Details of the synthesis and structural and physical characterization of oxides with the nominal composition NaTiO 2 are presented. The highest sodium loading we have achieved is Na 0.99(2) TiO 2 determined using diffraction methods. The results are not consistent with a localized moment d 1 system. High-resolution neutron diffraction shows that the transition observed previously between 200 and 250 K in the magnetic susceptibility has a structural, rather than purely magnetic, origin. At room temperature Na 0.99(2) TiO 2 has the R-NaFeO 2 structure, space group R3 hm (No. 166), with the hexagonal lattice parameters a ) 3.04537(1) Å, c ) 16.26210(4) Å, and Z ) 3. On cooling, a continuous, broad, nonhysteretic transition to a monoclinic phase occurs at about 250 K. Between 220 and 250 K, the two phases coexist, the precise temperature range of coexistence being correlated with the width of the Bragg reflections in different samples. At 5 K the monoclinic structure adopts the space group C2/m (No. 12) with a ) 5.24555(6) Å, b ) 3.02983(3) Å, c ) 5.73078(6) Å, β ) 108.536(2)°, and Z ) 2. The structural phase transition is apparently driven by an increase in Ti-Ti bonding on cooling. This leads to small differences in band structure above and below the transition which accounts for the room-temperature magnetic susceptibility of 4.7 × 10 -4 emu mol -1 almost halving between 250 and 200 K. The lowering of symmetry can be interpreted as a cooperative second-order Jahn-Teller distortion which is precipitated by the changes in Ti-Ti bonding.

The results of structural refinements on high-resolution powder neutron diffraction data for copper(II)-and copper(I)-exchanged zeolite Y and for the parent sodium zeolite Y, with composition Na 62 Si 128 Al 64 O 384 , are presented. Rietveld refinement of ambient-temperature data shows that the Cu(I) and Cu(II) cations are located on different sites in the two materials and that the structures differ considerably from that adopted by NaY. In Cu II Y, with composition Cu 33 Na 12 Si 130 Al 62 O 384 , the copper cations are located on the I A ′, I B ′, and II′ sites with occupancies of 0.552(4), 0.145(7), and 0.162(8), respectively, while some sodium, 0.36-(2), remains on site II. For Cu I Y, there is a redistribution of the cations on sites I A ′, I B ′, II′, and II to 0.39(2), 0.21(1), 0.24(1), and 0.18(2), respectively. In both zeolites, site I is vacant. The high-quality diffraction data provide direct evidence of structural changes associated with the redox behavior of the zeolite, including distortion of the framework and the location of charge-balancing species.