Evidences for the Formation of Chromium in the Unusual Oxidation State Cr(IV): II. Magnetic Properties and Low-Temperature X-Ray Investigations of the Nonstoichiometric Channel Compounds TlxCr5Se8 (0≤x≤1)

Magnetic measurements on TlxCr5Se8 (0≤x≤1) reveal that stoichiometric TlCr5Se8 is a three-dimensional antiferromagnet with a Néel temperature TN of about 55 K. In contrast, samples with a reduced Tl content show highly unusual magnetic properties that are without precedent in the literature of magnetically coupled systems: upon lowering the temperature from 300 K the susceptibility reaches a maximum at about 125 K and then steeply drops to a value comparable to that obtained at room temperature. The height of this maximum increases first with decreasing Tl abundance reaching its largest value at a composition Tl0.2Cr5Se8 and then decreases again for samples with x0.5 the predominant magnetic exchange is antiferromagnetic and becomes ferromagnetic for x