Ferroelectricity in the guanidinium compound [ C ( N H 2 ) 3 ] 4 <mml:mi …
2005; American Physical Society; Volume: 72; Issue: 5 Linguagem: Inglês
10.1103/physrevb.72.054122
ISSN1550-235X
Autores Tópico(s)Crystal Structures and Properties
ResumoA guanidinium compound, ${[\mathrm{C}{(\mathrm{N}{\mathrm{H}}_{2})}_{3}]}_{4}{\mathrm{Cl}}_{2}\mathrm{S}{\mathrm{O}}_{4}$, has been synthesized and characterized by x-ray diffraction, calorimetric, dilatometric, and dielectric measurements, and by optical examination in polarized light. At room temperature the initial crystal structure is orthorhombic, space group $Cmc{2}_{1}$, and it exhibits ferroelectric order. The crystal is built of two anionic sublattices with different ion valency, and guanidinium cations, which occupy three crystallographically different sites. On heating the crystal undergoes two first-order phase transitions: at ${T}_{1}=352\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ to the intermediate orthorhombic phase II, and at ${T}_{C}=356\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ to the paraelectric tetragonal phase I, space group $I\overline{4}2m$. The structure of the paraelectric phase is highly disordered and the prevailing part of this disorder persists in the crystal lattice after the reverse transition. Thus, at first the crystal transforms from the tetragonal phase to the disordered low-symmetry ferroelectric phase IV, which is metastable, and than slowly crystallizes to the initial, ordered phase III. Despite the prominent order-disorder contribution to the transition mechanism the ferroelectric polarization in ${[\mathrm{C}{(\mathrm{N}{\mathrm{H}}_{2})}_{3}]}_{4}{\mathrm{Cl}}_{2}\mathrm{S}{\mathrm{O}}_{4}$ is of displacive-type.
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