- 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 x<0.2. With decreasing Tl content the effective magnetic moment per Cr3+ decreases, indicating the successive formation of Cr4+ (3d2) centers. For samples TlxCr5Se8 with x>0.5 the predominant magnetic exchange is antiferromagnetic and becomes ferromagnetic for x<0.5. The magnetic data suggest a very similar spin structure for all samples except TlCr5Se8 below about 125 K. Low-temperature single-crystal X-ray investigations on TlxCr5Se8 (x=0.5 and 0.32) show a thermal expansion anomaly of the unit cell volume as well as of the b-axis starting at about 125 K, which is accompanied by an enlargement of one of the three Cr–Cr interatomic distances. The anomalous thermal behavior of the lattice parameters reflects competing antiferromagnetic and ferromagnetic exchange interactions. Detailed analysis of the Cr–Cr distances as a function of temperature gives evidence that the positive charge associated with the formation of Cr(IV) is delocalized between two chromium atoms at higher temperatures and trapped upon cooling on one Cr site. The localization of Cr(IV) centers induces net antiferromagnetic coupling. EPR investigations are in agreement with this picture. They show that in stoichiometric TlCr5Se8 the Cr3+ centers are antiferromagnetically coupled. For Tl-poor samples, the temperature dependence of geff follows the susceptibility curve, increasing upon cooling and passing a maximum at about 130 K.