Lead iodide
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Lead(II) iodide or lead iodide is a chemical compound with the formula . At
room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...
, it is a bright yellow odorless
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
line solid, that becomes orange and red when heated. It was formerly called plumbous iodide. The compound currently has a few specialized applications, such as the manufacture of
solar cells A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physics, physical and Chemical substance, chemical phenomenon.X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10  picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
and
gamma-ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nucleus, atomic nuclei. It consists of the shortest wavelength electromagnetic wav ...
detectors. Its preparation is an entertaining and popular demonstration in
chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
education, to teach topics such as
precipitation reaction In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...
s and
stoichiometry Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...
. It is decomposed by light at temperatures above , and this effect has been used in a patented
photographic Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed i ...
process. Lead iodide was formerly employed as a yellow pigment in some paints, with the name iodide yellow. However, that use has been largely discontinued due to its toxicity and poor stability.


Preparation

is commonly synthesized via a precipitation reaction between
potassium iodide Potassium iodide is a chemical compound, medication, and dietary supplement. It is a medication used for treating hyperthyroidism, in radiation emergencies, and for protecting the thyroid gland when certain types of radiopharmaceuticals are us ...
and
lead(II) nitrate Lead(II) nitrate is an inorganic compound with the chemical formula Pb( NO3)2. It commonly occurs as a colourless crystal or white powder and, unlike most other lead(II) salts, is soluble in water. Known since the Middle Ages by the name plumbum ...
()2 in water solution: :Pb(NO3)2 + 2 KI → PbI2 + 2 KNO3 While the potassium nitrate is soluble, the lead iodide is nearly insoluble at
room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...
, and thus
precipitate In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...
s out. Other soluble compounds containing lead(II) and iodide can be used instead, for example
lead(II) acetate Lead(II) acetate (Pb(CH3COO)2), also known as lead acetate, lead diacetate, plumbous acetate, sugar of lead, lead sugar, salt of Saturn, or Goulard's powder, is a white crystalline chemical compound with a slightly sweet taste. Like many other le ...
and
sodium iodide Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na+) and iodide anions (I ...
. The compound can also be synthesized by reacting
iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...
vapor with molten
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
between 500 and 700 °C. A thin film of can also be prepared by depositing a film of
lead sulfide Lead sulfide refers to two compounds containing lead and sulfur: *Lead(II) sulfide Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is s ...
and exposing it to iodine vapor, by the reaction :PbS + I2 → PbI2 + S The
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
is then washed with
dimethyl sulfoxide Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula ( CH3)2. This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds a ...
.


Crystallization

Lead iodide prepared from cold solutions usually consists of many small hexagonal platelets, giving the yellow precipitate a silky appearance. Larger crystals can be obtained by exploiting the fact that
solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubil ...
of lead iodide in water (like those of
lead chloride Lead chloride may refer to: * Lead(II) chloride (plumbous chloride), mineral name: cotunnite Cotunnite is the natural mineral form of lead(II) chloride with formula PbCl2. It was first described in 1825 from an occurrence on Mount Vesuvius, Napl ...
and lead bromide) increases dramatically with temperature. The compound is colorless when dissolved in hot water, but crystallizes on cooling as thin but visibly larger bright yellow flakes, that settle slowly through the liquid — a visual effect often described as "golden rain". Larger crystals can be obtained by
autoclaving An autoclave is a machine used to carry out industrial and scientific processes requiring elevated temperature and pressure in relation to ambient pressure and/or temperature. Autoclaves are used before surgical procedures to perform sterilizat ...
the with water under pressure at 200 °C. Even larger crystals can be obtained by slowing down the common reaction. A simple setup is to submerge two beakers containing the concentrated reactants in a larger container of water, taking care to avoid currents. As the two substances
diffuse Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
through the water and meet, they slowly react and deposit the iodide in the space between the beakers. Another similar method is to react the two substances in a
gel A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still dif ...
medium, that slows down the diffusion and supports the growing crystal away from the container's walls. Patel and Rao have used this method to grow crystals up to 30 mm in diameter and 2 mm thick. The reaction can be slowed also by separating the two reagents with a permeable membrane. This approach, with a
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
membrane, was used in September 1988 to study the growth of crystals in zero gravity, in an experiment flown on the
Space Shuttle Discovery Space Shuttle ''Discovery'' (Orbiter Vehicle Designation: OV-103) is one of the orbiters from NASA's Space Shuttle program and the third of five fully operational orbiters to be built. Its first mission, STS-41-D, flew from August 30 to Se ...
. can also be crystallized from powder by sublimation at 390 °C, in near
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
or in a current of
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
with some
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
. Large high-purity crystals can be obtained by
zone melting Zone melting (or zone refining, or floating-zone method, or floating-zone technique) is a group of similar methods of purifying crystals, in which a narrow region of a crystal is melted, and this molten zone is moved along the crystal. The molte ...
or by the Bridgman–Stockbarger technique. These processes can remove various impurities from commercial .


