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Two New Noncentrosymmetric (NCS) n = 3 Layered Dion-Jacobson (DJ) Perovskites: Polar RbBi2Ti2NbO10 and Nonpolar CsBi2Ti2TaO10 Hyung Gu Kim, T. Thao Tran, Woongjin Choi, Tae-Soo You, P. Shiv Halasyamani, and Kang Min Ok Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00778 • Publication Date (Web): 23 Mar 2016 Downloaded from http://pubs.acs.org on March 24, 2016

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Chemistry of Materials

Two New Noncentrosymmetric (NCS) n = 3 Layered Dion-Jacobson (DJ) Perovskites: Polar RbBi2Ti2NbO10 and Nonpolar CsBi2Ti2TaO10 Hyung Gu Kim,1 T. Thao Tran,2 Woongjin Choi,3 Tae-Soo You,3 P. Shiv Halasyamani,2 and Kang Min Ok1,* 1

Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea

2

Department of Chemistry, University of Houston, 112 Fleming Building, Houston, TX 77204-5003, USA

3

Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 28644, Republic of Korea

ABSTRACT: Two new noncentrosymmetric (NCS) n = 3 layered perovskites, RbBi2Ti2NbO10 and CsBi2Ti2TaO10, have been synthesized through high temperature solid state reactions. X-ray diffraction analyses suggest that RbBi2Ti2NbO10 crystallizes in the orthorhombic polar space group, Ima2 (No. 46), whereas CsBi2Ti2TaO10 crystallizes in the tetragonal nonpolar space group, P-4 (No. 81). Interestingly, CsBi2Ti2TaO10 is the first Dion-Jacobson (DJ) type layered perovskite with a NCS nonpolar space group. Powder second-harmonic generation (SHG) measurements with 1064 nm radiation reveal that both of the reported materials have SHG efficiencies of approximately 100 times that of α-SiO2 and are phase-matchable (type I). Piezoelectricity measurements suggest that the estimated d33 values for polar RbBi2Ti2NbO10 and nonpolar CsBi2Ti2TaO10 are 170 and 67 pm V-1, respectively. The net polarization direction arising from the constituent distorted polyhedra and the key component causing microscopic deformations in NCS structure are introduced. Polarization measurements are also presented along with other characterizations such as the UV-vis diffuse reflectance and infrared spectra, elemental analyses, and electronic structure calculations for the reported materials.

INTRODUCTION As one of the most versatile class of materials, perovskites have been continuously attracted by material’s scientists owing to their fascinating characteristics such as magnetic, superconductive, dielectric, thermoelectric, electrocatalytic, ferroelectric, and optical properties.1-14 Specific families of the perovskites with layered structures are generally classified as Ruddlesden-Popper (RP; A’2[An15-19 Aurivillius ((Bi2O2)[An-1BnO3n+1]),20-22 and Di1BnO3n+1]), on-Jacobson (DJ; A’[An-1BnO3n+1]) phases.23-26 While each of the different layered perovskites shares common twodimensional anionic slabs ([An-1BnO3n+1]), the motifs separating the layers (A’ or Bi2O2) and the subsequent offsetting of the layers are dissimilar. Many interesting abovementioned characteristics found from bulk ABO3 perovskites have been also similarly observed from layered perovskites.27-42 Interestingly, however, while the greater part of the known ABO3 perovskites crystallized in centrosymmetric (CS) space groups, a number of layered perovskites were found to crystallize in noncentrosymmetric (NCS) polar structures.43-51 Several representative NCS polar layered perovskites are Ca3Ti2O7 (RP),52 Bi4Ti3O12 (Aurivillius),53,54 and CsBiNb2O7 (DJ).44,55,56 Since very enchanting properties such as pyroelectricity and ferroelectricity may be expected from polar materials, layered perovskites with polar symmetry must be one of the most promising materials for thermal detectors, pollution monitors, and random-access memories. In addition, macro-

