Vaporization congruent

Liquid europium dichloride is known to vaporize congruently according to (1)... 

B203(/) vaporizes congruently to 8203(g), the boiling point reported for this case should equal the calculated volatilization temperature. 

Considering that dissociation occurs upon volatilization, the temperatures can be correlated extremely well on a In P vs (1/rd.voi) plot, where P is the total system pressure and T a.voi decomposition temperature, as the case dictates. Such a plot is shown in Fig. 11. Since the Clausius-Clapeyron relation for vapor pressure of pure substances shows an exponential dependence on temperature, TVoi was considered a pseudo-boiling point at the respective system pressure. For a substance that vaporizes congruently to its gaseous state, the slope of lines on a In P vs (l/Tvoi) plot represents the enthalpy of vaporization. Indeed, the enthalpy of vaporization calculated from the slope on a In P vs (l/TVoi) plot for the B-O2 system (360 kJ/mol) agrees exactly with the value calculated by using... 

The phrase boiling point must be used carefully for other reasons. Care must be exercised such that the boiling process is indeed defined. Two examples which illustrate this point are MgO and LiF. Magnesium oxide does not vaporize congruently (so that the total vapor pressure depends on the ambient oxygen potential), whereas LiF vaporizes congruently but monomer, dimer, and trimer species exist in the vapor. 

Mass spectrometric observations indicate that stoichiometric U02(s) vaporizes congruently, and that UOjCg) is the predominantly vaporizing... 

Niobium carbide vaporizes congruently into a vacuum at NbCo.748 and 2910° (Fries, 1962 Storms, 1962). At higher temperatures the composition drops, reaching NbCo.7js at 3130° (Storms, 1962). 

In general, the same precautions regarding crucible materials must be observed for the dihalides as for the trihalides, cf. ch. 32, section 2.1. Although noble metal containers were employed in much of the early work, platinum is particularly unsuited because of the formation of stable R-Pt alloys (Brewer et al., 1950 Bedford and Catalano, 1970). Several of the dihalides melt and vaporize congruently, cf. ch. 32, section 4.4.3, and can be purified by sublimation or distillation and single crystals can be grown from the melts. The majority of the reduced halides are thermally unstable and preparation of high purity samples is difficult. 

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