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Structure and Dynamics Investigations of Sr/Ca-Doped LaPO Proton Conductors 4

Amal al-Wahish, Usama al-Binni, Laurene Tetard, Craig A. Bridges, Krishna Kharel, Ozge GunaydinSen, Ashfia Huq, Janice L. Musfeldt, Mariappan Parans Paranthaman, and David G. Mandrus J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 15, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

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Structure and Dynamics Investigations of Sr/Ca-doped LaPO4 Proton Conductors ¨ Amal al-Wahish,∗,† U. al-Binni,‡ L. Tetard,¶ C. Bridges,§ K. Kharel,k O. Günaydın-S¸en,k A. Huq,⊥ J. L. Musfeldt,#,@ M. P. Paranthaman,§ and D. Mandrus@,4,∇ † University of Missouri Research Reactor, University of Missouri-Columbia, Columbia, Missouri 65211 , USA ‡Department of Physics and Astronomy, University of Washington, Seattle, Washington 98105, United States ¶NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826, USA §Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6100, USA kDepartment of Chemistry and Biochemistry, Lamar University, Beaumont, TX 77710, USA ⊥Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831,USA #Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA @Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA 4Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996-1200, USA ∇Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6100, USA E-mail: [email protected] 2 ACS Paragon Plus Environment

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Abstract Proton conductors loom out of the pool of candidate materials with great potential to boost hydrogen alternatives to fossil-based resources for energy. Acceptor doped lanthanum orthophosphates are considered for solid oxide fuel cells (SOFCs) for their potential stability and conductivity at high temperature. By exploring the crystal and defect structure of x% Sr/Ca-doped LaPO4 with different nominal Sr/Ca concentrations (x = 0 – 10) with Neutron powder diffraction (NPD) and X-ray powder diffraction (XRD), we confirm that Sr/Ca-doped LaPO4 can exist as self-supported structures at high temperatures during solid oxide fuel cell operation. Thermal stability, surface topography, size distribution are also studied to better understand the proton conductivity for dry and wet compounds obtained at sintering temperatures ranging from 1200 to 1400 ◦ C using a combination of scanning electron microscopy (SEM), Atomic Force Microscopy (AFM), Fourier transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS). The results confirm that Sr doped samples exhibit the highest proton conductivity of our samples and illustrate the impact of material design and versatile characterization schemes on the development of proton conductors with superior functionality.

Introduction Proton conduction is a fascinating phenomenon taking place in a broad range of systems from biochemical living organisms 1 to pure solid materials, including ampullae of Lorenzini jelly of Sharks, 2 solid bio-polymer electrolytes, 3 Nafion ionomers in Proton Exchange Membrane fuel cells (PEMFCs), and acceptor doped lanthanum orthophosphates electrolytes for solid oxide fuel cells (SOFCs). 4–10 As the worldwide demand in energy is growing, PEMFCs and SOFCs stand out as clean renewable energy technologies that have the potential to reduce the environmental impact of energy production. Nafion-based PEMFCs constitute environmentally friendly energy solutions, which could contribute to the world energy de-



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mands but still lack stability at high temperature. In principle, perfect proton electrolyte material should exhibit proton conductivity of the order of 0.01 S.cm−1 , as well as long-term stability in acidic atmospheres. 11 Unfortunately, to date, Nafion membranes are not stable above 280 ◦ C. 12,13 In addition, their industrial deployment is heavily hindered due to the cost associated with the need to use platinum (Pt) catalyst layers inside the PEMFCS to improve the systems efficiency. 14–16 As a result, an electrolyte that would be stable and inexpensive in intermediate-temperature (300-500 ◦ C) fuel cells and would offer high proton conductivity is highly sought-after. Discovery of commercially viable IT-SOFCs to replace Nafion-based polymer cells would revolutionize the fuel cell technology by eliminating the need for noble metal catalysts and by allowing in-situ reforming of renewable liquid fuels such as methanol or ethanol. 8,17 The chief obstacle hindering intermediate-temperature fuel cells is the lack of electrolyte with suitable stability and conductivity in the operating temperature range. Here we focus on the development of an electrolyte that is thermally and chemically stable, inexpensive, environmentally friendly, with proton conductivity on the order of 10−2 S.cm−1 around 400 ◦ C. Acceptor doped lanthanum orthophosphates LaPO4 have been reported to be promising candidates for intermediate-temperature fuel cell electrolytes 4–10 due to good conductivity, thermal stability, and thermal expansion properties. However, their proton conductivity remains lower than ideal, on the order of 10−4 S.cm−1 . 4,6,8–10 A number of syntheses and crystal growth conditions have been considered such as identifying the optimal heat treatment conditions. 9,10,18–22 However, the synthesis of the acceptor doped lanthanum orthophosphates, is challenging, due to the typical presence of a secondary phase, the limited dopant solubility and the segregation of Sr which restricts the proton conductivity in the orthophosphates. 22,23 In order to create charge-compensating oxygen vacancies, a fraction of the trivalent rare earth replaced with a divalent ion such as Sr or Ca. 18 These oxygen vacancies condense into pyrophosphate groups and formed by two tetrahedra sharing an oxygen ion. 24 When hydrating the pyrophosphate by post-annealing under water vapor, protons dissolve into


