Oxygen Level Dependent Lattice Dynamics of Na0.73CoO2-δ

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Oxygen Level Dependent Lattice Dynamics of Na0.73CoO2-δ P. H. Tsai,† R. Donelson,‡ T. T. Tan,† M. Avdeev,§ D. H. Yu,§ T. Stra¨ssle,| and S. Li*,† School of Materials and Engineering, The UniVersity of New South Wales, Sydney, NSW 2052 Australia, CSIRO DiVision of Process Science and Technology, Clayton, VIC 3168 Australia, The Bragg Institute, ANSTO, Lucas Heights, NSW 2234 Australia, and Laboratory for Neutron Scattering ETH Zurich and Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland ReceiVed: August 17, 2010; ReVised Manuscript ReceiVed: October 22, 2010

The optical and acoustic phonon branches of Na0.73CoO2-δ have been determined using Raman scattering and inelastic neutron scatterings, and their correlation with phononic thermal conductivity kph in terms of oxygenvacancy concentration δ was investigated. The experimentally observed phonon stiffening of the Ramanactive E1g mode suggests that oxygen vacancies may help stabilize texturing of Na ions that gives rise to higher kph with increasing δ. The generalized phonon density of states characterized using inelastic neutron scattering exhibits subtle stiffening of acoustic and optical phonons with δ, which appears to be responsible for the variations in kph(T) profile in the temperature range 323-923 K. 1. Introduction Oxygen deficiency plays an important role in determining the physical properties of electronic ceramic materials. For example, oxygen-vacancy concentration δ in strontium titanate SrTiO3-δ has been demonstrated to be the most decisive factor for determining its physical behaviors: the introduction of less than 0.03% oxygen vacancies not only induces insulator-metallic transition, but also alters its appearance from being colorless to bluish.1,2 The suppression of superconductivity in high-Tc superconductor YBa2Cu3O7-δ with increasing δ is another example.3 In addition, oxygen vacancies have also proved to be effective in scattering phonons,4 providing an alternative route for reducing lattice thermal conductivity and enabling the design of more efficient thermoelectric materials. The technological importance of lamellar-structured sodium cobaltates NaxCoO2 has recently emerged due to its superior thermoelectric properties.5 Although the electronic transport in this material has been extensively studied, the thermal transport, which is one of the critical factors determining the energy conversion efficiency, is less understood. Recent thermoelectric measurement on reduced Na0.73CoO2-δ reveals that thermopower can be enhanced by the introduction of oxygen vacancies, yet the effect is offset by the simultaneous increase in resistivity and thermal conductivity k. The anomalous k(δ) in this material is against the common understanding that vacancies in a crystal enhance phonon scattering and contribute inversely to the relaxation time (i.e., δ ∝ 1/τ). Therefore, lattice dynamical investigation is required in order to elucidate such exotic phenomenon in sodium cobaltates. Several lattice dynamical measurements have been performed owing primarily to the discovery of unconventional superconductivity in hydrated form of the compound NaxCoO2 · yH2O (x ≈ 0.35, y ≈ 1.3).6 Expectedly, the majority of the works focus on investigating the origin of superconductivity with relatively little attention on phonon transport behavior. In this work, the oxygen-vacancy concentration * Corresponding author. E-mail: [email protected]. † The University of New South Wales. ‡ CSIRO Division of Process Science and Technology. § The Bragg Institute. | ETH Zurich and Paul Scherrer Institute.

