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Übcr einige Arten der Gattung Guignardia Viala et Ravaz
Author(s) -
Reusser F. A.
Publication year - 1964
Publication title -
journal of phytopathology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.53
H-Index - 60
eISSN - 1439-0434
pISSN - 0931-1785
DOI - 10.1111/j.1439-0434.1964.tb03430.x
Subject(s) - biology , library science , citation , humanities , art , computer science
It has been attempted to grow Single crystals of Nb2Ü5 and Ta20s using Verneuil and hydrothermal methods as well as chemical transport reactions. Crystals belonging to the following phases have been obtained: aand y-Ta20s, a-, ß-, y-, £-, i;-Nb205, and also some suboxides of Nb20s. ? and t; are two new phases of Nb2Ü5. Monocrystals of ßand y-Nb20s are described here for the first time. In a series of preliminary experiments with a plasma torch, the Constitution of Nb20sand Ta2Cvpowders having passed through the plasma was investigated. Obser¬ vation on these materials seem to indicate that the transformation from the low temperature to the high temperature modiflcation of Nb2Üs may be reversible under high pressure. Pycnometric densities of crystals made by the Verneuil method gave values of 8.58 g/cm3 for a-Ta2Ü5 and 4.48 g/cm3 for a-NbaOs. X-ray analysis showed that the low temperature modiflcation of Ta2Ü5 (here called y) may occur in several "states" (called yi, y%, yi, yi) which differ in the position of weak powder lines. One of these (called yi) is probably isostructural with y-Nb205. X-ray investigations at high temperatures showed that at high temperatures an equilibrium dependent in O2-pressure between yand a-Ta20s is attained, whereby y is stabilized by increasing 02-pressure. From this it is assumed that y is the only modiflcation of stoichiometrio Ta20s, which is stable up to the melting point, while a is formed as a suboxide. When heated for long periods in air at temperatures higher than 1400°C, a-TagOs (monoclinic at room temperatures) is converted into a phase a*, which at room temperatures is slightly less ordered, but still monoclinic. Colourless, probably stoichiometrio NbaOs may show a powder pattern corresponding to a Nb2Ü5 suboxide phase, if it is produced from a suboxide by heating at relatively low temperatures. There are indications that S-NT^Os (described by Tbbao as probable suboxide) could have a broad ränge of homogeneity. ^-Nb2Ü5 normally occurs together with a-Nb20s; materials free of a appear to be stabilized by impurities or vacancies; single crystal photographs indicate a two dimensional disorder of the lattice.