Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX
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Intragranular Phase Proton Conduction in Crystalline Sn1−xInxP2O7 (x = 0 and 0.1) Cortney R. Kreller,*,† Hieu H. Pham,‡ Mahlon S. Wilson,† Rangachary Mukundan,† Neil Henson,§ Milan Sykora,∥ Monika Hartl,⊥ Luke Daemen,# and Fernando H. Garzon∇ Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ABSTRACT: Materials that exhibit fast ionic conduction in the intermediate temperature range are highly sought after for applications in fuel cells, electrolyzers, and sensors. Initial reports on tin pyrophosphate indicted that this material exhibited excellent protonic conductivity at 100−300 °C and that transport occurred via proton hopping through the bulk crystalline lattice. In this work, we conclusively show that the high conductivity reported by other research groups is not attributable to the bulk crystalline phase. The proton conduction mechanism of well-characterized Sn1−xInxP2O7 (x = 0 and 0.1) was investigated using ac impedance spectroscopy and inelastic neutron scattering. The crystalline MP2O7 phase possesses negligible bulk conductivity below 600 °C. Above 600 °C, the total conductivity exhibits Arrhenius behavior with an activation energy of ∼1 eV, with an increase in conductivity observed for the In-doped sample. Inelastic neutron scattering data indicates that no appreciable changes in proton concentration occur between hydrated and dehydrated samples of SnP2O7 while changes in proton vibrational mode amplitudes occur with indium doping. The vibrational modes identified for the two materials are consistent with atomistic models of the bulk crystalline conductivity mechanism, where our calculations show that doping of In does not increase the mobility; instead, it helps to incorporate protons. This is also consistent with the Arrhenius behavior of the conductivity in which the activation energy is very similar between the undoped and doped material but with the doped material showing a larger pre-exponential constant. Our modeling results indicate that the interoctahedra hop between two M− O−P bridges is the most dominant transport pathway irrespective of doping. This work helps resolve the ongoing discrepancies in the literature regarding the mechanism of proton conduction in this material system. There exist two distinct conduction mechanisms between stoichiometric Sn1−xInxP2O7 and excess polyphosphate containing Sn0.9In0.1P2+xO7±z. Intragranular proton transport through the bulk crystalline Sn1−xInxP2O7 material occurs only at elevated temperatures. An amorphous polyphosphate phase residing at the crystalline grain boundaries (not the intercrystalline grain boundaries of the metal pyrophosphate itself) is required to obtain the high conductivity at reduced temperatures reported in the literature.
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on the order of 10−3 S cm−1 at temperatures 600 °C) to form the MP2O7 phase along with the evaporation of excess P2O5. Several investigators have reported that this synthesis method is prone to leaving residual phosphoric acid intimately mixed with the MP2O7 crystalline phase. As the phosphoric acid phase is amorphous, it is virtually undetected by X-ray diffraction (XRD) measurements. Most investigations employ X-ray fluorescence (XRF) to measure the final P:M ratio, which should be 2 for a stoichiometric material without any residual phosphoric acid phase; however, small amounts of unreacted metal oxide precursors can significantly offset the final P:M ratio. For example, a SnP2O7 sample with 5% by mass unreacted SnO2 and 5% by mass excess phosphoric acid would yield a total measured P:M ratio of 2.02. Therefore, multiple techniques must be used in conjunction to fully characterize the composition of these samples. In a recent study,20 we synthesized Sn0.9In0.1P2O7 materials containing various amounts of an excess polyphosphate and examined how varying the excess polyphosphate content influenced material properties and total conductivity. We observed that total conductivity increased with increasing excess polyphosphate content (quantified via XRF), up to a critical temperature (∼400 °C) where conductivity dramatically decreasedthe onset temperature of loss in conductivity coincided with the loss of the excess polyphosphate material measured by thermogravimetric analysis (TGA). Lattice parameters extracted by whole-profile XRD pattern fitting revealed no systematic trend with excess polyphosphate content, indicating that the crystalline structure of the materials was not affected by the presence of the additional phosphate. The excess polyphosphate was also undetected in the diffraction patterns, indicating that this phase is amorphous. The results of our study strongly indicate that the presence of an excess polyphosphate phase is required to achieve high