Pressure-induced structural evaluation and insulator-metal transition in the mixed spinel ferrite $\mathrm{Z}{\mathrm{n}}_{0.2}\mathrm{M}{\mathrm{g}}_{0.8}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_4$

The effect of pressure on the electronic properties and crystal structure in a mixed spinel ferrite $\mathrm{Z}{\mathrm{n}}_{0.2}\mathrm{M}{\mathrm{g}}_{0.8}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_4$ was studied for the first time up to 48 GPa at room temperature using x-ray diffraction, Raman spectroscopy, and electrical transport measurements. The sample was cubic (spinel-type $Fd\overline{3}m$) at ambient pressure and underwent a pressure-induced structural transition to an orthorhombic phase $(\mathrm{CaT}{\mathrm{i}}_2{\mathrm{O}}_4-\mathrm{type}Bbmm)$ at 21 GPa. This structural transformation corresponded to a first-order phase transition that involved 7.5% molar volume shrinkage. The onset of the Mott insulator-metal transition (IMT) around 20 GPa was due to a spin crossover mechanism that led to the $\mathrm{F}{\mathrm{e}}^{3+}$ magnetic moment collapse. All the Raman modes disappeared at high pressures, which supported metallization. Analysis of structural and electrical transport measurements showed a simultaneous volume collapse and sharp IMT within a narrow pressure range. The orthorhombic high-pressure phase was found to have a higher conductivity than the cubic phase. The pressure dependence of the conductivity supported the metallic behavior of the high-pressure phase.

Figure 1

(a) Angle-dispersive XRD patterns for $\mathrm{Z}{\mathrm{n}}_{0.2}\mathrm{M}{\mathrm{g}}_{0.8}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_4$ at different pressures at RT. The ↑ depicts the appearance of new HP phase peaks, and ↓ indicates the disappearance of LP phase peaks. (b) Rietveld refinements for the new HP phases at 24, 34, 39, and 48 GPa, respectively.

Figure 2

(a) Unit cell volume as a function of pressure in the cubic and orthorhombic phases. (b) The XRD spectra of $\mathrm{Z}{\mathrm{n}}_{0.2}\mathrm{M}{\mathrm{g}}_{0.8}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_4$ in the decompression run.

Figure 3

Variation of unit cell parameters with $P$ for spinel and postspinel phases.

Figure 4

(a) Raman spectra of $\mathrm{Z}{\mathrm{n}}_{0.2}\mathrm{M}{\mathrm{g}}_{0.8}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_4$ at different pressures between 0 and 40 GPa, where Raman mode disappearance was observed at HP. (b) Deconvoluted spectra with Lorentzian fitting for Raman modes at (b-1) 0 GPa and at (b-2) 22 GPa. (b-3) Raman spectra for the same decompression cycle.

Figure 5

Pressure dependence of the Raman modes at different pressures between 0 and 33 GPa, where solid lines show linear fits corresponding to specific modes.

Figure 6

Electrical resistance behavior of $\mathrm{Z}{\mathrm{n}}_{0.2}\mathrm{M}{\mathrm{g}}_{0.8}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_4$ as a function of pressure at RT.

Figure 7

(a)–(d) Color variations observed through low-resolution optical microscopic images captured in $\mathrm{Z}{\mathrm{n}}_{0.2}\mathrm{M}{\mathrm{g}}_{0.8}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_4$ with increasing pressure. (e) Isobaric temperature-dependent resistivity for a few representative pressure values.

Figure 8

Variation of normalized resistance with normalized volume fraction.