Epitaxy of Rhodochrosite (MnCO3) on Muscovite Mica and Its Relation with Calcite (CaCO3).

The flatness of muscovite mica makes it a convenient substrate to study epitaxy. We have analyzed the growth of rhodochrosite (MnCO3) crystals in solution and on muscovite mica. Growth at high supersaturations occurs via the formation of amorphous MnCO3, which over time transforms into the crystalline form. In the presence of muscovite mica, epitaxial rhodochrosite crystals with a size of approximately 1 μm form. These crystals are kinetically roughened, because of the high supersaturation. The lattice match between MnCO3 and muscovite was found not to be the main reason for epitaxy. If the growth experiment is performed twice, the original epitaxial MnCO3 crystals are overgrown by many small crystallites. Similarly, spherical MnCO3 crystals with many overgrown facets can be formed on a muscovite surface that is exposed to humidity or by using a higher MnCO3 supersaturation. A comparison with calcite shows that epitaxy strongly depends on initial supersaturation for both carbonates. In contrast to previous studies, we find that at the right supersaturation, epitaxial calcite crystal growth is possible on freshly cleaved muscovite.

Introduction

In the past, muscovite mica was used as a cheap alternative to glass in windows in the Moscow area in Russia, as large sheets of the mineral could be obtained, hence the name given to the mineral. Nowadays, in addition to a wide range of industrial applications, muscovite is widely used in scientific experiments. For example, muscovite is used as a model mineral in order to understand enhanced oil recovery,[1,2] to study (hetero)epitaxy,[3−5] and to create ordered layers of (organic) molecules,[6−8] which in turn can act as a template for protein crystallization.[9,10] A special application for muscovite is found in the surface force apparatus, which is used to measure forces between surfaces.[11] The main reason for using muscovite in these applications is the extreme flatness of the surface obtained after cleavage of the muscovite (001) layers. It is possible to obtain atomically flat areas of over 1 cm2 without any steps.[12] Furthermore, the surface properties can be tailored by the exchange of the surface K+ ions with other cations.[13,14] In the crystal structure of muscovite mica (ideal chemical formula KAl2(Si3Al)O10(OH)2), a positive cationic K+ layer bridges two negatively charged SiO4 tetrahedral sheets. This layered structure can be easily cleaved through the cationic layer, which was recently suggested to occur at even numbers of layers.[15]

The growth of calcite (CaCO3) on muscovite mica was previously investigated. When grown on freshly cleaved muscovite, calcite crystals have a random orientation.[5] However, when muscovite is first exposed to humid air, small K2CO3 nanocrystallites form, which induce epitaxial growth of the calcite crystals.[5] Here, the growth of carbonates on muscovite (001) is further investigated by looking at the mineral rhodochrosite (MnCO3). Rhodochrosite is isostructural to calcite and is encountered in soil and sediment material, where it plays an important role in the Mn2+ geochemistry.[16−18] Furthermore, it has been extensively used as a template material for the formation of hollow-core capsules usable for drug delivery.[19−23] We expect the growth behavior of MnCO3 to be similar to CaCO3. We found, however, that rhodochrosite does crystallize epitaxially on freshly cleaved muscovite where calcite was thought not to. This seems conflicting, but we find that at sufficiently large supersaturation epitaxial crystals of calcite can be grown as well.

Experimental Methods

MnCO3 crystals were grown by adding equal amounts of an aqueous manganese chloride tetrahydrate solution (Sigma-Aldrich, ≥99% pure) to an aqueous sodium carbonate (Fisher Scientific, no indication of purity given) solution. If not stated otherwise, concentrations of 10 mM were used for both solutions, leading to a MnCO3 solution concentration of 5 mM. In the solution growth experiments, the suspension was filtered after a crystallization time varying from 0 min to several hours, prior to characterization by powder X-ray diffraction and microscopy. In the epitaxy experiments, muscovite mica (ASTM-V1 quality grade, S&J Trading Inc.) was freshly cleaved using Scotch tape. These muscovite samples were adhered to a sample holder by using petroleum jelly and placed upside down in a 24-well plate (ThermoFisher Scientific) filled with the solution. This was then placed in a closed compartment, typically for 2 h. Subsequently, the samples were washed two times in demineralized water for at least 30 s and dried in air. In some of the experiments, pieces of muscovite were, prior to crystallization experiments, exposed to humidity for 1 h in a Binder MKF 115 climate chamber, in which the relative humidity was set at 37.5%. A number of crystal growth experiments was performed twice; i.e., after separation of the specimen from the solution followed by washing and drying, the above procedure is repeated. CaCO3 crystals were grown in a similar way by using a 10 mM calcium chloride solution (Merck, >99.5% pure), or higher concentrations, when indicated. All experiments were performed at room temperature and without any pH adjustments.

