Topochemical Crystal Transformation from a Distorted to a Nearly

Aug 3, 2017 - Single crystals are indispensable for understanding the fundamental properties of materials, although obtaining them are often challengi...
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Topochemical Crystal Transformation from a Distorted to a Nearly Perfect Kagome Cuprate Hajime Ishikawa,*,† Takeshi Yajima, Atsushi Miyake, Masashi Tokunaga, Akira Matsuo, Koichi Kindo, and Zenji Hiroi Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan S Supporting Information *

ABSTRACT: Single crystals are indispensable for understanding the fundamental properties of materials, although obtaining them are often challenging. To investigate the magnetic properties of a kagomelattice antiferromagnet, we have performed a topochemical crystal transformation from a copper mineral volborthite with a distorted kagome lattice into another copper mineral vesignieite with an almost perfect kagome lattice. A millimeter-sized crystal of vesignieite, which is difficult to prepare via direct chemical reactions, has been successfully obtained. Magnetization measurements on the crystals reveal a unique magnetic order with weak-ferromagnetic moments lying within the kagome plane and successive anomalies in the magnetization curve, which would give important information on the magnetic order of the kagome-lattice antiferromagnet. The present results demonstrate a novel route via the topochemical crystal transformation in preparing a hard-to-obtain crystal.

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INTRODUCTION Magnetic materials with two-dimensional kagome lattices made of corner sharing triangles have attracted much attention, because the geometrical frustration causes unusual magnetic ground states such as quantum spin liquids1,2 or unique magnetic orders with topological spin textures such as skyrmions.3 Various copper minerals have been studied as candidates for the kagome-lattice antiferromagnet (KAF): herbertsmithite ZnCu3(OH)6Cl2,4−7 volborthite Cu3V2O7(OH)2·2H2O,8−11 vesignieite BaCu3V2O8(OH)212−18 and its Sr-analogue SrCu 3 V 2 O 8 (OH) 2 , 19 kapellasite ZnCu3(OH)6Cl2,20 haydeeite MgCu3(OH)6Cl2,21 KCu 3 As 2 O 7 (OH) 3 , 22 edwardsite Cd 2 Cu 3 (OH) 6 (SO 4 ) 2 · 4H 2 O, 23 barlowite Cu 4 (OH) 6 FBr, 24 centennialite CaCu3(OH)6Cl2·H2O,25 CdCu3(OH)6Cl2,26 and CdCu3(NO3)2(OH)6·H2O.27,28 Some of them including herbertsmithite may realize quantum spin liquids, while many others show certain magnetic orders. This is because there are often additional effects in actual compounds that are not taken into account in the simple KAF with only the nearest-neighbor (NN) Heisenberg-type antiferromagnetic interactions; examples of the additional effects are anisotropy in magnetic interactions caused by complex orbital arrangements, furtherneighbor interactions, and the Dzyaloshinskii−Moriya (DM) interaction. Among many kagome copper minerals, only a few compounds have been extensively studied. One of the experimental obstacles in studying actual compounds is the lack of high-quality single crystal samples. To study the magnetic properties of materials in detail, especially in the case © 2017 American Chemical Society

of frustrated quantum magnets, a high-purity and highcrystallinity crystal is required because the magnetic ground states could be sensitively influenced by disorder and defects. Moreover, a large single crystal is desirable because magnetic signals can be small as a result of strong quantum fluctuations. For neutron scattering experiments, which are widely used to determine the magnetic structure of a quantum magnet and to observe magnetic excitations, at least millimeter-sized single crystals are required. A large single crystal of a copper mineral with high crystallinity is often found in nature. However, it is usually contaminated by magnetic impurities such as Fe atoms, which tend to mask the intrinsic magnetic properties. Therefore, a synthetic crystal is required, though it is always hard to mimic a natural crystal growth process. In recent years, successful crystal growths have been done for a few kagome minerals, which enabled precise characterizations of the magnetic properties and brought about new perspectives in the KAF. For example, the signatures of the spin liquid ground states are observed in the neutron scattering5 and nuclear magnetic resonance (NMR)6 experiments in herbertsmithite. More recently, a structural distortion is observed in the torque magnetometry and electron spin resonance experiments.7 We have synthesized single crystals of volborthite (Figure 1, left) and discovered some novel magnetic phases in high magnetic fields by the Received: April 10, 2017 Revised: June 28, 2017 Published: August 3, 2017 6719

