Dielectric studies of lanthanum heptamolybdate crystals grown from gels

Results of dielectric measurements conducted on lanthanum heptamolybdate grown by a gel technique employing the system La(NO3)3MoO3NH4OHHNO3Na2SiO3 are presented. The variations of the dielectric constant ε′, dielectric loss tan δ and conductivity σ with the temperature and frequency are critically analyzed. The dielectric constant increases with temperature at lower frequencies (103 Hz), reaching a maximum value of 320 at 125 °C, after which it abruptly decreases. The dielectric loss tan δ and conductivity σ show both a temperature as well as a frequency dependence. The sharp increase in ε′ with temperature is attributed to total polarization resulting 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 of the type ϵ′=a0+a1T2-lnσ=a+b(T-T0)2 The non-linearities and anomalous behaviour of the dielectric constant near the transition temperature, supported by hysteresis studies, indicate the possibility of these materials ferroelectric.