Synthesis, Structure, and Magnetic Properties of NaTiO₂

Details of the synthesis and structural and physical characterization of oxides with the nominal composition NaTiO₂ are presented. The highest sodium loading we have achieved is Na_0.99(2)TiO₂ determined using diffraction methods. The results are not consistent with a localized moment d¹ 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)TiO₂ has the α-NaFeO₂ structure, space group R3̄m (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.