Abstract 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.

中文翻译：

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阳离子有序warwickite系统Mn2-Mg BO4的反铁磁性（x = 0.5、0.6和0.7）

摘要 对具有warwickite 结构的Mn2-xMgxBO4 (x = 0.5、0.6 和0.7) 单晶进行了X 射线衍射、热容量和磁测量。对于空间群为 P21/n 的所有样品，都发现了单斜对称性。M1 位点被三价 Mn 离子占据，而 M2 位点被二价 Mg 和 Mn 离子的混合物占据。观察到规则的阳离子和电荷分布，这对于异金属华威石来说是不寻常的。M1O6 的局部八面体畸变显示了对 Mg 含量的单调依赖性，并且与 Jahn-Teller 畸变一致。发现所有样品都经历了长程反铁磁排序，并且对于 x = 0.5、0.6 和 0.7，分别具有相当低的转变温度 TN = 16、14 和 13 K。局部八面体畸变的排序，