Uranate

{{Short description|Uranium oxyanion associated with a monovalent or divalent cation}} {{Distinguish|urination}} thumb|Uranates of calcium, strontium, barium and lead A '''uranate''' is a ternary oxide involving the element uranium in one of the oxidation states 4, 5 or 6. A typical chemical formula is M_xU_yO_z, where M represents a cation. The uranium atom in uranates(VI) has two short collinear U–O bonds and either four or six more next nearest oxygen atoms.<ref name=wells/> The structures are infinite lattice structures with the uranium atoms linked by bridging oxygen atoms.

Uranium oxides are the foundation of the nuclear fuel cycle ("ammonium diuranate" and "sodium diuranate" are intermediates in the production of uranium oxide nuclear fuels) and their long-term geological disposal requires a thorough understanding of their chemical reactivity, phase transitions, and physical and chemical properties.<ref>T. Vogt, D.J. Butterey, ''Complex Oxides. An Introduction''. World Scientific, 2019, https://www.worldscientific.com/doi/pdf/10.1142/9789813278585_fmatter</ref>

==Synthesis== A method of general applicability involves combining two oxides in a high temperature reaction.<ref name=p1269>{{Greenwood&Earnshaw2nd|page=1269}}</ref> For example, :Na₂O + UO₃ → Na₂UO₄ 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&nbsp;°C.<ref name=allpress>{{cite journal |last=Allpress |first=J.G. |year=1965 |title=The Crystal Structure of Barium Diuranate |journal=Journal of Inorganic and Nuclear Chemistry |volume=27 |issue=7 |pages=1521–1527 |doi=10.1016/0022-1902(65)80013-6}}</ref> :Ba[UO₂(ac)₃]₂ → BaU₂O₇ ... (ac=CH₃CO₂⁻)

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.<ref name="p1269" />

Uranates are insoluble in water and other solvents, so pure samples can only be obtained by careful control of reaction conditions.{{cn|date=May 2024}} {| class="wikitable" |+ !Formula !U-ox. state !Space group !Symmetry !Formula !U-ox. state !Space group !Symmetry !Formula !U-ox. state !Space group !Symmetry |- |Li₂UO₄ |VI |α: Fmmm, Pnma β: |orthorhombic hexagonal |BaU₂O₇ |VI |I4₁/amd |tetragonal |Sr₂UO₅ |VI |P2₁/c |monoclinic |- |Na₂UO₄ |VI |α: Cmmm β: Pnma |orthorhombic orthorhombic |SrU₂O₇ |VI | | |Li₆UO₆ |VI | |hexagonal |- |K₂UO₄ |VI |α: I4/mmm β: |tetragonal orthorhombic |CaU₂O₇ |VI | | |Ca₃UO₆ |VI |P2₁ |monoclinic |- |Cs₂UO₄ |VI |I4/mmm |tetragonal |MgU₃O₁₀ |VI | |hexagonal |Sr₃UO₆ |VI |P2₁ |monoclinic |- |MgUO₄ |VI |Imma |orthorhombic |Li₂U₃O₁₀ |VI |α: P2₁/c β: P2 |monoclinic monoclinic |Ba₃UO₆ |VI |Fm-3m |cubic |- |CaUO₄ |VI |R-3m |rhombohedral |SrU₄O₁₃ |VI | |monoclinic |NaUO₃ |V |Pbnm |orthorhombic |- |SrUO₄ |VI |α: R-3m β: Pbcm |rhombohedral orthorhombic |Li₂U₆O₁₉ |VI | |orthorhombic |KUO₃ |V |Pm3m |cubic |- |BaUO₄ |VI |Pbcm |orthorhombic |K₂U₇O₂₂ |VI |Pbam |orthorhombic |RbUO₃ |V |Pm3m |cubic |- |Li₂U₂O₇ |VI | |orthorhombic |Rb₂U₇O₂₂ |VI |Pbam |orthorhombic |CaUO₃ |IV | |cubic |- |Na₂U₂O₇ |VI |C2/m |monoclinic |Cs₂U₇O₂₂ |VI |Pbam |orthorhombic |SrUO₃ |IV | |orthorhombic |- |K₂U₂O₇ |VI |R-3m |hexagonal |Li₄UO₅ |VI |I4/m |tetragonal |BaUO₃ |IV |Pm3m |cubic |- |Rb₂U₂O₇ |VI |R-3m |hexagonal |Na₄UO₅ |VI |I4/m |tetragonal |Li₃UO₄ |V | |tetragonal |- |Cs₂U₂O₇ |VI |α: C2/m β: C2/m

