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The electrical conductivities of the sesquioxides, dioxides and a few non-stoichiometric oxides of Pr and Tb have been measured at various temperatures and the activation energies for the conduction process have been reported. The conductivity passes through a maximum in the composition range LnO1.70-LnO1.83 (Ln = Pr or Tb). The conductivity data are consistent with the hopping model for these semiconductors. Calculation of the Frohlich's coupling constant and transition probability in Pr6011 has shown that the intermediate polaron theory is applicable to these systems. The electrical conductivities of CeOzfY203 and CeOzfNd203 systems show a maximum in conductivity at a composition of about 30 mole % of Ln0l.s (Ln = Nd or Y). The reflectance spectra of Pr and Tb oxides show a minimum around a composition of LnO1.75.Recent studies have established the phase equilibria in the praseodymium + oxygen 1 . 2 and the terbium+ oxygen 39 4 systems and the crystallography of the non-stoichiometric phases between Ln2O3 and LnO2 have been described.5 There have been a few reports of the electrical properties of the rare earth oxides.6-8 Honig and coworkers 9 have reported conductivity and thermoelectric power (T.E.P.) data of a few non-stoichiometric praseodymium oxides and shown that the charge carriers responsible for the conductivity are the 4felectrons of the lanthanide ions. The electrical conductivities of the non-stoichiometric oxides of praseodymium and terbium as well as their sesquioxides and dioxides have presently been examined with a view to study the variation of the conductivity with the oxygen-to-metal ratio and establish the mechanism of the conduction process in these oxides.Since the non-stoichioinetric rare-earth oxides are bivalent oxides with the lanthanide ions in the + 3 and the + 4 valence states, it was considered interesting to study the conductivities of Ce02 + Y2Q3 and CeOz + Nd203 systems of varying composition and compare the results with those of the non-stoichiometric oxides. The systems were chosen since the radii of Ce4f and Y3+ (or Nd3+) ions correspond closely to those of Pr4f and Pr3f ions.The reflectance spectra of the praseodymium and terbium oxides have been studied since the reflectance maximum should depend on the proportion of Ln3f and Ln4f ions in the oxides as does the electrical conductivity. It was expected that the reflectance spectra would also show variation with the oxygen-to-metal ratio. EXPERIMENTAL Lindsay 9999 % Pr6011 and Tb4O7, ignited to coiistant weight, were used. C-Pr203 and C-Tb203 were prepared by the reduction of Pr6011 and Tb407 in a stream of dry hydrogen for 6 h at 6QO and 900°C respectively.3~ 10 PrO1.71 was prepared by heating Pr6O11 in vacuum (-1O-5rnmHg) at 490°C for 4h.11 Pro2 and TbO2 were prepared by the methods described earlier. 11

CHEBROLU RAMA MOHAN RAO and PRATAP NARAIN MEHROTRA. Can. J. Chem. 56.32 (1978).The transformation of aragonite to calcite (A-C) is an irreversible reconstructive transformation involving a change from 6 to 9 in the primary coordination. Cationic impurities like N a + , Mg2+, Zn2+, Y3 +, La3+, Nd3+, Gd3+, Dy3+ have been doped in pure aragoniteand their effect on the kinetics and energetics of its transformation to calcite is studied. By employing a DTA technique, the enthalpy and energy of activation for the transformation have been calculated. The ir spectra of aragonite and calcite in pure form and with various impurities have been studied and the effect of impurities on absorption bands discussed.

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