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Journal ArticleDOI

{100} → {111} morphological change in KCl crystals grown from Pb2+-doped aqueous solutions

12 Oct 2015-CrystEngComm (The Royal Society of Chemistry)-Vol. 17, Iss: 41, pp 7844-7855

AbstractKCl f.c.c. crystals generally exhibit a {100} habit when growing from pure aqueous solutions, a richer {100} + {111} morphology being obtained only under well-defined growth temperature and supersaturation. When increasing amounts (less than 2000 ppm) of Pb are put in a supersaturated solution, the KCl growth morphology undergoes a progressive change: {100} → {100} + {111} → {111}. Detailed growth patterns have been investigated by means of SEM and AFM, while EDS and XRF analyses allowed us to ascertain that Pb is not only adsorbed on the growing KCl surfaces but also selectively absorbed within the {111} growth sectors. Starting from recent and analogous findings, we tried to interpret the morphological change by means of a geometric and structural model of epitaxy between the {100} and the {111} forms of KCl and the most important forms of those compounds that could be adsorbed on them: PbCl2 (cotunnite), PbCl(OH) (laurionite–paralaurionite) and KCl·PbCl2 (challacolloite). Excellent lattice coincidences have been found, thus proving that the {111} KCl octahedron is largely privileged for adsorption/absorption to occur with respect to the {100} KCl cube. Based on this ground, simple kinetic considerations can be proposed to satisfactorily explain the observed morphology change.

Summary (2 min read)

ARTICLE Journal Name

  • Please do not adjust margins common cell =.
  • Concerning the absorption, challacolloite layers undergo some difficulties to be buried in the {111}-KCl growth sectors, except for the {111}-KCl/{010}-challacolloite interface.

Experimental

  • Cubic-octahedral KCl crystals were obtained following two growth routines: the first, to compare the results with those of Liang et al.,10 involves growth experiments performed, starting from KCl (Sigma-Aldrich analytical grade) aqueous solutions saturated at 40°C (solubility 40.05 g/100g water), by repeated crystallization from a starting temperature of 95°C.
  • According to the second routine, crystals were grown from a KCl solution saturated at 25°C and cooled down to 4°C, in the presence of variable amount of PbCl2.
  • The Pb 2+ concentration was adjusted from 0 to 2000 ppm, adding both analytical grade solid PbCl2 or Pb(CH3COO)2·3H2O.
  • Lead acetate was chosen because of its higher solubility with respect to lead chloride and in order to reduce the chlorine concentration in the starting solution, so avoiding the common-ion effect.
  • Chelating substances must be used being aware of their effect as habit modifiers.

Journal Name ARTICLE

  • When summarising, the authors proposed an epitaxial path to interpret both the morphological habit change of KCl in Pb doped aqueous solution and the consequent selective absorption of Pb in the growth sectors of the KCl octahedron.
  • This way seems to be promising and the authors are quantitatively improving their investigation, by X-ray diffraction and by calculating the adhesion energies involved in the best of the lattice coincidences they found.
  • The authors final aim is to verify, in a forthcoming paper, if the adsorbed foreign layers are able to generate anomalous mixed crystals like those the authors recently found in the systems: CaCO3 /(Li2CO3) zabuyelite and NaCl /H-CO-NH2 27.

XRF analysis

  • The elemental composition of some samples was mapped using an EDAX Eagle-III XPL µProbe, the instrument being equipped with a Rh X-ray tube and X-ray Poly-capillary Lens with a spot size of 30µm.
  • The working conditions were 40kV and 1mA, Ti 25 µm thick primary filter, resolution 128x100 pixels, dwell time=4s.
  • A KCl crystal grown from a cooled solution containing 500 ppm of Pb2+ (PbCl2) was used in order to obtain the Pb distribution inside the crystal.
  • The crystal as grown, showed welldeveloped cube faces and small complementary octahedron faces, corresponding to extended growth sectors of the cube and narrow growth sectors of the octahedron.
  • As one can observe, the Pb concentration is higher within the growth sectors of the octahedron and shows an oscillatory behavior mainly during the first stages of growth (close to the center of the crystal).

Chains in the 001 planes: the [100] and [010] PBCs.

  • The PbCl2 growth units do lye perfectly parallel to the 001 plane of the crystal, one half of them at z=(1/4)c0 and the other half at z=(3/4)c0, these two distributions being symmetry related by the inversion centers.
  • Accordingly, the outmost layer of the {001} form should be “spontaneously reconstructed”, obeying to the symmetry criterion the authors successfully started when dealing with {012} surfaces of calcite.

