Synthesis of Mn-bearing layered perovskite-type niobate and its delaminated nanosheet

Abstract

Layered perovskite-type compounds with the general formula MCa₂MnNb₃TiO₁₃ (M = K, Rb), in which Mn²⁺ is incorporated into the A-site of the perovskite layers, were synthesized via a unique conventional solid-state reaction. The synthesis was conducted by thoroughly mixing a layered perovskite precursor, MCa₂Nb₃O₁₀, with ilmenite-type MnTiO₃ in a stoichiometric molar ratio, followed by high-temperature calcination at 1373 K. Compositional and structural characterizations involving elemental analysis and Rietveld refinement confirmed that the perovskite slabs in the host layers are four [Nb,Ti]O₆ octahedra thick, with Ca²⁺ and Mn²⁺ ions occupying the A-sites. Ion exchange treatment with an acid solution effectively replaced the interlayer alkali metal ions (K⁺ or Rb⁺) with protons. Subsequent exfoliation by shaking in an aqueous tetrabutylammonium hydroxide solution yielded a light brown suspension. The colloidal material dispersed was deposited onto a Si substrate surface. Atomic force microscopy (AFM) revealed the presence of numerous micrometer-sized 2D nanosheets with a uniform thickness of approximately 2.0 nm, indicating complete exfoliation into single perovskite layers. Further in-depth analysis via in-plane and out-of-plane X-ray diffraction (XRD), as well as X-ray photoelectron spectroscopy (XPS), confirmed that the obtained 2D materials correspond to individual perovskite layers derived from the Mn-substituted parent phase.

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1 Introduction

A class of compounds known as layered perovskite oxides has garnered significant attention owing to a wide range of functional properties such as dielectricity, ferroelectricity, and photocatalytic activity based on their diverse compositions and structures.^1–5 Among them, compounds with the general formula MA_n−1B_nO_3n+1 (where M denotes alkali metals such as K, Rb, or Cs; A represents alkaline earth metals such as Ca or Sr; and B corresponds to transition metals such as Ti, Nb, or Ta) are classified as Dion–Jacobson-type layered perovskites.^6–9 A representative member of this series is KCa₂Nb₃O₁₀, which features a layered structure consisting of perovskite-type slabs composed of three corner-sharing NbO₆ octahedra stacked along the crystallographic c-axis. The interlayer region is occupied by alkali metal cations, which compensate for the negative charge of the perovskite layers. These interlayer cations are readily exchangeable under ambient conditions, enabling the intercalation of various organic molecules or metal complexes to form hybrid nanocomposites.^10–15 Furthermore, this ion-exchangeability allows for the exfoliation of the layered structure into unilamellar 2D nanosheets, which has been an active area of research in recent decades.^16–19 A series of related compounds with homologous structures, in which the thickness of the perovskite slabs varies with the number of NbO₆ octahedra, has also been reported.^7,16,20,21 These compounds are generally represented by the formula KCa₂Na_n−3Nb_nO_3n+1, where n indicates the number of octahedral layers in the perovskite slab. Phases with n = 3 or 4 can be synthesized via conventional solid-state reactions involving stoichiometric mixing of precursor oxides followed by high-temperature calcination (typically in the range of 1273–1473 K). However, such conventional methods are generally ineffective for synthesizing higher-order phases with n ≥ 5. Alternatively, these homologous phases can be obtained through a stepwise reaction approach. For example, heating a 1 : 1 molar mixture of KCa₂Nb₃O₁₀ and NaNbO₃ results in the formation of an n = 4 phase, in which one additional NbO₆ octahedral layer is incorporated into the original perovskite framework, as described in the eqn (1).

