Morphology Change of Nitratine (NaNO3) from Aqueous Solution, in the Presence of Li+and K+Ions

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[Crystal Growth & Design, Volume 15, Issue 11, 2015, DOI:

10.1021/acs.cgd.5b00920]

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Morphology Change of Nitratine (NaNO

) from

Aqueous Solution, in the Presence of Li

_{and K}

Ions

Raúl Benages-Vilau 1,*_{, Teresa Calvet}1_{, Linda Pastero}2_{, Dino Aquilano}2†_{, Miquel Àngel}

Cuevas-Diarte1

1 _{Departament de Cristal·lografia, Mineralogia i Dipòsits Minerals, Facultat de Geologia,}

Universitat de Barcelona (UB), c/Martí i Franquès s/n, 08028 Barcelona, Spain.

2 _{Dipartimento di Scienze della Terra, Università degli Studi, via Valperga Caluso 35, 10125}

- Torino, Italy.

ABSTRACT

It is usually accepted that the NaNO3 – nitratine growth morphology is built by only the

{10.4} form. In a previous paper we noticed that it can be changed by addition of selected impurities. Accordingly, a deeper study was carried out to fix the conditions for nitratine morphology change in aqueous solutions by the addition of impurities. We used standard crystallization methodologies: constant crystallization temperature (from supersaturated solutions) and slow evaporation. Both K+ _{and Li}+ _{ions can modify the nitratine growth shape.}

behavior of {10.4} faces in the presence of these ions and found that it shows a sudden decrease when an impurity is added. Finally, a simple model is proposed to describe how the morphology change takes place.

---*_{present address: Catalonia Institute for Energy Research (IREC), Jardins de les Dones de}

Negre 1 2ª pl., 08930 Sant Adrià de Besòs, Barcelona, Spain. rbenages@irec.cat.

1) INTRODUCTION

It is well known that impurities are technologically very important because they are basic for many modern devices. Single crystal with a high degree of perfection and well-defined behaviour (purity, controlled doping…) are needed as basic materials for applications such as: LED’s, lasers, microwave devices.1 _{Impurities are also important in nature: for example,}

they are responsible for different colours observed in quartz and other minerals.2

Commercially, some crystal habits are disliked because they give the crystalline mass a poor appearance; others make the product prone to caking, induce poor flow characteristics, or give rise to difficulties in handling or packaging of the material. For most commercial purposes a granular or prismatic habit are usually desired, but there are specific occasions when plates or needles may be needed.3

The presence of impurities in a growth medium can have a profound effect on the growth of the crystal. Some impurities can supress growth entirely or enhance it. Others may exert a highly selective effect, acting only on certain crystallographic faces and thus modifying the crystal habit. Some impurities can show an influence at very low concentration, less than 1 ppm, while others need to be in fairly large amounts.4 _{From the microscopic point of view,}

growth kinetics strongly depends on active impurities present in a crystallizing system, even in amounts that do not influence the properties of the bulk. The most stable faces (F faces in the Hartman-Perdok sense)5 _{grow slowly and usually build the equilibrium morphology of a}

crystal. Thus, in order to change the morphology a stabilization of other kind of faces is required (either stepped (S) or kinked (K) ones) so that they could decrease their growth rate, thus appearing in the growth morphology. In order to understand the specific adsorption of an impurity on a given crystal face, epitaxial relations between the face-substrate and the

ordered adsorbed phase could be often taken into account. Lattice misfits lower than 10% are usually required at the substrate/adsorbed phase interface, for an effective epitaxial

interaction to occur.6 _{Furthermore, in the case of nitrates, the anion stacking should be the}

same.

The adsorption of impurities substantially changes the average binding forces operating between particles along the surface of outmost crystal layers, so modifying the transport processes in the bulk and on the surface. They change the structure of the liquid and, hence, the diffusion of species modifies the face growth rate. If an impurity is strongly adsorbed in the kinks of a step ledge, the growth sites lose the opportunity of incorporating new material until it is desorbed from or buried by the growing crystal.5

As far as we know, the only work where an impurity effect was investigated in the system H2O - NaNO3 - KNO3 is that by Chernov and Sipyagin who determined the growth isotherms

for KNO3 in the presence of NaNO39 and found that the KNO3 growth rate decreases with the

NaNO3 concentration.

