High-pressure structural study of ${\mathrm{MnF}}_2$

Manganese fluoride $\left({\mathrm{MnF}}_2\right)$ with the tetragonal rutile-type structure has been studied using a synchrotron angle-dispersive powder x-ray diffraction and Raman spectroscopy in a diamond anvil cell up to 60 GPa at room temperature combined with first-principles density functional calculations. The experimental data reveal two pressure-induced structural phase transitions with the following sequence: rutile $\to {\mathrm{SrI}}_2$ type (3 GPa)$\to \alpha -{\mathrm{PbCl}}_2$ type (13 GPa). Complete structural information, including interatomic distances, has been determined in the case of ${\mathrm{MnF}}_2$ including the exact structure of the debated first high-pressure phase. First-principles density functional calculations confirm this phase transition sequence, and the two calculated transition pressures are in excellent agreement with the experiment. Lattice dynamics calculations also reproduce the experimental Raman spectra measured for the ambient and high-pressure phases. The results are discussed in line with the possible practical use of rutile-type fluorides in general and specifically ${\mathrm{MnF}}_2$ as a model compound to reveal the HP structural behavior of rutile-type ${\mathrm{SiO}}_2$ (Stishovite).

Figure 1

Schematic representations of: (a) rutile-type, (b) ${\mathrm{SrI}}_2$-type, (c) fluorite, and (d) $\alpha -{\mathrm{PbCl}}_2$-type crystal structures of ${\mathrm{MnF}}_2$. Blue and green spheres indicate Mn and F anions, respectively.

Figure 2

XRD patterns of ${\mathrm{MnF}}_2$ at various pressures. The patterns at 2.9 and 13.4 GPa correspond to a phase mixture of rutile-${\mathrm{SrI}}_2$ (HP-I) and ${\mathrm{SrI}}_2-\alpha -{\mathrm{PbCl}}_2$ (HP-II) phases.

Figure 3

Rietveld refinement results for ${\mathrm{MnF}}_2$ at: (a) 1.4 GPa, rutile structure, (b) 7.2 GPa, ${\mathrm{SrI}}_2$ type, and (c) 33 GPa, $\alpha -{\mathrm{PbCl}}_2$ type. Symbols correspond to the measured profile; the red solid lines represent the results of Rietveld refinements. The difference curves (blue curves) are shown also. Vertical ticks mark positions of Bragg peaks.

Figure 4

(a) Pressure dependence of the lattice parameters of ${\mathrm{MnF}}_2$ and (b) volume-pressure data for the rutile, ${\mathrm{SrI}}_2$ type and $\alpha -{\mathrm{PbCl}}_2$ type of ${\mathrm{MnF}}_2$. Experimental and calculated values are shown with solid and open symbols, respectively.

Figure 5

(a) Selected interatomic distances for the rutile, ${\mathrm{SrI}}_2$ type, and $\alpha -{\mathrm{PbCl}}_2$ type of ${\mathrm{MnF}}_2$ as a function of pressure shown with black, red, and blue symbols, respectively; Mn-F and Mn-Mn distances are noted with solid squares and circles, respectively, for the two high pressure phases. (b) Bar diagram of the various Mn-F distances up to $4\AA$ for the three different phases of ${\mathrm{MnF}}_2$; distances with very close values are grouped together for clarity. See text for details.

Figure 6

Calculated enthalpy differences for the rutile, fluorite, ${\mathrm{SrI}}_2$-type, and $\alpha -{\mathrm{PbCl}}_2$-type phases of ${\mathrm{MnF}}_2$ as a function of pressure. The enthalpy of the rutile phase is taken as the reference.

Figure 7

Raman spectra of ${\mathrm{MnF}}_2$ at various pressures. The black, red, and blue vertical ticks mark the calculated frequency of the Raman peaks for rutile, ${\mathrm{SrI}}_2$-type, and $\alpha -{\mathrm{PbCl}}_2$-type phases of ${\mathrm{MnF}}_2$, respectively.

Figure 8

Bar diagram showing the pressure stability intervals of the different structural modifications of ${\mathrm{MnF}}_2$ and ${\mathrm{SiO}}_2$. Critical pressures values are obtained with XRD from this study for ${\mathrm{MnF}}_2$ and XRD results or theoretical predictions for ${\mathrm{SiO}}_2$ (see the references given in the text).