Uranyl oxalate trihydrate, UOzCzO4.3H20, crystals are monoclinic with lattice parameters a = 5.623 (5), b=17.065 (2), c=9.451 (3)/~ and ,8=98.74 (1) °. The space group is P21/C with Z=4. 513 visually estimated reflexions were corrected for absorption and the structure refined by Fourier and least-squares methods to R = 0"079. Each uranium atom exists as a linear (O-U-O) 2+ ion with five secondary oxygen atoms coordinated to it in a perpendicular plane. The average distances are U-O~= 1-63 (4) and U-On = 2"49 (4)/~. The oxalate groups are planar and tetradentate, each bridging two uranyl ions. Only one water molecule is coordinated to the uranium atom. The structure is consistent with the thermal behaviour of the compound.

Pu(S0 4 ) 2 • 4 H 2 0 has been prepared in two polymorphic forms, a-and β-. The α-form crystals are orthorhombic, space group Fddd, with eight formula units in a cell of dimensions a = 26.527(19), b = 11.995(9) and c = 5.687(4) Â. The ß-form crystals are also orthorhombic, space group Prima with four formula-units in a cell of dimensions a = 14.544(10), b = 10.980(8) and c = 5.667(3) Λ. The structure of the ß-form has been determined from three dimensional X-ray data obtained on Weissenberg films. Least-squares refinement has led to a value of the conventional R index of 0.089 for 476 observed reflections. The coordination geometry about the Pu(IV) ion is an archemedian square -antiprism formed by oxygen atoms from four sulphate groups and four water molecules. Each sulphate group is shared by two Pu(IV) ions. Hydrogen bonds operate between the water molecules and sulphate oxygen atoms not bonded to plutonium. The structure of the α-form derived from powder data shows that it differs from the ^-structure only in the functional nature of the hydrogen bonds. The a-to ^-transformation can be achieved by slow heating upto 120°C.

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Thermal decomposition of the amorphous coprecipitate of yttrium and aluminium hydroxides forming yttrium aluminium garnet has been investigated employing thermal analyses, X-ray diffraction and IR spectroscopy. On heating, the coprecipitate progressively loses water forming a stable but highly disordered hydroxy garnet which crystallizes at -1180 K and decomposes to YAG at -1290 K. Nucleation of the crystalline phase appears to begin at ~800 K.