Thermal Expansion and Second Harmonic Generation Response of

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Thermal Expansion and Second Harmonic Generation Response of the Tungsten Bronze Pb2AgNb5O15 Kun Lin,† Pifu Gong,‡ Jing Sun,† Hongqiang Ma,† You Wang,† Li You,§ Jinxia Deng,† Jun Chen,† Zheshuai Lin,*,‡ Kenichi Kato,∥ Hui Wu,⊥,# Qingzhen Huang,⊥ and Xianran Xing*,† †

Department of Physical Chemistry and §State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China ‡ Beijing Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ∥ RIKEN SPring-8 Center, Hyogo 679-5148, Japan ⊥ Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, United States # Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742-2115, United States S Supporting Information *

ABSTRACT: The incorporation of transition metal element Ag was performed to explore negative thermal expansion (NTE) materials with tetragonal tungsten bronze (TTB) structures. In this study, the structure and thermal expansion behaviors of a polar TTB oxide, Pb2AgNb5O15 (PAN), were systematically investigated by high-resolution synchrotron powder diffraction, high-resolution neutron powder diffraction, transmission electron microscope (TEM), and high-temperature X-ray diffractions. The TEM and Rietveld refinements revealed that the compound PAN displays (√2aTTB, √2bTTB, 2cTTB)-type superstructure. This superstructure within the a−b plane is caused by the ordering of A-site cations, while the doubling of the c axis is mainly induced by a slight tilt distortion of the NbO6 octahedra. The transition metal Ag has larger spontaneous polarization displacements than Pb, but the Pb−O covalence seems to be weakened compared to the potassium counterpart Pb2KNb5O15 (PKN), which may account for the similar Curie temperature and uniaxial NTE behavior for PAN and PKN. Powder second harmonic generation (SHG) measurement indicates that PAN displays a moderate SHG response of ∼0.2 × LiNbO3 (or ∼100 × α-SiO2) under 1064 nm laser radiation. The magnitudes of the local dipole moments in NbO6 and PbOx polyhedra were quantified using bond-valence approach. We show that the SHG response stems from the superposition of dipole moments of both the PbOx and NbO6 polyhedra.

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Pb2KNb5O15 (PKN).13 The NTE along the polar b axis (only occurs in Pb-based ones among the TTBs) was found to be caused by both Pb−O covalence and the cooperated rotations of the NbO6 octahedra induced by A(Pb)−O covalence, and little contribution was addressed from the ionic alkali cations K+.13 The partial replacing K using Li, which has larger electronegativity and smaller ionic radius (rK+ = 1.51 Å; rLi+ = 0.92 Å; CN = 8), could enhance the A−O polarization (and hence the NTE) along the polar b axis, but the resultant tilt of the NbO6 octahedra away from the layered c axis contributes extra positive thermal expansion along this direction.14,15 To achieve stronger NTE and volumetric NTE, one possible route is to substitute K with a cation that possesses moderate ionic radius (larger than Li+) and larger electronegativity. Then, the Ag+, a transition metal ion with electrons [Kr]4d10 and a radius of 1.28 Å (CN = 8),15 was taken into account.

INTRODUCTION The search for negative thermal expansion (NTE) materials is still a challenging target, because the NTE behavior, in which one or more cell dimensions contract while heating, only occurs in a very limited variety of materials.1,2 It is well-known that the NTE is generally structurally related, especially in frameworkinduced NTE materials, for example, ZrW2O8,3,4 ScF3,5,6 or Fe[Co(CN)6].7 Interestingly, there is one type of NTE that occurs in functional materials, and these NTEs were shown to be dominated by their physic properties such as ferroelectricity, superconductivity, magnetism, electron configuration change, and so on.1,8,9 The important branch, ferroelectricity-induced NTEs, is mainly achieved in the PbTiO3-based perovskites, which is caused by the spontaneous polarization (ferroelectricity) induced distortion of oxygen octahedra along the polar c axis.10−12 Recently, we reported the uniaxial NTE induced by the combination of ferroelectricity and framework effects in a Pbbased tetragonal tungsten bronze (TTB) structured material © 2016 American Chemical Society

Received: November 23, 2015 Published: February 29, 2016 2864

DOI: 10.1021/acs.inorgchem.5b02702 Inorg. Chem. 2016, 55, 2864−2869

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Figure 1. SAED patterns along different directions. The reflections are indexed using average structure (√2aTTB, √2bTTB, cTTB) set. X-ray diffraction (XRD), high-temperature XRD (HTXRD), and TEM experiment methods have been reported elsewhere.14

