Phonon raman scattering in A(2)Mn(2)O(7) (A=Tl,In,Y)

Raman scattering of optical phonons in polycrystalline A(2)Mn(2)O(7) (A = Y , In, and Tl) are reported as a function of temperature. For A = In and Tl a hardening of the two lowest energy modes is observed below T-c that we as sociate to a phonon modulation of the exchange interaction. These data sugg est superexchange as the mechanism responsible for magnetic ordering in the se systems. All but one of the Raman modes in the In- and Tl-based compound s appear at lower frequencies when compared with those in the Y-based one. This frequency shift may be due to the strong hybridization of the Tl(In) 6 s(5s) orbital with O 2p and Mn 3d, claimed by recent band-structure calcula tions on these materials. [S0163-1829(99)01733-6].