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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Chemical transport experiments are a valuable aid of great potential in the synthesis and thermochemical characterization of solids. Compounds of interest can often only be crystallized by this method. The use of a transport balance allows very detailed observations, particularly of the course of the experiment over time (simultaneous or sequential deposition of multiple-phase solids). The computer program CVTRANS enables a quantitative thermochemical description of the experiment based on the cooperative transport model. A compilation of recent results illustrates the development and efficiency of the method. The transport of phosphides, anhydrous phosphates and sulfates, and rare-earth oxide and oxohalide compounds are treated in detail.

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Durch formale Substitution von je zwei Nb5+ durch Nb4+ (bzw. W4+) und W6+ sind – ausgehend von Nb8W9O47 – gemischtvalente Niob‐Wolframoxide unterschiedlicher Nb/W‐Verhältnisse denkbar, von denen Zusammensetzungen Nb8‐nW9+nO47 mit n = 1 bis 5 durch isothermes Erhitzen (1 250°C) der entsprechenden Gemenge von NbO2 (bzw. WO2) mit Nb2O5 und WO3 phasenrein dargestellt werden konnten. Diese Produkte kristallisieren alle in der Struktur des Nb8W9O47 und sind somit als Mischphase (0 ≤ n ≤ 5) aufzufassen, wie die röntgenographische (Pulvermethode) und durchstrahlungs‐elektronenmikroskopische Untersuchung belegt. Präparate mit niedrigerem Nb‐Gehalt (n > 5) erhielten wir dagegen nicht mehr phasenrein.