Synthesis, Direct Formation under High Pressure, Structure, and

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Synthesis, Direct Formation under High Pressure, Structure, and Electronic Properties of LiNbO3‑type Oxide PbZnO3 Daisuke Mori,*,† Kie Tanaka,† Hiroyuki Saitoh,‡ Takumi Kikegawa,§ and Yoshiyuki Inaguma*,† †

Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan Quantum Beam Science Center, Japan Atomic Energy Agency, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan § Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan ‡

S Supporting Information *

ABSTRACT: A novel LiNbO3-type (LN-type) lead zinc oxide, PbZnO3, was successfully synthesized under high pressure and temperature. Rietveld structure refinement using synchrotron powder X-ray diffraction (XRD) data demonstrated that LN-type PbZnO3 crystallized into a trigonal structure with a polar space group (R3c). The bond valence sum estimated from the interatomic distances indicated that the sample possesses a Pb4+Zn2+O3 valence state. Polarization could evolve as a result of the repulsion between constituent cations because PbZnO3 does not contain a stereochemical 6s2 cation or a Jahn−Teller active d0 cation. Distortion of ZnO6 octahedra resulting from cation shift is comparable with that of d0 TiO6 in ZnTiO3 and MnTiO3 with LN-type oxides, which leads to stabilization of the polar structure. PbZnO3 exhibited metallic behavior and temperature-independent diamagnetic character. In situ XRD measurement revealed that the formation of LN-type PbZnO3 occurred directly without the formation of a perovskite phase, which is unusual among LN-type materials obtained by high-pressure synthesis.

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This is also the first case in which both the Pv and LN phases could be obtained at ambient pressure. The LN-type structure is polar and can be described as a derivative of the Pv-type structure since both compounds possess three-dimensional corner-sharing MO6 octahedra.14 In the field of earth science, LN-type compounds are considered unquenchable Pv-type phases under high pressure.15−29 Pioneering work conducted by Syono et al.15,16 and Sleight and Prewitt29 focused on the functional properties of LN-type oxides using high-pressure synthesis. From this, novel LN-type compounds, such as ZnSnO3,30 (In1−xMx)MO3 (x ≈ 0.111− 0.176; M = Fe0.5Mn0.5),31,32 CdPbO3,33 GaFeO3,34 LiOsO3,35 Mn2FeMO6 (M = Nb, Ta),36 ZnTiO3,37,38 Zn2FeTaO6,39 ScFeO3,40 and MnTaO2N,41 have been developed that exhibit remarkably attractive properties, such as second harmonic generation,30,37,42 ferroelectricity,43 ferroelectric metals,35 polar ferromagnets,40 and helical spin order.41 Furthermore, dielectric and magnetic coupling behaviors have been reported for wellknown FeTiO344 and MnMO3 (M = Ti, Sn).45 These findings indicate that research on the topic of LN-type oxides can deliver materials with desirable properties based on naturally occurring polar structures. In this study, we focused on PbZnO3 as a novel PbMO3 compound. Zn is able to stabilize Pb4+, which is uncommon in the PbMO3 series, as the divalent cation Zn2+ is stable and has a similar ionic radius to Ni2+ in PbNiO3.46 Furthermore, the

INTRODUCTION Lead 3d transition metal oxides have attracted widespread attention and are of significant scientific interest due to their functional properties. Their structure, physical properties, and valence state depend on the transition metal ion. PbTiO3 is a well-known ferroelectric material whose polarity is derived from Pb2+ with a lone pair of 6s2 electrons and second-order Jahn− Teller active Ti4+ ions.1−3 PbTiO3 has a tetragonal structure and is the only ternary lead 3d transition metal perovskite (Pv) obtainable at ambient pressure. Conversely, other lead 3d transition metal Pv oxides, such as PbVO3, PbCrO3, PbMnO3, PbFeO3, and PbNiO3, are obtained under high-pressure conditions. PbVO3 with V4+ has the same structure as PbTiO3, but with a large tetragonal distortion originating from the d1 cations,4,5 and exhibits two-dimensional magnetism attributed to d xy orbital ordering.6,7 PbCrO3 with Cr4+ crystallizes into a cubic structure8 with periodic deficiencies of Pb and O ions.9 Meanwhile, the charge disproportionation into Pb2+ and Pb4+ can be determined for PbFeO3 using X-ray photoelectron spectroscopy.10 PbFeO3 has an orthorhombic unit cell of 6a x 2a x 2a (where a is the pseudocubic subcell length), but its detailed structure has not yet been determined. PbMnO3 with Mn4+ has a hexagonal structure11 and transforms via 6H to a 3C Pv structure with increasing applied pressure.12 Pv-type PbNiO3 is the first example from the Pb4+M2+O3 series with the Pv containing a tetravalent A-site cation.13 Of greater interest is that the Pv phase transforms to the LiNbO3 (lithium niobate, LN) phase upon heat treatment at ambient pressure. © XXXX American Chemical Society

