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

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