HE lattice parameter of the Tho, unit cell was measured as T part of a study of binary systems of Tho, and typical fissionproduct oxides in nuclear fuels. The structure of Tho,, determined by Goldschmidt andThomassen,' is of the fluorite type, with an fcc lattice, space group Oh5(Frn3rn), and 4 molecules/unit cell. The lattice parameter of the unit cell has been determined by several workers; the latest and accepted value was reported by Swanson and Tatge,, who made their measurements on 99.99%-pure at 26°C and gave a theoretical density of 9.991 g/cm3. No information other than chemical purity was given to characterize the Tho,.It is now recognized that oxides of equivalent purity can have different properties, depending on the precursor from which they were prepared and the heat treatment to which they were subjected. Such a dependence is especially true for Th0,.334 The Tho, used in the present work was prepared from thorium oxalate which was precipitated from a solution of thorium nitrate (99.95%)* by oxalic acid (analytical-reagent grade)' at 21°C. The oxalate was washed with distilled water and calcined in air at 600°C for 1 h. Pellets were pressed from the powder and sintered at 1700°C in dry N, for 72 h.The sintered Tho, was ground and mixed with 50 wt% CaF,. The resultant mixture was placed in a 0.5-mm lithium boride capillary and X-rayed in a Debye-Scherrer camera (Straumanis modification) at 26°C with CuKa radiation, at 40 kV and 20 mA, for 1.5 h. The film was read with a film reader* and a microdensitometer,s the former to obtain diffraction angles and the latter to obtain relative intensities. The readings obtained from the X-ray diffraction photograph were corrected for systematic errors by alternative procedures. (1) Thea, values were plotted vs the function '/z(cos2 f3/f3)+(cos2 B/sin O ) , as suggested by Taylor and Sinclair5 and Nelson and Riley.6 The line of best fit, calculated by the method of least-squares, was extrapolated to give the value at 0=90". (2) The d values for the internal standard, CaF,, were calculated from its known a, value, and the difference from the measured values (Ad) was found. Values of (Adld) were again plotted vs the cosine function, and the resulting points were fitted to a straight line. Percentage corrections read from this line were applied to give the correctedd values and, hence, the correcteda,. The final values for a, given by the two procedures are, respectively, 5.5958 and 5.5957. The results are summarized in Table I. The Tho, examined in the present work was highly crystalline; the sintered grain size ranged from 20 to 200 p m . The smaller unit cell determined for this powder is consistent with a structure more highly crystalline than that examined by Swanson and Tatge.,
