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

Results of dielectric measurements carried out on mixed rare earth (didymiu m-a mixture of La, Nd, Pr and Sm) heptamolybdate grown by gel encapsulation technique are reported. The dependence of dielectric constant epsilon', di electric loss (tan delta) and conductivity cy on temperature (30-150 degree sC) and frequency of the applied field (10(2)-10(7)) Hz is analyzed. The va riation of dielectric constant with frequency is discussed with reference t o three frequency ranges viz., low frequency range (10(2)-7 x 10(2)) Hz, mi ddle frequency range (7 x 10(2)-4 x 10(6)) Hz and high frequency range (4 x 10(6)-10(7)) Hz. The dependence in these three regions is reported to be m arkedly different. At low frequency (10(2)-7 x 10(2)) Hz the dielectric con stant sharply increases with temperature, reaches a maximum value of 100.07 at 75 degreesC, after which it falls abruptly. The dielectric loss (tan de lta) and conductivity epsilon' are also strongly dependent on frequency and temperature. The sharp rise of C with temperature is explained to be due t o a major contribution towards total polarization from a space charge effec t. The observed data on dielectric constant and conductivity finds a good a greement with the respective polymeric expressions of the type epsilon' = A (0) + A(1)T(2) and - ln sigma = A(2) + A(3)(T - T-0(2))). The non-lineariti es and anomalous behaviour of the dielectric constant near the transition t emperature suggests the possibility of these materials being ferroelectric. The transition temperature gets diffused at higher frequencies. though a t ransition does take place at the same temperature irrespective of the frequ ency. (C) 2001 Elsevier Science B.V. All rights reserved.