From (S = 1) Spin Hexamer to Spin Tetradecamer by CuO Interstitials in A2Cu3O(CuO)x(SO4)3 (A = alkali)
Résumé
Na, K)2Cu3O(SO4)3 compounds form structural chains of Cu6 hexameric units with nominal S=1 spins due to the interplay between inner strong antiferromagnetic and ferromagnetic exchanges. We show here that the lattice relaxation after the replacement of alkali by bigger Rb and Cs ones is accompanied by the insertion of neutral CuO species into (Rb,Cs)2Cu3O(CuO)x(SO4)3 phases. Structurally, interstitial CuO link two next Cu6 units in longer Cu14 tetradecameric ones. For A=Cs (x=0.5), the cationic ordering is perfect inside a doubled-cell superstructure. Magnetically, the original Cu14 units consist of frustrated fragments of S=1/2 spin-ladder, with ferromagnetic rung-like but antiferromagnetic leg-like and next-nearest neighbors (NNN) couplings. It returns S=1 Cu14 spin-clusters, effective around 100K. Our DFT calculations and susceptibility fits also show at low temperature, that they interact in 2D-lattices, despite the existence of short inter Cu-Cu distances between two next clusters along pseudo 1D-chains.
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