The Crystal Structure of MnF3 Revisited

. We correct the crystal structure of MnF 3 , of which the space group was reported as monoclinic C 2/ c (no. 15) with a = 8.9202, b = 5.0472, c = 13.4748 Å, β = 92.64°, V = 606.02 Å 3 , Z = 12, mS 48, T not given, likely 298 K. In the structure model proposed here, we use a unit cell of one third of the former volume. The ruby red crystals of MnF 3 were synthesized by a high-pressure/high-temperature method, where MnF 4 was used as a starting material. As determined on a single


Introduction
Manganese(III) fluoride is a ruby red colored compound, which was first mentioned in 1867. [1]The first positive proof that pure MnF 3 was synthesized was given in 1900 from Moissan. [2][4] In 1957, Hepworth and Jack published some details about the crystal structure. [5,6]From powder X-ray data they could deduce a monoclinic cell in space group C2/c, which contained twelve Mn and 36 F atoms.Additionally, they reported the coordinates of all atoms.In 1993, Schrötter and Müller reacted a prefluorinated mixture of SmF 3 and NH 4 MnF 3 at 250 °C in an autoclave filled with 100 % F 2 at 200 bar.The obtained violet product was sealed in a gold ampoule and heated up to 650 °C for 21-28 days and cooled down with 40 K per day.The ruby red crystals of MnF 3 were collected and the crystal structure was determined on the basis of the previously described one. [7]According to Müller and co-worker, MnF 3 crystallizes with twelve formula units in the unit cell (a = 8.9202, b = 5.0472, c = 13.4748Å, β = 92.64°,V = 606.02Å 3 , Z = 12, mS48, T likely 298 K) with two crystallographically independent manganese and five independent 1 crystal, MnF 3 crystallizes in the monoclinic space group I2/a (no.15) with a = 5.4964 (11), b = 5.0084 (10), c = 7.2411 (14) Å, β = 93.00(3)°,V = 199.06(7)Å 3 , Z = 4, mS16, T = 183(2) K.The crystal structure of MnF 3 is related by a direct group-subgroup transition to the VF 3 structure-type.We performed quantum chemical calculations on the crystal structure to allow the assignment of bands of the obtained vibrational spectra.fluorine atoms. [7]Both Mn atoms were coordinated by six fluorine atoms in shapes of distorted octahedra due to the Jahn-Teller effect as the electron configuration of the Mn 3+ cations is [Ar]3d 4 .The coordination octahedra were connected through corners to each other and built a three-dimensional network.
[7] Here, we present the crystal structure of MnF 3 containing only one symmetry independent Mn and only two symmetry independent F atoms.

Results and Discussion
We synthesized MnF 3 by compressing MnF 4 , on which we will report in the near future, [8] inside a platinum capsule using a multi-anvil press (for details see Experimental Section).Likely, MnF 4 decomposed thermally [9] to MnF 3 due to the reaction temperature of 500 °C and the released fluorine reacted with the capsule.We obtained ruby red single crystals of MnF 3 beside ruby red powder (see Figure 1) and determined the crystal structure from X-ray diffraction data.

Crystal Structure
The volume of our unit cell is with 199.06(7)[7] The lattice parameters determined on a single crystal are a = 5.4964 (11), b = 5.0084 (10), c = 7.2411(14) Å, β = 93.00(3)°,V = 199.06(7)Å 3 , Z = 4, mS16, T = 183(2) K. Details of the structure solution and refinement are given in Table 1 and the atomic coordinates are given in Table 2. Seven crystallographically independent atoms were required in the previous structure model, whereas in ours only three symmetry independent atoms are necessary (Table 2).The previous crystal structure can be transformed from mC to mI via the basis transformation a I = -1/ Comparing the atomic coordinates of the previous structure model (Table 3) with those of the new structure model proposed here (Table 2), one recognizes that the previous Mn(1) and Mn(2) atoms are superimposed by the lattice transformation.Also, the atomic coordinates of the previous atoms F(1) and F(5), as well as those of F(2), F(3), and F(4) become alike and correspond to the atoms F(1) and F(2) of the new structure model, respectively.It is interesting to note that this relation has already been recognized by Hepworth and Jack [5] and it is unclear why neither they, nor the later report on the single crystal structure, used the smaller unit cell for the structure description. [7]Thus, the crystal structure of MnF 3 is best described with only three symmetry independent atoms in the smaller mI unit cell instead of seven in the larger mC unit cell.

