X-Ray Study on the Lattice Strains of Ferroelectric Iron Iodine BoraciteFe3B7

Abstract

The accurate lattice parameters and the extremely small lattice strains of iron iodine boracite, Fe3B7013I, have been successfully measured over the paraelectric and the ferroelectric regions by using special x-ray di6'raction 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 Fe337013I that the electrostrictions prevail over the linear strain induced through a torsional piezoelectric constant b36 of the paraelectric phase. The electrostrictive constants are approximately two orders of magnitude larger than those of KH&PO4. 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 b36 vanishes.

X-ray dilatometric study of phase transitions of ferroelectric (NH4)2BeF4

“…The lattice parameters a, b, and c were measured by using the high-precision X-ray dilatometer (see, e.g., [7]) with the two-dimensional reciprocal lattice method [8] ; the exact positions of specific X-ray reflections were determined from their two-dimensional intensity contours. The reflections used for the measurements of the lattice parameters were SOO(0 = 55"), 000(26"), 006(51"), 602(42"), and 602(42").…”

The specific heat anomaly of ferroelectric Co-I-boracite, Co3B7O13I

“…The shift of the transition temperature due to hydrostatic pressure, hTtr/dp, can be predicted from the Clausius-Clapeyron equation: A T /dp = T t r tr AV is the volume change at the transition and L is the latent heat. ConsiderTng the AV value for Co-I-B being of the order of w -2 2 , a s it is given in (3) for Fe-I-B, and putting L x AQ, one obtains A T /dp = -4x10 deg cm /dyn, i. e. -4x t r x10 deg/atm.…”

“…The effect of hydrostatic pressure up to 10 atm has been measured on A T -3 3 tr of Co-I-B in (4): on heating, AT /dp = -8 . 1~1 0 deg/atm.…”

Temperature-dependence of the lattice parameters of nickel-bromine boracite between 25 and 200°C

“…Secondly, the spontaneous strains of the boracites are of considerable interest in their own right. So far, the temperature-dependence of the lattice parameters has been measured for only two boracites, FeI (Kobayashi, Mizutani, Schmid and Schachner, 1970;Kobayashi, Sat0 and Schmid, 1972) and CuCl (Uesu, Kobayashi, Anjoh and Schmid, 1978). The behaviour of these two crystals was qualitatively quite different, with the orthorhombic-cubic transition in CuCl boracite appearing to be far more second-order like than that in FeI boracite.…”