Phonon anomalies versus magnetic ordering in CuO

We report the first measurements of FIR and MIR reflectivity of single-crys tal monoclinic antiferromagnetic copper monoxide (CuO) in a wide temperatur e range (7-300 K) when b-axis and ac-plane polarized modes are completely s eparated and excited in transverse geometry. Previously reported softening of the A(u)(3) mode at the NC el transition is confirmed and found to be up to two times more significant (about 10%). For the first time an emergence of several new IR-active lines at low temperatures is observed. The highes t frequency new mode at 690 cm(-1) strongly hardens and strengthens with co oling. The data are discussed in terms of strong spin-phonon coupling and p ossible charge disproportionation at low T. (C) 2000 Elsevier Science B.V. All rights reserved.