conductivity at low−intermediate temperatures. It is worth emphasizing that the grain boundary phase responsible for the high conductivity in prior reports is the excess polyphosphate phase and not the intercrystalline grain boundaries in the metal pyrophosphate itself. Nevertheless, previous reports of the correlation between the total measured conductivity and aliovalent doping levels raises interest in the possibility of interactions between the polyphosphate phase and the support matrix that serve to enhance the conductivity. Substitution of native lattice tetravalent cations with trivalent cations results in negatively charged defect sites that are compensated by the formation of positively charged defects. Some authors have proposed that the defect chemistry of doped MP2O7 is similar
to that of the proton conducting perovskites, where the positively charged defects are oxygen vacancies, which can incorporated protons into the lattice in the form of hydroxide sites when exposed to water vapor:1 x • V •• O + H 2O(g) + OO → 2OH O
Alternatively, it has been proposed that the formation of pyrophosphate vacancies is more energetically favorable as the strength of the A−O bond is less than that of the P−O bond (532 and 599 kJ/mol, respectively) in diatomic molecules. In this scenario, the defect chemistry of doped MP2O7 is similar to that of LaPO4, where the positively charged defects are pyrophosphate vacancies, which can incorporate protons upon exposure to water vapor in the form of hydrogen phosphate groups.3−8 The present study focuses on the theoretical study, synthesis, and characterization of the crystalline Sn1−xInxP2O7 (x = 0 and 0.1) material in order to better understand the defect chemistry and conductivity mechanism of the crystalline material in the absence of the fast proton conducting amorphous polyphosphate phase, as well as to firmly establish that the mechanism of proton hopping through the bulk of the crystalline material is not responsible for the high conductivities measured at low− intermediate temperatures in MP2O7 systems. First-principles calculations were performed using the climbing image nudged elastic band method (CI-NEB) to determine the minimumenergy path (MEP) and activation energy for proton diffusion.21,22 Stoichiometric materials synthesized via the low temperature solution precipitation method were characterized with XRD, XRF, and TGA to identify the phase and composition of the material. Vibrational spectroscopy, including Fourier transform infrared spectroscopy (FTIR) and inelastic neutron scattering (INS) were used to probe vibrational modes. The spectra of the crystalline materials are briefly compared to those of the materials with excess polyphosphate content (Sn0.9In0.1P2+xO7±z). The high temperature conductivity measured via ac impedance is discussed in terms of the vibrational modes identified via INS and compared to atomistic models of the bulk crystalline conductivity mechanism.
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EXPERIMENTAL METHODS Sample Preparation. Powders with varying excess polyphosphate content were synthesized via the solution precipitation method.23 Indium and tin chlorides were mixed with diammonium phosphate, the pH was adjusted through the addition of ammonium hydroxide, and the resulting solution was gelled at room temperature followed by calcination at 650 °C. The modified synthesis route allows for more precise gravimetric determination of phosphate content and forms the MP2O7 cubic (Pa3̅ primitive unit cell) phase in shorter times at high temperature, thus preserving the volatile phosphates. Sample Characterization. XRD patterns were measured using a Siemens Model D5000 diffractometer with Cu Kα radiation and a graphite diffracted beam monochromator. The data was analyzed using MDI JADE 9.6 whole profile/Rietveld refinement software. XRF (Spectrace Instruments QuanX) coupled to a fundamental parameter model, using tin oxide and indium phosphide standards, was used for elemental analysis to calculate the ratio of phosphorus to metal (P:M, where M = In + Sn). Thermal analysis was carried out using a Netzsch STA 449C thermogravimetric analyzer. Sample mass loss was recorded during heating under dry nitrogen at 5 °C/min to B
DOI: 10.1021/acs.jpcc.7b06060 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 1. (a) Cubic structure of SnP2O7. SnO6 are displayed as blue octahedra and PO4 are green tetrahedra. Oxygen ions (not shown) are located at corners of polyhedra. (b) Bulk proton diffusion in tin pyrophosphates: intratetrahedron hop between two Sn−O−P bridges (hop 1), intraoctahedron hop between two Sn−O−P bridges (hop 2), and intratetrahedron hop between Sn−O−P and P−O−P bridges (hop 3). (c) Interpolyhedra proton diffusion: interoctahedra hop between two Sn−O−P bridges (hop 4), and intertetrahedra hop between Sn−O−P and P−O− P bridges (hop 5).