The aim of this paper is to answer the question: what are the conditions for NaNO3

morphology change? To do that we have used potassium and lithium nitrates, lithium hydroxide and lithium carbonate which appear to be the most promising additives. This manuscript is divided in three parts. First, experimental observation of the NaNO3

morphology change by the action of different additives is described. Then, we show the variation of the growth rate of {10.4} form of NaNO3 in the presence of KNO3 or LiNO3.

Finally, we try to explain the morphology change using simple 2D-epitaxial relationships.

2) MATERIALS AND METHODS

NaNO3 (analytical grade) was provided by Quality Chemicals.8 Its purity was checked by

ionic coupled plasma with a mass detector (Perkin Elmer Optima 3200 RL). X-ray diffraction pattern (Panalytical X'pert Pro diffractometer at room temperature with Bragg-Brentano geometry with a hybrid monochromator and a X'Celerator Detector) confirmed that the

powder supplied by Quality Chemical was nitratine, i.e. the low temperature phase of sodium nitrate (II-NaNO3). KNO3 and LiNO3 (chemically pure) were also provided by Quality

Chemicals. Finally, Li2CO3 (purity >99%) from Acros Organic was also used. All the

products were used without further purification.

The obtained crystals were visualized by a Nikon SMZ 1500 magnifying glass with ProgRess speed XTcore _{3 CCD camera. Selected crystals were characterized by Raman spectroscopy and}

scanning electron microscopy in Centres Científics i Tecnològics of the Universitat de Barcelona (CCiT-UB). For NaNO3 and KNO3 powders Raman spectroscopy was carried out

at room temperature with the Jobin Yvon T64000 Raman spectrometer. The liquid nitrogen cooled CCD detector was calibrated against TiO2. Nominal laser power was 400 mW with a

514 nm wavelength. Five measurements, of ten seconds each, were performed from 24 to 1700 cm-1_{. Peak position and intensity were measured with Origin Software. HR800 Raman}

microscope was used for some sample characterizations. Hitachi S-4100 field-emission and H-2300 scanning electron microscope (FE-SEM) at room temperature were used to take images of crystals.

For crystallization experiments we adopted, as an approximation, pure NaNO3 solubility data

since no information about NaNO3-KNO3 and LiNO3-NaNO3 systems is available. The most

reliable solubility data for NaNO3 show a linear dependence (molar fraction XT vs absolute

temperature T), when 277 K < T < 325 K:

XT = 0.0022  T - 0.1757 (1)

as comes out from a sound paper by Xu and Pruess (Fig. A6, pg. 46). 10

First, we crystallised from supersaturated solutions at constant temperature. For each set of experiments we fix the total mass of the system (MT = 25 or 30 g) and the crystallization

temperature (Tgrowth = 293K). Then, we varied the ratio NaNO3/impurity (Na/X) and the

equilibrium temperature (Teq), which results in a change of the solution supersaturation:

 = [c(Teq) - c(Tgrowth)] / c(Tgrowth) (2)

where c stays for solution concentration.

For every experiment we weighted the required amount of each component with a precision of 0.1 mg. Solids were dissolved at a temperature that allowed complete dissolution; then, solutions were filtered with filter paper in a polypropylene Petri capsule and sealed with Parafilm®_{. Finally, crystallizations were carried out at 293 K for 2 days in order to minimise}

water evaporation; then, photographs were recorded. We use the same procedure when adding KNO3, LiNO3 and Li(OH) as impurities.

Alternatively, we let KNO3 or LiNO3 doped solutions to evaporate at 291 K for several

months to observe if new faces appear.

For Li2CO3 doped solutions a cooling crystallization method was used. We weighed the

required amount of NaNO3 and Li2CO3. After dissolving sodium nitrate at room temperature,

pH was allowed to reach a value close to 7-8, by CO2 bubbling. Thus, solutions were placed

at 277 K to obtain crystals.

