MANIFESTATION OF THE COOPERATIVE JAHN-TELLER EFFECT IN THE RAMAN-SPECTRA OF BA2CUXZN1-XWO6 MIXED-CRYSTALS

Raman spectra of Ba2CuxZn1-xWO6 mixed crystals (x = 0.0 to 0.4 and x = 1) are reported and shown to display considerable enhancement of Rama n intensity for the WO6 vibration with E(g)(O-h) symmetry when increas ing the concentration of Cu2+. Ab initio calculations of Raman frequen cies and intensities for a W(OH)(6) model cluster indicate that the A( 1g)(D-4h) component of this vibration mag gain considerable intensity via mixing with the totally symmetric stretching A(1g) (O-h) vibration mhen lowering the symmetry from O-h to D-4h, thus reflecting an indir ect influence of Jahn-Teller distorted CuO6 centers on neighboring WO6 octahedra. Similarly striking is the change of the Raman spectrum in the energy region of the T-2g modes, when switching from the cubic pha se with disordered local distortions in the CuO6WO6 frame to the long- range cooperative Jahn-Teller order in Ba2CuWO6. It is concluded that Raman spectra can be used as a rather sensitive probe for detecting Ja hn-Teller distortions in mixed crystals, thus providing a valuable add ition to other spectroscopic techniques. (C) 1997 Academic Press.