X-Ray Study on the Lattice Strains of Ferroelectric Iron Iodine BoraciteFe3B7O13I

The accurate lattice parameters and the extremely small lattice strains of iron iodine boracite, ${\mathrm{Fe}}_3$ ${\mathrm{B}}_7$ ${\mathrm{O}}_{13}$I, have been successfully measured over the paraelectric and the ferroelectric regions by using special x-ray diffraction methods. The spontaneous strains in the ferroelectric state could be separated into two parts, a linear strain (pure shear) and quadratic ones (electrostrictions). It is a conspicuous feature of the electromechanical properties of ${\mathrm{Fe}}_3$ ${\mathrm{B}}_7$ ${\mathrm{O}}_{13}$I that the electrostrictions prevail over the linear strain induced through a torsional piezoelectric constant $b_{36}$ of the paraelectric phase. The electrostrictive constants are approximately two orders of magnitude larger than those of K${\mathrm{H}}_2$P${\mathrm{O}}_4$. Another phase transformation at a lower temperature was disclosed by the present diffractometry. At this temperature a special rhombohedral lattice is formed, and the piezoelectric constant $b_{36}$ vanishes.