Effect of high pressure on the crystal and magnetic structure and on the raman spectra in Pr$_{0.7}$Ba$_{0.3}$MnO$_3$manganite

The crystal and magnetic structure and the Raman spectra in Pr$_{0.7}$Ba$_{0.3}$MnO$_{3}$ manganite have been studied by the neutron diffraction technique at pressures up to $5$ GPa as well as by the X-ray diffraction and Raman spectroscopy at pressures up to $30$ GPa. The pressure dependence is determined for the lattice parameters, unit cell volume, Mn–O bond lengths in the orthorhombic structure of the $Imma$ symmetry, and bending and stretching vibration modes for oxygen octahedra. In the low-temperature range at pressure $P = 1.9$ GPa, the magnetic transition from the initial ferromagnetic (FM) ground state ($T_{\rm C}=197$ K) to the A-type antiferromagnetic (AFM) state ($T_N= 153$ K) has been revealed. The FM and AFM phases coexist at pressures up to 5.1 GPa and exhibit negative and positive values of the pressure coefficient for the Curie and Nйel temperature, respectively $dT_{\rm C}$/$dP = - 2.3$ K/GPa and $dT_N$/$dP = 8$ K/GPa). The pressure dependence of the Curie temperature in Pr$_{0.7}$Ba$_{0.3}$MnO$_{3}$ differs drastically from that observed in other manganites of nearly the same composition with the orthorhombic $Pnma$ and rhombohedral $R\bar{3}c$ structures, where the FM phase is characterized by the positive values of $dT_{\rm C}$/$dP$. The structural mechanisms of these phenomena are discussed.