Structure-dependent microwave dielectric properties of   (1-x)La(Mg1/2Ti1/2)O3 - xLa2/3TiO3 ceramics
  Structure and dielectric characterization were performed on the (1-x)La(Mg1/2Ti1/2)O3 - xLa2/3TiO3 perovskite ceramics with the actual composition 0<=x<=0.52. The sequence of structure transformations was detected as x is increased: P21/n (0<=x<0.3) - Pnma (0.3<=x<0.37) - Imma (0.37<=x<0.4) - I2/a (0.4<=x<=0.49) - R-3c (0.49<x<=0.52). The structure evolution from the tilt configuration a+b-b- to a-a-a- are considered in terms of competing the repulsive energy and the Madelung energy. Relative permittivity of the ceramics was measured as a function of temperature and LT content. Temperature coefficient of the resonant frequency was found to pass zero-value at the narrow compositional range between x=0.49 and 0.52, where the discontinuous I2/a - R-3c crossover occurs. Temperature and compositional variation of these fundamental microwave dielectric parameters are discussed in respect to type of the phase transitions (continuous/discontinuous) between the structures. 
A.N. Salak, D.D. Khalyavin, P.Q. Mantas, A.M.R. Senos and V.M.   Ferreira