dependence of phonon behavior with lattice thermal conductivity in Na0.73CoO2-δ was investigated to provide new insights into the thermal transport in sodium cobaltates. 2. Experimental Methods Detailed experimental procedures for the fabrication of polycrystalline Na0.73CoO2-δ samples with various δ can be found elsewhere.7 In this work, six samples with δ ∼ 0.00, 0.09, 0.11, 0.13, 0.18, and 0.21 were fabricated and labeled according to their δ values. Phonon excitations were characterized by a Raman spectrometer employing an Ar ion laser (514 nm) as the excitation source. Due to the highly hygroscopic nature of NaxCoO2, the samples are considered to be unstable with decomposition into Co3O4 in ambient environment.8 Therefore, the samples were characterized immediately after fabrication to avoid degradation. While only those optical phonons at the Γ-points are visible in Raman spectra, inelastic neutron scattering is capable of characterizing both acoustic and optical phonons throughout the zone. Measurements of the generalized phonon density of states have been employed using the FOCUS cold neutron time-of-flight spectrometer at PSI, Switzerland. The incident neutron wavelength was chosen to be 4 Å. Standard corrections for empty sample holder, detector efficiency, 1/ω term, polarization and thermal population factor were applied. 3. Results and Discussion Figure 1 illustrates the unpolarized Raman spectra for polycrystalline Na0.73CoO2-δ samples with δ ∼ 0.00, 0.09, 0.11, 0.13, 0.18, and 0.21 at room temperature. The δ ∼ 0.09 sample unambiguously shows two phonon modes at 459 and 574 cm-1 corresponding to Raman-active in-plane E1g and out-of-plane A1g vibrations of oxygen atoms, respectively, in excellent agreement with that reported for single crystal β-Na0.7CoO2 (hexagonal space group P63/mmc).9 An increase in oxygen content leads to subtle softening of the E1g and A1g modes. On the other hand, equilibrating the samples at lower oxygen partial pressure and higher temperatures increases δ and shifts the E1g and A1g modes to higher frequency. The origin for such phenomena will be discussed later. The addition of oxygen vacancies above 0.18 leads to simultaneous suppression of E1g

10.1021/jp107774y  2010 American Chemical Society Published on Web 11/22/2010

Lattice Dynamics of Na0.73CoO2-δ

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Figure 1. Unpolarized Raman spectra for polycrystalline Na0.73CoO2-δ with δ ∼ 0.00, 0.09, 0.11, 0.13, 0.18, and 0.21. The vertical dash lines show the peak positions for the δ ∼ 0.09 sample.

Figure 3. (a) Generalized phonon density of states (no correction for Debye-Waller factor), and (b) oxygen-vacancy dependent phononic thermal conductivity kph of Na0.73CoO2-δ with various δ. The kph was obtained after subtracting electronic thermal conductivity from total thermal conductivity measured employing laser flash technique.

Figure 2. Oxygen-vacancy concentration dependence of Raman-active in-plane E1g/Eg and out-of-plane A1g vibrations of oxygen atoms. The insets illustrate the schematic drawings of the corresponding oxygen motions in CoO6 octahedra. The shaded area represents the coexistence of β- and R-NaxCoO2-δ.

and A1g modes and appearance of two new peaks at 484 and 584 cm-1 attributable to Raman-active in-plane Eg and out-ofplane A1g vibrations of oxygen atoms from trigonal-structured R-NaCoO2 (space group R3jm) respectively.10 Raising δ above 0.21 completely suppresses the E1g and A1g modes from hexagonal structure, leaving merely the higher frequency Eg and A1g modes from the trigonal structure. Our preliminary neutron diffraction study, which reveals that the transition from hexagonal to trigonal phase is accompanied by the formation of CoO resulting in an increased Na content x, confirms this transition. As δ approaches a critical value δ*, which corresponds to the minimum oxygen-vacancy concentration required to trigger phase transition, the charge associated with further removal of oxygen ions can no longer be compensated by the reduction of Co4+/3+ ions. Instead, it is accomplished by forming Na aggregates. Consequently, the crystal stabilizes into Na-rich trigonal phase which results in the formation of CoO. The δ dependence of the E1g/Eg and A1g phonon modes is summarized in Figure 2 and the corresponding vibrational motions of the two phonon modes are depicted in the inset. The shaded area indicates the crossover region from β- to R-phase (i.e., coexistence of β- and R-phases) as δ increases. It