DOI: 10.1021/acs.chemmater.7b01448 Chem. Mater. 2017, 29, 6719−6725

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Chemistry of Materials

molecules and CuO4(OH)2 octahedra. As a result, a proposed magnetic model for volborthite is far different from the simple KAF model with identical NN interactions (Figure 4a).11 On the other hand, herbertsmithite has Zn ions between the kagome layers and the dx2‑y2 orbitals carrying spins are arranged with keeping the 3-fold symmetry in the kagome layers. Vesignieite has VO4 tetrahedra and the Ba ions between the kagome layers (Figure 1, right). In this case, either d3z2−r2 or dx2‑y2 orbitals are considered to carry spins; there is a controversy on this issue as mentioned in the next paragraph. If the d3z2−r2 orbitals carry spins, vesignieite would have a regular kagome lattice to become a “structurally perfect” spin1/2 KAF as in herbertsmithite.4,14 The structural and magnetic properties of vesignieite remain still controversial despite a number of reports. Two similar structural models with the d3z2−r2 orbitals carrying spins are reported: one is in the monoclinic space group C2/m and the other in the trigonal space group R−3m, which are based on the single crystal X-ray diffraction (XRD) experiments on natural13 and synthetic14 single crystals, respectively. In the R−3m structure, the position of the VO4 tetrahedra is disordered. As a result of this randomness, the Cu ions form the perfect kagome lattice. In the C2/m structure, in contrast, the VO4 tetrahedra are ordered so as to cause a tiny distortion of the kagome lattice. Recently, another structural model with the trigonal space group P3121 is proposed based on the powder synchrotron XRD and neutron diffraction experiments.15 Different from what is expected for the C2/m and R−3m models, the dx2‑y2 orbitals carry spins in the P3121 structure. Moreover, a peculiar orbital state is proposed: the orbital is static at one of the two Cu sites, while it is dynamically fluctuating at the other site.15 As a result, the kagome lattice can be distorted. Concerning the magnetic properties of vesignieite, a certain magnetic transition is commonly observed at 9 K.14−16 In addition, a magnetization plateau-like state was observed at around the height of 2/5 of the full moment in a polycrystalline sample,17 which is apparently different from the 1/3 magnetization plateau state expected for the simple KAF.2 However, the spin structure of the ordered state and the origin of the unusual magnetization curve are not known. We would like to clarify these issues of vesignieite, which would shed light on the magnetic order of the KAF. As in the cases of herbertsmithite and volborthite, investigations using single crystals of vesignieite are required. However, a large crystal had not been obtained thus far, and characterizations were carried out using powder samples; the largest crystal reported is a thin hexagonal plate with approximately 0.1 mm length in edge,14 which is too small to perform detailed physical property measurements. We have attempted a direct synthesis of vesignieite under various hydrothermal conditions, but failed to obtain larger single crystals. In the present study, we performed a SCSC transformation from volborthite into vesignieite, taking into account the structural similarity between the two compounds (Figure 1). In fact, it was already reported that a powder sample of vesignieite was prepared by annealing a powder of volborthite in Ba(CH3CHOO)2 (aq).15 In addition, we already have large single crystals of volborthite.9 Through an optimization of reaction conditions, we have successfully obtained a high-quality and millimeter-sized crystal of vesignieite as shown in Figure 1; in terms of mineralogy, such a crystal may be called “vesignieite pseudomorph after

