Optical properties of natural topaz

Pacharasak Limthavornlit5480115

Introduction

Why studied?◦determine the quality of jewelry topaz

Benefit◦thermoluminesence dosimetry

measuring ionizing radiation from the intensity of visible light emitted from a crystal in the detector when the crystal is heated◦for human radiation protection◦for measurement of dose in both medical and industrial

processes

Introduction

Benefit◦evaluation of physical and chemical condition

of formation of mineral

What is Topaz?◦Aluminium flurosilicate with fairly constant

chemical composition Al2SiO4(OH,F)2

Major variation◦[OH]/[F] concentration

Introduction

SiO4 group linking octahedral chains of Al in Zigzag fashion parallel to crystalline structure

Introduction

Four of six anions surrounding Al3+ ion belong to SiO4 tetrahedron

Remaining 2 anions are F- and OH- group

Introduction

Topaz crystallizes in orthorhombic system with 4 molecules per unit cell

But the OH/F substitution turns its symmetry into triclinic◦The lattice parameters are

a=4.6499 A b=8.7968 A c=8.3909 A

Introduction

Most of natural topaz has a very low commercial value ◦colorless ◦weak coloration

Blue topaz is now one of the gem industry’s most commercialized gemstones

(FACT)Brazil is one of the most important producers of topaz in the world

Introduction

Differently from other gems, the color induced in topaz by gamma radiation is not stable

How to make color1. the incorporation of transition impurities2. Irradiation by neutron

Material & methods

IR, Raman, UV-VIS absorption spectra of natural topaz and blue topaz crystals from Ukraine before and after neutron irradiation

The sample thickness was about1.8-3.4 mm

neutron irradiation◦performed by 5MW water-water reactor◦ the fluence of fast neutron with energy>0.1 MeV was in

the range of 1014 -5.1017 cm-2 ◦A cadmium filter was used for absorption of thermal

neutrons◦The sample temperature did not exceed 40 C during the

neutron irradiation

Material & methods

FT-IR using VERTEX 80v with actively aligned UltraScanTM interferometer at room temp, provides peak spectral resolution

Material & methods

Raman Spectrometer Advantage 785 with 785 nm

Material & methods

UV-VIS absorption spectra◦The standard technique

double-beam spectrophotometer operating in spectral region of 190-1100 nm

Optical measurements before and after irradiation were carried out at the temperature 300 K

Result & Discussion

Result & Discussion

(1) Before: 230 nm(2) After: the absorption band 230 nm

intensity increases and appear bands with maxima at 305, 430, 620 nm and crystal obtain blue color

(3) Natural blue: there was identified an intensive band at 230 nm and less intensive bands at 430, 620 nm

Result & Discussion

Two line around 230 and 305 nm belong to a single defect such as silanone (=Si=O)◦suggested in another paper basing on similar

lines observed in spodumene( LiAl(SiO3)2)

We also know from other paper that the blue color induced by neutrons 1. independent of the origin of topaz2. correlated with an O- defect

Result & Discussion

(About 620 nm) correlated with an O- center interacting with two Al ions◦This O- center is produced by irradiation in the

hydroxyl sites which substitutes for fluorine

they compare excitation spectra of Cr3+ Fe2+ Mn2+ show bands associated with the blue defects in neutron damaged topaz in 620 nm

The band 430 nm also belongs to impurity Cr3+ ion

Result & Discussion

Result & Discussion

Depending on its origin topaz always contain hydroxyl groups which substitute for fluorine atoms

The ratio [OH]/[OH+F] varies from almost 0 to 30%

OH– stretching mode is observed on wave number around 3650 cm-1

Result & Discussion

(1)before irradiation of colourless topaz◦observed two bands around 4809 and 4787,

which are the combination of main (OH) stretching and Al-OH bending modes

◦Fast neutron irradiation reduces this band intensity and band became symmetrical with maximum 4802 cm-1.

Result & Discussion

After fast neutron irradiation bands intensities decrease, and only the 3650 band intensity increases.

Neutron irradiation produces ◦ the electron◦hole defects in topaz crystals

Probably happen two things◦ interact with impurities and hydroxyl ions◦reduce the local symmetry

Result & Discussion

Result & Discussion

276-293 cm-1, 561 cm-1, 858cm-1, 929cm-1, 1006cm-1 are assigned to the various Si-O vibration modes in SiO4 group

235-380 cm-1 are considered to be caused by symmetric Si-O ring deformation

300-400 cm-1 are satellite bands

Result & Discussion

The series of sharp bands in 330-500 cm-1 range are assigned with stretching & bending mode of

AlO6 Octahedral coupling with bending mode SiO4 tetrahedral

325 cm-1 are responded to Al-F stretching modes

Based on their [13], [16], [19], [20] references

Result & Discussion

The colorless before irradiation & natural blue has the same raman spectrum but intensities of some band differ

After fast neutron irradiation, the board band with maxima at 650 are observed

The narrow line intensities decrease with fast neutron fluence increasing(curve 3 & 4)

Result & Discussion

Discussion of Raman 1. The narrow line intensities decrease2. Board band appear

May be connected with topaz lattice disorder

Conclusion

IR, Raman, UV-VIS of natural topaz crystal from Ukraine before and after exposure

Fluence of fast neutron in the range of 1014 -5.1017 cm-2 (E>0.1 MeV)

Conclusion

Assumption1. (UV)620nm band may associated with the

presence of Cr3+,Fe2+,Mn2+ impurities

2. Broad band with maxima at 650cm-1 were observed in Raman spectra for topaz radiated by fast neutrons may be connect with lattice disorder

3. Interaction between radiation defect and impurity ions during fast neutron irradiation lead to appearance of additional absorption band in IR, Raman, UV