The discovery rate of truly frustrated magnets has decreased. Conventional high-temperature synthesis often yields well-ordered antiferromagnets, and materials with nearly canceled interactions are scarce. Herein, we show that low-temperature chemistry can convert a modest, partially magnetic spinel into an antiferromagnet with a diamond network. The key step is a topochemical ion‑exchange, which preserves the Ti₃O₈ framework.

The starting compound was Li₂CoTi₃O₈, a distorted spinel in which Li⁺ and Co²⁺ share tetrahedral A sites and the octahedral B sublattice has a 1:3 ordering of Li⁺ and Ti⁴⁺. This was considered magnetically quiet. Two treatments at 230 °C with CoSO₄ were used to remove Li⁺ and insert Co²⁺ ions, without breaking the Ti₃O₈ framework. The product Co₂Ti₃O₈ crystallized in the chiral space group P4₁32 (or its enantiomer P4₃32). Additionally, all A sites were occupied by Co²⁺, forming a diamond network, while the Ti⁴⁺ and vacancy ordering at the B sites remain.

This exchange rebuilt magnetic connectivity. In Li₂CoTi₃O₈, only half of the A sites carried Co²⁺; therefore, the Co sublattice did not percolate in three dimensions (the magnetic sites failed to form a connected 3D network) and the magnetic susceptibility showed only weak Curie–Weiss behavior. After the exchange, every A site hosted Co²⁺, the network became connected, and frustration became apparent. The magnetic susceptibility followed the Curie–Weiss law with an antiferromagnetic Weiss temperature of approximately −27 K (the negative value indicates that antiparallel interactions dominate and the interaction scale is set near 27 K). However, long‑range order appeared only at T_N ≈ 4.4 K, implying that the spins preferred to order at a much higher temperature but were obstructed by competing pathways and geometry until the temperature reached approximately six times the lower value, which indicates strong frustration. The heat capacity data show that approximately 70% of the spin entropy was released at temperatures above T_N, which is consistent with the substantial short-range correlations that occur well before the long-range order sets in. In pulsed fields of up to approximately 50 T, the magnetization showed four step‑like increases. A molecular‑field analysis revealed J₂/J₁ ≈ 0.5, a regime in which spiral ground states are theoretically predicted on the diamond network.

Therefore, vacancy-ordered spinels with composition A₂B₃X₈ provide a useful platform for building and testing 3D frustrated magnets. By tuning the cation order within a known framework, one can vary the exchange pathways and directly probe the models of magnetic frustration.

(Written by Yuya Haraguchi on behalf of all authors)

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