Pure anhydrous cerium tetrafluoride is best prepared by fluorinating cerium dioxide at 350-500�. A monohydrate can be obtained from aqueous solution in a variety of ways, but it cannot be dehydrated without decomposition. It loses water "zeolitically" in vacuum, showing relatively small changes in lattice parameters for loss of 70% of its water content. Thereafter the lattice collapses, forming well-crystallized cerium trifluoride and poorly crystallized "anhydrous" cerium tetrafluoride. The refractive indices of anhydrous monoclinic cerium tetrafluoride have been measured and its fluorine dissociation pressure at 500� shown to be less than 0.5 mm. At higher temperatures the tetrafluoride sublimes incongruently, and at 835-841� it melts with extensive decomposition into a fluorine-poor liquid and a fluorine-rich vapour. Cerium tetrafluoride is easily reduced to the trifluoride by ammonia and by water vapour at low temperatures. At higher temperatures it is quantitatively converted by water vapour to cerium dioxide and hydrogen fluoride. When heated with cerium dioxide it is reduced to the trifluoride with liberation of oxygen.
X-ray diffraction and halogen nuclear quadrupole resonance (n.q.r.) methods show that Rb3ZnBr5, (NH4)3ZnBr5, and CsZnI5 are isostructural with orthorhombic (NH4)3ZnCl5, and that Rb3ZnCl5 has the tetragonal Cs3CoCl5 structure. ZnI2,2NH3 is isomorphous with the corresponding chloride and bromide diammines. Rb2ZnBr4 and (NH4)2ZnBr4 have structures of Cs2ZnBr4 type, although the rubidium salt shows some stacking faults. In the complex chlorides 35Cl n.q.r. frequencies are grouped about 9 MHz, while the bromides have 81Br frequencies grouped around 60 MHz. The � → 3/2 transitions of 127I in the iodides are in the region of 76 MHz. The diammine compounds ZnBr2,2NH3 and ZnI2,2NH3 have frequencies close to the mean values for the ZnBr42- and ZnI42- ions respectively, but in the hydrates ZnCl2,l⅓H2O,ZnBr2,2H2O, β-KZnBr3,2H2O, and KZnI3,2H2O the halogen frequencies are increased. N.q.r. and X-ray data are also reported for the compound Cs3CuCl5, formed by quenching the melt. This compound slowly disproportionates at room temperature into Cs2CuCl4 and CsCl.
The compound a-K5ThF, has not previously been described. The compound K,ThF, is stable only a t high temperatures and has been shown to disproportionate a t lower temperatures into a mixture of P-K,ThF, and P-K,ThF,. Discordant views in the literature as to the formula of the compound of highest thorium fluoride content have been discussed, and the formula KTh,F,, for this compound confirmed. Partial solid solution has been shown to occur between KTh,F,, and ThF,. Optical properties have been recorded for the majority of the compounds listed.Systematic precipitation studies in the systems KF-Th(NO,),-H,O and KN0,-Th(NO,),-HF-H,O have shown that cationic complexes of thorium and fluorine strongly influence the composition of the precipitated potassium thorium fluorides. The hydrated compounds a-K,ThF,,nH,O, KTh,F,,nH,O, and ThF,,nH,O have been isolated from solution, but the compound KThF,,nH,O could not be isolated in a substantially pure state. Lattice water in the precipitated phases can be removed a t 300" without appreciable loss of fluorine.THE preparation of hydrated potassium thorium fluorides approximating in composition to K,ThF,, KThF, (probably a mixture of K,ThF, and KTh,F9), and KTh,F, (Rosenheim, Samper and Davidsohn, 2. autorg. Chem., 1903, 35, 424) suggests a considerable degree of complexity for the binary system KF-ThF,. Ample confirmation of this complexity was obtained from preliminary studies of the system and from the results of Dergunov and
A tentative phase diagram has been proposed which shows that the system CeF4-CeF3 is essentially eutectic in character. There is no solid solution in the terminal phases. The melting behaviour of the tetrafluoride is complicated by its volatility and by its decomposition into a fluorine-rich vapour and a fluorine-poor liquid. Clarification of phase relationships requires that the system be regarded as portion of the system CeF3-F2.
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