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phosphates, forming hydrogen phosphate where the protons stay at PO4 tetrahedra in the acceptor doped lanthanum orthophosphates. The presence of oxygen vacancies or protonic defects in a lattice can cause displacement or relaxation of the positions of the surrounding lattice ions. 7 A theoretical study of the proton conduction mechanism in LaPO4 by a first-principles approach that supports proton preferential binding to sites around the PO4 tetrahedra. 25 The proton-transfer mechanism in LaPO4 on the basis of the density functional calculations 26 are rotation around an oxygen ion, oscillatory motion between adjacent tetrahedra, transfer between oxygen ions within a single PO4 tetrahedron (intratetrahedral), and jumps between tetrahedra (intertetrahedral). 26 The intertetrahedral transfer with 0.6 -1.0 eV potential barriers is the dominant proton-transfer process. The energy barrier for forward or backward proton transfer mechanism between two adjacent PO4 is below 0.1 eV. A detailed of Quasi-Elastic Neutron Scattering (QENS) study of proton dynamics in the 4.2% Ca-doped LaPO4 reveals fast proton diffusion at intermediate temperatures. 6 Based on our previous studies on 4.2% Sr/Ca-doped LaPO4, 4,6 we design here an electrolyte formed with x%Sr/Ca-doped LaPO4 materials, where x = 0 – 10, sintering at different temperatures (from 1200, 1350 and 1400 ◦ C) and in dry and wet conditions, to better understand connections between the crystal composition, its structure, defect structure and the dynamics of proton conduction. A combination of results from X-ray diffraction (XRD), neutron powder diffraction (NPD), scanning electron microscopy (SEM), Atomic Force Microscopy (AFM), Fourier transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS) are reported. The NPD and XRD measurements are used to determine the structural parameters, and interatomic distances for each sample. The XRD shows that in the hydrated 4.2%Sr/Cadoped LaPO4 most of the proton-oxygen site preference is located on the particular corners of the PO4 tetrahedra, that is, the O2 sites, while the Neutron Powder Diffraction (NPD) shows an average bond length distortion. The thermal stability of the samples is studied by collecting NPD as a function of temperature to access thermal expansion coefficients



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(TECs). 4 While the FTIR measurements reveal the contrast of hydration level of the dry and wet 4.2% Sr-doped LaPO4. AFM and the SEM examine the surface structure of the samples. We obtained dense ceramics that are useful as electrolytes for SOFCs at high temperature. Proton conductivity of Sr/Ca-doped LaPO4 under dry and humid conditions are investigated with EIS. The effect of heat treatment on the crystal structure and the dynamics of the protons is discussed. We also explore the structure of the secondary phase and its effects on proton conductivity. Surprisingly, the proton conductivities of the 4.2% Ca-doped LaPO4 show less response to higher sintering temperature than the 4.2% Sr-doped LaPO4 samples. The absence of significant change of the proton conductivities of the 4.2% Ca-doped LaPO4 at 1200 and 1350 ◦ C suggests that Ca resists to leave the LaPO4 structure. These results indicate that Sr is leaving the LaPO4 structure and create a secondary phase faster than the Ca during the sintering process.

Experiment 1, 4.2, 8 and 10 % Sr-doped LaPO4 samples are prepared by co-precipitation (CPT) 27 using the following precursors: Lanthanum (III) nitrate hexahydrate (99.9%-La (REO), Strem Chemicals) La(NO3)3•6H2O, Strontium nitrate (99+%, ACS, Acros Organics) Sr(NO3)2 and Ammonium phosphate (99+%, Acros Organics) (NH4)2HPO4 ( Note: for the 8% Sr-doped sample, we used very pure precursors upto 99.999%). CPT is used instead of solid state reaction (SSR) 6 due to the low solubility of the alkaline earth metals into lanthanum orthophosphate (LaPO4), the doped alkaline earth metals and its concentration. The solubility of Sr into LaPO4 by SSR has previously been reported to be 1.9 mol% without detecting any secondary phase. 10 Precursor solutions at 0.2 M are prepared. A solution of (NH4)2HPO4 is mixed with the solution of the lanthanum and strontium nitrates drop by drop at a speed of 5 ml/min, at room temperature, with constant stirring. After filtering the gel, the product


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is dried at 100◦ C for two days. The dried material is crushed using an agate mortar. The powders are calcined in air for 12 h in a Pt crucible at 800◦ C with a heating rate of 2◦ C/min. The material is then finely crushed using an agate mortar, followed by dry ball milling for 15 min. The process is repeated several times. After pelletizing under a load of 7 metric tons, one set of samples was sintered at 1200◦ C in air for 4 h to obtain relatively dense pellets (a relative density up to 97% of the X-ray density is achieved for samples). Another batch of the samples were sintered at 1400◦ C. Powder X-ray diffraction (XRD) was used to study the samples for phase identification and structure refinement. The initial structural characterization was conducted with a Bruker AXS D2 Phaser diffractometer with Cu Kα radiation (λ = 1.541 ˚ A) at a range of 10◦ -90◦ with a scan rate of 0.02◦ s−1 . Data for Rietveld refinement collected using a Panalytical X-pert Pro diffractometer with Cu Kα radiation in the range of 5◦ -130◦ with a scan rate of 0.0167◦ s−1 . The data analysis performed using FullProf software. 28 Scanning Electron Microscopy (SEM) is performed on a Zeiss ULTRA-55 FEG SEM with secondary electron (SE) detection mode at voltage 10 kV and 5 kV for higher resolution. Sintered pellets similar to those used in EIS measurements prior to electrode fabrication are examined. Infrared spectra are obtained in transmission (400 - 4000 cm−1 ) at room temperature using a Nicolet iS50 FT-IR spectrometer at Lamar university. The absorption is calculated 1 ln(T (ω)), where T (ω) is the measured transmittance, h is the loading, and d as α(ω) = − hd

is the thickness. The crystal structures of the doped and undoped lanthanum orthophosphate (LaPO4) was measured by neutron powder diffraction (NPD). The atomic displacement parameters (ADPs), structural parameters, interatomic distances and angles calculated from NPD spectra. The stability of samples as function of temperature also measured with NPD. 4 Time-offlight (TOF) powder neutron diffraction data collected using the POWGEN diffractometer at the Spallation Neutron Source facility. The data are collected using center wavelengths (CWLs) of 1.333 ˚ A that cover d-spacing from 0.41 to 5.3 ˚ A with temperature ranging from



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room temperature to 800◦ C. Rietveld refinement of the data are performed using the FullProf suite. 28 Electrochemical impedance spectroscopy (EIS) is used to study the proton conductivity of the x%Sr/Ca doped lanthanum orthophosphates (x = 4.2, 8, and 10). The details of the EIS experiment setup is previously reported. 6 The samples are sintered at two different temperatures: 1200 and 1400 ◦ C. Each sample is measured in dry and wet (2.2 mol% water content) air.

