From (S = 1) Spin Hexamer to Spin Tetradecamer by CuO Interstitials in A₂Cu₃O(CuO)(SO₄)₃ (A = alkali)

(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 larger Rb and Cs ones is accompanied by the insertion of neutral CuO species into (Rb,Cs)₂Cu₃O(CuO)(SO₄)₃ phases. Structurally, interstitial CuO links the next two Cu₆ units in longer Cu₁₄ tetradecameric ones. For A = Cs (x = 0.5), the cationic ordering is perfect inside a double-cell superstructure. Magnetically, the original Cu₁₄ units consist of frustrated fragments of an S = ¹/₂ spin ladder, with ferromagnetic rung-like but antiferromagnetic leg-like and next-nearest neighbor couplings. It returns S = 1 Cu₁₄ spin clusters, effective around 100 K. Our density functional theory calculations and susceptibility fits also show that at low temperatures they interact in two-dimensional lattices, despite the existence of short inter-Cu-Cu distances between the next two clusters along pseudo-one-dimensional chains.