When we sought to introduce Li+ _{ions in the crystallizing solution, problems appeared with}

LiNO3 because it is a deliquescent compound. Thus, we decided to use Li(OH) and Li2CO3 to

check if lithium has some implication in morphology change of nitratine. As a matter of fact, morphology change of calcite due to the presence of zabuyelite (Li2CO3) has been studied

from both theoretical11 _{and experimental points of view.}12-14 _{As calcite and II-NaNO} 3 are

For crystal growth measurements we used the device built for measuring the pure NaNO3

growth rate described elsewhere,15 _{while here we added different amounts of KNO}

3 or LiNO3

as impurities to determine the variation in the advancement rate of the crystal faces. In this set of measurements, we maintained constant both the equilibrium and the

crystallization (growth) temperature, while impurity content was raised, i.e. we were working at Teq = 292.7 K and (Teq-Tgrowth)= 0.3 K. Accordingly, one could estimate from relation (2) the

supersaturation  of the mother solution.

It is worth remembering here, that for each measurement a new crystal seed must be used due to the change in the equilibrium temperature when an impurity is added. We have determined

the growth rate

R

hydrodynamic conditions, i.e. with a face of the form either perpendicular or parallel to the

solution flow ( R104N _{and R}104 p _R

104P _{respectively), as done by Benages-Vilau et al.}

for pure nitratine study.15 _{When a new KNO}

3 (or LiNO3) addition was required, we extracted

some solution from the reservoir and then dissolved in it the required amount of impurity. We heated the extracted amount of solution in order to dissolve all the new solids formed

(salting-out effect), and finally it was returned back into the reservoir. The equilibrium temperature was rechecked and adjusted. ICP-OES were used to control the concentration of species in each step.

3) RESULTS

Crystallization of sodium nitrate from pure aqueous solutions always gives the {10.4} form.7,15 _{This should not be surprising because it is the most stable form (as we have}

determined by surface energy calculation),7 _{even if a thermodynamic property could not}

results obtained by addition of impurities in the crystallizing solution. First, we used KNO3

and LiNO3, then other lithium salts were employed to obtain NaNO3 habit modification and,

finally, we describe the growth rate behaviour of NaNO3 in the presence of K+ and Li + in the

solution.

3.1. Systems with K+ _{or Li}+ _ions.

We describe here the results obtained when crystallizing at 293 K. We have worked at three NaNO3 supersaturations between 2.3 % and 4.2 %, corresponding to a temperature gradient

of up to 9K. We used several Na/K or Na/Li molar ratios from 1000 to 25 as detailed in table 1, where we have marked with an X (KNO3 impurity) or + (LiNO3 impurity) the conditions in

which modification is observed; XX or ++ marks larger proportion of modified crystals.

Table 1. Experimental conditions for obtaining NaNO3 modified crystals by

KNO3 addition. (See the text for symbols)

σ (%) Na/impurity 2,3 3,3 4,2 25 X + + + 40 + ++ 50 X X + + 100 X +++ + 200 X XX + + + 500 X XX

+

1000 XX

+ +

From Table 1 one can deduce that the best conditions for obtaining a significant quantity of modified crystals, either by KNO3 or LiNO3 addition, are related to a relatively high

supersaturation and low impurity content. It is worth noting here that we have not obtained morphological modifications of all crystals in any batch (see figure 1 and 2). In fact, only a minor concentration of modified crystals was usually observed, since only the bottom (00.1) face generally appears in some crystals. Moreover, LiNO3 results to be a better morphology

modifier that KNO3, as it roughly comes out from Table 1, when comparing the amount (18)

of symbols + with the amount (12) of symbols X.