RESULTS AND DISCUSSION The unique features of LaPO4, including its high temperature stability, low thermal conductivity, and high thermal expansion coefficient are strongly associated with the structure of LaPO4. 29,30 Figure 1(left) presents the monoclinic monazite LaPO4 which crystallizes in space group P 21 /n (No.14) 1. The La3+ ion is coordinated with nine oxygen ions which share corners with PO4 tetrahedra,, as seen in Figure 1(right). The XRD data presented in Figure 2 strongly suggests a lack of impurity phases in 1 and 4.2% Sr/Ca-doped LaPO4 at 1200 ◦ C, which indicates that 1 and 4.2% Sr/Ca have been successfully doped into the

Figure 1: Left: the unit cell of LaPO4 which has a monoclinic P 21 /n structure containing four formula units. The unit cell contains La3+ cations (yellow) and isolated PO4 tetrahedra (O in grey and P in blue). Right: LaO9 coordination polyhedra (This picture modified for clarity). 8 ACS Paragon Plus Environment

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lattice. The Sr- doped samples sintering temperature at 1200 ◦ C with higher doping levels more than 8%, produced impurity peaks, possibly from Sr(PO3)2 secondary phase. The lattice parameters are derived from a Rietveld refinement of the XRD data. 28 Table 1 lists the XRD cell parameters, the angle between c and a axis β and cell volume for x% Sr/Ca-doped LaPO4, x = 0, 1 and 4.2, at room temperature. The lattice parameters are similar to those of LaPO4, as expected given the similarity in ionic radii of the La3+ and Ca2+/ Sr2+ cations. We delve into the crystal and defect structures of aliovalent 1 and 4.2 % (Sr, Ca) doping of LaPO4 materials sintering temperature at 1200 ◦ C. The inter-ionic bond lengths (˚ A) and the angles (◦ ) between the phosphorus ions and the oxygen ions in the LaPO4 samples are determined from the XRD spectra acquired at room temperature (RT). The average bond lengths between the lanthanum and oxygen ion as well as the distance between two phosphorus ions are listed in Table 2. The maximum inter-ionic bond length in 4.2% Sr A and 1.609(13) ˚ A, respectively. and Ca-doped LaPO4 is found to be for P O2 = 1.613(19) ˚ For lower doping with 1% Sr and Ca doped LaPO4, P O3 is found to be the maximum inter-ionic bond length with 1.640(30) ˚ A and 1.589(14) ˚ A, respectively. The difference in inter-ionic bond length P O implies that the proton in the 4.2% Sr/Ca -doped LaPO4 likely resides on O2, while the proton preference site is O3 for 1% Sr/Ca-doped LaPO4. Table 1: The XRD cell parameters (a,b,c), the angle between c and a axis β and the cell volume (V) for x%Sr/Ca-doped LaPO4 (x = 0, 1, and 4.2) sintering temperature at 1200◦ C, measured at room temperature. Samples LaPO4 4 4.2%Sr 4 1%Sr 4.2%Ca 4 1%Ca

a (˚ A) 6.8396(0) 6.8388(5) 6.8394(0) 6.8389(4) 6.8409(0)

b (˚ A) 7.0761(0) 7.0760(5) 7.0722 (0) 7.0718(4) 7.0752(0)

c (˚ A) 6.5099(0) 6.5117(6) 6.5122 (0) 6.5119(4) 6.5149 (0)

β(◦ ) 103.2746(11) 103.2893(6) 103.3067(11) 103.2954(4) 103.3051(7)

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V (˚ A ) 306.644(7) 306.673(4) 306.538(7) 306.498(3) 306.866(5)

The grain size of the materials prepared into a pellet form were studied by SEM. The images of 0 and 8%Sr/Ca- doped LaPO4 are presented in Figure 3 and 4 and show a clear effect of composition on the grain size. The grain sizes of the samples range from 1 to 2 µm for LaPO4 (the pores are up to a few hundred nanometers), 0.3 to 1.5 µm for 1% Ca-doped 9 ACS Paragon Plus Environment


LaPO4, 0.5 to 2.7 µm for 1% Sr-doped LaPO4, 1.0 to 2.5 µm for 4.2% Ca-doped LaPO4, 4 0.1 to 4.5 µm for 4.2% Sr-doped LaPO4 4 and 0.99 to 6.6 µm for 8% Sr-doped LaPO4. The Srdoped LaPO4 samples have larger and broader grain size distribution than Ca-doped LaPO4 of the same doping level. The vibrational modes of the materials are studied with IR spectroscopy. The transmittance and absorption of hydrated (blue) and dry (red) 4.2% Sr-doped LaPO4 in the 1600-400 cm−1 range is presented in Figure 5 along with their mode assignments. The asymmetric and

R Rwp

2

= = =

Intensity (arb. units)

(b) 1% Sr 8.18 11 4.52

R = 10.8 Rwp = 14.4 2 = 7.88

2θ (deg.)

2θ (deg.)

