A2+ Cation Control of Chiral Domain Formation in A(TiO)Cu4(PO4)4

Jan 14, 2016 - Our proposed mechanisms linking the chirality strength and domain formation can account for the different occurrence frequency of chira...
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A2+ Cation Control of Chiral Domain Formation in A(TiO)Cu4(PO4)4 (A = Ba, Sr) Kenta Kimura,* Masakazu Sera, and Tsuyoshi Kimura Division of Materials Physics, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan S Supporting Information *

A(TiO)Cu4(PO4)4 (A = Ba, Sr), hereafter called BTCPO and STCPO, respectively. We show that these materials are isostructural to the previously reported Ba(VO)Cu4(PO4)4 (called BVCPO) with tetragonal space group P4212.9 A full description of the crystal structure is given in the Supporting Information (SI). Surprisingly, the chiral domain size in the present system was found to strongly depend on the A-site cations. We explain this behavior in terms of the quantitative chirality strength based on the difference of the atomic positions in the chiral and nearest-achiral crystal structures. Single crystals of BTCPO and STCPO were synthesized from Na2Mo2O7 flux, as detailed in the SI. Plate-shaped single crystals of typical dimensions ∼5 × 5 × 0.5 and ∼3 × 3 × 0.3 mm3 for BTCPO and STCPO, respectively, were obtained. These crystals are transparent blue and have flat shiny faces corresponding to the c plane (Figure 1a,b). Powder X-ray diffraction (XRD)

ABSTRACT: Single crystals of two novel tetragonal chiral materials, A(TiO)Cu4(PO4)4 (A = Ba, Sr), were grown from Na2Mo2O7 flux, and their crystal and chiral domain structures were characterized. Polarized-light microscopy studies of the chiral domain structures in the crystals show that Ba(TiO)Cu4(PO4)4 mostly hosts a multidomain state, while a monodomain state predominantly appears in Sr(TiO)Cu4(PO4)4. To explain this striking difference, we quantified the chirality strength of these materials by comparing atomic positions in the chiral and nearestachiral crystal structures, revealing larger chirality strength in Sr(TiO)Cu4(PO4)4 than in Ba(TiO)Cu4(PO4)4. Our proposed mechanisms linking the chirality strength and domain formation can account for the different occurrence frequency of chiral domains in this system.

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hirality, a label defining the handedness of a system without improper mirror symmetry, has long been an intensive subject in a variety of fields such as chemistry, biology, particle physics, and solid-state physics.1,2 Its great importance in solidstate physics lies in the potential unique functionalities of chiral materials such as optical activity,3,4 piezoelectricity,5 and recently discovered unconventional magnetism and associated magnetotransport and magnetoelectric phenomena.6 Associated with these functionalities, tremendous efforts have been devoted to controlling the chirality by applying external stimuli, with some success particularly in molecular-based systems.7 Thus, it is natural to regard chirality as a “ferroic” property, analogous to magnetization in ferromagnets. An important feature of ferroic materials is the formation of domains and domain walls. Domain controllability is essential for application not only because the presence of domains generally degrades the ferroic properties but also because domain walls can provide novel functionalities absent in a mother material.8 In ferromagnets, it is well-known that the spontaneous domain formation is driven to reduce the magnetostatic energy. In contrast, there is no general guideline regarding a formation mechanism of chiral domains in chiral materials because the chirality itself does not have a counterpart to the magnetostatic energy in ferromagnets. To elucidate key factors for chiral domain formation, it is important to explore suitable chiral systems where the chiral domain state can be changed as a function of controllable parameters such as external stimuli and chemical substitution. Here, we report the synthesis, structural characterization, and observation of chiral domains in two new chiral materials, © XXXX American Chemical Society

Figure 1. Transmission optical microscopy images of single crystals of BTCPO (a) and STCPO (b). Transmission polarized-light microscopy images of BTCPO (c) and STCPO (d) taken by a monochromatic camera. A 450-nm-band-pass filter (fwhm, ±20 nm) was placed before the sample. Configurations of a polarizer and an analyzer are denoted by red and blue bars, respectively, along with the analyzer angle θA. Sketch of the domains of BTCPO (e) and STCPO (f) crystals, where the blue and red regions correspond to the levo (L) and dextro (D) rotatory domains, respectively.

profiles collected from crushed single crystals of BTCPO and STCPO revealed that all peaks can be indexed by the BVCPOtype crystal structure (Figure S1, SI), which, in collaboration with single-crystal XRD results (SI), allow us conclude that they are isostructural to BVCPO. A useful indicator of the chirality in a material is optical rotation, which can be observed by polarized-light microscopy. Figure 1c shows transmission polarized-light microscopy images Received: November 13, 2015

