Effects of A -site ordering on the Mn local structure and polar phases of RBaMn2O6 (

We have investigated the temperature dependence of the Mn local structure in $A$-site ordered $R\mathrm{Ba}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ ($R=\mathrm{La}$, Nd, Sm, and Y) perovskites, in parallel with their disordered counterparts, ${R}_{0.5}{\mathrm{Ba}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$, by means of x-ray emission (XES) and x-ray absorption spectroscopy (XAS) The end member $\mathrm{LaBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ shows a nearly regular $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedron independent of temperature. With decreasing the $R$ ionic radius in the ordered samples, the XAS results indicate that a local distortion develops in the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedron at the low-temperature charge-localized and polar phases. For $\mathrm{NdBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$, this local distortion is tiny, indicating the absence of charge segregation at the Mn site. This is followed by a bigger local distortion anticipated for $\mathrm{SmBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ in its respective charge-localized and polar phase and finally, the biggest local distortion for the smallest $A$-site cation ordered compound, $\mathrm{YBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$, for which it even persists above the polar charge-localization transition temperatures. The high-resolution XAS spectra confirm the presence of charge segregation between two nonequivalent Mn sites in the low-temperature polar phase of Sm and Y ordered samples. Thus, our XAS study suggests a displacive mechanism for the charge-localization and polar transitions in the Nd and Sm ordered samples while a combination of displacive and order-disorder contributions is revealed for $\mathrm{YBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$. Besides, calorimetric measurements confirm the combination of the two mechanisms, order-disorder and displacive, for the ordered Sm and Y compounds. On the other hand, the $A$-site disordered ${R}_{0.5}{\mathrm{Ba}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ samples with $R$ cations smaller than Nd present a significant static (temperature-independent) local disorder, which explains why polar charge-localization transitions are not developed in these samples. Finally, we correlate our results about the Mn local structure and character of the transitions with the macroscopic magnetic and electric behavior of both $A$-site ordered and disordered compounds.