(c) 4.2% Ca

(d) 4.2% Sr

R Rwp

2



(a) 1% Ca


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= 7.98 = 11.0 = 7.04

2θ (deg.)

R Rwp

2

= 11.7 = 15.3 = 8.87

2θ (deg.)

Figure 2: XRD Rietveld refinement results for (a) 1% Ca-doped LaPO4, (b) 1% Sr-doped LaPO4; (c) 4.2% Ca-doped LaPO4 and (d) 4.2% Sr-doped LaPO4 samples at room temperature.


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˚) between the phosphorus ions and the oxygen ions Table 2: The inter-ionic bond lengths (A in the LaPO4 samples calculated from XRD measurements performed at RT, average bond lengths between the Lanthanum and Oxygen ions and distance between two Phosphorus ions are listed for each sample. Samples are sintered at 1200◦ C. Doped sample are sintered under water vapor. P P P P La P

200 nm

2 μm

˚) O1 (A O2 (˚ A) O3 (˚ A) O4 (˚ A) O (˚ A) P (˚ A)

LaPO4 1.500(20) 1.480(30) 1.470(30) 1.629(18) 2.570(11) 4.110(9)

1%Ca 1.534(11) 1.567(14) 1.589(14) 1.576(10) 2.620(13) 4.110(9)

4.2% Ca 1.494(11) 1.609(13) 1.549(13) 1.547(10) 2.603(11) 4.105(7)

(a) LaPO4

1% Sr 1.458(14) 1.633(17) 1.640(30) 1.573(13) 2.654 (15) 4.127(13)

(b) LaPO4

2 μm

(c) 1% Ca-doped LaPO4

2 μm

4.2% Sr 1.426(18) 1.613(19) 1.580(30) 1.591(14) 2.449(21) 4.113(12)

(d) 1% Sr-doped LaPO4

Figure 3: SEM images of (a) and (b) LaPO4, (c) 1% Ca-doped LaPO4 ; and (d) 1% Sr-doped LaPO4 samples at room temperature. The samples sintering temperature at 1200◦ C.



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symmetric stretching of P O4−3 occur in the 1400 to 668 cm−1 range in the form of a broad band with distinct shoulders. The bands at 991.0, 951.0 cm−1 and at 1091.5 are assigned to P O4−3 symmetric and asymmetric stretching modes. The P O4−3 asymmetrical bending vibrations exhibit four bands in the 668 to 411 cm−1 region. The absorption spectra of the wet samples are higher than for dry samples. The absorption difference between the dry and the wet samples are observed in the 972.1–1554.7 cm−1 range. The absorption spectra confirmed that hydration of the wet sample can be correlated to the conductivity and XRD results. The increase in the stretching modes may be correlated to the protonic defects. The strong absorption modes are due to the increase in bond length in the P–O–H vibrations and is related to the presence of the proton. The proton induces a remarkable distortion of

200 nm

1 µm

(a) 4.2%Ca-doped LaPO4

2 µm

(b) 4.2%Sr-doped LaPO4

(c) 8%Sr-doped LaPO4

2 µm

(d) 8%Sr-doped LaPO4

Figure 4: SEM images of (a) 4.2% Ca-doped LaPO4 , (b) 4.2% Sr-doped LaPO4 , (c) and (d) 8% Sr-doped LaPO4 samples at room temperature. The samples sintered at 1200◦ C. 12 ACS Paragon Plus Environment

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the PO4 tetrahedra; therefore, significant stretching of the bridging P-O bonds is observed and showed longer P-O bond lengths. Our FTIR results support the XRD observations of the increase of P-O bond lengths of the hydrated doped phosphate samples. Table 3: Cell parameters (a,b,c), the angle between c and a axis is β) and cell volume (V) measured by NPD for x%Ca-doped LaPO4 (x = 0, 1 and 4.2) at room temperature. Samples LaPO4 1% Ca (D2O) 4.2% Ca (D2O)

3

a (˚ A) 6.840

b (˚ A) 7.075

c (˚ A) 6.512

β(◦ ) 103.28

V (˚ A ) 306.71

6.840

7.074

6.514

103.296

306.713

6.841

7.072

6.514

103.30

306.69

NPD measurements are more sensitive to light elements and oxygen positions than XRD. The NPD spectra obtained at 300 K on LaPO4, deuterated 1%Ca-doped LaPO4, and deuterated 4.2% Ca-doped LaPO4 4 are presented in Figure 6 . The NPD cell parameters and cell volume (V) of x% Ca-doped LaPO4 (x = 0, 1 and 4.2) at room temperature are listed in Table 3. Cell parameters obtained by NPD and XRD are in good agreement. Our measurements allow us to investigate the structure and defect structure of 0, 1 and 4.2%Ca-doped LaPO4 materials. The inter-ionic bond lengths (˚ A) between the phosphorus and the oxygen 2 .5 k

S y m m e tric s tre tc h in g 3 P O 4 9 5 1 .7 c m

S tre tc h in g m o d e 3 -1 P O 4 9 9 1 .0 c m

A s s y m e tric b e n d in g 5 7 5 .0 a n d 5 3 4 .6 c m

4 .2 %

S r(d ry )

A b s o rp tio n (c m

4 .2 %

-1

)

T r a n s m itta n c e ( a .u )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S r(w e t)

2 .0 k

1 .5 k

1 .0 k

-1

-1

A s y m m e tric s tre tc h in g 3 -1 P O 4 1 0 9 1 .5 c m

4 .2 %

A s s y m e tric b e n d in g -1 6 1 5 .0 c m

S r(w e t)

