Magnetoelastic couplings in the distorted diamond-chain compound azurite

We present results of ultrasonic measurements on a single crystal of the distorted diamond-chain compound azurite Cu sub(3)(CO sub(3)) sub(2) (OH) sub(2). Pronounced elastic anomalies are observed in the temperature dependence of the longitudinal elastic mode c sub(22) which can be assigned to the relevant magnetic interactions in the system and their couplings to the lattice degrees of freedom. From a semiquantitative analysis of the magnetic contribution to c sub(22) the magnetoelastic coupling G = [partialdifferential]J sub(2)/[partialdifferential] sub([epsilon ]b) can be estimated, where J sub(2) is the intradimer coupling constant and sub( epsilon b) the strain along the intrachain b axis. We find an exceptionally large coupling constant of G ~ 3650 K highlighting an extraordinarily strong sensitivity of J sub(2) against changes of the b-axis lattice parameter. These results are complemented by measurements of the hydrostatic pressure dependence of J sub(2) by means of thermal expansion and magnetic susceptibility measurements performed both at ambient and finite hydrostatic pressure. We propose that a structural peculiarity of this compound, in which Cu sub(2) O sub(6) dimer units are incorporated in an unusually stretched manner, is responsible for the anomalously large magnetoelastic coupling.