Figure 1. Changes in the appearance of crystal (upper row) and the crystal structure (lower row) before (left) and after (right) the topochemical reaction from volborthite to vesignieite. The crystal of volborthite before the reaction is twinned with a twin boundary at the middle; the arrowhead shape is made of the two triangular domains with the ab planes along the paper. After annealing at 200 °C in Ba(NO3)2 (aq), the overall shape is preserved with keeping the orientation of the kagome layers. In the crystal structure, the green and yellow units represent CuO4(OH)2 octahedra and VO4 tetrahedra, respectively. The blue and red balls represent H2O molecules and Ba ions, respectively.

high-field magnetization and NMR experiments, which were not observed in the previous low-quality samples.10 It is preferable to synthesize a large single crystal of the target material directly as for herbertsmithite and volborthite. However, finding an optimal condition for the crystal growth usually requires a number of trial-and-error and becomes more difficult when the number of component elements increases; two cation elements for the two compounds, while three for vesignieite. A possible alternative method is to obtain a crystal of a target material via a crystal of another material with a similar chemical composition and/or crystal structure. In the field of mineralogy, this process is known as pseudomorphosis. In the pseudomorphosis, a resulting mineral crystal maintains the shape of the original mineral crystal which does not occur in nature; it is called the pseudomorph. In materials science, artificial pseudomorphosis has been known as the single-crystalto-single-crystal (SCSC) transformation.29,30 The SCSC transformation has been mainly reported for compounds with rigid frameworks or layered structures. For example, an SCSC transformation occurs in various metal−organic frameworks by the exchange of gas or solvent molecules. Moreover, single crystals of layered materials such as the quantum magnet (CuCl)LaNb 2 O 7 31 and the superconductor (Li 1−x Fe x )OHFeSe32 were obtained by ion-exchange reactions from precursor single crystals of CsLaNb2O7 and K0.8Fe2Se2, respectively. Many copper minerals with kagome lattices commonly have layered structures comprising “kagome layers” made of edge sharing CuO4X2 octahedra (X = OH, Cl), where the Cu ions form a kagome lattice. Depending on the structural building block between the kagome layers, various arrangements of the spin-carrying Cu 3d orbitals appear in the kagome layer. In volborthite, the layers are separated by pairs of corner sharing VO4 tetrahedra and H2O molecules (Figure 1, left). The dx2‑y2 orbitals carry spins in the distorted kagome lattice without the 3-fold symmetry; the large deviation from the regular kagome lattice is likely caused by the hydrogen bonding between H2O 6720

DOI: 10.1021/acs.chemmater.7b01448 Chem. Mater. 2017, 29, 6719−6725

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Chemistry of Materials volborthite”. Magnetization measurements on thus obtained crystals reveal a unique magnetic order with a weak ferromagnetic (WF) moment lying within the kagome plane, which may be related to the ordering of vector chirality on the kagome lattice. Moreover, the presence of successive anomalies in the magnetization curve at around 5 and 40 T, which was not observed in powder samples, is evidenced.

