Abstract

The effect of applying hydrostatic pressure in the layered-perovskite A${\mathrm{MnF}}_{4}$ (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 ${\mathrm{CsMnF}}_{4}$ is tetragonal with space group P4/n, ${\mathrm{RbMnF}}_{4}$ is orthorhombic with space group Pmab and ${\mathrm{KMnF}}_{4}$ is monoclinic with space group P${2}_{1}$/a. ${\mathrm{CsMnF}}_{4}$ was found to undergo a first-order structural phase transition, from tetragonal to orthorhombic symmetry at ${\mathit{P}}_{{\mathit{c}}_{1}}$=1.4\ifmmode\pm\else\textpm\fi{}0.2 GPa. At pressures in excess of ${\mathit{P}}_{{\mathit{c}}_{2}}$=6.3\ifmmode\pm\else\textpm\fi{}1 GPa, for the Cs derivative, and ${\mathit{P}}_{{\mathit{c}}_{3}}$=4.5\ifmmode\pm\else\textpm\fi{}1 GPa, for the Rb derivative, the symmetry appears to be monoclinic. Moreover, the critical unit-cell volumes associated with ${\mathit{P}}_{{\mathit{c}}_{1}}$, ${\mathit{P}}_{{\mathit{c}}_{2}}$, and ${\mathit{P}}_{{\mathit{c}}_{3}}$ are slightly higher than the ambient pressure unit-cell volumes of ${\mathrm{RbMnF}}_{4}$ for ${\mathit{P}}_{{\mathit{c}}_{1}}$ and ${\mathrm{KMnF}}_{4}$ for ${\mathit{P}}_{{\mathit{c}}_{2}}$ and ${\mathit{P}}_{{\mathit{c}}_{3}}$. 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 A${\mathrm{MnF}}_{4}$ series. The tetragonal to orthorhombic transition of ${\mathrm{CsMnF}}_{4}$ has been found to be inhibited when NaCl is used as an internal pressure calibrant. The partial substitution of Cs by Na in ${\mathrm{CsMnF}}_{4}$ at ${\mathit{P}}_{{\mathit{c}}_{1}}$ has been shown to be a likely explanation for this behavior. The anisotropic broadening of the Bragg peaks for pressures higher than ${\mathit{P}}_{{\mathit{c}}_{1}}$ has been analyzed in terms of microstrain affecting the ${\mathrm{CsMnF}}_{4}$ 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. \textcopyright{} 1996 The American Physical Society.

Highlights

  • The AMnF4 ͑AϭCs, Rb, K, Naseries of layered perovskites has been shown to exhibit a very interesting structural and magnetic behavior

  • 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 crystallizes in the tetragonal space group P4/n with aϭ7.9440͑6͒ Å and cϭ6.3376͑9͒ Å,4,5 RbMnF4 has been refined in the orthorhombic space group Pmab with aϭ7.8051͑8͒ Å, bϭ7.7744͑8͒ Å, and cϭ6.0432͑6͒ Å, and

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Summary

SEPTEMBER 1996-II

Pressure-induced structural phase transitions in the AMnF4 series „A‫؍‬Cs, Rb, K... studied by synchrotron x-ray powder diffraction: Correlation between hydrostatic and chemical pressure. Pressure-induced structural phase transitions in the AMnF4 series „A‫؍‬Cs, Rb, K... Studied by synchrotron x-ray powder diffraction: Correlation between hydrostatic and chemical pressure. The effect of applying hydrostatic pressure in the layered-perovskite AMnF4 ͑AϭCs, Rb, Kseries 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. A correlation between hydrostatic and chemical pressure can be established from the structural point of view for the AMnF4 series. 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. The equation of state associated with the different high-pressure phases has been calculated including compressibilities. ͓S0163-1829͑96͒06534-4͔

INTRODUCTION
EXPERIMENTAL DETAILS
STRUCTURAL PHASE TRANSITIONS INDUCED BY PRESSURE
Orthorhombic to monoclinic
Pressure correlations
INHIBITION OF THE TETRAGONAL TO
EQUATION OF STATE AND COMPRESSIBILITIES
Full Text
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