Relaxor-like Dielectric Behavior in Stoichiometric Sillenite Bi12SiO20

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Relaxor-like Dielectric Behavior in Stoichiometric Sillenite Bi12SiO20 Yu Hu and Derek C. Sinclair* Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, United Kingdom ABSTRACT: A combination of fixed frequency dielectric measurements (103 to 106 Hz) and impedance spectroscopy reveal evidence of (nonferroelectric) relaxor-like behavior in the temperature range ∼600−750 K for the stoichiometric sillenite phase Bi12SiO20 (space group I23, a = 10.1034(5) Å). Variable temperature Raman Spectroscopy reveals significant changes associated with the Bi−O framework vibrations, and the relaxor-type behavior is attributed to noncorrelated local dipole moments associated with the BiO5E (E = 6s2 lone pair) units in the framework. Sillenites, therefore, represent another family of Bi-based oxides to exhibit relaxor-like dielectric behavior. KEYWORDS: Bi-based relaxors, sillenites, impedance spectroscopy, Raman spectroscopy

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INTRODUCTION Sillenites are a wide ranging family of materials that are structurally related to γ-Bi 2 O 3 1 with general formula Bi12MxO20±δ, where M represents ion(s) on a tetrahedral site with an average oxidation state from 2+ to 5+ and ionic radius from 0.1 Å (B3+) to 0.98 Å (Pb2+).2 The ideal stoichiometric sillenite contains a tetravalent M-cation of ionic radius, r = 0.31 Å, forming geometrically regular MO(3)4 tetrahedra3 and a fully occupied oxygen sublattice (i.e., 20),2 Figure 1a. The

M-ions,8 such as Bi12B3+O19.59 and Bi12(Bi3+0.03V5+0.89□0.08)O20.27,2 respectively. In general, sillenites have a pseudo-body-centered cubic unit cell and the average structure belongs to the noncentrosymmetric space group I23 (no. 197), Figure 1a. Cubic systems of this type can be piezoelectric but not ferroelectric, as they are not polar. The complex three-dimensional (3D) framework structure is formed by Bi−O distorted polyhedra, BiO5E, where Bi3+ ions are coordinated with five oxygen ions as well as its stereochemically active 6s2 lone electron pairs (denoted as E), Figure 1b. Pairs of these distorted Bi octahedra edge-share through two oxygen ions, O1(b) and O1(c), and corner-share with O(3) ions from adjacent MO(3)4 tetrahedra; these chains form a 3D network by corner-sharing through the other two oxygen ions, O1(a) and O(2).2 The O(4) site is empty in ideal Bi12MO20 but is reported to be partially occupied for oxygenexcess sillenites.2 Stoichiometric sillenites such as Bi12M4+O20, where M = Si, Ge, and Ti, have long been recognized for their electro-optic properties;10−12 however, in recent years, their dielectric properties have attracted attention for applications in low temperature cofired ceramics (LTCC) technology due to their low sintering temperatures (commonly ≤850 °C), chemical compatibility with silver electrodes and low dielectric loss and temperature-stable permittivity, εr, of ∼35−40, near room temperature.13,14 Limited research has been performed on the dielectric properties of sillenites as a function of temperature and frequency. To date, Valant and Suvorov15 reported Bi12(B3+1/2P5+1/2)O20 to exhibit a broad, frequency dispersive, “relaxor-like” εr anomaly at ∼−80 to ∼100 °C, which was

Figure 1. Structure of (a) ideal Bi12M4+O20 sillenite viewed along the (001) plane, where yellow, blue, and red spheres represent Bi3+, M4+, and O2− ions, respectively; black lines indicate the body-centered cubic unit cell of the structure. (b) A fragment of the structure to show the various crystallographic sites of sillenites.

