Pressure effects onNaMnF4: Structural correlations and Jahn-Teller effect from crystal-field spectroscopy

Abstract

This work investigates the optical absorption spectrum of the NaMnF 4 -layered perovskite and its variation with pressure. The spectrum basically consists of three broadbands located at 1.916, 2.263, and 2.817 eV, which correspond to the crystal-field ͑CF͒ transitions 5 B 1g → 5 ⌫ i (⌫ i ϭA 1g , B 2g , and E g ) with the Jahn-Teller-͑JT-͒ distorted MnF 6 3Ϫ complex (Mn 3ϩ d 4 configuration͒. In addition, there are two spin-flip 5 B 1g → 3 B 1g peaks at 2.397 and 2.890 eV, which are activated by the exchange mechanism. Their variation with pressure reveals that the JT energy does not change significantly with pressure: ‫ץ‬E JT ‫ץ/‬ Pϭ0.8 meV/GPa. Furthermore, the variation of the JT tetragonal splitting of the parent octahedral e g and t 2g , termed ⌬ e and ⌬ t , respectively, clearly indicate that ‫⌬ץ‬ e ‫ץ/‬ PӶ‫⌬ץ‬ t ‫ץ/‬ P, although ⌬ e Ϸ4⌬ t . The CF energies and their pressure shift are explained in terms of local structural changes within the MnF 6 3Ϫ complex induced by pressure. The structural correlation analysis reveals that the reduction of the MnF 6 3Ϫ JT distortion is smaller than the expected one on the basis of the crystal volume reduction, thus indicating tilt phenomena. This interpretation is supported by the decrease of in-layer Mn-F-Mn superexchange, such as is derived from the optical spectra. We demonstrate that the equatorial and axial distances decrease from 1.839 to 1.808 Å and from 2.167 to 2.107 Å, respectively, in the 0-10 GPa range.

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

“…Note the antiferrodistortive structure shown by the ͑MnF 6 ͒ 3− octahedra in the a , b layer. 11,17 the three transition energies in D 4 ͑four in D 2 ͒ are related to the equatorial Mn-F distance, R eq , and the tetragonal and rhombic normal coordinates, Q and Q , describing the low-symmetry distortion of the ͑MnF 6 ͒ 3− octahedra ͑Fig. 1͒.…”

“…The high-pressure spectroscopy setup has been described elsewhere. 17 The absorption spectra were taken with the light propagating perpendicular to the c layers, and are nearly isotropic as corresponds to antiferrodistortive layers of ͑MnF 6 ͒ 3− even in the monoclinic highpressure phase.…”

Structural Correlation in Jahn–Teller Systems of Cu2+and Mn3+under Pressure

“…The A 2 CuCl 4 (A=Rb,C n H 2n+1 NH 3 ; n=1-3) [1][2][3][4][5][6][7][8][9] and AMnF 4 (A: Rb, Na, Tl, Cs) [10][11][12][13][14][15][16]) layered compounds present a large variety of interesting physical phenomena associated with antiferrodistortive (AFD) structure displayed by the Jahn-Teller (JT) axially elongated CuCl 6 4-and MnF 6 3-complexes, respectively. They have attracted interest as two-dimensional ferromagnets with T C 10 K 1,3,7,8,10,11) , as related material to high T C superconductors and colossal magnetoresistors 17) , as an organic/inorganic hybrid layered system, and as piezochromic materials 5,13) .…”

“…They have attracted interest as two-dimensional ferromagnets with T C 10 K 1,3,7,8,10,11) , as related material to high T C superconductors and colossal magnetoresistors 17) , as an organic/inorganic hybrid layered system, and as piezochromic materials 5,13) . In the last years an intense activity aimed to investigate the behavior of these materials under hydrostatic pressure, in particular, for exploring pressure-induced structural transitions associated with the disappearance of the AFD structure and tilting phenomena [3][4][5][6][7][8]14 . The former effect is noteworthy since it must lead to a switch in the 2D magnetic behavior from ferromagnetic to antiferromagnetic.…”

“…XRD powder diagrams often exhibit intensity deviations due to preferred orientation, twining, texture and dynamical JT effects that can hinder analysis of the diffracted intensity associated with JTD. The use of local probes like EXAFS 7) or optical absorption (OA) spectroscopy [13][14][15][16] is crucial for an appropriate local structural determination. XAS studies on LaMnO 3 20) and OA in NaMnF 4 4) are examples illustrating the usefulness of local probes to detect structural effects unrevealed by XRD.…”