Patent Number: 
Section: claims

1. A process for synthesizing a compound C1 selected from the group consisting of mixed peroxides and hydroxo-peroxides of an actinyl and of at least one cation X1, wherein:the actinyl has formula AnO2q+ where An is an actinide selected from the group consisting of uranium and neptunium, and q equals 1 or 2;the cation X1 is a doubly, triply or quadruply charged metal cation, wherein the metal differs from An;which process comprises a reaction in a solvent of a salt of the cation X1 with a compound C2 selected from the group consisting of mixed peroxides and hydroxo-peroxides of the actinyl and of at least one singly charged cation X2, whereby compound C2 is converted to compound C1 by a replacement of the cation X2 by the cation X1. 2. The process of claim 1, wherein the cation X1 is a cation of an alkaline-earth metal, a cation of a post-transition metal, a cation of a transition metal, a cation of a lanthanide or a cation of an actinide. 3. The process of claim 2, wherein the cation X2 is a cation of an alkaline metal, a cation of a transition metal or a polyatomic cation. 4. The process according to claim 1, wherein the reaction of the cation X1 with compound C2 comprises adding a solution of the salt of the cation X1 to compound C2 to obtain a reaction medium, and leaving the reaction medium to stand for sufficient time to obtain the replacement of the cation X2 by the cation X1. 5. The process of claim 4, wherein the solution of the salt of the cation X1 is an aqueous solution. 6. The process of claim 1, further comprising a synthesis of compound C2. 7. The process of claim 6, wherein the synthesis of compound C2 comprises a reaction of a first aqueous solution comprising a salt of the actinide An with an alkaline second aqueous solution comprising a salt or hydroxide of the cation X2 and hydrogen peroxide. 8. The process of claim 7, wherein the synthesis of compound C2 comprises adding the first solution to the second solution under agitation to obtain a reaction medium and leaving the reaction medium to stand for sufficient time to obtain the formation of compound C2. 9. The process of claim 1, wherein compound C1 has general formula (I):(X1m+)r1[(AnO2q+)n(O22−)p−x(OH−)2x](2p−qn)−  (I)where:m equals 2, 3 or 4;n is an even integer of 2 or higher;x is an integer of 0 or higher;p is an integer higher than x; andn, p and r1 are such that 1.5≤p/n≤2 and 0<r1=(2p−qn)/m. 10. The process of claim 9, wherein compound C2 has general formula (II):(X2+)r2[(AnO2q+)n(O22−)p−x(OH−)2x](2p−qn)−  (II)where:0<r2=2p−qn. 11. The process of claim 9, wherein n is an even integer ranging from 2 to 60. 12. The process of claim 1, wherein the cation X1 is a cation of an actinide or lanthanide. 13. The process of claim 1, wherein the cation X2 is an ammonium cation. 14. A process for synthesizing a mixed oxide of an actinide An selected from the group consisting of uranium and neptunium, and of at least one metal able to form a doubly, triply or quadruply charged metal cation X1, the metal differing from An, which process comprises:synthesizing a mixed peroxide or hydroxo-peroxide of an actinyl of formula AnO2q+ where q equals 1 or 2, and of at least the cation X1, the synthesis comprising a reaction in a solvent of a salt of the cation X1 with a compound C2 selected from the group consisting of mixed peroxides and hydroxo-peroxides of the actinyl and of at least one singly charged cation X2, whereby compound C2 is converted to compound C1 by a replacement of the cation X2 by the cation X1; andcalcining the mixed peroxide or hydroxo-peroxide thus synthesized. 15. A mixed peroxide or hydroxo-peroxide of an actinyl and of at least one cation X1, wherein:the actinyl has formula AnO2q+ where An is an actinide selected from the group consisting of uranium and neptunium, and q equals 1 or 2;the cation X1 is a doubly, triply or quadruply charged metal cation, wherein the metal differs from An;the peroxide or hydroxo-peroxide has following general formula (I):(X1m+)r1[(AnO2q+)n(O22−)p−x(OH−)2x](2p−qn)−  (I)where:m equals 2, 3 or 4;n is an even integer of 2 or higher;x is an integer of 0 or higher;p is an integer higher than x; andn, p and r1 are such that 1.5≤p/n≤2 and 0<r1=(2p−qn)/m. 16. The peroxide or hydroxo-peroxide of claim 15, wherein n is an even integer ranging from 2 to 60. 17. The peroxide or hydroxo-peroxide of claim 15, wherein the cation X1 is a cation of an actinide or lanthanide. 18. The process of claim 10, wherein n is an even integer ranging from 2 to 60.