Successive structural phase transitions in monoclinic thallium monosulphide

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Abstract
The structural phase transitions in newly found monoclinic TlS have been studied using single-crystal X-ray diffraction methods. The room temperature ferroelectric phase has a commensurate structure; the satellite reflections are observed at qc=(0, 0, 1/4) suggesting fourfold lattice modulations along the c axis. The intermediate phase between 318.6 K and 341.1 K is incommensurate; the satellites shift from qc to qi=(0.04, 0, 1/4). In the high-temperature paraelectric phase above 341.1 K, these satellite reflections disappear. A phenomenological treatment of the successive phase transitions in TlS is given. The temperature dependence of the dielectric constant is also discussed. The experimental results are compared with those for TlGaSe2.

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