Herein, we designed, synthesized, and characterized a composition with chemical formula Pb2AgNb5O15 (PAN). Although PAN had been previously synthesized by Soviet Union scholars in the 1980s,16 the detailed physical properties are not investigated, and its crystal structure is not determined yet. In this study, we systematically investigated the crystal structure and thermal expansion behavior of PAN by means of high-angular resolution synchrotron X-ray powder diffraction (SPD), high-resolution neutron powder diffraction (NPD), transmission electron microscope (TEM), and various temperature X-ray powder diffraction experiments. They show that PAN possesses a (√2aTTB, √2bTTB, 2cTTB)-type superstructure with a polar space group Cm2m (different from other TTBs). However, the NTE behavior was not obviously enhanced as expected. In addition, the powder second harmonic generation (SHG) measurement showed that PAN adopts an SHG response of ∼0.2 × LiNbO3 under 1064 nm laser radiation and is not type-1 phase-matchable. The influence of Ag on the polarization as well as thermal expansion was discussed in terms of chemical bond, and the origin of the SHG response was discussed in terms of polyhedral dipole moment contributions. The present work enriched our knowledge of the structure− property relationship of TTB structured materials and could be helpful in exploring NTE and nonlinear optical materials with TTB structures.

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RESULTS AND DISCUSSION Structure Descriptions. The product was verified by powder XRD, indicating an orthorhombic TTB-type structure with cell parameters of (√2aTTB, √2bTTB, cTTB). Then, TEM equipped with selected area electron diffraction (SAED) was performed to determine the space group. The SAED patterns shown in Figure 1 were captured by continuously tilting the sample stage with different angles using a double-tilt sample stage. As can be seen from Figure 1, strong main reflections accompanied by very weak superstructure reflections can be observed. The extinction condition of the main reflections and the existence of SHG signals (discussed below) indicate that the average structure adopts polar space group Cm2m (No. 38) or Cmm2 (No. 35), but Cmm2 was excluded due to the polar axis along the b axis (evidenced by the negative thermal expansion along this direction). The superstructure reflections of (0, 0, l/2), where l = 2n + 1, implies the doubling of the c axis. Taking the superstructure reflections into consideration, then, a new structure model (c′ = 2c, a subgroup of Cm2m) was established, and it also adopts the space group Cm2m (Im2m (44); Ic2m (46) was excluded by systematic extinction condition h + k = 2n). To the best of our knowledge, the (√2aTTB, √2bTTB, 2cTTB)-type superstructure with space group Cm2m was reported for the first time in TTB family, in comparison with PKN (Cm2m: √2aTTB, √2bTTB, cTTB),13 Ba2NaNb5O15 (Cmm2: √2aTTB, √2bTTB, 2cTTB),20 Pb2K0.5Li0.5Nb5O15 (Bb21m: 2√2aTTB, √2bTTB, 2cTTB),14 Sr2NaNb 5 O 15 (Im2a: 2√2a TTB , 2√2b TTB , 2c TTB ), 21,22 and Ba0.5−xTaO3−x (P21212: aTTB, 3bTTB, cTTB).23 In addition to the (√2aTTB, √2bTTB, 2cTTB) superstructure, some more complex superstructures were also observed during the TEM experiments (Figure S1), which could be caused by the fluctuation of the chemical composition during the solidstate synthesis procedure. For simplification, these superstructures were not considered. Subsequently, high-angular resolution SPD17 and highresolution NPD experiments were conducted to further analyze the structural details.24,13,14 First, trial Rietveld refinements showed that the average structure (namely, (√2aTTB, √2bTTB,

EXPERIMENTAL SECTION

Polycrystalline samples were synthesized by solid-state method using PbO, Ag2CO3, and Nb2O5 (4N) as raw materials. High-resolution NPD data were collected on the BT-1 diffractometer at the Center for Neutron Research at the National Institute of Standards and Technology using a Cu monochromator (λ = 1.5403 Å) at room temperature (RT). High-angular resolution SPD data (RT) were collected at the BL44B2 beamline17 of SPring-8 synchrotron radiation facility using a constant wavelength of 0.500 26 Å and a large Debye− Scherrer camera. Structural refinements were completed using JANA2006 software.18 Powder SHG measurement was performed on a Kurtz-NLO system19 using a pulsed Nd:YAG laser with an incident wavelength of 1064 nm. The polycrystalline samples were ground and sieved into distinct particle size ranges (23−38, 38−52, 52−75, 75−105, 105−125, 125−150, 150−180, 180−250, and 250− 350 μm). For comparison, the known SHG materials, LiNbO3, were also ground and sieved into the same particle size ranges. The powder 2865