closest known examples of Bi12MO20 are with M = Si4+ (r = 0.26 Å), Ge4+ (r = 0.39 Å), and Ti4+ (r = 0.42 Å)4,5 with the most ‘perfect’ crystal lattice being Bi12GeO20 based on structural refinement of neutron diffraction6 and Raman spectroscopy data.7 Significant deviations from this ideal structure and composition have been widely reported, and nonstoichiometric sillenites with an oxygen deficiency (20) can be prepared depending on the valence of the © 2012 American Chemical Society

Received: September 27, 2012 Revised: December 3, 2012 Published: December 11, 2012 48

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RESULTS A. Structural Characterization. The XRPD pattern of Bi12SiO20 (not shown) was fully indexed on ICDD file 80-627 (space group I23) with a refined lattice parameter of 10.1034(5) Å, confirming the powder to be phase-pure. RS of Bi12SiO20 in the frequency range 40−1000 cm−1 at 298 and 773 K are shown in Figure 2. The peak intensities of the RS

attributed to a random distribution of the B and P atoms over the tetrahedral sites. It is well-known that the sillenite framework can be highly distorted. This can occur in ‘ideal’ stoichiometric sillenites such as Bi12SiO20 due to a mismatch between the ‘ideal’ and actual ionic radius of the M-site cation for the framework structure and due to the presence of the 6s2 electron lone pair on the Bi atoms. Such lone pairs occupy a volume in space similar in size to an O2− ion and protrude into the framework space available in the vicinity of the O4 site, see Figure 1b. The stereochemical activity of the electron lone pairs has a significant role in the bonding between the Bi and O atoms and therefore influences the electronic structure, vibrational modes, and dielectric properties. It is not surprising, therefore, that sillenites exhibit a variety of electro-optic and electrical properties including Second Harmonic Generation (SHG) and high relative permittivity and, in the case of some nonstoichiometric compositions, mixed oxide-ion-electronic conduction.16 Here, we report the dielectric properties of the stoichiometric sillenite Bi12SiO20 as a function of frequency (103 to 106 Hz) at different temperatures (10 to 773 K). A frequency-dependent dielectric dispersion phenomenon above room temperature (RT) is observed and analyzed by a combination of Raman spectroscopy and impedance spectroscopy. The electrical data reveal this ‘ideal’ stoichiometric sillenite with geometrically regular tetrahedral SiO4 units to, surprisingly, exhibit (nonferroelectric) relaxor-like dielectric relaxation behavior above room temperature. Equivalent circuit analysis of the impedance data in combination with high temperature Raman spectroscopy data are used to elucidate the possible origin of the dielectric dispersion, which is attributed to dielectric lattice relaxation of the distorted BiO5E units in the sillenite framework structure.

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Figure 2. Raman spectra of Bi12SiO20 at 298 and 773 K.

data at 773 K were normalized by the strongest peak at ∼538 cm−1 from the spectrum recorded at RT. The Raman peak positions are inversely proportional to the mass of the groups of bonded atoms; therefore, the vibrational modes of framework Bi polyhedra, BiO5E, dominate the lower frequency RS data (i.e. predominantly below ∼650 cm−1), whereas the vibration modes of the SiO4 tetrahedra occur at higher frequencies. There is a switchover in the most intense peak in the RS spectra with increasing temperature. The Raman active vibration at ∼538 cm−1 is the largest peak in the spectrum at 298 K, whereas at 773 K the ∼89 cm−1 peak dominates the spectrum, Figure 2. These peaks are assigned as the breathing of framework O1 atoms (∼538 cm−1) and the elongating vibrations of framework Bi−O3, Bi−O2 vibrations (∼89 cm−1).17 The associated peak shape and height (i.e. full width half maximum (fwhm)) can be used to provide structural information. The fwhm values of the ∼538 cm−1 peak at 298 and 773 K are 13.2 and 33.7 cm−1, respectively, whereas the fwhm of the ∼89 cm−1 peak at 298 and 773 K remains the same, with a value of 7.5 cm−1. The broadening and downshifting of the ∼538 cm−1 peak with increasing temperature indicates the interatomic distances between Bi and O1 atoms in the bipyramidal groups of the framework are getting larger and this part of the framework of the structure is becoming increasingly disordered and/or flexed with increasing temperature. Similar broadening of peaks at 144, 166, 276, and 328 cm−1 indicate temperature dependent dynamics of the O2 atoms. In contrast, there is minimal change in the position or fwhm of the peak at ∼89 cm−1 with temperature indicating much more rigidity associated with Bi−O2 and Bi−O3 bonds. For the latter, this can be linked to the sharing of O3 atoms between the Bi and Si atoms and reveals the influence of the rigid tetrahedral SiO4 units on the flexibility of the framework structure. B. Thermal Analysis. DSC was performed on Bi12SiO20 with a heating and cooling rate of 10 °C/min in the temperature range from ∼400 to 1123 K, Figure 3. Two pairs of reversible peaks with endothermic processes during