KCa₂Nb₃O₁₀ + NaNbO₃ → KCa₂NaNb₄O₁₃ (1)

Similarly, by adjusting the molar ratio of NaNbO₃ to 2, 3, or 4 during thermal treatment, a series of homologous layered oxides with n = 3–7 was successfully synthesized. Owing to the fact that the thickness of the perovskite layers in these compounds can be precisely tuned in increments of approximately 0.4 nm—corresponding to the height of a single NbO₆ octahedron—this system has been extensively studied as a model platform for investigating structure-property relationships in layered perovskite oxides. We exfoliated the compounds with n = 3 to 6 into single-layer nanosheets and investigated their dielectric properties. As a result, we observed a stepwise increase in the relative dielectric constant from ∼200 to ∼450 with increasing, indicating that these nanosheets exhibit excellent dielectric performance despite their nanometer-scale thickness.^20,22 Furthermore, as indicated in the eqn (2a) and (2b), analogous layered structures can be obtained by employing other perovskite-type oxides, such as CaTiO₃ or SrTiO₃, demonstrating the versatility of this synthetic approach.⁸

KCa₂Nb₃O₁₀ + CaTiO₃ → KCa₃Nb₃TiO₁₃ (2a)

KSr₂Nb₃O₁₀ + SrTiO₃ → KSr₃Nb₃TiO₁₃ (2b)

In a manner analogous to the previously reported approach, homologous phases with the general formula KCa_n−1Nb₃Ti_n−3O_3n+1 (n = 4, 5) can be synthesized by tuning the molar ratio of CaTiO₃ in the reaction mixture. In these cases, Ca or Sr ions are incorporated into the A-site of the perovskite layers in place of Na ions as in the eqn (1), while Ti is co-substituted with Nb at the B-site. These results indicate that, although the thermodynamic stability of such homologous layered perovskites generally decreases with increasing slab thickness – making their synthesis via conventional solid-state methods more difficult – the reactions represented by eqn (1) and (2) enable the formation of higher-order members in the homologous series. This is likely facilitated by the layered perovskite matrix serving as a structural template, allowing for the controlled insertion of additional NbO₆ or TiO₆ octahedra and thereby incrementally increasing the perovskite slab thickness on a layer-by-layer basis. This templating effect may provide a route to access novel layered perovskite compounds with unique compositions and architectures that are otherwise unattainable by conventional synthetic methods. In the present study, we explored whether the synthetic strategy described by eqn (1) and (2) could be extended to ilmenite-type MnTiO₃. While layered perovskite oxides exhibit considerable diversity in composition and structure, their A-sites are predominantly occupied by alkali or alkaline earth metal ions. The successful integration of such transition metals into the layered perovskite framework would significantly broaden the functional landscape of these materials, particularly with respect to their magnetic and redox-related properties.

2 Experimental

2.1 Reagents and materials

Chemicals—K₂CO₃, Rb₂CO₃, CaCO₃, Nb₂O₅, TiO₂, and MnO (all ≥99.9% purity, supplied by Rare Metallic Co.)—were used for the synthesis. All other chemicals used in this study were of analytical grade or higher. Ultrapure water (Milli-Q, resistivity >18 MΩ cm) was used throughout all experimental procedures. The layered perovskite compounds KCa₂Nb₃O₁₀ and RbCa₂Nb₃O₁₀, serving as the basic frameworks, were synthesized by mixing K₂CO₃ or Rb₂CO₃, CaCO₃, and Nb₂O₅ in a molar ratio of 1.1 : 4 : 3, followed by calcination at 1473 K for 10 h in a platinum crucible.^6,21 The ilmenite-type oxide MnTiO₃ was prepared by calcining a stoichiometric mixture of MnO and TiO₂ at 1473 K for 12 h.²³ The target compounds, KCa₂MnNb₃TiO₁₃ and RbCa₂MnNb₃TiO₁₃, were synthesized by solid-state reaction between KCa₂Nb₃O₁₀ or RbCa₂Nb₃O₁₀ and MnTiO₃ in a 1 : 1 molar ratio, followed by two successive calcination steps at 1373 K for 96 h each. To exchange the interlayer K⁺ or Rb⁺ ions with protons, 5 g of the resulting product was stirred in 200 cm³ of 5 M HNO₃ aqueous solution for 72 h.

