High pressure optical study of KTbP2Se6

We have observed a pressure induced phase transition in KTbP2Se6, at about 9.2 GPa which was accompanied by a discontinuous jump of the absorption edge of about 0.5 eV into the red. We have proposed that the high pressure phase is due to charge transfer from the selenium to the phosphorous accompanied by the formation of a Se-Se bond. In order to exclude a model which involves a 4f-5d charge transfer in Tb, leading to the intervalence charge transfer from Tb3+ P4+,-Tb4+P3+ we have measured the pressure dependence of the luminescence of the crystal field split levels of the Tb-ion. A comparison of the pressure dependence of the luminescence spectra of KTb(MoO4)(2) and KTbP2Se6, shows that such a charge transfer does not occur in KTbP2Se6. (C) 2002 Elsevier Science Ltd. All rights reserved.