Study On Pyroelectric Properties Mechanism Of Schorl-dravite Tourmalines

The tourmaline was widely used in environmental protection, chemical industry, health and other areas because its pyroelectric properties, infrared radiations and anion producing characteristics, and has gained a lot of research results. The pyroelectric property of tourmaline is the earliest found and the most important property. However, the lack of basic research and scientific explaination of the mechanism of those characteristics of tourmaline were exist in the past, and it limit the further development and application of tourmaline functional materials.In this paper, the pyroelectric property of the four Fe^-Mg tourmalines with different iron contents which heated treatment at 850℃after 48h, 72h were determined experimentally at room temperature. The result of experimental shows the pyroelectric coefficient decreases with increasing total Fe content, and increases with the heat treatment time increased. In addition, single^-crystal X^-ray diffraction structure data of the four tourmalines with different iron contents were collected at room temperature and 173 K. By calculating and comparing the cell parameters, the electric dipole moment and the distorted structure parameters of the tourmalines, it is found that iron content and the elevated temperatures have different influence on crystal structure and the electric dipole moment, in which the X and T polyhedra having a greater impact. Secondly, The vibrational spectra features of the tourmalines with different iron contents and with heat treatment time have been analyzed by using polarized Raman and infrared spectra from ^-195℃to 450℃. The results showed that Raman spectra of the radical [Si₆O₁₈]^12-, [BO₃]^3- and [OH]^- in tourmaline parallel to c^-axis are more sensitive than that perpendicular to c^-axis, Raman spectrum of [OH]^- is significantly polarized parallel to c^-axis direction; however, the influence of Fe content and temperature on the Raman spectra of the three radicals were different. The isomeric schif （IS）, quadrupole splitting （QS）, width in half maximum （Γ） andγ^-ray transmission rate of the four tourmaline simples and one heat treated after tourmaline were fitted. By comparing and analyzing these parameters , it was found Fe in the four tourmalines have two valence states and three locations, i.e. Fe²⁺ （Y）, Fe²⁺ （Z）, Fe³⁺ （Y）. After being heated at 850℃for 48h and 72h, Fe in the tourmalines have one valence states and three locations, i.e. (Fe³⁺ （Y）, Fe³⁺ （Z）, Fe^2+-Fe³⁺ （Y-Z）). With increasing of heating time, Fe²⁺ in Y sites was completely oxidized to Fe³⁺, but a small amount of Fe³⁺ shifted to Z sites, which resulted in the contraction of Y polyhedron while expansion on Z polyhedron. It is further clarified the influence of Fe valence and location on the pyroelectric property.The hysteresis loops of Fe-Mg tourmaline were measured at room temperature, Hysteresis loop measurement shows that the sample exhibited a little leakage in electric conduction. It is the first time how the content of iron and the time of heat treatment affect the hysteresis loop was researched; in addition, by observing the electric domains distribution of tourmaline by TEM, the ferroelectric properties of tourmaline is proven.