Applications

Lead iodide is a precursor material in the fabrication of highly efficient
solar cells A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physics, physical and Chemical substance, chemical phenomenon.dimethylformamide Dimethylformamide is an organic compound with the formula ( CH3)2NC(O)H. Commonly abbreviated as DMF (although this initialism is sometimes used for dimethylfuran, or dimethyl fumarate), this colourless liquid is miscible with water and the majo ...
or dimethylsulfoxide, is applied over a
titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolubl ...
layer by
spin coating Spin coating is a procedure used to deposit uniform thin films onto flat substrates. Usually a small amount of coating material is applied on the center of the substrate, which is either spinning at low speed or not spinning at all. The substrate ...
. The layer is then treated with a solution of
methylammonium iodide Methylammonium iodide in an organic halide with a formula of CH3NH3I. It is an ammonium salt composed of methylamine and hydrogen iodide. The primary application for methylammonium iodide, sometimes in combination with other methylammonium ha ...
and annealed, turning it into the
double salt A double salt is a salt that contains two or more different cations or anions. Examples of double salts include alums (with the general formula ) and Tutton's salts (with the general formula ). Other examples include potassium sodium tartrate, ammo ...
methylammonium lead iodide , with a
perovskite Perovskite (pronunciation: ) is a calcium titanium oxide mineral composed of calcium titanate (chemical formula ). Its name is also applied to the class of compounds which have the same type of crystal structure as (XIIA2+VIB4+X2−3), known as ...
structure. The reaction changes the film's color from yellow to light brown. is also used as a high-energy photon detector for gamma-rays and X-rays, due to its wide band gap which ensures low noise operation. Lead iodide was formerly used as a paint pigment under the name "iodine yellow". It was described by
Prosper Mérimée Prosper Mérimée (; 28 September 1803 – 23 September 1870) was a French writer in the movement of Romanticism, and one of the pioneers of the novella, a short novel or long short story. He was also a noted archaeologist and historian, and a ...
(1830) as "not yet much known in commerce, is as bright as
orpiment Orpiment is a deep-colored, orange-yellow arsenic sulfide mineral with formula . It is found in volcanic fumaroles, low-temperature hydrothermal veins, and hot springs and is formed both by sublimation and as a byproduct of the decay of another a ...
or chromate of lead. It is thought to be more permanent; but time only can prove its pretension to so essential a quality. It is prepared by precipitating a solution of acetate or nitrate of lead, with potassium iodide: the nitrate produces a more brilliant yellow color." However, due to the toxicity and instability of the compound it is no longer used as such. It may still be used in art for bronzing and in gold-like
mosaic A mosaic is a pattern or image made of small regular or irregular pieces of colored stone, glass or ceramic, held in place by plaster/mortar, and covering a surface. Mosaics are often used as floor and wall decoration, and were particularly pop ...
tiles.


Stability

Common material characterization techniques such as
electron microscopy An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...
can damage samples of lead (II) iodide.
Thin film A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ap ...
s of lead (II) iodide are unstable in ambient air. Ambient air oxygen oxidizes iodide into elemental
iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...
: :2 PbI2 + O2 → 2 PbO + 2 I2


Toxicity

Lead iodide is very toxic to human health. Ingestion will cause many acute and chronic consequences characteristic of
lead poisoning Lead poisoning, also known as plumbism and saturnism, is a type of metal poisoning caused by lead in the body. The brain is the most sensitive. Symptoms may include abdominal pain, constipation, headaches, irritability, memory problems, inferti ...
. Lead iodide has been found to be a carcinogen in animals suggesting the same may hold true in humans. Lead iodide is an inhalation hazard, appropriate respirator should be used when handling powders of lead iodide.


Structure

The structure of , as determined by
X-ray powder diffraction An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30  ...
, is primarily hexagonal close-packed system with alternating between layers of lead atoms and iodide atoms, with largely ionic bonding. Weak van der Waals interactions have been observed between lead–iodide layers. The most common stacking forms are 2H and 4H. The 4H polymorph is most stable in samples grown from the melt, by precipitation, or by sublimation, whereas the 2H polymorph is usually formed by sol-gel synthesis. The solid can also take an R6 rhombohedral structure.


See also

*
Lead(II) bromide Lead(II) bromide is the inorganic compound with the formula PbBr2. It is a white powder. It is produced in the burning of typical leaded gasolines.Michael J. Dagani, Henry J. Barda, Theodore J. Benya, David C. Sanders "Bromine Compounds" in Ullma ...
*
Lead(II) chloride Lead(II) chloride (PbCl2) is an inorganic compound which is a white solid under ambient conditions. It is poorly soluble in water. Lead(II) chloride is one of the most important lead-based reagents. It also occurs naturally in the form of the mi ...
*
Tin(II) iodide Tin(II) iodide, also known as stannous iodide, is an ionic tin salt of iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at st ...
* Bridgman–Stockbarger technique *
Perovskite solar cell A perovskite solar cell (PSC) is a type of solar cell that includes a perovskite-structured compound, most commonly a hybrid organic–inorganic lead or tin halide-based material as the light-harvesting active layer. Perovskite materials, such a ...


References


Cited sources

*


External links


Toxic Substances Portal – Lead
{{Iodides Iodides Lead(II) compounds Metal halides Semiconductor materials