5 0 0 .0

4 .2 %

S r(d ry ) A s s y m e tric b e n d in g -1 5 6 3 .0 c m

1 6 0 0

1 4 0 0

1 2 0 0

1 0 0 0

8 0 0

F re q u e n c y (c m

-1

6 0 0

4 0 0

0 .0 1 6 0 0

)

1 4 0 0

1 2 0 0

1 0 0 0

F re q u e n c y (c m

8 0 0

-1

6 0 0

)

Figure 5: Infrared transmittance (left) and absorption (right) of 4.2% Sr-doped LaPO4 in wet (blue) and dry (red) conditions. IR mode assignments are indicated in the absorption graph. 13 ACS Paragon Plus Environment

4 0 0


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ions, lanthanum and oxygen ions, phosphorus and phosphorus, and the angles (◦ ) between the phosphorus ions and the oxygen ions in the samples at 300 K are extracted from the NPD data. A summary of the values is presented in Table 4. A slight change is seen in the inter-ionic bond lengths between the phosphorus and the oxygen ions in the NPD experiment at 300 K. Our NPD investigations of tetrahedral distortion by protonic defects indicate an average distortion. Table 5 gives fractional atomic coordinates (x,y,z) and the equivalent isotropic displacement parameters B of dry deuterated 0, 1 and 4.2 % Ca-doped LaPO4 at RT. ˚) and angles (◦ ) between the phosphorus ions and Table 4: The inter-ionic bond lengths (A the oxygen ions in the LaPO4 samples, the average bond lengths between the lanthanum and oxygen ion and the distance between two phosphorus ions calculated from the NPD data acquired at 300 K are listed for each sample. All the samples are sintered at 1200◦ C. LaPO4 P O1 P O2 P O3 P O4 La O (˚ A) ˚ P P (A) (O1 P O2) (O1 P O3) (O1 P O4) (O2 P O3) (O2 P O4) (O3 P O4)

1.534 1.556 1.537 1.526 2.593 4.110 105.10 113.63 112.26 107.78 113.80 104.41

1% Ca (D2O) 1.536 1.554 1.537 1.526 2.570 4.111 105.15 113.53 112.16 107.91 113.91 104.32

4.2% Ca (D2O) 1.533 1.552 1.534 1.525 2.541 4.104 105.15 113.54 112.13 107.91 113.95 104.31

4.2% Ca (Dry) 1.529 1.548 1.529 1.518 2.530 4.088 105.04 113.45 112.28 107.85 113.99 104.37

Temperature-dependent NPD on deuterated 4.2% Ca-doped LaPO4 and hydrated 4.2% Sr-doped LaPO4 were measured 4 under dry air up to 800 ◦ C and 500 ◦ C, respectively as shown in figures 7 and 8. Thermal expansion coefficients (TECs) calculated are on the order of several functional solid oxide fuel cell materials 7.55 − 10.70 × 10−6 K−1 , 4,31 as previously reported for LaPO4. 32 In our previous neutron diffraction study, hydration of 4.2%Sr-doped LaPO4 is more 14 ACS Paragon Plus Environment

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Table 5: : Fractional Atomic Coordinates (x, y and z) and equivalent isotropic displacement 2 parameters (B in ˚ A ) measured at RT on deuterated and dry 0, 1 and 4.2 % Ca-doped LaPO4 . x%Ca

0

1/Deuterated

4.2/Deuterated

4.2/Dry

La P O2 O1 O4 O3 La/Ca P O2 O1 O4 O3 La/Ca P O2 O1 O4 O3 La/Ca P O2 O1 O4 O3

x 0.28185(9) 0.30440(16) 0.38077(13) 0.24933(14) 0.12904(16) 0.47435(15) 0.28179(9) 0.30452(16) 0.38078(14) 0.24945(14) 0.12898(17) 0.47414(16) 0.28179(10) 0.30462(17) 0.38088(15) 0.24951(15) 0.12904(18) 0.47419(17) 0.28179(10) 0.30435(17) 0.38093(15) 0.24952(16) 0.12902(18) 0.47419(17)

y 0.15978(11) 0.16388(18) 0.33129(17) 0.00775(14) 0.21528(13) 0.10833(13) 0.15959(12) 0.16413(19) 0.33137(17) 0.00773(15) 0.21518(13) 0.10832(14) 0.15956(13) 0.16420(20) 0.33143(19) 0.00775(16) 0.21513(14) 0.10839(15) 0.15948(13) 0.16430(20) 0.33154(19) 0.00788(16) 0.21536(14) 0.10829(15)

z 0.10088(11) 0.61245(16) 0.49708(15) 0.44541(16) 0.71024(17) 0.80155(18) 0.10078(11) 0.61234(17) 0.49701(16) 0.44554(17) 0.71014(17) 0.80153(19) 0.10069(12) 0.61235(18) 0.49708(17) 0.44548(18) 0.71016(18) 0.80150(20) 0.10073(12) 0.61241(18) 0.49693(17) 0.44534(18) 0.71031(18) 0.80140(20)


B 0.355(16) 0.25(2) 0.63(2) 0.53(2) 0.52(3) 0.66(3) 0.340(16) 0.24(2) 0.59(2) 0.49(3) 0.50(3) 0.64(3) 0.32(2) 0.22(2) 0.58(3) 0.50(3) 0.49(3) 0.63(3) 0.33(2) 0.20(2) 0.61(3)) 0.51(3) 0.47(3) 0.65(3)


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noticeable in the diffraction pattern than 4.2%Ca-doped LaPO4. 4 This result supports that Sr-doped LaPO4 is a better host for protons than the Ca-doped LaPO4. This, in turn, leads to a higher proton conductivity in Sr-doped LaPO4. In order to further characterize the proton dynamics, we investigate the proton conductivity of different Sr concentrations in the LaPO4 structure by the EIS. The conductivities, σ(S cm−1 ), are defined as:

σ(T ) =

q 2 cD(T ) kB T

(1)

where q, kB , c, D(T ) and T are the charge per ion (1.6 × 10−19 C), Boltzmann constant, charge concentration (cm−3 ), diffusion coefficient (cm2 s−1 ), and the absolute temperature (K), respectively. Figure 9 shows the AC conductivity of the samples, determined by using impedance spectroscopy in dry and wet condition. The conductivity plotted vs. (1000/T ), which satisfies the Arrhenius relationship (Equation 2):

Ea σT = A exp − kB T

(2)

where A is a pre-exponential constant and Ea is the activation energy. Our previous investigations have confirmed a higher conductivity in the 4.2%Sr doped case than 4.2%Ca doped materials. 4 Our observations, over a range of doping levels in this work, show that changing the acceptor concentration and the sintering temperature can control the grain size, grain boundary impedance and the density of proton conductor materials. It is also interesting to compare the conductivity of the Sr -doped LaPO4 samples prepared with different sintering temperatures (1200 and 1400 ◦ C) and with different concentrations. In figure 9 (a) and (b), we compare the 4.2%Sr-doped materials with 10 and 8%Sr, respectively, all synthesized by co-precipitation (CPT) and sintered at 1200 ◦ C. We find that increasing the doping does not significantly increase the proton conductivity. The


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conductivity of 4.2%Sr remains slightly higher than that of the 10 and 8%Sr-doped materials in the temperature range of 327 to 527 ◦ C. The proton conductivity of the 8% Sr-doped LaPO4, sintering temperature at 1200 ◦ C, extended to 298 ◦ C in water vapor atmosphere. Above the 635 ◦ C, the 8% Sr have slightly higher proton conductivity than the 4.2% Sr. The 8% Sr-doped samples synthesised with a high pure precursors upto 99.999% compare to the rest of the samples. Using very pure precursors may give a new feature of the samples such as extending the proton conductivity below 300 ◦ C. Figure 9 (c) shows 8% Sr-doped LaPO4 sintered at two different temperature 1200 and 1400 ◦ C. The conductivity of the 8% Sr at 1200 ◦ C is slightly higher than for 8%Sr sintered at 1400 ◦ C. A very weak peak is detected in the XRD of the 8% Sr-LaPO4 when sintered at 1400 ◦ C as shown in Figure 10. This secondary phase peak is hindered the proton conductivity. The proton conductivities of 4.2% Ca sintered at 1200 and 1350 ◦ C did not change significantly, as shown in Figure 9 (d). In terms of conductivity, the Sr-doped LaPO4 samples are more sensitive to the change in sintering temperature than the Ca-doped LaPO4 samples. The sintering temperature had a dramatic effect on the density and grain size. 33 The density and the grain sizes of the Sr-doped samples sintered at 1400 ◦ C have a higher densities and a larger grain sizes than the ones sintered at 1200 ◦ C. The density of 10%Sr-doped increased with the increase of sintering temperature, from 4.57 g/cm3 at 1200 ◦ C to 4.96 g/cm3 at 1400 ◦ C. Figure 10 shows the XRD data obtained at room temperature for LaPO4 sintered at 1000 ◦ C, 10%Sr-doped LaPO4 samples sintered at 800, 1000, 1200, and 1400 ◦ C, and 8% Sr-doped LaPO4 sintered at 1400 ◦ C. Around 2θ = 24◦ , a very tiny peak appears for the 8%Sr at 1400 ◦ C and for 10%Sr at different sintering temperatures. A very weak secondary phase is detected above the 8% Sr-LaPO4 doping level, which is found to hinder the proton conductivity. This weak peak probably may be related to amorphous phase or to poorly crystalline Sr(PO3)2) secondary phase. 22 Given our synthesis method, the solubility limit of Sr into LaPO4 is around 8% and the sintering temperature should not exceed 1200◦ C. As the 8%Sr-doped LaPO4 samples exhibited the highest proton conductivity by our



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synthesis method. As a result, we investigated the behavior of the materials at the nanoscale using Atomic Force Microscopy (AFM). The topography of the sample is presented in figure 11 (a). The sample was dry when inserted in the AFM chamber. The measurements were carried out under controlled environment by flowing an inert gas in the chamber. Figure 11 (b) illustrates local variations in the conductivity of the material, which will have to be further investigated in follow up studies. The results suggest that local variations in the materials may affect their performance.

CONCLUSIONS The concentration of acceptor in lanthanum orthophosphates, as well as the sensitivity of the heating treatments during the synthesis procedure, determine phase purity and eventually the proton conductivity of the samples. In this study, we synthesized different acceptor concentration samples at different sintering temperatures. We employed NPD, XRD, SEM, AFM, FTIR and EIS to study proton conductors in the acceptor doped lanthanum orthophosphates. NPD and XRD were used to examine the structure and defect structure of aliovalent the Sr and Ca doping on LaPO4 materials. We determined the inter-ionic bond lengths (˚ A) between the phosphorus/lanthanum ions and the oxygen ions, and the angles (◦ ) between the phosphorus ions and the oxygen ions. We obtained the fractional atomic coordinates and the equivalent isotropic displacement parameters (B) of 0, 1 and 4.2 % Ca-doped LaPO4 , for deuterated and dry samples at RT. The structural analysis using XRD and NPD at elevated temperatures of up to 800 ◦ C, confirm that the dense electrolyte 4.2%Sr/Ca-doped LaPO4 samples have the potential to exist as self-supported structures at high temperatures during solid fuel cell operation. We investigated the proton conductivity of Sr/Ca-doped LaPO4 with EIS. The conductivities of the hydrated and dry samples were determined as function of temperature. We compared the conductivity with different nominal Sr concentrations. Also, we successfully