EXPERIMENTAL SECTION

Sillenite, Bi12SiO20, was prepared by using a conventional solid state reaction method. Reagents Bi2O3 (Sigma-Aldrich, 99.9%, 180 °C) and SiO4 (Alfa Aesar, 99.5%, 600 °C) were dried at the indicated temperatures, weighed (∼10 g total), mixed with acetone in an agate mortar, ground manually by pestle for 30 min, and then heated in a gold foil boat at 840 °C for 48 h with intermittent grinding every 12 h to aid the reaction. Single phase Bi12SiO20 was detected by X-ray powder diffraction (XRPD) using a Stoë STADI P diffractometer (Cu Kα1 radiation, 1.54059 Å) with a small linear position sensitive detector operating in transmission mode, step size 0.01° and 2θ range 10−70°. Silicon was used as a standard to determine accurate lattice parameters. Pellets were pressed uniaxially, fired at 890 °C for 6 h, and coated with sputtered Au on opposite pellet faces for electrical characterization. Pellet density was 90 % of the theoretical value. Radiofrequency (rf) fixed frequency capacitance and impedance spectroscopy (IS) measurements at low temperatures (∼10 to 320 K) used a He cryocooler with an Agilent 4294A impedance analyzer and at high temperatures (∼320 to ∼800 K) used a tube furnace with a HewlettPackard 4192A impedance analyzer. All data were corrected for sample geometry prior to analysis. Equivalent circuit analysis of IS data was performed using ZView software. Differential scanning calorimetry (DSC) was performed using a Netzsch DSC404C instrument operating from 400 to 1123 K with a heating and cooling rate of 10 K/min. Raman spectroscopy (RS) was performed using a Renishaw inVia micro-Raman spectrometer with an Ar-laser (514.5 nm) and recorded in backscattering geometry. A laser power of 10 mW was focused on a ∼2 μm spot on the sample. A Linkam THMS600 cell was used for obtaining data above room temperature (298−773 K). All RS data were corrected for the Bose-Einstein thermal factor. 49

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Figure 3. DSC trace of Bi12SiO20 powder.

