The Dilemma of the High-Spin Persistence Into the Mbar Range of Some Ferric-Metal Oxides

The fate of the strongly correlated $d-d$ Mott-Hubbard (MH) insulators at extreme conditions of pressure is determined by two main reactions: ($i)$ correlation breakdown due to broadening leading to bands overlap of the \textit{empty-filled }band resulting in metallization consequently loss of magnetic moment, and, (\textit{ii}) spin crossover due to the augmented \textit{crystal-field} (10Dq $\sim$ $r^{-5})$ which in the case of the Fe$^{3+}$ - oxides results in $S=$5/2 \textgreater $S=$1/2 transition. The experimental observation of these high pressure phenomena using Diamond-Anvils-Cells and the experimental methods of resistance and $^{57}$Fe M\"{o}ssbauer effect at varying (P,T) and Synchrotron XRD at RT. This presentation will focus on the recent discovered cases of some Fe$^{3+}M$O where the \textit{high-spin} state prevails into the Mbar region; showing no signs of correlation breakdown. The persistence of correlated, HS states to such pressures cannot be explained. This will be preceded by a short introduction to the experimental methods and cases of pressure-induced spin-crossover or MH transitions.