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synthesized the samples up to 8% Sr-doped LaPO4 (sintered 1200 ◦ C) compact samples with no trace of secondary phase. The 4.2 and 8% Sr doped samples (sintered 1200 ◦ C) sport the highest proton conductivity by our synthesis method with no trace of a secondary phase. A secondary phase was observed on the acceptor doped lanthanum orthophosphates when the sintering temperature exceeded the 1200 ◦ C (doping level is more than 8%) in our synthesis methods. The samples 8 and 10 % Sr-doped LaPO4 sintered at 1400 ◦ C slightly indicated a secondary phase that reduced their proton conductivity compared to 4.2% Sr but stayed in the same order. However, the signature of the secondary phase is not strongly detected in the XRD data of 8 and 10% Sr sintered at 1400 ◦ C. The proton conductivity did not increase to the order of 0.01 S.cm−1 as increasing the Sr concentration above the 8%. Although the 4.2% Sr-doped LaPO4 samples have larger and broader grain size distribution, higher proton conductivity and higher thermal expansion than 4.2% Ca-doped LaPO4, surprisingly the latter’s proton conductivities did not change significantly at higher sintering temperature. We may conclude that the Sr extrudes from the LaPO4 unit cell and creates a secondary phase with the phosphorus and the oxygen relatively more easily than the Ca.

Acknowledgements This work was supported by NSF Grant Nos. DGE-1069091 and DMR-1508249 (A.A.W.), the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Mate¨ rials Sciences and Engineering Division (A.A.W., C.B., M.P.P., and D.M.). K.K andO. Gwere ¨ supported by Welch Foundation (V-0004). J. M was supported by DE-FG02-01ER45885. The NPD measurements and Time-of-flight (TOF) powder neutron diffraction collected using the POWGEN diffractometer were carried out at Oak Ridge National Laboratory’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, office of science, US Department of Energy. The XRD and SEM measurements were conducted at the Center for Nanophase Materials Sciences (pro-



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posal CNMS2013-310), which is sponsored by the Division of Scientific User Facilities, Office of Basic Energy Sciences, U.S. Department of Energy. Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Author contributions A.A.W. and D.M. developed the plan and the projects. A.A.W. synthesized and character¨ Gmeasured ized the materials by XRD, SEM. and EIS. L.T. measured the AFM. K.K and O. ¨ the FTIR. The error and statistical analysis done by U.A.B. A.A.W., C.B., and A.H. performed the NPD measurements. A.A.W. analyzed the NPD and XRD findings and discussed the data with C.B. and A.H. We thank L. Tetard and S. Tang for assistance with the SEM measurements.The paper was written by A.A.W. with input from all coauthors.

References (1) Teissie, J.; Prats, M.; LeMassu, A.; Stewart, L. C.; Kates, M. Lateral Proton Conduction in Monolayers of Phospholipids from Extreme Halophiles. Biochemistry 1990, 29, 59–65. (2) Josberger, E. E.; Hassanzadeh, P.; Deng, Y.; Sohn, J.; Rego, M. J.; Amemiya, C. T.; Rolandi, M. Proton Conductivity in Ampullae of Lorenzini Jelly. Sci. Adv. 2016, 1–6.


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(3) Chai, M. N.; Isa, M. I. N. Novel Proton Conducting Solid Bio-polymer Electrolytes Based on Carboxymethyl Cellulose Doped with Oleic Acid and Plasticized with Glycerol. Sci. Rep. 2016, 6, 27328–27335. (4) al Wahish, A.; al Binni, U.; Bridges, C. A.; Tang, S.; Bi, Z.; Paranthaman, M. P.; Huq, A.; Mandrus, D. In Situ X-ray and Neutron Diffraction of the Rare-Earth Phosphate Proton Conductors Sr/Ca-Doped LaPO4 at Elevated Temperatures. Chem. Mater. 2016, 28, 7232–7240. (5) al Wahish, A.; Armitage, D.; al Binni, U.; Hill, B.; Mills, R.; Jalarvo, N.; Santodonato, L.; Herwig, K. W.; Mandrus, D. A New Apparatus Design for High Temperature (up to 950 ◦ C) Quasi-Elastic Neutron Scattering in a Controlled Gaseous Environment. Rev. Sci. Instrum. 2015, 86, 095102–9. (6) al Wahish, A.; Jalarvo, N.; Bi, Z.; Herwig, K. W.; Bridges, C.; Paranthaman, M. P.; Mandrus, D. Quasi-elastic Neutron Scattering Reveals Fast Proton Diffusion in CaDoped LaPO4 . J. Phys. Chem. C 2014, 118, 20112–20121. (7) Phadke, S.; Nino, J. C.; Islam, M. S. Structural and Defect Properties of the LaPO4 and LaP5 O14 -based Proton Conductors. J. Mater. Chem. 2012, 22, 25388–25394. (8) Norby, T. Solid-state Protonic Conductors:

Principles, Properties, Progress and

Prospects. Solid State Ion. 1999, 125, 1–11. (9) Gallini, S.; Hänsel, M.; Norby, T.; Colomer, M. T.; Jurado, J. R. Impedance Spectroscopy and Proton Transport Number Measurements on Sr-Substituted LaPO4 Prepared by Combustion Synthesis. Solid State Ion. 2003, 162-163, 167–173. (10) Amezawa, K.; Tomii, Y.; Yamamoto, N. High Temperature Protonic Conduction in LaPO4 Doped with Alkaline Earth Metals. Solid State Ion. 2005, 176, 135–141.