It is worth outlining that the morphology changes we describe here, are nothing else than the results of a preliminary investigation and that, within this work, we haven’t pretensions to obtaining “morphodromes” of NaNO3 in the presence of variable amounts of K+ or Li+. To

do that, two main constraints could be fulfilled, according to the original definition of “morphodromes” , owing to the action of impurities as specific habit modifiers: 16

i) The impurity could really be a foreign substance (x) with respect to the crystal composition (a glycine molecule or a ferri-cyanide ion with respect to the crystallizing NaCl, as an example17, 18₎

ii) In a morphodrome diagram each crystal morphology should have its own domain of existence defined by two parameters (the supersaturation  of the solution and the concentration cx of the foreign substance). Then, a rigorous solubility

diagram: crystal-impurity-solvent is needed.

In our case, both potassium and lithium can form nitrates that are iso- or quasi- isostructural with nitratine and the probability of forming “anomalous mixed crystals” is not a priori

excluded; furthermore, three supersaturation values are not sufficient to draw a reliable morphodrome. Finally, as we just mentioned, no available data exist for the systems:

NaNO3-KNO3 -H2O and LiNO3-NaNO3 -H2O . Thus, our supersaturation values are nominal

ones; nevertheless, these limitations do not affect the validity of our morphological

observations and the results on the kinetic effects of Li and K on the growth rate of nitratine single crystal (Section 4).

In Fig. 1a two modified rhombohedra are laying on the {00.1} form, due to KNO3 addition

(σ=3.3 % and Na/K=500); further, a 3-fold axis holds perpendicular to the snapshot.

Similarly, in figure 1b modified crystals by LiNO3 addition (σ = 3.3 % and Na/Li = 100) are

shown.

Figure 1. Modified NaNO3 crystals obtained at Tgrowth=293K, crystallization time 2 days. a)

Na/K = 500, σ = 3.3%; b) Na/Li = 100, σ = 3.3 %.

Several batches were allowed to slowly evaporate at 291 K (Fig. 2a,b); the time lapse between the experiment preparation and the observation can be as much as 8 months. The results showed, as expected, big rhombohedra. In this case, growth features such as macrosteps and curved crystals (probably due to partial dissolution in the final stages) are observed. Moreover, modified morphologies are detected in most of experiments, in secondary crystal generation. In general, the two faces belonging to the {00.1} pinacoid

along with the cleavage rhombohedron are observed when KNO3 was added as an impurity

(Fig. 2a); but only one out of the two {00.1} faces is observed in experiments where the impurity was LiNO3 (Fig. 2b). Realizing that much of NaNO3 is already crystallized and

KNO3 and LiNO3 concentration has not fluctuated, we can assume to have higher Na/K or

Na/Li ratios in these experiments than in the previous ones. Remembering that the {00.1}- NaNO3 is a polar form, as in calcite (see, for detail, the S.I.), the fact that both (00.1) and

(00. ´1 ) faces appear in the presence of KNO3 can be due to a higher adhesion energy of the

KNO3 adsorbed layer on the NaNO3 {00.1} form with respect to those of LiNO3.

Figure 2. Modified NaNO3 crystals obtained by slow water evaporation at Tgrowth=291 K.

Initial conditions: a) Na/K=10, σ = 2.3%. b) Na/Li = 10, σ = 1.4 % and;

Further, we selected a fully modified crystal obtained by slow evaporation (over 4 months, at 291K) of a solution with (Na/K) = 10 and initially supersaturated at σ = 2.3% (Fig. 3, the insert). There one can see the micro-Raman spectra of i)-the as grown modified crystal in solution, ii) - a dry {10.4} NaNO3 rhombohedron, iii) -NaNO3 and KNO3 powder products. It

should be noted that the modified crystal has mainly a KNO3 environment, as the strongest

peak intensity is due to this configuration. We can assign the following peaks: ν4=725 cm-1

and ν1=1068 cm-1 for NaNO3 and ν4=716 cm-1 and ν1=1049 cm-1 for KNO3.19 Additionally, the

line broadening can be interpreted in different ways: first, the crystal has some internal stress; second, the formation of solid solution can occur; finally, K-nitrate could be adsorbed

on the Na-nitrate surface. The unit cell of this crystal was determined by single crystal diffraction using 25 independent reflection spots collected by a CAD4 detector between 20.9º and 42.7º. The indexation reproduces a pure II-NaNO3 cell; therefore we can assess that

KNO3 did not enter the NaNO3 structure while it was adsorbed on the NaNO3 surfaces.