The crystallographic orientation of muscovite mica was determined using a Leica DMRX optical polarization microscope in conoscopy mode (see Supporting Information S1 and S2).[9,24] Scanning electron microscopy images were acquired using a PhenomWorld Phenom scanning electron microscope and a JEOL 6330 Cryo field emission scanning electron microscope. For these measurements, a thin gold layer was applied to the samples using a Cressington 108auto sputter coater. As a measure of the epitaxial crystal size, the distance between one corner of the triangular crystal to the middle of the opposite side was used. The crystal sizes of at least two different batches were measured, and for each batch seven random positions were evaluated.

Throughout this article, the hexagonal setting of MnCO3 and CaCO3 is used. The (001) crystal plane in the hexagonal setting corresponds to the (111) plane in the rhombohedral setting, whereas the (104) plane in the hexagonal setting corresponds to the (221) plane in the rhombohedral setting.

The chemical speciation of the solutions and corresponding MnCO3 and CaCO3 saturations were determined using the PHREEQC software.[25] The solution was treated as a closed system with a temperature of 25 °C. Input species were Ca2+/Mn2+ (1 equiv), CO32– (1 equiv), Na+ (2 equiv), and Cl– (2 equiv), where 1 equiv corresponds to the MnCO3 and CaCO3 concentrations mentioned in the text. The saturation index (SI) was calculated by the software according to SI = log10({A}{B}/Ksp), with {A} and {B} the actual activities of the ions and Ksp the solubility product. The driving force of crystallization () is related to the SI via SI, with k, the Boltzmann constant and T, the temperature (in K). The calculated values can be found in Table .

Table 1

Calculated Values of pH, Driving Force and Total Concentration of Dissolved Mn2+/Ca2+ for Several MnCO3 and CaCO3 Concentrations, Using the PHREEQC Software

concentration (mM)	pH	driving force ()	dissolved Ca2+/Mn2+ (mM)
MnCO3
0.2	9.1	5.2	0.01
0.5	9.1	6.4	0.01
5	9.1	8.9	0.02
25	9.1	10.6	0.02
50	9.2	11.3	0.02
250	9.2	13.0	0.04
CaCO3
0.5	9.9	2.8	0.13
1.7	9.9	4.7	0.14
5	9.9	6.1	0.16
25	9.9	8.0	0.21
50	9.9	8.7	0.26
250	9.9	10.5	0.48

Results and Discussion

MnCO3 Growth in Solution

In order to understand the epitaxial growth of MnCO3, we first performed experiments in solution under similar experimental conditions. To obtain MnCO3, solutions containing 10 mM of MnCl2 and 10 mM of Na2CO3 were mixed, which immediately leads to the precipitation of white particles in solution. Powder X-ray diffraction (PXRD) measurements performed directly after mixing both compounds showed no crystalline peaks, as is shown in Figure a, but an amorphous signal is visible around 18°. We therefore conclude that these particles are amorphous (A-MnCO3). The same conclusion was drawn in a previous study, in which the amorphous phase was found to contain slightly more than one water molecule per MnCO3 moiety.[26]

Figure 1

(a) PXRD measurements in which the crystallization time (tc) is varied. If not mentioned otherwise, the waiting time (tw) after filtration is kept as short as possible, i.e., several minutes, before the PXRD is conducted. The reference rhodochrosite diffractogram is calculated from the crystal structure.[29] Diffractograms are given a vertical offset for clarity. (b) and (c) SEM images of MnCO3 crystals from a 5 mM solution after a crystallization time of 2 h and a waiting time of 2 days.