The KCl / challacolloite-KCl⋅2PbCl interfaces

  • Table 5 illustrates the lattice coincidences between {001} and {111} forms of KCl and the morphologically most important forms of challacolloite.
  • Further, it is worth outlining that the shape of the 2D-coincidence cell of the {111}-KCl/{001}-challacolloite interface deviates by only 0.81° from the perfect hexagonality, which fully agrees with the trigonal symmetry of the {111}- KCl surfaces.

Discussion and conclusions

  • According to the partial and preliminary considerations just drawn from Tables 2, 4 and 5, an order of priority could be proposed about the probability of 2D epitaxy between KCl crystals and the compounds that could potentially crystallize from Pb doped solutions: a).
  • The KCl octahedron is largely favoured, with respect to the cube, for epitaxially adsorbing all the examined compounds.
  • The authors kinetic considerations are supported by remembering that: i) the normal growth kinetics R{111} and R{001} of the F faces {111} and {001} is ruled by the advancement rate of the macro-steps shown in Fig. 4 and in the S.I.
  • Moreover, the percent misfit of the common areas is 0.78 and 0.50 for {111}KCl/{010}PbCl2 and {111}KCl/{001}PbCl2, respectively, which means that the relaxation needed to adapt the KCl and PbCl2 structures at the epitaxial interface should be necessarily low.
  • Ii) Laurionite-paralaurionite largely favor the Pb absorption in the {111}KCl growth sectors, especially in the cases: {111}KCl/{001}PbCl(OH) and {111}KCl/{100}PbCl(OH); instead, cotunnite can favor the Pb capture only at the interface {111}KCl/{100}PbCl2, while challacolloite can be hardly absorbed.

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09 August 2022
AperTO - Archivio Istituzionale Open Access dell'Università di Torino
Original Citation:
100→111 Morphological Change on KCl Crystals Grown from Pb2+doped aqueous solutions
Published version:
DOI:10.1039/C5CE01425E
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[CrystEngComm, 17, 2015, 7844 – 7855]
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[http://pubs.rsc.org/en/journals/journalissues/ce#!issueid=ce017041&type=current]

