Simple Shape-Selective Control of Germanium Pyroxene Crystals

Article pubs.acs.org/crystal

Simple Shape-Selective Control of Germanium Pyroxene Crystals Stanislav Ferdov* Department of Physics, University of Minho, 4800-058 Guimarães, Portugal S Supporting Information *

ABSTRACT: Molecular recognition at interfaces is essential approach for growth of crystals with selectively exposed facets. In general, this process is guided by impurities or solvents that favor the development of surfaces with particular reticular density. However, the specific agents that direct the crystal habit of one system often remain irrelevant for another, and thus, the control of crystal surface of many materials is still poorly known. Here we show selective resizing of prismatic crystal facets of germanium clinopyroxene (NaFeGe2O6) only by a simple switch between nitrate (Fe3+), chloride (Fe3+), and sulfate (Fe2+) sources of iron. The observed effect is explained by the combination of different rates of ion transport to the crystal surface and binding affinity of the hydrolyzed derivatives of the iron precursors that cap the crystal surface during the crystallization. Furthermore, this work reveals the first mild hydrothermal synthesis of germanium clinopyroxene.

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synthesis of vanadium silicate with pyroxene structure,26 we now reveal a mild hydrothermal synthesis of germanium pyroxene with aegirine structure.

INTRODUCTION Ability to control the shape of a crystalline solid is of fundamental importance for modern science. A common method to manipulate the crystal habit is the addition of capping agents that preferentially adsorb on specific crystallographic planes.1 Such preferential adsorption affects the growth rate of a crystal in an anisotropic way, resulting in modification of the final crystal shape.2 This approach is well-documented for the number of materials, but the synthetic protocols are usually valid for the particular system. For example, in the wellstudied system of simple oxide the addition of F− ions in the synthesis batch can selectively modify either {111} or {001} facets in the case of alfa-Fe2O33 and TiO2 (anatase),4 respectively. In more complex compounds the information about the control of crystal shape is less abundant or simply missing. Such is the case of the class of pyroxens. Pyroxenes are one of the most studied minerals in geology and mineralogy as they are important indicators for the conditions of rock formation on Earth.5 In materials science the pyroxens are emerging quasi-one-dimensional magnets and multiferroics.6−10 In spite of the longtime research, the synthetic protocols for pyroxens are poorly established, and the relationships between the synthesis conditions and crystal morphology are unknown. A revision of the literature showed that the germanium pyroxenes are typically prepared by a conventional solid-state reaction between the corresponding oxides at temperatures ranging between 800 and 1000 °C (LiCrGe2O6, NaCrGe2O6, LiFeGe2O6, NaFeGe2O6, CaFeGe2O6, NaVGe2O6)11−23 or by high-temperature (580−591 °C) hydrothermal synthesis (LiVGe2O6, NaVGe2O6).24 Some members as NaMnGe2O6 can be synthesized only by combination of high temperature and high pressure.25 Here we demonstrate control of the aspect ratio of iron germanium pyroxene crystals by a simple change of the iron precursor. Additionally, after the first mild hydrothermal © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Aggregates of prismatic crystals of NaFeGe2O6 with different aspect ratios were prepared by the hydrothermal method at identical molar compositions, temperature, and time for crystallization where the only difference was between the chemical sources of iron. The syntheses were performed from solutions with the following molar composition: 5.4 Na2O−0.2 Fe2O3−4.1 GeO2−1000 H2O. The iron precursors were iron sulfate (Fe2+SO4·7H2O), iron nitrate (Fe3+(NO3)3·9H2O), and iron chloride (Fe3+Cl3) (Sigma-Aldrich). All three syntheses were carried out by the following procedure: (1) 0.48 g of GeO2 (SigmaAldrich) was dissolved a solution of 0.48 g of NaOH (Sigma-Aldrich) and 10.05 g of H2O. Subsequently, (2) each one of the three iron sources (0.12 g of FeSO4·7H2O; 0.18 g of Fe(NO3)3·9H2O; and 0.07 g of FeCl3) (Sigma-Aldrich) was dissolved in 10.09 g of H2O and then added to a solution with composition (1). The obtained batches were stirred for 20 min, poured in Teflon-lined stainless steel autoclaves, and heated at 230 °C for 6 days. After being cooled with tap water, the obtained powders were filtered, washed with distilled water, and dried at room temperature. The pH was measured by indicator strips (Macherey-Nagel). The samples were examined by powder X-ray diffraction (XRD) using a Bruker D8 Discover diffractometer that worked with CuKα1,2 radiation. The diffractograms were measured in θ/2θ scan regime in the 2θ range 10−50° and 10−110°, step 0.04°, and time per step 1 and 15 s. The images of the samples were collected by scanning electron microscopy (SEM) using NanoSEM - FEI Nova 200 (FEG/SEM). Received: May 23, 2016 Revised: June 23, 2016

