A large dielectric relaxation was observed in the crystallized glass containing a large amount of V
2O
5. The main crystalline precipitate in the crystallized glass 70V
2O
5⋅30P
2O
5 was V
2O
5 crystal. To clarify the origin of the large dielectric relaxation, the dielectric properties of the crystallized glass were studied and compared with those of polycrystalline V
2O
5 obtained by two different methods. The ε′ of crystallized glass 70V
2O
5⋅30P
2O
5 is two or three times larger than that of the glass for the same composition. The sintered and oriented V
2O
5 polycrystalline materials show a large dielectric relaxation; the value of the static dielectric constant in the specimen sintered at 620°C for 5h is 1000. The ε′ of polycrystalline V
2O
5 changed with the sintering temperature and with the direction of the crystal axis in the oriented specimen. The results obtained were analyzed on the basis of the equivalent circuit model for a double-layer dielectric. According to this model, the ε′ and σ of the crystal (the direction of
b-axis) of the specimen oriented perpendicular to the electric field were 63 and 7.06×10
-4(mho/cm), respectively. The σ of this sample is nearly equal to the value of the
b-axis in the V
2O
5 single crystal. The σ of the grain boundary is 6.60×10
-7(mho/cm) and is equal to that of amorphous V
2O
5. In the sintered specimen, the ε′ values of the crystal are about 10-13; the ε′ value of the grain boundary layer increased with increasing sintering temperature. The change of ε′ in the sintered specimen is thus related to the thickness of the grain boundary. The ε′ of the precipitated V
2O
5 crystal in the crystallized glass is nearly equal to that of the crystals for the sintered V
2O
5 polycrystalline materials. The thickness corresponding to the grain boundary in the crystallized glass is larger than that of the sintered polycrystalline material, The ε′ in the sintered specimen and crystallized glass is influenced not only by the V
2O
5 crystal properties but also by the properties of the grain boundary between V
2O
5 crystals. The analysis by the double-layer dielectric model led to the conclusion that the large dielectric relaxation of the crystallized glass is not due to the contribution of the direction of the preferential crystal axis in V
2O
5 crystal, but should be attributed to the effect of the inter-surface polarization between the highly conducting crystal (V
2O
5) and the poorly conducting matrix.
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