Dystectic

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Phase diagram of a melt of components A and B. Two eutectics E (blue) and one dystectic D (red) occur due to the formation of the mixed crystal A n B m .

A local maximum of the liquidus line in a melting diagram is called a dystectic or a dystectic point (Greek δύς dys- bad, τήκω teko melt) .

Origin of name

The term dystectic is derived from the poor suitability of intermetallic compounds as materials , since these produce coarse crystalline structures with low strength.

conditions

A prerequisite for the observation of a dystectic is that two (more rarely several) immiscible components A and B form a mixed crystal with a constant stoichiometric composition A n B m . This is in equilibrium with the melt at the dystectic point . It follows from this that the mixed crystal must be a congruent melting compound: It therefore has a defined melting temperature and must therefore not decompose below the melting point , which would result in the formation of a peritectic . The mixed crystal cannot form mixed phases with components A and B, so that two eutectics are formed . This is due to the different crystal structure of the starting components and the mixed crystal (example: Mg : hexagonal , Ge : diamond structure , Mg 2 Ge : fluorite structure ).

A special feature of a dystectic compared to a eu or peritectic is that when the compound A n B m decomposes in the melt, the liquidus line runs steadily at the dystectic point . In the case of disintegrating mixed crystals, however, a discontinuous liquidus line is observed.

Occurrence

Dystectics can be observed for a wide variety of two- or multi-component systems. A dystectic point often occurs when metals are melted when they form intermetallic compounds when they cool (example: formation of CaAl 2 from a calcium - aluminum melt).

Compounds such as GaAs or Mg 2 Ge show, however, that dystectics are not limited to intermetallic compounds. Often a dystectic point can even be observed with mixtures of non-metal compounds, provided that associations are formed . Examples of this are the mixtures of resorcinol / urea or phenol / aniline .

application

Dystectics can be used to prove the existence of certain compounds. The hydrates of sulfuric acid could be detected using the dystectics in the boiling diagram.

Individual evidence

  1. ^ Gert Blumenthal, Dietmar Linke, Siegfried Vieth: Chemistry: Basic knowledge for engineers . Springer Verlag , Wiesbaden 2007, ISBN 978-3-8351-9047-4 , chap. 9 , p. 134 ( limited preview in Google Book search).
  2. Melting diagram. In: Spektrum.de - Lexicon of Chemistry. Retrieved January 5, 2018 .
  3. a b Andreas Heintz: Thermodynamics of Mixtures: mixed phases, interfaces, reactions, electrochemistry, external force fields . Springer Verlag, 2017, ISBN 978-3-662-49924-5 , chap. 1 , p. 64–65 ( limited preview in Google Book Search).
  4. Erwin Riedel, Christoph Janiak: Inorganic Chemistry . 8th edition. de Gruyter , Berlin / New York 2011, ISBN 978-3-11-022566-2 , chap. 2 , p. 195 .
  5. ^ A b Gerd Wedler, Hans-Joachim Freud: Textbook of Physical Chemistry . 6th edition. Wiley-VCH , 2012, ISBN 978-3-527-32909-0 , chap. 2 , p. 398 ( limited preview in Google Book search).
  6. Guenter Gauglitz, Manuela Reichert: phase equilibria - dystectic. In: chemgapedia.de. Retrieved January 4, 2018 .
  7. ^ Peter Atkins, Julio de Paula, James Keeler: Physical Chemistry . 11th edition. Oxford University Press , 2018, ISBN 978-0-19-876986-6 , chap. 5 ( limited preview in Google Book search).
  8. Erich Meister: Basic practical course in physical chemistry: theory and experiments . 2nd Edition. vdf Hochschulverlag , 2012, ISBN 978-3-7281-3709-8 , chap. 5 , p. 85–87 , doi : 10.3218 / 3709-8 ( limited preview in Google book search).
  9. ^ AF Holleman (original), Egon and Nils Wiberg: Inorganic Chemistry . 103rd edition. tape 2 . de Gruyter, Berlin / Boston 2016, ISBN 978-3-11-049573-7 , chap. 11 , p. 1653 .