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DOI: 10.1021/acs.inorgchem.5b02622 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry of a thin-plate crystal of BTCPO (∼0.1 mm thickness), along with the polarizer−analyzer configuration. It is found that the analyzer rotation by a small angle +θA from the cross-polarized configuration divides the image into bright and dark regions, and the counterrotation (−θA) reverses the contrast. The contrast is affected by neither sample rotation nor flipping, indicating that the contrast arises from opposite optical rotation, that is, opposite chirality. By definition,3 the blue and red regions in a sketch of the domain structure (Figure 1e) correspond to the levo (L) and dextro (D) rotatory domains, respectively. Importantly, the area of the chiral domains in our single crystals is ∼1 mm2 on average and ∼5 mm2 in maximum. Monodomain crystals are extremely rare when crystals with an area of more than 1 × 1 mm2 are examined. Corresponding images and a sketch of the STCPO crystal (∼0.1 mm thickness) are shown in Figure 1d,f. In sharp contrast to BTCPO, STCPO crystals predominantly show a monodomain state, and hence the domain size often exceeds 5 × 5 mm2 in area, much larger than that of the BTCPO crystals. To quantitatively illustrate this difference, we examined the chiral domains of 50 pieces of randomly selected single crystals of BTCPO and STCPO with shiny flat faces. The area of these crystals ranged from 1 × 1 to 2 × 2 mm2 and the thickness from ∼0.1 to 0.4 mm. The result shown in Figure 2a

atomic arrangement characterizing the chirality is a possible origin for the different chiral domain states. To test this possibility, we quantify the chirality strength by comparing atomic positions in the chiral and nearest-achiral crystal structures. Figure 3a,c shows the crystal structures of C+

Figure 2. (a) Bar chart showing the number of BTCPO and STCPO crystals with a multidomain state and a monodomain state of levo (L) and dextro (D) rotation. (b) Rotation angles of each atom of BTCPO (blue circles), BVCPO (gray triangles), and STCPO (red squares).

and C− for BTCPO, respectively, viewed along the c axis.11 By averaging these structures, we obtain a nearest-achiral structure with space group P4/nmm (Figure 3b). Note that the crystal structure can be viewed as an alternating array of two square parts, α and β, each of which has a 4-fold rotational axis (4-axis) at its center (Figure 3a−c). Comparing structural fragments in square α of Figure 3a−c (Figure 3d−f, respectively) reveals that, in the chiral phase, all of the atoms rotate in the same direction around the 4-axis from the achiral phase, while the rotation direction is opposite between the C+ and C− structures. Moreover, the rotation direction of square β is opposite to that of square α. Thus, the symmetry-breaking atomic shift from the achiral structure can be characterized by the antiferroic rotation of atoms around the 4-axis, as denoted by curved arrows in Figure 3a,c. As a result, the rotation angle around the 4-axis, φ, must be a good quantitative descriptor of the chiral phase, i.e., the chirality strength. Because the rotation angle differs among atoms, we define φi for individual atoms i. As an example, φi for O(3) is depicted in Figure 3d. On the basis of the structural parameters of the C+ structure (Table S2, SI), we calculated φi for each atom. The result shows that all φi values are larger for STCPO than for BTCPO (Figure 2b), meaning that the former has stronger chirality than the latter. This is attributable to the different ionic radii of the Sr and Ba ions because increasing φi of all atoms in STCPO can shorten a bond length between the smaller Sr ion and its nearest coordinated O(2) with minimum changes in other cation− oxygen bond lengths (Figure S2, SI). Additionally, φi for BVCPO

Figure 3. (a−c) Levo (C+), dextro (C−), and nearest-achiral crystal structures of BTCPO viewed along the c axis. The Ba, Ti, and O(4) atoms not related to chirality symmetry breaking are omitted for clarity. In the achiral structure, O(2) and O(5) are equivalent and, hence, are labeled as O(2,5). The gray dotted line represents the unit cell. The black solid lines divide the crystal structure into two square parts, α and β. The filled square represents a 4-fold rotational axis (4-axis). (d−f) Fragments of the crystal structure in square α. The solid gray lines in part e represent mirror planes perpendicular to the ab plane, while the dotted gray lines in parts d and f indicate broken mirror planes due to atomic shift from the achiral structure. The rotation angle of O(3) atoms about the 4-axis is denoted in part d.

