Color Enhancement by Diffusion of Beryllium in Dark Blue Sapphire

Kyungjin Kim，Yongkil Ahn

1. Division of Crafts Design, Kongju National University, 56 Gongjudaehak-Ro, Gongju-Si,Chungnam Province 314-701, Republic of Korea 2.Department of Materials Science and Engineering, Hanyang University, 17 Haengdang-dong,Seongdong-gu, Seoul 133-791, Republic of Korea

Color Enhancement by Diffusion of Beryllium in Dark Blue Sapphire

Kyungjin Kim1，Yongkil Ahn2

1. Division of Crafts Design, Kongju National University, 56 Gongjudaehak-Ro, Gongju-Si,Chungnam Province 314-701, Republic of Korea 2.Department of Materials Science and Engineering, Hanyang University, 17 Haengdang-dong,Seongdong-gu, Seoul 133-791, Republic of Korea

Diffusion of beryllium was performed on dark blue sapphire from China and Australia.The samples were heated with beryllium as a dopant in a furnace at 1 600 ℃ for 42 h in air.After beryllium diffusion, samples were analyzed by UV-Vis, FTIR, and WD-XRF spectroscopy.After heat-treatment with Be as a catalyst, the irons of the ferrous state were changed to the ferric state.Therefore, reaction of Fe2+/Ti4+IVCT was decreased.The absorption peaks at 3 309 cm-1attributed to OH radical were disappeared completely due to carry out heat treatment.Consequently, the intensity of absorption band was decreased in the visible region.Especially, decreased absorption band in the vicinity of 570 nm was responsible for the lighter blue color.Therefore, we confirmed that the dark blue sapphires from China and Australia were changed to vivid blue.

Blue sapphire; Beryllium; Diffusion; UV-Vis； FTIR； WD-XRF spectroscopy

Introduction

Isovalent ions for Al3+are Cr3+and Fe3+in corundum and aliovalent ions are Be2+, Fe2+, Ti4+, and Si4+.The deep blue color of sapphire is attributed to 50 ppm Fe2+/Ti4+pairs in corundum[2].If Fe2+ion is located adjacent to Ti4+ion in corundum lattice, they interact for adjusting the ionic valence of the structure.Consequently, the Fe2+is moved to Fe3+by losing one electron, and the Ti4+is translated to Ti3+by receiving one electron : Fe2++Ti4+→Fe3++Ti3+.This interaction is called intervalence charge transfer (IVCT).In order to be occurred this process in corundum, energy of 2.11 eV is required.Therefore, the blue color is attributed to the absorption band at 588 nm[3].

Fe3+ions have the weak bands at 540, 700, and 1 050 nm, and the narrow peak at 388 nm, while the peaks at 377 and 450 nm have been attributed to Fe3+-Fe3+pairs[4-5].

As an acceptor, Mg2+can be charge compensated by Ti4+or an oxygen vacancy or a hole[6-7].If Mg2+charge compensates Ti4+, Mg2+-Ti4+pair results in colorless.If corundum is grown or heat treated at low oxygen partial pressures, it will be nearly colorless because the charge compensation is by oxygen vacancies.Especially, high oxygen partial pressure would contribute to a strong orangy yellow color, due to charge compensation of Mg2+by holes forming the trapped-hole color center.However, in natural sapphires, which contain a variety of trace elements, the hole would associate with Cr3+or Fe3+preferentially.

In natural sapphire, there are various blue sapphires ranging from dark to pale color.If the dark blue sapphires can be altered to vivid blue, it will be very significant investigation.Because the vivid blue sapphire is selling for the highest price in the gem market.Therefore, the purpose of this work is to investigate the change of dark color in dark blue sapphires from China and Australia.

