CONTRIBUTIONS TO THE INVESTIGATION OF INO RGANIC NONSTOICHIOMETRIC COMPOUNDS .44. NEW ELECTRON-MICROSCOPIC INVESTIGATION OF THE METASTABLE NB2O5 MODIFICATION OX1C

Oxidation of the blue-black NbOx phases (2.4 < x < 2.5) in air or a di luted oxygen atmosphere leads all together to 21 metastable colourless (partly yellowish) Nb2O5 modifications. These can be described as com pounds having block structures like the NbOx phases themselves. At 200 degrees C the oxidation process to the metastable phases proceeds qui te slowly; at higher temperatures all polymorphs are finally transform ed to H-Nb2O5. Electron diffraction patterns of the polymorph, which c an be obtained by heating crystals of Nb22O54 in atmospheric environme nt at about 200 degrees C (Ox1C), show extra spots in the c direction compared to Nb22O54: This oxidation product is quite sensitive to the focused electron beam, so the reduction of the crystals back to Nb22O 54 can only be avoided by reducing its intensity. Finally HRTEM images of thin crystal areas showing a superstructure were obtained. Compari ng those characteristic image contrasts to calculated images of differ ent point defect models one finds, that mainly one point defect is res ponsible for the superstructure. By migration of one Niobium atom two interstitial oxygen sites are created. So a structure model can be ach ieved leading to the formula Nb44O110.