Na(Nb1−xVx)O3 System and ``Ferrielectricity''

Following a report of ferroelectricity at room temperature in Na(Nb1−xVx)O3, crystals of pure NaNbO3 and Na(Nb1−xVx)O3 have been investigated electrically, optically, and by x-ray diffraction. Attempts to introduce amounts of vanadium larger than x≈0.005 into the crystals were unsuccessful. The room temperature phase of pure NaNbO3 has been reported to be antiferroelectric, that of NaVO3 nonferroelectric but perhaps also antiferroelectric. The vanadium-doped crystals appear to be antiferroelectric in the room temperature as grown state. A transition to a ferroelectric phase can be induced in both the vanadium-doped and the pure crystals by a sufficiently high field (of the order of 50 kv/cm), called the transition field, applied normal to the orthorhombic b axis. In one type of crystal plate, when the field-induced transition into the ferroelectric phase occurs, a crystallographic reorientation is also observed. Some crystals revert to the nonferroelectric phase when the transition field is removed. All of the crystals investigated while in the ferroelectric phase were found to have the length of the b axis twice the length of the simple perovskite cube, whereas in the antiferroelectric phase, the length of the b axis is four times that of the simple perovskite cube. In so far as the phenomena studied in the present investigation are concerned, it can be stated that the vanadium-doped crystals behave in most respects like pure NaNbO3 crystals. In particular, no evidence was found for an unbalanced antidipole configuration, i.e., ``ferrielectric'' structure, proposed by another investigator in an earlier paper. Further differences between the content of that paper and the present results are pointed out.