Received: September 8, 2015

A

DOI: 10.1021/acs.inorgchem.5b02049 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry tolerance factor of PbZnO3 (0.77) is comparable to that of PbNiO3 (0.79). Thus, we considered that it would be interesting to investigate whether PbZnO3 would adopt a Pvor LN-type structure. Here, we examined the conditions for synthesis under high pressure and temperature, resulting in the successful synthesis of PbZnO3 with a LN-type structure. The crystal structure, electronic properties, and formation behavior of PbZnO3 under high pressure are discussed.

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EXPERIMENTAL PROCEDURES

PbZnO3 was synthesized by a solid-state reaction under high pressure. PbO2 (>99.999% in purity) and ZnO (>99.99% purity) were mixed with KClO3 (>99.5% purity) as an oxidizing agent. The mixture was sealed in a Au capsule and heated at 1000−1200 °C for 30 min under 5−7.5 GPa using a multianvil-type high-pressure apparatus (NAMO 2001, TRY engineering). The details are described in our previous work.13,47 The obtained sample was washed with distilled water and nitric acid to remove the resultant KCl and unidentified impurity phase, respectively. Phase identification was carried out by powder X-ray diffraction (XRD) measurements using a Rigaku RINT2100 diffractometer with monochromatic Cu Kα radiation. The sample composition was determined by inductively coupled plasma (ICP) spectroscopy with a HORIBA Ultima 2 ICP optical emission spectrometer. Synchrotron XRD (SXRD) data were collected on a Debye−Scherrer-type powder diffractometer with an imaging-plate-type detector installed in the beamline (BL02B2) at SPring-8. The wavelength of the incident synchrotron radiation was fixed at 0.41846 Å. The sample was packed into a glass capillary with a 0.1 mm outer diameter. The structural parameters were refined by Rietveld analysis using the RIETAN-FP program,48 and the crystal structure was drawn using the VESTA program.49 The electronic resistivity was measured by the four-probe direct current method at a temperature range of 10−300 K using a sintered pellet formed at 1000 °C under 6 GPa after washing. Temperature dependence of magnetic susceptibility was measured from 5 to 300 K in an external magnetic field of 1 kOe using a superconducting quantum interference device (SQUID) magnetometer (MPMS, Quantum design). To elucidate the formation behavior of PbZnO3, in situ XRD measurements were conducted under high pressure and temperature using a cubic multianvil-type high-pressure apparatus (MAX80) installed in the beamline (NE5C) at the Photon Factory Advanced Ring (PF-AR) of the High Energy Accelerator Research Organization (KEK). A cell assembly similar to that used for high-pressure synthesis in the laboratory with a 6 mm side length was adopted. White beam Xray radiation with an energy of 20−140 keV was used to irradiate the sample through the high-pressure cell and was detected with a Ge solid-state detector fixed at 2θ = 5.0°. The sample was pressed at room temperature and then heated.

Figure 1. (a) XRD patterns for PbZnO3 synthesized at (a) 1000−1200 °C under 7.5 GPa and (b) 1100 °C under 6 GPa with various amounts of KClO3. (b) XRD patterns for PbZnO3 synthesized at (a) 1000− 1200 °C under 7.5 GPa and (b) 1100 °C under 6 GPa with various amounts of KClO3.