2.2 Exfoliation

To achieve exfoliation into individual nanosheets, 0.4 g of the acid-treated layered compound HCa₂MnNb₃TiO₁₃·1.5H₂O was dispersed in 100 cm³ of an aqueous tetrabutylammonium hydroxide (TBAOH) solution and agitated using a mechanical shaker at 170 rpm for 7 days. The concentration of TBAOH was adjusted to be stoichiometrically equivalent to the amount of exchangeable protons in HCa₂MnNb₃TiO₁₃·1.5H₂O.

2.3 Film fabrication

To characterize the obtained nanosheets by atomic force microscopy (AFM) and in-plane/out of plane X-ray diffraction (XRD), the nanosheets were deposited onto Si substrates using the Langmuir–Blodgett (LB) technique. Prior to deposition, the Si substrates were cleaned by immersion in a HCl/CH₃OH (1 : 1 v/v) solution for 30 min, rinsed thoroughly with Milli-Q water, immersed in concentrated H₂SO₄ for another 30 min, and finally washed again with Milli-Q water. A 10 cm³ aliquot of the nanosheet suspension was centrifuged at 1500 rpm for 10 min to remove unexfoliated aggregates. The supernatant (5 cm³) was then diluted with water to a final volume of 250 cm³. This diluted suspension was spread onto the water surface in a LB trough, and a monolayer film of nanosheets was transferred onto the substrate at a surface pressure of 10 mN m⁻¹ with a lifting speed of 0.0167 mm s⁻¹.

2.4 Characterizations

Powder XRD measurements were carried out using a Rigaku Ultima IV diffractometer equipped with graphite monochromatized Cu Kα radiation source (λ = 0.15405 nm), and a scintillation counter. Crystal structure was analyzed by Rietveld refinement with the program JANA2006.²⁴ AFM observations were performed using an AFM5000II (Hitachi High-Tech Corp.). In-plane XRD and X-ray absorption near edge structure (XANES) measurements were conducted at beamline BL-6C of the High Energy Accelerator Research Organization (KEK) synchrotron radiation facility. XANES spectra were acquired in transmission mode using pelletized samples prepared by mixing each powder sample with hexagonal boron nitride (h-BN). X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI Quantes system (ULVAC-PHI). The samples for XPS measurements were prepared by drop-casting a diluted nanosheet suspension onto Si substrates, allowing them to stand for 10 min, removing the excess liquid, and drying the substrates prior to measurement. For chemical analysis, a weighed amount of sample was dissolved in a mixed acid solution of concentrated H₂SO₄ and HF, and the resulting solution was examined using inductively coupled plasma (ICP) atomic emission spectrophotometry (1700HVR, Seiko Instruments Inc.) and atomic absorption spectrophotometry (SpectrAA-20, Varian, Inc.). Water content was deduced from weight loss up to 1273 K using thermogravimetric-differential thermal analysis (TG-DTA, Rigaku TGA-8120).

3 Results and discussion

To evaluate the feasibility of the synthesis route described in eqn (3a) and (3b), equimolar amounts of MnTiO₃ were mixed with polycrystalline KCa₂Nb₃O₁₀ and RbCa₂Nb₃O₁₀. The resulting mixtures were subjected to two successive heat treatments at 1473 K for 96 h each.

KCa₂Nb₃O₁₀ + MnTiO₃ → KCa₂MnNb₃TiO₁₃ (3a)

RbCa₂Nb₃O₁₀ + MnTiO₃ → RbCa₂MnNb₃TiO₁₃ (3b)