γ: P6/mmc |monoclinic monoclinic

hexagonal |Ca₂UO₅ |VI |P2₁/c |monoclinic |Na₃UO₄ |V |Fm-3m |cubic |}

==Uranium(VI)==

===Structures=== thumb|160px|left|CaUO₄ structure<ref name="loopstra">{{cite journal |last1=Loopstra |first1=B. O. |last2=Rietveld |first2=H. M. |year=1969 |title=The structure of some alkaline-earth metal uranates |journal= Acta Crystallographica Section B |volume=25 |issue=4 |pages=787–791 |doi=10.1107/S0567740869002974 |bibcode=1969AcCrB..25..787L }}</ref> thumb|right|BaUO₄ structure<ref name="loopstra"/> All uranates(VI) are mixed oxides, that is, compounds made up of metal(s), uranium and oxygen atoms. No uranium oxyanion, such as [UO₄]²⁻ or [U₂O₇]²⁻, is known. Instead, all uranate structures are based on UO_n polyhedra sharing oxygen atoms in an infinite lattice.<ref name=wells/> 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.<ref name=wells>{{cite book |last=Wells |first=A. F. |title=Structural Inorganic Chemistry |edition=3rd |year=1962 |publisher=Clarendon Press |location=Oxford |pages=966–969 |isbn=978-0-19-855125-6}}</ref>

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 D_3d 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.<ref name=wells/>

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.<ref name=wells/>

thumb|MgUO₄ structure.<ref>{{cite journal |last1=Zachariasen |first1=W. H. |title=Crystal chemical studies of the 5f-series of elements. XXI. The crystal structure of magnesium orthouranate |journal=Acta Crystallographica |date=1 December 1954 |volume=7 |issue=12 |pages=788–791 |doi=10.1107/S0365110X54002459|bibcode=1954AcCry...7..788Z |doi-access=free }}</ref>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.<ref name=wells/>

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, M_xU₂O₇; 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.<ref name=allpress/>

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, K₂U₂O₇ and K₂U₄O₁₃.<ref>{{cite journal |last1=Van Egmond |first1= A. B. |last2=Cordfunke |first2=E. H. P. |year=1976 |title=Investigations on potassium and rubidium uranates |journal=Journal of Inorganic and Nuclear Chemistry |volume=38 |issue=12 |pages=2245–2247 |doi=10.1016/0022-1902(76)80203-5}}</ref> The uranate structures in these compounds differ in the way the UO_x structural units are linked together.

===Properties and uses=== thumb|left|130px|A drum of yellowcake <!-- This is a hydroxylamine complex of the uranyl ion. I don't think that it occurs in yellow cake. thumb|α-UO₂(NH₂O)₂·4H₂O structure.<ref name=amur1/> --> Yellowcake is produced in the separation of uranium from other elements, by adding alkali to a solution containing uranyl salts.<ref>{{cite journal |last=Hausen |first=D. M. |year=1961 |title=Characterizing and classifying uranium yellow cakes: A background |journal=JOM |volume=50 |issue=12 |pages=45–47 |doi=10.1007/s11837-998-0307-5 |bibcode=1998JOM....50l..45H|s2cid=97023067 }}</ref>

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.<ref>{{cite journal |last1=Ainscough |first1=J. B. |last2=Oldfield |first2=B. W. |year=1962 |title=Effect of ammonium diuranate precipitation conditions on the characteristics and sintering behaviour of uranium dioxide |journal=Journal of Applied Chemistry |volume=12 |issue=9 |pages=418–424 |doi=10.1002/jctb.5010120907}}</ref> In other studies it was found to approximate to the gross formulas 3UO₃·NH₃·5H₂O,<ref>{{cite journal |last=Cordfunke |first=E. H. P. |year=1962 |title=On the uranates of ammonium—I: The ternary system NH₃---UO₃---H₂O |journal=Journal of Inorganic and Nuclear Chemistry |volume=24 |issue=3 |pages=303–307 |doi=10.1016/0022-1902(62)80184-5}}</ref> <!-- UO₂(NH₂O)₂·3H₂O<ref>{{cite journal |journal=Acta Crystallographica Section B |year=1977 |volume=33 |pages=2997–3000 |doi=10.1107/S0567740877010127 |title=A low-temperature neutron and X-ray diffraction study of UO₂(NH₂O)₂·3H₂O |first=H. W. W. |last=Adrian |first2=A. |last2=Van Tets}}</ref> or UO₂(NH₂O)₂·4H₂O.<ref name=amur1>{{cite journal |journal=Acta Crystallographica Section B |year=1978 |volume=34 |pages=88 |doi=10.1107/S0567740878002356 |title=A low-temperature neutron diffraction study of α-UO₂(NH₂O)₂·4H₂O |first=H. W. W. |last=Adrian |first2=A. |last2=Van Tets}}</ref> --> 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.<ref>{{cite journal | last1=Stuart | first1=W. I. |last2=Whateley |first2=T. L. | year=1969 |title=Composition and structure of ammonium uranates |journal=Journal of Inorganic and Nuclear Chemistry |volume=1 |issue=6 |pages=1639–1647 |doi=10.1016/0022-1902(69)80378-7 |hdl=10238/379|hdl-access=free }}</ref> thumb|A collection of uranium glassware

