Prolonged mixed phase induced by high pressure in MnRuP

Hexagonally structured MnRuP was studied under high pressure up to 35 GPa from 5 to 300 K using synchrotron x-ray diffraction. We observed that a partial phase transition from hexagonal to orthorhombic symmetry started at 11 GPa. The new and denser orthorhombic phase coexisted with its parent phase for an unusually long pressure range, $\mathrm{\Delta }P\approx 50\mathrm{GPa}$. We attribute this structural transformation to a magnetic origin, where a decisive criterion for the boundary of the mixed phase lies in the different distances between the Mn-Mn atoms. In addition, our theoretical study shows that the orthorhombic phase of MnRuP remains steady even at very high pressures up to $\sim 250\mathrm{GPa}$, when it should transform to a new tetragonal phase.

Figure 1

The fitting of the ADXRD data for MnRuP powders at room temperature and different pressures. The wavelength of the incident x-ray beam is 0.4133 Å. (a) Data collected before the phase transition at 8.5 GPa. (b),(c) Data collected after the transition at 11.5 and 16.8 GPa, respectively. (d) Volumes per formula unit as a function of pressure indicating a new MnRuP phase appearance at 11 GPa (dashed line). The symbols are the experimental data: purple points indicate the hexagonal phase, and pink squares denote the orthorhombic phase. The solid lines are the calculated third-order Birch-Murnaghan equation of state (EoS) fit to the experimental data. (e) Pressure dependence of $d$ spacing.

Figure 2

(a) The comparison of the Mn $K$-edge XANES spectra for selected manganese materials at ambient pressure and room temperature. The spectra of the Mn metal and MnP taken from [21, 28] compared to the MnRuP data. The inset shows the Mn $K$-edge spectrum for MnRuP alone, with its features marked by a sequence of letters. (b) An expanded view of the energy derivative of the MnRuP XANES spectra, showing the absence of any detectable Mn valence transition in the given pressure range.

Figure 3

Pressure dependence of the temperature and weight fraction in MnRuP. The symbols represent the data extracted by two-phase fitting. The round and square symbols belong to the hexagonal and orthorhombic phases, respectively. The dashed curves are linear fits for the hexagonal and orthorhombic phase data.

Figure 4

Crystal symmetry phase diagram of MnRuP in the pressure range from 0 to 300 GPa.

Figure 5

Pressure dependence of the first-nearest-neighboring (1NN) and second-nearest-neighboring (2NN) Mn-Mn distances in the low-pressure phase of MnRuP. The 1NN distance decreases nonlinearly while the 2NN distance shows a linear decrease with increasing pressure. The dashed lines are guides to the eye.