High-pressure synthesis and characterization of ${\text{PrMn}}_7{\text{O}}_{12}$ polymorphs

We report the synthesis and the characterization of ${\text{PrMn}}_7{\text{O}}_{12}$, a new manganite with multiple (quadruple) perovskite structure of general chemical formula: $A{A}_3^{\prime }{B}_4{\text{O}}_{12}$. This family of manganites is extremely interesting and is attracting a great attention; thanks to its structural peculiarity (and complexity), it might help the comprehension of the ordering phenomena (charge, orbital, spin), which is one of the most challenging issues in the strongly electron correlated rare-earth oxides. Like the majority of the materials having similar structure, ${\text{PrMn}}_7{\text{O}}_{12}$ is a metastable compound, requiring high pressure synthesis. Contrary to other reported isostructural compounds, ${\text{PrMn}}_7{\text{O}}_{12}$ crystallizes in two different forms with rhombohedral $\left(R\text{−}3\right)$ and monoclinic $\left(I2/m\right)$ symmetry, the latter characterized by a distortion of the perovskite structure lattice that depends on the synthesis conditions. The approximate stability fields of the two ${\text{PrMn}}_7{\text{O}}_{12}$ phases have been defined in the $P/T$ space, allowing the synthesis of almost single phase samples functional to physical characterization. For the monoclinic phase we succeeded in the growth of crystals sufficiently large to perform structural refinement by single crystal x-ray diffraction data; the rhombohedral structure was instead refined by Rietveld method applied to powder x-ray diffraction data. Although the two phases differ slightly from the crystallographic point of view, physical characterizations reveal surprisingly different properties, in particular for what concerns the magnetic behavior. The differences of the two structures might be explained with a different electronic configuration of Mn, implying the partial occupation of ${\text{Mn}}^{3+}$ in low spin state on the $B$ site of the rhombohedral polymorph.

Figure 1

${\text{PrMn}}_7{\text{O}}_{12}$ phase diagram in the $P/T$ space.

Figure 2

(Color online) Powder XRD pattern of monoclinic (red, top) and rhombohedral (blue, bottom) ${\text{PrMn}}_7{\text{O}}_{12}$. Impurities phases are marked with symbols. The inset shows the $2\theta$ range of $33.0°–35.5°$, particularly indicative for the detection of rhombohedral (blue, straight line) and monoclinic (red, dotted line) phases.

Figure 3

(Color online) Powder XRD patterns, in the $2\theta$ range $33.0°–35.5°$, of monoclinic ${\text{PrMn}}_7{\text{O}}_{12}$ samples synthesized at different temperatures. The monoclinic cell shows different distortions as a function of synthesis conditions; dotted line is a guide for the eyes and follows the deviation of $\beta$ from $90°$.

Figure 4

(Color online) Rhombohedral (left) and monoclinic (right) structure viewed along the [111] direction in terms of pseudocubic perovskite cell. Symmetry-equivalent cations are identified by the same colors (gray tones).

Figure 5

Field cooled (FC) and zero field cooled (ZFC) magnetic susceptibility of ${\text{PrMn}}_7{\text{O}}_{12}$ in the monoclinic phase $\left(H=100\text{Oe}\right)$.

Figure 6

Field cooled (FC) and zero field cooled (ZFC) magnetic susceptibility of ${\text{PrMn}}_7{\text{O}}_{12}$ in the rhombohedral phase $\left(H=100\text{Oe}\right)$.

Figure 7

Field cooled inverse magnetic susceptibility for ${\text{PrMn}}_7{\text{O}}_{12}$ in the monoclinic and rhombohedral phase. The slopes in the paramagnetic region yield $4.73{\mu }_B$ and $4.54{\mu }_B$ per magnetic ion.

Figure 8

Field dependence of magnetization of ${\text{PrMn}}_7{\text{O}}_{12}$ in the monoclinic phase at selected temperatures.

Figure 9

(Color online) dc electrical resistivity vs temperature for ${\text{PrMn}}_7{\text{O}}_{12}$ polymorphs.