is discernible from the figure that the removal of oxygen atoms shifts both phonon modes to higher frequency. The stiffening of A1g mode can be attributed to the compression of trigonal oxygen antiprisms along the c-axis direction, which is evidenced by a slight decrease in the distance between oxygen atoms (dO-O) located at the nearest neighboring CoO2 sheets.11 The c-axially compressed oxygen antiprisms inevitably lead to longer dO-O along the in-plane (i.e., ab axis) direction, which should shift the in-plane E1g mode to lower frequency. However, our results show that the introduction of oxygen vacancies shifts the phonon mode to higher frequency. The E1g mode has been demonstrated to be sensitive to the ordering of Na ions and shifts to higher frequency in the presence of such “electronic texture”.12 The organizational principle for patterning of Na ions is the stabilization of multivacancy clusters that order longrange at certain fractional fillings (i.e., occupancies for the two different Na sites).13 Neutron diffraction data reveals that the evolution of crystal structure with δ is almost identical to that with Na content and relies on the redistribution of Na ions between the two sites, suggesting that tri- or quadri- vacancies of Na ions may be established by oxygen deficiency. Indeed, the stiffening of the E1g mode with δ from Figure 2 may be indicative of the enhancement of Na-ordering by oxygen vacancy formation. This argument is further consolidated by the anomalous dependence of phononic thermal conductivity kph on δ, showing that kph climbs with δ and the effect is most prominent at lower temperatures, as illustrated in Figure 3b. The kph was obtained after subtracting the electronic thermal conductivity from the total thermal conductivity measured using laser flash technique. The origin of superior thermoelectric properties of sodium cobaltates arises from the fact that the crystalline CoO2 layers are responsible for electron conductance, while the highly amorphous Na layer, which is intercalated between CoO2 sheets along the c-axis direction, hinders the phonon transport. The ordering of the initially disordered Na layer is expected to enhance thermal conductivity, which has been confirmed experimentally.14 Therefore, the enhancement

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of kph with oxygen vacancies may be attributed to the enhanced Na-ordering resulting from oxygen deficiency. The generalized phonon density of states (GDOS) for polycrystalline Na0.73CoO2-δ samples with δ ∼ 0.00 and 0.09 at 400 K is shown in Figure 3a. The reason for choosing the samples with δ ∼ 0.00 and 0.09 for GDOS measurement is because the coexistence of β- and R-NaxCoO2-δ was found in the sample with δ > 0.13, resulting in difficult investigation on the sole effect of δ on GDOS of β-phase which is favorable for thermoelectric applications. Phonon bands in the energy range 0-40 meV are attributed to acoustic phonons whereas those in the range 50-80 meV are to optical phonons. The E1g (57 meV) and A1g (71 meV) Raman-active phonon modes manifest themselves in the optical region. The cutoff energy of onephonon process is ∼80 meV above which lattice anharmonicity sets in. From the figure, it is discernible that oxygen vacancies induce phonon stiffening throughout most of the region (i.e., 10 e E < 65 meV) as indicated by arrows. This is in good agreement with our oxygen-vacancy dependent phononic thermal conductivity kph measurement, as illustrated in Figure 3b. The figure shows that kph declines with increasing temperature and decreasing δ, reaches a plateau at 473 K and begins to climb at 673-873 K depending on δ. The kph for all samples converges at intrinsic temperature 923 K (equivalent to 79.5 meV) above which anharmonic (i.e., phonon-phonon) interaction dominates and is in excellent agreement with the one-phonon cutoff energy. In the temperature range 323-473 K, thermal conductance is governed by the normal process (N-process) and does not give rise to thermal resistance. Therefore, kph in this temperature range follows the GDOS profile in the corresponding energy range (i.e., 27-40 meV). In other words, the acoustic phonon bands are primarily responsible for the high kph of each sample in the temperature range 323-473 K. The subsequent plateau (473-573 K) suggests the absence of phonon bands in agreement with our GDOS measurement. The subtle decrease of kph with increasing temperature of the plateau in the absence of phonon bands is characteristic of Umklapp scattering (U-process) that gives rise to thermal resistance. An upsurge of kph takes place at 673-873 K (58-75 meV) depending on δ, which directly relates to the phonon-hardening that arises from oxygen deficiency as shown in Figure 1 and 2. The Raman-active E1g (Eg) and A1g (A1g) bands for β- (R-) phase are speculated to increase kph similar to the acoustic phonon branches that raise kph in the lower temperature regime, albeit long-wavelength phonons are usually responsible for heat conductance in solid materials. The forces associated with the U-process that tends to lower kph compete with optical bands that tend to augment kph. The relatively flat kph(E) in this region implies that the two forces are comparable. The phonon-hardening of these optical bands with increasing δ raises the temperature or energy at which kph begins to climb, as evidenced in Figure 3b. An upward kink at 62 meV was only observed in δ ∼ 0.14 and 0.16 samples. This energy roughly corresponds to the Eg in-plane oxygen vibration of the R-structure, suggesting δ* ∼ 0.14.