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EXPERIMENTAL SECTION

We obtained arrowhead-shaped single crystals of volborthite by the hydrothermal method as reported:9 a large crystal is usually twinned and made of two triangular plates parallel to the ab plane (Figure 1). They were grown after a long hydrothermal reaction for about 30 days at 170 °C. It is known that there are two crystal polymorphs for volborthite with the space group C2/m and C2/c at room temperature; the C2/c structure has a doubled unit cell compared with the C2/m structure.9 We examined some of the precursor crystals by the single crystal XRD experiments and confirmed that they have the C2/c structure. For a topochemical reaction, approximately 10 mg of the precursor volborthite crystals were sealed in a Teflon-lined stainless autoclave of 45 cc in volume together with 10 cc of deionized water and 0.5 g of Ba(NO3)2 (99.9%, Raremetallic). The autoclave was heated at 200 °C for 24 h to complete the topochemical reaction. After the reaction, the color of the whole crystal had completely changed from dark green to light green, while the shape of the crystal is maintained. The obtained crystals were further annealed hydrothermally at 550 °C and 150 MPa for 12 h in a sealed Au tube, as done for a powder sample.16 The chemical analysis was performed by the inductively coupled plasma atomic emission spectroscopy (ICP-AES, JY138 KH ULTRACE, HORIBA) using a few crystals dissolved in HNO3(aq). The obtained atomic molar ratios are Cu/V = 3:1.99 for precursor volborthite and Ba/Cu/V = 1:2.87:1.99 for the annealed SCSC crystal of vesignieite. The result suggests that the transformation is almost complete, but there might be a small Cu deficiency in the annealed SCSC crystal of vesignieite. Single crystal XRD experiments were performed by using a diffractometer with an imaging plate (R-AXIS RAPID, Rigaku) with Mo Kα radiation. The single crystal XRD data are refined by using the SHELX software.33 Powder XRD experiments were performed in a diffractometer with Cu Kα radiation (RINT 2000, Rigaku) at room temperature. Magnetization was measured at temperatures between 1.8 and 300 K and in magnetic fields up to 7 T in a SQUID magnetometer (MPMS3, Quantum Design). High-field magnetization measurements up to 72 T were performed in 4 ms pulsed magnetic fields generated by a nondestructive magnet at International MegaGauss Science Laboratory in ISSP.

Figure 2. (a) XRD pattern of topochemically obtained crystals of vesignieite: annealed SCSC crystal measured with the scattering vector perpendicular to the crystal surface (top, magenta), powder patterns from crushed annealed SCSC crystal (second, red) and crushed asprepared SCSC crystal (third, blue) and the calculated powder patterns for the R−3m structure of vesignieite (fourth, black) and for the C2/c structure of volborthite (bottom, green). (b) Evolutions of the (006) and (015) peaks via hydrothermal annealing at 550 °C and 150 MPa for 12 h; as-prepared and annealed SCSC crystals in blue and red colors, respectively. The full width at half-maximum of the (006) reflection is 0.12° in both of the samples, while those of the (015) reflection are 0.31° for the as-prepared crystal and 0.22° for the annealed SCSC crystal.

the reaction, i.e., the reaction has occurred topochemically. Note that most of the XRD peaks are broad in the as-prepared SCSC crystal and become sharper in the annealed SCSC crystal. Specifically, the already sharp 00l reflections remain almost the same, while other peaks become sharper (Figure 2b). This result suggests that the crystallinity is improved by the high-temperature hydrothermal annealing. We performed single crystal XRD experiments for a small piece of crystal with approximately 0.05 × 0.05 × 0.02 mm3 size cut from an annealed SCSC crystal. The XRD data obtained at 293 K can be refined in the R−3m model of vesignieite with a = 5.9415(5) Å, c = 20.829(2) Å, and with small R values (R1 = 0.029, wR2 = 0.061; the cif file is attached as Supporting Information) which are close to those reported for the synthetic single crystal.14 A test refinement with the occupancy at the Cu site as a free parameter results in the site occupancy of 0.99(1), suggesting Cu site is fully occupied. The reason for the discrepancy between ICP-AES and single crystal XRD results is not clear, but the Cu deficiency is at most 4%. Although our XRD data are compatible with the R−3m model, it is difficult to exclude the possibility of the other structures. The crystal becomes less transparent after the transformation (Figure 1) suggesting that domains have been introduced. However, note

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RESULTS AND DISCUSSION Figure 2 shows powder XRD patterns of finely ground crystals after the SCSC transformation. One is an “as- prepared SCSC crystal” and the other is after hydrothermal annealing at 550 °C, which is designated as “annealed SCSC crystal”. They are compared with the calculated powder patterns of R−3m structure of vesignieite and the C2/c structure of volborthite. The absence of peaks from the precursor volborthite indicates that a certain chemical reaction has occurred on the entire crystal. The powder pattern of the annealed crystals is similar to that calculated for the R−3m structure of vesignieite14 with a trigonal unit cell of a = 5.939(1) Å and c = 20.830(1) Å: the possibility of the C2/m or the P3121 structures will be discussed later. Moreover, in the XRD pattern measured on a millimetersized annealed SCSC crystal with the scattering vector normal to the plate surface (first row in the Figure 2a), only 00l (l = 3n: n is an integer) reflections are observed. This means that the orientation of the kagome planes is perfectly maintained over 6721

