First-principles study of pressure-induced structural phase transitions in MnF₂

Abstract

In this work we report a complete structural and magnetic characterization of crystalline MnF under pressure obtained using first principle calculations. Density functional theory was used as the theoretical framework, within the generalized gradient approximation plus the Hubbard formalism (GGA+U) necessary to describe the strong correlations present in this material. The vibrational, the magnetic exchange couplings and the structural characterization of MnF in the rutile ground state structure and potential high pressure phases are reported. The quasiharmonic approximation has been used to obtain the free energy, which at the same time is used to evaluate the different structural transitions at 300 K. Based on previous theoretical and experimental studies on AF compounds, ten different structural candidates were considered for the high pressure regime, which led us to propose a path for the MnF structural transitions under pressure. As experimental pressure settings can lead to non-hydrostatic conditions, we consider hydrostatic and non-hydrostatic strains in our calculations. According to our results we found the following sequence for the pressure-driven structural phase transition in MnF: rutile (P4/mnm) → α-PbO-type (Pbcn) → dist. HP PdF-type (Pbca) → dist. fluorite (I4/mmm) → cotunnite (Pnma). This structural path is correlated with other phase transitions reported on other metal rutile fluorides. In particular, we found that our proposed structural phase transition sequence offers an explanation of the different paths observed in the literature by taking into account the role of the hydrostatic conditions. In order to get a deep understanding of the modifications of MnF under pressure, we have analyzed the pressure evolution of the structural, vibrational, electronic, and magnetic properties for rutile and for each of the high pressure phases.

Quest for Compounds at the Verge of Charge Transfer Instabilities: The Case of Silver(II) Chloride †

“…And since they tend to collapse fast under even moderate pressures, it could be that other polymorphic forms, such as the ribbon one, would become competitive at rather low pressures, even preceding the transformation to the layered form. The previously documented pressure-induced transformations of CuF 2 [47] and AgF 2 [11] as well as a large body of data for transition metal difluorides and dichlorides (see also [48] and references therein) seem to suggest this scenario as a viable one.…”

“…The common tangent method [17,46] allows for a rather crude estimate for the formation pressure of AgCl 2 of 35 GPa (at T → 0 K), and likely even higher pressures at elevated temperature. The more precise estimate requires calculations in the function of pressure to be performed, also including the ribbon polymorph, which should exhibit substantial compressibility, and several viable high-pressure polymorphs [11,47,48]. Moreover, while drawing the computed volume-based conclusions one should always remember that despite great performance of HSE06 functional for describing crystal and electronic structure of solids, the reproduction of van der Waals interactions is still imperfect.…”

The Jahn-Teller Distortion at High Pressure: The Case of Copper Difluoride

“…This compound is one of the simplest binary connections containing the Cu 2+ cation. It belongs to the family of metal difluorides, which have been extensively studied at high pressure [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]. Due to the operation of the JT effect CuF 2 adopts at ambient conditions a rarely-encountered crystal structure found only in one other compound (CrF 2 ) [38].…”

Uniaxial pressure induced compensated half metal in Cr‐substituted iron(II) fluoride

“…This indicates that the obtained CHM could be a promising candidate for spintronics devices in real application. It is also noticed that the absolute value of T N depends on U , similar to the case of MnF 2 . However, this does not have influence on the final conclusion.…”