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Figure 2. Rietveld refinement patterns of PAN using (√2aTTB, √2bTTB, cTTB) (a, b) and (√2aTTB, √2bTTB, 2cTTB) (c, d) model sets. (a−d) The patterns for SPD (a, c) and NPD (b, d), respectively. (inset, a) The unidentified weak peaks (◆) indicate minor unknown phases. (inset, c) The arrows (↓) indicate calculated superstructure reflections that cannot be experimentally observed.

cTTB)) could fit the SPD data well (Rwp = 2.78%, GOF = 4.68, Figure 2a), but it poorly fits the NPD data (Rwp = 11.81(1)%, GOF = 2.87, see Figure 2b). Interestingly, the superstructure model fits the NPD data better (Rwp = 9.58%, GOF = 2.34, Figure 2d); however, no obvious improvement occurs when fitting the SPD data (Rwp = 2.71%, GOF = 4.58, Figure 2c). The superstructure reflections were hardly observed in the SPD profile, neither did the single-crystal diffraction experiments (see Figure S2). As it is known, the X-ray scattering factors increases monotonically with ionic electron numbers and are sensitive to the heavy elements, the absence of strong superstructure reflections in the SPD profile should be caused by the ordering of light O atoms, which are, conversely, sensitive to the neutrons. Finally, the superstructure model was determined by refining against the combined SPD and NPD data. The resultant parameters are summarized in Table 1. Figure 3 describes the final crystal structures of PAN. Viewing along the c axis, the A-site Pb/Ag atoms adopt nearly Figure 3. Perspective view of the crystal structure of PAN determined by NPD and SPD at RT from [001] (a) and [100] (b) directions. The structure of PAN consists of corner-sharing NbO6 octahedral framework and tunnel A-site cations,14 and only three NbO6 octahedra were drawn in (a). The dashed frame labels the basic TTB cell dimension.

Table 1. Selected Crystal Data for the Final Structure of Pb2AgNb5O15 chemical formula Z cell parameters

space group A-site occupancies

Ap1 Ap2, Ap3 At

Pb2AgNb5O15 8 a = 17.6093(2) Å b = 17.9063(2) Å c = 7.7247(1) Å V = 2435.74(5) Å3 Cm2m (No. 38) Pb Ag (16.4(6)%), Pb (83.6(6)%) Ag (83.6(6)%), Pb (16.4(6)%)

the same positions within the x−y plane, while the NbO6 octahedra distort slightly and tilt from the c axis by an angle of Δθ ≈ 7°, smaller than that of PKLN of Δθ ≈ 10°,14 which could be the origin of superstructure (c′ = 2cTTB). It is wellknown that there are three types of pentagonal A sites (Ap1, Ap2, and Ap3), and one type of quadrangular site (At) in the TTB structures.14,25 The cross-substitutions of Pb and Ag 2866