highlights the highly different occurrence frequency of chiral domains in BTCPO and STCPO crystals. This striking result is reproducible from batch to batch. (For STCPO, the number imbalance between the L and D rotatory crystals is observed in all of the batches examined in this study, while which one dominates is batch-dependent.) Because Sr and Ba ions have similar electronic and chemical properties, the result is linked to structural differences due to their different ionic radii. To elucidate structural differences, we performed singlecrystal XRD measurements on BTCPO and STCPO and refined their crystal structures. The Flack parameter was refined to establish the sign relationship between the optical rotation and structural chirality.10 Four small monodomain single crystals (L and D crystals of BTCPO and STCPO) were used for the measurements. The results are summarized in Tables S1 and S2. Within the error bar, the Flack parameters of the L and D crystals converged at 0 and 1, respectively, which indicates single chirality and thus establishes the one-to-one relationship between the optical rotation and chirality in A(TiO)Cu4(PO4)4. Here, we define the sign of the chirality of the L and D crystals as plus (C+) and minus (C−), respectively. Notably, as expected, there are finite differences in the structural parameters of BTCPO and STCPO. Because the lattice constants differ by only ∼1%, the B

DOI: 10.1021/acs.inorgchem.5b02622 Inorg. Chem. XXXX, XXX, XXX−XXX

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calculated from the previously reported structural data9 indicates that BVCPO has stronger chirality than BTCPO. Thus, the present chiral system, A(BO)Cu4(PO4)4 (A = Ba, Sr; B = Ti, V), offers a highly unique arena in which the chirality strength can be tuned by both A- and B-site cations. Here, we also evaluate the chirality strength using a method known as continuous chirality measures (CCM),12 by which the chirality is quantified based on the distance of the atoms to the nearest-achiral structure. The values of CCM calculated using the Internet service13 are fully consistent with the above-mentioned chirality strength (Figure S3, SI). The derived chirality strength can explain our observation that weakly chiral BTCPO crystals host a multidomain state, as follows. There are two possible mechanisms that link the chirality strength and chiral domain formation: (i) Structural phase transition. When the chirality is weaker, there might be a reversible structural phase transition from a chiral to an achiral structure at a high temperature. In this case, chiral multidomains can be formed upon cooling through the transition to the chiral phase. The presence of such an achiral−chiral phase transition in BTCPO is experimentally suggested by the change in the chiral domain patterns before and after heating above 700 °C (Figure S4, SI). In contrast, a domain pattern change was not observed in STCPO up to 1000 °C, suggesting no such phase transition. (ii) Domain wall energy. When the chirality is weaker, the domain wall energy should be smaller because the atomic arrangement of the C+ structures becomes less different from that of the C− structure. This apparently promotes chiral domain formation. Thus, both mechanisms i and ii based on the chirality strength can well explain our observation. Although, at present, we cannot identify which scenario (or both) is the case, the chirality strength must play a crucial role for chiral domain formation. An in situ high-temperature study on chiral domains using XRD and polarized-light microscopy would give crucial insight into the mechanism. Although quantitative chirality has been widely used to describe the chemical properties of molecular-based materials,14 there are only a small number of reports discussing a correlation between the quantitative chirality (and similar noncentrosymmetricity) and functional properties of materials such as optical rotatory power15 and second-harmonic-generation efficiencies.16 To our knowledge, the present study is the first to demonstrate that the quantitative chirality strength is a key factor for chiral domain formation. This finding may be useful for chiral domain engineering, and examining whether this is applicable to other chiral systems is of great interest. Moreover, the present discovery of the novel correlation triggers widespread investigations on the importance of the chirality strength in a variety of chiral phenomena. In summary, we have successfully synthesized two novel chiral materials, A(TiO)Cu4(PO4)4 (A = Ba, Sr), and revealed that the quantitative chirality strength of these materials can be tuned by the A cation. We have observed striking differences in the occurrence frequency of chiral domains in the two materials, which can be well explained in terms of the quantitative chirality strength. This result not only potentially provides a helpful guide for chiral domain engineering but also stimulates further exploration of chiral phenomena associated with the quantitative chirality strength. Finally, the correlation between the chirality strength and other material properties in this system such as the optical properties and magnetism carried by Cu2+ ions is particularly interesting.

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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.inorgchem.5b02622. Experimental details, description of the crystal structure, calculation of CCM, and domain observation after heating (PDF) X-ray crystallographic file in CIF format (CIF) X-ray crystallographic file in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Y. Wakabayashi for helpful discussion. This work was partially supported by JSPS KAKENHI Grants 26610103 and 24244058.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02622 Inorg. Chem. XXXX, XXX, XXX−XXX