Tsai et al. 4. Conclusions In summary, the correlation of oxygen-vacancy concentration with crystal structure and lattice dynamics of Na0.73CoO2-δ was studied in detail. The experimental results suggest that the introduction of oxygen vacancies shifts the Raman-active E1g and A1g optical phonon modes, corresponding respectively to in-plane and out-of-plane vibrations of oxygen atoms, to higher frequency. The stiffening of the E1g mode may result from the texturing of Na ions with increasing oxygen-vacancy concentration, which accounts for the anomalous dependence of thermal conductivity. The correlation between generalized phonon density of states and phononic thermal conductivity reveals that subtle stiffening of acoustic and optical phonon bands with increasing oxygen-vacancy concentration is responsible for the variations in phononic thermal conductivity profile in the temperature range 323-923 K. The experimental results provide new insights into the phonon transport mechanism in sodium cobaltates. Acknowledgment. The authors would like to acknowledge Dr. Peter Lemmens for his valuable advice in Raman characterization. This work was financially supported by Australian Research Coucil Discovery Program (Grant No. DP0988687) and partially based on experiments performed at the Swiss Spallation Neutron Source SINQ, Paul Scherrer Institute, Villigen, Switzerland. Travel funding was provided by Australian AMRF program. References and Notes (1) Frederikse, H. P. R.; Thurber, W. R.; Hosler, W. R. Phys. ReV. 1964, 134, A442. (2) Mannhart, J.; Schlom, D. G. Nature 2004, 430, 620. (3) Farneth, W. E.; Bordia, R. K.; McCarron, E. M., III; Crawford, M. K.; Flippen, R. B. Solid State Commun. 1988, 66, 953. (4) Yu, C.; Scullin, M. L.; Huijben, M.; Ramesh, R.; Majumdar, A. Appl. Phys. Lett. 2008, 92, 191911. (5) Terasaki, I.; Sasago, Y.; Uchinokura, K. Phys. ReV. B 1997, 56, R12685. (6) Takada, K.; Sakurai, H.; Takayama-Muromachi, E.; Izumi, F.; Dilanian, R. A.; Sasaki, T. Nature 2003, 422, 53. (7) Tsai, P. H.; Norby, T.; Tan, T. T.; Donelson, R.; Chen, Z. D.; Li, S. Appl. Phys. Lett. 2010, 96, 141905. (8) Lemmens, P.; Scheib, P.; Krockenberger, Y.; Alff, L.; Chou, F. C.; Lin, C. T.; Habermeier, H. U.; Keimer, B. Phys. ReV. B 2007, 75, 106501. (9) Iliev, M. N.; Litvinchuk, A. P.; Meng, R. L.; Sun, Y. Y.; Cmaidalka, J.; Chu, C. W. Phys. C: Supercond. 2004, 402, 239. (10) Yang, H. X.; Xia, Y.; Shi, Y. G.; Tian, H. F.; Xiao, R. J.; Liu, X.; Liu, Y. L.; Li, J. Q. Phys. ReV. B 2006, 74, 094301. (11) Oxygen-vacancy concentration dependent crystal structure of NaxCoO2-δ will be published elsewhere. (12) Wu, T.; Liu, K.; Chen, H.; Wu, G.; Luo, Q. L.; Ying, J. J.; Chen, X. H. Phys. ReV. B 2008, 78, 115122. (13) Roger, M.; Morris, D. J. P.; Tennant, D. A.; Gutmann, M. J.; Goff, J. P.; Hoffmann, J. U.; Feyerherm, R.; Dudzik, E.; Prabhakaran, D.; Boothroyd, A. T.; Shannon, N.; Lake, B.; Deen, P. P. Nature 2007, 445, 631. (14) Lee, M.; Viciu, L.; Li, L.; Wang, Y.; Foo, M. L.; Watauchi, S.; Pascal, R. A., Jr.; Cava, R. J.; Ong, N. P. Nat. Mater. 2006, 5, 537.

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