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DOI: 10.1021/acs.cgd.6b00773 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Article

RESULTS AND DISCUSSION Figure 1 shows SEM images of NaFeGe2O6 crystals synthesized by Fe(NO3)3·9H2O as a source of iron. The presence of nitrate

Figure 2. SEM images of the as-synthesized NaFeGe2O6 obtained by FeSO4·7H2O as a source of iron. (a) Aggregates of prismatic crystals with a length of more than 400 μm. (b) Single prismatic crystal with a high aspect ratio: 50/1. (c) Tip of twinned prismatic crystalthe circle shows a V-shaped sign of swallow tail twinning.

Figure 1. (a) SEM image of the as-synthesized NaFeGe2O6 obtained by Fe(NO3)3·9H2O as sources of iron. (b) Bundle-like aggregates of prisms with average length of 90 μm and width of about 10 μm in the middle, and about 40 μm in the periphery of the particle. (c) Closely packed prismatic crystals with two types of pyramidal facets (marked by arrows).

anions contributes to the crystallization of bundle-like aggregates of prisms with an average length of 90 μm, width of about 10 μm in the middle and about 40 μm in the periphery of the particle (Figure 1a,b). Because of spatial limitations some of the prismatic crystals also tend to be narrower in the middle and broader at the periphery of the particle (Figure 1c). A majority of the crystals show irregular intergrowth along their prismatic facets, and some of the surfaces are stepwise, which is a clear sign for twinning. There are two types of pyramidal facets that share contact surfaces with adjacent crystals (Figure 1c). When the synthesis of NaFeGe2O6 is carried out by FeSO4· 7H2O the crystals appear as elongated branches or euhedral prisms (Figure 2a,b) with an aspect ratio that acquires average values of about 30−50/1. The length of the prisms varies between 10 and 60 μm, and the width is between 0.3 and 2 μm. A majority of the prisms form aggregates whose lengths reach more than 400 μm. A closer look at the edge between pyramidal and prismatic facets reveals negative V-shaped signs of swallow tail twinning (Figure 2c). These observations demonstrate that the presence of sulfate anions effectively limits the lateral growth of prismatic facets, and as a result, pyroxene crystals with higher aspect ratio are grown. When the source of iron is FeCl3, the run product appears as shorter (50−70 μm) and wider (8−10 μm) single or multiple twinned crystals with well-defined prismatic facets (Figure 3a− c). Among them there is a distinctive combination between penetration and contact twins. Although the beta angle (107.56°) in the monoclinic unit cell of the clinopyroxene is far larger than 90° the sample shows pseudo cross-shaped penetration twins similar to that of the mineral staurolite (Figure 3b). Some of the pyramidal facets show a rough surface which is a sign of recrystallization (Figure 3c). These observations indicate that the Cl− anions favor the development of prismatic facets with a lower aspect ratio (5−7/1).

Figure 3. (a) SEM image of single and twinned prismatic crystals of the as-synthesized NaFeGe2O6 obtained by FeCl3 as sources of iron. (b) Combination of pseudo cross-like interpenetration and contact twinning. (c) Some of the pyramidal facets are not smooth and show signs of recrystallization.

In summary, the difference between the crystal morphologies obtained from nitrate (Fe3+)- and chloride (Fe3+)-containing solutions indicates that the anion part of the iron precursor plays a more important, shape-directing role than the initial oxidation state of the iron ion. Furthermore, pH values of the three solutions before and after the synthesis remained extremely high (around 14). Thus, the lack of clear difference between the alkalinity of the three batches suggests that the different solubility of the iron precursors in combination with the capping ability of the dissolved species influence the rate of ion transport to the crystal sturface. Correlating the order of solubility in water of the iron precursors (Fe(NO3)3·9H2O > FeCl3 > FeSO4·7H2O) and the obtained crystal morphologies, one can suggest that lower solubility of the chloride and sulfate sources contribulte to lower nucleation rate and formation of less aggregated crystals when compared with the nitrate-assisted synthesis. Another observation reveals that the crystals with the B

