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The structures of Na2UCI 6 and Li2UC16 have been determined by powder neutron diffraction at room temperature in the presence of NaCI and LiCI respectively as impurity. Na2UCI 6 crystallizes in space group P3ml, with a= 11-8062(9), c=6.3243(2) A, and Z = 3. The best description of Li2UCI 6 was found in space group P63/mmc, a = 11.1915(5), c = 6"0356(1)A, and Z = 3, with uranium and lithium ions disordered over the available octahedral sites. The results give the first illustration of the relative ease of analysing a twophase mixture using a Rietveld profile program adapted to refine simultaneously more than one phase.

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RCsumC.-Nous prtsentons une etude thtorique sur les proprietts des dtfauts dans le chlorure de strontium pur et dope par cations monovalents. Nous examinons les defauts intrinskques trouvant que l'energie de formation d'un defaut du type Frenkel anionique est en bon accord avec les valeurs experimentales. Nous montrons que les defauts du type cation alkalin interstitiel sont importants aux basses temperatures dans des cristaux dopes par chlorides alkalins.

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