Non Ergodic Aging in an Orientational Glass

Abstract
In K_1-xLi_xTaO₃, smail Li⁺ ions are randomly substituted to large K⁺ ions. They take off-centre positions creating therefore electric dipoles and elastic quadrupoles 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>2.5%, at low temperatures the system is frozen in a disordered phase called orientational glass. Measurements of the dielectric constant ε 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 details 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 ε 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.