scopic NCS materials that are lacking inversion symmetry including nonpolar crystal classes can reveal other invaluable features such as piezoelectricity and secondharmonic generation (SHG).57 As other polar oxides, most of NCS polar layered perovskites are composed of locally asymmetric second-order Jahn-Teller (SOJT) distortive cations, namely, lone pair cations (Bi3+) and d0 transition metal cations (Ti4+, Nb5+, W6+) under octahedral environment.58-64 Here, the octahedral rotations occurring through a hybrid improper or trilinear coupling mechanism should be the driving force for the transitions from nonpolar lattices to polar structures.49,65,66 Previously, we have demonstrated that the NCS properties are significantly influenced by the net polarization occurring from the asymmetric polyhedra of lone pair cations, i.e., Bi3+, in a couple of polar layered perovskites such as Bi4Ti3O12 and CsBiNb2O7.67,68 Our continuous efforts to replace the nonpolarizable cation, La3+, in CS DJ materials, RbLa2Ti2NbO10 and CsLa2Ti2TaO10,69 with lone pair cation, Bi3+, resulted in two novel NCS layered DJ series, namely, polar RbBi2Ti2NbO10 and nonpolar CsBi2Ti2TaO10. It should be pointed out that the assembly of asymmetric units for NCS nonpolar CsBi2Ti2TaO10 has never been observed before. In this paper, detailed solid state syntheses, structural determinations, NCS properties measurements, spectroscopic characterizations, and electronic structure calculations of two new n = 3 NCS DJ series will be presented.

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EXPREIMENTAL SECTION Reagents. Rb2CO3 (Alfa Aesar, 99%), Cs2CO3 (Alfa Aesar, 99%), Bi2O3 (Alfa Aesar, 99%), TiO2 (Kanto, 99.5%), Nb2O5 (Alfa Aesar, 99.5%), and Ta2O5 (Aldrich, 99%) were used as received. Synthesis. Single crystals of CsBi2Ti2TaO10 were grown by high temperature solid state reactions. 2.3 mmol of Cs2CO3, 3.0 mmol of Bi2O3, 6.0 mmol of TiO2, and 1.5 mmol of Ta2O5 were thoroughly ground and contained in a platinum crucible. The reaction mixture was heated to 1000 °C for 24 h and cooled to 700 °C for 80 h before being quenched to room temperature. After cooling, the product mixture was washed with deionized water and filtered. Colorless plate crystals of CsBi2Ti2TaO10 were obtained in about 30% yield along with polycrystalline Bi2Ti2O770,71 from the reaction mixture. Pure polycrystalline samples of RbBi2Ti2NbO10 and CsBi2Ti2TaO10 were prepared through standard solid state reactions. Stoichiometric amounts of A2CO3 (A = Rb and Cs; 50% excess), Bi2O3, TiO2, and M2O5 (M = Nb and Ta) were mixed together intimately with agate mortars and pestles, and pressed into pellets. The pellets were heated to 1000 °C for RbBi2Ti2NbO10 and 900 °C for CsBi2Ti2TaO10 for 24 h and cooled to room temperature. The reaction mixtures were reground, repelletized, and reheated to the same temperatures for further 24 h and cooled to room temperature. The heating and regrinding processes were repeated 3 times until pure phases are obtained. The products were washed with deionized water and dried in a drying oven at 80 °C. The newly synthesized NCS layered perovskites materials have been deposited to Noncentrosymmetric Materials Bank (http://ncsmb.knrrc.or.kr). X-Ray Diffraction (XRD). Single crystal X-ray diffraction data of CsBi2Ti2TaO10 were collected using a Bruker SMART BREEZE 1K CCD diffractometer with graphite monochromated Mo Kα radiation. A colorless plate crystal of CsBi2Ti2TaO10 (0.013 × 0.044 × 0.094 mm3) was attached on a glass fiber using a glue for data collection. A narrow-frame method was utilized to obtain the frames with scan widths of 0.30° in omega and an exposing time of 10 s/frame. After integration, the diffraction data were solved and refined with SHELXS-97 and SHELXL-97, respectively.72,73 Although a satisfactory solution and a reasonable crystallographic model were obtained, the refinement resulted in a rather higher R value of 9.14% with unsatisfactory oxygen displacement parameters possibly attributed to the very thin plate-like crystal morphology containing extremely heavy atoms. Thus, the Rietveld refinement using powder X-ray diffraction data (PXRD) was utilized for the structural refinement of CsBi2Ti2TaO10. The PXRD data were obtained from a Bruker D8-Advance diffractometer using Cu Kα radiation with 40 KV and 40 mA at room temperature. The polycrystalline samples for both of the reported compounds were mounted on sample holders and the diffraction data were obtained in the 2θ range of 5−100° with a step size of 0.02° and a step time of 4 s. The collected diffraction patterns were refined using the Rietveld method with the program GSAS.74 The