Fig. 1 displays XRD patterns of the obtained samples. The basal reflection peaks indicate an interlayer spacing of approximately 1.87 nm for both samples, representing an expansion of about 0.4 nm relative to the parent compounds KCa₂Nb₃O₁₀ and RbCa₂Nb₃O₁₀ (1.48 nm; see Fig. S1). The observed expansion was similar to that occurred in the reactions of (1) and (2), suggesting the formation of the target phase, in which the host perovskite layers are expanded by an additional TiO₆ octahedral unit. Most diffraction peaks observed for the K-containing sample could be indexed to a C-centered orthorhombic unit cell, while those for the Rb-containing sample corresponded to a body-centered lattice. The refined lattice parameters are summarized in Table 1. However, certain reflections were attributed to supercell structures—2 × 2 × 2 for the former and 2 × 2 × 1 for the latter. Such superstructures have been reported in various layered perovskite-type oxides.^6,25 Taking these superstructures into account, all diffraction peaks were successfully assigned to an n = 4 member of a homologous series, indicating that the target phase was obtained as a single-phase product. As a control experiment, direct synthesis of the target phase was attempted using stoichiometric mixtures of carbonates and oxides (Rb₂CO₃, CaCO₃, MnO, TiO₂, Nb₂O₅). The results revealed the formation of a multiphase product containing Mn₃O₄, CaMnO₃, and other impurities (Fig. S2). These results suggest that the synthetic route described in eqn (3), involving precursor phases of lower n values, is more effective for preparing the higher-order target phase.

Fig. 1 Powder XRD patterns of (a) KCa₂MnNb₃TiO₁₃ and (b) RbCa₂MnNb₃TiO₁₃.

Table 1 Lattice parameters of layered perovskite

Nominal composition	a/nm	b/nm	c/nm
a The upper column lists the lattice constants for the fundamental lattice, while the lower column indicates the lattice parameters based on the superlattice structures, if any.
KCa2MnNb3TiO13
(Fundamental)	0.39309(9)^a	0.38415(8)^a	3.7311(6)
(Superlattice)	0.7872(1)^a	0.7684(1)^a	3.7306(4)
RbCa2MnNb3TiO13
(Fundamental)	0.3828(1)^a	—	1.8658(6)
(Superlattice)	0.7657(3)^a	—	1.8659(7)
KCa2Nb3O10	3.8799(8)	0.7719(1)	2.9547(5)

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Next, the obtained materials were treated with acid solutions to replace the interlayer K⁺ or Rb⁺ ions with protons. Fig. 2 presents XRD patterns of the acid-treated samples. The lattice parameters of the sample derived from the K-form were determined to be a = 0.38374(5) nm and c = 2.0092(3) nm, indicating an expansion of the interlayer spacing by approximately 0.13 nm. A similar XRD profile was also observed for the acid treatment sample prepared from the Rb form (Fig. S3). This variation in lattice constants is consistent with those reported for Dion–Jacobson-type layered perovskites such as KCa₂Nb₃O₁₀, and is attributed to the intercalation of oxonium ions into the interlayer region.⁷ Moreover, TG-DTA revealed the presence of approximately 1.5 mol of interlayer water per formula unit (Fig. S4), which is also in good agreement with previously reported acid-exchanged products of KCa₂Nb₃O₁₀.²⁶ Table 2 summarizes the results of elemental analysis before and after the proton exchange process.

Fig. 2 Powder XRD pattern of the acid-treated product from KCa₂MnNb₃TiO₁₃.

Table 2 Elemental analysis results and molar ratios of each layered perovskite

Nominal composition	K or Rb	Mn	Ca	Nb	Ti
The values are given in wt%. The values in parentheses represent the molar ratios normalized with Nb = 3.00.
KCa2MnNb3TiO13	6.7 (1.00)	10.5 (1.03)	15.7 (1.95)	57.2 (3.00)	10.8 (0.95)
Acid-exchanged product from K-form	3.3 (0.31)	9.6 (0.89)	16.5 (1.93)	60.7 (3.00)	11.1 (0.91)
RbCa2MnNb3TiO13	12.3 (0.99)	10.0 (1.06)	15.0 (2.02)	52.8 (3.00)	10.3 (0.97)
Acid-exchanged product from Rb-form	0.6 (0.04)	10.9 (1.02)	16.8 (1.99)	60.1 (3.00)	11.6 (0.96)