heating and corresponding exothermic processes during cooling were observed at ∼920 and 1103 K, respectively, which indicates a plausible reversible γ → β → δ → β → γ phase transition sequence occurring in this sample as observed in other sillenites18 and doped Bi2O3 compounds. XRPD confirmed the sample retained the same (γ) sillenite phase on post-DSC measurements. This shows Bi12SiO20 to be a thermodynamically stable phase below ∼900 K that transforms to other Bi2O3-type polymorphs above ∼900 K. C. Electrical Characterization. (1). rf Fixed Frequency Measurements. The temperature dependence of the permittivity (εr) and dielectric loss (tan δ) at fixed frequencies of 1, 10, and 100 kHz and 1 MHz over the temperature range from 10 to 800 K are shown in Figure 4. A broad frequencydispersive region of εr and tan δ was observed above RT, with the peak maximum shifting to higher temperature with increasing frequency. In the εr plot, the 1, 10, and 100 kHz data exhibit a peak with an εr maximum of ∼335, 136, and 66 at temperatures of ∼597, 661, and 739 K, respectively. The corresponding capacitance maximum of the 1, 10, and 100 kHz data is 2.97 × 10−11, 1.20 × 10−11, and 5.84 × 10−12 F cm−1, respectively. The data at 1 MHz does not show the presence of a peak maximum within the measured temperature range; instead, it exhibits an increase in εr from ∼37 at 200 K to ∼45 at 800 K, corresponding to an increase in capacitance from ∼3.28 × 10−12 to ∼3.98 × 10−12 F cm−1, respectively. In the tan δ plot, the 1, 10, and 100 kHz data exhibit corresponding peaks with a tan δ maximum of ∼3.4, 3.6, and 4.1 at ∼543, 619, and 746 K, respectively. (2). Impedance Spectroscopy. Impedance data of Bi12SiO20 in the formats of Z* plots and combined −Z″, M″ spectroscopic plots, Figure 5a−f, are shown at selected temperatures of 623, 673, and 773 K. Two semicircular arcs are present in Z* plots, Figure 5a and c, which correspond to the two Debye peaks in the −Z″ spectroscopic plots, Figure 5b and d at 623 and 673 K, respectively. The magnitude of the lower frequency semicircular arc in the Z* plots decreases rapidly with increasing temperature and eventually merges with the higher frequency arc at 773 K, Figure 5e and f. Spectroscopic plots of C′ at various temperatures between 10 and 773 K, Figure 5g, also reveal the existence of two plateaux: one at higher frequency and one at lower frequency. The higher frequency C′ plateau is frequency-independent and exists over

Figure 4. Temperature dependence of (a) permittivity and (b) dielectric loss for Bi12SiO20. Insets are an expanded view of lower temperature data between 10 and 400 K.

almost the entire measured temperature range with C′high ∼ 3.6 pF cm−1 (ε′high ∼ 41), whereas the lower frequency C′ plateau is frequency-dependent and is present in the measured frequency range above 573 K with the capacitance plateau decreasing significantly with increasing temperature: C′low ∼ 24 pF cm−1 (ε′low ∼ 272) at 623 K to C′low ∼ 11 pF cm−1 (ε′low ∼ 126) at 773 K at 1 kHz. The frequency-independent high frequency C′ plateau corresponds to the large, higher frequency semicircular arc observed in the Z* plots, Figure 5a, c, and e, and the high frequency Debye peak in the combined −Z″, M″ spectroscopic plots, Figure 5b, d, and f, which is attributed to the bulk response of the sample with C′ ∼ 3.6 × 10−12 F cm−1 (ε′ ∼ 41). The frequency-dependent lower frequency C′ plateau corresponds to the smaller, lower frequency semicircular arc in the Z* plots, Figure 5a and c, and the low frequency peak in the −Z″ spectra, Figure 5b and d and is attributed to an intrinsic bulk relaxation of the sample. Justification for these assignments will be provided from equivalent circuit fitting analysis presented later. (3). Equivalent Circuit Fitting. Various equivalent circuits using resistors, R, capacitors, C, and constant phase elements, CPE, were used to try and fit the data including the brickwork layer model (BLM) of two parallel RC (or R-CPE) elements connected in series that is used to model grain and grain boundary responses. This equivalent circuit is commonly used 50

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Figure 5. Z* plots and combined −Z″, M″ spectroscopic plots at 623 K (a, b), 673 K (c, d), and 773 K (e, f) and C′ spectroscopic plots at various temperatures (g) for Bi12SiO20 ceramic.