After a crystallization time of approximately 15 min, peaks start to emerge in the diffractogram. These peaks, corresponding to crystalline MnCO3, i.e., rhodochrosite, become more intense when the crystallization time is increased. Visually, the transformation from the white amorphous form to the brown crystalline form can be observed as well. After a crystallization time of approximately 25 min, a slight brown color becomes apparent, which agrees with the PXRD measurements. After approximately 2 h, a stable brown color is reached, indicating a considerable amount of crystalline MnCO3. The transformation to crystalline MnCO3 does not stop when the amorphous particles are removed from the solution. After a crystallization time of 5 min in solution, followed by a waiting time of 2 days in air, intense peaks corresponding to rhodochrosite were observed (Figure a), whereas without this waiting time no crystals were present.

Figure b shows SEM images of MnCO3 crystals obtained after filtration of the solution after a crystallization time of 2 h, followed by a waiting time of 2 days. The smaller crystals of 1–2 μm in size appear rhombohedral. Larger, more spherical particles of about 4–8 μm were also observed. The latter possess many small overgrown facets, as can be seen in Figure b. Figure c shows a similar crystal of which a part was detached during the filtration process, allowing the observation of the interior of the crystal. The core of the crystal appears hollow. Needle-shaped crystals grow from the inside toward the surface, where they end well-faceted. It is likely that nucleation and growth of the crystallites take place on an amorphous particle, while growth material is supplied by the supersaturated solution. In time, the amorphous particle dissolves, leading to a hollow core. The crystalline phase grows outward by means of evolutionary selection.[27,28] According to Van der Drift’s model,[27] the texture of polycrystalline layers on top of a planar or spherical substrate is determined by the fastest growing directions after some period of growth. In the case of MnCO3, this is the [001] top direction and following Van der Drift’s evolutionary selection model, the polycrystalline spheres are bound by the [001] apexes of the crystallites pointing outward from the center as shown in Figure c.

Epitaxial MnCO3 on Muscovite Mica

Morphology and Orientation

Epitaxial growth of MnCO3 was achieved by mixing equimolar 10 mM solutions of Na2CO3 and MnCl2 (yielding a 5 mM MnCO3 solution) in the presence of muscovite mica. Scanning electron microscopy images of these epitaxial crystals are shown in Figure . Nearly all crystals observed are oriented epitaxially (99 ± 1%). The size and density of the crystals are not sufficient to use PXRD to determine the MnCO3 “contact face”, i.e., the MnCO3 crystal plane parallel to the muscovite surface. Even an exposure time of more than 100 h did not reveal any detectable MnCO3 reflections, as these are obscured by the mica background X-ray scattering. However, from the 3-fold symmetry and by looking at similarities with epitaxial calcite (CaCO3) on muscovite mica[5] (Supporting Information S2), it can be concluded that the MnCO3 contact face is the (001) plane. Also, careful measurements of the lozenge-shaped faces of several MnCO3 crystals revealed average values of 102.5° for the obtuse angles and 76.1° for the acute angles. This agrees well with the angle of 103.02° and 76.98° as calculated for the {104} faces of the R3̅c rhodochrosite structure using a = 4.773 Å and c = 15.67 Å in the hexagonal setting. So, the MnCO3 crystallites are bounded by {104} faces, similar to CaCO3. The (pseudo)hexagonal symmetry of the muscovite mica (001) surface, in combination with the 3-fold symmetry of the MnCO3 crystals, gives rise to two distinguishable—opposite—MnCO3 orientations. The detailed measurements of the epitaxial properties of the MnCO3 crystallites on muscovite mica are summarized in the Supporting Information (S3).

Figure 2

Scanning electron microscopy images of epitaxial MnCO3 crystals on muscovite mica for a 5 mM concentration and after 2 h of crystallization time.

