DIELECTRIC STUDIES OF LANTHANUM HEPTAMOLYBDATE CRYSTALS GROWN FROM GELS

Results of dielectric measurements conducted on lanthanum heptamolybda te grown by a gel technique employing the system La(NO3)(3)-MoO3-NH4OH -HNO3-Na2SiO3 are presented. The variations of the dielectric constant epsilon', dielectric loss tans delta and conductivity sigma with the temperature and frequency are critically analyzed. The dielectric cons tant increases with temperature at lower frequencies (10(3) Hz), reach ing a maximum value of 320 at 125 degrees C, after which it abruptly d ecreases. The dielectric loss tan delta and conductivity sigma show bo th a temperature as well as a frequency dependence. The sharp increase in epsilon' with temperature is attributed to total polarization resu lting from a space charge effect. The functional relationship between the dielectric constant, conductivity and temperature is determined by fitting the observed data with the respective polymeric expressions o f the type epsilon' = a(0) + a(1)T(2) -ln sigma a + b(T-T-0)(2) The no n-linearities and anomalous behaviour of the dielectric constant near the transition temperature, supported by hysteresis studies, indicate the possibility of these materials being ferroelectric.