Lattice-Mediated Propagation of Photoinduced Phase Transition in Co–Fe Cyanide

Abstract

Spin-crossover compounds show many photoinduced phase transition (PIPT) phenomena, reflecting the strong coupling between the electron, spin, and lattice degrees of freedoms. In these compounds, the photoinduced electronic excitation triggers the intersystem of crossing in Fe2þ or Co3þ with a high probability, and alters the spin state of the transition metals (light-induced excited spin state trapping: LIESST). The spin state transition further causes local lattice deformation as a result of the variation in the ionic radii of Fe2þ or Co3þ.14) The resultant local lattice distortion modifies the electronic structure of the surrounding sites, causing a nonlinear response of the system. Ogawa et al. reported the threshold and incubation behaviors in the photoinduced phase transition of [Fe(2-pic)3]Cl2 EtOH (2-pic = 2-aminomethyl-pyridine) at 2.2K. Liu et al. reported a dynamical phase transition in Nafion–[Fe(Htrz)3] (Htrz = 1,2,4-4H-triazole) and an optical hysteresis in [Fe(pyz)6](BF4)2 (ptz = 1-propyltetrazole) at 77K. Several researchers theoretically investigated the photoinduced dynamics beyond the mean-field approximation, even though the approach had been successful in several situations. In this Note, we reported the mesoscale lattice effect on PIPT of a Co–Fe cyanide, Na0:27Co[Fe(CN)6]0:713.8H2O, at 200K by a microscopic spectroscopy. The compound shows a spin-crossover-type PIPT from the low-spin (LS) phase with LS Co3þ and LS Fe2þ to the high-spin (HS) phase with HS Co2þ and LS Fe3þ, accompanying a significant lattice expansion of 3%. We carefully investigated the PIPT dynamics at each position around the photoexcitation spot, which is 75 m in diameter. We found that the PIPT at each position takes place simultaneously within 0.2 s, even though the density (nHS) of the HS sites is widely distributed in the range of 0–0.6. The distribution of the linear expansion coefficient ( L=L) within the HS region suggests that the propagation of the PIPT region is mediated by a coherent lattice expansion. A thin film of Na0:27Co[Fe(CN)6]0:713.8H2O, 1310 nm in thickness, was electrochemically synthesized on an indium–tin oxide (ITO) transparent electrode. The chemical composition was determined by inductively coupled plasma (ICP) and CHN organic elementary analyses. The Xray diffraction pattern revealed that the compound is facecentered cubic with a lattice constant of a 1⁄4 10:34 A. The surface and cross-sectional scanning electron microscopy (SEM) images revealed that the film consists of rodlike crystals, 600 nm in length and 130 nm in area. The film shows a thermally induced LS–HS phase transition at 230K (1⁄4 Tc) in the warming run, and the film changes from violet (LS phase) to red (HS phase). The photoinduced dynamics was recorded with a laboratory-built microscopy system equipped with a charge-coupled device (CCD) camera. We used the second harmonics (532 nm) of the yttrium aluminum garnet (YAG) laser as the excitation light source. The spot size of the excitation light was 75 m, and the excitation density was 6.6 photon s 1 site . A notch filter was inserted to eliminate the scattering from the excitation light. The magnitudes of nHS at each time and position were determined by the chromaticity in the microscopy moving image. The spatial resolution of the system was 5 m. To increase the signal-to-noise ratio, the chromaticity was averaged within the 10 10 pixels around each position. Figure 1 shows examples of the microscopy images of the Na0:27Co[Fe(CN)6]0:713.8H2O film under photoirradiation at 200K at various times. The bright region corresponds to the HS region. No macroscopic PIPT is observed at (b) 41.7 s. The PIPT starts around the central region of the excitation spot at (c) 43.3 s, and then expands outside at (d) 43.5 s. A similar incubation behavior of the PIPT was observed in [Fe(2-pic)3]Cl2 EtOH. One may notice that the PIPT region at (d) 43.5 s extends beyond the excitation spot [broken-line circle in Fig. 1(a)]. The inset of Fig. 2(b) shows a prototypical example of the time dependence of nHS. (a) 0 .0 s