Why are vaporous substances difficult to handle during zone melting?

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zone melting Figure 1: Schematic representation of (A) normal freezing, (B) zone refining

In the context of zone melting, vaporous substances are difficult to handle primarily due to the large change in volume that occurs upon vaporization. This significant volumetric expansion necessitates that the charge must move along the containing tube during the process, which complicates the practical application of the technique.1

While zone melting is a highly effective method for purifying materials by manipulating impurities through a moving liquid zone, the handling of substances that transition between vapor and solid phases remains a distinct technical challenge compared to standard liquid-solid transformations.1