DIELECTRIC CHARACTERISTICS OF PURE & MIXED RARE EARTH (La,Nd) HEPTAMOLYBDATE CRYSTALS GROWN BY GEL ENCAPSULATION TECHNIQUE. Sushma Bhat,S. K. Khosa, P. N. Kotru, R. P. Tandon^*, Department of Physics, University of Jammu, Jammu-180004 (India)^*, National Physical Laboratory, New Delhi, India

Single crystal growth of single rare earth heptamolybdates (where R=La,Nd) and mixed (La-Nd) is achieved using the system R (NO₃)₃ - MoO₃-NH₄ OH - HNO₃-Na₂SiO₃ by gel encapsulation technique. The crystals are characterized using X- ray and electron diffraction, IR spectroscopy, SEM and optical microscopy. Dependence of dielectric constant, dielectric loss and conductivity of the samples subjected to different frequencies of the applied a-c fields at different temperatures is studied. The dielectric constant of the materials increase sharply, attains a peak value then decreases rapidly, as the temperature is raised. Dielectric loss and conductivity are strongly temperature and frequency dependent. The sharp rise of dielectric constant with temperature at 10³ Hz for T<T_o (T_o being the transition temperature) is attributed to the contribution to a major contribution from space charge polarization. The functional relationship between the dielectric constant ([[epsilon]]) and conductivity ([[sigma]]) and temperature T is theoretically linked to analytical expressions of the type [[epsilon]] = a_o+ a₁T² and -ln [[sigma]] = a+b(T-T_o)² for T<T_o. The indications of these materials being ferroelectric are suggested by the non-linearities and anamolous behaviour of dielectric constant near the transition temperature and further supplemented by the results of preliminary hystereses experiments.