Uranate
Encyclopedia
A uranate is a ternary
oxide
involving the element uranium
in one of the oxidation state
s +4, +5 or +6. A typical chemical formula
is MxUyOz, where M represents a cation. The uranium atom in uranates(VI) has two short, co-linear, U-O bonds and either four or six more next nearest oxygen atoms. The structures are infinite lattice structures with the uranium atoms linked by bridging oxygen atoms. "Ammonium diuranate
" is an intermediate in the production of uranium oxide nuclear fuel
s.
Another method is the thermal decomposition of a complex, such as an acetate
complex. For example,
microcrystalline barium diuranate, BaU2O7 was made by thermal decomposition of barium uranyl acetate at 900 °C.
Uranates are insoluble in water and other solvents, so pure samples can only be obtained by careful control of reaction conditions.
Naturally occurring uranates are rare and always occur as secondary minerals. Examples include curite
, becquerelite and soddyite named after prominent radiochemists Pierre Curie
and Marie Curie
, Henri Becquerel
and Frederick Soddy
, respectively.
Uranates can be prepared by adding alkali
to an aqueous solution of a uranyl
salt. However, the composition of the precipitate that forms is variable and depends on the chemical and physical conditions used.
, such as [UO4]2– or [U2O7]2–, is known. Instead, all uranate structures are based on UOn polyhedra sharing oxygen atoms in an infinite lattice. The structures of uranates(VI) are unlike the structure of any mixed oxide of elements other than actinide elements. A particular feature is the presence of linear O-U-O moieties
, which resemble the uranyl
ion, UO22+. However, the U-O bond length varies from 167 pm, which is similar to the bond length of the uranyl ion, up to about 208 pm in the related compound α-UO3, so it is debatable as to whether these compounds all contain the uranyl ion. There are two principal types of uranate which are defined by the number of nearest-neighbour oxygen atoms in addition to the "uranyl" oxygens.
In one group, including M2UO4 (M=Li, Na, K) and MUO4 (M=Ca, Sr) there are six additional oxygen atoms. Taking calcium uranate, CaUO4, as an example, the six oxygen atoms are arranged as a flattened octahedron
, flattened along the 3-fold symmetry axis
of the octahedron which also runs through the O-U-O axis (local point group
D3d at the uranium atom). Each of these oxygen atoms is shared between three uranium atoms, which accounts for the stoichiometry, U + 2×O + 6×1/3 O = UO4. The structure has been described as a hexagonal layer structure. It can also viewed as a distorted fluorite
structure in which two U-O distances have decreased and the other six have increased.
In the other group, exemplified by barium uranate, BaUO4, there are four additional oxygen atoms. These four oxygens lie in a plane and each is shared between two uranium atoms, which accounts for the stoichiometry, U + 2×O + 4×1/2 O = UO4. The structure may called a tetragonal
layer structure.
Magnesium uranate, MgUO4, has a quite different structure. Distorted UO6 octahedra are linked into infinite chains; the "uranyl" U-O bond length is 192 pm, not much shorter than the other U-O bond length of 218 pm.
A number of so-called diuranates are known. They fall into two categories, compounds of exact composition, synthesized by combination of metal oxides or thermal decomposition of salts of uranyl complexes and substances of approximate composition, found in yellowcake
. The name refers only to the empirical formula
, MxU2O7; the structures are completely different from ions such as the dichromate ion. For example, in barium diuranate, BaU2O7, UO6 octahedral units are joined by sharing edges, forming infinite chains in the directions of the crystallographic a and b directions.
Uranates with more complicated empirical formulas are known. Essentially these arise when the cation:uranium ratio is different from 2:1 (monovalent cations) or 1:1 (divalent cations). Charge-balance constrains the number of oxygen atoms to be equal to half the sum of charges of the cations and uranyl groups. For example, with the cation K+, compounds with K:U ratios of 2, 1 and 0.5 were found, corresponding to empirical formulas K2UO4, KUO3 and K2U4O13. The uranate structures in these compounds differ in the way the UOx structural units are linked together.
Halo-uranates are compounds in which one oxygen atom is replaced by two halogen
atoms. For example, CaU2O6F2 is described as an "isotype" of the oxide U3O8. In the structure of the oxyfluoride, UO2F2, there is the same hexagonal layer structure as in CaUO4. Other anions, such as nitrate
, can also be present in uranates.
Yellowcake
is produced in the separation of uranium from other elements, by adding alkali
to a solution containing uranyl salts.