DOI: 10.1021/acs.chemmater.7b01448 Chem. Mater. 2017, 29, 6719−6725

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Chemistry of Materials

Figure 3. (a,b) Temperature dependences of magnetic susceptibility χ of an annealed SCSC crystal of vesignieite measured at 1 T (a) and 0.1 T (b). The black and red lines represent data sets for H∥ab and H⊥ab, respectively. In panel a, the χ of a volborthite crystal measured at H = 5 T and ∥ab (along the twin boundary) is shown by the blue line. The green dashed lines on the data sets of vesignieite indicate Curie−Weiss fittings described in the text. (c) Magnetization curves of a vesignieite crystal at 1.8 and 20 K below 7 T in H∥ab (black) and H⊥ab (red). (d) High-field magnetization curve of annealed SCSC crystals of vesignieite up to 72 T (H⊥ab, red) and its field derivative (gray). The measurements were performed using a stack of approximately 20 crystals. The magnetization curve of volbortite single crystals (H⊥ab, blue) is shown for comparison: the data for volborthite is taken from ref 10.

The small difference in μeff should come from a difference in the g values: g = 2.26 and 2.34 for H∥ab and H⊥ab, respectively. The χ of the vesignieite SCSC crystal is significantly different from that of volborthite, indicating that the magnetic interactions in the common kagome layers are quite different between the two compounds, which is due to differences in the bond distances and bond angles depending on the interlayer species. At low temperatures below 20 K, a large anisotropy is observed depending on the field direction: the χ increases steeply at H∥ab, while it shows a broad peak at H⊥ab. At lower temperatures, a sharp peak appears at TN = 9 K at H = 0.1 T (Figure 3b), where a magnetic transition is reported in previous studies. Note that the sharpness of the transition showed a large sample dependence in the previous studies: a sharp peak was observed in a small single crystal14 and in a powder sample postannealed at 550 °C,16 while a much broader peak was observed in powder samples prepared at lower temperatures.12,15 The transition observed in our SCSC crystal is as sharp as those of the former samples,14,16 suggesting that our crystals are of high quality from the viewpoint of magnetism. It is noted that the transition is slightly broad in the as-prepared SCSC crystal and becomes sharper in the annealed SCSC crystal. The large anisotropy in χ at low temperatures becomes evident in the magnetization curves shown in Figure 3c. In H∥ab, a steep increase with a small hysteresis is observed at around 0.2 T. The small net magnetic moment estimated by the linear interpolation of the magnetization curve above 2 T is 0.018 μB−Cu−1, which corresponds to 1.6% of the full moment considering the g-value. In sharp contrast, such a WF moment

that the crystal is not a polycrystal made of randomly oriented domains but a pseudocrystal with mosaicity, which consists of domains with the kagome planes nearly parallel to each other and with slightly misoriented in-plane directions; this is evidenced by the reasonably small R values in the refinements. A test refinement of the single crystal XRD data assuming the C2/m model gives slightly larger R values (R1 = 0.035, wR2 = 0.066) than for the R−3m model, while that assuming the P3121 model did not provide a reasonable result. If the crystal had lower symmetry structures, the seemingly high symmetry of R−3m could be a result of averaging out over domains with lower symmetry. Anyway, the crystal is not suitable for a complete structural determination, but can be used to study the anisotropy of the magnetic properties for magnetic fields parallel and perpendicular to the kagome plane thanks to the pseudo-single-crystal form. The temperature dependence of the magnetic susceptibility χ of one annealed SCSC crystal of vesignieite is shown in Figure 3a; that of volborthite is also shown for comparison. We could obtain two data sets at H∥ab and H⊥ab. They are nearly identical at high temperatures and can be fitted by the Curie− Weiss law with a constant term χ0, χ = C/(T + Θ) + χ0, where the C and Θ are the Curie constant and the Weiss temperature, respectively. Good fittings between 150 and 300 K are obtained with the parameters C = 0.483(16) and 0.513(17) cm3 K mol−Cu−1 for H∥ab and H⊥ab, respectively, and the common Θ = 71(5) K and χ0 = −3(3) × 10−5 cm3 mol−Cu−1. The Curie constants correspond to the effective magnetic moments μeff of 1.96 and 2.02 μB for H∥ab and H⊥ab, respectively. Note that the obtained Θ and μeff are similar to those reported in the powder samples: Θ = 77−85 K and μeff = 1.98−2.03 μB.12,14,15 6722