Journal Name
ARTICLE
This journal is © The Royal Society of Chemistry 20xx J. N am e., 2013, 00 , 1-3 | 1
Please do not adjust margins
Please do not adjust margins
a.
Dipartimento di Scienze della Terra, Università degli Studi, via Valperga Caluso 35,
I-10125, Torino, Italy.
† To whom correspondence should be addressed: dino.aquilano@unito.it
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
{
{{
{100}
}}
}→
→{
{{
{111}
}}
} Morphological Change on KCl Crystals Grown from
Pb
2+
doped aqueous solutions
L. Pastero,
a
R. Cossio
a
and D. Aquilano
a
KCl f.c.c. crystals generally exhibit {100} habit when growing from pure aqueous solutions, a richer {100} + {111}
morphology being obtained only under well-defined growth temperature and supersaturation. When increasing amounts
(less than 2000 ppm) of Pb are put in supersaturated solution, the KCl growth morphology undergoes a progressive
change: {100} → {100} + {111} → {111}. Detailed growth patterns have been investigated by means of SEM and AFM, while
EDS and XRF analyses allowed to ascertain that Pb is not only adsorbed on the growing KCl surfaces, but also selectively
absorbed within the {111} growth sectors. Starting from recent and analogous findings, we tried to interpret the
morphological change by means of a geometric and structural model of epitaxy between the {100} and {111} forms of KCl
and the most important forms of those compounds that could be adsorbed on them: PbCl
2
(cotunnite), PbCl(OH)
(laurionite-paralaurionite) and KCl·PbCl
2
(challacolloite). Excellent lattice coincidences have been found, so proving that
the {111} KCl octahedron is largely privileged for adsorption/absorption to occur with respect to the {100} KCl cube. Based
on this ground, simple kinetic considerations can be proposed to satisfactorily explain the observed morphology change.
Introduction
Ninety years ago Gaubert
1
first suggested that the habit
change of a crystal and the oriented deposit of crystals of a
given species, on a crystal of a different species, are nothing
else than two phenomena generated by the same cause.
Bunn
2
and Royer
3a-e
tried to verify this hypothesis. Royer,
investigating crystals with simple and well known structure,
first demonstrated that a habit change should occur when the
2D lattices of the new appeared face and the one of the
“crystallizing impurity” show close parametric size.
Starting from the findings by Retgers,
4
Royer hypothesized that
the
{
100
}
{
100
}
+
{
111
}
habit change underwent by KCl
crystallizing in the presence of PbCl
2
occurred because “… the
2D lattice cell of the new
{
111
}
form shows the same size of
the 2D cell of one of the faces of the crystalline impurity
introduced in the mother phase…”. As a matter of fact, the
original Royer’s intuition was that the ratio (b
0
/a
0
)=1.706
between the cell parameters on the 001 plane of the
orthorhombic PbCl
2
is very close to the value √3=1.732 which
represents, in turn, the parametric ratio of the rectangular cell
that can be determined on the 111 plane of the KCl crystal. In
other words, Royer outlined that the pseudo-hexagonal
symmetry of the
{
001
}
form of PbCl
2
fits with the trigonal one
of the KCl-
{
111
}
octahedron, so favoring the
{
100
}
{
111
}
habit change.
3d,e
The same reasoning was applied to interpret
the
{
100
}
{
111
}
morphological transition of both KBr and KI
crystals growing in the presence of the orthorhombic PbBr
2
and of the hexagonal PbI
2
, respectively. However, the
coincidence between host and guest lattices is a necessary but
not sufficient constraint to obtain a change of habit, as shown
by Royer itself.
3c
It has been also well known that the
{
100
}
+
{
111
}
habit change
of KCl and NaCl crystals in the presence of minor amounts of
Pb ions in aqueous solution was recorded in two-dimensional
diagrams (supersaturation vs impurity concentration) called
“morphodromes”, obtained on both growth
5-8
and dissolution
morphology
9
through in situ and ex-situ observations.
Later on,
10
careful in-situ observations showed that in KCl
crystals grown from pure aqueous solution the {100} form
exhibits square growth layers bounded by straight <001>
steps, when the relative supersaturation of solution,
σ=(c
solution
/c
saturation
)-1, is lower than 0.01 and it transforms to a
hopper-form when σ reaches 0.015; here c
solution
and c
saturation
represent the concentration of the solution at supersaturation
and saturation, respectively. In the presence of Pb ions the
<001> steps become less stable and truncated by diagonal
<110> steps; further, the advancement rate of the steps
decreases whereas their height increases with the Pb
concentration. Thus, the {111} octahedron starts to appear, in
the presence of Pb ions, associated with the appearance of the
<110> steps on the cube faces. The earlier stages of the
octahedron occurrence are followed by the appearance of
growth layers on the octahedron surfaces originating