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synergic rotations. Considering the ionic radii rAg+ (1.28 Å) < rPb2+ (1.29 Å) < rK+ (1.51 Å) (CN = 8),15 the small Ag+ tends to occupy the small quadrangular At site (83.6%), and thus more Pb exists in the pentagonal Ap sites, which is believed to promote the NTE.13,14 However, the larger PS displacement of Ag (compared to Pb and K) seems to enhance the A−O covalence, which might contribute to NTE and polarization as well. However, the NTE or macroscopic polarization was not obviously enhanced as expected by the replacement of alkali metal K with alkali earth metal Ag. This contradiction may be related to the electron structures of Ag. To understand the electron structure and chemical bond characteristics between these two materials, the partial density of state (PDOS) projected on the constitutional atoms of both PAN and PKN were calculated based on the density functional theory. As shown in Figure S6, the band just below the Femi level (−6 to 0 eV) is predominately composed of O-2p, Ag-4d, and Nb-4d states, revealing the Ag−O and Nb−O covalence for PAN and that for PKN was dominated by O-2p, Nb-4d, and Pb-6s states, revealing the Pb−O and Nb−O covalence. Thus, it is suggested that compared to PKN, the covalence between Ag and O was strengthened, while those of Pb−O were weakened in PAN. The former tend to contribute to NTE as well as polarization, but the latter do the opposite, which might account for the similar macroscopic polarization and uniaxial NTE behavior for PAN and PKN. Second Harmonic Generation Properties. The noncentrosymmetric structure of PAN motivates us to investigate its nonlinear optical (NLO) properties. The as-synthesized material was light gray in color. UV−vis absorption spectrum suggests that PAN is wide band gap semiconductor with optical band gap of ∼2.64 eV (see Figure S7). Polycrystalline powder SHG properties were studied by a Kurtz-NLO method, and it shows (Figure 5a) PAN displays a mediate SHG response of ∼0.2 × LiNbO3 in the 105−125 μm particle size range under 1064 nm laser radiation and is ∼100 × α-SiO2.19,26,27 Additional measurement (Figure 5b) demonstrates that the SHG intensity decreases with particle size, suggesting that PAN is not type-1 phase-matchable.19,26 Ferroelectric hysteresis loops were also observed (Figure S8). The polarization reversal, evidenced by current maximum of ca. ±40 kV/cm in Figure S8, indicates that PAN is not only polar but also ferroelectric. From the structural point of view, the polarization originates from summation of local dipole moments (LDMs) in the unit cell.28−32 For PAN, the LDMs could come from the distortions of three types of polyhedra: NbO6, AO12, and AO15 (A = Pb, Ag). The space group Cm2m indicates a specific direction of polarization along the b-axis for PAN; that is, both the a- and ccomponents of the net dipole moments cancel out to be zero. To better understand the synergistic effect of LDMs from different types of polyhedra, the LDMs for each cation site were calculated using the bond-valence approach.33 As shown in Table 3, we found that the y-components of LDMs associated with PbOx is 5.54 Debye (D) in average, much larger than that

atoms at each site were tested, and the Ap1 turned out to be exclusively occupied by Pb (4d) (∼99%), while the Ap2, Ap3, and At sites were occupied by both Pb and Ag atoms (see Table 1 and Figure 3). Thus, relative to the basic TTB cell, the √2type orthorhombic variant superstructure was formed by the ordering of A-site cations within the a−b plane. Besides, the Ag+ cations display larger spontaneous polarization (PS) displacements (Figure 3) than Pb2+, in contrast to the K+ that have smaller PS displacements than the Pb2+. In addition, the Pb in Ap2 and Ap3 sites split by a mirror (x = 0.5) in the large pentagonal tunnels with a distance of ∼0.6(1) Å, which could associate with the lone pair stereochemical character of Pb2+. Thermal Expansion and Thermal Stability Studies. The as-synthesized PAN material is thermally stable above 800 °C (Figure S3). High-temperature XRD experiment reveals an orthorhombic to tetragonal transition at ∼450 °C (Figure 4)

Figure 4. Temperature dependence of cell parameters of PAN. The solid lines are guides for the eyes, and the error bars are too small to be distinguished.

agreeing with Filipyev’s report.16 But this transition cannot be detected by thermal analysis (Figure S3), implying little latent heat release during the phase transition. The evolution of cell parameters with temperature is shown in Figure 4. Similar to Pb2KNb5O15 (PKN)13 and Pb2K0.5Li0.5Nb5O15 (PKLN),14 PAN displays a uniaxial NTE along the b axis but positive thermal expansions along the other two axes (Figure S4). The coefficients of thermal expansion (CTEs) are summarized in Table 2. It shows that compared to PKN and PKLN, with Ag+ replacing K+, the NTE along the polar b axis is only slightly changed (from −1.56(0) to −1.69(1) × 10−5 °C−1) and is much smaller than that of PKLN (−2.36(1) × 10−5 °C−1). This trend is in accordance with the orthorhombic distortion (b/a; Figure S5) and ferroelectric Curie temperature (TC; see Table 2), suggesting the similar polarization in PKN and PAN. On the other hand, the CTE of PAN along the c axis is just between PKN and PKLN, agreeing well with the degree of tilt of NbO6 octahedra (0°, 7°, and 10° for PKN, PAN, and PKLN). As mentioned above, the NTE along the polar b axis is caused by A−O covalence and its induced NbO6 octahedra

Table 2. Average Coefficients of Thermal Expansion along the a, b, and c Axes from Room Temperature to Curie Temperature

PAN PKN PKLN

TC, °C

αb, 1 × 10−5 °C−1

αa, 1 × 10−5 °C−1

αc, 1 × 10−5 °C−1

αV, 1 × 10−5 °C−1

450 460 500

−1.69(1) −1.56(0) −2.36(1)

1.29(0) 1.69(0) 1.60(0)

2.18(1) 1.62(0) 2.73(1)

1.77(1) 1.35(0) 1.97(2)

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Figure 5. (a) Oscilloscope traces of the SHG signal of PAN compared with LiNbO3 (particle size: 105−125 μm). (b) Particle size dependence of SHG intensity for PAN.

