KI
Potassium iodide · iodide of potassium, kalii iodidum, potassium monoiodide
Potassium iodide (KI) is an ionic salt made of potassium cations and iodide anions. It is widely used as a source of iodine in nutrition and medicine. It is a thermodynamically stable insulator with a wide band gap, and under ambient conditions it crystallizes in the face-centered cubic rock-salt (NaCl-type) structure.
About Potassium iodide
What is potassium iodide and what structure does it adopt?
Potassium iodide is a binary alkali halide made of potassium cations (K+) and iodide anions (I-) held together by ionic bonding. Under ambient conditions it crystallizes in the face-centered cubic rock-salt (sodium chloride) structure, in which each potassium ion sits at the center of an octahedron of six iodide ions, and each iodide is surrounded by six potassium ions in the same way. At elevated pressure it undergoes a reversible phase transition to the denser, eight-coordinate cesium chloride-type structure.
What is potassium iodide used for?
Potassium iodide is used in nutrition, medicine and chemistry. It is an iodine source for iodized salt and dietary supplements that prevent iodine deficiency. In medicine it is used as a thyroid-blocking agent in radiological emergencies, where it prevents the thyroid from taking up radioactive iodine. It is also used in some treatments of thyroid disease. In the laboratory and in industry, KI is a common source of iodide ions for analytical chemistry and synthesis, and it dissolves elemental iodine by forming triiodide solutions. It is also a raw material for making other iodide compounds.
How stable is KI and how easy is it to synthesize?
Potassium iodide is a thermodynamically stable compound. Industrially, it is made by reacting potassium hydroxide with iodine. That reaction also produces potassium iodate, which is removed or reduced back to iodide. A second route avoids iodate: iron reacts with iodine to form an iron iodide, which is then treated with potassium carbonate. KI can also be made by neutralizing hydriodic acid with a potassium base. Over time, air and light slowly oxidize iodide to traces of free iodine, which gives the salt or its solutions a yellowish tint.
Is KI a conductor, semiconductor, or insulator?
Potassium iodide is an electronic insulator with a wide band gap. The valence band comes mainly from filled iodine p orbitals, and the conduction band mainly from empty potassium states. Because the gap is large, room temperature produces almost no thermally excited electronic charge carriers. Pure crystalline KI therefore does not conduct electricity electronically. The small conductivity seen in the melt or in crystals with defects comes from moving ions.
How does KI compare to other compounds in its class?
KI belongs to the alkali iodide series, together with lithium, sodium, rubidium and cesium iodide. Like lithium, sodium and rubidium iodide, it has the six-coordinate rock-salt structure at ambient pressure. Cesium iodide is different: it takes the eight-coordinate cesium chloride structure. Among the potassium halides, the band gap gets smaller from fluoride to chloride to bromide to iodide, so KI has a narrower gap than KF or KCl but is still a wide-gap insulator.
Key Properties
Cross-validated computational properties for Potassium iodide, aggregated across 3 databases.
Band GapEnergy needed to move an electron from the valence band to the conduction band. Lower or zero values tend to behave more metallic; larger gaps are more insulating or semiconducting.
Energy Above HullThermodynamic distance from the most stable set of competing phases. 0 eV/atom is on the convex hull; small positive values may still be experimentally accessible.
StabilityA plain-language summary of the best reported energy-above-hull result. It reflects whether the lowest-energy structure is on, near, or far from the stability hull.
StructuresCount of reported calculated crystal structures for this formula, including alternate polymorphs, source databases, and observed space groups.
Cross-Source DFT Agreement
How well independent DFT databases agree on the thermodynamics of KI. Tight agreement means computed properties can be trusted without re-running calculations.
Agreement ScoreA normalized confidence score summarizing how closely independent DFT databases agree. Higher scores mean tighter cross-source agreement.
Hull SpreadDifference between the highest and lowest energy-above-hull values reported by comparable sources. Smaller spread means less thermodynamic disagreement.
Sources ComparedNumber and names of computational sources with comparable entries for this formula.
Space Group ConsensusWhether independent sources predict the same crystal symmetry for the lowest-energy structure.
Reported Structures
Lowest-energy structures reported for KI, ranked by energy above hull.
| Space GroupSymmetry classification of the crystal arrangement. The number is the international space-group index. | Crystal SystemBroad lattice family, such as cubic, tetragonal, monoclinic, or triclinic, derived from unit-cell symmetry. | Band Gap (eV)Electronic gap calculated for this specific reported structure, measured in electronvolts. | E above hull (eV/atom)Thermodynamic distance from the convex hull for this structure, normalized per atom. Lower is generally more stable. | E/atom (eV)Computed total energy normalized per atom. Use energy above hull, not this value alone, when comparing stability. | Density (g/cm³)Mass per relaxed crystal volume, reported in grams per cubic centimeter. |
|---|---|---|---|---|---|
| Fm-3m (No. 225) | cubic | 4.09 | 0.0000 | -1.582 | — |
| — | — | 3.98 | 0.0000 | -1.584 | — |
| Fm-3m (No. 225) | cubic | 3.85 | 0.0000 | -2.998 | 3.10 |
| — | — | 4.11 | 0.0007 | -1.583 | — |
| — | — | 3.98 | 0.0009 | -1.583 | — |
| — | — | 3.90 | 0.0192 | -1.565 | — |
| Cmcm (No. 63) | orthorhombic | 3.75 | 0.0193 | -2.978 | 2.97 |
| P63/mmc (No. 194) | hexagonal | 4.00 | 0.0197 | -1.563 | — |
| Pnma (No. 62) | orthorhombic | 3.73 | 0.0202 | -2.978 | 2.95 |
| P63/mmc (No. 194) | hexagonal | 4.47 | 0.0204 | -2.977 | 3.09 |
| — | — | 3.55 | 0.0329 | -1.551 | — |
| Pm-3m (No. 221) | cubic | 3.63 | 0.0411 | -1.541 | — |
Applications
Where Potassium iodide is used.
Frequently Asked Questions
Common questions about Potassium iodide, answered from cross-validated data and the published literature.
What is the chemical formula of potassium iodide?
The chemical formula of potassium iodide is KI, which means it contains potassium and iodide ions in a one-to-one ratio.
Is KI soluble in water?
Why does KI turn yellow over time?
What crystal structure does KI adopt at ambient conditions?
What is the role of potassium iodide in radiation emergencies?
What is KI?
What is KI used for?
What is the band gap of KI?
Is KI a metal, semiconductor, or insulator?
Is KI thermodynamically stable?
What is the crystal structure of KI?
How many polymorphs of KI are known?
What elements does KI contain?
Where does the data for KI come from?
Data sources & attribution
- jarvis — Data from JARVIS (jarvis.nist.gov), NIST. Cite: Choudhary et al., npj Comp. Mater. 6, 173 (2020). (LicenseRef-US-Gov-PD)
- oqmd — Data from the OQMD (oqmd.org). Cite: Saal et al., JOM 65, 1501 (2013); Kirklin et al., npj Comp. Mater. 1, 15010 (2015). (CC-BY-4.0)
- materials_project — Data from the Materials Project (materialsproject.org). Cite: Jain et al., APL Materials 1, 011002 (2013). (CC-BY-4.0)
- mpaloe — Data from MP-ALOE. Cite: Kuner et al., npj Comput. Mater. (2025), doi:10.1038/s41524-025-01834-9.
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