Phonon raman scattering in A2Mn2O7 (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 associate to a phonon modulation of the exchange interaction. These data suggest superexchange as the mechanism responsible for magnetic ordering in these systems. All but one of the Raman modes in the In- and Tl-based compounds 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) 6s(5s) orbital with O 2p and Mn 3d, claimed by recent band-structure calculations on these materials. [S0163-1829(99)01733-6].