shown in Figure 1b. The sample synthesized with 1 wt % KClO3 contained Pb2O3, ZnO, and an unknown phase as impurities. Although Pb2O3 derived from the reduction of PbO2 disappeared with increasing oxidizing agent, the amount of unknown phase increased, and PbO2 was observed in the sample with 2 wt % KClO3. Consequently, when using 1.5 wt % KClO3, almost a single phase was obtained with only a very small amount of unknown phase. The unknown impurity phase could be removed by washing with 3 M nitric acid for 1.5 h, resulting in a single phase of PbZnO3, which forms a gray powder. The XRD pattern of PbZnO3 (top of Figure 1b) could be indexed using a hexagonal lattice with a trigonal space group (R3c or R3c̅ ). The constituent cation ratio of PbZnO3 was confirmed to be Pb/Zn = 0.990(18) by ICP analysis. In the thermogravimetric-differential thermal analysis (TG-DTA) curves for PbZnO3 (shown in Figure S1), mass reduction and an endothermic peak were observed between 800 and 950 K. The XRD measurement after heating demonstrated that PbZnO3 was decomposed to PbO and ZnO (shown in Figure S2). Structural Studies. Structure refinement was carried out for the SXRD data of the PbZnO3 single phase synthesized at 1100 °C under 6 GPa. Since the SXRD pattern of PbZnO3 was quite similar to those of LN-type PbNiO313 and ZnTiO3,37 the LN-type structure with the R3c space group was adopted as the

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RESULTS AND DISCUSSION High-Pressure Synthesis of PbZnO3. PbZnO3 was synthesized at 1100 °C under various pressures. The sample synthesized under 3 GPa contained PbO2, Pb2O3, and ZnO without a new phase, while a new phase with some impurities was formed in the samples synthesized under pressures above 5 GPa. Subsequently, the temperature conditions were examined. The XRD patterns for PbZnO3 synthesized at 1000−1200 °C under 7.5 GPa are shown in Figure 1a. The sample synthesized at 1000 °C contained large amounts of PbO2 and ZnO with the new phase. Conversely, Pb2O3 and ZnO were detected in the samples synthesized above 1100 °C, indicating that the PbO2 starting material was reduced at high temperature. Therefore, the oxidizing conditions under high pressure and temperature were examined. The XRD patterns for PbZnO3 synthesized at 1100 °C under 6 GPa with various amounts of KClO3 are B

DOI: 10.1021/acs.inorgchem.5b02049 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry structure model. Fractional coordinate z of Zn was fixed at zero. The occupation parameters were fixed at unity according to the results of ICP analysis. Figure 2 displays the observed and

Figure 3. Crystal structure of LiNbO3-type PbZnO3.

Table 2. Selected Bond Lengths, Bond Angles, Distortion of Octahedra, Bond Valence Sum (BVS), and Calculated Polarizations for LiNbO3-type PbZnO3 bond length/Å

Figure 2. Observed (+) and calculated (solid green line) powder SXRD patterns, their differences (solid blue line), and peak positions (|) from Rietveld analysis of LiNbO3-type PbZnO3.

Zn−O average Zn−O Pb−O

calculated SXRD patterns and their differences for PbZnO3. The small difference and sufficiently low R-factor demonstrated that PbZnO3 possesses a trigonal polar space group (R3c). However, refinement using the R3̅c space group with the corundum-type structure and R3̅ with the ilmenite-type structure could not achieve an acceptable quality of fit between the observed and calculated patterns. The refined structural parameters for PbZnO3 are listed in Table 1. The crystal structure of PbZnO3 is illustrated in Figure 3. The selected bond lengths, bond angles, distortion of octahedra, bond valence sum (BVS), and calculated spontaneous polarizations are summarized in Table 2. The distortion of PbO6 and ZnO6 octahedra for PbZnO3 are estimated by the equation ⎛ d − d ⎞2 ∑ ⎜ i av ⎟ dav ⎠ i=1 ⎝

average Pb−O bond angle/deg. Zn−O−Zn Pb−O−Pb distortion of octahedra Δ

(×3) (×3) (×3) (×3)

132.1 (2) 129.27(18)

ZnO6 PbO6 BVS (deviation from assumed valence)

3.23 × 10−3 0.81 × 10−3

Zn (+2) Pb (+4) calculated polarization/μC cm−2

1.89 (−6%) 4.23 (+6%)

P

113

6

1 Δ= 6

2.274(4) 2.030(4) 2.152 2.239(5) 2.115(5) 2.177

(1)

where di is the individual cation−O bond length and dav is the average bond length. Spontaneous polarization in a simple point-charge model is calculated by employing the structural parameters according to the formula P = (∑ qiΔzi)/V

