Influence of lone-pair cations on the germanate anomaly in glass

A fragment of the network in a thallium

A fragment of the network in a thallium germanate glass
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Neutron diffraction has been used to prove that lone pair cations can change their behaviour as glass composition is changed.

Lone-pair cations in glasses are important due to the strong non-linear optical properties which they impart to the glass. Cations with a non-bonding (lone-pair) of electrons, such as lead or thallium, can adopt two different types of environment: a symmetric, highly coordinated environment, or a much more optically active, asymmetric, low coordination environment. It has long been believed that lone-pair cations change their behaviour as the glass composition is changed. However, there has been little evidence on this point until a few recent studies which have mostly not
found signs of this.

We have used neutron diffraction to study a series of thallium germanate glasses. We have shown firstly that the lone-pair thallium cations undergo a change in behaviour from a symmetric
to an asymmetric environment. Secondly, we have shown that this change is associated with the germanate anomaly – a growth and subsequent decline in the number of higher coordinated germanium VGe sites in the glass.

ER Barney, AC Hannon (ISIS), N Laorodphan, D Holland (University of Warwick)

Research date: August 2011

Further Information

Contact: Dr ER Barney, emmacruddace@gmail.com
Further reading: ER Barney et al., J. Phys. Chem. C in press (DOI: 10.1021/jp202279b)

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