Figure 2. XRD patterns of SnP2O7 and Sn0.9In0.1P2O7 powders. Inset shows crystallite size (d) and lattice parameter (a) extracted from whole pattern fitting structure refinement.
800 °C. Previous TGA analysis of samples heated to 1400 °C verified that all of the excess polyphosphate phase is volatilized by 800 °C; between 800 and 1000 °C no further mass loss is observed, which is then followed by the decomposition of the SnP2O7 phase with continued increase in temperature. For FTIR analysis, samples were mixed, 5% by mass, with KBr and pressed into thin pellets. Spectra were measured from 800 to 4000 cm−1 in a Thermo Scientific Nicolet 8700 FTIR spectrometer. Samples were measured via INS in both a dried state and a hydrated state. The stoichiometric samples were dried at 650 °C in dry nitrogen for 6 h and humidified at 650 °C in Ar bubbled through water at room temperature for 6 h. The Sn0.9In0.1P2+xO7±z was dried at 200C in a vacuum oven for 6 h and humidified at 200 °C with Ar bubbling through room temperature H2O for 3 h. The SnP2+yO7+z was dried in flowing Ar at 250 °C for 5 h and then at 400 °C for 6 h. The sample was humidified at 200 °C in the vacuum oven with a bottle of water inside the oven and Ar bubbling through H2O at 80 °C flowing into the oven, only for 30 min. Samples were loaded into vanadium holders and sealed using an indium gasket. Neutron vibrational spectra were collected on the Filter Difference Spectrometer (FDS) at the Lujan facility of the Los Alamos Neutron Science Center (LANSCE). Electrochemical Measurements. For conductivity measurements, the powders were ground and pressed into pellets
using a 13 mm die. The pellets were pressed uniaxially at 20 000 psi, and then isostatically to 50 000 psi. The density of the pressed pellets was 75% relative to crystalline SnP2O7. The pellets were not sintered, and conductivity measurements were made on the as-pressed pellets. Electrical contact was made via spring-loaded Pt foil current collectors on either side of the pellet, and the samples were placed inside a single-chamber quartz tube. The ac impedance measurements were performed using a Solartron 1260 frequency response analyzer in the frequency range 106−1 Hz. Impedance data was taken in dry N2 and N2 humidified by bubbling the gas stream through water at room temperature (pH2O = 0.04 bar). Atomistic Modeling. Early crystallographic studies suggested tin(IV) pyrophosphate at high temperatures exhibits a cubic structure with corner-sharing SnO6 octahedra and PO4 tetrahedra. Each PO4 tetrahedron shares a corner with another PO4 to form P2O7 groups.24 A unit cell of this cubic structure contains four tin pyrophosphate formula units (SnP2O7) and a 180° P−O−P bond angle is constrained by the symmetry Figure 1. Later work suggested that a phase transition into a tripled (3 × 3 × 3) pseudocubic superlattice with bent P2O7 units could be observed at room temperature.9,25 In this work, we use the cubic structure as the model for our computational study. C
DOI: 10.1021/acs.jpcc.7b06060 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Density functional theory (DFT),26 as implemented in the Vienna Ab initio Simulation Package (VASP),27 was employed to perform the first-principles calculations. The ionic and crystal properties were described by the electron projectoraugmented-wave methods28 with the Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation (GGA) exchange−correlation29 and a plane-wave cutoff of 770 eV. For k-space sampling, we used 2 × 2 × 2 and 1 × 1 × 1 Monkhorst−Pack grids30 for the plane wave basis in the unit cell (4 formula units: Sn4P8O28) and in the 2 × 2 × 2 supercell (32 formula units: Sn32P64O224), respectively. Details on the use of supercell types for each calculation are given in the corresponding subsection. The minimum-energy path (MEP) and activation energy for proton diffusion were obtained by using the climbing image nudged elastic band method (CINEB).21,22 Eight interpolated images between the initial and final states were used in most of our NEB calculations, and 16 images were used in several cases. Charge compensation for proton incorporation into undoped Sn(IV)P2O7 was accomplished by the introduction of a homogeneous negative background charge throughout the cell. In the case when the substitution of trivalent dopants such as In(III) and Ga(III) is introduced (denoted as InSn ′ and GaSn ′ in Krö ger−Vink notation), the positive charge of the incorporated proton is naturally compensated by the In′Sn or GaSn ′ effective negative charge. In calculations of structural vacancies (O2−, PO43−, and P2O74−), the overall charge neutrality of the supercell is maintained by introducing a respective number of In′Sn.
