Thermally activated dynamics of the tilts of the CuO6octahedra, hopping of interstitial O, and possible instability towards the LTT phase in La2CuO

Abstract

The anelastic spectrum ͑dynamic Young's modulus and elastic energy absorption͒ of La 2 CuO 4ϩ␦ has been measured between 1 and 700 K with 0Ͻ␦Ͻ0.02. The spectrum of stoichiometric La 2 CuO 4 in the lowtemperature orthorhombic ͑LTO͒ phase is dominated by two intense relaxation processes which cause softenings of 16% around 150 K and 9% below 30 K at f ϳ1 kHz. The relaxation at 150 K is attributed to the presence of a fraction of the CuO 6 octahedra which are able to change their tilted configuration by thermal activation between orientations which are nearly energetically equivalent, possibly within the twin boundaries. The relaxation below 30 K is governed by tunneling, and involves a considerable fraction of the lattice atoms. It is proposed that the double-well potentials for the low-temperature relaxation are created by the tendency of the LTO phase to form low-temperature tetragonal ͑LTT͒ domains, which however are not stabilized like when La is partially substituted with Ba. On doping with excess O, the relaxation rates of these processes are initially enhanced by hole doping, while their intensities are depressed by lattice disorder; an explanation of this behavior is provided. Excess O also causes two additional relaxation processes. The one appearing at lower values of ␦ is attributed to the hopping of single interstitial O 2Ϫ ions, with a hopping rate equal to Ϫ1 ϭ2ϫ10Ϫ14 exp(Ϫ5600/T) s. The second process is slower and can be due to O pairs or other complexes containing excess O. ͓S0163-1829͑98͒03210-X͔

Study of interstitial oxygen mobility in RuSr2GdCu2O8 by internal friction

“…The intensities of the peaks H1 and H2 is increased with respect to outgassing and this increase is due to the formation of oxygen vacancies, as observed by Cordero et al [14] in the 650-750 K range at kHz frequencies. The peak T can be the same peak observed by Cordero et al [15] in La 2 CuO 4+␦ and attributed to the tilt of CuO 6 octahedra. These peaks will not be the subject of this paper.…”

Anelastic spectroscopy study of iron carbonate scales from CO2 corrosion of steel

“…Indeed, as the Fe-O 6 octahedra are all interconnected, the tilting of one of them would involve a collective and coordinated dynamics of the octahedra chains. This proposed model would be similar to what was observed for the Cu-O 6 octahedra motion in LaCuO 4 giving rise to an anelastic peak with an activation energy of 0.23 eV [7]. However, the prefactor of the relaxation time we presently obtained is not typical of the dynamics of extended defects (∼10 −12 s).…”

Ferroelectricity in underdoped La-based cuprates

“…In the case of lightly oxygen doped La 2 CuO 4+x 15 it was proposed that the non-stoichiometric oxygen ions occupy interstitial positions in the La 2 CuO 4 unit cell, causing a displacement in the apical oxygen ions of the CuO 6 octahedra, which are the building blocks of the La 2 CuO 4 unit cell. Therefore, local-scale structural CuO 6 distortions take place breaking spatial inversion symmetry and resulting in the formation of local electric dipoles1617. These dipoles localize around the oxygen interstitials forming charge clusters, which couple and freeze at low temperature giving rise to a measurable polar state.…”