Vibrational Spectrum of the Anion /Fe(CN)6/4−in Ferroelectric K4Fe(CN)6.3H2O

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Abstract
The crystals A4M(CN)6. 3H2O, where A=K, NH4 and M=Fe, Mn, Ru, Os, exhibit a paraelectric to ferroelectric phase transition below room temperature. Potassium ferrocyanide trihydrate K4Fe(CN)6. 3H2O (KFCT) has been subjected to several spectroscopic investigations1–4 but the information about its vibrational spectrum is rather incomplete. The infrared absorption study of KFCT performed by Blinc et al.1 is mainly devoted to the temperature behavior of the O-H bond vibrations at the phase transition around −26°C. Recently Brehat and Hadni2 have reported the far infrared spectra of hydrated and deuterated KFCT single crystals from 10 to 250 cm−1. The infrared spectra of KFCT, examined as a pouder film and mulls, in the 250–4000 cm−1 region have been measured by Nakagawa and Shimanouchi3. However, because of the centrosymmetric nature of the crystal space group a lot of modes do not appear in the infrared absorption. One of us has studied the low-frequency Raman spectrum of hFCT using the 4880 Å line of an argon laser. No polarized spectra of the Raman active internal vibrations have been reported so far. Due to the interest and importance of KFCT as a farroelectric, further work has been undertaken by us on its Raman spectrum in the region of 200–2200 cm−1. There t h e characteristic C[tbnd]N stretching, metal-ligand stretching and metal-C[tbnd]N bending frequencies are expected to be found.