DOI: 10.1021/acs.cgd.6b00773 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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lowest aspect ratio were obtained by monatomic, monovalent anion (Cl−), and the crystals with the highest aspect ratio were grown in the presence of polyatomic and bivalent anion (SO42−). Finally, although the exact interactions between the hydrolyzed derivatives of iron precursors and the growing crystals facets can be speculated, the specific effect on the final morphology is clearly demonstrated. Figure 4 shows the powder XRD patterns of NaFeGe2O6 prepared by different precursors of iron ions. The morphology

Figure 5. Experimental (black) and simulated (red) powder XRD patterns for the Rietveld refinement of NaFeGe2O6.

interconnected by another chain of GeO4 tetrahedra that share common corners. Both chains are developed along the c-axis (Figure 6). The iron octahedron is distorted by three different

Figure 4. Powder XRD patterns of NaFeGe2O6 prepared by using FeSO4·7H2O (a), FeCl3 (b), and Fe(NO3)3·9H2O (c) as sources of iron.

of the sample synthesized in the presence of sulfate anions implies extreme texturing, and the diffractogram show a very strong preferential orientation along reflection (310) whose relative intensity is almost three times higher than the expected one (PDF No: 34-0068). Furthermore, other reflections as (1̅11) almost vanished, and (110) acquires a higher than the expected intensity (Figure 4a). These data suggest that the extremely elongated prismatic facets expose surfaces with higher reticular density. As expected from the crystal morphology the samples prepared in the presence of chloride and nitrate ions (Figure 4b,c) showed no preferential orientation. Because of the relatively higher crystallinity the crystals grown from nitrate-containing batches (Figure 4c) were chosen for detailed structural studies. The structural refinement was performed by the method of Rietveld using TOPAS (Bruker AXS TOPAS V3.0) software package. The starting model was the structure of iron germanium pyroxene (NaFeGe2O6) prepared by a solid-state reaction.21 The background was described by the Chebyshev polynomial function with 10 coefficients. The profile of the XRD pattern was described by the fundamental parameters approach (FPA) where the intensity profile is obtained from the X-ray emission, instrument and specimen dependent parameters. The variables that were dependent on the diffractometer were used together with a full axial divergence model. The Rietveld refinement fit of NaFeGe2O6 is shown in Figure 5. The final parameters of the refinement are summarized in Tables S1 and S2, while the selected bond distances and angles are shown in Table S3. Similar to previous reports,21 the refined XRD pattern shows that NaFeGe2O6 crystallizes in a typical pyroxene-type monoclinic structure composed of zigzag arranged chains of FeO6 octahedra that share common edges. These edges are

Figure 6. Structural model of the refined crystal structure of NaFeGe2O6.

Fe−O distances that range from 1.94 to 2.10 Å. The interconnection between the octahedra is achieved by an edge developed between the longest and shortest Fe−O bond. In a fair agreement with the previously reported structure of iron germanium pyroxen, the GeO4 tetrahedra show bond lengths (Ge−O) ranging from 1.73 to 1.77 Å.21 Sodium ion occupies small cavities between the two types of chains where it acquires a coordination of eight oxygen atoms. The refined structure was validated by the bond valence sums (SPuDS software) of Fe (2.96 v. u.), Ge (4.02 v. u.), and Na (1.02) which are in a fair agreement with the theoretical values of 3, 4, and 1, respectively.

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CONCLUSIONS This work reports for the first time a synthesis approach that controls the aspect ratio of germanium pyroxene crystals. The method is based on a simple change between three different C

DOI: 10.1021/acs.cgd.6b00773 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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precursors of iron ions. It is found that the initial batch that contains bivalent anion as SO42− tends to yield prismatic crystals with a higher aspect ratio than the batch that includes monovalent Cl− or NO3− anions. As a result, aggregates or single crystals with different aspect ratios based on selective resizing of the prismatic facets are achieved. Additionally, less water-soluble iron prcursors as FeCl3 and FeSO4·7H2O contribute to the formation of crystals that are less aggregated than the crystals obtained by a more soluble iron source (Fe(NO3)3·9H2O). This work also introduces for the first time a mild hydrothermal synthesis of germanium clinopyroxens. Finally, detailed structural information from powder XRD data is provided.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00773. Summarized results from the Rietveld refinement (PDF) Accession Codes

CCDC 1481681 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Tel. +351 253 510 468. Fax + 351 253 510 461. E-mail: sferdov@fisica.uminho.pt. Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fundaçaõ para a Ciência e a Tecnologia (FCT) − “Investigador 2013 (IF/01516/2013)”.

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REFERENCES

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DOI: 10.1021/acs.cgd.6b00773 Cryst. Growth Des. XXXX, XXX, XXX−XXX