ARTICLE
In the crystal structure of MnF 3 , the Mn(1) atom is surrounded by six fluorine atoms in an octahedron-like shape (Figure 2).The Mn-F distances are 2 ϫ 1.8124( 6) Å [Mn(1)-F(2)], 2 ϫ 1.9037(4) Å [Mn(1)-F(1)], and 2 ϫ 2.0806(6) Å [Mn(1)-F(2)] and are given in Table 4.The Mn-F distances are comparable to those reported by Hepworth and Jack (1.79, 1.91, 2.09 Å) as well as those by Schrötter and Müller (1.8173, 1.9124, 2.0878 Å). [5,7] The distortion of the coordination polyhedron around the Mn atom from O h symmetry may be explained with the Jahn-Teller effect due to the [Ar]3d 4 electronic configuration of the Mn 3+ ion.The distorted octahedron is vertex-linked to six other MnF 6 octahedra so that all fluorine atoms are coordinated by two Mn atoms.Thus, a three-dimensional network results, which can be described with the Niggli formula 3 ϱ [MnF 6/2 ].The crystal structure can be derived from the VF 3 structure-type as shown in the Bärnighausen-tree in Figure 3.
For comparison with the previously reported Coulomb component of the lattice energy of the previous MnF 3 structure model, we carried out MAPLE calculations.The Madelung constant of the current structure model is 8.4054 compared with 8.3311 for the previous one. [7]The Coulomb component of the lattice energy is now 6442 kJ•mol -1 and, as may be expected, quite similar to the previously obtained 6437 kJ•mol -1 . [7]The Coulomb energy is in good agreement with the result (6427 kJ•mol -1 ) obtained from the calculated lattice energies of NaMnF 4 (7474 kJ•mol -1 ) and NaF (1047 kJ•mol -1 ) and deviates only 0.2 %. [11,12] Table 5 lists the motifs of mutual adjunction, effective coordination numbers (ECoN), mean fictive ionic radii (MEFIR), and the calculated charge distribution.Due to the Jahn-Teller distortion and the relatively large Mn(1)-F(2) distance of 2.0806(6) Å, the ECoN values for these two atoms differ slightly from the above described coordination number.The CHARDI calculations show a charge of approximately -1 for the fluorine atoms and, as expected, +3   for the manganese atom.Thus, the assignment of the oxidation states -1 and +3, respectively, is supported.We also carried out quantum chemical calculations at the DFT-PBE0/TZVP level of theory (for details see Experimental Section) for the crystal structure and obtained optimized lattice parameters a = 5.558, b = 5.075, c = 7.320 Å, β = 92.58°,V = 206.26Å 3 , Z = 4 (T = 0 K).The calculated lattice parameters agree well with the experimentally obtained ones and are only slightly larger, which leads to a volume increase of the unit cell of circa 4 %.The calculated atomic positions are given in Table 6 and deviate only slightly from the experimentally observed positions (see Table 2).The calculated Mn-F distances are 1.82, 1.92, 2.11 Å and agree well with the respective, experimentally observed atom distances of 1.8124(6), 1.9037(4), 2.0806(6) Å.

Powder X-ray Diffraction
The powdered sample was transferred into a silica-capillary and a powder X-ray diffraction pattern was recorded at 293 K. Bärnighausen-tree which shows the relationship between the VF 3 structure and the MnF 3 structure.Data for VF 3 are from the literature. [10]e pattern is shown in Figure 4 and only small impurities of boron nitride, from the crucible that contains the Pt capsule, could be detected.The Rietveld refinement shows that at room temperature the same crystal structure is present as at 183 K.The refinement details are given in Table 7 and the refined atom positions and bond lengths are available from Table 8 and Table 9.

Vibrational Spectroscopy
The experimentally obtained and theoretically calculated vibrational spectra are in good agreement.The recorded IR spectrum is not of the highest quality due to the small amount of the sample.The broad band at 546 cm -1 can be interpreted as an overlap of the two calculated bands at 592 and 515 cm -1 (Figure 5).In the measured Raman spectrum, all bands except those at 200 and 175 cm -1 agree well with the calculated one (Figure 6).These two bands overlap and show a broad band in the recorded spectrum.The band assignment is given in Table 10.

Conclusions
Ruby red crystals of MnF 3 were synthesized by a high-pressure/high-temperature method by decomposition of MnF 4 .

