Dielectric characteristics of gel grown mixed rare earth (didymium) heptamolybdate crystals

Results of dielectric measurements carried out on mixed rare earth (didymium—a mixture of La, Nd, Pr and Sm) heptamolybdate grown by gel encapsulation technique are reported. The dependence of dielectric constant ε′, dielectric loss (tan δ) and conductivity σ on temperature (30–150 °C) and frequency of the applied field (102–107) Hz is analyzed. The variation of dielectric constant with frequency is discussed with reference to three frequency ranges viz., low frequency range (102–7×102) Hz, middle frequency range (7×102–4×106) Hz and high frequency range (4×106–107) Hz. The dependence in these three regions is reported to be markedly different. At low frequency (102–7×102) Hz the dielectric constant sharply increases with temperature, reaches a maximum value of 100.07 at 75 °C, after which it falls abruptly. The dielectric loss (tan δ) and conductivity σ are also strongly dependent on frequency and temperature. The sharp rise of ε′ with temperature is explained to be due to a major contribution towards total polarization from a space charge effect. The observed data on dielectric constant and conductivity finds a good agreement with the respective polymeric expressions of the type ε′=Ao+A1T2 and −lnσ=A2+A3(T−To2). The non-linearities and anomalous behaviour of the dielectric constant near the transition temperature suggests the possibility of these materials being ferroelectric. The transition temperature gets diffused at higher frequencies, though a transition does take place at the same temperature irrespective of the frequency.