Displacive structural phase transitions and the magnetic ground state of quadruple perovskite YMn7O12

We present an experimental study of the structural and magnetic phase transitions of the quadruple perovskite ${\mathrm{YMn}}₇{\mathrm{O}}₁₂$. On cooling below 640 K a displacive structural phase transition occurs, lowering the crystal symmetry from cubic $(Im\overline{3})$ to monoclinic $(I2/m)$, driven by long range B site ${\mathrm{Mn}}³⁺$ orbital order. A second, unconventional structural phase transition takes place on further cooling below 200 K, which is characterized by static atomic displacements $(P{2}₁/n)$ as well as a dipolar-glass-like state of yttrium. This low temperature structural transition is driven by the underbonding of yttrium, which is present due to the rigid framework of octahedra tilts unique to the quadruple perovskites that primarily accommodates A site manganese ions. A ground state ferrimagnetic structure was also determined, similar to that found in other $R{\mathrm{Mn}}₇{\mathrm{O}}₁₂$ compounds that do not present a low temperature structural phase transition.