Table 3. Magnitude (in Debye) of the Polyhedral Local Dipole Momentsa for Pb2AgNb5O15 along the Polar b-Axis Direction Pb1O15 Pb1′O15 Pb2O15 Pb2′O15 Pb3O15 Pb3′O15 PbtO12 Pbt′O12 ave total weight a

8.51 5.66 −0.70 4.86 6.05 7.63 3.87 −0.62 5.54 88.62 0.55(1)

−3.36 1.92 0.47 0.59 1.97 0.34

Ag2O15 Ag2′O15 Ag3O15 Ag3′O15 AgtO12 Agt′O12

Nb1O6 Nb2O6 Nb3O6 Nb4O6 Nb5O6 Nb6O6

0.95 7.62 0.41(1)

3.67 −5.42 1.52 0.47 2.33 4.80

1.65 65.94 0.05

For the average and total LDMs calculations, the site multiplicities and occupancies were considered.

addition, the as-synthesized PAN material displays an SHG response of ∼0.2 × LiNbO3, which is dominated by dipole moments superposition of PbOx and NbO6 polyhedra. This study also implies Pb-based TTBs to be potential NLO materials.

of NbO6 and AgOx polyhedra with 1.65 and 0.95 D, respectively. The LDMs for different NbO6 octahedra, induced by second-order Jahn−Teller distortion (SOJT), constructively add along the b-axis, while the ones for both PbOx and AgOx polyhedra are partially offset (by the negative magnitudes of the b-component). The total weight of contributions from PbOx, NbO6, and AgOx polyhedra are 55%, 41%, and 5%, respectively. Thus, the SHG response of PAN mainly originates from the lone pair containing PbOx polyhedra and the SOJT-induced distortions of the NbO6 octahedra, and the contribution from AgOx is neglectable. The SHG response is the nonlinear response of noncentrosymmetric materials to the laser. Ferroelectric materials belong to non-centrosymmetric and polar materials and are SHG active. For Pb-based bronzes, the uniaxial NTE along the polar b axis is associated with the release of ferroelectric polarization. Thus, both NTE and SHG responses are closely related to ferroelectric polarization, suggesting their intrinsic physical correlation. The SHG signal may be used as probe to forecast and explore NTE materials in ferroelectrics.

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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.5b02702.

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CONCLUSIONS In summary, the transition metal Ag was introduced into the TTB-type compositions to investigate the thermal expansion behavior. The crystal structure of the TTB composition Pb2AgN5O15 was determined by combined SPD, NPD, and TEM studies. It shows that PAN displays a superstructure with cell dimensions of (√2aTTB, √2bTTB, 2cTTB). The Ag could form Ag−O bond but, in return, the Pb−O bond seems to be weakened, which may result in similar polarization and thermal expansion behavior for Pb2AgN5O15 and Pb2KN5O15. In

Experimental details, computational methods, references, SAED patterns, thermal analysis curves (TG-DSC), thermal expansion data, UV diffuse reflectance spectrum, P−E curves. (PDF) Crystallographic information file. (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (X.X.) *E-mail: [email protected]. (Z.L.) Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest. 2868

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant Nos. 91022016, 21031005, 21231001, 91422301, 21590793), Program for Changjiang Scholars and Innovative Research Team in University (IRT1207), and the Fundamental Research Funds for the Central Universities, China (Grant No. FRF-SD-13-008A). The synchrotron radiation experiments were performed at the BL44B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2015B1127). We thank Prof. J. Sun, Prof. G. Li, and Z. Zhou (Peking Univ.) for their help with the single-crystal diffraction measurements. We also thank Prof. K. M. Ok (Chung-Ang Univ.) for his useful discussion on the dipole moment calculations.

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ABBREVIATIONS PAN, Pb2AgNb5O15; PKN, Pb2KNb5O15; PKLN, Pb2K0.5Li0.5 -Nb5O15; CTE, coefficient of thermal expansion; NTE, negative thermal expansion

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