ELASTIC PROPERTIES OF RbNH4SO4 CRYSTALS

Oleh Markevych, Vasyl Stadnyk, Oleh Bovgyra, Roman Lys, Igor Matviishyn, Oleh Onufriv

Abstract


Background. The elastic properties of a material are of great importance in solid-state physics, as they significantly influence the optical properties of crystals.

Materials and Methods. The elastic properties of rubidium ammonium sulfate (RbNH4SO4) crystals are investigated in this work. The calculation of elastic properties was carried out using the CASTEP program.

Results and Discussion. Analysis of the elastic coefficients matrix of RbNH4SO4 reveals that this crystal belongs to the rhombic system with pronounced elastic anisotropy. The crystal is mechanically stable, with the greatest rigidity along the Y–direction and the smallest along the X–direction. The most stable shear is in the XZ plane, and the least stable is in the XY plane. The calculation of the Cauchy relations revealed a significant deviation (Cij > Ckl), indicating the predominance of non-central forces in interatomic interaction and a significant contribution of the covalent or ionic component of the bond in the RbNH4SO4 lattice.

The spatial distributions of Young's modulus, Young's modulus under universal compression, shear modulus, and Poisson's ratio were calculated. The degree of anisotropy in the elastic properties of the RbNH4SO4 crystal was estimated, confirming its significant anisotropy.

Conclusion. The Born criteria calculation showed that the crystal is mechanically stable. The calculation of the Cauchy relations revealed the predominance of non-central forces in the interatomic interaction and the presence of a significant contribution of the covalent or ionic component of the bond in the RbNH4SO4 lattice.

The presence of rigid tetrahedral groups [SO]_4^2- causes the formation of covalent directed bonds within these groups. It is shown that the polarizability of ions is important: rubidium ions (Rb+) and sulfate groups  [SO]_4^2-  have large electron shells that deform during mechanical compression of the crystal. This leads to the formation of induced dipole moments. The complex nature of the forces in the RbNH4SO4 crystal is a consequence of the fact that the crystal consists not of point charges, but of polarizable ions and molecular complexes [SO]_4^2- with their own internal covalent bond. This makes the material elastically anisotropic and sensitive to the direction of the applied load.

Keywords: crystal, anisotropy, elastic coefficients, Young's modulus, universal compression modulus, Poisson's ratio.


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References


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DOI: http://dx.doi.org/10.30970/eli.34.15

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