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refinement for RbBi2Ti2NbO10 was performed in the space group, Ima2 (No. 46) with a starting model similar to the recently reported polar DJ phase, CsBi2Ti2NbO10.51 The crystal structure of CsBi2Ti2TaO10 was refined with an initial model obtained from our single crystal X-ray diffraction data. The unit-cell parameters, scales, zero-point errors, peak shape parameters, profile parameters, atomic coordinates, and isotropic displacement parameters were refined. As seen in the Rietveld refinement profiles, the structural refinements have been successfully achieved (see Figure 1). The insets and distinct Bragg positions of each material found in Figure 1 clearly suggest that RbBi2Ti2NbO10 and CsBi2Ti2NbO10 crystallize in different space groups. The refinement results along with crystallographic data of RbBi2Ti2NbO10 and CsBi2Ti2NbO10 are summarized in Table 1.

Figure 1. Final Rietveld plots of (a) RbBi2Ti2NbO10 and (b) CsBi2Ti2TaO10. The observed (×), calculated (red solid line), and difference profiles (blue solid line) are shown along with the reflection positions (magenta vertical bars).

Infrared (IR) Spectroscopy. IR spectra were recorded on a Thermo-Scientific Nicolet IS10 FT-IR spectrometer in the spectral range of 400−4000 cm-1, with the samples contacted by a diamond attenuated total reflectance (ATR) crystal. Ultraviolet-Visible (UV-vis) Light Diffuse Reflectance Spectroscopy. UV-vis diffuse reflectance spectra were obtain on a Varian Cary 500 scan UV-vis-NIR spectrometer in the range of 200−2500 nm at room temperature. To transform the reflectance data into absorbance, the Kubelka-Munk function was adopted.75,76


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Thermal Analysis. The thermal properties of the title compounds were characterized by a Scinco TGA-N 1000 thermal analyzer. Each sample was loaded in alumina crucibles and heated to 900 °C at 10 °C min-1 under flowing argon. Table 1. Summary of Crystallographic Data and Refinement Results for RbBi2Ti2NbO10 and CsBi2Ti2TaO10 RbBi2Ti2NbO10

CsBi2Ti2TaO10

fw

852.47

996.60

space group

Ima2 (No. 46)

P-4 (No. 81)

a (Å)

30.4905(12)

3.86107(9)

b (Å)

5.41718(18)

3.86107(9)

c (Å)

5.4591(2)

15.5895(7)

V (Å )

3

901.69(6)

232.407(15)

Z

4

1

T (K)

298.0(2)

298.0(2)

λ (Å)

1.5406

1.5406

a Rp

0.0891

0.0845

0.1119

0.1078

b

Rwp a

b

2

2 1/2

Rp = Σ |Io - Ic| / Σ Io. Rwp = [Σ w |Io - Ic| / Σ w Io ] .

Scanning Electron Microscopy (SEM)/EnergyDispersive Analysis by X-Ray (EDX). SEM/EDX analyses were performed with Hitachi S-3400N/Horiba energy Ex-250 instruments. RbBi2Ti2NbO10 and CsBi2Ti2TaO10 reveal approximate Rb (or Cs):Bi:Ti:Nb (or Ta) ratios of 1.0:2.0:1.9:0.9 and 1.0:1.8:1.8:1.0, respectively. Powder Second-Harmonic Generation (SHG) Measurements. SHG measurements with ground samples were carried out on a modified Kurtz-NLO system with a DAWA Q-switched Nd:YAG laser using a 1064 nm radiation. In order to study the type I phase-matching characteristics, polycrystalline materials were sieved into specific particle size ranges (20−45, 45−63, 63−75, 75−90, 90−125, 125−150, 150−200, and >200 μm) and packed into different capillary tubes. Crystalline α-SiO2 and LiNbO3 were also graded into the same particle size ranges to make pertinent SHG efficiency comparisons. An account of the detailed methodology used has been published earlier.77 Piezoelectric Measurements. Converse piezoelectric measurements were carried out using a Radiant Technologies RT66A piezoelectric system containing a Precision Materials Analyzer, TREK high-voltage amplifier (model 609E−6), MTI 2000 Fotonic Sensor, and Precision HighVoltage Interface. Polycrystalline RbBi2Ti2NbO10 and CsBi2Ti2TaO10 were mixed with poly(vinyl acetate) (PVA) binder and pressed into half-inch diameter and ~1 mm thick pellets. The pellets were sintered at 900 °C for 10 h. After applying silver paste to both sides of the pellets as electrodes, the pellets were further cured at 200 °C for 3 h before the measurements. A maximum voltage of 500 V was applied to the prepared pellets.

