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It is becoming clear that determination of the abundance of Si using lines ofSi II and Si III can lead to quite discordant results in mid to late B-typestars. The difference between the Si abundances derived from the two ion statescan exceed one dex in some cases.  We have carried out a study intended to clarify which kinds of B starsexhibit this discrepancy, to try to identify regularities in the phenomenon,and to explore possible explanations such as abundance stratification bycomparing models to observed spectra.  We used spectra from the ESPaDOnS spectropolarimeter and FEROS spectrograph,supplemented with spectra from the ESO and ELODIE archives, of magnetic Bp,HgMn, and normal B-type stars ranging in effective temperature from about 10500to 15000 K. Using these spectra, we derived abundances using the spectrumsynthesis program ZEEMAN which can take into account the influence of magneticfields. For each star, accurate abundances of Si II, Si III, Ti, Cr, and Fewere derived.  All magnetic Bp stars in our sample show a discordance between the derivedabundances of the first and second ions of silicon, with the latter beingbetween 0.6 - 1.7 dex higher. The same behaviour is observed in thenon-magnetic stars but to a much smaller extent: Si III is enhanced by between0.3 - 0.8 dex compared to Si II.We do not detect the discrepancy in threestars, HD 22136 (normal), HD 57608 (HgMn) and HD 27295 (HgMn); these are theonly stars in our sample for which the microturbulence parameter issignificantly different from zero, and which therefore probably have convectionoccurring in their atmospheres.  We find that vertical stratification of silicon in the atmospheres of B-typestars may provide an explanation of this phenomenon, but our detailedstratification models do not completely explain the discrepancies, which may,in part, be due to non-LTE effects.

Abundances determined using Si II and Si III in B-type stars: evidence for stratification