Figure 3. FTIR spectra for Sn1−xInx P2+yO7±z samples with P:M ratios of ∼2.0 (nominal), and excess polyphosphate ratios of 3.0 and 3.3 for Sn0.9In0.1P2O7 and SnP2O7, respectively.
previously been attributed to protons incorporated into the bulk of Sn1−xInxP2O7,8,31 but we believe that these features are associated only with the excess polyphosphate phase in agreement with work by Xu et al.12 that showed these features were only present for SnP2O7 with additional H3PO4. All samples, with and without the excess polyphosphate phase, exhibit strong absorbance peaks at 980, 1017, and 1180 cm−1 which can be assigned to the asymmetric vibration mode of bridge P−O−P, and the symmetric and asymmetric vibration modes of PO412,32,33 in the MP2O7 bulk crystalline phase. These low frequency features were examined more closely with INS in order to better understand proton incorporation into the bulk of the crystalline pyrophosphate material. INS provides more detailed information than IR spectroscopy as neutrons may impart momentum transfer on any molecular vibrations without the selection rule limitations of infrared adsorption. Due to the large inelastic neutron scattering cross section for hydrogen, the presence of small quantities of protons is easily detected. Figure 4 shows the INS difference spectra obtained by subtracting the spectra of the dehydrated samples from the spectra of the rehydrated materials for samples of varying compositions. The solid line is a filtered version of the data. It was obtained by the application of a Lee filter to smooth the data.34 The difference spectra for both the SnP 2 O 7 and Sn0.9In0.1P2O7 samples with excess P:M exhibit large features at 800 and 1230 cm−1 that are absent in the stoichiometric samples. The mode near 800 cm−1 is the P−O−P stretch in polyphosphates. This mode would normally be very weak in the neutron spectrum. The motion of hydrogen dominates the intensity of the spectrum. The P−O−P stretch is observable only because hydrogen is attached to the phosphate group and moves with the phosphate units. As such it “highlights” an otherwise weak mode. The band around 1230 cm−1 is normally attributed to the PO stretch. Since it appears strongly in the spectrum, the PO bond appears to be involved with protons
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RESULTS AND DISCUSSION Materials Characterization. The solution precipitation method was used to synthesize 20 g each of undoped and 10 mol % In doped SnP2O7 powders of nominal composition (P:M = 2). This 20 g batch was used for all of the analysis reported herein. The elemental compositions of the powders were measured with XRF coupled with a fundamental parameters model using InP and SnO2 standards. The P:M ratios were determined to be 2 and 2.02 for the SnP2O7 and Sn0.9In0.1P2O7 powders, respectively. The In:Sn + In ratio for the Sn0.9In0.1P2O7 was measured as 0.09. The SnP2O7 sample exhibited a mass loss of −0.3% measured via TGA in dry nitrogen at 800 °C, while no mass loss was observed for the Sn0.9In0.1P2O7 sample. XRD patterns are shown in Figure 2. The patterns are well fit by the cubic Pa3̅ structure of SnP2O7. All of the features in the pattern can be accounted for by the proposed 3 × 3 × 3 superstructure commonly reported for this material. Whole pattern fitting structure refinement (JADE MDI) was used to extract the crystallite size and lattice parameters, shown in the inset table of Figure 2. The value of 7.9404 obtained for the undoped material is in good agreement with the room temperature lattice parameter of 7.9444 measured by both X-ray and neutron diffraction reported by Gover et al.9 The lattice parameter is observed to increase with substitution of the larger In cation, appearing to follow Vergard’s law as reported by other authors.11 FTIR spectra for the SnP2O7 and Sn0.9In0.1P2O7 of nominal composition are shown in Figure 3 along with the spectra for SnP2O7 and Sn0.9In0.1P2O7 with excess polyphosphate phase (P:M 3.3 and 3.0, respectively). It is worth noting that only the spectra of the materials containing excess polyphosphate phase exhibit features at high wavenumbers that can be assigned to ν(OH), ν(P−O−H), and ν(OH). These features have D
DOI: 10.1021/acs.jpcc.7b06060 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 5. Arrhenius plot of conductivity of SnP2O7 (open symbols) and Sn0.9In0.1P2O7 (filled symbols). Squares represent dry N2 test gas; circles represent N2/H2O 0.04 atm wet test gas.