DOI: 10.1021/acs.chemmater.7b01448 Chem. Mater. 2017, 29, 6719−6725

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Chemistry of Materials

Figure 4. (a) Magnetic interactions in the kagome net proposed for volborthite,11 which contain ferromagnetic NN interactions J1 (red solid lines) and J″ (red dashed lines), antiferromagnetic (AF) NN interaction J′ (blue solid lines), and AF NNN interaction J2 (blue arcs). The strongest AF J′ stabilizes the 1/3 plateau state at high magnetic fields. (b) Possible magnetic interactions of vesignieite, which contain AF NN interactions and DM interactions. The D-vector of the DM interaction is made of the out-of-plane component Dz and the in-plane component Dp:32 the spin product in D·[Si × Sj] is defined on the triangles in the direction of the red arrows. (c, d) Possible magnetic structures of KAF stabilized by the DM interaction: (c) PVC structure stabilized for Dz > 0 and (d) NVC structure stabilized for Dz < 0. In the presence of the easy-axis depicted by the red dashed lines, the NVC order can show in-plane weak ferromagnetism, while the PVC order does not.

stabilized. In the case of Dz < 0, another 120° structure without out-of-plane moment, so-called inverse triangular structure (Figure 4d), is stabilized. The former and the latter are characterized by positive vector chirality (PVC) and negative vector chirality (NVC), and thus are called the PVC and NVC structures, respectively.28 Provided that vesignieite is described in terms of a regular KAF model with DM interactions, the absence of out-of-plane WF moment indicates that the NVC structure is realized unless Dp = 0 accidentally. The previous electron spin resonance study on the powder sample of vesignieite suggests the presence of sizable Dp,18 and may support the NVC scenario. Moreover, it is known that the NVC structure can show an in-plane WF moment caused by the easy-axis anisotropy within the kagome plane as shown in the Figure 4d.35,36 Thus, the in-plane WF moment observed in vesignieite is consistent with the NVC structure. A similar NVC structure with in-plane WF moments has been observed in the intermetallic compound Mn3Sn,35,36 and, more recently, in the copper mineral CdCu3(NO3)2(OH)6·H2O.28 In the previous zero-field Cu NMR experiments on the powder sample of vesignieite; however, the PVC structure was proposed because a single internal field was observed:16 two kinds of internal fields should be observed in the NVC structure from the symmetry of the spin structure. This controversy could be explained if the two internal fields happen to have close magnitudes or if one of them is too small to be observed because of a large anisotropy with a sign change in the hyperfine coupling. Effects of possible lattice distortions in the SCSC crystal of vesignieite on its magnetic properties are not clear. We speculate that a combination of small lattice distortion and DM interaction stabilizes the slightly modified PVC or NVC structures and the presence of the WF moment confined in the kagome plane is better explained by the NVC structure.

