The high temperature drift mobility (ad) of charge carriers in nonstoichiometric cerium dioxide (CeOz_,X) has been calculated by combining the electrical conductivity and nonstoichiomet~ data on the basis of the oxygen vacancy model with correct ionization state. The electrical conductivity was measured by a four-probe d.c. technique and the nonstoichiometry by thermogravimetric analysis. The dilute solution model of the point defects is valid up to x = 0.03. From the magnitude of pd and its temperature dependence, the charge carriers in CeO,_, are proposed to be small-poiarons formed by localization of electrons at cerium sites and the charge transport process is proposed to occur by a hopping mechanism. The observed temperature dependence of s,+ is in accord with that derived by Holstein and Friedman for small-poiaron transport by the hopping mechanism. The activation energy of mobility is found to increase with increasing x as expected for the hopping model.

Electrical conductivity of the sintered Nb2Os-doped CeO2 specimens was measured between 800 ~ and 1329~ in the P02 range 10-19-1 arm. A polarization technique was used to determine the predominant electrical conduction mode in these specimens. Thermogravimetric analysis was done to measure the weight loss in selected specimens. The solid solubility limit of Nb205 in CeO2 is between 0.60 and 0.80 m/o, and it does not vary significantly with temperature or Po2. The Nb2Os-doped CeO.2 is an n-type semiconductor. At high Po2, Nb ~ + ions are incorporated in CeO2 by both possible charge-com-

Good-quality hexagonal NbSe 2 single crystals were prepared. In 2H-NbSe 2 , superconducting and charge density wave (CDW) transitions were found at T s = 7.4 K and T c = 35 K respectively as reported previously. We have noticed that these two transitions are changed to T c = 42 K and T s = 6.5 K, in 4H-NbSe 2 . Thermopower has shown clear anomaly at CDW transitions. The anisotropic upper critical field was calculated as ∼3 and 6.3 for 2H-and 4H-single crystals around t = 0.81, where t = T /T s , from resistivity and explained in terms of coherence length. From the relation, H c2 (T ) = H c2 (0)[1 − t 2 ], H l c2 (0) was calculated as ∼8.15 T and 16.98 T at t = 0.84 in 2H-NbSe 2 and 4H-NbSe 2 respectively. However, H t c2 (0) = 2.68 T for both single crystals.

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A technique for fabricating low-loss (on the order of 0.1 dB/cm) integrated optical waveguides in amorphous SiO2-based material by nitrogen ion implantation is reported. By comparing the results of nitrogen implantation and oxygen implantation in SiO2, the mechanism of waveguide formation in the nitrogen-implanted waveguides is shown to be chemical doping effect of the nitrogen dissolved in amorphous SiO2.