The spin-Hamiltonian parameters (g factors gǁ, g⊥ and hyperfine structure constants

AǁA⊥) of tetragonal MIC16

- (MI = Mo, W) centers in cubic Cs2MCl6 (M = Zr, Hf):

MI

5+ crystals were calculated from high-order perturbation formulas based on a

two-mechanism model. These contain the contributions to SH parameters from

both the crystal-field mechanism concerning the crystal-field excited states and the

charge-transfer mechanism concerning the charge-transfer excited states. In the

calculations, two possible defect models were suggested for the tetragonal MIC16

-

centers. In model I, one Cl- ion shifts towards the MI

5+ impurity by ΔZI along the

C4 axis due to electrostatic repulsion between the Cl- ion and a neighbouring

charge compensator, monovalent anion, at the interstitial site. In model II, two Clions

were displaced towards MI

5+ by ΔZII along the C4 axis due to the Jahn-Teller

effect. The calculated results showed that, to obtain a good fit between calculated

and experimental spin-Hamiltonian parameters, the required displacement ΔZI

(which was about twice the displacement ΔZII) was too large to be regarded as

reasonable. Thus, model II seems more likely. The contributions of the chargetransfer

mechanism to spin-Hamiltonian parameters were important and, for high

valence state dn ions in crystals, the exact calculations of the spin-Hamiltonian parameters should take both crystal-field and charge-transfer mechanisms (the

latter was omitted in the widely used crystal-field theory) into account.

Defect Model and Spin-Hamiltonian Parameters for the Tetragonal Mo5+ and W5+

Centers in Cs2ZrCl6 and Cs2HfCl6 Crystals. W.C.Zheng, Y.Mei, W.Q.Yang:

Philosophical Magazine, 2009, 89[20], 1621-8