is absent for H⊥ab. This means that the WF moment lies within the kagome plane. The WF moment is absent at 20 K (Figure 3c) and appears below TN, indicating that the origin is a spin canting in the AF order: it is absent in the precursor volborthite. The anisotropy of χ below 20 K may indicate that a short-range spin correlation is already present above the TN. We have carried out the high-field magnetization measurements up to 72 T to explore a field-induced transition. Figure 3d shows a magnetization versus H curve in H⊥ab; that of volborthite is shown for comparison. Volborthite shows a clear 1/3 magnetization plateau above 30 T,10 which is likely stabilized by the unique pattern of F and AF interactions in the kagome lattice as shown in Figure 4a. In vesignieite, in contrast, we have observed two anomalies in the magnetization curve at around 5 and 40 T (Figure 3c,d): the slope of the curve increases around 5 T, while it decreases around 40 T. At 40 T, where the field derivative of the magnetization curve dM/dH drops, the magnitude of magnetization coincides approximately 1/3 of the full moment considering the g-value. Above 55 T, the dM/dH becomes flat, indicating that the magnetization increases almost linearly. Thus, in our data, a 2/5 magnetization plateau previously reported in the powder sample is absent.17 Instead, there are two field-induced transitions at around 5 and 40 T possibly reflecting certain changes in the spin structure. The origin of the successive magnetic anomalies in vesignieite is not clear at present. We consider that a DM interaction, which would be important in vesignieite as discussed in the next paragraph, has transformed the 1/3 magnetization plateau2 into another magnetic state. The in-plane WF moment revealed in the present study gives a clue to narrowing down the possible magnetic structure. In the previous studies, it was pointed out that a main perturbation from the simple KAF model with NN interaction is the DM interaction in vesignieite.18 In the regular kagome lattice, the D-vector D of the DM interaction D·[Si × Sj] is composed of the out-of-plane component Dz and the in-plane component Dp as depicted in Figure 4b.34 The Dz stabilizes coplanar 120° spin structures, while the Dp induces out-of-plane WF moment. A theoretical study has shown that, in the presence of both Dz and Dp, either of the two 120° structures shown in Figures 4c and d is stabilized depending on the sign of Dz.34 In the case of Dz > 0, 120° structure with an out-of-plane WF moment, so-called the q = 0 structure (Figure 4c), is

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CONCLUSION We have successfully obtained a single crystal of a KAF vesignieite by a topochemical pseudomorphosis from volborthite. Discovered in the crystal is the presence of an in-plane WF moment and two anomalies in the magnetization curve, which have not been observed in previous studies using powder samples. It is suggested that a NVC structure is stabilized below 6723

DOI: 10.1021/acs.chemmater.7b01448 Chem. Mater. 2017, 29, 6719−6725

Article

Chemistry of Materials

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9 K owing to the DM interactions. Our pseudocrystal of vesignieite would be useful for further experiments to investigate the unique magnetic order of kagome magnet. Particularly, inelastic neutron scattering experiments could reveal magnetic excitations, which are not powder-averaged but spatially resolved and would make it possible to determine the magnetic interactions involved. More generally, our study demonstrates that a topochemical crystal transformation is efficient for the synthesis of a hard-to-obtain crystal, which would be of great advantage for investigations of their properties.

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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.chemmater.7b01448. Single crystal XRD data and refinement result of vesignieite annealed SCSC crystal (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hajime Ishikawa: 0000-0002-1303-9460 Present Address †

H.I.: Institute for Functional Matter and Quantum Technologies, University of Stuttgart, Stuttgart, Germany) Author Contributions

The manuscript was written mainly by H.I., T.Y., and Z.H. H.I. performed crystal growth. H.I. and T.Y. performed XRD experiments. H.I., A.M., M.T., A.M., and K.K. performed magnetization measurements. Funding

H. I. was supported by research fellowship of Japan Society for the Promotion of Science (JSPS) DC2 and its Program for Leading Graduate Schools (MERIT). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate Ryutaro Okuma, Masataka Iwaki, Makoto Yoshida, Hikaru Takeda, and Masashi Takigawa for discussions about the magnetic structure of vesignieite. We appreciate Masayoshi Koike for the ICP-AES experiments.

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REFERENCES

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DOI: 10.1021/acs.chemmater.7b01448 Chem. Mater. 2017, 29, 6719−6725

Article

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DOI: 10.1021/acs.chemmater.7b01448 Chem. Mater. 2017, 29, 6719−6725