Analysis of the spin lattice model for the spin-gapped layered compounds Na3Cu2SbO6 and Na2Cu2TeO6 on the basis of electronic structure calculations

The spin lattice model for the spin-gapped layered magnetic solids Na3Cu2SbO6 and Na2Cu2TeO6 was examined by evaluating the three spin exchange interactions of their Cu2MO6 (M = Sb, Te) layers in terms of spin dimer analysis based on extended Huckel tight binding calculations and mapping analysis based on first principles density functional theory electronic band structure calculations. For both compounds, our calculations show that the two strongest spin exchange interactions, that is, the Cu-O center dot center dot center dot O-Cu super-superexchange (J(2)) and the Cu-O-Cu superexchange (J(1)) interactions, form alternating chains that interact weakly through the Cu-O-Cu superexchange (J(3)) interactions. The dominant one of the three spin exchange interactions is J(2), and it is antiferromagnetic in agreement with the fact that both of the compounds are spin gapped. For Na3Cu2SbO6 and Na2Cu2TeO6, the superexchange J(1) is calculated to be ferromagnetic, hence, leading to the alternating chain model in which antiferromagnetic and ferromagnetic spin exchange interactions alternate. This picture does not agree with the recent experimental analysis, which showed that the temperature-dependent magnetic susceptibilities of both compounds should be described by the alternating chain model in which two antiferromagnetic spin exchange interactions of different strengths alternate.