ARTICLE Journal Name
2 | J. N ame., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins
Please do not adjust margins
alternately from opposite corners or edges of the face.
Successively, growth spirals start to appear from the central
portion of the octahedron faces and their step become thicker
with increasing Pb concentration in the mother solution. The
overall growth morphology is summarized in Fig. 1 where both
crystal habit and surface micromorphology are represented as
a function of the solution supersaturation and Pb
concentration.
10
Figure 1.
Morphodrome of KCl crystals growing at different
supersaturation values and under different Pb concentration
(ppm) in solution; label
s
indicates the presence of growth
spirals on the growing faces. Inspired and elaborated from
reference 10. See S.I., Fig. 1 for details.
Epitaxial growth experiments were carried out on KCl seed
crystals, having initial
{
100
}
+
{
111
}
habit, immersed in a
KCl+PbCl
2
solutions where the concentration of Pb ions ranged
from 0.8 to 1% . It was observed that “…small crystallites with
an elongated prismatic habit corresponding to the one of PbCl
2
crystals, grew in epitaxial orientation on
{
111
}
, and less clearly
on
{
100
}
faces. On both faces, the elongation of PbCl
2
crystallites is parallel to the set of <110> directions of KCl. The
epitaxial relation between PbCl
2
and KCl is thus confirmed”.
Based on this argument, it was concluded that “…the habit
change of KCl crystals, from cubic to octahedral, obtained in
the presence of Pb ions in solution, takes place probably
because the Pb ions precipitate in the form of PbCl
2
crystallites
preferentially along the <110> steps of the growth layers
running on the
{
100
}
flat faces. This reduces the advancing
rate of the growth layers and results in a piling-up of <110>
steps; hence, the originally kinked
{
111
}
form (K-type, in the
sense of Hartman-Perdok
11
) changes to a stepped form (S-
type
11
). As a result, small
{
111
}
faces appear that become
larger by the spiral growth mechanism.
10
Unfortunately, the epitaxial growth of PbCl
2
crystallites along
the <110> directions of KCl was not proved by means of
photographic evidence.
In the present paper, KCl crystals were nucleated and grown
from aqueous solutions in the presence of increasing Pb
concentrations (from 0 to 2000 ppm), under controlled
crystallization temperature and supersaturation, with the aim
at determining the mechanisms ruling out the morphological
transition:
{
100
}→{
100
}
+
{
111
}
. Keen attention is paid to the
reticular relationships between the
{
111
}
-KCl substrate and
the adsorbed matter that could deposit on it, in the form of
epitaxial 2D-phases related to those 3D-phases, like PbCl
2
(cotunnite), PbCl(OH) (laurionite, para-laurionite) and
KCl·2PbCl
2
(challacolloite), which could potentially precipitate
in the growth solution under suitable supersaturation
conditions. We are confident in this epitaxial approach, owing
to the recent examples of habit change we found on the
following epitaxial couples: Li
2
CO
3
(zabuyelite) / CaCO
3
(calcite),
12
BaCO
3
(witherite) / SiO
2
(quartz)
13a,b
and NaCl / H-
CO-NH
2
(formamide).
14
Experimental
Cubic-octahedral KCl crystals were obtained following two
growth routines: the first, to compare the results with those of
Liang et al.,
10
involves growth experiments performed, starting
from KCl (Sigma-Aldrich analytical grade) aqueous solutions
saturated at 40°C (solubility 40.05 g/100g water), by repeated
crystallization from a starting temperature of 95°C. According
to the second routine, crystals were grown from a KCl solution
saturated at 25°C and cooled down to 4°C, in the presence of
variable amount of PbCl
2
. The Pb
2+
concentration was adjusted
from 0 to 2000 ppm, adding both analytical grade solid PbCl
2
or Pb(CH
3
COO)
2
·3H
2
O. Lead acetate was chosen because of its
higher solubility with respect to lead chloride and in order to
reduce the chlorine concentration in the starting solution, so
avoiding the common-ion effect. Moreover, its chelating
properties are useful to limit the precipitation of crystalline
PbCl
2
when lead concentration rises and, consequently, to
preserve the requested lead concentration. Chelating
substances must be used being aware of their effect as habit
modifiers. Aiming at excluding the eventual surface poisoning
effect due to the presence of acetate ions in solution, all
experiments were carried both in pure chloride and acetate
solutions. KCl precipitation was induced by cooling down the
solution to 34°C, in order to reproduce and compare our
results with those published by Lian et al.
10
and obtained at
relative supersaturation σ = 0.03, by imposing a temperature
gradient ΔT=6°C. We adopted as well the same starting
temperature and Pb
2+
concentrations chosen by Lian et al. to
relate the habit changes of KCl to the σ value and to the Pb
concentration.
Experiment
code
T
saturation
(°C) T
growth
(°C) Pb
2+
/K
+
molar
ratio
KPC 40 34 0 - 0.0013
nKPC 25 4 0 - 0.0015
SEM – AFM Imaging and EDS analysis
The overall crystal morphology was observed by means of a
Scanning Electron Microscope Cambridge S-360 (EHT 30 kV,
wd 5mm, current probe 100 pA). An Electron Dispersion
Spectrometer Oxford INCA Energy 200 was used to get the