When the alkali used is ammonia, so-called ammonium diuranate, known in the industry as ADU, is the main constituent of yellowcake. The exact composition of the precipitate depends to some extent on the conditions and anions that are present and the formula (NH4)2U2O7, is only an approximation. The precipitates obtained on addition of ammonia to uranyl nitrate solution under different conditions of temperature and final pH, when dried, were considered as loosely bound compounds with an ammonia/uranium ratio of 0.37 containing varying amounts of water and ammonium nitrate. In other studies it was found to approximate to the gross formulas 3UO3·NH3·5H2O,
The asymmetric stretching frequency
of the uranyl ion was found to decrease with increasing NH4+ content. This decrease is continuous and no band splitting was observed, indicating that the ammonium uranate system is homogeneous and continuous.
ADU is an intermediate in the production of uranium oxides to be used as nuclear fuel
; it is converted directly into an oxide by heating. βUO3
is produced at about 350 °C and U3O8 is obtained at higher temperatures. When the alkali used is sodium hydroxide, so-called sodium diuranate, SDU, is produced. This can also be converted into an oxide. Another choice of alkali is magnesium oxide
, making magnesium diuranate
, known as MDU.
Oxides and uranates of uranium(VI) have been used in the past as yellow ceramic glazes as in Fiesta and to make yellow-green uranium glass
. Both of these applications are abandoned due to concern regarding radioactivity
of the uranium. Uranates are important in radioactive waste management.
. MI7UO6 structures are based on a hexagonally close-packed array of oxygen atoms. In all cases the uranium is at the centre of an octahedron
of oxygen atoms. Few other compounds of uranium(V) are stable.
and uranium dioxide
in an atmosphere that contains absolutely no oxygen. It has a cubic crystal structure (space group
Pmm). An extensive series of non-stoichiometric mixed oxides exists. The UO2/PuO2 system is important in the fuel of fast breeder reactors. Cerium uranium blue
, of variable formula CexU1–xO2, has a structure similar to the fluorite
structure.
Ternary compound
In chemistry, a ternary compound is a compound containing three different elements. An example of this is sodium phosphate, Na3PO4. The sodium ion has a charge of 1+ and the phosphate ion has a charge of 3-. Therefore, three sodium ions are needed to balance the charge of one phosphate ion. Another...
oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
involving the element uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
in one of the oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
s +4, +5 or +6. A typical chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
is MxUyOz, where M represents a cation. The uranium atom in uranates(VI) has two short, co-linear, U-O bonds and either four or six more next nearest oxygen atoms. The structures are infinite lattice structures with the uranium atoms linked by bridging oxygen atoms. "Ammonium diuranate
Ammonium diuranate
Ammonium diuranate or ' , is one of the intermediate chemical forms of uranium produced during yellowcake production. The name 'yellowcake' originally given to this bright yellow substance, now applies to mixtures of uranium oxides which are actually hardly ever yellow...
" is an intermediate in the production of uranium oxide nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
s.
Synthesis
A method of general applicability involves combining two oxides in a high temperature reaction. For example,- Na2O + UO3 → Na2UO4
Another method is the thermal decomposition of a complex, such as an acetate
Acetate
An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
complex. For example,
microcrystalline barium diuranate, BaU2O7 was made by thermal decomposition of barium uranyl acetate at 900 °C.
- Ba[UO2(ac)3]2 → BaU2O7 + ... (ac=CH3CO2–)
Uranates are insoluble in water and other solvents, so pure samples can only be obtained by careful control of reaction conditions.
Naturally occurring uranates are rare and always occur as secondary minerals. Examples include curite
Curite
Curite is a lead uranium oxide mineral with formula: Pb38O86·3. It is named after the French physicist Pierre Curie, who is known for his work on radioactivity....
, becquerelite and soddyite named after prominent radiochemists Pierre Curie
Pierre Curie
Pierre Curie was a French physicist, a pioneer in crystallography, magnetism, piezoelectricity and radioactivity, and Nobel laureate. He was the son of Dr. Eugène Curie and Sophie-Claire Depouilly Curie ...
and Marie Curie
Marie Curie
Marie Skłodowska-Curie was a physicist and chemist famous for her pioneering research on radioactivity. She was the first person honored with two Nobel Prizes—in physics and chemistry...
, Henri Becquerel
Henri Becquerel
Antoine Henri Becquerel was a French physicist, Nobel laureate, and the discoverer of radioactivity along with Marie Curie and Pierre Curie, for which all three won the 1903 Nobel Prize in Physics.-Early life:...
and Frederick Soddy
Frederick Soddy
Frederick Soddy was an English radiochemist who explained, with Ernest Rutherford, that radioactivity is due to the transmutation of elements, now known to involve nuclear reactions. He also proved the existence of isotopes of certain radioactive elements...