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. β-UO₃ is produced at about 350&nbsp;°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.<ref>{{cite book |last=Skelcher |first=Barrie |title=The Big Book of Vaseline Glass |year=2002 |publisher=Schiffer Publishing |location=Atglen, PA |isbn=978-0-7643-1474-2}}</ref> Uranates are also important in radioactive waste management.<ref>{{cite book |last1=Saling |first1=James H. |last2=Fentiman |first2=Audeen W. |title=Radioactive Waste Management |url=https://books.google.com/books?id=FxP3sNrYsW4C&pg=PA2 |access-date=2011-02-12 |edition=2 |year=2002 |publisher=Taylor & Francis |location=New York |isbn=978-1-56032-842-1 |page=2}}</ref>

==Uranium(V)==

Several series of uranates(V) have been characterized. Compounds with the formula M^IUO₃ have a perovskite structure. Compounds M^I₃UO₄ have a defect rock-salt structure. M^I₇UO₆ 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. Also M^IIIUO₄ have been recently synthesized and characterized (M^III=Bi, Fe, Cr etc.).<ref>{{cite journal |doi=10.1039/C6DT00735J |pmid=27063438 |title=Further insights into the chemistry of the Bi–U–O system |journal=Dalton Transactions |volume=45 |issue=18 |pages=7847–7855 |year=2016 |last1=Popa |first1=Karin |last2=Prieur |first2=Damien |last3=Manara |first3=Dario |last4=Naji |first4=Mohamed |last5=Vigier |first5=Jean-François |last6=Martin |first6=Philippe M. |last7=Dieste Blanco |first7=Oliver |last8=Scheinost |first8=Andreas C. |last9=Prüβmann |first9=Tim |last10=Vitova |first10=Tonya |last11=Raison |first11=Philippe E. |last12=Somers |first12=Joseph |last13=Konings |first13=Rudy J. M.|doi-access=free }}</ref><ref>{{Cite journal |doi=10.1039/C6DT00066E |pmid=26854913 |title=U(v) in metal uranates: A combined experimental and theoretical study of MgUO₄, CrUO₄, and FeUO₄ |journal=Dalton Transactions |volume=45 |issue=11 |pages=4622–4632 |year=2016 |last1=Guo |first1=Xiaofeng |last2=Tiferet |first2=Eitan |last3=Qi |first3=Liang |last4=Solomon |first4=Jonathan M. |last5=Lanzirotti |first5=Antonio |last6=Newville |first6=Matthew |last7=Engelhard |first7=Mark H. |last8=Kukkadapu |first8=Ravi K. |last9=Wu |first9=Di |last10=Ilton |first10=Eugene S. |last11=Asta |first11=Mark |last12=Sutton |first12=Stephen R. |last13=Xu |first13=Hongwu |last14=Navrotsky |first14=Alexandra|osti=1256103 |url=https://www.osti.gov/biblio/1256103 }}</ref> Few other compounds of uranium(V) are stable.<ref name=p1269/> <!-- Li₃UO₄, Li₇UO₆ -->

==Uranium(IV)==

Barium uranate, BaUO₃, is made from barium oxide and uranium dioxide in an atmosphere that contains absolutely no oxygen. It has a cubic crystal structure (space group Pm{{overline|3}}m).<ref>{{cite journal |last1=Barrett |first1=S. A. |last2=Jacobson |first2=A. J. |last3=Tofield |first3=B. C. |last4=Fender |first4=B. E. F. |year=1982 |title=The preparation and structure of barium uranium oxide BaUO_3+x |journal= Acta Crystallographica Section B |volume=38 |issue=11 |pages=2775–2781 |doi=10.1107/S0567740882009935 |bibcode=1982AcCrB..38.2775B }}</ref>

==References== {{reflist|30em}}

==Further reading== *{{cite book |last1=Burns |first1=C. J. |last2=Neu |first2=M. P. |last3=Boukhalfa |first3=H. |last4=Gutowski |first4=K. E. |last5=Bridges |first5=N. J. |last6=Roger |first6=R. D. |title=Comprehensive Coordination Chemistry II |year=2004 |publisher=Elsevier |isbn=978-0-08-043748-4 |pages=189–345 |chapter=Chapter 3.3, The Actinides |doi=10.1016/B0-08-043748-6/02001-6}}

Category:Uranates