Polarization Measurements. Polarization measurements on polar RbBi2Ti2NbO10 were performed on a Radiant Technologies RT66A ferroelectric test system containing a TREK high-voltage amplifier at temperatures between 40 and 170 °C in a Delta 9023 environmental test chamber. A more detailed explanation of the measurements used has been published.77 Electronic Structure Calculations. Theoretical calculations have been performed for the two title compounds by using the Stuttgart TB-LMTO47 program with the atomic sphere approximation (ASA) method.78-82 Given that a mixed-atomic occupation cannot be exploited in theoretical calculations for a practical reason, the idealized compositions of RbBi2Ti2NbO10 and CsBi2Ti2TaO10 having separated Ti and Nb sites or Ti and Ta sites were used, respectively. The local density approximation (LDA) was applied for exchange and correlation.78-82 A scalar relativistic approximation was taken to treat all relativistic effects, except spin-orbit coupling. In the ASA method, the space was filled with the overlapping Wigner-Seitz (WS) atomic spheres.78-82 The symmetry of the potential inside each WS sphere was considered spherical, and a combined correction was used to take into account the overlapping part.83 The radii of WS sphere were obtained by requiring the overlapping potential to be the best possible approximation to the full potential and were determined by an automatic procedure.83 This overlap should not be too large because the error in kinetic energy introduced by the combined correction was proportional to the fourth power of the relative sphere overlap. The used WS radii are listed as follows: Rb = 2.329 Å, Bi = 1.714 Å, Ti = 1.167 Å, Nb = 1.200 Å, and O = 0.865−1.203 Å for RbBi2Ti2NbO10; and Cs = 2.321 Å, Bi = 1.814 Å, Ti = 1.278 Å, Ta = 1.424 Å, and O = 0.985−1.113 Å for CsBi2Ti2TaO10. The basis sets included 5s, 5p, 4d, and 4f orbitals for Rb; 6s, 6p, 5d, and 4f orbitals for Cs; 6s, 6p, 6d, and 5f orbitals for Bi; 4s, 4p, and 3d orbitals for Ti; 5s, 5p, 4d, and 4f orbitals for Nb; 6s, 6p, 5d, and 5f orbitals for Ta; and 3s, 2p, and 3d orbitals for O. The Rb 5p, 4d, and 4f; Cs 6p, 5d, and 4f; Bi 6d and 5f; Ta 5f; Nb 4f; and O 3s and 3d orbitals were treated by the Löwdin downfolding technique.84 The kspace integration was conducted by the tetrahedron method,85 and the self-consistent charge density was obtained using 266 and 282 irreducible k-points in the Brillouin zone for RbBi2Ti2NbO10 and CsBi2Ti2TaO10, respectively.

RESULTS AND DISCUSSION Structures. Both RbBi2Ti2NbO10 and CsBi2Ti2TaO10 are classified as members of n = 3 Dion-Jacobson (DJ) series with the general formula of A’[An-1BnO3n+1]. An archetypal representation of n = 3 DJ phase is shown in Figure 2. The structure of the DJ series consists of two main parts, i.e., perovskite blocks and alkali metal separators. As can be seen in Figure 2, the perovskite blocks ([An-1BnO3n+1]) are composed of corner-shared octahedra, [BO6] (B = Ti4+, Nb5+, and Ta5+) and A site cations (A = Bi3+ for both RbBi2Ti2NbO10 and CsBi2Ti2TaO10). Alkali metal separators,



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A’ (A’ = Rb+ and Cs+) reside in between the anionic layers and maintain the charge balance.

Figure 2. Ball-and-stick and polyhedral representation of n = 3 Dion-Jacobson phase, A’[An-1BnO3n+1].

