Although Pb 2+ in PbBr 2 has the same coordination number (8) as Tl 1+ in TlBr, there is a significant difference in their Pauling ionic radii: r(Tl 1+)=1.15 and r(Pb 2+)=1.43 Å. Pb 2+ was selected as a dopant in order to check the influence of ions with crystallochemical parameters, which are significantly different from the host ion, Tl 1+.

However, sharp differences in crystal structure of InBr (layered chains) and TlBr (cubic) make formation of the continuous solid solution (InBr) x–(TlBr) 1− x problematic. Therefore, indium is an isoelectronic impurity with a geometric size similar to thallium, and will not lead to a significant distortion of the crystal lattice or of the electronic system of the host crystal. Indium was selected because it is in the same column of the periodic table as thallium and has a similar Pauling ionic radius: r(Tl 1+)=1.15 and r(In 1+)=1.04 Å.

In the current work we use In 1+ and Pb 2+ ions, in the form of InBr and PbBr 2, respectively, as dopants. Therefore, selection of appropriate impurities, that not only make crystals mechanically harder but also maintain the good spectroscopic properties of TlBr, is an important scientific and applied task. However, while these impurities can improve some properties, these same impurities can degrade others. It is possible to increase crystal hardness by doping crystals with impurities that can “lock” dislocations in place. As a result, any promising physical properties of “perfectly chemically pure” crystals can be killed by unsuitable mechanical treatment during sample preparation. One of the major problems of TlBr crystals is their mechanical softness, which leads to easy plastic deformation, even during handling of samples. The interest in TlBr is due to its high average atomic number, high density, and wide band gap. Over the past six decades, considerable effort has been expended in developing TlBr crystals for use as radiation detectors (see, for instance, Refs.

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