Structural Evidence for Ferroelectricity in Eu2GeS4

✍ Scribed by Markus Tampier; Dirk Johrendt

• Publisher: Elsevier Science
• Year: 2001
• Tongue: English
• Weight: 396 KB
• Volume: 158
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.2001.9123

✦ Synopsis

A ferroelectric phase transition was detected in monoclinic europiumthiogermanate Eu 2 GeS 4 (T ‫؍‬ 298 K: a ‫؍‬ 6.639(1) A > , b ‫؍‬ 6.674(1) A > , c ‫؍‬ 8.146(1) A > , ‫؍‬ 108.19(4)3, Z ‫؍‬ 2) by means of temperature-dependent single-crystal X-ray structure determinations. At 298 K, the structure is polar with the space group P2 1 ( -Eu 2 GeS 4 ), but above T C +335 K it is centrosymmetric with the space group P2 1 /m ( -Eu 2 GeS 4 ). Main contributions to the transition are an antidistortive tilt of the thiogermanate tetrahedra and shifts of the europium ions, whose coordination numbers change from 7 to 6(؉2). Small opposing displacements of europium and sulfur ions (0.09+0.11 A > ) produce a permanent polarization parallel to the b axis. The phase transition corresponds to a condensing optical mode of A u symmetry. From the analysis of thermal displacement parameters above T C , it follows that the transition mechanism is partially displacive due to shifts of the europium sites and partially order+disorder due to the ordering of two sulfur atom sites. Below T C , the evolution of the order parameters results in a critical exponent + 1 4 , which is phenomenological interpreted within the Landau theory. Similarities and di4erences between the phase transitions of Eu 2 GeS 4 and of isostructural halide ferroelectrics are discussed. From the structural viewpoint, Eu 2 GeS 4 should be ferroelectric below T C +335 K and paraelectric above this temperature.