Alq J Sci. 2025;1(2):157-171 5 ajs.2520 https://doi.org/10.69667/ Alqalam Journal of Science لعلـومم لقلـــة الل https://alqalam.utripoli.edu.ly/index.php/AR Copyright Author (s) 2025. Distributed under Creative Commons CC-BY 4.0 Received: 02-07-2025 - Accepted: 01-09-2025 - Published: 10-09-2025 157 Original article Raman Light Scattering Study of the Isotopic Construction Effects on the Lattice properties -TO and LO Phonon modes- of CuCl Nadir Driza* , Ola Mohammed , Asma Elgade Department of Physics, Faculty of Arts and Sciences, Elmarj, University of Benghazi, Benghazi, Libya Corresponding E-mail: nadir.driza@uob.edu.ly Abstract This work expands on our research on isotope effects on phonons to include compound semiconductors. In this article, we concentrate our study on the variation of the lattice properties, such as lattice dynamics and electronic structure, due to the effects of isotope substitution on the CuCl Raman spectra. We fulfilled that by examining 35Cl, 37Cl, 63Cu, and 65Cu. Unlike elemental semiconductors, compounds whose constituents are isotopically substituted have a substantial dependence on the phonon branch and wave vector q for changes in phonon frequencies and atomic displacements. In the case of CuCl, the substitution of heavy or light atoms has distinct effects on the acoustic and visual branches of the phonon dispersion relations. The significant mass difference between copper and chlorine causes the eigenvectors of the acoustic vibrations to be dominated by copper displacements, while the optic vibrations are dominated by chlorine displacements. Consequently, the phonon frequencies in the visual and acoustic branches can be altered nearly independently, especially at the X point, by employing isotope replacement. At low temperatures, this enables us to examine the effectiveness of the anharmonic decay channels of Γ-point optical phonons into lower-lying acoustic bands. Keywords. Isotopic, Lattice Properties, TO Phonon Mode, LO Phonon Mode, Raman Light Scattering. Introduction Isotopes are atoms with the same number of protons and electrons but a different number of neutrons. Isotopes differ in mass but are chemically identical. Early theoretical works have established that only properties dependent on nuclear mass are altered in isotopes. The most obvious instance of such dependence is the mass-dependent effect on the harmonic lattice vibrational frequency, which is represented as 1/√ . Isotope-dependent properties also include properties that are affected by changes in unit cell volume, atomic hopping mobility, and anharmonicities—for example, thermal conductivity, thermal expansion, melting temperature, nuclear magnetic resonance, and superconducting phase transition temperature. The electronic band structures were once believed to remain identical in isotopes under such changes. However, this belief has been justified only by atomic-spectra data. Evidence has revealed that in the case of molecular spectra, the effect of mass involves differences between isotopes through the mechanism of electron–phonon coupling [Watanabe, 2009, P1426] [Kragh, 2012, P179]. The presence of an isotope effect is an effective indicator of phonon-mediated superconductivity in materials and constitutes support for the Bardeen–Cooper–Schrieffer (BCS) theory of superconductivity [Gweon, 2004, P187, P188] [Cheng, 2017, P1-4], [Hodovanets, 2013, P1748-1753] [Bud'ko, 2001, P1877, P1878]. The ionicity of copper chloride is   = 0.75. Of all the binary compounds that crystallize in the zincblende structure at room temperature and pressure, it is therefore one of the semiconductors that shows the highest ionicity [Phillips, 1970, P332, P346] [Alhaddad, 2025, P9] [Göbel, 1997, P210]. The six-fold coordinated rock- salt structure is preferred above a critical value of   [Ono, 2020, P1], which is   = 0.785(10). The physical properties of CuCl, which are near phase transitions, show several peculiarities that have been thoroughly studied. For example, the linear expansion coefficient at low temperatures is strongly negative, which is associated with the negative mode-Grüneisen parameters of the acoustic branches at the zone edges. A strongly anharmonic lattice potential has been linked to the expansion coefficient and the tiny elastic shear constants c44 and cs = (c11 – c12)/2, which decrease with increasing hydrostatic pressure [Varshney, 2016, P1, P2]. The inverted spin-orbit splitting in CuCl [Hodges, 2007, P1] and changes in the band gap with isotope substitution [Yu, 2004, P1-P4] show that the admixture of the copper d electrons to the chlorine p levels is essential for the valence band structure [Quevedo, 2020, P10]. Even at low temperatures, the mean square atomic displacement of Cu in CuCl is substantial, and it keeps rising as the temperature rises. Though not as noticeable, CuBr and CuCl exhibit comparable behavior [Wang, 2023, P1,2]. The mean square atomic displacement of Cu in CuCl is large even at low temperature, and it continues to increase with increasing temperature. CuBr and Cu2 show a similar behavior, although not as pronounced [Wang, 2023, P1,2]. There are three models to describe the large mean square displacements. Those models have been considered as the following: first model is isotopic thermal vibrations of copper atoms within a harmonic model [Majzlan, 2023, P1-P3], second model describes displacements along the tetrahedral axes as a result of asymmetric anharmonic thermal vibrations [Aree, 2022, P1], and the third model clarifies a