Journal Name ARTICLE
This journal is © The Royal Society of Chemistry 20xx J. N am e., 2013, 00 , 1-3 | 3
Please do not adjust margins
Please do not adjust margins
qualitative elemental mapping (EHT 15 kV, wd 25mm, current
probe 2.5 nA). Surface detailed morphology was observed by
Atomic Force Microscopy using a DME Dual Scope Microscope
(alternated contact mode, silicon nitride Au coated probes
with typical resonant frequency 170 kHz and typical force
constant 40 N/m).
XRF analysis
The elemental composition of some samples was mapped
using an EDAX Eagle-III XPL µProbe, the instrument being
equipped with a Rh X-ray tube and X-ray Poly-capillary Lens
with a spot size of 30µm. The working conditions were 40kV
and 1mA, Ti 25 µm thick primary filter, resolution 128x100
pixels, dwell time=4s.
A KCl crystal grown from a cooled solution containing 500 ppm
of Pb
2+
(PbCl
2
) was used in order to obtain the Pb distribution
inside the crystal. The crystal as grown, showed well-
developed cube faces and small complementary octahedron
faces, corresponding to extended growth sectors of the cube
and narrow growth sectors of the octahedron. The crystal was
dry-polished in order to ensure the planarity of the surface to
be mapped. KK, ClK and PbL lines were used.
The Pb distribution is shown in Figure 2. As one can observe,
the Pb concentration is higher within the growth sectors of the
octahedron and shows an oscillatory behavior mainly during
the first stages of growth (close to the center of the crystal). In
correspondence of the growth sectors of the cube the
concentration of lead is smooth and quite uniform, decreasing
during the late stages of growth. The Pb distribution in the
crystal bulk is related to the preferential absorption on the
surfaces of the octahedron. This leads to a rise of lead
concentration inside the octahedron growth sectors, since
adsorption/absorption occurs onto the octahedron terraces.
On the contrary, the smooth distribution inside the cube
sectors is due to the lack of Pb absorption onto the cube
terraces. Here the absorption could occur only on the ledges
Figure 2
. The SEM image of the sample, used as a
morphological reference for the Pb distribution inside the
crystal ( left side). Pb concentration, in ppm (right side).
of the macrosteps running on the cube faces and having the
structure of the octahedron facets, as it will be detailed
in following
chapter.
KCl crystals grown in the presence of Pb ions: the overall
morphology
As expected, the simply cubic habit observed in pure aqueous
solution progressively changes to
{
100
}
+
{
111
}
and then to
the dominating
{
111
}
, as much as c
Pb
increases (Fig. 3).
Figure 3.
Observed habit of KCl grown (
T=6°C) in the
presence of increasing percentage (c
Pb
) of Pb ions in solution.
From left to right: c
Pb
= 0, 500, 1000, 2000 ppm.
{
100
}
, grey
color;
{
111
}
, orange. The
{
111
}
form increases its importance
with c
Pb
.
Figure 4.
SEM images of KCl crystals grown from Pb doped
solutions. The octahedron dominates the cube (top-left). The
surface growth pattern of a cube face is made by layers
running parallel to the diagonals of the face, i.e. by <100>
macrosteps (top-right). Trigonal 3D islands nucleate on the
octahedron faces: the filling up of the islands starts from their
periphery (bottom-left). Islands, once completely filled, show
their terraces parallel to the
{
111
}
substrate (bottom-right).
From the overall surface patterns of both cube and octahedron
faces it follows that:
i)
On the cube faces, the lead presence induces, even at
a low concentration, patterns built by <100>
macrosteps which are nothing else than thin ledges
having the slopes of the anticlockwise sequence of
the cube faces: (100), (010) and (001), as detailed in
the S.I., Fig.2 left. It is worth noting as well that,
contrary to Lian et al. conclusions,
10
the presence of
PbCl
2
crystallites aligned along the <100> macrosteps
is excluded (Fig. 4 top).
ii)
The surfaces of both
{
100
}
and
{
111
}
forms don’t
show growth spirals, at SEM resolution level.
iii)
Beyond a critical supersaturation, growth islands
appear on the octahedron only. These 3D hillocks are
regularly oriented with respect to the face edges. As

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42 citations


Journal ArticleDOI
Abstract: This paper deals with the influence of silicate species on the morphogenesis of BaCO 3 (witherite), one of the main component of “silica/carbonate biomorphs”. The size of barium carbonate crystals decreases progressively and significantly with the increasing amount of Na-metasilicate (Na-MTS) in the crystallization environment. When the Na-MTS amount in the aqueous mother solution is less than 500 ppm, single well-shaped micrometric BaCO 3 crystals are obtained. Higher Na-MTS concentrations (4000–5000 ppm) produce polycrystalline structures built by pseudo-hexagonal nano-rods. X-ray powder diffraction (XRPD) diagrams show the decrease of crystal size with Na-MTS concentration and point out as well that silicate groups can be slightly absorbed into the barium carbonate lattice. The last part of the paper is devoted to the first finding, in laboratory, of aragonite polycrystalline structures belonging to the family of “silica/carbonate biomorphs”.

42 citations