Critical effects of the light scattering intensity at an order–disorder phase transition: KCN

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Abstract
The temperature dependence of the Raman spectrum of potassium cyanide between 140 and 220 cm−1 was measured in the two low temperature phases. With decreasing temperature the band at 190 cm−1 grows out of a broad feature of the spectrum and becomes distinct below the transition temperature at 83 °K. We suppose that the Raman spectrum of KCN in the measured energy range is disorder induced above 83 °K, and thus of low intensity. Below the transition temperature only the intensity of the Raman allowed modes increases. We analyze the data by assuming that fluctuating clusters of the ordered low temperature phase enhance the intensity of these modes. From the spectra we determine the critical exponents ν=0.68 and β=0.33 and estimate a lower limit for the correlation length.