600 750 900 1050 0 2000 4000 6000 8000 10000 In te ns ity ( a rb itr ar y un its ) Frequency (cm-1₎

wet modified rhombohedron dry rhombohedron powder NaNO3 powder KNO3

Figure 3. From top to bottom: micro-Raman spectra (10) of a wet modified rhombohedron

(black), wet rhombohedron (red). Powdered sodium nitrate (green) and potassium nitrate (blue) are also presented.

3.2. Systems with other lithium compounds.

Li(OH) has been added to the growth solution in a similar series of experiments, at the crystallization temperature of 291 K. Also in this case crystal morphology was modified (Fig. 4, left side). The best conditions for the appearance of new faces were σ = 3.3 % and Na/Li = 100 or less (ppm range).

When Li2CO3 was added to the growth solution, crystallization was done by cooling, as

explained in Section 2. The best conditions to obtain modified NaNO3 crystals is to prepare a

Li2CO3 (at pH 7.5-8.2, adjusted by CO2 bubbling). Finally, a slow cooling rate was preferred

to obtain big crystals sitting on a (00.1) face (Fig.4, right side).

Figure 4. {10.4}+{00.1}morphology of NaNO3 obtained in the presence of: Li(OH) (SEM

picture, left) and Li2CO3 (optical microscopy, crystal size1cm, right).

It has been proved that Li+ _{and K}+ _{ions can modify to some extent sodium nitrate}

morphology, but the best result was obtained by adding Li2CO3. Consequently, one may ask:

is CO32- ion responsible for morphology change? Some preliminary experiments have been

made with sodium carbonate as impurity, and we have also obtained morphology changes. But deeper experimentation is needed to prove this statement.

4) GROWTH RATE DETERMINATION

In this section we deal with the isothermal growth rate of NaNO3 (at constant ), while

increasing the impurity content. The {10.4} growth rate of both perpendicular and parallel faces in respect to the solution flow is plotted in Fig. 5. We can clearly see that the growth rate decays exponentially for both faces. For LiNO3, fewer additions have been made because

the decrease in the growth rate resulted to be evident since the first attempt.

As anticipated in the Introduction, an impurity can increase or decrease the face growth rate due to its (transitory) adsorption. Both Na+ _{and K}+ _{are highly poisoning the (10.4) face}

reduced below 5·10-7 _{cm/s, in order to appear in the final crystal morphology. This agrees}

with the results quoted in our previous sections: lithium nitrate is the most effective in changing sodium nitrate morphology.

Figure 5. Growth rate of {10.4}faces of NaNO3. Black plus symbol perpendicular to the

flow, red point parallel to the flow.

5) DISCUSSION

5.1. Why the epitaxial model can be adopted to explain the morphology change of NaNO3.

It is widely known that the most effective adsorption (temporary or not) of an impurity on a crystal face occurs when epitaxial relationships do set up between the 2D adsorbed phase and the crystalline substrate. In this case, the interface energy (adsorbed phase/substrate) results to be strongly affected and its lowering modifies the equilibrium shape of the crystal;

moreover, the kinetics of the interested face is affected as well, because the flat terraces lying in between the running steps hinder the advancement rate of the steps themselves. It follows that it is reasonable searching for epitaxial relationships when an impurity is supposed to induce the appearance of a new face on a growing crystal.

m=dguest−dhost

d_{host (3)}

where dhost are the surface parameters of NaNO3 while dguest are those of the adsorbed

impurity. Lower the m value, higher the probability of an ordered impurity adsorption. Additionally, a similar stacking of anions and cations of the growing crystal and impurity made the adsorption process easier. Table 2 gives the cell parameters of three nitrates.

Table 2. Cell parameters (in Å) of selected nitrates

Compoun d Space Group a0 b0 c0 Ref . LiNO3 _R

3

3

As we have obtained the additional {00.1} form where nitrate groups maintain parallel in both host and guest structures, we examined two potential epitaxies: II-NaNO3 (00.1) ||

-KNO3 (001) and II-NaNO3 (00.1) || LiNO3 (00.1).