to fit IS data of electroceramics based on two arcs in Z* plots as presented in Figure 5; however, such a model gave a poor fit to the data (not shown), and the magnitude and temperature dependence of the C values extracted for the lower frequency response were inconsistent with those expected from a grain boundary response; therefore, the BLM circuit made no physical sense. It was clear, therefore, that a parallel-type

equivalent circuit may be more appropriate compared to the series-type equivalent circuit based on the BLM. The equivalent circuit in Figure 6a accurately fits the IS data over the temperature range of 573−773 K and the measured frequency range of 103−106 Hz, as shown in examples for the IS data set at 673 K, Figure 6b−f. All the fitted values of the circuit parameters R1, C1, Cx, and CPE (A and n), Table 1, show 51

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Figure 6. (a) Equivalent circuit used to fit the IS data. Fitting results (+) from the IS experimental data points (o) for Bi12SiO20 at 673 K to the equivalent circuit shown in (a) with (b) Z* plot; (c) Y′, (d) C′, (e) −Z″/M″, and (f) tan δ spectroscopic plots.

Table 1. Fitted Values of Bi12SiO20 from 573 to 773 Ka temp. (K) 573 591 610 623 658 673 698 723 743 773 a

R1 (Ω cm) 5030700 3192800 1971700 1388800 596350 427730 226740 145860 101940 69007

C1 (pF cm−1)

Cx (F cm−1)

3.54 3.54 3.53 3.53 3.52 3.51 3.51 3.51 3.51 3.48

6.32 4.73 3.50 3.11 1.75 1.59 1.07 7.64 5.54 4.33

× × × × × × × × × ×

−11

10 10−11 10−11 10−11 10−11 10−11 10−11 10−12 10−12 10−12

A (Ω−1 cm−1 rad−1) 6.01 6.32 7.05 7.91 9.80 1.12 1.40 1.63 1.84 1.60

× × × × × × × × × ×

10−8 10−8 10−8 10−8 10−8 10−7 10−7 10−7 10−7 10−7

n 0.12 0.13 0.14 0.15 0.17 0.18 0.19 0.20 0.20 0.23

Errors associated with the fitted circuit parameters R1, C1, Cx, A, and n are < ±0.4%, < ±0.3%,< ±1.5%,< ±8%, and < ±2.5%, respectively.

reasonable physical significance: R1 is the total dc resistance of the sample that obeys the Arrhenius law; C1 with a value of pico-farads is a typical bulk capacitance (ε ∼ 40)19 and has little temperature dependence; the series combination of CPE and Cx (Cole−Cole branch) represents polarization associated with lattice relaxation process(es), where Cx represents a blocking capacitance associated with the local polarization behavior and the CPE with n in the range from ∼0.12 to 0.23 represents a predominantly leakage-resistivity element (low n) based on Z*CPE = [A(jω)n]−1 = [Aωn(cos(nπ/2) + j sin(nπ/2))]−1.20

A Y′ spectroscopic plot, Figure 6c, shows a frequency independent plateau attributed to the dc conductivity, R−1, at lower frequency (f < 104 Hz), with frequency-dependent power law behavior at higher frequencies (f > 104 Hz). The dc plateau dominates the data at higher temperatures, whereas the power law response dominates the data at lower temperatures. The corresponding C′ spectroscopic plot, Figure 6d, shows a frequency independent plateau at higher frequency ( f > 105 Hz), attributed to the magnitude of the bulk capacitance C1, with the power law response at low frequencies ( f ∼ 104 Hz). 52