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For the as-synthesized samples, the elemental molar ratios, normalized to Nb = 3, were approximately (K or Rb) : Ca : Mn : Nb : Ti = 1 : 2 : 1 : 3 : 1 for both the K- and Rb-containing compositions, which is consistent with the nominal target stoichiometry. In contrast, after acid treatment, the elemental ratios of Ca, Mn, Nb, and Ti in the Rb-containing sample remained largely unchanged, whereas Rb became nearly undetectable, indicating that Rb⁺ ions were almost completely exchanged with oxonium (H₃O⁺) ions. On the other hand, in the K-containing sample, approximately 30% of the initial K⁺ ions remained after the proton-exchange treatment, and the contents of Ca and Mn were lower than the ideal stoichiometry. This behavior has also been observed in the synthesis of compounds such as KCa₂NaNb₄O₁₃, described by eqn (1), and is understood as a result of partial incorporation of K⁺ ions into the A-sites of the perovskite layers during solid-state synthesis. The excess Ca²⁺ and Mn²⁺ ions may then be accommodated in the interlayer region, and involve the proton exchange. In contrast, due to the larger ionic radius of Rb⁺, such incorporation into the A-sites was not the case, which leads to a composition closer to the ideal layered perovskite structure in the Rb-containing sample. XANES measurements were performed on both the K-form and H-form samples. Comparison with reference Mn oxides confirmed that Mn remains in the divalent oxidation state throughout the solid-state synthesis and subsequent the proton-exchange processes (Fig. S5). This result is consistent with charge balance based on the nominal composition.

Based on these findings, Rietveld refinement was carried out for more quantitative structural characterization. Since the K-containing sample required a doubled unit cell along the c-axis relative to the Rb-containing sample, and considering the structural complexity arising from the solid solution behavior of K⁺, Ca²⁺, and Mn²⁺ occupying both the interlayer and A-sites in the perovskite framework, structure refinement was performed on the Rb-containing sample, which is expected to have a simpler and more ordered structure. Specifically, superstructure features suggested by lattice parameter analysis were disregarded, and instead a model based on a fundamental layered perovskite framework was adopted, referring to previously reported structurally similar compounds.²⁷ Refined occupancies and atomic displacement parameters may be affected by this simplification of the subcell model, but are reliable enough to discuss the basic feature of the structure. In this model, Ca²⁺ and Mn²⁺ occupy the A-sites within the host perovskite layers at a molar ratio of 2 : 1 as a whole, while Nb⁵⁺ and Ti⁴⁺ occupy the B-sites at a molar ratio of 3 : 1. As shown in Fig. 3 and Table 3, satisfactory refinement results were obtained, confirming the successful synthesis of the targeted layered perovskite structure with Mn²⁺ incorporated into the A-sites. In this refinement, fitting was performed with MnTiO₃ included as an impurity phase. Its refined with a phase fraction of 3.3%, indicates the almost full reaction of MnTiO₃ according to eqn (3b). Although the initial structural model assumed a solid solution of Nb⁵⁺ and Ti⁴⁺ with a molar ratio of 3 : 1, the refinement process revealed a pronounced redistribution of Nb⁵⁺ ions. Specifically, Nb⁵⁺ progressively accumulated at the B-site positions located closer to the surface of the perovskite layer, ultimately resulting in 100% occupation of these sites by Nb⁵⁺ (denoted as Nb(2)). In contrast, the B-sites situated nearer to the center of the perovskite layer (Nb(1), Ti(1)) converged to a mixed-occupancy configuration in which Nb⁵⁺ and Ti⁴⁺ form a solid solution. This site preference is plausibly attributed to electrostatic considerations based on Pauling's principle.²⁸ In the present perovskite-related structure, the A-sites within the perovskite layer are occupied by divalent Ca²⁺/Mn²⁺ cations, while the interlayer region is filled with monovalent Rb⁺ cations. Consequently, Nb⁵⁺ cations are expected to preferentially occupy B-sites adjacent to the interlayer region to stabilize the local charge balance, leading to the observed enrichment of Nb⁵⁺ near the perovskite layer surface.

Fig. 3 Structure refinement of X-ray powder diffraction data for RbCa₂MnNb₃TiO₁₃. Blue: observed, red: calculated, green: Bragg reflections, black: the difference.

