Temperature And Polarization Dependence Of Optical-Absorption Edge Of Gadolinium Molybdate

The optical absorption spectrum of gadolinium molybdate (GMO) was measured as a function of photon energy and polarization between 40 and 205 \\ifmmode^\\circ\\else\\textdegree\\fi{}C for absorption coefficients between 10 and 300 ${\\mathrm{cm}}^{\\ensuremath{-}1}$. GMO undergoes a simultaneous ferroelectric-ferroelastic transition at 159 \\ifmmode^\\circ\\else\\textdegree\\fi{}C; no discontinuity in the absorption spectrum was observed at this temperature. No differences were observed between spectra for light polarized along the $a$ or $b$ crystal axes, but the spectrum shifted to higher energy for light polarized along the $c$ axis and shifted uniformly with temperature. Two absorption bands, each consisting of at least three separate lines, were superimposed on the optical absorption edge. The absorption coefficients were found to obey a modified form of Urbach's rule which, within experimental error, can be described equally well by either of two equations. The implications of these two equations are discussed.