Crystal form (host) II-NaNO3 2D-lattice of the host form(Å) Crystal form (guest) -KNO3 2D-lattice of the guest form(Å) 2D- misfit (host/guest)  Notes 00.1 [010]= 5.07 _001 [100] = 5.4119 + 6.74 good misfit [210] = 8.7815 [010] = 9.1567 + 4.27 good misfit 2D-area (Å2_{) 44.522 49.555 + 11.30 compatible} misfit d00.6 = 2.803 d002 = 3.21065 14.53 incompatible

Table 3a. Lattice coincidences between 00.1of II-NaNO3 and 001 of -KNO3 . Crystal form (host) II-NaNO3 2D-lattice of the host form(Å) Crystal form (guest) LiNO3 2D-lattice of the guest form(Å) 2D- misfit (host/guest)  Notes 00.1 [010]= 5.07 _00.1 [010] = 4.692 8.05 good misfit [100] = 5.07 [100] = 4.692 8.05 good misfit 2D-area (Å2_{) 22.2611 19.0654 16.76 compatible} misfit d00.6 = 2.803 d00.6 = 2.5358 10.54 compatible misfit

Table 3b. Lattice coincidences between 00.1of II-NaNO3 and 00.1 of LiNO3.

Tables 3a,b show that if the 00.1 form of nitratine is assumed as a substrate, then both pinacoids: 00.1 of -KNO3 and 00.1 of LiNO3 can easily stay in epitaxy with it. These

good lattice coincidences are not surprising because:

iii) i) 00.1of II-NaNO3 and 00.1 of LiNO3 show the same A3 symmetry at the

common interface and a low parametric misfit (8.05%).

iv) ii) 00.1of II-NaNO3 and 001 of -KNO3 show an even better parametric

misfit, notwithstanding an orthorhombic symmetry should coexist (in this case) with a trigonal one at the common interface (Fig.6). Actually, if new lattice vectors are considered in the 00.1 plane of -KNO3 , then one can find a

pseudo-hexagonal supercell , having two sides 10.824 Å long = 2[010] (-KNO3) and four sides 10.636 Å long = [110] (-KNO3). Further, the obliquity of this 2D supercell is very low, being built by four angles of 120.58° and by two angles of 118.83°. Concerning the misfit between the areas of the cells of substrate and

adsorbed phase, the value of 11.3% remains obviously confirmed, coherently with Table3a. This misfit is slightly higher with respect to that indicated in the

Introduction to get a good geometrical condition for epitaxy; 6 nonetheless, it is not epitaxially incompatible, owing to the fact that geometry constraints do not take into account the improvable lattice adaptability, due to surface relaxation.

5.1.1. The surface structures at the interfaces: {00.1}NaNO3/{00.1}LiNO3 and {00.1}NaNO3 /{001}β-KNO3.

Once the necessary lattice constraints have been fulfilled for epitaxy to occur, one should verify the compatibility of host and guest phases at the level of the interface structure. When dealing with the {00.1}NaNO3/{00.1}LiNO3 interface, there is no difficulty for

adapting the guest to the host phase, since they show the same structure. As a consequence, the same surface reconstruction will be needed for both {00.1}NaNO3 and {00.1}LiNO3

surface profiles, according to the successful model recently proposed in order to make stable the polar {00.1}surfaces of the isostructural calcite (CaCO3). 24

The treatment of the {00.1} NaNO3/{001} KNO3 interface is slightly more complex. In fact,

one should refer to the structure refinement by Adiwidjaja and Pohl, 23 _{in order to obtain}

definitive information on the symmetry and configuration of the outmost layer that the {001}KNO3 form exposes to the mother phase. This new structure belongs to a space group

Cmc21 with a supercellhaving parameters (in Å): a0= 10.825; b0= 18.351 and c0= 6.435. The

repeat period of the attachment energy related to the form {001} is d002, each slice of this

thickness being polar, as intrinsically ensues from the screw diad parallel to the z axis. According to a preliminary investigation on the KNO3 theoretical growth morphology, we can

assess that the {001} character is that of a flat face; in fact two strong PBCs run within a d002

The {001} surfaces can be terminated only by the _NO−¿

3

¿ groups, contrary to a recent proposed hypothesis 25_{; moreover, its surface polarity is slight, and hence its surface profile}

does not need to be reconstructed. All this means that 2D d002 layers of KNO3 can be easily

form on the two {00.1} faces of the growing NaNO3 crystals.

