Effect of Pressure on the Polymorphism and Melting Points of the Thallous Halides

The phase diagrams of the thallous halides were studied to 40 kbar by means of differential thermal analysis. Orthorhombic TlF II transforms at 81°C to TlF I, which was found to crystallize with a tetragonal distortion of the NaCl-type structure. The space group is $D_{4h}^{}{}_{}{}^{17}-\frac{I4\mathrm{}}{\mathrm{mmm}}$, with $a_0=3.771\mathrm{}$, $c_0=6.115\mathrm{}$ Å at 135°C. TlF I is slightly denser than TlF II. The TlF II-I transition line ends in a triple point I-III-II at 8.3 kbar, 59°C. TlF III is a high-pressure phase that appears at 12.6 kbar at 22°C. The TlF III-II transition line rises slightly with pressure. The melting curve of TlF I rises less steeply with pressure than those of the other thallous halides.