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sillenite Bi12(B1/2P 1/2)O20 with the relaxation behavior occurring over the temperature range ∼200 to 400 K and attributed to random occupation of B and P atoms on the Msites.15 What is clearly surprising in our study is that relaxor-like behavior can be obtained from a near ideal sillenite that contains only Si atoms on the M-sites, Figure 4. Additionally, the relaxor-like behavior is observed (for equivalent rf measurements) at much higher temperatures, over the range ∼600 to 750 K, and the dielectric loss is more than an order of magnitude greater than that reported for Bi12(B1/2P1/2)O20. The origin of the relaxor-type behavior is therefore different in these two compounds and in the case of Bi12SiO20 the framework must play a significant role. It is noteworthy from the DSC results that Bi12SiO20 is thermodynamically stable with a sillenite-type structure below ∼900 K, Figure 3, and therefore, the relaxor-type behavior observed at rf values occurs significantly below the structural transformations to β- and subsequently δ-Bi2O3-type structures at elevated temperatures. The RS data in Figure 2 show significant temperature dependence associated with Bi−O1 vibrations (∼538 cm−1) whereas there is negligible change in Bi−O2/O3 vibrations (∼89 cm−1). The downshifting and broadening of the ∼538 cm−1 peak with increasing temperature indicates increasing distortion of the framework and increasing interatomic distances between the Bi and O1 atoms. The peak at ∼89 cm−1 remains unchanged, indicating little change in the Bi−O3 and Bi−O2 bond lengths with increasing temperature. The bridging O3 atoms between the Si and Bi atoms constrain the movement of the Bi atoms; however, the connectivity of adjacent Bi atoms via O1 atoms (site 1b and 1c) and the presence of the 6s2 electron lone pair on the Bi atoms permits considerable structural flexibility associated with the BiO5E bipyramids, Figure 1b. We suggest the high temperature relaxor-type behavior observed for Bi12SiO20 is associated with local polarization effects due to the flexibility of the framework structure and, in particular, with small but noncorrelated movement of the Bi atoms in the framework to create local dipole moments. In the case of M = Si, this is observed via rf electrical measurements above room temperature; however, in principle, it should be present in other sillenites. The temperature range over which this relaxor-type behavior can be observed by rf measurements will depend on the stoichiometry and size of the M-site cation(s) of the sillenite phase. Further studies are in progress to confirm this hypothesis. It is worth commenting that the relaxor-type behavior observed at lower temperatures for Bi12(B1/2P1/2)O20 may well be associated with the mixed occupation of B3+ and P5+ on the M-sites as suggested by Valant and Suvorov15 but that higher temperature neutron powder diffraction measurements are required to observe the framework relaxation effect observed here for Bi12SiO20. This is also currently under investigation. It is important to understand how we observe this relaxortype effect using electrical measurements and this requires examination of the equivalent circuit, Figure 6a, that successfully modeled the IS data, Figures 5 and 6. At low temperatures, Bi12SiO20 is a dc insulator with R1 > 10 G ohms and the resistivity component of the CPE is also exceedingly large. As a result, negligible current flows in the top and bottom arms of the equivalent circuit in Figure 6a and it therefore reduces to a single capacitor, C1 and this explains the temperature independent capacitance (εr ∼ 40) and very low loss in the temperature range 10−300 K, Figure 4. Above 400

Below the power law response, there is a frequency dependent plateau at lower frequency ( f ∼ 103−104 Hz), attributed to the polarization arising from the dielectric relaxation of the lattice, Cx. At f < 103 Hz, there is a continuous increase in the data with decreasing frequency that is attirbuted to space-charge polarization21,22 associated with the sample−electrode interface. This response is beyond the scope of the present work, and no attempt has been made to include it in the equivalent circuit analysis/fitting, hence the poor agreement between the low frequency experimental and fit data in Figure 6d. The temperature dependence of extracted C1 and Cx from the fitted values is shown in Figure 7. The magnitude of C1 is

Figure 7. Temperature dependence of fitted capacitances C1 and Cx.

temperature-independent, with a value of ∼3.5 pF cm−1 (εr ∼ 40) from 573 to 773 K, whereas Cx decreases by an order of magnitude with increasing temperature from 6.32× 10−11 F cm−1 (εr ∼ 714) at 573 K to 4.33 × 10−12 F cm−1 (εr ∼ 49) at 773 K. An Arrhenius plot of the long-range bulk conductivity (where σ = 1/R1), Figure 8, is extracted from the fitted values of R1 by equivalent circuit analysis and the associated activation energy is 0.84(1) eV.

