Jan 28, 2016 - observed as an endothermic peak at 466 K (see DSC heating trace in Figure S1, .... In superionic α phase, the signature of motion is reflected in ...
Article pubs.acs.org/JPCC
Atomic-Level Characterization of Dynamics of Copper Ions in CuAgSe Chenglong Shi,† Xuekui Xi,*,† Zhipeng Hou,† Enke Liu,† Wenhong Wang,† Shifeng Jin,‡ Yue Wu,§ and Guangheng Wu† †
State Key Laboratory for Magnetism, Beijing National Laboratory for Condensed Matter Physics Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China § Department of Physics and Astronomy, University of North Carolina, Chapel Hill 27599, United States S Supporting Information *
ABSTRACT: To examine the ionic migrations on atomic level in metal chalcogenide phonon glasses, 63Cu solid state nuclear magnetic resonance (NMR) spectroscopy was performed in an archetype CuAgSe compound. The observed characteristic motional narrowing effect in the superionic state provide clear evidence for the model of single ion-hopping instead of freeion-like motion. We further find, for the first time, that thermal conductivity is strongly correlated with cation dynamics revealed by NMR within the range of temperatures investigated. This investigation provides better understanding how microscopic structure affects thermal transport properties which is of current interest in transition-metal chalcogenide based fast ion conductors.
1. INTRODUCTION Mixed (ionic-electronic) conductors have unique thermal transport properties including high thermoelectric and nonlinear electric conductive properties which can be applied in a new generation of magnetic field sensor, thermoelectric transducer, or solid state (nonvolatile) memory devices.1−7 Recently, more attention has been paid on exotic phonon scattering processes in ionic conductors.8,9 Phonon wavelength and mean free paths have been theoretically suggested be closely related to the local dynamics of ions. Despite of the importance of the knowledge of local dynamics of ions in this type of materials, this structural property remains largely unknown. Conventional diffraction experiments on local structure and dynamics is challenging probably due to the presence of atomic disorder. In this regard, solid state nuclear magnetic resonance (NMR) spectroscopy is one wellestablished technique that provides a powerful tool to study structural dynamics on the time scale from 10−9 to 102 seconds. NMR line width which is a direct measure of diffusion hopping time (τc ≈ 10−5 s) of the selective nuclei will decrease with temperature if the probed nuclei are in rapid motion rather than frozen in solid.10−12 Motional narrowing usually occurs when the hopping rate τc−1 becomes comparable to nuclear dipolar broadening (Δω)d in a rigid sublattice.13−16 One recent example involves the study of CuAgSe and Cu2Se chalcogenides which exhibit Type-I superionic transition (in the notion of Boyce and Huberman17) with sharp changes of volume during the transition.18,19 The high mobility of both © 2016 American Chemical Society
cations in its superionic state can be inferred from fast ion exchange reactions18 and high value of ionic conductivity which increases by orders of magnitude on passing through the superionic transition. CuAgSe also exhibits abnormal electronic and thermal transport properties including switchable charge carriers and extremely high electron mobility.20,21 The thermal conductivity (κ) in its superionic state is only ∼0.25 W m−1 K−1 above superionic transition temperatures (Tc). Compared to other state-of-the-art thermoelectrics, this value is remarkably low and close to the amorphous limit.21 It has been proposed that the phonons are strongly scattered and softened by ionic motion, exhibiting a phonon-liquid like thermal conducting behavior.4,22 This proposal is inspired from the understanding of phonon scattering mechanism of rattling ions in skutterudites and clathrates. For better understanding of the phonon scattering mechanism in the superionic conductors, the very first step is to accurately describe the dynamic behavior of cations. Diffraction scattering indicates the average amplitude of thermal vibrations of cations is relatively high compared with interatomic distances in superionic α-CuAgSe,23 indicating the melting of sublatice according to Lindemann criterion. A freeion-like (correlated AgAg or CuAg motion) model was then suggested for describing the cation motion.23,24 This model implies a crystalline cage be immersed in a viscous liquid Received: December 15, 2015 Revised: January 27, 2016 Published: January 28, 2016 3229
DOI: 10.1021/acs.jpcc.5b12296 J. Phys. Chem. C 2016, 120, 3229−3234
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The Journal of Physical Chemistry C consisting of Cu and/or Ag ions. Alternatively, a single-ion hopping behavior was proposed.19,25 Single cation would like to move with a discrete set of velocities from site to site on a regular sublattice rather than in correlated motion. The ratio of ion jumping over residence time was estimated to be 1/40 in electrochemical ionic conductivity measurements.19 The hopping motion of ions in superionic conductors was also frequently suggested in theoretical considerations and molecular dynamics (MD) simulations.26−28 In spite of the above results about the dynamics of cations in metal chalcogenide ionic conductors, the correlation of dynamics of ion with thermal conductivity has never been established, and solid experimental evidence is highly desirable. In this investigation, we performed temperature-dependent 63/65Cu NMR studies on CuAgSe, an archetype of superionic conductors. The observed motional narrowing of 63Cu NMR central lines with temperature indicates that cation migration across superionic transition of CuAgSe seems to be more accurately described by the single-ion hopping model. Moreover, thermal conductivity is found to be strongly correlated with cation dynamics in terms of 63Cu NMR central line width within the range of temperatures investigated. This correlation should provide better understanding of the role of ionic motion on thermal transport properties in this type of superionic conductors.
