Spectrum-sensitive phonon wipeout due to a fluctuating spin state in a ${\text{Fe}}^{2+}$ coordination polymer

Raman scattering in the spin-crossover system $\left[\text{Fe}\left(\text{pmd}\right)\left({\text{H}}_2\text{O}\right){\left\{\text{Au}{\left(\text{CN}\right)}_2\right\}}_2\right]\cdot {\text{H}}_2\text{O}$ reveals a complex three-phase spin-state transition in contrast to earlier observations in magnetization measurements. We observe different spin-state phases as a function of temperature and electromagnetic radiation in the visible spectral range. There exists a fluctuating spin-state phase with an unexpected wipeout of the low-frequency phonon-scattering intensity. Furthermore we observe one phase with reduced symmetry that is attributed to a cooperative Jahn-Teller effect. Pronounced electron-phonon interaction manifests itself as a strong Fano resonance of phonons related to $\left\{{\text{FeN}}_6\right\}$ and $\left\{{\text{FeN}}_4{\text{O}}_2\right\}$ coordination octahedra. Density-functional theory supports this interpretation.

Figure 1

(Color online) Fragment of the coordination polymer $\left[\text{Fe}\left(\text{pmd}\right)\left({\text{H}}_2\text{O}\right){\left\{\text{Au}{\left(\text{CN}\right)}_2\right\}}_2\right]\cdot {\text{H}}_2\text{O}$ displaying the asymmetric unit (top), the expanded version of the archetypal network structure of ${\text{CdSO}}_4$ (middle), and the triple interpenetration of identical networks (bottom). Color code: O, red; N, blue; C black; Au, gray; and Fe, pink.

Figure 2

(Color online) Room-temperature Raman spectrum of a $\left[\text{Fe}\left(\text{pmd}\right)\left({\text{H}}_2\text{O}\right){\left\{\text{Au}{\left(\text{CN}\right)}_2\right\}}_2\right]\cdot {\text{H}}_2\text{O}$ (1Au) single crystal. The inset shows a sketch of the spin-state diagram including a light-induced excitation with a 514 nm laser line.

Figure 3

(Color online) Temperature-dependent Raman spectra of the $\left[\text{Fe}\left(\text{pmd}\right)\left({\text{H}}_2\text{O}\right){\left\{\text{Au}{\left(\text{CN}\right)}_2\right\}}_2\right]\cdot {\text{H}}_2\text{O}$ measured in the frequency regimes of 0–550 and $2150–2200{\text{cm}}^{-1}$. The inset shows details of the spectra in the FS state phase.

Figure 4

(Color online) Temperature-dependent Raman spectra in the frequency region of internal modes of the pyrimidine.

Figure 5

(Color online) Temperature dependence of integrated Raman intensities $R_{\chi }^{″}$ for selected low-frequency phonon lines (33 and $91{\text{cm}}^{-1}$, open squares in upper and lower plot, respectively) and the susceptibility ${\chi }_{M}T$ without light irradiation (Ref. 23) (triangles, upper right scale) and under laser $\left(\lambda =540\text{nm}\right)$ irradiation (triangles, lower right scale).

Figure 6

(Color online) Temperature dependence of the phonon line positions in the frequency regions of the $\left\{\text{Fe}\left(1\right){\text{N}}_6\right\}$ and $\left\{\text{Fe}\left(2\right){\text{N}}_4{\text{O}}_2\right\}$ octahedrons (left) and C-N stretchinglike vibrations (right).

Figure 7

(Color online) Raman spectra of the $\left[\text{Fe}\left(\text{pmd}\right)\left({\text{H}}_2\text{O}\right){\left\{\text{Au}{\left(\text{CN}\right)}_2\right\}}_2\right]\cdot {\text{H}}_2\text{O}$ excited by a red 647 nm laser line.

Figure 8

(Color online) Fano resonance in the PIHS phase of $\left[\text{Fe}\left(\text{pmd}\right)\left({\text{H}}_2\text{O}\right){\left\{\text{Au}{\left(\text{CN}\right)}_2\right\}}_2\right]\cdot {\text{H}}_2\text{O}$.

Figure 9

(Color online) High-temperature Raman spectra of $\left[\text{Fe}\left(\text{pmd}\right)\left({\text{H}}_2\text{O}\right){\left\{\text{Au}{\left(\text{CN}\right)}_2\right\}}_2\right]\cdot {\text{H}}_2\text{O}$ single crystal in the frequency region of C-N stretchinglike vibrations.