a closed crucible for 10 h. Tao et al. report values of approximately 1.1 × 10−7 and 2.1 × 10−6 S cm−1 in ambient air at 600 °C for SnP2O7 and Sn0.92In0.08P2O7, respectively, while Phadke et al. report values of approximately 3.2 × 10−7 S cm−1 for SnP2O7 and 1.8 × 10−6 S cm−1 for Sn0.9In0.1P2O7 at 600 °C in unhumidified air. The slightly higher values for the In-doped material could be due to the contribution of humidification from ambient air, as the prior work did not specifically investigate the influence of humidification; however, the conductivity measured in humidified N2 in this study was still lower at 2.6 × 10−7 S cm−1. Another explanation for the variation in the reported values could be the variation in pellet density. The microstructure of a fractured SnP2O7 pellet is shown in Figure 6. The unsintered pellets characterized in this
Figure 4. INS difference spectra (hydrated − dehydrated) from bottom to top: SnP2O7, SnP2+yO7+z, Sn0.9In0.1P2O7, Sn0.9In0.1P2+yO7±z.
as well. The lower frequency band 350−500 cm−1 observed in all samples is most likely a phosphate torsion mode. This feature is very weak in the difference spectrum of hydrated and dry stoichiometric SnP2O7 indicating little change in proton content. The difference spectrum for the Sn0.9In0.1P 2O7 stoichiometric sample shows comparatively greater intensity in this region, suggesting proton incorporation upon hydration. It is evident that, with the exception of the low frequency band, the samples with excess phosphate show the greatest changes in vibrational spectra upon hydration. Conductivity Measurements. The conductivities of the stoichiometric SnP2O7 and Sn0.9In0.1P2O7 materials were measured in dry and slightly humidified (pH2O = 0.04 bar at room temperature) N2 over 600−900 °C. A single semicircular arc was observed when the impedance data was plotted in Nyquist form precluding the separation of bulk grain versus crystalline grain boundary effects. The resistance obtained by fitting the single arc to a simple RC equivalent circuit was used to calculate the conductivity. The measured conductivities are plotted in Arrhenius form in Figure 5. In dry environments, the In-doped material exhibited slightly higher conductivity than the undoped SnP2O7. Upon humidification, the conductivity of Sn0.9In0.1P2O7 increased slightly, while that of SnP2O7 was unchanged. In dry environments at 600 °C, the conductivities of SnP2O7 and Sn0.9In0.1P2O7 were 1.1 × 10−7 and 2.9 × 10−7 S cm−1, respectively. These values are in close agreement with values reported by Tao et al.11 for pellets sintered at 1000 °C for 2 h and by Phadke et al.10 for pellets sintered at 1400 °C in
Figure 6. Scanning electron microscope image of fractured SnP2O7 pellet.
work had a calculated relative density of only 75%, and the scanning electron microscopy image shows a wide distribution in grain size with distributed porosity. In contrast, the sintering procedure used by Phadke et al. resulted in pellets with relative densities of 90−95%. Nevertheless, extrapolation of the high temperature conductivity predicts values on the order of 10−12 S cm−1 at 200 °C, many orders of magnitude lower than the 10−2−10−1 S cm−1 reported in the literature. The activation energy for conduction calculated from the Arrhenius equation is ∼1 eV for both the undoped and 10 mol % In-doped materials in both dry and humidified environments. E
DOI: 10.1021/acs.jpcc.7b06060 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C The In doping is thus most likely increasing the intergranular proton concentration via water hydrolysis on vacancies35 without appreciably changing the conduction mechanism. The high temperature activation energies of conduction of the stoichiometric materials are in contrast to samples with excess phosphate that display activation energies in the range 0.2−0.3 eV that are highly dependent upon the state of humidification at low temperatures (600 °C). This work serves to clarify the literature controversial claims about the wide variation in proton conductivity observed in the investigations performed by many research teams. As shown in our previous work,20 the presence of an amorphous polyphosphate phase is likely responsible for the high conductivity at low−intermediate temperatures (100−300 °C) reported in the literature. While the high activation barrier to diffusion in the crystalline MP2O7 precludes its use in practical applications, the hybrid MP2O7/ excess polyphosphate phase material system continues to show promise as an intermediate temperature proton conducting electrolyte.