1.70%, respectively, compared with PbNiO3. The BVS estimated from interatomic distances demonstrated that the sample valence state is represented as Pb4+Zn2+O3, similar to PbNiO3. It was found that PbZnO3 is a unique polar material containing 6s0 and d10 cations, and that it does not contain second-order Jahn−Teller active cations (d0 transition metal) or stereochemical lone pair cations (6s2; Pb2+, Bi3+), which induce polarization. Therefore, the polar structure would originate from the repulsion between constituent cations. The calculated polarization of 113 μC cm−2 was greater than that of PbNiO3 (95 μC cm−2). The distortion of Pb4+O6 octahedra attributable to the cation shift from the noncentrosymmetric position was comparable to that of PbNiO3 (1.07 × 10−3), whereas the distortion of ZnO6 was much greater than that of NiO6 in PbNiO3 (8.26 × 10−5). This difference can be

(2)

i

where V is the unit cell volume, qi is the charge on the ith atom, and Δzi is the displacement along the c-axis of the ith atom from its centrosymmetric position, corresponding to z = 1/4 for the A cation and z = 1/12 (0.0833) for oxygen in ABO3 with the LN-type structure. The lattice parameters of PbZnO3 were greater than those of PbNiO3, and the expansion of the lattice in PbZnO3 was anisotropic.13 That is, the lattice parameters a and c of PbZnO3 were enlarged by 0.98% and Table 1. Structural Parameters of LiNbO3-type PbZnO3a

a

atom

site

g

x

y

z

B/Å2

Pb Zn O

6a 6a 18c

1 1 1

0 0 0.0375(10)

0 0 0.3617(7)

0.28186(6) 0 0.0558(4)

0.500(7) 0.546 (18) 0.67(7)

R3c space group (No. 161), Z = 6, a = 5.41548(2) Å, c = 14.33010(11) Å, Rwp = 3.03%, Rp = 2.30%, S = 1.17, RB = 2.24%, and RF = 1.59%. C

DOI: 10.1021/acs.inorgchem.5b02049 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Pb−O bond distance in PbZnO3 was comparable to that of CdPbO3 (2.175 Å), while the Pb−O−Pb bond angle in PbZnO3 was much smaller than that of CdPbO3 (140.3°). It indicated that the Pb−O−Pb bond angles are little involved in the electric resistivity of PbZnO3 and CdPbO3. The firstprinciple calculations within DFT using hybridized-functional approximation (HSE) revealed that CdPbO3 has a semiconducting ground state with a finite band gap of 0.94 eV.51 It is in contrast to LiOsO3 with the metallic ground state in nature attributed to Os and O orbitals lying on Fermi level.35 Moreover, PbZnO3 and CdPbO3 showed large residual resistivity. Therefore, the metallic behavior of PbZnO3 is most likely due to either cation or oxygen nonstoichiometry in an undetectable level by the Rietveld refinements. The magnetic susceptibility exhibited a temperature-independent diamagnetic character of −5 × 10−4 emu mol−1 as expected from the PbZnO3 valence state (shown in Figure S3). Formation Behavior under High Pressure. Figure 5 depicts the in situ XRD patterns of PbZnO3 under 8 GPa. The

attributed to the anisotropic lattice expansion of PbZnO3 compared to PbNiO3. The distortions of ZnO6 in PbZnO3 (3.24 × 10−3), LN-type ZnSnO330 (5.03 × 10−3), and ZnTiO337 (3.81 × 10−3) were greater than the distortions of MO6 octahedra formed by other cations in LN-type materials, such as MnO6 in MnTiO345 (1.51 × 10−3) and MnSnO345 (2.69 × 10−3) and SnO6 in ZnSnO3 (4.30 × 10−4) and MnSnO3 (2.55 × 10−4). Meanwhile, the distortions of ZnO6 were comparable to those of TiO6 in ZnTiO 3 (4.74 × 10 −3 ) and MnTiO 3 (3.77 × 10 −3 ). Additionally, the distortions of ZnO6 were also comparable to that of NbO6 (4.00 × 10−3) in well-known LiNbO3-type oxide LiNbO3 (shown in Tables S1, S2).50 That is, Zn is potentially capable of adapting to the distortion of octahedra corresponding to cation shift. Moreover, as the LN-type structure has a cation arrangement of A−B−Vac−A−... (Vac = vacancy) along the c-axis, the repulsion between A and B cations would relax upon cation shift toward the vacancy site in the octahedra formed by cations and O, which would lead to reinforcement of the polarity. Consequently, the polarity of PbZnO3 can be attributed to repulsion between the cations and the capability of Zn to tolerate the acentric shift. Physical Properties. Figure 4 shows the temperature dependence of electronic resistivity for PbZnO3 with LN-type