Figure 6. Projection along the [001] direction of the structure of the orthorhombic KNO3

(left) and of the rhombohedral NaNO3 (right). The comparison outlines the strong similarity

between the symmetry of the pseudo-hexagonal KNO3 and that of NaNO3. Further, the KNO3

parametric ratio (b0/a0)=1.695, while for the NaNO3 supercell (b’/a’)=1.732, the parametric

misfit between the two structures, in their basal plane, being 2.18% only.

Benages-Vilau et al.7 _{have calculated the surface energy values of several NaNO}

3 faces. The

most stable Na terminated (00.1) face has a relaxed surface energy of 0.695 J m2 _{while for}

the (10.4) it is 0.160 J m2_{. Thus, the impurity addition should lower the surface energy of}

(00.1) to about 0.220 J m2 _{in order the NaNO}

3-(00.1) face to appear in the equilibrium

morphology. This is quite reasonable, having considered that the energetic analysis7 _was

6) CONCLUSIONS

In this paper we deal with the morphology modifications of sodium nitrate crystals due to the addition of the following impurities: potassium and lithium nitrate, lithium hydroxide and lithium carbonate. Results show that all these compounds are able to change, at different extent, the NaNO3 morphology. They often induce the appearing of the {00.1} faces which

truncate the classic {10.4} rhombohedron of NaNO3. When crystals are nucleated and grown

from mass precipitation, their population is so large that quantitative data about the ratio between the areas of the {00.1} and {10.4} forms of NaNO3 cannot be reasonably obtained;

nevertheless, the effectiveness of the chosen impurities as habit modifiers has been

qualitatively evaluated. Moreover, to deepen quantitatively the understanding of the impurity effect, we measured on single crystals the growth rate of {10.4} sodium nitrate faces (under controlled T, supersaturation and solution flow) and under the addition of potassium and lithium nitrate.

Here, we summarize the best conditions to obtain the observed modifications.

For crystallization at constant temperature with addition of potassium nitrate, lithium nitrate or lithium hydroxide the best conditions are supersaturations between three and four per cent, with an atomic ratio sodium/impurity less than 100. Contrarily, for evaporation experiments it is better to use higher sodium/potassium and sodium/lithium ratios. Additionally, we have obtained a fully modified batch by using a saturated Li2CO3 solution as an impurity and

σ~1% for NaNO3.

On the other hand, it comes out that potassium and lithium nitrate lower the growth rate of {10.4} form of sodium nitrate exponentially. As the {00.1} form also appears in some experiments, its growth rate could be lowered as well to a such extent to appear in the growth morphology. To model the mechanism of this dramatic change, we proposed that the {00.1}

form of sodium nitrate may be stabilized by an ordered adsorption of impurities (2D-epitaxy) that depresses its growth rate.

Supporting information

The two projections of the ideal structures of Calcite and Nitratine along the [100] direction (Fig. SI1); the superposition of the two [001] projections of KNO3 and NaNO3 (Fig. SI2).

This material is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES

1. Benz, K.W.; Bauser, E. in III-V Semiconductors; Springer-Verlag, Berlin 1980, Vol. 3, 1-48.

2. Sunagawa, I. Forma 1999, 14, 147-166.

3. Boerrigter, S. X. M. Modelling of Crystal Morphology. Growth Simulation on Facets in

Arbitrary Orientations, 1972, Thesis-University of Nijmegen, The Netherlands.

4. Mullin, J. W. Crystallization 1972, Butterworths, London.

5. Hartman, P. Structure and Morphology. In Crystal growth: an introduction; Hartman, P., Ed.; North-Holland: Amsterdam, 1973, Vol. 1, pp 367-402.

