Dedicated with sincere good wishes to Professor Wilhelni Klemm on his 70th birthdayNiobium pentoxide crystallizes in numerous modifications. The present paper describes their mode of formation, structures, and thermal properties.It should be particularly noted that the frequently coincident terms "modification" and "thermodynamic phase" a r e independent of one another. The definition does not exclude the possibility thatas discussedseveral modifications may occur in one phase, or sevsral phases may belong to the same modification. Obviously, however, this will not usually occur.The form Nb205-I1 observed by Slznfer and Royr31 and identified with Brnuer's M-Nb205 [21 is mentioned with the M-form. However, Nb205-I1 is, according to the interplanar [ 3 ] M .

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referred to but will not be discussed. The involvement of a hitherto unknown gaseous species may be indicated by the experimental data that do not agree with model calculations at all.We will report new possibilities of measuring the mass flux inside a closed ampule. The correlation of thermochemical calculations with experimental data is particularly proven in the systems containing Cr, Mo, W, or their compounds.

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