Structural evidence for ferroelectricity in Eu

A ferroelectric phase transition was detected in monoclinic europium thiogermanate Eu(sub 2)GeS(sub 4) (T=298 K: a=6.639(1)(angstrom), b=6.674(1)(angstrom), c=8.146(1)(angstrom),(beta)=108.19(4)(sup o), 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(sub 1) ((alpha)-Eu(sub 2)GeS(sub 4)), but above T(sub C)(approx)335 K it is centrosymmetric with the space group P2(sub 1)/m ((beta)-Eu2GeS4). 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(angstrom)) produce a permanent polarization parallel to the b axis. The phase transition corresponds to a condensing optical mode of A(sub u) symmetry. From the analysis of thermal displacement parameters above T(sub 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(sub C), the evolution of the order parameters results in a critical exponent(beta)(approx) 1/4, which is phenomenological interpreted within the Landau theory. Similarities and differences between the phase transitions of Eu(sub 2)GeS(sub 4) and of isostructural halide ferroelectrics are discussed. From the structural viewpoint, Eu(sub 2)GeS(sub 4) should be ferroelectric below T(sub C)(approx)335 K and paraelectric above this temperature