For the enhancements of dark blue, it was accomplished by the diffusion of beryllium into the sapphire.Beryllium can be diffused deeply into the sapphire lattice due to its faster ionization and small radius.After heat treatment, Be diffused samples wasn’t toxic because Be ions entered into the lattice.Therefore, the diffused sapphire crystal is very stable, like a natural sapphire.Furthermore, Be behaves as a catalyst for the heat-treatment[8].The iron of the ferrous state changes to the ferric state in the oxidizing atmosphere.It results in decreased charge interaction of Fe2+/Ti4+pair.This can also reduce the dark blue.Therefore, in this study, we demonstrated the improvement of blue color through Be diffusion in dark blue sapphires.

1 Experimental procedures

Dark sapphires used in this study originated in Shandong, China and New England, Australia.13 pieces of the pristine samples were sliced to a thickness of 3 mm using a diamond sawing machines.Their surfaces were polished using three processes by turns: silicon carbide cc-220cw, cc-2000cw abrasive paper, and alumina (Al2O3) powder.In an ultrasonic bath, the polished sapphires were cleaned by distilled water for 10 minutes[9].The samples were embedded into mixed Al2O395% and BeO 5% powder without any flux.For the protection of ourselves from poison, this experiment was conducted after wearing dust and cartridge respirator.

They were heated in a horizontal alumina tube furnace at 1 600 ℃ for 42 h in air.After beryllium diffusion, each samples were analyzed by using ultraviolet-visible (UV-Vis), Fourier-transform infrared (FTIR) spectroscopy in order to confirm the changed colors.Their chemical compositions were analyzed by wavelength dispersive X-ray fluorescence (WD-XRF) spectroscopy.UV-Vis spectra were measured using a Jasco Co.These spectra were analyzed in the spectral region 300 to 750 nm at a scanning speed of 100 nm per min.IR spectra were measured using a Jasco Co.4100 type A Fourier transform IR spectrometer in the spectral region from 4 000 to 2 000 cm-1with a resolution of 4 cm-1.The chemical compositions were detected using a Shimadzu Co.XRF-1700 with an Rh target at 40 kV and 95 mA.

2 Results and discussion

Table 1 shows various trace and main elements of blue sapphire from China and Australia.From the analysis of elements detected by WD-XRF, we investigated the difference and correlation between impurities of sapphires formed in two deposits.The deposits of China and Australia were derived from basalt but the chemical composition was a little different[10].Sapphires from China contained more abundant Fe than sapphires from Australia.From UV-Vis spectra of these sapphires, we could confirm the result of chemical composition.

Table 1 Chemical composition of two deposits by WD-XRF.SC- series are China; SA-series are Australia (Unit: Wt.%)

Fig.re 1 shows UV-Vis spectra of China and Australia sapphires.The above spectra of Fig.1 are UV-Vis spectra of China sapphires.Australia sapphires show in the below spectra of Fig.1.The absorption peaks of Fig.1 (a), (c), (e) and (g) are attributed to a Fe3+/Fe3+pair in sapphire.These peaks appeared at 378 and 453 nm.On the other hand, Fe3+ion is responsible for the absorption peaks at 387 nm by thed5d—dtransition in Fig.1 (b) and (f)[11].The blue color was caused by Fe2+/Ti4+pair.These absorption bands occurred in the vicinity of 570 nm as shown in Fig.1 (d) and (h).The absorption peaks of China sapphires show higher than those of Australia sapphires below the 400 nm.Because China sapphires contain more amounts of Fe2O3 than Australia sapphire.Therefore, China sapphires showed darker and slightly greenish blue.

Fig.1 UV-Vis absorption spectra of two deposits.Six dark blue sapphires from China in above diagram and seven dark blue sapphires from Australia below diagram

Fig.re 2 shows the FTIR spectra in the mid-infrared region between 4 000 and 2 000 cm-1.The samples of two deposits showed absorption peaks at similar positions.They appeared dominant absorption in the vicinity of 3 309, 2 923, 2 851 and 2 349 cm-1but they occurred slightly higher in sapphires from Australia.There was additional peak at 3 233 cm-1as shownin Fig.2(f).The absorption peak at 3 309 cm-1is attributed to OH radical.No.SA-1 of sapphire from Australia showed very high intensity at 3 309 cm-1as shown in Fig.2(e).Absorption bands appeared at 2 923 cm-1which were caused by C—H vibrations as shown in Fig.2(b) and (g)[12].