RbBi2Ti2NbO10 crystallizes in the orthorhombic polar NCS space group, Ima2 (No. 46) with a similar structure to that of CsBi2Ti2NbO10 at room temperature.51 Within the anionic framework of the perovskite block, [Bi2Ti2NbO10]-, two unique B-sites exist, where Ti(1)4+/Nb(1)5+ and Ti(2)4+/Nb(2)5+ are statistically disordered to B(1)- and B(2)-sites with the refined ratios of 0.885(14)/0.115(14) and 0.554(13)/0.446(13), respectively (see Figure 3a). Both of the disordered Ti(1)4+/Nb(1)5+ and Ti(2)4+/Nb(2)5+ cations are bonded to six oxygen atoms in highly distorted octahedral coordination environment attributed to the SOJT effect. While Ti(1)/Nb(1)O6 octahedra reveal local C2 distortions, i.e., two short [1.70(5) Å], two intermediate [1.99(3) Å], and two long [2.18(5) Å] Ti4+/Nb5+(1)−O bonds, Ti(2)/Nb(2)O6 octahedra exhibit local C4 distortions, i.e., one short [1.72(3) Å], one long [2.46(3) Å], and four intermediate [1.79(7)−2.10(6) Å] Ti(2)/Nb(2)−O bonds (see Figure 3a). A-sites of the perovskite blocks are occupied by Bi3+ cations. The unique Bi(1)3+ cation is coordinated by six oxygen atoms with the Bi(1)−O bond distances ranging from 2.31(5) to 2.70(6) Å. The weak Bi−O interactions between Bi(1)3+ and five more oxygen atoms in the distance ranges of 2.95(5)−3.23(4) Å are also observed. Bi(1)3+ cations are in asymmetric environment owing to their lone pairs. Finally, the alkali metal separator, Rb(1)+ cations interacting with eight oxygen atoms with contact lengths of 3.03(10)−3.22(10) Å are found between the layers.

Figure 3. Ball-and-stick and polyhedral representations of (a) NCS polar RbBi2Ti2NbO10 and (b) NCS nonpolar CsBi2Ti2TaO10. RbBi2Ti2NbO10 contains C2 distortive Ti(1)/Nb(1)O6 octahedra and C4 distortive Ti(2)/Nb(2)O6 octahedra, whereas CsBi2Ti2TaO10 possesses Ti(1)/Ta(1)O6 octahedra with unconventional distortions and C4 distortive Ti(2)/Ta(2) octahedra.

CsBi2Ti2TaO10 is another new n = 3 DJ family that crystallizes in the tetragonal nonpolar NCS space group, P-4 (No. 81). Two unique B-site cations, Ti(1)4+/Ta(1)5+ and Ti(2)4+/Ta(2)5+ are disordered to B(1)- and B(2)-sites with the ratios of 0.543(6)/0.457(6) and 0.694(6)/0.306(6), respectively, in the perovskite block. However, the distortive environments of the B-cations are different to those of polar RbBi2Ti2NbO10. Especially the local distortive environments of Ti(1)/Ta(1)O6 octahedra are completely different: two axial Ti(1)/Ta(1)−O(1) bond lengths are a bit longer [2.01(6) Å] compared to those of four equatorial Ti(1)/Ta(1)−O(2) bonds [1.975(10) Å]. Interestingly, all the Ti(1)/Ta(1)−O bond distances facing each other in Ti(1)/Ta(1)O6 octahedra are same, although they are distorted (see Figure 3b). The distortions observed from Ti(2)/Ta(2)O6 octahedra can be considered as the result from the SOJT: the observed Ti(2)/Ta(2)−O bond distances are one short [1.86(3) Å], one long [2.49(6) Å], and four intermediate [1.944(5)−2.029(10) Å]. The A-site cation, Bi(1)3+ interacts with 10 oxygen atoms in the range of 2.43(2)−2.76(3) Å. The larger A’ cation, Cs(1)+ resides in between the layers interacting with 12 oxide ligands with contact lengths of 3.085(15)−4.02(4) Å. It should be noted that CsBi2Ti2TaO10 is the first example of nonpolar NCS layered DJ type perovskite revealing an interesting distortive MO6 octahedral coordination environment.


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Infrared (IR) spectroscopy. The IR spectra of RbBi2Ti2NbO10 and CsBi2Ti2TaO10 reveal Bi−O, Ti−O, Nb−O, or Ta−O vibrations. The peaks found at ca. 510−480 cm-1 and 880−908 cm-1 are attributed to Bi−O and Ti−O vibrations, respectively. Bands observed from ca. 910−955 cm-1 and 541−621 cm-1 may be attributable to M=O and M−O (M = Nb or Ta) vibrations. The assignments are consistent with vibrations for formerly reported mixed metal oxides.86,87 IR spectra for both materials can be found in the Supporting Information.

values are similar to those of polar BaTiO3 (d33 = 191 pm V-1, S.G.: P4mm) and PbZrxTi1-xO3 (0 ≤ x ≤ 1; PZT, d33 = 110−223 pm V-1, S.G.: R3c), and nonpolar LiIO3 (d33 = 92 pm V-1, S.G.: P6322).88

UV-vis diffuse reflectance spectroscopy. The absorption data obtained by the UV-vis diffuse reflection spectra of RbBi2Ti2NbO10 and CsBi2Ti2TaO10 were calculated with the Kubelka-Munk function, i.e., F ( R) =

(1 − R) 2 K = 2R S

in which R is the reflectance, S is the scattering, and K is the absorption. The first appearances of the optical absorption obtained by the extrapolation of the linear section of the ascending curves to 0 in the (K/S) versus E plots of RbBi2Ti2NbO10 and CsBi2Ti2TaO10 were shown at 3.42 and 3.53 eV, respectively (see the Supporting Information). The observed data are very similar to those of previously reported DJ families.68 Thermal Properties. In order to study the thermal stabilities for the reported materials, thermogravimetric analyses (TGA) were performed for the reported materials. TGA diagrams revealed that both RbBi2Ti2NbO10 and CsBi2Ti2TaO10 did not show any weight losses up to 1000 °C. To investigate any phase transitions, powder X-ray diffraction (PXRD) measurements at higher temperatures were carried out. Interestingly, RbBi2Ti2NbO10 and CsBi2Ti2TaO10 transformed to cubic pyrochlore phases at 1030 and 1000°C, respectively (see the Supporting Information). Noncentrosymmetric (NCS) Properties Measurements. Both polar RbBi2Ti2NbO10 and nonpolar CsBi2Ti2TaO10 crystallize in NCS space groups; thus, SHG properties for the materials have been investigated. RbBi2Ti2NbO10 and CsBi2Ti2TaO10 revealed similarly strong SHG efficiencies of about 100 times that of α-SiO2. Further SHG measurements as a function of particle size on the graded polycrystalline samples of the reported compounds suggest that both Rb and Cs phases are type I phase-matchable SHG materials (see Figure 4). More detailed analysis to investigate the origin of the SHG responses are described later. Since the NCS space groups, Ima2 (RbBi2Ti2NbO10) and P-4 (CsBi2Ti2TaO10) are the right symmetries for not just SHG but piezoelectricity, converse piezoelectric measurements for both samples were also carried out. When voltages were applied to the materials, strains that are parallel to the polarization directions were obtained attributed to the materials’ macroscopic deformations in the measurements. The estimated d33 values for polar RbBi2Ti2NbO10 and nonpolar CsBi2Ti2TaO10 are 170 and 67 pm V-1, respectively (see the Supporting Information). The

Figure 4. Plots of SHG efficiencies as a function of particle size for RbBi2Ti2NbO10 and CsBi2Ti2TaO10. The curves are drawn to guide the eye and are not fit to the data.

Polarization measurements were also performed for polar RbBi2Ti2NbO10. Loops were observed in the ferroelectric measurements; however, the polarization saturation was not completely achieved perhaps attributed to the dielectric loss originating from the energetically unfavorable polarization reversals associated with the lone pair cation, Bi3+ and the electrical leak arising from the high porosity of the pellet (see the Supporting Information). Pyroelectric coefficient for polar RbBi2Ti2NbO10 is about 3.0 µC m-2 K-1, which is similar to those of polar materials, such as tourmaline (4.0 µC m-2 K-1) and ZnO (9.4 µC m-2 K-1).89 Structure-NCS Properties Relationships. The measured NCS properties for NCS polar RbBi2Ti2NbO10 and NCS nonpolar CsBi2Ti2TaO10 can be explained by careful examinations of the structures. Especially with polar RbBi2Ti2NbO10, the net polarization direction can be found by determining the orientations of the polarizations of constituent distorted polyhedra. The edge-type distortions in Ti(1)/Nb(1)O6 octahedra along the local C2 direction reveal two short, two intermediate, and two long Ti(1)/Nb(1)−O bonds. Since the C2 distortions of Ti(1)/Nb(1)O6 octahedra occur along the same direction, a net moment along the [00-1] direction is observed (see Figure 5a). Another distorted octahedra, Ti(2)/Nb(2)O6, show the local C4 distortions with one short, four intermediate, and one long Ti(2)/Nb(2)−O bonds. The moment incorporated with Ti(2)/Nb(2)O6, however, cancels in effect, since their polarizations point toward opposite directions, [100] and [-100] (see Figure 5a). Finally, as seen in Figure 5a, the polarization arising from the alignment of asymmetric lone pair cation, Bi3+, is observed along the [00-1] direction. When taken all the polarizations as a whole, a net moment is produced along the [00-1] direction from polar RbBi2Ti2NbO10. The net polarization is, in



