The magnetic structure of sodium biphenyl bistriglyme

Abstract

In NaBp.2Tg (Bp = biphenyl; Tg = triglyme = CH₃O(CH₂CH₂O)₃CH₃) layers of paramagnetic biphenyl anions are separated by layers of alkali-glyme complexes. EPR showed that this layered system behaves as a pseudo-two-dimensional magnetic system. It was also suggested that in the plane of the paramagnetic biphenyl anions exchange coupling constants of different signs may be found. In this paper we prove the presence of these exchange couplings with different sign by theoretical calculations. First of all, the EPR resonance field shifts below 4 K are explained on the basis of a localized and delocalized point-dipole approximation. It is concluded that positive as well as negative exchange constants are present and that the positive interactions follow a zig-zag path parallel to the b-axis. Second, the powder susceptibility measured from 11 to 260 K was calculated using mean field theory for a cluster of three spins. A satisfactory agreement was reached between the experimental and calculated susceptibilities with J ₁ = 0 K, J ₂ = 13·8 K and J ₃ = -6.9 K. Finally, the exchange coupling constants between the three discernible biphenyl anion pair configurations have been calculated. Qualitative agreement with the above mentioned exchange constants was found. All results, therefore, confirm the idea of a magnetic lattice with exchange couplings of positive as well as negative sign.