Response of Gd2Ti2O7 and La2Ti2O7 to swift-heavy ion irradiation and annealing

Abstract

Swift heavy ion (2 GeV 181 Ta) irradiation-induced amorphization and temperature-induced recrystallization of cubic pyrochlore Gd 2 Ti 2 O 7 (Fd 3m) are compared with the response of a compositionally-similar material with a monoclinic-layered perovskite-type structure, La 2 Ti 2 O 7 (P2 1 ). The averaged electronic energy loss, dE/dx, was 37 keV/nm and 35 keV/nm in Gd 2 Ti 2 O 7 and La 2 Ti 2 O 7 , respectively. Systematic analysis of the structural modifications was completed using transmission electron microscopy, synchrotron X-ray diffraction, Raman spectroscopy, and small-angle X-ray scattering. Increasing ion-induced amorphization with increasing ion fluence was evident in the X-ray diffraction patterns of both compositions by a reduction in the intensity of the diffraction maxima concurrent with the growth in intensity of a broad diffuse scattering halo. Transmission electron microscopy analysis showed complete amorphization within ion tracks (diameter: $10 nm) for the perovskite-type material; whereas a concentric, core-shell morphology was evident in the ion tracks of the pyrochlore, with an outer shell of disordered yet still crystalline material with the fluorite structure surrounding an amorphous track core (diameter: $9 nm). The radiation response of both titanate oxides with the same stoichiometry can be understood in terms of differences in their structures and compositions. While the radiation damage susceptibility of pyrochlore A 2 B 2 O 7 materials decreases as a function of the cation radius ratio r A /r B , the current study correlates this behavior with the stability field of monoclinic structures, where r La /r Ti > r Gd /r Ti . Isochronal annealing experiments of the irradiated materials showed complete recrystallization of La 2 Ti 2 O 7 at 775°C and of Gd 2 Ti 2 O 7 at 850°C. The annealing behavior is discussed in terms of enhanced damage recovery in La 2 Ti 2 O 7 , compared to the pyrochlore compounds Gd 2 Ti 2 O 7 . The difference in the recrystallization behavior may be related to structural constraints, i.e., reconstructing a low symmetry versus a high symmetry phase.

Structural response of titanate pyrochlores to swift heavy ion irradiation

“…smaller than those measured with HRTEM for all compositions studied. This is not surprising for reasons described earlier and is consistent with previous studies employing the peak deconvolution method for determining amorphous fractions [3,9,33]. An alternative method for analyzing the amorphous fraction using XRD, described in detail in [11], involves consideration of only the relative change in crystalline peak area before and after irradiation of the same sample.…”

Thermoelectric Study of La2Ti2-xNbxO7 (0 ≤ x ≤ 0.25) Ceramics

“…7. Crystal structure of Lanthanum dititanate, La2Ti2O7 [13]. Blue, red, and green balls correspond to La3+, O2-and Ti4+ ions, respectively.…”

“…The study of electrical properties of lanthanum dititanate, La 2 Ti 2 O 7 (LTO) ferroelectrics started over thirty years ago [12]. LTO belongs to the perovskite-like layered structure family with a homologous series , where A = rare earth (RE) elements, B = titanium and n (= 4 in this case of LTO) is the number of octahedral units in the perovskite layers [13,14]. [12,13,16], non-linear optical and photocatalytic properties [12,13,15,16] and nds application in gas turbines at high temperature ( 1000 o C) [15].…”

“…The structure of LTO consists of discontinuous layers of corner sharing TiO 6 octahedra in the unit cell [13,14] with La occupying interstitial positions between the octahedral ( Fig. 1) [13]. Pure LTO has a wide bandgap (E g ) of 3-4 eV [17,18].…”

Acceptance of the 2015 Roebling Medal of the Mineralogical Society of America

“…Udo Becker had just joined the faculty at Michigan and brought much to the collaboration, particularly from the computational perspective-both in radiation effects (Wang et al 2013) and uranium chemistry (Renock et al 2013;Wang et al 2013Wang et al , 2014. This team is now dispersed, but the work continues at the University of Michigan, the University of Tennessee, and Stanford Park et al 2015;Lang et al 2015).…”