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fact, the origin of the observed NCS properties of RbBi2Ti2NbO10. The polar direction does not exist in NCS nonpolar CsBi2Ti2TaO10. However, as demonstrated by Ti(1)/Ta(1)O6 and Ti(2)/Ta(2)O6 octahedra in Figure 5b, one can easily find the -4 axis along the c-direction, which unambiguously confirms that CsBi2Ti2TaO10 crystallizes in NCS structure. Thus, the NCS properties of CsBi2Ti2TaO10 are originating from the microscopic deformations in the asymmetric unit cell. The larger cations, Cs+ containing different contact distances with oxide ligands compared to Rb+ might influence the separating layers. The cation size effect in turn significantly affects the local distortive environment of BO6 octahedra as well as macroscopic space groups.90-92

Figure 6. DOS and COHP curves for (a) CsBi2Ti2TaO10 and (b) RbBi2Ti2NbO10. Total DOS (black-bold line), PDOS (darkgray region), Bi PDOS (gray region), Ti (orange region), Ta or Nb PDOS (blue region), and O (magenta region). COHP curves represent (a) the averaged Ta−O (bold line) and Ti−O (dashed line) interactions, or (b) the averaged Nb−O (bold line) and Ti−O (dashed line) interactions. EF (dashed vertical-line) is the energetic reference (0 eV).

Figure 5. Ball-and-stick models representing (a) the net polarization arising from polar RbBi2Ti2NbO10 and (b) NCS nonpolar environment of CsBi2Ti2TaO10.

Electronic Structures. The series of comprehensive theoretical investigations have been performed using the TB-LMTO method to interpret electronic structures and chemical bonding of two title compounds. For a practical reason, two mixed-atomic sites (Wyckoff 1a and 2e) in each compound were treated as the single-atom occupied sites: the Wyckoff 1a sites were assigned with either Nb or Ta for each compound, and the Wyckoff 2e sites were assigned with Ti for both compounds resulting in the idealized compositions of RbBi2Ti2NbO10 and CsBi2Ti2TaO10. The rest of structural information including lattice parameters and atomic coordinates were extracted from powder X-ray diffraction data.


