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Alkali metal actinide complex halides : thermochemical and structural considerations J. Fuger To cite this version: J. Fuger. Alkali metal actinide complex halides : thermochemical and structural considerations. Journal de Physique Colloques, 1979, 40 (C4), pp.C4-207-C4-213. 10.1051/jphyscol:1979465. jpa- 00218861 HAL Id: jpa-00218861 https://hal.archives-ouvertes.fr/jpa-00218861 Submitted on 1 Jan 1979 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE Colloque C4, supplément au n° 4, Tome 40, avril 1979, page C4-207 Alkali metal actinide complex halides: thermochemical and structural considerations J. Fuger Institute of Radiochemistry, University of Liege, Sart Tilman, B-4000 Liege, Belgium Résumé. — L'auteur passe en revue l'état actuel de nos connaissances dans le domaine de la thermodynamique des complexes halogènes d'actinides avec les ions alcalins, en portant une attention toute spéciale aux dérivés chlorés et bromes. Lorsque les données thermodynamiques et structurales sont accessibles, il tente de déduire l'évolution de l'énergie de la liaison actinide-halogène au sein d'une série de composés isomorphes ou analogues. Enfin, la variation énergétique au cours de la formation du complexe halogène à partir des halogénures binaires d'actinides et de métaux alcalins est prise pour base en vue de prévoir la stabilité de composés nouveaux, spécialement ceux pour lesquels l'halogénure binaire d'actinide n'a pas été préparé ou est de faible stabilité. Diverses méthodes de préparation sont évoquées. Abstract. — The present status of our information on the thermodynamics of the actinide halogeno-complexes with alkali metal ions is reviewed, with special emphasis on chloro- and bromo-derivatives. Where enough thermodynamic and structural data are available, attempts are made to deduce the evolution of the energetics of the actinide-halogen bonds along a series of isomorphous or analogous compounds. The energy change upon the formation of the halogeno-complexes from binary actinide halides and alkali metal halides is discussed with the aim of predicting the stability of new compounds, especially those for which the corresponding binary actinide halides have not been characterized or are of low stabilities. Possible preparative routes for such compounds are also outlined. 1. Introduction. — For many years the halogeno- instance via aqueous rather than via dry method) or complexes of the actinides, as well as those of the to handle (because they are less hygroscopic) than lanthanides and of d transition and main group the binary halides. elements have received considerable interest from In the present paper we shall restrict our conside­ the inorganic and the physical chemist. It is probably rations to halogeno-complexes and oxyhalogeno- significant to indicate that at the Second Internatio­ complexes involving alkali metal ions ('), as thermo­ nal Conference on the Electronic Structure of the dynamic data on complexes with organic univalent Actinides in Wroclaw (1976) four papers were de­ cations are simply not existent and since our overall voted to this topic : these papers, however, were information on complexes with other metal ions are essentially oriented toward spectral and magnetic of relative paucity. studies, leading to energy levels of the actinide cation in a highly symmetrical environment of halide 2. Fluoro-complexes. — A very large number of anions. Structural chemistry of these compounds actinide fluoride complexes have been characte­ has also received a lot of attention and has shown rized, in which the actinide cation is displaying a that quite often the coordination about the actinide coordination which can vary from 6 to 9, and prepa­ in a complex halide is not the same as in the binary rative, structural and spectral studies on these halide. On the other hand our information on the compounds have adequately been reviewed [1-3]. thermochemical properties is quite fragmentary : Our knowledge on the enthalpies of formation of This may appear surprising since such data are such species is so far restricted to a number of needed to fully understand the very existence of uranyl compounds [4-6] with the general formulae such compounds. The stabilization observed upon M;U02F;, M'(U02)2F5 and M;(U02)2F9 (M' variously formation of such complex halides from the binary Na, K, Rb and Cs). We have also at disposal salts is itself a source of valuable information with information on the complex compounds of UFf) with respect to the obtention of new compounds for NaF : NaUF„ Na2UF8 and the controversial which the binary salt either cannot be obtained or is Na3UF9 [7-10] ; however, quantitative thermodyna- of very low stability. Finally, on a practical point of view, these halogeno-complexes often provide the actinides in a wide choice of oxidation states, in the (') The ammonium cation, which often behaves like an alkali metal ion will not be considered here. We have also disregarded form of compounds which are easier to prepare (for the numerous alkali metal hydrated complexes. