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.