Barium perrhenate

Experimentally, the compound consisting of the preformed ion and its counter ion (such as barium perrhenate for perrhenate emission) does not produce ions when heated—instead, only neutral species sublime. It must be embedded in a suitable matrix and then heated. The limited experimental evidence collected to date indicates that the ion to be emitted must have significantly greater mobility and/or vapor pressure in this matrix than its counter ion, allowing diffusion of the ion of interest. When the temperature gets sufficiently hot the ion migrates to the... 

Schriewer MS, Jeitschko W (1993) Preparation and crystal structure of the isotypic orthorhombic strontium perrhenate halides Sr5(Re05)3X (X = chloride, bromide, iodide) and structure refinement of the related hexagonal apatite-like compound barium perrhenate chloride (Ba5(Re05)3Cl). J Solid State Chem 107 1-17... 

This equation has the advantage over the previous one that the barium containing molecule on the right of the equation retains a partial ionic character that permits it to serve as a transfer station for additional migration of perrhenate anions. 

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