(Template for Misasa2017 & Core-Mantle Coevolution symposium. Remove this before upload.)
Low-frequency micro-Raman spectroscopy and its application to mineral sciences
M Kanzaki1* 1Institute for Planetary Materials, Okayama University, Misasa, Tottori 682-0193, Japan.
*email:mkanzaki@okayama-u.ac.jp
Introduction: Because of advent of simple andcheap Raman system with single monochromatorplus CCD detector, low-frequency Raman (below~200 cm-1) becomes nearly obsolete in last twodecades. This is due to lower frequency limit ofRaman filters (+ dichroic filter) used for those
spectrometers. However, this situation has changed
in last few years, as Ondax Inc. introduced new
Raman filter which allows measurement down to ~5
cm-1. Simultaneously, terahertz spectroscopy(corresponding far-infrared region) is now available
to non-specialists. Therefore, new era of terahertzregion spectroscopy has arrived.
Although low-frequency region is not quite
necessary for phase identification of materials (oneof most used applications), it contains richinformation of dynamics and local structure ofmaterials. Boson peak in glasses and soft mode ofphase transition are a few well known examples. Inthe talk, a few examples of applications to minerals
are given to demonstrate ability of low-frequency
Raman spectroscopy.
Experimental: For micro-Raman measurement,a home-build spectrometer with singlemonochromator and CCD detector was used. Two
Ondax SureBlock filters are adopted to the system
to obtain low-frequency Raman spectra. We also
employed a Ondax NoiseBlock filter, which works
as a dichroic mirror. 488 nm solid laser with laser
power of 80 mW was used. See [1] for details of theinstrument.
For high-temperature measurements, a wireheater was used, and temperature was calibrated
against 5~6 standard materials with known meltingpoints [2].
Results and Discussion: First example is softmode of a-AlPO4-moganite. This is a high-pressure
phase synthesized at 5 GPa, 1500 ˚C [3]. Similar to
SiO2-moganite and other silica phases such as quartz,temperature-induced phase transition is expected.
This supposed transition was studied using present
micro-Raman system up to 600 ˚C. Two lowfrequency modes (60 and 74 cm-1) were detected atroom temperature. The 74 cm-1-mode significantly
shifted toward 0 cm-1 with temperature as shown inFig. 1. This behavior is well fitted with an orderparameter equation as shown in Fig. 1, confirmingthat the mode is soft mode. The transition
temperature (Tc) was determined as 415 ˚C. Firstprinciples calculation of vibrational modes of aAlPO4-moganite predicted two low-frequency
modes, and atomic displacements for the 74 cm-1-mode matched well toward to route for the expected
high-temperature structure.

Figure 1 Soft mode Raman shift of a-AlPO4-moganite
versus temperature (sold circles). The solid line
represents curve fitting using an order parameterequation (ws = A|Tc-T|b).

Second example is low-frequency peak observedin natural minerals. We found a very intense (andbroad) low-frequency Raman peak in a CO2containing melanophlogite as shown in Fig. 2 [4].
Melanophlogite is one of rare silica minerals with aclathrate structure. Heating study of this sample
revealed that this feature is due to CO2 in thestructure and is likely due to librational andtranslational modes of CO2 molecules in cages, asdegassing reduced its intensity. Degassing behavior
which is related to diffusion of CO2 in cages of thestructure, can be studied by observing this and also
CO2 vibrational peaks [4]. Low-frequency peaks arealso found in several zeolites which are likely
originated from molecular H2O clusters in the large
cages of zeolites.

Figure 2 Raman spectrum of CO2-containing
melanophlogite from Fortunillo, Italy at ambientconditions.

References:[1] http://www.misasa.okayamau.ac.jp/pukiwiki/index.php?Micro-Ramanspectroscopy [2] Kanzaki, M., J. Mineral. Petrol.
Sci., 113, 126 (2018). [3] Kanzaki, M. and Xue, X.
Inorg. Chem., 51, 6164 (2012). [4] Kanzaki, M., J.
Mineral. Petrol. Sci., 114 (2019) in press.