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Figure 9. NEB calculations on the proton-hopping barriers in cases of structural deficiency as illustrated in Figure 8 for (a) pathway 1−2 and (b) pathway 1−3. The 2 × 2 × 1 supercell (160 atoms) was used for all VO cases, and the 2 × 2 × 2 supercell (320 atoms) was used for VPO4 and VP2O7. Substitution of tin by indium was incorporated to balance the charge of vacancy and proton addition. Unit formulas are Sn0.75In0.25P 2O7 (nondefective), Sn0.81In0.19P 2O7−2δ (with VO), Sn0.88In0.19P2−δO7−4δ (with VPO4), and Sn0.84In0.16P2−2δO7−7δ (with VP2O7), respectively (in which δ = 0.03).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Cortney R. Kreller: 0000-0003-2180-2494 Present Addresses †
C.R.K., M.S.W., and R.M.: MPA-11, Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. ‡ H.H.P.: NASA Ames Research Center, Moffett Blvd., Mountain View, CA 94035-1000, USA. § N.H.: Analytical Chemistry of Nuclear Materials Group, National Security Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA. ∥ M.S.: C-IIAC, Inorganic Isotope and Actinide Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. ⊥ M.H.: European Spallation Source, Tunavagen 24, S-22363, Lund, Sweden. # L.D..: Oak Ridge National Laboratory, Spallation Neutron Source, Oak Ridge, TN 37831, USA. ∇ F.H.G.: Department of Chemical and Biological Engineering Materials Sciences Division, University of New Mexico, Albuquerque, NM 87131, USA.
formed. The MEPs of the 1−3 pathway are shown in Figure 9b. In general the structural deficiency does not facilitate proton conduction. Again, the energy barrier for pathway 1−3 is reduced with VO(O4). However, the direct transfer from O1 to O3 is not feasible; instead, a go-around hop was suggested. The proton initially transfers by a low-energy jump from O1 to O2 and then the continuing transport to O3 is made by a lowbarrier intraoctahedron hop (Figure 9b). This observation suggests that an oxygen vacancy could interfere with the migration path of proton in a detrimental way, most likely due to electrostatic interaction.
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CONCLUSIONS The solution precipitation method was used to synthesize Sn1−xInxP2O7 (x = 0 and 0.1) with precise stoichiometry of the bulk crystalline phase with and without excess amorphous polyphosphate phase as confirmed by combined XRD and XRF analysis. The protonic incorporation and conductivity of these materials was investigated using ac impedance spectroscopy and inelastic neutron scattering. The crystalline MP2O7 with no excess polyphosphate phase possesses negligible bulk conductivity below 600 °C. Above 600 °C, the total conductivity exhibits Arrhenius behavior with activation energy of ∼1 eV, with an increase in conductivity observed for the In-doped sample. Additionally, the undoped sample exhibits no change in conductivity upon humidification while the conductivity of the Sn0.9In0.1P2O7 sample increased by 1/2 order of magnitude upon humidification. This behavior is consistent with the observation from INS that only the In-doped SnP2O7 material showed features associated with proton incorporation in the difference spectra of dried and humidified samples. The experimental results are supported by atomistic modeling that shows that trivalent doping serves to increase proton incorporation or carrier concentration, as shown by the increase in the pre-exponential constant in the Arrhenius plot of the conductivity of Sn0.9In0.1P2O7, without altering the conduction mechanism which is consistent with the observed constant activation energy. Taken together, these results show
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support of the Los Alamos Laboratory Directed Research and Development program funded by the U.S. Department of Energy, 20120003DR. This work has benefited from the use of the Manuel Lujan, Jr. Neutron Scattering Center at Los Alamos National Laboratory and funding from the U.S. Department of Energy’s Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE Contract No. DE-AC52-06NA25396.
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DOI: 10.1021/acs.jpcc.7b06060 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C (49) Lan, R.; Tao, S. W. Conductivity of a New Pyrophosphate Sn0.9Sc0.1(P2O7)(1-Delta) Prepared by an Aqueous Solution Method. J. Alloys Compd. 2009, 486, 380−385.
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DOI: 10.1021/acs.jpcc.7b06060 J. Phys. Chem. C XXXX, XXX, XXX−XXX
