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We report X-ray structural studies of the metal-insulator phase transition in bismuth ferrite, BiFeO3, both as a function of temperature and of pressure (931 oC at atmospheric pressure and ca. 45 GPa at ambient temperature). Based on the experimental results, we argue that the metallic gamma-phase is not rhombohedral but is instead the same cubic Pm3m structure whether obtained via high temperature or high pressure, that the MI transition is second order or very nearly so, that this is a band-type transition due to semi-metal band overlap in the cubic phase and not a Mott transition, and that it is primarily structural and not an S=5/2 to S=1/2 high-spin/low-spin electronic transition. Our data are compatible with the orthorhombic Pbnm structure for the beta-phase determined definitively by the neutron scattering study of Arnold et al .[Phys. Rev. Lett. 2009]; the details of this beta-phase had also been controversial, with a remarkable collection of five crystal classes (cubic, tetragonal, orthorhombic, monoclinic, and rhombohedral!) all claimed in recent publications., Comment: 5 figures

The effect of applying hydrostatic pressure in the layered-perovskite AMnF4 (A=Cs, Rb, K) series has been studied using energy-dispersive synchrotron x-ray powder diffraction at pressures between ambient and 20 GPa. At ambient pressure CsMnF4 is tetragonal with space group P4/n, RbMnF4 is orthorhombic with space group Pmab and KMnF4 is monoclinic with space group P21/a. CsMnF4 was found to undergo a first-order structural phase transition, from tetragonal to orthorhombic symmetry at Pc1=1.4±0.2 GPa. At pressures in excess of Pc2=6.3±1 GPa, for the Cs derivative, and Pc3.=4.5±1 GPa, for the Rb derivative, the symmetry appears to be monoclinic. Moreover, the critical unit-cell volumes associated with Pc1, Pc2, and Pc3 are slightly higher than the ambient pressure unit-cell volumes of RbMnF4 for Pc1 and KMnF4 for Pc2 and Pc3. Hydrostatic pressure has been found to have a similar effect on the crystal symmetry of the series as the decreasing of the radius of the alkaline ion from Cs to Rb and K. A correlation between hydrostatic and chemical pressure can therefore be established from the structural point of view for the AMnF4 series. The tetragonal to orthorhombic transition of CsMnF4 has been found to be inhibited when NaCl is used as an internal pressure calibrant. The partial substitution of Cs by Na in CsMnF4 at Pc1 has been shown to be a likely explanation for this behavior. The anisotropic broadening of the Bragg peaks for pressures higher than Pc1 has been analyzed in terms of microstrain affecting the CsMnF4 lattice due to Na incorporation. A substitutional reaction has been shown to be a competitive process, versus a structural phase transition, that enables the system to return to equilibrium after applying pressure on it. Finally, the equation of state associated with the different high-pressure phases has been calculated including compressibilities., We would like to thank the C.I.C.Y.T. for Grants No. MAT91-681, MAT94-43, and MAT95-1490-E, and U.K. Science & Engineering Research Council for providing Synchrotron Radiation beam time under the E.C. Large Scale Facilities Programme.

The structural behaviour of the AMnF4 (A = Cs, Rb and K) layered perovskites has been studied as a function of the external pressure using energy dispersive powder diffraction. Two structural phase transitions, a sharp one at 1.4±0.2 GPa accompaigned by a large discontinuity in the unit cell volume and a smooth one at 6.3±1 GPa showing no discontinuity in the unit cell volume, have been detected for CsMnF4 when silver is used as pressure calibrant. The crystal symetries of the high pressure phases are, respectively, orthorhombic Pmab and monoclinic P21/a. These symmetries nicely correspond to those observed at ambient pressure for RbMnF4 and KMnF4, respectively. Strong differences are observed in the structural behaviour of CsMnF4 when NaCl is used as pressure calibrant. Changes in crystal symmetry and discontinuity in the unit cell volume are not observed. However, for pressures higher than 1.25±0.2 GPa anisotropic broadening of Bragg peaks is strongly noticeable. These results are interpreted in terms of a pressure induced substitution reaction of Cs ions by Na ones within the CsMnF4 lattice.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., Pressure-induced transformations in the structural and magnetic properties of CsMnF4 are reported. This behavior is analyzed in the framework of magnetostructural correlations within the layered perovskite AMnF4 (A=Cs,Rb,K) series by using magnetic susceptibility and synchrotron x-ray powder-diffraction techniques as a function of temperature and hydrostatic pressure., We would like to thank C.I.C.Y.T. for Grants Nos. MAT94-43 and MAT91-681, U.K. Science Ez. Engineering Research Council for providing Synchrotron Radiation beam time under the E.C. Large Scale Facilities Programme, CNPq and the Programa de Cooperacion Cientifica con Iberoamerica.