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Simultaneous differential thermal analysis and thermogravimetric analysis were conducted on crystalline Sb2O3 identified by x-ray diffraction to be initially in its low-temperature cubic form, senarmontite. Samples were run under air or argon flowing at 250ml/min at heating rates ranging from 10°C/min to 160°C/min with quartz as a reference material.

Thermal Analysis of Sb2O3

Figure 3 – Simultaneous differential thermal and thermogravimetric analysis of Sb2O3 under Argon at a heating rate of 20°C/min.

Figure 2 – Differential thermal analysis of Sb2O3 under Argon at a heating/cooling rate of 10°C/min.

Figure 1 – Differential thermal analysis of Sb2O3 in air at various heating rates.

When run under an atmosphere of flowing air (Fig 1), it was observed that the oxidation of Sb2O3 to Sb2O4 could be suppressed by increasing the heating rate to reveal an endotherm due to melting; mass loss was also increased from 6% at 20°C/min to 68% at 160°C/min, as more unoxidised Sb2O3 volatilised.

Thermal analysis under a flowing argon atmosphere allowed a slower heating rate to be used (Fig 2) revealing a second endotherm prior to the melting peak, probably from the transition to the other crystalline phase of Sb2O3, valentinite, which consists of chains of [SbO3] trigonal pyramids.

Further examination at a higher heating rate (Fig 3) resolved a third endothermic peak in the same region, with corresponding changes in mass loss of the sample. Peaks 1 and 2 are believed to be either the senarmontite-valentinite transition followed by valentinite melting, or the melting of small particulate senarmontite prior to the phase transition and general melting at peak 3.

Vitreous Sb2O3:Cl

A 50g batch was prepared from 50 mol% Sb2O3 (99.6%, Alfa Aesar) and 50 mol% SbCl3 (99.9+%, Aldrich). A lidded alumina crucible containing the mixed powders was placed into a furnace pre-heated to 1000°C, held at temperature for 5-10 minutes until molten and fuming, then agitated and splat-quenched between two cooled copper plates.

The resulting glass was ~1mm in thickness, pale yellow in colour and translucent with some evidence of deposits (presumably chlorine-rich) on or near the surface.

A sample of the Sb2O3:Cl glass formed by splat-quenching. Energy dis-persive x-ray analysis suggests that the glass contains 6-18 at.% Cl.

Differential thermal analysis of the newly-made (October 2004) glass and an older sample (September 2003) was performed under the same conditions used for the commercial Sb2O3 powder.

The results show (Fig 4) that the onset temperatures of phase transitions appear to be affected by atmosphere (more specifically, the presence of oxygen). This relationship was not observed in the crystalline Sb2O3, where oxidation also began at a higher temperature.

Comparisons between the two glasses and senarmontite (Fig 5) demonstrate that the crystal phase change and melting endotherms occur both earlier and more widely separated when Sb2O3 is crystallised from vitreous material. These features may be the result of factors such as the chlorine content, sample age and particle size (Table 1). The shapes of the endotherms also indicate that several features are overlapping one another, suggesting a similar spread of peaks to those found in Sb2O3.

The glass transition temperatures were determined to be 277±3°C for the September 2003 glass and 283±3°C for the October 2004 sample. These values were independent of the atmosphere used for the analysis.

Figure 4 – Comparison of differential thermal analysis patterns at 20°C/min for Sb2O3:Cl glass under different atmospheres. Peaks 1 & 2 are thought to be the crystalline phase transition with peaks 3 & 4 corresponding to melting.

Sample

Mean particle size / μm

Approx. Cl content / at.%

Sb2O3 (senarmontite)

4

Sb2O3:Cl glass (Sep ’03)

7.5

8

Sb2O3:Cl glass (Oct ’04)

15

12

Figure 5 – Comparison of differential thermal analysis patterns for Sb2O3 and Sb2O3:Cl glasses at 20°C/min under Argon.

Table 1 – Some physical parameters of the samples investigated.

Phase Transitions in Antimony Oxychloride Glasses

Robin Orman, Diane Holland Department of Physics, University of Warwick, Coventry, CV4 7AL, UK

Alex Hannon ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK

Abstract

An investigation of the phase transitions of Sb2O3 (senarmontite) and vitreous Sb2O3:Cl has been carried out using thermal analysis techniques. A glass transition temperature of 280±6°C has been measured and similarities between the phase transitions of the two materials are apparent. However, factors such as atmosphere, particle size, sample age and chlorine content (of the glass) have also been identified as variables potentially affecting the behaviour of the systems. Further thermal and structural investigations are planned.

Introduction

Whilst past attempts to form pure antimony oxide glass by melt-quenching have proven difficult, there has been some success at forming a chlorine-stabilised specimen [1,2]. Since heavy metal antimony oxychloride glasses have already been considered for potential use in infra-red transmission [3], and other antimony oxyhalide glasses appear to show similar optical characteristics [4], further study of the properties of this chlorine-stabilised antimony oxide glass seems justified.

Thermal analysis techniques have been used to examine phase transitions of the material with comparisons drawn to those of the crystalline Sb2O3 system; further investigations utilising other techniques are ongoing.

References

1. J A Johnson, D Holland, J Bland, C E Johnson, M F Thomas, J. Phys.: Condens. Matter15 (2003), 755-764.

2. A C Hannon, R G Orman, D Holland, J. Non-Cryst. Solids (in preparation).

3. M R Sahar, M M Ahmed, D Holland, Inst. Phys. Conf. Ser.111 (1990), 449-458.

4. B Dubois, H Aomi, J J Videau, J Portier, P Haggenmuller, Mater. Res. Bull.19 (1984), 1317-1323.

Conclusions and Further Work

Thermal analysis of the vitreous Sb2O3:Cl system has shown that it has phase transitions similar to those of crystalline Sb2O3 (senarmontite), although apparently influenced by factors including the presence of oxygen in the surrounding atmosphere, and potentially particle size, sample age and chlorine content. Further thermal analysis studies to determine the impact of these variables is planned, together with Raman and NMR/NQR structural investigations.

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