Structure and spectroscopy of Pa4+ defects in CS2ZrCl6. An ab initio theoretical study

In this paper we present the results of spin-orbit relativistic ab initio m odel potential embedded cluster calculations on (PaCl6)(2-) embedded in a r eliable representation of the Cs2ZrCl6 host. Totally symmetric local distor tions and vibrational frequencies are calculated for all the states of the 5f(1) and 6d(1) manifolds, as well as the corresponding 5f<->6d transition energies and the shape of the 5f(Gamma (8u))<--6d(<Gamma>(8g)) fluorescence band. An excellent overall agreement with available experimental data is o bserved which allows us to conclude that the quality of the spin-orbit oper ators used is very high for actinide elements, as was already known for tra nsition metal and lanthanide elements. Furthermore, it is concluded that th e structural and spectroscopic information produced here is very reliable a nd that the 6d(Gamma'(8g)) state is around 10 000 cm(-1) higher in energy t han it was thought; our calculations suggest a value of 30000 cm(-1) for th e 10Dq parameter of Pa4+ in Cs2ZrCl6, would be compatible with the lower li mit of 20000 cm(-1) accepted for Ce3+ in Cs2NaYCl6,. (C) 2001 American Inst itute of Physics.