Pressure-induced spin collapse of octahedrally coordinatedMn3+in the tetragonal hydrogarnet henritermieriteCa3Mn2[SiO4]2[O4H4]

Abstract

The high-pressure behavior of natural henritermierite garnet with close to end-member composition ${\mathrm{Ca}}_{3}{\mathrm{Mn}}_{2}[{\mathrm{SiO}}_{4}{]}_{2}[{\mathrm{O}}_{4}{\mathrm{H}}_{4}]$ was studied at pressures up to 80 GPa using single-crystal synchrotron x-ray diffraction, Raman spectroscopy, and quantum-mechanical calculations based on density functional theory. An isosymmetric phase transition was observed in the pressure range between 55 and 70 GPa, which is associated with a gradual high-spin to low-spin electronic transition in ${\mathrm{Mn}}^{3+}$ and a pronounced reduction of the Jahn-Teller distortion of the ${\mathrm{Mn}}^{3+}{\mathrm{O}}_{4}({\mathrm{OH})}_{2}$ octahedra. In the high-pressure phase the Jahn-Teller distortion is totally suppressed and the ${\mathrm{Mn}}^{3+}$ is in a low-spin configuration. Experimental structural data before and after the phase transition are in excellent agreement with the theoretically predicted structural compression of the high-spin and low-spin phases, respectively. While the overall unit-cell volume is reduced by about $1.5%$ across the phase transition, a collapse of about $4\text{--}5%$ of the ${\mathrm{MnO}}_{6}$ octahedral volume is observed. The high-spin phase shows a bulk modulus $B=101$(1) GPa and its pressure derivative ${B}^{\ensuremath{'}}=4.5$(1). The bulk moduli of the coordination polyhedra are ${B}_{{\mathrm{MnO}}_{6}}=178$(2) GPa, ${B}_{\mathrm{Ca}1{\mathrm{O}}_{8}}=101.2$(5) GPa, ${B}_{\mathrm{Ca}2{\mathrm{O}}_{8}}=88.4$(8) GPa, ${B}_{{\mathrm{SiO}}_{4}}=337$(5) GPa, and ${B}_{{\mathrm{O}}_{4}{\mathrm{H}}_{4}}=29$(1) GPa for the high-spin phase. Mode Gr\uneisen parameters range between 0.34 and 0.94. The computed spin-pairing energy is $\ensuremath{\approx}3.6$ eV at 0 GPa.