Structural diversity and phase transitions in Mg$_{2-x}$Mn$_{1+x}$BO$_5$ ($x=0.0,0.2,0.4$) borates with a mixed valence

Single and polycrystalline samples of Mg$_{2-x}$Mn$_{1+x}$BO$_5$ ($x=0.0,0.2,0.4$) oxyborates have been obtained for the first time by spontaneous crystallization from the solution–melt and through a solid-state reaction. X-ray diffraction studies have shown that compounds are crystallized with increasing manganese content in the ludwigite (space group $Pbam$)–hulsite (space group $P2/m$)–orthopinakiolite (space group Pbam) series and belong to the “3 Å wallpaper” borate family. A common property of materials is the presence of octahedral complexes (walls) consisting of manganese ions with a mixed valence at even crystallographic sites. The dc magnetization and specific heat of the Mg$_{2-x}$Mn$_{1+x}$BO$_5$ ($x=0.0,0.2,0.4$) compounds have been studied for the first time. These studies have shown that cooling is accompanied by magnetic transitions, which are due to the ordering of several magnetic subsystems.