Muscovite mica (001) has a rectangular surface unit cell with lattice parameters a = 5.1906 Å and b = 9.0080 Å,[30] which is illustrated in Figure a. Figure b shows the MnCO3 contact face, i.e., the (001) plane. As this face is polar, two different terminations can be expected: a carbonate termination or a manganese termination. In our case, it is more likely that the Mn2+ layer is found directly above mica, as muscovite mica has a negative intrinsic charge caused by isomorphous substitution.[31] The rectangular centered MnCO3 surface unit cell, drawn in Figure b, is an alternative for the rhombohedral setting and has lattice parameters a = 4.773 Å and b = 8.267 Å.[29] From the fact that the {104} MnCO3 side facets point toward the muscovite b-axis and its hexagonal equivalents, it follows that both rectangular surface unit cells coincide (see Supporting Information S1 and S3). The Mn2+ cations could be located above the center of the muscovite hexagonal cavity, replacing the K+ cations initially present at the muscovite surface. Both the a and b lattice parameters of MnCO3 are 8% smaller than those of muscovite, resulting in an area that is 16% smaller than the area of the muscovite surface unit cell. Remarkably, this lattice mismatch is larger than that of calcite in the same setting (∼4% for both lattice parameters, resulting in an area mismatch of ∼8%).[29] Calcite, which is isostructural to rhodochrosite, is reported not to exhibit epitaxy directly on freshly cleaved muscovite, but epitaxy of calcite can be achieved in the presence of K2CO3 nanocrystallites on the muscovite surface.[5] This means that for MnCO3 the lattice match is not the main reason for epitaxy. A similar conclusion was drawn regarding the epitaxy of calcite on muscovite.[5]

Figure 3

(a) Top view of the muscovite mica crystal structure. The black square indicates the centered unit cell with a = 5.1906 Å and b = 9.0080 Å.[30] (b) The MnCO3 (001) plane. The rhombohedral unit cell displays the 3-fold symmetry observed in the grown crystals. An alternative centered rectangular unit cell with a = 4.773 Å and b = 8.267 Å is drawn as well.[29] The K+ cations in (a) and Mn2+ cations in (b) are shown in purple, O in red, Si in yellow, and C in gray. Panels (a) and (b) are not to the same scale. (c) Scanning electron microscopy images of epitaxial MnCO3 crystals on muscovite mica after 2 h of crystallization time.

Growth

The first epitaxial MnCO3 crystals on the muscovite mica surface are observed after a crystallization time between 15 and 75 min, whereas the amorphous phase in the bulk solution immediately forms when the two solutions are brought together. The solubility of A-MnCO3 is higher than that of crystalline MnCO3,[32] which results in a supersaturated solution with respect to crystalline MnCO3. This leads to sustained growth of crystalline MnCO3 on the mica substrate. This is an example of Ostwald’s rule of stages, where first the least stable form is formed, followed by crystallization of the stable form.[33,34] The nucleation barrier for the amorphous phase is lower than that of the crystalline phase. Rapid nucleation of the amorphous phase is followed by slow nucleation and growth of the crystalline phase. If both phases would have a similar nucleation barrier, crystals would form immediately because of the high supersaturation. The evolution is schematically illustrated in Figure a, in which the supersaturation is shown as a function of time. When the two solutions are brought together, a large supersaturation is created. This rapidly decreases due to the formation of A-MnCO3 dispersed in solution. After the nucleation of crystalline MnCO3 in solution and at the muscovite surface, A-MnCO3 dissolves leading to a plateau in concentration, followed by a decrease in MnCO3 concentration if A-MnCO3 is completely dissolved, as was earlier observed for CaCO3.[35] This is confirmed by the growth rate measurements displayed in Figure b. Initially, the crystals show a linear increase in size over time, as expected for constant supersaturation. Then the growth speed decreases as the solution depletes and approaches the equilibrium concentration of MnCO3. The crystals continue to grow until the equilibrium concentration is reached after approximately 24 h. The epitaxial crystals observed at different crystallization times all possess the same morphology and only differ in size (Figure c). At concentrations above approximately 2.5 mM, immediate precipitation of A-MnCO3 occurred, whereas at lower concentrations, no precipitation of A-MnCO3 nor growth of MnCO3 on muscovite was observed. The high supersaturation in this experiment leads to kinetic roughening, resulting in rounded edges of the MnCO3 crystals, as can been seen in Figures and 3c.

Figure 4

(a) Schematic illustration of the MnCO3 solution concentration as a function of time. After the rapid precipitation of A-MnCO3, the supersaturation is determined by the solubility of A-MnCO3. Then, crystallization takes place, but as long as A-MnCO3 is not yet dissolved, the driving force for crystallization remains constant. When (nearly) all A-MnCO3 is dissolved, the concentraton of MnCO3 decreases until the saturation concentration of MnCO3 is reached and crystallization stops. (b) Average crystal size of epitaxial MnCO3 crystals for the 5 mM initial concentration as a function of time. The solid line is a guide to the eye. Error bars indicate the standard deviation. (c) Selection of SEM images of MnCO3 crystals on muscovite mica at different crystallization times. Note that the figures have different scale bars.

