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Results obtained from the dielectric studies of neodymium heptamolybdate crystals grown in the system Nd(N03)3 -MOO, -NH40H -HNO, -Na,SiO, by gel encapsulation technique are presented. The variation of dielectric constant (E'), dielectric loss (tan 6) and conductivity (a) with frequency at different temperatures is studied. The dielectric constant of the material increases sharply, attains a peak value and then decreases rapidly, as material's temperature rises from room temperature to higher degrees. The temperature at which the peak value is attained is the transition temperature of the material. Dielectric loss (tan b) follows almost a similar behaviour. The conductivity (u) is also found to be temperature-and frequency-dependent. This sharp rise in E' is attributed to the contribution from space charge polarization. Theoretically obtained data using the expression E' = a, + a , T z and -In a = a + b(T -T. )' for T < To) fits very well with the experimentally obtained data indicating that space charge polarization increases as some power of temperature larger than one.

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Thermal behaviour of gel-grown pure and mixed rare earth tartrates of yttrium and samarium is investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal behaviour suggests that the materials are unstable at lower energies and pass through various stages of decomposition, decomposing to respective rare earth oxides which remain stable on further heating. It is estimated that both pure yttrium and pure samarium tartrate crystals carry eight waters of hydration, while mixed yttrium samarium tartrate crystals carry six waters of hydration. Critical examination of TG and DSC curves shows that the initial decompositions are endothermic and the latter are exothermic. Thermal kinetics of these materials has been worked out using Horowitz-Metzger, Piloyan-Novikova and Coats-Redfern equations. Application of these equations to these materials yields values of activation energy, order of reaction and frequency factor which are in reasonably good agreement. Keywnrds. Yttrium samarium tartrate; thermal behaviour; solid state reaction kinetics.