Pressure-induced Jahn-Teller suppression and simultaneous high-spin to low-spin transition in the layered perovskiteCsMnF4

Abstract

The interplay between the orbital ordering and the spin state in Jahn-Teller Mn 3+ governing the optical, magnetic, and transport properties in the layered CsMnF 4 perovskite is investigated. Such electronic effects are strongly coupled to the lattice and thus can be modified by external pressure. However, there is very little understanding of the structural conditions which are required to attain spin crossover in connection with the electronic structure of Mn 3+ . The distortion, spin state, and tilting of ͑MnF 6 ͒ 3− octahedra in the insulating ferromagnet CsMnF 4 are jointly studied by high-pressure optical spectroscopy. The insulating character of CsMnF 4 allowed us to explore the electronic structure associated with the 3d levels of Mn 3+ in the 0 -46 GPa pressure range, an information which is obscured in most oxides due to metallization at high pressure. We show that the spin-crossover transition, related to the spin change, S =2→ 1, in Mn 3+ , takes place at 37 GPa with the simultaneous suppression of the axially elongated distortion associated with the Jahn-Teller effect. The wide stability pressure range of the Jahn-Teller distortion and high-spin state is explained in terms of crystalfield models including the Jahn-Teller stabilization energy. On this basis, we discuss the interplay between spin transition and Jahn-Teller effect comparing present findings with other results attained in Mn 3+ , Ni 3+ , and Co 3+ systems.

Pressure-induced spin transition and site-selective metallization in CoCl2

“…1) shows that, for CoCl 2 ( B ≈ 80 meV at 50 GPa 15,30 ), the HS-LS ( 4 T 1 , S = 3/2 → 2 E, S = 1/2) transition should occur at Δ SCO = 1.7 eV. Importantly, the LS state may be affected by a strong Jahn-Teller effect, providing an additional lattice relaxation energy which, in principle, could reduce the SCO crystal-field strength triggering metallization at unexpectedly low pressure 19,30 . Alternatively, high pressure could, in turn, suppress the Jahn-Teller distortion causing the HS-LS to occur at higher pressures than expected, or even disappear if CoCl 2 transforms into a fluorite-type structure ( d 3 -like Co 2+ ) 20 .

Figure 1( a ) Structural phase-transition sequence in CoCl 2 under high-pressure conditions determined by density-functional-theory calculations and confirmed experimentally by x-ray diffraction.

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

“…Large compression generally leads to the reduction of the JT distortion; it was found, however, that in compounds containing both Cu 2+ and Mn 3+ cations (the latter has a 3d 4 configuration and is JT-active in the high-spin state) this distortion is surprisingly robust. In LaMnO 3 JT-distorted domains persist up to the insulator-to-metal transition at 34 GPa [18], while for CsMnF 4 it was found that the effect is quenched only above 37 GPa when Mn 3+ cations enter the low-spin state [17]. The JT distortion seems to be even more stable in the case of divalent copper [19,20].…”

“…Even subtle distortions in the first coordination sphere of the JT-active cation can lead to large changes in material properties, as exemplified by the case of Ag 2+ -bearing fluorides [10,11]. Due to the fluxional nature of the Jahn-Teller effect many studies have been devoted to tuning this distortion either by chemical substitution [12][13][14][15][16], or by high external pressure [17][18][19][20][21][22].…”

Structural and magnetic characterization of the elusive Jahn-Teller active NaCrF3

“…In fluorides JT ions are well known for showing interesting phenomena under external stimuli. Alkali ternary manganese (III) fluorides with formula A x MnF 3+x (with A = Na, K, Cs) show significant structural diversity, adopting zero-, one-, and two-dimensional vertex-sharing arrangements of the octahedral units depending on the value of x [5,6].…”