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(11) Fontaine, M.-L.; Larring, Y.; Haugsrud, R.; Norby, T.; Wiik, K.; Bredesen, R. Novel High Temperature Proton Conducting Fuel Cells: Production of La0.995 Sr0.005 NbO4− δ Electrolyte Thin Films and Compatible Cathode Architectures. J. Power Sources 2009, 188, 106–113. (12) Li, Q.; He, R.; Jensen, J. O.; Bjerrum, N. J. Approaches and Recent Development of Polymer Electrolyte Membranes for Fuel Cells Operating above 100 C. Chem. Mater. 2003, 15, 4896–4915. (13) Surowiec, J.; Bogoczek, R. Studies on the Thermal Stability of the Perfluorinated Cation-exchange Membrane Nafion-417. J. Therm. Anal. Calorim. 1988, 33, 1097– 1102. (14) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity Benchmarks and Requirements for Pt, Pt-alloy, and Non-Pt Oxygen Reduction Catalysts for PEMFCs. Appl. Catal., B 2005, 56, 9 – 35. (15) Sealy, C. The Problem with Platinum. Mater. Today 2008, 11, 65 – 68. (16) Afsahi, F.; Mathieu-Potvin, F.; Kaliaguine, S. Impact of Ionomer Content on Proton Exchange Membrane Fuel Cell Performance. Fuel Cells 2016, 16, 107–125. (17) Malavasi, L.; Fisher, C. a. J.; Islam, M. S. Oxide-ion and Proton Conducting Electrolyte Materials for Clean Energy Applications: Structural and Mechanistic Features. Chem. Soc. Rev. 2010, 39, 4370–87. (18) Norby, T.; Christiansen, N. Proton Conduction in Ca- and Sr-Substituted LaPO4 . Solid State Ion. 1995, 77, 240–243. (19) Hatada, N.; Nose, Y.; Kuramitsu, A.; Uda, T. Precipitation Behavior of Highly Srdoped LaPO4 in Phosphoric Acid Solutions. J. Mater. Chem. 2011, 21, 8781 – 8786.


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(20) Colomer, M. T.; Delgado, I.; Ortiz, A. L.; Farias, J. C. Microwave-assisted Hydrothermal Synthesis of Single-crystal Nanorods of Rhabdophane-type Sr-doped LaPO4nH2O. J. Am. Ceram. Soc. 2014, 97, 750–758. (21) Phadke, S. R.; Nino, J. C. Conductivity Enhancement in Lanthanum Phosphates. J. Am. Ceram. Soc. 2011, 94, 1817–1823. (22) Ohtaki, K. K.; Heravi, N. J.; Leadbetter, J. W.; Morgan, P. E.; Mecartney, M. L. Extended Solubility of Sr in LaPO4 Monazite. Solid State Ion. 2016, 293, 44 – 50. (23) Tyholdt, F.; Anton Horst, J.; Jrgensen, S.; Stvold, T.; Norby, T. Segregation of Sr in Sr-doped LaPO4 Ceramics. Surf. Interface Anal. 2000, 30, 95–97. (24) Amezawa, K.; Maekawa, H.; Tomii, Y.; Yamamoto, N. Protonic Conduction and Defect Structures in Sr-doped LaPO4 . Solid State Ion. 2001, 145, 233–240. (25) Toyoura, K.; Hatada, N.; Nose, Y.; Tanaka, I.; Matsunaga, K.; Uda, T. ProtonConducting Network in Lanthanum Orthophosphate. J. Phys. Chem. C 2012, 116, 19117–19124. (26) Yu, R.; De Jonghe, L. C. Proton-Transfer Mechanism in LaPO4 . J. Phys. Chem. C 2007, 111, 11003–11007. (27) Gallini, S.; Jurado, J. R.; Colomer, M. T. Synthesis and Characterization of Monazitetype Sr:LaPO4 Prepared Through Coprecipitation. J. Eur. Ceram. Soc. 2005, 25, 2003– 2007. (28) Carvajal, J. R. Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction. Physica B 1993, 192, 55–69. (29) Mogilevsky, P.; Boakye, E. E.; Hay, R. S. Solid Solubility and Thermal Expansion in a LaPO4 –YPO4 System. J. Am. Ceram. Soc. 2007, 90, 1899–1907.



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Page 25 of 30 The Journal of Physical Chemistry

Figure 6: NPD spectra obtained from measurements carried out at RT on (a) deuterated 1%Ca-doped LaPO4, (b) deuterated 4.2%Ca-doped LaPO4, and (c) LaPO4


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Page 26 of 30

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Page 28 of 30

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2

1 5

0

00

(1

4

aP

(8

00

(1

40

(1

r r L

(1

r

(1

S r

S S r %

S

S

% % %

10 10

10

10

In te n s ity (A rb . U n its )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8%

Page 29 of 30

Figure 10: The X-ray powder diffraction data obtained at room temperature for LaPO4, 10%Sr-doped LaPO4 sintered at 800, 1000, 1200, and 1400 ◦ C, and 8% Sr-doped LaPO4 sintered at 1400 ◦ C. 29 ACS Paragon Plus Environment


Figure 11: Nanoscale properties of 8 % Sr-doped LaPO4 sintered at 1200 ◦ C): (a) topography and (b) local variations in conductivity.

-3

4 .2 % 4 .2 %


-4

4 .2 % 4 .2 %


-4

-5

-5

-6

-6

-7 -8

-3

L o g ( σ( S / c m ) )

L o g ( σ( S / c m ) )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

1 0 % 1 0 %

0 .8

-7


1 .0

8 % 8 %

1 .2

1 .4

1 .6

-8

1 .8

0 .8


1 .0

1 .2

1 .4

1 0 0 0 /T (K )

1 0 0 0 /T (K )


1 .6

1 .8

Ca ... - ACS Publications

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