Spectroscopic Investigations of the Structural Phase Transition in Gd₂(Ti_1-yZr_y)₂O₇ Pyrochlores

Abstract

The Gd2(Ti1 - y Zr y )2O7 pyrochlore series undergoes a structural phase transition from pyrochlore (Fd3̄m) to defect fluorite (Fm3̄m) that can be driven compositionally by increasing the Zr content or thermally by sintering Zr-rich compositions at temperatures above 1550 °C. Our results demonstrate that ion-beam irradiation can also drive the structural phase transition for Zr-rich compositions. In an effort to understand the effects of composition and ion-beam irradiation on this phase transition, powder X-ray diffraction, polarized Raman, reflection infrared, and X-ray absorption spectroscopy experiments were conducted on Gd2(Ti1 - y Zr y )2O7 pyrochlores prior to and following irradiation with 2 MeV Au2+ ions to a fluence of 5 ions/nm2. Analysis of the vibrational and X-ray absorption data suggests that the structural integrity of the pyrochlore structure is based on distorted corner-shared TiO6 or ZrO6 octahedra. The vibrational spectra indicate that both anion and cation disorder precede the compositionally driven phase transition, but cation disorder appears to dominate the irradiation-driven transition. Analyses of the extended X-ray absorption fine structure of the Ti and Zr K edges and the Gd LIII edges reveal a significant change in the Gd local environment upon irradiation and with increasing Zr content. The Ti and Zr local environments are less affected by irradiation or compositional change but show evidence of increasing disorder that can be attributed to rotations about shared polyhedral edges and corners.

Phonon and crystal field excitations in geometrically frustrated rare earth titanates

“…58 and Vandenborre et al 67 Earlier works either interchanged the mode assignment within this band 57,59 or ascribed the whole band to only one of these modes. 60,61 However, our temperature and angle dependent measurements confirm the assignment made by Saha et al 58 The last expected phonon, an F 2g mode, has been either not accounted for, 57 combined with the A 1g mode in one band, 58,62 or ascribed to low intensity peaks around 105, 60 450, 59 or 680 cm −1 ͑Ref. 61͒ in previous works.…”

“…The figure depicts the room-temperature ͑RT͒ parallel polarization spectra of the rare earth titanates and the corresponding modes. The assignment of the mode symmetries is based on comparison with assignments in previous works on these [57][58][59][60][61][62] and related [62][63][64][65][66][67] compounds ͑see Table II͒, on aforementioned measurements ͑not shown͒ of the angular dependence of mode intensities and on the temperature dependence of the modes ͑vide infra͒.…”

Holmium hafnate: An emerging electronic device material

“…The observed large linewidth can be attributed to the static atomic displacement from their ideal positions as a result of lattice strain due to the sample's thermal history and more significantly due to compositional substitution by atoms with mismatched cationic radii and valencies. 15 The temperature dependent vibrational spectra (data shown for 83 K and 1200 K) depicted in Fig. 1(b) were found to be nearly identical to that obtained at room temperature (with a nominal softening with increasing temperature) confirming that the HHO defect-fluorite phase is quite stable; hence, it is a promising material for various applications in a remarkably wide temperature range.…”

Temperature-dependent Raman and x-ray studies of the spin-ice pyrochloreDy2Ti2O7and nonmagnetic pyrochlore

“…The assignment of various modes have been done following the previously reported results of ref. [21,22] (see Table- cm −1 (N7, E g ), 458 cm −1 (N8, F 2g ), 520 cm −1 (N9, F 2g ) and 609 cm −1 (N10, F 2g ). Some of the previous studies [23,24,25] labeled the mode M9(N9) as A 1g + F 2g , and overlooked the weak mode near 450 cm −1 .…”