Raman based crystallographic orientation mapping with qRICO: first applications for geological materials characterisation
Authors: Bowen Jacob , Heinig Mario , Reischig Peter , Bachmann Florian , Ilchenko Oleksii , Pilgun Yuriy , Barbee Olivia 
Published: February 2023

The relationship&#160;between Raman peak intensity and crystal orientation via knowledge of the:&#160;Raman tensor of a given vibrational mode, incident laser light and Raman&#160;scattered light polarisation vectors is well established. Thanks to Loudon&#8217;s&#160;work in 1964 the Raman tensor structure is known for all 32 crystal classes[1]. Many researchers&#160;have&#160;exploited this to determine Raman tensor coefficients to study the nature of&#160;semiconductor and covalently bonded materials e.g.[2,3]&#160;using polarised Raman&#160;microscopy with the aim to determine crystal orientation locally.In 2019&#160;Ilchenko&#160;et al. demonstrated the feasibility of&#160;quantitatively mapping crystallographic orientation of polycrystalline&#160;materials in 2D, and in 3D exploiting the confocal&#160;nature of a Raman microscope[4].&#160;The novelty of this work overcomes the need to serially collect Raman spectra&#160;at each map pixel for multiple combinations incident and&#160;scattered polarization&#160;needed to compute the local crystal orientation. A new generation of this&#160;technology, quantitative Raman imaging of crystallographic orientation&#160;(qRICO),&#160;rapidly collects Raman spectra of up to 20 combinations of incident and&#160;scattered polarization in a simultaneous manner.Development work using ideal semiconductor&#160;materials has demonstrated that qRICO delivers the ability to produce&#160;crystallographic images of sample microstructures with&#160;sample stage step /&#160;pixel sizes down to 0.5 &#181;m, contiguous scanning areas on the order of 10 x 10&#160;cm and crystal orientation accuracy better than 2&#176;. Thus, qRICO&#160;provides access&#160;to a very wide range of the microstructure length scales seen in geological&#160;materials and is amenable to typical geological specimen dimensions and&#160;shapes.&#160;Fundamentally qRICO is not limited to polished planar sample surfaces and is&#160;not restricted to surface studies for transparent materials.In this work, in addition to non-natural polycrystalline&#160;materials, we will present high resolution as well as large area map examples&#160;of orientation mapping results on natural&#160;diamonds containing defect&#160;structures, polycrystalline quartz particles and multiphase petrographic thin&#160;slices. These examples will be used to illustrate the potential of&#160;qRICO for&#160;understanding geological materials in terms of grain boundaries, phase&#160;boundaries, orientation gradients, and crystallographic orientations and texture&#160;in relation&#160;to the conventional information contained in the underlying Raman&#160;spectra such as chemical gradients and internal stress.[1]&#160; R. Loudon, The Raman effect in crystals, Adv. Phys. 13 (1964) 423&#8211;482. https://doi.org/10.1080/00018736400101051.[2]&#160; C. Kranert, C. Sturm, R. Schmidt-Grund, M. Grundmann, Raman tensor elements of &#946;-Ga2O3, Sci. Rep. 6 (2016) 35964. https://doi.org/10.1038/srep35964.[3]&#160; 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.