In an attempt to increase the size of the crystals, the growth experiment was performed twice. Surprisingly, we observed that the original {104} facets became overgrown, as can be seen in Figure . The epitaxy and {104} facets of the MnCO3 crystals are still visible, but many small well-faceted crystallites have grown on top of the original {104} facets. Also in this second crystallization step, because of the high supersaturation, amorphous material precipitates from solution, judging from the white color of the precipitated solid. It is unclear if amorphous particles adhere to the surface of the original MnCO3 crystals and then transform into small MnCO3 crystallites or if they first transform into the crystalline form followed by attachment to the MnCO3 crystals. In both cases, this can lead to the formation of the observed nonepitaxial faceted crystallites on the original crystals. In the second cycle, no nucleation barrier or a very low nucleation barrier for crystallization has to be overcome before growth can occur on top of the existing MnCO3 crystals. This leads to multifaceted growth. However, this is not the case for newly formed crystals in the second cycle, which are the small crystals observed in Figure a.

Figure 5

(a–c) Scanning electron microscopy images of epitaxial MnCO3 crystals on muscovite mica after performing two crystal growth cycles of 2 h.

Epitaxy of calcite on muscovite mica can be achieved by exposing muscovite to humid air prior to the growth experiment.[5] The exposure to humid air is known to lead to epitaxial K2CO3 nanocrystals, formed by a combination of muscovite surface K+ ions and CO2 from the air.[36,37] Stephens et al. hypothesize that these K2CO3 crystallites act as a carbonate source attracting Ca2+ to the surface, leading to the epitaxial nucleation of CaCO3 crystals on muscovite mica exposed to humid air.[5] We have also investigated if the presence of K2CO3 crystallites on the muscovite surface has an effect on the growth of MnCO3. On the substrates exposed to humid air, still a number of small, epitaxial MnCO3 crystals can be seen, but in contrast to freshly cleaved mica, also larger, nearly spherical MnCO3 crystals are formed (Figure a,b). In contrast to the crystals grown by performing two crystal growth cycles on bare muscovite (Figure ), the larger crystals grown on muscovite exposed to humid air do not have underlying {104} facets of the epitaxial MnCO3 crystals and therefore are nearly spherical. It is likely that the smaller crystals nucleate on the bare muscovite surface and will form epitaxial MnCO3 crystals similar to those on a freshly cleaved muscovite surface, whereas when nucleation takes place on top of the K2CO3 crystallites existing on the muscovite mica exposed to humid air, large spherical crystals form.

Figure 6

Scanning electron microscopy images of MnCO3 crystals grown on muscovite mica (a) and (b) exposed to humid air followed by 2 h of crystallization time and (c) with a concentration of 25 mM instead of 5 mM on freshly cleaved muscovite mica.

MnCO3 crystals grown at higher supersaturations on freshly cleaved muscovite, often while stirring the solution, show a similar shape as the multifaceted spherical crystals. The high supersaturation can be achieved by using a high concentration (Figure c) or by adding solvents in which MnCO3 is less soluble.[19,23,38,39] In Figure c, a concentration of 25 mM is used, which leads to formation of exclusively spherical MnCO3 crystals and no epitaxial crystals. At these higher supersaturations, nucleation can take place at the bare muscovite or on amorphous particles adsorbed at the muscovite surface, similar to crystallization in solution (Figure b,c). In both cases, the high supersaturation leads to fast and evolutionary growth which results in the multifaceted crystals.

