Pressure-Induced Phase Transition in Zn–Fe Prussian Blue Lattice

Abstract

Recently, Mastuda et al. reported negative thermal expansion in a series of Prussian blue type cyanides, (Cs,Rb)xM[Fe(CN)6]yzH2O (M is a transition metal). The compound belongs to a face-centered cubic (Fm 3m; Z 1⁄4 4), in which M and the [Fe(CN)6] unit form a rock-salt lattice bridged by the cyano groups. The alkaline metals and water molecules are accommodated in the nanopores of the lattice. The coefficient ( d ln a=dT) of the thermal expansion systematically decreases with an increase in the lattice constant a: 1⁄4 þ1:49 10 K 1 for Rb0:49Zn[Fe(CN)6]0:803.8H2O (a 1⁄4 9:943 A) and 1⁄4 0:59 10 K 1 for Rb0:64Zn[Fe(CN)6]0:882.3H2O (a 1⁄4 10:392 A). The negative thermal expansion as well as the systematic change of were ascribed to the thermally-induced rotational displacement of the [Fe(CN)6] units. Such a rotational instability of the [Fe(CN)6] units may cause a significant structural response to external stimuli, e.g., hydrostatic pressure, magnetic field, electric field, and photo-irradiation. In this Note, we report pressure effects on the Prussian blue lattice with the negative thermal expansion, Rb0:85Zn[Fe(CN)6]0:951.5H2O (abbreviated as RZF) and Cs0:64Zn[Fe(CN)6]0:883.5H2O (CZF). We observed prominent kink structures in the Raman shifts of the CN stretching modes and a at Pc 1⁄4 1:0GPa for RCF and at Pc 1⁄4 1:4GPa for CZF. These observations suggest a pressure-induced phase transition driven by the rotational instability of the [Fe(CN)6] units. Powder samples of RZF and CZF were prepared by reacting an aqueous solution of ZnCl2, RbCl, CsCl, and K3[Fe(CN)6]. Chemical compositions of the prepared samples were determined by inductively coupled plasma (ICP) and standard microanalytical methods. The high-pressure Raman spectroscopy was performed at 300K using a diamond anvil cell (DAC) and a microscopic Raman system. The sample powders were sealed in a gasket hole of the DAC, 100 mm in thickness and 300 mm in diameter. The pressure-transmitting medium was Daphne 7373 oil (made by Idemitsu), and applied pressure P was determined by the R1 luminescence line 6) of a small peace of a ruby. The excitation energy was 2.54 eV and excitation area was 30 mm in diameter. Scattering light was detected with a single monochromator equipped with a charge-coupled device (CCD). The high-pressure synchrotron-radiation x-ray powder diffraction measurements were performed at the BL10XU beamline of SPring-8. The powder samples were sealed in a gasket hole of a specially-designed DAC, 100 mm in thickness and 300 mm in diameter. The wavelength of the incident x-ray was 0.41131 A. The 2 range used in the Rietveld refinement was 2–30 degree. Figure 1 shows Raman scattering spectra of (a) RZF and (b) CZF at 300K against P. The higher-lying and lowerlying CN stretching bands are ascribed to A1g and Eg modes of the [Fe(CN)6] units with Oh symmetry. 4) In (a) RZF, both the A1g and Eg modes show a significant blue shift with an increase in P in the low-P region. The Raman frequencies, however, becomes nearly constant in the high-P region. A similar behavior is also observed in (b) CZF. In Fig. 2(a), we plotted the Raman shifts of the CN stretching modes of RZF against P. Solid and broken straight lines are the least-square-fitted results in the low-P and high-P regions, respectively. The Raman frequency linearly increases with an increase in P in the low-P region (P Pc 1⁄4 1:0GPa; upward arrows), but becomes nearly constant in the high-P region (P Pc). A kink structure is observed in (c) CZF at Pc 1⁄4 1:4GPa, even though the data points in the high-P region are rather scattered. We further investigated the x-ray powder diffraction patterns of RZF and CZF against P. With increase of P, the diffraction peaks shift to the high-2 side and their line widths become broader. All the diffraction peaks can be indexed in the cubic setting (Fm 3m) in both below and above Pc. We observed neither peak splitting nor extra peak above Pc. The magnitude of a was refined by the Rietveld method, and plotted in Figs. 2(b) and 2(d). In (b) RZF, a kink structure is observed around Pc 1⁄4 1:0GPa, as indicated 215