CRYSTAL-CHEMISTRY OF CATION ORDER-DISORDER IN PSEUDOBROOKITE-TYPE MGTI2O5

Effects of caticon order-disorder on the pseudobrookite-type MgTi2O5 s tructure have been studied with single-crystal X-ray diffraction on fi ve synthetic crystals (labeled P600, P700, P800, P1000, and P1400) ana nealed a; 600, 700, 800, 1000, and 1400 degrees C, respectively. The d isorder parameters (X = Ti ins the M1 site) determined for five sample s range from 0.070(5) to 0.485(5). Unit-cell dimensions a and c increa se with X, whereas b decreases. The decrease in b with increasing cati on disorder results primarily from the marked reduction in the O3-O3 d istance within the M1 octahedron, whereas the variation of n and c is controlled by the size of the M2 octahedron. From P600 to P1400, the m ean M1-O length shortens by 2.35%; in contrast, the mean M2-O distance lengthens by 1.16%. as a consequence, the M1 and M2 sites have a near ly identical octahedral volume in P1400. With increasing cation disord er, the degree of distortion of the M1 octahedron is reduced significa ntly, while that of M2 remains essentially unchanged, Cation disorder is also accompanied by straightening of the bands consisting of the M1 and M2 octahedra. There is a close correlation between atomic isotrop ic displacement factors (B-iso) and cation order-disorder: B-iso facto rs of all atoms in more disordered samples are larger than those in le ss disordered ones, (C) 1998 Academic Press.