The sapphires were heat-treated with Be as a dopant.For comparison before with after Be diffusion, they were analyzed by UV-Vis spectroscopy in the region of 350～750 nm.Figure 3 and 4 shows the UV-Vis spectra of sapphires from China and Australia, respectively.

Fig.2 FTIR absorption spectra of two deposits.Six dark blue sapphires from China in above diagram and seven dark blue sapphires from Australia below diagram

After heat-treatment, the absorption bands of blue sapphires from China and Australia appeared to be low in comparison with natural samples.Especially, decreased absorption band in the vicinity of 570 nm was responsible for the lighter blue color.Decreased absorption band and peak can be explained by behavior of ions.Be behaves as a catalyst for the heat-treatment.Therefore, the movement of ions will be promoted in the lattice (that is, ions will be moved to stable state by charge compensations and change of valance state).The iron of the ferrous state changes to the fersic state after heat-treatment in the oxidizing atmosphere.It resulted in decrease of Fe2+/Ti4+pair.This had an effect on reduction of the dark blue.However, the absorption spectra of diffused sapphires have been increased above 700 nm.Furthermore, they have risen more sharply in the blue sapphires from China because of containing more amounts of Fe2O3than Australia sapphire (that is, the ferrous state was converted to more amounts of ferric state in the China sapphires).Because the broad bands at 700 and 1 050 nm, have been assigned to single Fe3+ions.

Aspects of the relationship between Fe, Ti, and Mg can be discussed by natural and diffused sapphire.In the corundum structure, aluminum cations are only filled with two-thirds of the oxygen anions sites.The trace elements (that is, isovalent ions for Al3+are Cr3+and Fe3+in corundum and aliovalent ions are Be2+, Fe2+, Mg2+, Ti4+, and Si4+) are in the interstitial and vacant sites in natural condition.If the electrons of these ions are excited by outer energy, isovalent (transition metals) and aliovalent ions can be explained by the crystal field and charge transfer.In the color mechanism, the most important factor is the position of these cations in corundum lattice.

Fig.3 UV-Vis absorption spectra of Shandong Province, China in the pristine state (solid), after beryllium diffusion (dashed line) in the range of 350～750 nm

Fig.4 UV-Vis absorption spectra of New England, Australia in the pristine state (solid), after beryllium diffusion (dashed line) in the range of 350～750 nm

As shown in Table 1, the dark blue sapphires from China and Australia contained more amounts of Mg than Ti except SA-2, 3 and 7.If all the Ti charge compensated Mg, the samples would appear yellow or colorless.But they were dark blue color.Furthermore, the dark blue samples were changed to vivid blue after heat treatment with Be.Therefore, this means that Mg2+wasn’t charge compensated by Ti4+or an oxygen vacancy or a hole.Furthermore, in natural sapphires, which contain a variety of trace elements, the hole would associate with Cr3+or Fe3+preferentially.

As shown in Fig.5, if Ti and Mg make a cluster (MgTiO3) occupying two nearest neighbor Al sites, it can describe that titanium has been charge compensated by magnesium.But their reaction is not easy for charge compensation because these two ions have to exist in the same region of the lattice.

On the other hand, natural and heat treated samples were blue color.This shows that Fe2+charge compensated Ti4+.Fe2+/Ti4+IVCT are much easier than it of Mg2+/Ti4+because the dark blue sapphires contain lots of Fe.

Fig.5 Inter valance charge transfer between ions in corundum lattice

Fig.re 6 shows the mechanism of color change.Left in Fig.6 depicts the transfer of electron before Be diffusion.This is reaction of Fe2+/Ti4+pair which is called intervalence charge transfer (IVCT).Fe2+/Ti4+pair contributes to the blue color.On the other hand, right diagram shows both reactions of titanium and iron in sapphire lattice after Be diffusion.If beryllium is diffused in dark sapphire, Be-induced trapped holes will be formed in the sapphire lattice.An electron escaped from the outermost orbital enters to hole in the valence band.If the electron trapped in hole is excited by outer energy, Fe4+ion alters to Fe3+as received the electron.This mechanism can be explained as follows

Fe3+-O-·h+-Be2+→Fe4+-O2--Be2+→Fe3+-

O-·h+-Be2+

(1)

This equation was responsible for a little yellowish in blue sapphire.

