like) pattern - unusual for a metal. This reflects the covalent character of the Ga₂ dimers in the orthorhombic structure: the material lacks the slip planes characteristic of close-packed metallic structures (FCC or HCP) that enable ductile deformation. Solid gallium is brittle in the engineering sense: it fractures without significant plastic deformation.

This brittleness is one reason solid gallium ingots are not fabricated into structural components. The metal's commercial form is always either ingot feedstock (for further processing into compound semiconductors), liquid (for alloy formulation and liquid metal applications), or precursor chemical (TMGa, GaCl₃) for vapor deposition. The solid metal is a handling form, not a structural material.