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Dielectric constant measurements of a K0.989Li0.011TaO3 crystal are performed in the orientational glass low-temperature phase. Aging and ergodicity breaking are studied through isothermal decays and temperature cycles. New results are obtained on the response of the sample to temperature jumps: three processes, with very different time scales, are put in evidence. This allows to propose a new interpretation of previous experiments and, in particular, to present a new solution of the paradox observed for the negative temperature cycles.( * ) Associated with the Centre National de la Recherche Scientifique.

: In K1-xLi,Ta03, small Li+ ions are randomly substituted to large K+ ions. They take off-centre positions creating therefore electric dipoles and elastic quadnrpoles at random sites. This disordered system presents frozen random interactions and frustration. For x > 2.5 %. thermal hysteresis signals a structural transition with a first order character towards a ferroelectric low temperature phase. For x c 2.5 %, at low temperatures the system is frozen in a disordered phase called orientational glass. Measurements of the dielectric constant E and of the elastic constant c provide a lot of information about the nature of the transition and the relaxation of the Li+ ions which has been studied in detaik and analyzed in the form of Argand diagrams exhibiting a very particular (strophoidal) shape. After zero field cooling from 100 K to 4.2 K the time evolution of E and c during 80,000 s has been followed. The behaviour of the system (amplitude and asymptot) strongly depends on the cooling rate. The fact that the asymptotic limit depends on the initial conditions shows that ergodicity is broken in this system.

The complex dielectric constant of the orientational glass (SCT) with x = 0.0055 was measured as a function of time at 4.2 K, for frequencies ranging from 1 kHz to 1 MHz, after different thermal histories. Both the in-phase component and the out-of-phase component clearly exhibit aging as shown by the cooling rate dependence of their evolution. Moreover, a weak dependence has been seen for the asymptotic values of both components at very long time. Hence, evidence exists for ergodicity breaking. This is in contrast to what was seen for spin glasses, and similar to what was observed for the series (KLT) of orientational glasses studied previously. However, when temperature jumps are imposed on the SCT sample, the response differs both from that of spin glasses and from that of the KLT series.