23 Na NMR study of the local order in the Na 1/2 Bi 1/2 TiO 3 structure in a weak magnetic field

The orientation dependences of the second-order quadrupole shifts of the central component in the 23 Na NMR spectrum were studied in the temperature range 293–760 K. The profile of the spectral distribution is calculated using various models of the Na 1/2 Bi 1/2 TiO 3 structure. The calculations agree with the experimental data for the monoclinic structure of a polar cluster with two Na displacement components: a displacement along the [111] p direction and a small displacement statistically or dynamically disordered over six equally probable [100] p -type directions. Tetragonal-phase nuclei and monoclinic clusters with a very small displacement component along the [111] p direction are found to coexist and have close energies over the temperature range 580–610 K. The results obtained provide new information concerning the character of the diffuse phase transition at 610 K.