Can FTIR spectroscopy identify the crystal structure of a sample?

June 17, 2020


Yes. FTIR spectroscopy can detect absorption bands associated with lattice vibrations in a crystal, allowing differences in structure to be identified.

Generally, absorption bands due to lattice vibrations appear in the low wavenumber region. Although spectra measured in the mid-infrared region (4000 to 400 cm-1) using standard FTIR spectroscopy can distinguish differences in crystallinity, much more detailed information can be obtained in the far-infrared region (400 to 10 cm-1).

The FT/IR-6000 series allows continuous measurements in the mid-infrared to far-infrared regions by combining a broadband beam splitter (6000 to 20 cm-1) and a DLATGS (broadband) detector (15000 to 30 cm-1). It is possible to perform measurements quickly and easily because there is no need to replace any optical elements, which is usually required when performing measurements over a wide wavenumber range.

Figure 1  ATR spectra of CaCO3 with different crystal structures  (y-axis is offset for clarity)

Figure 1 shows an example of CaCO3 spectra with different crystalline structures. Water vapor peaks in the far-infrared region can overlap with from the sample, a full vacuum FTIR instrument (Figure 2) that can remove water vapor quickly and reliably from the infrared light path is strongly recommended.  In addition to a system that incorporates a mid-to-far IR broadband beam splitter and a DLATGS (broadband) detector for vacuum-FTIR measurements, the ease of sample preparation ATR PRO ONE (Figure 3) was used for this measurement. The ATR prism was made from diamond with a wide transmission wavenumber range of 10000 to 30 cm-1. It can be seen from the spectra in Figure 1 that the difference in crystallinity clearly appears as a difference in band shape in the far-infrared region.

Required Products and Software

Instrument : FT/IR-6600FV 
Light source : High-intensity ceramic source
Beam splitter : Mid-to-far IR broadband beam splitter
Detector : DLATGS (broadband) detector
Window : None (opened by “Gate Valve with Window Auto-Changer”)
Accessory : ATR PRO ONE 
Prism : Diamond (wide bandwidth)

About the Author

Spectroscopy Group