Anharmonic lattice dynamics of ${\text{Ag}}_2\text{O}$ studied by inelastic neutron scattering and first-principles molecular dynamics simulations

Inelastic neutron scattering measurements on silver oxide (${\text{Ag}}_2\text{O}$) with the cuprite structure were performed at temperatures from 40 to 400 K, and Fourier transform far-infrared spectra were measured from 100 to 300 K. The measured phonon densities of states and the infrared spectra showed unusually large energy shifts with temperature, and large linewidth broadenings. First principles molecular dynamics (MD) calculations were performed at various temperatures, successfully accounting for the negative thermal expansion (NTE) and local dynamics. Using the Fourier-transformed velocity autocorrelation method, the MD calculations reproduced the large anharmonic effects of ${\text{Ag}}_2\text{O}$, and were in excellent agreement with the neutron scattering data. The quasiharmonic approximation (QHA) was less successful in accounting for much of the phonon behavior. The QHA could account for some of the NTE below 250 K, although not at higher temperatures. Strong anharmonic effects were found for both phonons and for the NTE. The lifetime broadenings of ${\text{Ag}}_2\text{O}$ were explained by anharmonic perturbation theory, which showed rich interactions between the Ag-dominated modes and the O-dominated modes in both up- and down-conversion processes.

Figure 1

Cuprite structure of ${\text{Ag}}_2\text{O}$, showing standard cubic fcc unit cell and O-Ag-O links that pass between cubes.

Figure 2

Neutron weighted phonon DOS of ${\text{Ag}}_2\text{O}$ with the cuprite structure from ARCS experimental data (black dots) and MD simulations (red curves) at temperatures from 40 to 400 K. The dashed spectrum corresponds to the 40 K experimental result, shifted vertically for comparison at each temperature. Vertical dashed lines are aligned to the major peak centers at 40 K from experiments, and are numbered at the top. The incident energy was 100 meV for panel (a), and 30 meV for panel (b).

Figure 3

Shifts of centers of peaks in the phonon DOS, relative to data at 40 K. The filled symbols are experimental data, open symbols (green) are from MD-based QHA calculations, and solid curves (red) are from MD calculations. Indices 1, 2, 3 correspond to the peak labels in Fig. 2, and are also represented by the triangle, square, and circle, respectively, for experimental data and QHA calculations.

Figure 4

(a) FT-IR absorbtion spectra of ${\text{Ag}}_2\text{O}$ with the cuprite structure at 300 K. (b), (c) Enlargement of two bands at selected temperatures.

Figure 5

Temperature dependence of lattice parameter from experimental data in Ref. [1], quasiharmonic calculations and MD calculations, expressed as the relative changes with respect to their 40 K values, i.e., $a\left(T\right)/a\left(40\text{K}\right)-1$.

Figure 6

Phonon modes simulated by MD and projected on the $\Gamma$ point, at temperatures and pressures as labeled. The normal-mode frequencies calculated from harmonic lattice dynamics are shown as vertical dashed lines in red. The group symmetry for each mode is shown at the bottom.

Figure 7

(a) Temperature dependent frequency shifts of the Ag-dominated $F_{1u}^{\left(1\right)}$ mode and the O-dominated $F_{2g}$ and $F_{1u}^{\left(2\right)}$ modes from FT-IR (black), compared with the MD simulated peaks (red) such as in Fig. 6. (b) The lifetimes of the corresponding modes at temperatures from 40 to 400 K, from FT-IR (black) and the MD simulated peaks (red).

Figure 8

(a) Calculated phonon dispersion along high-symmetry directions of ${\text{Ag}}_2\text{O}$ with the cuprite structure. $\Gamma (0,0,0)$, $M(0.5,0.5,0)$, $X(0.5,0,0)$, $R(0.5,0.5,0.5)$. (b) The TDOS spectra $D\left(\omega \right)$ at 40 K (dashed) and 400 K (solid). The down-conversion and up-conversion contributions are presented separately as black and green curves, respectively.

Figure 9

Temperature dependence of the inverse of the quality factors $1/\mathcal{Q}$ for the $F_{1u}^{\left(1\right)}$, $A_{2u}$, $F_{2g}$, and $F_{1u}^{\left(2\right)}$ phonon modes from FT-IR (black squares), MD calculations (red circles), and the theoretical fittings with a full calculation of the TDOS. The unit of the fitting parameter $|C_j^{\left(3\right)}{|}^2$ is 10${}^{-1}$ eV${}^{-1}$.