Figure 1. (a) A schematic diagram showing the average structure of cubic α-AgCuSe. green, gray and brown spheres are Se2− in 4(a), Ag+/ Cu+ in octohedral 32(f) and Cu+ in tetragonal 8(c) sites, respectively. (b) Projected plane along cubic [100] direction. The red sites represent the sites that can be occupied by Cu cations (only partially occupied), and the green sites represent the sites fully occupied by Se atoms. The silver sites are partially occupied by both Ag and Cu ions in α-AgCuSe. The dotted lines show the possible hoping paths of Cu ions.
2. EXPERIMENTAL SECTION CuAgSe crystals were grown and characterized by X-ray diffraction (XRD) with Cu Kα radiation on a Bruker X-ray diffractometer. Details on growth procedures can be found elsewhere. Cu2Se crystals were purchased from Alfa Chemical Co. Ltd. Figure1a illustrates the structure of α-CuAgSe with possible sites that could be occupied by cations in a unit cell. The perspective view of this crystal structure in the (110) plane is shown in Figure 1b. 63Cu NMR spectra of powdered crystals were obtained through Fourier transformation of spin−echo signal with a Bruker Avance III 400 HD spectrometer in a magnetic field of 9.39 T. Hahn echo pulse sequence was used with a typical pulse delay for the crystals around 200 ms. The first pulse length was set to be 1.5 μs at low temperatures and 6.5 μs for α-CuAgSe at high temperatures, less but close to 90 time. All 63Cu NMR shifts were referenced to CuCl powders (−332 ppm at 298 K). The phase transition behaviors were characterized by differential scanning calorimeter method on a DSC 214 Polyma, NETZSCH. During DSC measurements, the sample was first cooled down to 173 K from 298 K at a cooling rate of 20 K/min, then heated to 473 K at a heating rate of 10 K/min. After staying at 473 K for 5 min, the sample was cooled down to 173 K again, with a cooling rate of 10 K/min. 3. RESULTS Figure 2 shows the observed and calculated powder XRD patterns of CuAgSe crystals at various temperatures from 298 to 543 K. For clarity, only three patterns collected at 298, 440, and 465 K are displayed, respectively. At 300 K, the XRD pattern can be well indexed with an orthorhombic cell in space group Pmmn, as for β-CuAgSe [JCPDS 10−0451]. The three peaks at 2θ = 41−43° labeled by black stars are from β′CuAgSe [JCPDS 25−1180]. As shown in Figure 2a, a structural transition from β to cubic α-phase (space group, Fm3̅m) occurs at the onset of ∼468 K on heating. This transition is also observed as an endothermic peak at 466 K (see DSC heating
Figure 2. X-ray powder patterns of CuAgSe at various temperatures. Lattice parameters for full-profile Rietevield refine of tetrahedral β phase at 298 K: a = 4.1 Å, b = 4.05 Å, and c = 6.31 Å; orthorhombic β phase: a = 4.1 Å, b = 20.35 Å, and c = 6.31 Å. For face centered cubic α phase at 523 K, a = 5.8 Å.
trace in Figure S1, Supporting Information). The structures and site occupations of the β and α phases were obtained by Rietveld refinement and were consistent with previous 3230
DOI: 10.1021/acs.jpcc.5b12296 J. Phys. Chem. C 2016, 120, 3229−3234
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The Journal of Physical Chemistry C
Figure 3. (a) and (b) 63Cu NMR spectra of CuAgSe at different temperatures. (c) 63Cu NMR line shift as a function of temperatures for β-CuAgSe. The solid line is the fitting by following the two-band model described in the context. (d) 63Cu NMR central line-width versus the inverse of the temperature. The experimental data are fitted according to the Arrhenius relation.