, respectively.
Uranates can be prepared by adding alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
to an aqueous solution of a uranyl
Uranyl
The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula [UO2]2+. It has a linear structure with short U-O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands are bound to the uranyl ion in an equatorial plane...
salt. However, the composition of the precipitate that forms is variable and depends on the chemical and physical conditions used.
Structures
All uranates(VI) are mixed oxides, that is, compounds made up of metal(s), uranium and oxygen atoms. No uranium oxyanionOxyanion
An oxyanion or oxoanion is a chemical compound with the generic formula AxOyz− . Oxoanions are formed by a large majority of the chemical elements. The formulae of simple oxoanions are determined by the octet rule...
, such as [UO4]2– or [U2O7]2–, is known. Instead, all uranate structures are based on UOn polyhedra sharing oxygen atoms in an infinite lattice. The structures of uranates(VI) are unlike the structure of any mixed oxide of elements other than actinide elements. A particular feature is the presence of linear O-U-O moieties
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
, which resemble the uranyl
Uranyl
The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula [UO2]2+. It has a linear structure with short U-O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands are bound to the uranyl ion in an equatorial plane...
ion, UO22+. However, the U-O bond length varies from 167 pm, which is similar to the bond length of the uranyl ion, up to about 208 pm in the related compound α-UO3, so it is debatable as to whether these compounds all contain the uranyl ion. There are two principal types of uranate which are defined by the number of nearest-neighbour oxygen atoms in addition to the "uranyl" oxygens.
In one group, including M2UO4 (M=Li, Na, K) and MUO4 (M=Ca, Sr) there are six additional oxygen atoms. Taking calcium uranate, CaUO4, as an example, the six oxygen atoms are arranged as a flattened octahedron
Octahedron
In geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex....
, flattened along the 3-fold symmetry axis
Reflection symmetry
Reflection symmetry, reflectional symmetry, line symmetry, mirror symmetry, mirror-image symmetry, or bilateral symmetry is symmetry with respect to reflection. That is, a figure which does not change upon undergoing a reflection has reflectional symmetry.In 2D there is a line of symmetry, in 3D a...
of the octahedron which also runs through the O-U-O axis (local point group
Molecular symmetry
Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...
D3d at the uranium atom). Each of these oxygen atoms is shared between three uranium atoms, which accounts for the stoichiometry, U + 2×O + 6×1/3 O = UO4. The structure has been described as a hexagonal layer structure. It can also viewed as a distorted fluorite
Fluorite
Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...
structure in which two U-O distances have decreased and the other six have increased.
In the other group, exemplified by barium uranate, BaUO4, there are four additional oxygen atoms. These four oxygens lie in a plane and each is shared between two uranium atoms, which accounts for the stoichiometry, U + 2×O + 4×1/2 O = UO4. The structure may called a tetragonal
Tetragonal crystal system
In crystallography, the tetragonal crystal system is one of the 7 lattice point groups. 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 square base and height .There are two tetragonal Bravais...
layer structure.
Magnesium uranate, MgUO4, has a quite different structure. Distorted UO6 octahedra are linked into infinite chains; the "uranyl" U-O bond length is 192 pm, not much shorter than the other U-O bond length of 218 pm.
A number of so-called diuranates are known. They fall into two categories, compounds of exact composition, synthesized by combination of metal oxides or thermal decomposition of salts of uranyl complexes and substances of approximate composition, found in yellowcake
Yellowcake
Yellowcake is a kind of uranium concentrate powder obtained from leach solutions, in an intermediate step in the processing of uranium ores. Yellowcake concentrates are prepared by various extraction and refining methods, depending on the types of ores...
. The name refers only to the empirical formula
Empirical formula
In chemistry, the empirical formula of a chemical compound is the simplest positive integer ratio of atoms of each element present in a compound. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. The empirical formula is used as standard for most ionic...
, MxU2O7; the structures are completely different from ions such as the dichromate ion. For example, in barium diuranate, BaU2O7, UO6 octahedral units are joined by sharing edges, forming infinite chains in the directions of the crystallographic a and b directions.
Uranates with more complicated empirical formulas are known. Essentially these arise when the cation:uranium ratio is different from 2:1 (monovalent cations) or 1:1 (divalent cations). Charge-balance constrains the number of oxygen atoms to be equal to half the sum of charges of the cations and uranyl groups. For example, with the cation K+, compounds with K:U ratios of 2, 1 and 0.5 were found, corresponding to empirical formulas K2UO4, KUO3 and K2U4O13. The uranate structures in these compounds differ in the way the UOx structural units are linked together.