Comparison with Calcite

Given the different observed behavior in epitaxial growth between MnCO3 (our results) and CaCO3,[5] we now take a detailed looked at these two systems. Earlier we saw that the lattice mismatch is not the main cause of epitaxy for these carbonates. When comparing MnCO3 and CaCO3, many physical parameters are similar. Both Mn2+ and Ca2+ adsorb at the muscovite surface with similar occupancies.[14] Also, the hydration energy of Mn2+ and Ca2+ is comparable.[40] The ionic radius of Mn2+ is somewhat smaller than that of Ca2+ (0.83 and 1.0 Å respectively).[41] It is known that cations adsorb at different heights at the muscovite interface, depending on hydration energy, valency, and size,[42−46] but the differences in these parameters are small in this case. The solubility, on the other hand, shows a large difference. The solubility product of rhodochrosite is approximately a factor 200 lower than that of calcite.[47,48] As presented above, in some of our experiments the solution concentration is determined by the presence of amorphous material. A rough estimate of the solubility of A-MnCO3 is 0.2 mM, which was determined by diluting A-MnCO3 suspensions until a clear solution was obtained. This is approximately a factor eight lower than for amorphous CaCO3 (1.7 mM).[49] On the basis of the above, the initial supersaturation of MnCO3 and CaCO3 for a 5 mM solution is and , respectively (Table ). The supersaturation during growth in the presence of the amorphous phase is for MnCO3 and for CaCO3. The experiments that result in epitaxial MnCO3 are thus performed at a much higher initial supersaturation than used in the experiments on CaCO3 growth reported earlier. To test if similar high (super)saturations for CaCO3 would yield epitaxial calcite crystals on freshly cleaved muscovite, we performed experiments using a concentration of 50 mM. This gives a driving force () that is similar to that of a 5 mM MnCO3 solution (). Using this CaCO3 concentration of 50 mM, epitaxy of CaCO3 was indeed achieved, as is shown in Figure a. The number of epitaxial CaCO3 crystals is smaller than when grown on muscovite exposed to humidity.[5] Moreover, most of these crystals are overgrown.

Figure 7

Scanning electron microscopy images of CaCO3 crystals grown on muscovite mica after 2 h of crystallization time at a concentration of (a) 50 mM and (b) 250 mM.

As the amorphous phase precipitates, the concentration will rapidly reach the solubility of the amorphous phase. After this time, the supersaturation is expected to be the same as for a concentration of 5 mM, only a larger amount of material precipitates. For 5 mM CaCO3 solutions, no epitaxial calcite is formed on freshly cleaved muscovite, while such growth occurs for 50 mM solutions. Since both concentrations are above the solubility of amorphous CaCO3, we conclude that the high initial concentration is important and that the nuclei for epitaxial CaCO3 are likely already formed in the initial phase. The high supersaturation may result in sufficiently large areas with a high local Ca2+ concentration in the geometry as sketched in Figure b that lead to nucleation. Further growth of the oriented nuclei leads to the observed epitaxial crystals. The same mechanism likely occurs occurs for MnCO3. While in both cases the supersaturation is similar, for MnCO3 the equilibrium concentration is much lower. This implies a higher surface free energy,[50] resulting in a higher activation barrier for secondary 3D nucleation, which leads to epitaxial crystals without the overgrowth (2), in contrast to CaCO3, where we do see this overgrowth (Figure a).

At a concentration of 250 mM and higher, all calcite crystals are overgrown, as can be seen in Figure b. Because of the overgrowth, it is unclear if the underlying crystals are epitaxial with the muscovite substrate. The calcite crystals differ in morphology from the spherical overgrown MnCO3 crystals in Figure c, but both form at high concentrations. Besides the overgrown calcite crystals, also vaterite crystals are formed (Supporting Information S4). Vaterite is known to be an intermediate phase for the formation of calcite.[51,52]

When the growth experiment is performed twice with 5 mM CaCO3, the calcite crystals continued to grow in a regular fashion, in contrast to MnCO3 (Figure ), as can be clearly seen from the persistence of the planar {104} facets (Supporting Information S5). Even when the two growth cycles with CaCO3 are performed with a 50 mM concentration (for both cycles) no differences in morphology are observed compared to the 5 mM case (Supporting Information S5). This again shows the effect of the solubility and thus supersaturation of both carbonates on the crystal growth and morphology.

Conclusion

We have demonstrated the epitaxial growth of MnCO3 crystals on the muscovite surface. These epitaxial crystals are kinetically roughened because of the high supersaturation. By repeating the crystallization procedure, many small crystals grow on the pre-existing epitaxial MnCO3 crystals, of which the underlying epitaxy remains visible. The formation of large spherical polycrystalline MnCO3 crystals is induced by the presence of K2CO3 crystallites formed by exposure of muscovite mica to a humid environment. Similar results can be achieved by using a higher MnCO3 supersaturation. Also epitaxy of CaCO3 on freshly cleaved muscovite was achieved by performing experiments at a similar supersaturation as that of MnCO3, showing that the initial supersaturation and not lattice match is the major parameter in achieving epitaxy.