Table 3 Structural parameters from crystal structure refinement for RbCa₂MnNb₃TiO₁₃ in space group P4/mmm (123)

	x	y	z	Occ.	U	Site
a = 0.3837 nm, c = 1.872 nm, V = 0.27557(1) nm^3, Rp = 8.03%, Rwp = 11.04%, Re = 4.54%, S = 2.43.
Nb(1)	0	0	0.10461(13)	0.5	0.0037(7)	2
Nb(2)	0	0	0.32929(10)	1.0	0.0149(7)	2
Ti(1)	0	0	0.10461(13)	0.5	0.0037(7)	2
Ca(1)	0.5	0.5	0	0.89(6)	0.034(3)	1
Ca(2)	0.5	0.5	0.2207(2)	0.56(3)	0.046(2)	2
Mn(1)	0.5	0.5	0	0.11(6)	0.034(3)	1
Mn(2)	0.5	0.5	0.2207(2)	0.44(3)	0.046(2)	2
Rb(1)	0.5	0.5	0.5	1.0	0.0505(16)	1
O(1)	0	0	0	1.0	0.076(2)	1
O(2)	0	0.5	0.1034(8)	1.0	0.076(2)	4
O(3)	0	0	0.1977(8)	1.0	0.076(2)	2
O(4)	0.5	0	0.3051(6)	1.0	0.076(2)	4
O(5)	0	0	0.4136(10)	1.0	0.076(2)	2

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Subsequently, we examined the exfoliation of the prepared layered perovskite into unilamellar nanosheet. When the acid-treated product derived from the Rb-form, HCa₂MnNb₃TiO₁₃·1.5H₂O, was vigorously agitated with an aqueous TBAOH solution, the mixture transformed into a light brown colloidal suspension (Fig. S6). The colloidally dispersed sample was deposited onto a Si substrate via the LB technique and subsequently examined by AFM (Fig. 4). The AFM images revealed a large number of rectangular, ultrathin, sheet-like structures. The image provides further evidence for uniform and unilamellar nature of the nanosheet. The height profile histogram (Fig. 4c) exhibited two distinct peaks: one corresponding to the height of the Si substrate and the other to that of the nanosheets, indicating a uniform thickness for the exfoliated perovskite layers. The nanosheet thickness was estimated to be approximately 2.0 nm. This value is in close agreement with the theoretical thickness of 1.859 nm, calculated from the outermost O–O distance in a single perovskite layer as determined by Rietveld refinement, thereby confirming successful exfoliation into individual monolayers. The slightly larger experimental thickness compared to the calculated value is commonly observed in various nanosheet systems and is attributed to the presence of surface-adsorbed water molecules.^29–32

Fig. 4 AFM observation data of Ca₂MnNb₃TiO₁₃ nanosheets deposited on a Si substrate. (a) and (b) topographic image and (c) height histogram.

To examine the crystal structure, in-plane XRD measurements were performed on the monolayer film of Ca₂MnNb₃TiO₁₃ nanosheets deposited on the substrate. As shown in Fig. 5, sharp diffraction peaks indicative of high crystallinity were observed, and these peaks could be indexed to a 2D square lattice with a lattice parameter of a = 0.38372(4) nm. This result clearly demonstrates that the fundamental perovskite lattice is preserved after exfoliation into nanosheets.

Fig. 5 In-plane XRD pattern of a monolayer film of Ca₂MnNb₃TiO₁₃ nanosheets deposited on a Si substrate using the LB method.

The out-of-plane XRD pattern was measured in the Bragg–Brentano geometry using a powder X-ray diffractometer (Fig. 6) and exhibited characteristic oscillatory scattering features. The continuous nature of the diffraction profile suggests the absence of repeating periodicity along the thickness (c-axis) direction. The observed oscillations are attributed to structural architecture across the nanosheet thickness. To validate this interpretation, the scattering intensity along the z-direction of monolayer perovskite nanosheets was calculated based on the atomic coordinates obtained from the aforementioned Rietveld refinement, according to the eqn (4) and (5) below.^21,33 The resulting simulated profile closely reproduced the experimental profile.