investigation.9 Moreover, Rietveld refinement of the broad peak in the background of XRD pattern at 523 K reveals that there exists a considerable degree of disorder. Similar ion disordering phenomena were also observed from the diffusive scattering pattern of Ag2Te and Cu2−xSe (x = 0, 0.15, 0.25) metal chalcogenides and the other family of superionic conductors,17,28−31 indicating that ion disordering is a common structural feature in the superionic state. Figure 3a displays static 63Cu (spin I = 3/2) NMR powder spectra of CuAgSe compound at different temperatures. Only central lines can be observed in nonsuperionic β-CuAgSe from 203 to 457 K. The central line width (full width at half magnitude, Δv) is about 30 kHz at 203 K, which is much larger than that from the pure magnetic dipolar broadening in copper metal (9 kHz). The low temperature central lines can be fitted when taking consideration of various broadening contributions from quadrupolar, dipolar, and chemical shift anisotropy. The quadrupole coupling constant, CQ, can be obtained from line shape analysis with SIMPSON using DMFIT software.32 From 203 to 283 K, the line shape is dominated by second-order quadrupole broadening with CQ ranges from 5 MHz (203 K) to 4 MHz (283 K). Quadrupolar interactions have been averaged out at higher temperatures. The decrease of CQ is concomitant with the reduction of 63Cu central line width. Similar observations of central line narrowing from the decrease of second order quadrupole coupling were also noted in other ionic conductors.33−37 When approaching 348 K, the line shape seems to be overtaken by chemical shift anisotropy.38 Characteristic band structural information on β-CuAgSe can be revealed from the analysis of 63Cu isotropic shifts. The very small shifts at low temperatures, as shown in Figure 3b, indicate
Cu s partial density of states at the Fermi energy is close to zero or in a deep valley, in agreement with DFT calculations showing a large dispersion of the s bands near the Fermi level. Figure 3b also shows that 63Cu isotropic shifts decrease remarkably when heated above 300 K and then nearly level off at higher temperatures. This trend of shifts with temperatures can be explained by a two-band model39 in which a highly dispersive band (s-band in this case) across the Fermi level and a less dispersive band are well separated away from Fermi level. Electrons in less dispersive band were thermally excited into dispersive band with increasing temperatures, which leads to the shift of resonances. The electronic band gap size was determined to be 0.29 eV by fitting the shifts against temperatures with the equation K = K 0 + C T e−Eg / kBT ,39 where, K, a combination of orbital and temperatureindependent isotropic shifts; C, constant for a given semimetal or semiconductor with narrow band gap; Eg, the excitation energy (separation of band edge within the vicinity of the Fermi level). The above results are consistent with the semimetallic character in which both electrons and holes are present at the Fermi level, as indicated from temperature dependence of carrier concentration, Seebeck coefficients20 and our firstprinciples band structure computations. The line shape of the 63Cu NMR spectra in CuAgSe changes dramatically when passing through the superionic transition. Anisotropical central lines are abruptly transformed into symmetric lines positioned at −139 ppm (reference: 63CuCl powder, −332 ppm at 298 K) with line width ∼5 kHz. The transition temperature, 463 K is in good accordance with superionic transition temperature, 466 K determined from DSC 3231
DOI: 10.1021/acs.jpcc.5b12296 J. Phys. Chem. C 2016, 120, 3229−3234
Article
The Journal of Physical Chemistry C
simultaneously. The central line width Δv = (25/64)(2I + 3) × e4q2Q2/[4I2(2I − 1)h2] if the broadening is entirely from second order interaction, where Q, quadrupole moment; I, spin quantum number, and other symbols have their normal meanings. The ratio of line width 65Δv/63Δv = 1 at 468 K, which deviates from (65Q/63Q)2 = 0.86. The above results indicate the vanishing quadrupole effects and point to the conclusion that the line broadening originates from heterogeneous instead of homogeneous second order quadrupolar broadening.12 The site distribution as revealed by NMR is consistent with the scenario of cation disordering in the superionic phase.17 In this regard, it seems that the line width of a heterogeneously broadened NMR central line is a measure of disorder on atomic scale. The copper ion dynamics revealed by 63/65Cu NMR in this investigation is of importance for understanding the unconventional thermal transport properties. In general, the total intrinsic