The Energetics of Li Off-Centering in K1−x_{1-x}Lix_xTaO3_3; First Principles Calculations

K$_{1-x}$Li$_{x}$TaO$_3$ (KLT) solid solutions exhibit a variety of interesting physical phenomena related to large displacements of Li-ions from ideal perovskite A-site positions. First-principles calculations for KLT supercells were used to investigate these phenomena. Lattice dynamics calculations for KLT exhibit a Li off-centering instability. The energetics of Li-displacements for isolated Li-ions and for Li-Li pairs up to 4th neighbors were calculated. Interactions between nearest neighbor Li-ions, in a Li-Li pair, strongly favor ferroelectric alignment along the pair axis. Such Li-Li pairs can be considered "seeds" for polar nanoclusters in KLT. Electrostriction, local oxygen relaxation, coupling to the KT soft-mode, and interactions with neighboring Li-ions all enhance the polarization from Li off-centering. Calculated hopping barriers for isolated Li-ions and for nearest neighbor Li-Li pairs are in good agreement with Arrhenius fits to experimental dielectric data.Comment: 14 pages including 10 figures. To Physical Review B. Replaced after corrections due to referees' remark

Aging and memory effects in beta-hydrochinone-clathrate

The out-of-equilibrium low-frequency complex susceptibility of the orientational glass methanol(73%)-beta-hydrochinone-clathrate is studied using temperature-stop protocols in aging experiments . Although the material does not have a sharp glass transition aging effects including rejuvenation and memory are found at low temperatures. However, they turn out to be much weaker, however, than in conventional magnetic spin glasses.Comment: 5 pages RevTeX, 6 eps-figures include

Dielectric and elastic-response investigations of polar motion in potassium-sodium tantalate

The polar instability of K1-xNaxTaO3 is associated to critical linear and nonlinear phenomena. Data on linear dielectric and elastic susceptibilities show a critical slowing down from 109 to 107 Hz of the Ta5+ motion on approaching the critical temperature. Polydispersive susceptibility at low frequency and critical nonlinearity are evidence for freezing of a disordered configuration only slightly below the (extrapolated) critical temperature. Standard models of dilute glasses do not account for this near-coincidence. Instead, we search for a scenario in which both the ferroelectric and the frustrated interactions between impurity dipoles are enhanced by the critical correlation of lattice dipolar displacements.L'instabilité polaire de K1-xNaxTaO3 est associée à une variation critique des réponses linéaire et non linéaire. Les résultats sur les susceptibilités diélectriques et élastiques montrent un ralentissement critique de 109 à 107 Hz de l'oscillation des ions Ta5+ en s'approchant de la température critique. La polydispersion de la susceptibilité à basses fréquences et la divergence de la susceptibilité non linéaire sont des indications en faveur du gel d'une configuration désordonnée à une température légèrement inférieure à la température critique (extrapolée). Les modèles habituels des verres dilués n'expliquent pas cette quasi-coïncidence. Par contre, nous cherchons un scénario dans lequel les interactions de type ferroélectrique et les interactions frustrées entre les impuretés dipolaires sont accrues par la corrélation entre les distorsions dipolaires du réseau