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Theoretical Investigation of Color Alterations in Ruby and Spinel due to Nitrogen and Oxygen Ion Bombardment

December 9, 2020.

 

       Thailand has enjoyed a unique position as the world’s major ruby export and ruby enhancement hub. Revenue from gemology industry has continued to grow albeit a general decline in economy. Currently it is second only to the automobile industry in terms of GDP. Thai rubies, also known as “Tubtim Siam,” are known for their vibrant blood-red color and are the benchmark of high-quality rubies. In addition, Thailand also has an inspection system and has an institute to certify the quality of gemstones such as diamonds, topaz, and various soft gems, which can check whether that piece of gems has undergone improvement. The most popular process to improve the color of gemstones is heat treatment (also known as gemstone cooking), a process of adding value to the corundum family of gemstones such as ruby, topaz, and other soft gems by heating in the range of 1,000 to 2,000°C for several hours to several days. In addition, there are other discoloration processes such as diffusion treatment, which is performed by sintering the gemstone that has been cut with the pigment, causing the color to enter the gemstone at a very shallow distance; irradiation with electromagnetic waves using the upper purple wavelengths; and using an electron beam or other small particles projected onto the gemstone. However, the color changes with this process tend to fade away quite quickly. In addition, the radiotherapy byproduct is difficult to verify with existing laboratory techniques.

 

 

Figure 1 Crystal structure of corundum (Al2O3). Here Al and O are represented by blue and red spheres respectively.

 

       Corundum or alpha aluminum oxide ( -Al2O3) is a very useful ceramic substance as it can be used in many applications, such as dielectric materials, heat-resistant material, catalysts, optical devices, etc. [1-6] This is because this material has a very high mechanical strength, has a high melting point, does not react with other substances and has excellent optical properties. In general, these physical properties can change significantly, which may be made for better or worse when defects are formed inside the crystal. These defects may be unintentional or may be caused by deliberate addition of other substances into the material. For example, diffusion of vacancies or interstitials can greatly influence the high temperature mechanical properties of aluminum oxide [7-10]. Likewise, the optical properties of aluminum oxide are also dependent upon the types of defects that occur within the material. This could be intrinsic defects or due to impurities such as Chromium and Titanium [11-13]. Thus, fundamental study of these defects is of utmost importance in improving and developing the material to be used in various applications in the future.

 

       Recently there has been growing evidence that color-enhanced rubies and spinels start emerging in the market. Although color enhancement is not new, generally this is done using the standard heat treatment which can be verified, and accordingly certified, in the laboratory. These alleged color-enhanced rubies and spinels are however undetectable by known techniques. Should this continue, these types of gem stones could flood the market, depreciate virgin stones, and potentially topple the industry. In a preliminary study [16], it is believed that these rubies and spinels are enhanced by implanting of oxygen and/or nitrogen ions. Generally heavy-ion implantation is known to affect only few hundreds of layers of atoms (less than one micron) from the surface. These layers would eventually be scrubbed out during polishing, making any implantation traces disappear. It is therefore of paramount economic and scientific interests to learn how the effect of implantation could penetrate the whole bulk as is evident by a thorough change in color of the treated stones.

 

 

Figure 2 Penetration depth of O ions at various incoming energies impinging upon different crystallographic planes. It is clear that ions can penetrate much more deeply along a certain direction than another.

 

       Advances in computational physics allow us to investigate the possibility of changing the color of rubies with ion beams. The two main problems we would like to study are: 1) how deep the ion beams penetrate the crystal, and 2) how they alter the colors of a ruby. We will consider ion beams with energy in the 10-500 keV range, which is what’s used in the laboratory. The ion penetration distance can be calculated using a binary collision approximation, which divides the ion-nucleus section of the calculation. By determining the scattering integral between ion particles and the nucleus using various collision parameters, the solution gives the scattering angle and the energy loss to nuclei, electrons, and phonons. We found that if the ions were fired in a specific direction such as [0, -1,1], the ions could penetrate three times deeper into the lattice than in an amorphous target. The penetration depth is still considered very shallow (only 1.5 microns at 500 keV), but what's interesting is that the ion beam energy is transferred mostly to electrons in descending amounts with each penetration distance. These energies are sufficient to cause or change the defects present in corundum.

 

 

Figure 3 (left) shows the loss of ion energy to phonons as a function of penetration depths for various ion energies; (right) shows the energy loss to ionizations as a function of penetration depths for various ion energies.

 

 

Figure 4 Ti and Fe can replace two adjacent Al atoms in two different ways called face-sharing and edge-sharing resulting in various Ti/Fe complex defects.

 

       We can use first principle calculation based on density functional theory to examine the energy of different types of defects, which allows us to explain how different types of defects affect the optical properties of the corundum. We have looked at the possibility of defects caused by a wide range of dopants, including iron, titanium and chromium substituting aluminum and oxygen sites. We found that the complex defects resulting from the replacement of two adjacent aluminum atoms with titanium and iron of type TiIII/FeIII were very stable, and thus were very likely to occur naturally. This type of complex defect is what produces the blue color [17]. It is our assumption that, by firing ions into ruby, the electrons that receive energy from the beam could subsequently give enough energy to decouple these complex defects. This eventually leads to the reduction of blue color in ruby, resulting in the crystal appearing more red.

 

       As with other theoretical investigations, all of these theoretical postulates are pending approval from further experimental work.

 

References

 

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[7] R. M. Cannon, W. H. Rhodes, and A. H. Heuer, Journal of the American Ceramic Society 63, 46 (1980).

[8] A. H. Heuer, N. J. Tighe, and R. M. Cannon, Journal of the American Ceramic Society 63, 53 (1980).

[9] K. P. D. Lagerlof, T. E. Mitchell, and A. H. Heuer, Journal of the American Ceramic Society 72, 2159(1989).

[10] A. H. Heuer, and K. P. D. Lagerlof, Philos Mag Lett 79, 619 (1999).

[11] K. J. Caulfield, R. Cooper, and J. F. Boas, Physical Review B 47, 55 (1993).

[12] A. Stashans, E. Kotomin, and J. L. Calais, Physical Review B 49, 14854 (1994).

[13] G. Emilie et al., Journal of Physics: Condensed Matter 17, 5467 (2005).

[14] R. W. Hughes, Corundum, London: Butterworth-Heinemann (1990).

[15] T. Themelis, The heat treatment of ruby and sapphire, USA: Gemlab Inc, (1992).

[16] S. Intarasiri et al. Surface & Coatings Technology 203, 2788 (2009).

[17] S. Na-Phattalung et al., accepted for publication in Physica Status Solidi B (2020).

 

Principle investigators

 

Assoc. Prof. Dr. Surachate Limkumnerd        Department of Physics, Chulalongkorn University

Assoc. Prof. Dr. Jiraroj T-Thienprasert           Department of Physics, Kasetsart University

 

Reported by

 

Assoc. Prof. Dr. Surachate Limkumnerd

Physics of Energy Materials Research Unit, Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand.