Abstract:
A thorough analysis of the surface configurations of NH4H2PO4
(ADP) at the atomic/molecular level has revealed a much larger number of
PBCs and F-slices than obtained previously by Aquilo and Woensdregt
(AW) in 1984 [J. Crystal Growth 69 ( 1984) 527] and 1987 [J.
Crystal Growth 83 (1987) 549] by means of visual inspection of structure
projections. The computational results of these relatively early
applications of the Hartman-Perdok (WP) theory may be called into question
insofar as they arise from an incomplete set of theoretical data.
Specifically, the ADP forms {031} and {211}, {21-1} classified earlier
by AW as S and K forms, respectively, are here
demonstrated to be genuine F forms. The present theoretical results
improve agreement with experiment: (i) The theoretical F character of
{031}, a form thought by AW to be important in connection with tapering,
is reconciled with the experimental observation by AW, according
to which (031) grows as flat faces. (ii) The interpretation of an experimental
observation by Dam et al, in 1986 [J. Crystal Growth 74 (1986)
118] of a series of small flat faces on zone [1-11] was dismissed by
AW in 1987 as theoretically impossible, on the grounds
that only one F face, the (022), was known to occur on that zone. The
fact established here that (121) = {21-1} is also an F
face located on the same zone invalidates that objection. The theoretical
F character of (121) agrees with the recent experimental observation
of Verheijen et al., according to which (121) is located on the [1-11]
Bat face band. The aim of the present work is to (i)
remove some degree of confusion currently in the literature as regards
interpretation of experimental results, and (ii) provide a tool enabling
a
more accurate and adequate investigation of the growth mechanism and
morphology of ADP type structures than has thus far been available.