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979465 C4-208 J. FUGER mic data on fluoride complexes of uranium in a More recently similar data were obtained on the valency state other than six or on complex fluorides newly characterized compounds Cs,NpBr6 and of other actinides are essentially lacking except for Cs,PuBr6 by Magette and Fuger [19] and Niffle and studies on the interaction of alkali metal fluorides Fuger [20]. Table I lists the recently assessed with PuF, [I 11. values [21] for the standard enthalpies of formation Under such conditions the establishment of at 298 K of the various actinide (IV) chloro- and thermodynamic interrelationships with fluoro- bromo-complexes, according to reaction (1) complexes seems premature. 3. Chloro- and bromo-complexes. - Here, our information is much broader and, in fact, as early as 1911, Chauvenet [12] obtained results on the enthal- 4 MW4+x(c), AH: (1) pies of formation of various chloro-complexes of thorium (IV) with the general formula M'ThCl,, where M', M and X, are, respectively, the alkali M:ThC16 and M:ThCl,, from the comparison of the metal, the actinide and the halogen in their standard enthalpies of solution in water of the binary halides state at 298 K : crystalline (c), liquid (1) and gaseous of these chloro-complexes. In a study involving the (g). These values are consistent with the latest various actinide (IV) compounds of the type auxiliary data recommended by CODATA [22] or Cs,MC16 (M = Th to Pu inclusive) Fuger and compatible with the CODATA selection 1231. Ta- Brown [13, 141 obtained results in good agreement ble I lists also the best values for the enthalpies of with those of Chauvenet for the thorium salt. On the formation, AHcomp,,,,from the binary salts, ac- other hand, Martynova et al. and Vdovenko et al. cording to reaction (2) reported results for the enthalpies of formation of various alkali metal uranium (IV) chloro- complexes 115, 161 and bromo-complexes [17, 181. Table I. - Thermodynamic data associated with the formation of quadrivalent actinide chloro- and bromo -complexes at 298 K (kJ . mol-I). Original references (*) on Compound - AH: which data are based - - - LiThC1, 1 619.6 + 4.2 Chauvenet [I23 KuC1, 1481.1 + 3.3 Martynova [15] ;Vdovenko [16] RbUCl, 1 497.9 2 4.2 Vdovenko [16] CsUCl, 1 518.4 +- 4.2 Vdovenko [16] Li,ThCl, 2 038.9 & 6.3 Chauvenet [I21 Li,UC16 1 831.3 + 3.8 Vdovenko [16] Na,ThC1, 2 041.4 + 6.3 Chauvenet [I21 Na2UCl, 1 848.1 + 3.8 Martynova [I51 ;Vdovenko [16] NaKUCl, 1891.623.8 Martynova [15] K,ThCl, 2 110.8 + 6.3 Chauvenet [I21 K2uc16 1931.8 & 3.8 Martynova [15] ;Vdovenko [16] Rb,ThCI, 2 157.3 rt 6.3 Chauvenet [I23 Rb,UCl, 1 956.0 2 3.8 Vdovenko [ 163 Rb4ThC1, 3 063.5 t 8.0 Chauvenet [l2] Rb,UCl, 2 828.0 rt 4.2 Vdovenko 1161 Cs,ThCl, 2 147.6 r 2.1 Chauvenet [12] ;Fuger, Brown [13] Cs,PaC16 2029 + 13 Fuger, Brown [I41 Cs,UCl, 2 011.2 + 4.2 Fuger, Brown [13] ;Vdovenko [16] Cs,NpCl, 1 977.4 t 1.7 Fuger , Brown [13] Cs,PuCl, 1 972.8 + 2.9 Fuger ,Brown [13] Cs4ThC1, 3 053.1 + 8.0 Chauvenet [12] Na,UBr6 1 529.7 t 2.5 Vdovenko [I81 K,UBr6 1 632.6 % 2.5 Vdovenko [l8] Rb,UBr6 1 653.3 2 3.3 Vdovenko 1171 Cs,UBr, 1 710.0 % 3.3 Vdovenko [l7] Cs,NpBr, 1 682.5 + 1.9 Magette, Fuger [19] Cs,PuBr6 1 694.1 + 3.6 Niffle , Fuger [20] CsU,C1, 2 535.1 t 8.0 Vdovenko [16] (*) When there are more than two authors, only the first one is indicated. ALKALI METAL ACTINIDE COMPLEX HALIDES C4-209 The evaluation of AH,,,,,, is obtained from the the enthalpies of formation of the tetravalent actini- comparison of the enthalpies of solution of the de ions [26]. complex halides and of the binary halides in the As our information on the thermochernistry of the same media. In a number of instances the reported Cs2MX, compounds is now relatively abundant we data arise from measurements by two groups of have attempted to gain some insight on the evolution authors.
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