Here, F_00l and I_00l denote the structure factor and the scattering intensity of a single-layer perovskite nanosheet along the 00l direction, respectively; f_j and z_j denote the atomic scattering factor and the z-coordinate of the j-th atom. θ is the diffraction angle and λ is the X-ray wavelength (Cu Kα, λ = 0.15405 nm). This agreement supports the conclusion that the perovskite layer is composed of four corner-sharing [Nb,Ti]O₆ octahedra, indicating that the perovskite framework is retained along the layer normal.

Fig. 6 Out-of-plane XRD pattern of Ca₂MnNb₃TiO₁₃ nanosheets monolayer on a Si substrate using the LB method. Red: observed, cyan: calculated. The part indicated by the arrow corresponds to the hump relating to the Si substrate.

Fig. 7 shows the wide-scan XPS spectrum of Ca₂MnNb₃TiO₁₃ nanosheets obtained from the Rb-containing precursor (Fig. S7). The elemental composition of each nanosheet, determined from the integrated peak areas and normalized to Nb = 3.00, is summarized in Table 4.

Fig. 7 X-ray photoelectron spectroscopy of Ca₂MnNb₃TiO₁₃ nanosheet.

Table 4 Elemental ratios of each Ca₂MnNb₃TiO₁₃ nanosheet estimated from XPS

	K	Rb	Ca	Mn	Nb	Ti
From K-form	0.05	—	2.15	0.88	3.00	1.17
From Rb-form	—	0.00	2.03	1.11	3.00	1.10

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XPS analysis detected Mn signals in nanosheets derived from both K- and Rb-containing layered perovskites. The nanosheets obtained from the Rb-based precursors exhibited no detectable Rb signal, and the elemental ratios of Ca, Nb, Ti, and Mn were consistent with those determined by elemental analysis after ion exchange. This confirms that the perovskite layer framework was preserved during the exfoliation process. In contrast, nanosheets derived from the K-containing samples exhibited residual K signals and a lower Mn content. These findings are in agreement with the post-ion-exchange elemental analysis, suggesting that a fraction of the K ions is incorporated into the perovskite layers and remains even after acid treatment and delamination.

4 Conclusions

A new class of layered perovskite oxides, KCa₂MnNb₃TiO₁₃ and RbCa₂MnNb₃TiO₁₃, was successfully synthesized by calcining the precursors KCa₂Nb₃O₁₀ or RbCa₂Nb₃O₁₀ with illmenite-type MnTiO₃. Compositional and structural characterizations confirmed that these compounds belong to the Dion–Jacobson series of layered perovskites. The original three-layer perovskite slabs composed of corner-sharing NbO₆ octahedra were expanded through the incorporation of MnTiO₃, forming a novel four-layer perovskite structure comprising mixed [Nb,Ti]O₆ octahedra. To the best of our knowledge, there have been no prior reports of Dion–Jacobson type layered perovskite oxides containing Mn ions at the A-site, and the synthetic route developed in this study offers a promising strategy to expand the compositional diversity of layered perovskites. Furthermore, proton exchange followed by exfoliation enabled the isolation of perovskite nanosheets with a thickness corresponding to four octahedral layers, while retaining the original perovskite framework. The preservation of the perovskite architecture in the exfoliated nanosheets was verified by AFM, in-plane and out-of-plane XRD, and XPS analyses. These findings demonstrate that the present synthetic approach is an effective strategy for the rational design of functional layered perovskite oxides and their corresponding nanosheet derivatives.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data supporting this study are available within the article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6dt00131a.

Acknowledgements

This work was supported by World Premier International Research Center Initiative (WPI), Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and CREST of the Japan Science and Technology Agency (JST) (grant no. JPMJCR22B1), Japan. The in-plane XRD measurements were performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2024G501). This work was supported by “Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)” of the MEXT. Proposal Number JPMXP1225NM5176. We also acknowledge technical supports by the Surface and Bulk Analysis Unit in NIMS for ICP-OES.

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