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The resultant DOS and COHP curves for the two compounds are displayed in Figure 6. In particular, both compounds show an overall strong orbital mixing of five components over the entire energy window. Since the TBLMTO calculation underestimates a size of bandgap, we should pay more attention to the relative sizes of those bandgaps rather than the absolute values. As can be estimated, both bandgaps are observed near the Fermi level (EF), and the sizes of those are very similar to each other. These comparable sizes of bandgaps are in a good agreement with two experimentally measured values of 3.53 and 3.42 eV for CsBi2Ti2TaO10 and RbBi2Ti2NbO10, respectively. As seen in Figure 6a, the valence band region in total DOS (TDOS) of CsBi2Ti2TaO10 can roughly be divided into three sections according to the orbital distributions: 1) the section between ca. -12 and -11 eV contains a major contribution from Bi 6s with a small portion from O 2p, 2) the section between ca. -8 and -1 eV presents a large contribution from O 2p well mixed with Ta 6s and 5d, Ti 4s and 3d, and Bi 6p, and 3) the section between ca. -1 and EF includes a mixed contribution of O 2p and Ti 3d with some portion of Cs 6s. The conduction band beyond the bandgap starts with a dominant contribution of Bi 6p states. Interestingly, the overall shape of a TDOS curve for CsBi2Ti2TaO10 nicely represents a typical DOS curve for an octahedral complex consisting of a central metal and six O ligands found in a perovskite. In particular, the second and the third sections in the valence band region illustrate the σ- and π-bond interactions of the octahedral [TaO6/2] and [TiO5/2O] geometries. For the local octahedral coordination geometry, total 18 p states from six O ligands form two sets of group orbitals, respectively, in order to build the σ- or π-bond interactions with orbitals from a central metal. In addition, the Eg and T2g sets of dorbitals in a central metal can be major participants for the bond formation through those σ- and π-type interactions, respectively. With a help of some additional bonding as well as antibonding states between the s or p orbitals from a central metal and several remaining group orbitals from six O ligands, the overall shape of a TDOS curve can be outlined. Some minor differences between the TDOS from a typical perovskite and the title compound should be attributable to the long-range ordered layered perovskite structure of CsBi2Ti2TaO10 containing a terminal oxygen in the [TiO5/2O] octahedron. Two COHP curves shown in Figure 6 verify these bond interactions between a central metal and surrounding six O ligands. The Ta−O COHP clearly shows the stronger σbond interactions via the Eg states of a central metal between ca. -8 and -5 eV and the relatively weaker π-bond interactions via the T2g states of the metal between ca. -5 and -1.5 eV. On the other hand, the Ti−O COHP displays the overall relatively weaker bonding interactions than the Ta−O COHP due to the longer bond distances. Moreover, given the fact that two averaged Ti−O distances along the axial- and the equatorial-directions are somewhat different from each other in the [TiO5/2O] moiety, the Eg states observed between -6.5 and -1.5 eV are more

dispersed than the T2g states appearing between -1.5 eV and EF. DOS and COHP curves for RbBi2Ti2NbO10 resemble to those for CsBi2Ti2TaO10 (see Figure 6b). However, there exist two noticeable differences in RbBi2Ti2NbO10 comparing to those in CsBi2Ti2TaO10: one is the significantly lowered energy levels of both bonding and antibonding states in the Nb PDOS, and the other one is the less dispersed Eg states from the Ti d-orbitals. Firstly, the lowered energy levels of bonding and antibonding states in the Nb PDOS are mostly contributed by the Eg states of the Nb dorbitals and can be attributed to the relatively shorter Nb−O distances (averaged) in the [NbO6/2] octahedron than the Ta−O distances (averaged) of the [TaO6/2] octahedron in CsBi2Ti2TaO10. Moreover, the noticeably lowered eg* states in PDOS display a strongly enhanced antibonding character for the Nb−O COHP just above EF (see Figure 6b). Secondly, the Eg states of Ti d-orbitals, which are observed in both of the PDOS and the Ti−O COHP, are less dispersed than those in CsBi2Ti2TaO10. The rationale for this phenomenon can be provided by the similar Ti−O distances (averaged) along the axial- and the equatorial-directions in the [TiO5/2O] octahedron of RbBi2Ti2NbO10 resulting in a more compact Eg states.

CONCLUSIONS Two new n = 3 Dion-Jacobson families of layered perovskites, RbBi2Ti2NbO10 and CsBi2Ti2TaO10, have been synthesized through solid state reactions. Structural analyses suggest that RbBi2Ti2NbO10 exhibits an NCS polar structure composed of perovskite blocks of Ti/NbO6 octahedra and Bi3+ cations, which are separated by Rb+ cations. With RbBi2Ti2NbO10, a net polarization is achieved toward the [00-1] direction as a result of the alignment of moments arising from the distorted Ti(1)/Nb(1)O6 octahedra and asymmetric BiO6 groups. The first NCS nonpolar DJ phase, CsBi2Ti2TaO10 exhibits nonlinear optical properties as well as piezoelectricity attributed to the asymmetric crystal structure. The powder SHG measurements on RbBi2Ti2NbO10 and CsBi2Ti2TaO10 reveal that both materials are phase-matchable (type I) and possess strong SHG efficiencies of about 100 times that of α-SiO2.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx. Atomic coordinates and equivalent isotropic displacement parameters, IR spectra, UV-vis diffuse reflectance spectra, TGA diagrams, piezoelectric measurement data, polarization and pyroelectric measurement data (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].



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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014M3A9B8023478). TTT and PSH thank the Welch Foundation (Grant E-1457) and the National Science Foundation (DMR-1503573) for support.

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