Now i am working on a organic salt which is having Orthorhombic structure in room temperature and it exhibits a structural phase transition at 426K. I have calculated 288 Raman modes for Orthorhombic and 141 for Monoclinic structure, but when i have done temperature dependence Raman there is no change in the Raman spectrum at phase transition temperature. what are the possible reason for this?
I have conformed the phases with xrd data. Orthorhombic(as Prepared) phase is transforming to monoclinic by a irreversible phase transition at 426K so i have collected the as prepared phase and heated phase XRD data which are indicating clearly about structural change. I have recorded the Raman spectra of both as prepared and heated phase where also no change in spectrum. I have calculated the Raman modes by computational methods.
In our lab we are using micro Raman spectrometer (Renishaw, UK, model in via) . I have used 514.5nm laser excitation for measurements, the lowest wave number cutoff in this instrument is 70 cm-1 with 1.5 cm-1 resolution.
according the measurements of raman spectrum, there have significant shifts for some raman peaks such as 805cm-1 and 834cm-1，916 cm-1 in DIPAB. the shifts may be only 0.5-1cm-1. the reason may be that the anions an cations have the similar structure. the significant shift may raise from the different in hydrogen bonding structure such as bond length. theoretical calculation may ignore the influence taking by hydrogen bonding---a weaker correlation.