  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.