When Be is diffused in dark blue sapphire containing titanium, an electron left from the outermost orbital of Ti3+ion enters to Be-induced trapped hole in the valence band.If the electron trapped in hole is excited by outer energy again, Ti4+ion changes to Ti3+as received the electron.This mechanism can be expressed as follows:

Ti3+-O-·h+-Be2+→Ti4+-O2--Be2+→Ti3+-

O-·h+-Be2+

(2)

This equation results in the colorless.

Fig.6 Band diagram showed movements of electron in dark blue sapphire after beryllium diffusion

Consequently, after Be diffusion, dark blue sapphires were changed to light blue because amount of titanium reacting to Fe2+ion were reduced.Namely, IVCT reaction of Fe2+/Ti4+pair contributing to the blue color is reduced.Therefore, absorption band is reduced in the entire region of UV-Vis spectra as shown in Fig.3 and 4.If small amounts of beryllium are diffused in dark blue sapphire, equation (1) will not be occurred due to react with titanium earlier than iron.Furthermore, as the Be2+/Ti4+IVCT, Be diffusion would be a limited impact because beryllium and titanium ions have to be in the same region of the lattice.

Fig.re 7 shows FTIR spectra in the region of 4 000 to 2 000 cm-1.The solid line and dashed line are FTIR spectra of China [Fig.7 (a) and (b)] and Australia [Fig.7 (c) and (d)] sapphires before and after heat-treatment, respectively.The absorption peaks in the vicinity of 3 309, 2 923 and 2 851 cm-1were disappeared after Be diffusion.But the absorption peak at 2 349 cm-1was remained.Intensities of absorption were decreased totally after Be diffusion.Especially, the absorption peaks at 3 309 cm-1attributed to OH radical were disappeared completely due to carry out heat treatment.

Fig.7 FTIR absorption spectra of China and Australia sapphires in the pristine state (solid), after beryllium diffusion (dashed line) in the range of 4 000～2 000 cm-1

3 Conclusions

Sapphires from China showed more Fe and less Si as compared with sapphires from Australia whereas Australia sapphires contained more aliovalent ions, i.e.Na+, K+, Mg2+, Ca2+, Si4+and Ti4+.In UV-Vis spectra, the absorption peaks of China sapphires showed higher than those of Australia sapphires below the 400 nm.Because China sapphires contain more amounts of Fe2O3than Australia sapphire.Therefore, China sapphires show darker and slightly greenish blue.

After heat-treatment with Be as a catalyst, the intensity of absorption band was decreased in the full region.Especially, in the vicinity of 570 nm, decreased absorption band was responsible for the lighter blue color.However, the absorption spectra of diffused sapphires have been increased above 700 nm.Furthermore, they have risen more sharply in the blue sapphires from China because of containing more amounts of Fe2O3than Australia sapphire (that is, the ferrous state was converted to more amounts of ferric state in the China sapphires).Because the broad bands at 700 and 1 050 nm, have been assigned to single Fe3+ions.

In conclusion, reduced Fe2+/Ti4+IVCT resulted in the lighter blue color.After heat-treatment with Be, we confirmed that the dark blue sapphires from China and Australia were changed to vivid blue selling for the highest price to the gem market.

Acknowledgements: This work was supported by the research grant of the Kongju National University in 2011.

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2015-03-06，

2015-07-10)

O657.3

A

10.3964/j.issn.1000-0593(2016)05-1623-06

Received：2015-05-16; accepted：2015-09-25

Biography：Kyungjin Kim, Professor e-mail: kkj007@hanmail.net