Oxygen-deficient SrTiO3−x, X = 0.28, 0.17, and 0.08. Crystal growth, crystal structure, magnetic, and transport properties

The grossly nonstoichiometric perovskites SrTiO_3−x with x = 0.28, 0.17, and 0.08 were prepared from a reaction of Sr₂TiO₄, TiO, and TiO₂ at 1500°C. For x = 0.28 relatively large single crystals were obtained. Also for this sample the crystal symmetry was found to depend on the rate of cooling from the reaction temperature and the annealing conditions. Rapidly cooled samples are tetragonal a = 3.9177(3) Å, c = 3.8878(5) Å. Samples annealed in vacuum at temperatures of 1000 to 600°C are cubic a = 3.9075(3) Å with no change in cell volume. Single crystal data from a tetragonal sample indicate slight preferential occupancy of one oxygen position in P4/mmm. No evidence for any supercell due to defect ordering could be seen by TEM in either cubic or tetragonal samples. The x = 0.28 crystals show metallic resistivity, (300 K) = 6 × 10⁻⁴ ohm-cm and temperature-independent paramagnetism, χ_m = 118 × 10⁻⁶ cm³ mole⁻¹. Hall effect data from 300 to 4.2 K analyzed on a single carrier model give a temperature-independent n-type carrier density of 2.4 × 10²¹ cm⁻³. This is a factor of 3.9 less than that expected if the creation of each oxygen vacancy results in the production of two carriers in a single band. Hall data for x = 0.17 and 0.08 samples give similar results corresponding to densities of 2.1 and 1.4 × 10²¹ cm⁻³, respectively, in the same temperature range. These densities are 2.7 and 1.9 times less than the expected single-band value, respectively. Such results point to a two-band model with a large effective mass in one of the bands.