Raman based crystallographic orientation mapping with qRICO: first applications for geological materials characterisation

The relationship between Raman peak intensity and crystal orientation via knowledge of the: Raman tensor of a given vibrational mode, incident laser light and Raman scattered light polarisation vectors is well established. Thanks to Loudon’s work in 1964 the Raman tensor structure is known for all 32 crystal classes[1]. Many researchers have exploited this to determine Raman tensor coefficients to study the nature of semiconductor and covalently bonded materials e.g.[2,3] using polarised Raman microscopy with the aim to determine crystal orientation locally.

In 2019 Ilchenko et al. demonstrated the feasibility of quantitatively mapping crystallographic orientation of polycrystalline materials in 2D, and in 3D exploiting the confocal nature of a Raman microscope[4]. The novelty of this work overcomes the need to serially collect Raman spectra at each map pixel for multiple combinations incident and scattered polarization needed to compute the local crystal orientation. A new generation of this technology, quantitative Raman imaging of crystallographic orientation (qRICO), rapidly collects Raman spectra of up to 20 combinations of incident and scattered polarization in a simultaneous manner.

Development work using ideal semiconductor materials has demonstrated that qRICO delivers the ability to produce crystallographic images of sample microstructures with sample stage step / pixel sizes down to 0.5 µm, contiguous scanning areas on the order of 10 x 10 cm and crystal orientation accuracy better than 2°. Thus, qRICO provides access to a very wide range of the microstructure length scales seen in geological materials and is amenable to typical geological specimen dimensions and shapes. Fundamentally qRICO is not limited to polished planar sample surfaces and is not restricted to surface studies for transparent materials.

In this work, in addition to non-natural polycrystalline materials, we will present high resolution as well as large area map examples of orientation mapping results on natural diamonds containing defect structures, polycrystalline quartz particles and multiphase petrographic thin slices. These examples will be used to illustrate the potential of qRICO for understanding geological materials in terms of grain boundaries, phase boundaries, orientation gradients, and crystallographic orientations and texture in relation to the conventional information contained in the underlying Raman spectra such as chemical gradients and internal stress.

[1]  R. Loudon, The Raman effect in crystals, Adv. Phys. 13 (1964) 423–482. https://doi.org/10.1080/00018736400101051.
[2]  C. Kranert, C. Sturm, R. Schmidt-Grund, M. Grundmann, Raman tensor elements of β-Ga2O3, Sci. Rep. 6 (2016) 35964. https://doi.org/10.1038/srep35964.
[3]  X. Zhong, A. Loges, V. Roddatis, T. John, Measurement of crystallographic orientation of quartz crystal using Raman spectroscopy: application to entrapped inclusions, Contrib. Mineral. Petrol. 176 (2021) 89. https://doi.org/10.1007/s00410-021-01845-x.
[4] O. Ilchenko, Y. Pilgun, A. Kutsyk, F. Bachmann, R. Slipets, M. Todeschini, P.O. Okeyo, H.F. Poulsen, A. Boisen, Fast and quantitative 2D and 3D orientation mapping using Raman microscopy, Nat. Commun. 10 (2019) 5555. https://doi.org/10.1038/s41467-019-13504-8.