Bismuth(III) oxide is perhaps the most industrially important compound of
bismuth
Bismuth is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs ...
. It is also a common starting point for bismuth chemistry. It is found naturally as the mineral
bismite (monoclinic) and sphaerobismoite (tetragonal, much more rare), but it is usually obtained as a by-product of the smelting of
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...
and
lead
Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is density, denser than most common materials. Lead is Mohs scale of mineral hardness#Intermediate ...
ores. Dibismuth trioxide is commonly used to produce the "
Dragon's eggs
''Dragon's Egg'' is a 1980 hard science fiction novel by American writer Robert L. Forward. In the story, Dragon's Egg is a neutron star with a surface gravity 67 billion times that of Earth, and inhabited by cheela, intelligent creatures ...
" effect in
fireworks
Fireworks are a class of low explosive pyrotechnic devices used for aesthetic and entertainment purposes. They are most commonly used in fireworks displays (also called a fireworks show or pyrotechnics), combining a large number of devices ...
, as a replacement of
red lead
Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a second ...
.
[
]
Structure
The structures adopted by differ substantially from those of arsenic(III) oxide
Arsenic trioxide, sold under the brand name Trisenox among others, is an inorganic compound and medication. As an industrial chemical, whose major uses include in the manufacture of wood preservatives, pesticides, and glass. As a medication, i ...
, , and antimony(III) oxide, .[Wells, A.F. (1984) ''Structural Inorganic Chemistry''. 5th. London, England: Oxford University Press. p.890 ]
Bismuth oxide, has five crystallographic polymorphs. The room temperature phase, α- has a monoclinic
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...
crystal structure. There are three high temperature phases, a tetragonal
In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a squar ...
β-phase, a body-centred cubic γ-phase, a cubic
Cubic may refer to:
Science and mathematics
* Cube (algebra), "cubic" measurement
* Cube, a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex
** Cubic crystal system, a crystal system w ...
δ- phase and an ε-phase.
The room temperature α-phase has a complex structure with layers of oxygen atoms with layers of bismuth atoms between them. The bismuth atoms are in two different environments which can be described as distorted 6 and 5 coordinate respectively.
β- has a structure related to fluorite
Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon.
The Mohs sca ...
.
γ- has a structure related to that of ( sillenite), where a fraction of the Bi atoms occupy the position occupied by SiIV, and may be written as .
δ- has a defective fluorite
Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon.
The Mohs sca ...
-type crystal structure in which two of the eight oxygen sites in the unit cell are vacant.
ε- has a structure related to the α- and β- phases but as the structure is fully ordered it is an ionic insulator. It can be prepared by hydrothermal means and transforms to the α- phase at 400 °C.
The monoclinic
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...
α-phase transforms to the cubic
Cubic may refer to:
Science and mathematics
* Cube (algebra), "cubic" measurement
* Cube, a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex
** Cubic crystal system, a crystal system w ...
δ- when heated above 729 °C, which remains the structure until the melting point, 824 °C, is reached. The behaviour of on cooling from the δ-phase is more complex, with the possible formation of two intermediate metastable
In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy.
A ball resting in a hollow on a slope is a simple example of metastability. If the ball i ...
phases; the tetragonal
In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a squar ...
β-phase or the body-centred cubic γ-phase. The γ-phase can exist at room temperature with very slow cooling rates, but α- always forms on cooling the β-phase. Even though when formed by heat, it reverts to α- when the temperature drops back below 727 °C, δ- can be formed directly through electrodeposition and remain relatively stable at room temperature, in an electrolyte of bismuth compounds that is also rich in sodium or potassium hydroxide so as to have a pH near 14.
Conductivity
The α-phase exhibits p-type electronic conductivity (the charge is carried by positive holes) at room temperature which transforms to n-type conductivity (charge is carried by electrons) between 550 °C and 650 °C, depending on the oxygen partial pressure.
The conductivity in the β, γ and δ-phases is predominantly ionic with oxide ions being the main charge carrier. Of these δ- has the highest reported conductivity. At 750 °C the conductivity of δ- is typically about 1 S cm−1, about three orders of magnitude greater than the intermediate phases and four orders greater than the monoclinic
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...
phase.
δ- has a defective fluorite-type crystal structure in which two of the eight oxygen sites in the unit cell are vacant. These intrinsic vacancies are highly mobile due to the high polarisability of the cation sub-lattice with the 6''s''2 lone pair
In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bondIUPAC '' Gold Book'' definition''lone (electron) pair''/ref> and is sometimes called an unshared pair or non-bonding pair. L ...
electrons of . The Bi–O bonds have covalent bond character and are therefore weaker than purely ionic bonds, so the oxygen ions can jump into vacancies more freely.