Figure 8. Arrhenius plot of bulk conductivity extracted from the fitting values of R1.

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DISCUSSION The variable temperature rf measurements of εr and tan δ of Bi12SiO20 ceramic ranging from 1 kHz to 1 MHz, Figure 4, reveal a broad frequency-dispersive region above RT, which is more ordinarily found in disordered solids such as relaxor ferroelectrics22,23 and dipolar glasses.24 However, relaxor-type behavior has been reported previously for the stoichiometric 53

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ACKNOWLEDGMENTS We thank the EPSRC for funding (EP/G005001/1). We thank Dr. J. Pokorny for training with variable temperature Raman Spectroscopy.

K, Bi12SiO20 starts to exhibit significant long-range (dc) electronic conduction, and therefore, current can pass via R1 in the equivalent circuit. In addition, ac current can flow through the Cole−Cole branch (CPE-Cx) of the circuit. This branch is the relaxation process associated with short-range orientation of the dipoles associated with the BiO5E units in the framework. The resistive component of the CPE becomes smaller due to increased thermal energy and ease of movement of the Bi atoms with increasing temperature. CPE’s are commonly used to model IS data but often suffer from a lack of physical significance. Relaxor behavior is strongly dependent on CPE’s, which can range from resistive (n close to zero) to capacitive (n close to one) depending on the origin of the effect. Unfortunately, we are not in a position to offer a definitive microscopic model for the CPE-Cx arm of the equivalent circuit, but it is linked inherently to the co-operative nature of the vibrations and distortions associated with the framework structure that ultimately influence the polarization behavior in even the simplest of sillenites such as Bi12SiO20. The data presented in Figure 4 is another example of relaxoreffects in a Bi-based compound. Care is required to distinguish bulk relaxor-like behavior from that associated with the frequency response of complex equivalent circuits that contain contributions from bulk, grain boundary, and electrode−sample interface impedances.25,26 The most commonly reported relaxors are Bi- and Pb-based compounds and many of them are ferroelectric based on noncentrosymmetric cells such as Pb(Zn1/3Nb2/3)O3;27 however, nonferroelectric examples based on cubic symmetry do exist28−30 such as the pyrochlore phase (Bi3/2Zn1/2)(Zn1/2Nb3/2)O7, space group Fd3m. Not all Bibased oxides exhibit relaxor-type behavior, 26 and this demonstrates the importance of the structure-type and local bonding between the Bi and O atoms in controlling the stereochemically active lone pair based on an admixture of the Bi 6s and 6p orbitals and O 2p orbitals. Co-operative movements of atoms in the sillenite framework is complex, as there are competing vibrations associated with BiO5 E bipyramids connected via the MO4 tetrahedral units, Figure 1a. Nevertheless, based on the results presented here, local dipole moments can arise in simple, stoichiometric sillenites such as Bi12SiO20 based on macroscopic cubic symmetry.

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REFERENCES

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CONCLUSIONS Bi12SiO20 was successfully prepared by a traditional solid state reaction method, and a combination of XRPD and DSC showed it to be thermodynamically stable below ∼900 K. Bi12SiO20 is categorized as an ‘ideal’-type sillenite; however, rf fixed frequency and IS measurements showed it to exhibit (nonferroelectric) relaxor-like behavior in the range ∼600−750 K. IS data were successfully modeled by an equivalent circuit consisting of a parallel connection of a R, C, and Cole−Cole branch (CPE in series with a C). Variable temperature RS data revealed significant changes associated with Bi−O framework vibrations and the relaxor-type behavior (as modeled by the Cole−Cole branch in the equivalent circuit) attributed to noncorrelated local dipole moments associated with the BiO5E units in the framework.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. 54

dx.doi.org/10.1021/cm3031363 | Chem. Mater. 2013, 25, 48−54