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Fluorides: Solid-State Chemistry

  1. Werner Massa

Published Online: 15 DEC 2011

DOI: 10.1002/9781119951438.eibc0070

Encyclopedia of Inorganic and Bioinorganic Chemistry

Encyclopedia of Inorganic and Bioinorganic Chemistry

How to Cite

Massa, W. 2011. Fluorides: Solid-State Chemistry. Encyclopedia of Inorganic and Bioinorganic Chemistry. .

Author Information

  1. Philipps-Universität Marburg, Marburg, Germany

Publication History

  1. Published Online: 15 DEC 2011

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Abstract

Inorganic solid fluorides and fluorometalates play a unique role in inorganic chemistry. The preparation, mostly using aqueous or anhydrous HF or even elemental F2, requires special conditions: no glass or silicate containing vessels can be used. Their structural chemistry is governed by the outsider role of the fluoride anion. It is the smallest and least polarizable anion and its mainly ionic compounds follow the rules of hard spheres packing better than any other class of substances. Nearly all metal ions except those with the largest ionic radii form fluoride structures with octahedral [Mn+F6](6−n)– units. If the metal:fluorine ratio is less than 6, these octahedra share common vertices or, in rare cases, edges or even faces to account for electroneutrality, the more the lower the oxidation state of the metal. In this way, a variety of 1D chain, 2D net, and 3D framework structures is formed that can be classified into a few large structure families. In binary fluorides, for example, VF5 has an octahedral cis-chain structure, VF4 a layer structure based on a square net of corner-connected octahedra, VF3 has a ReO3-related 3D structure with all corners of octahedra connected, and VF2 has the rutile-type structure with partial edge-sharing of octahedra. In ternary or quaternary compounds, for example, alkali or alkaline earth fluorometalates, the same principles are valid for the anionic substructure while the counter cations fill interstices and stabilize at the same time the structure. The main structure families are grouped according to the basic principles of octahedral nets. These are the ReO3-related trifluorides, the fluoride perovskites AMF3, their relatives elpasolites A2A′MF6, and cryolites A3MF6, as well as some layer structures with square nets. A second large family is based on mixed octahedral nets including triple units: it includes for example, the pyrochlores, the weberites, the hexagonal and tetragonal tungsten bronze structures, and some derivable layer structures. Recent developments deal with ‘tailoring’ of low-dimensional or framework structures using ‘soft-chemistry’ methods. The relatively simple structural chemistry of fluorides invoked their use as model systems, for instance, for the investigation of electronic effects like Jahn–Teller ordering, or for magnetic investigations. Besides the cryolite Na3AlF6 used in Al production, some fluorides are of technical interest owing to their special optical properties (e.g. laser optics, fluoride glasses), their ability for fluoride ion conductivity, or their role as catalysts for fluorinations or halogene exchange in organic systems.

Keywords:

  • fluorides;
  • fluorometalates;
  • crystal structures of fluoro compounds;
  • magnetic exchange interactions;
  • superexchange in fluorides;
  • ionic conductivity in fluorides;
  • glasses based on fluorides;
  • hydrothermal synthesis of fluorides;
  • catalysis by fluorides