The arrangement of oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...
atoms within the unit cell of δ- has been the subject of much debate in the past. Three different models have been proposed. Sillén (1937) used powder X-ray diffraction on quenched samples and reported the structure of was a simple cubic
Cubic may refer to:
Science and mathematics
* Cube (algebra), "cubic" measurement
* Cube, a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex
** Cubic crystal system, a crystal system w ...
phase with oxygen vacancies ordered along <111>, the cube body diagonal. Gattow and Schroder (1962) rejected this model, preferring to describe each oxygen site (8c site) in the unit cell as having 75% occupancy. In other words, the six oxygen atoms are randomly distributed over the eight possible oxygen sites in the unit cell. Currently, most experts seem to favour the latter description as a completely disordered oxygen sub-lattice accounts for the high conductivity in a better way.[
Willis (1965) used neutron diffraction to study the fluorite () system. He determined that it could not be described by the ideal fluorite crystal structure, rather, the fluorine atoms were displaced from regular 8c positions towards the centres of the interstitial positions.][ Shuk et al. (1996)][ and Sammes et al. (1999)][ suggest that because of the high degree of disorder in δ-, the Willis model could also be used to describe its structure.
]
Use in solid-oxide fuel cells (SOFCs)
Interest has centred on δ- as it is principally an ionic conductor. In addition to electrical properties, thermal expansion
Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.
Temperature is a monotonic function of the average molecular kinetic ...
properties are very important when considering possible applications for solid electrolytes. High thermal expansion
Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.
Temperature is a monotonic function of the average molecular kinetic ...
coefficients represent large dimensional variations under heating and cooling, which would limit the performance of an electrolyte. The transition from the high-temperature δ- to the intermediate β- is accompanied by a large volume change and consequently, a deterioration of the mechanical properties of the material. This, combined with the very narrow stability range of the δ-phase (727–824 °C), has led to studies on its stabilization to room temperature.
easily forms solid solutions with many other metal oxides. These doped systems exhibit a complex array of structures and properties dependent on the type of dopant, the dopant concentration and the thermal history of the sample. The most widely studied systems are those involving rare earth metal oxides, , including yttria, . Rare earth metal cations are generally very stable, have similar chemical properties to one another and are similar in size to , which has a radius of 1.03 Å, making them all excellent dopants. Furthermore, their ionic radii decrease fairly uniformly from (1.032 Å), through (0.983 Å), (0.938 Å), (0.912 Å) and (0.89 Å), to (0.861 Å) (known as the " lanthanide contraction"), making them useful to study the effect of dopant size on the stability of the phases.
has also been used as sintering additive in the -doped zirconia system for intermediate temperature SOFC.
Preparation
The trioxide can be prepared by ignition of bismuth hydroxide
Bismuth hydroxide () is non-fully characterised chemical compound of bismuth. It is produced as white flakes when alkali is added to a solution of a bismuth salt and is usually described as bismuth oxide hydrate or bismuth hydrate.
Uses
Bismuth h ...
.[ Bismuth trioxide can be also obtained by heating bismuth subcarbonate at approximately 400 °C.]
Reactions
Atmospheric carbon dioxide or dissolved in water readily reacts with to generate bismuth subcarbonate.[ Bismuth oxide is considered a basic oxide, which explains the high reactivity with . However, when acidic cations such as Si(IV) are introduced within the structure of the bismuth oxide, the reaction with do not occur.][
Bismuth(III) oxide reacts with a mixture of concentrated aqueous sodium hydroxide and bromine or aqueous potassium hydroxide and bromine to form sodium bismuthate or potassium bismuthate, respectively.]
Usage
Medical devices
Bismuth oxide is occasionally used in dental materials to make them more opaque to X-rays than the surrounding tooth structure. In particular, bismuth (III) oxide has been used in hydraulic silicate cements (HSC), originally in " MTA" (a trade name, standing for the chemically-meaningless " mineral trioxide aggregate") from 10 to 20% by mass with a mixture of mainly di- and tri-calcium silicate powders. Such HSC is used for dental treatments such as: apicoectomy, apexification, pulp capping, pulpotomy, pulp regeneration, internal repair of iatrogenic perforations, repair of resorption perforations, root canal sealing and obturation. MTA sets into a hard filling material when mixed with water. Some resin-based materials also include an HSC with bismuth oxide. Problems have allegedly arisen with bismuth oxide because it is claimed not to be inert at high pH, specifically that it slows the setting of the HSC, but also over time can lose color by exposure to light or reaction with other materials that may have been used in the tooth treatment, such as sodium hypochlorite.
Radiative cooling
Bismuth oxide was used to develop a scalable colored surface high in solar reflectance and heat emissivity for passive radiative cooling. The paint was non-toxic and demonstrated a reflectance of 99% and emittance of 97%. In field tests the coating exhibited significant cooling power and reflected potential for the further development of colored surfaces practical for large-scale radiative cooling applications.
References
Further reading
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{{DEFAULTSORT:Bismuth(Iii) Oxide
Bismuth oxide
Pyrotechnic oxidizers
Sesquioxides