Antiferromagnetism of the cation-ordered warwickite system Mn2-xMgxBO4 (x = 0.5, 0.6 and 0.7)

Single crystals of Mn2-xMgxBO4 solid solutions were obtained using flux synthesis. Regular cation and charge distributions are observed. The Mn2-xMgxBO4 system is the first example of a heterometallic warwickite showing long-range magnetic order. [Display omitted] •Single crystals of Mn2-xMgxBO4 (x = 0.5, 0.6, and 0.7) solid solutions were obtained using flux synthesis.•A monoclinic P21/n symmetry is found for all samples.•Regular cation and charge distributions over two non-equivalent metal sites are observed.•DC magnetization and heat capacity measurements revealed an AFM ordering for all samples. X-ray diffraction, heat capacity and magnetic measurements are performed on single crystals of Mn2-xMgxBO4 (x = 0.5, 0.6 and 0.7) with the warwickite structure. The monoclinic symmetry is found for all samples with the space group P21/n. The M1 site is occupied by trivalent Mn ions while the M2 site is occupied by a mixture of divalent Mg and Mn ions. Regular cation and charge distributions are observed, which is unusual for heterometallic warwickites. The local octahedral distortions of M1O6 show the monotonic dependence on the Mg content and are in accordance with the Jahn-Teller distortion. All samples are found to undergo long-range antiferromagnetic ordering with rather low transition temperatures of TN = 16, 14 and 13 K for x = 0.5, 0.6, and 0.7, respectively. The ordering of local octahedral distortions, caused by the strong electron-phonon interaction of the trivalent Mn ions, is proposed to stabilise the cationic ordering and, as a result, the long-range magnetic ordering in the material.