Halo-uranates are compounds in which one oxygen atom is replaced by two halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
atoms. For example, CaU2O6F2 is described as an "isotype" of the oxide U3O8. In the structure of the oxyfluoride, UO2F2, there is the same hexagonal layer structure as in CaUO4. Other anions, such as nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
, can also be present in uranates.
Properties and uses
Yellowcake
Yellowcake is a kind of uranium concentrate powder obtained from leach solutions, in an intermediate step in the processing of uranium ores. Yellowcake concentrates are prepared by various extraction and refining methods, depending on the types of ores...
is produced in the separation of uranium from other elements, by adding alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
to a solution containing uranyl salts.
When the alkali used is ammonia, so-called ammonium diuranate, known in the industry as ADU, is the main constituent of yellowcake. The exact composition of the precipitate depends to some extent on the conditions and anions that are present and the formula (NH4)2U2O7, is only an approximation. The precipitates obtained on addition of ammonia to uranyl nitrate solution under different conditions of temperature and final pH, when dried, were considered as loosely bound compounds with an ammonia/uranium ratio of 0.37 containing varying amounts of water and ammonium nitrate. In other studies it was found to approximate to the gross formulas 3UO3·NH3·5H2O,
The asymmetric stretching frequency
Infrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...
of the uranyl ion was found to decrease with increasing NH4+ content. This decrease is continuous and no band splitting was observed, indicating that the ammonium uranate system is homogeneous and continuous.
ADU is an intermediate in the production of uranium oxides to be used as nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
; it is converted directly into an oxide by heating. βUO3
Uranium trioxide
Uranium trioxide , also called uranyl oxide, uranium oxide, and uranic oxide, is the hexavalent oxide of uranium. The solid may be obtained by heating uranyl nitrate to 400 °C. Its most commonly encountered polymorph, γ-UO3, is a yellow-orange powder.-Production and use:There are three methods...
is produced at about 350 °C and U3O8 is obtained at higher temperatures. When the alkali used is sodium hydroxide, so-called sodium diuranate, SDU, is produced. This can also be converted into an oxide. Another choice of alkali is magnesium oxide
Magnesium oxide
Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...
, making magnesium diuranate
Magnesium diuranate
Magnesium diuranate is a compound of uranium. It is known in the uranium refining industry as "MDU" and forms the major part of some yellowcake mixtures. Yellowcakes are an intermediate product in the uranium refining process....
, known as MDU.
Oxides and uranates of uranium(VI) have been used in the past as yellow ceramic glazes as in Fiesta and to make yellow-green uranium glass
Uranium glass
Uranium glass is glass which has had uranium, usually in oxide diuranate form, added to a glass mix before melting. The proportion usually varies from trace levels to about 2% by weight uranium, although some 19th-century pieces were made with up to 25% uranium.Uranium glass was once made into...
. Both of these applications are abandoned due to concern regarding radioactivity
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
of the uranium. Uranates are important in radioactive waste management.
Uranium(V)
Three series of uranates(V) have been characterized. Compounds with the formula MIUO3 have a perovskite structure. Compounds MI3UO4 have a defect rock-salt structureCubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
. MI7UO6 structures are based on a hexagonally close-packed array of oxygen atoms. In all cases the uranium is at the centre of an octahedron
Octahedron
In geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex....
of oxygen atoms. Few other compounds of uranium(V) are stable.
Uranium(IV)
Barium uranate, BaUO3, is made from barium oxideBarium oxide
Barium oxide, BaO, is a white hygroscopic compound formed by the burning of barium in oxygen, although it is often formed through the decomposition of other barium salts.It reacts with water to form barium hydroxide.-Uses:...
and uranium dioxide
Uranium dioxide
Uranium dioxide or uranium oxide , also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used...
in an atmosphere that contains absolutely no oxygen. It has a cubic crystal structure (space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
Pmm). An extensive series of non-stoichiometric mixed oxides exists. The UO2/PuO2 system is important in the fuel of fast breeder reactors. Cerium uranium blue
Cerium uranium blue
Cerium uranium blue is the name given to solid solutions of cerium oxide, CeO2, and uranium oxide, UO2, of variable composition from 0 to 100% uranium.- Synthesis and properties :...
, of variable formula CexU1–xO2, has a structure similar to the fluorite
Fluorite
Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...
structure.