6. Herman, M. A.; Richter, W.; Sitter, H. Epitaxy: physical principles and technical

implementation; Springer- Berlin, 2004, 522.

7. Benages-Vilau, R.; Costa, E.; Bruno, M.; Cuevas-Diarte, M. A.; Calvet i Pallas, T.; Aquilano, D. Cryst. Res. Technol., 2011, 46(8), 773-778.

8. Quality Chemicals http://www.qualitychemicals.com/cas/index.html. 9. Sipyagin, V. V.; Chernov, A. A. Kristallografiya 1972, 5, 1003-1009.

10. Xu, T.; Pruess, K. Thermophysical Properties of Sodium Nitrate and Sodium Chloride

Solutions and their Effects on Fluid Flow in Unsaturated Media. 2001. E. O. Lawrence

Berkeley National Laboratory, Berkeley, CA (US). DOI 10.2172/790019.

11. Massaro, F. R.; Pastero, L.; Costa, E.; Sgualdino, G.; Aquilano, D. Cryst. Growth Des.

12. Pastero, L.; Costa, E.; Bruno, M.; Rubbo, M.; Sgualdino, G.; Aquilano, D. Cryst. Growth

Des. 2001, 3, 485-490.

13. Pastero, L.; Aquilano, D.; Costa, E.; Rubbo, M. J. Crys. Growth, 2005, 275(1-2), e1625-e1630.

14. Pastero, L.; Aquilano, D. Cryst. Growth Des., 2008, 8(9), 3451-3460.

15. Benages-Vilau, R.; Calvet, T.; Cuevas-Diarte, M. A.; Rubbo, M.; Aquilano, D. Cryst.

Growth Des., 2013, 13 (8), 3419–3428

16. Kern, R. Bull. Soc. Franç, Min. Crist. 1953, 76, 356

17. Redouté, M.; Boistelle, R.; Kern, R. C. R. Acad. Sc. Paris, 1965, 260, 2167

18. Bienfait, M.; Boistelle, R.; Kern, R. Adsorption et Croissance Cristalline, Coll. Intern.

CNRS n°152, 1965, 515

19. Nakamoto, K. Inorganic Compounds, in Infrared and Raman Spectra of Inorganic and

Coordination Compounds. 1986, 4th ed. New York, Wiley.

20. Wu, X.; Fronczek, F.; Butler, L. G., Inorg. Chem. 1994, 33, 1363. 21. Paul, G. L.; Pryor, A. W. Acta Cryst. B, 1972, 15, 2700-2702. 22. Nimmo, J. K.; Lucas, B. W., J. Phys. C, 1973, 201-211.

23. Adiwidjaja G. ; Pohl D. Acta Crystallogr.,2003, C59, i139-i140.

24. Bruno, M; Massaro, F. R.; Prencipe, M.; Aquilano, D. CrystEngComm, 2010, 12, 3626-3633.

25. Løvvik, O.M.; Jensen, T.L.; Moxnes, J.F.; Swang , O.; Unneberg , E. Comp. Mater. Sci.,

2010, 50, 356-362

26. Bruno, M.; Massaro, F. R.; Pastero, L.; Costa, E.; Rubbo, M.; Prencipe, M.; Aquilano, D. Cryst. Growth Des., 2013, 13, 1170-1179.

Morphology Change of Nitratine (NaNO

) from

Aqueous Solution, in the Presence of Li

_{and K}

Ions

Raúl Benages-Vilau 1,*_{, Teresa Calvet}1_{, Linda Pastero}2_{, Dino Aquilano}2†_,

Miquel Àngel Cuevas-Diarte1

The growth morphology of NaNO3 from pure solution is usually built by only the {10.4}

form. The nitratine morphology change by the addition of K+ _{and Li}+ _{as surface specific}

impurities is experimentally investigated. The presence of lithium and potassium in solution suddenly changes the growth rate of the rhombohedron. A 2D-epitaxial model is proposed to explain the morphology change.