Figure 5. In situ XRD patterns under 8 GPa at various temperatures for LiNbO3-type PbZnO3. The characteristic X-ray radiations of tungsten (W Kα) are due to the beam slit or collimater.

diffraction patterns obtained at room temperature, 600 °C, 800 °C, 1000 °C, and room temperature after heating are displayed in sequence starting from the bottom. The Bragg reflections derived from β-PbO2 as a starting material with Au as the sample container and the characteristic X-ray radiation of Au (Au Kα2) were observed in the XRD pattern under 8 GPa at room temperature. The peak broadening of β-PbO2 was caused by lattice strain and/or particle size reduction upon application of pressure.13,52 The rutile-type β-PbO2 transformed to orthorhombic α-PbO2 upon heating to 600 °C, which is consistent with a previous report.53 Above 800 °C, the reflection of α-PbO2 disappeared and the reflection of PbZnO3 appeared. No reflections derived from other phases were observed. Therefore, the in situ XRD measurement under high pressure and temperature showed that the formation of LN-type PbZnO3 occurred directly above 800 °C without the formation of a Pv phase. In general, LN-type oxides are considered to be unquenchable phases of the Pv phase that are stable under high pressure and are obtained via formation of the Pv phase. In fact, Ross et al. reported that LN-type MnTiO3 reversibly transforms to an orthorhombic Pv phase between 2

Figure 4. Temperature dependence of electronic resistivity for LiNbO3-type PbZnO3 with PbNiO3 and CdPbO3.

PbNiO3 and CdPbO3. PbZnO3 exhibited electronic resistivity of 3.5 × 10−2 Ω cm at room temperature and metallic behavior, in which the electronic resistivity decreased with decreasing temperature. The electronic resistivity of PbZnO3 was smaller than that of PbNiO3,13 which exhibited semiconducting behavior. The Pb−O bond distance and Pb−O−Pb angle in LN-type PbNiO3 are 2.175 Å and 126.6°,13 respectively, which are comparable to those of PbZnO3. Therefore, the difference in electronic resistivity between PbZnO3 and PbNiO3 could not be explained exclusively in terms of the overlap between Pb and O orbitals. Therefore, it may be ascribed to not only the covalency of the Pb−O bond but also the widespread and unoccupied 4s and 4p orbitals of Zn. Meanwhile, PbZnO3 exhibited similar behavior of resistivity and had comparable electronic resistivity to CdPbO3. It is speculated that the conduction mechanism of PbZnO3 is similar to CdPbO3. The D

DOI: 10.1021/acs.inorgchem.5b02049 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and 3 GPa.19 The Pv phases of PbNiO313 and ScFeO340 have also been observed under high-pressure conditions. Therefore, the formation behavior of LN-type PbZnO3 is quite rare among LN-type oxides obtained by high-pressure synthesis. The investigation of LN-type compounds exhibiting functional properties using high-pressure synthesis has been conducted primarily for compounds in which the Pv phase is likely to be stable under high pressure. The findings of the present study provide another perspective on the development of functional LN-type materials based on their naturally occurring polarization.

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CONCLUSION LN-type PbZnO3 was successfully synthesized under pressures above 6 GPa at 1100 °C. This is uncommon in terms of the emergence of polarization in materials containing only nonstereochemical 6s0 and second-order Jahn−Teller inactive d10 cations. Polarization was caused by the repulsion between constituent cations and reinforced by the capability of Zn for adapting to the cation shift in MO6 octahedra. The metallic behavior of PbZnO3 is most likely attributed to either cation or oxygen nonstoichiometry. In situ XRD studies under high pressure and temperature revealed the unusual formation of the LN-phase of PbZnO3. These findings provide another approach to the development of novel functional LN-type oxides based on their polarity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02049. Figures S1−S3 and Tables S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The powder SXRD measurement of SPring-8 was performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposal No. 2011B1453). The in situ XRD measurement under high-pressure and temperature was conducted under the approval of Photon Factory Program Advisory Committee (Proposal No. 2010P102, 2011G681) and was carried out for preliminary study at BL14B1 in SPring-8 under the Common-Use Facility Program of JAEA (Proposal No. 2011B3615). This work was supported by a Grant- in-Aid for Scientific Research (No. 21360352, 24360275, 15H04128 to Y.I.) of the Japan Society for the Promotion of Science and the Promotional Project for Development of a Strategic Research Base for Private Universities: Matching Fund Subsides of the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02049 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b02049 Inorg. Chem. XXXX, XXX, XXX−XXX