thermal conductivity (ktot) consists of three terms, including lattice thermal conductivity (kL), electronic thermal conductivity (ke), and bipolar thermal conductivity (kb). Bipolar conduction process is expected for semimetals but only at elevated temperatures.43,44 kL (= ktot − ke) can be estimated from the Wiedemanne-Franz law ke = LσT, where the Lorenz number L = 2.1 × 10−8 V−2 K−2. Rather than taking advantages of thermal boundary resistance through multiphase alloying and nanostructure engineering, extremely low intrinsic thermal conductivity can be also realized in pristine materials with atomic-layered structure by selecting anharmonic chemical bonds. In addition to this Bragg scattering mechanism, phonon crystals/glasses with local resonant domains or “rattling” ions realized in clathrates and skururrites represent an alternate mechanism for lowering lattice thermal conductivity. From Figure 4a, one can see that the lattice thermal conductivity is strongly correlated with 63Cu NMR central line width within the range of temperatures from 203 to 483 K. The lattice thermal conductivity data were derived from literature.9,21,22 At low temperatures, the temperature dependence of thermal conductivity can be well described by Slack’s T−1 law due to intrinsic phonon−phonon interactions. The new observation here from this correlation is that the structural dynamics in terms of jumping rate of cations probed by 63Cu NMR has a strong imprint on the temperature dependence of the thermal transport properties. As mentioned earlier, NMR line width provides an estimate for the appearance of “static” for atoms on the time scale of 1/Δv, that is, τc ≈ 2 × 10−4 s in this case. This strong correlation suggests low frequency (tens of kHz) phonons play an essential role in heat conduction. This correlation can be understood: (1) when Cu atoms are bonded to Se atoms by strong covalent and/or ionic bonds in orthorhombic β-CuAgSe, the phonon mean free path (MFP) is relatively long in the periodic sublattice structure. (2) As the migration of cation occurs, the periodicity of cation sublattice will be strongly disturbed by atomic displacement or motion. The phonon MFP will be reduced as a consequence of this disordering.45 (3) The CuAgSe compound with weakly bond cations embedded in the rigid Se sublattice is structurally similar to a composite with structure unit consisting of a hard core and soft surface-layer. The local resonances from weakly bonded ions result in phonon spectral gaps and soft phonon modes.46 Beyond phase transition, one remaining issue to be addressed is the feature on the line width below Tc in the temperature range between 350 and 430 K. The central line width increases with increasing temperature in the temperature
measurements. The remarkable feature is that with further increasing temperature beyond Tc, the line width monotonically narrows, as shown in Figure 3a−c. NMR line width provides an estimate for the appearance of “static” for atoms on the time scale of 1/Δv, that is, τc ≈ 2 × 10−4 s in this case. For random hopping in a thermally activated process, the line width is related to a correlation time which follows:37 ⎛ E ⎞ Δv = Δv0 exp⎜ − a ⎟ ⎝ RT ⎠
(1)
where Ea is the activation energy of the system and Δv0 the frequency prefactor which linked to the line width in the rigid lattice. A fit of experimental data to this equation over Tc up to 483 K yields Ea = 0.16 ±0.01 eV, as shown in Figure 3d, which is comparable to the value of 0.15 eV obtained from ionic conductivity measurement.19 In addition, no changes of the NMR spectra could be found in heating and cooling cycles many times from 203 to 483 K and back. The highly reversible nature of superionic transition (β → α) is also accompanied by thermal hysteresis (∼10 K). No bifurcation of NMR peaks occurs, suggesting the absence of phase coexistence during the transition.
4. DISCUSSIONS 63 Cu NMR central line width is a direct measure of correlation time for the random hopping of copper ions with respect to their nearest neighbors. Motionally narrowed 63Cu NMR lines, as demonstrated in Figure 3a, provide clear evidence for high mobility of copper ions relative to their nearest neighbors. Motions of atoms/ions such as Ag+ within the vicinity of the Cu nuclei being observed will gradually average out the electric field gradients. The decreasing effective quadrupolar interactions eliminate the line broadening from second order quadrupole couplings. This explains the gradually narrowing effects with increasing temperatures in the nonsuperionic β phase. In superionic α phase, the signature of motion is reflected in two ways: one is the motionally narrowed line width (