Experimental Section
Synthesis of MnF 4 : The MnF 4 that was used to obtain single crystals of MnF 3 was synthesized by the direct fluorination of MnF 2 in a stream of 10 % (v/v) F 2 (Solvay, Ͼ 99.0 %) in argon (5.0, Praxair) with a flow of approximately 2 mL•min -1 . [13,14]At the synthesis temperature of 550 °C, MnF 4 sublimed and was collected at a water-cooled, goldcoated cooling finger that was made out of Monel.The greyish product Synthesis of MnF 3 : Single crystals of MnF 3 were synthesized via a high-pressure/high-temperature approach.As starting material, MnF 4 was used and transferred into a platinum capsule (99.95 %, Ögussa, Vienna, Austria).Subsequently, the platinum capsule was inserted into a boron nitride crucible (Henze Boron Nitride Products AG, Lauben, Germany), which was placed into an 14/8 assembly.Handling of the starting material, as well as the preparation of the assembly was carried out under argon atmosphere (MBraun Inertgas-System GmbH, Germany).The 14/8 assembly was placed in the center of eight tungsten carbide cubes (Hawedia, Marklkofen, Germany), which transferred the pressure from a 1000 t multi-anvil press utilizing a Walker-type module (Max Voggenreiter GmbH, Mainleus, Germany) to the sample.[17] MnF 4 was compressed to 5.5 GPa within 140 min and kept at that pressure during the heating program.Within 10 min, the sample was heated to 500 °C, kept at that temperature for 30 min, and subsequently cooled to 200 °C within 60 min.Afterwards the heating was switched off and the sample was quenched to room temperature.Upon completion of the heating program, the sample was decompressed within 330 min.Recovery of the sample was carried out in an inert gas atmosphere.Single Crystal X-ray Diffraction: A single crystal of the sample was isolated under perfluoropolyalkylether using a polarization microscope.The crystal was mounted onto a Bruker D8 Quest diffractometer (BRUKER, Billerica, USA).The measurement was carried out at 183(2) K and a molybdenum radiation source (Mo-K α radiation, λ = 0.7107 Å) was used.The diffractometer is equipped with a Photon 100 detector and an Incoatec microfocus X-ray tube (Incoatec, Geesthacht, Germany).Intensity data was corrected by a multi-scan absorption correction using SADABS 2014/5. [18]2][23] Powder X-ray Diffraction: The powder sample was filled into a 0.3 mm silica capillary and measured in Debye Scherrer mode.For the analysis, a Stoe Stadi P diffractometer (Stoe, Darmstadt, Germany) was used in transmission geometry.The diffractometer operates with Mo-K α1 radiation (λ = 0.7093 Å) and a Ge(111) primary beam monochromator.Diffraction data were recorded in a range of 2.0 to 40.4°2 θ with a step size of 0.015°by a Mythen 2 DCS4 detector.The Rietveld refinement was performed with Jana2006. [24]ystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK.Copies of the data can be obtained free of charge on quoting the depository number CCDC-1979304 and CCDC-1979305 (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).

ARTICLE
IR Spectroscopy: Infrared spectra were measured on a Bruker Alpha Platinum FT-IR spectrometer using the ATR Diamond module with a resolution of 4 cm -1 .The spectrometer was located inside a glovebox under argon (5.0, Praxair) atmosphere.For data collection, the OPUS 7.2 software was used. [25]man Spectroscopy: The Raman spectrum was recorded with a Confocal Raman Microscope S+I MonoVista CRS+, using the 633 nm excitation line of an integrated diode laser (resolution Ͻ 1 cm -1 ; range 60 to 2700 cm -1 ).[26] The sample was measured inside a glass vessel.

Quantumchemical Calculations:
The structural properties of MnF 3 were investigated using the CRYSTAL17 program package. [27]Both, the atomic positions as well as the lattice parameters were fully optimized using the PBE0 hybrid density functional method. [28,29]Triplezeta-valence + polarization (TZVP) level basis sets derived from the molecular Karlsruhe basis sets, [30] were applied (see supporting information for full basis set details). [31,32]For spin-polarized calculations, an antiferromagnetic ordering of Mn III atom spins was derived by low-

Figure 1 .
Figure 1.Picture of ruby-red crystals and powder of MnF 3 under a visible light microscope.
3 a c + 1/3 c c , b I = Z.Anorg.Allg.Chem.2020, 1-8 www.zaac.wiley-vch.de© 2020 The Authors.Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2 b c , c I = -2/3 a c -1/3 c c and an origin shift of +1/4 for x, y, and z (the indices I and c correspond to the mI and mC unit cells, respectively).A comparison of atomic coordinates of the previous crystal structure before and after transformation to mI is shown in Table 3.The lattice parameters after transformation are a I = 5.4996, b I = 5.0472, c I = 7.2855 Å, β I = 92.69°andare comparable to the lattice parameters determined on the single crystal.

Figure 2 .
Figure 2. Crystal structure of MnF 3 .The displacement ellipsoids are shown at the 70 % probability level at 183 K.

Z 3 Figure 3 .
Figure3.Bärnighausen-tree which shows the relationship between the VF 3 structure and the MnF 3 structure.Data for VF 3 are from the literature.[10]

Figure 4 .
Figure 4. Observed (black) and calculated powder X-ray pattern (red) of MnF 3 after Rietveld refinement.The calculated reflection positions are indicated by the vertical bars below the pattern.The curve at the bottom represents the difference between the observed and the calculated intensities.The greyish region was excluded due to impurities of boron nitride from the crucible.R p = 7.17, R wp = 9.56 (not background corrected R values), S = 1.38.

Table 1 .
Selected crystallographic data and details of the structure determination of MnF 3 .

Table 2 .
Atomic coordinates and equivalent isotropic displacement parameters U iso for MnF 3 .

Table 4 .
Selected interatomic distances d and their multiplicities m for MnF 3 .

Table 6 .
Quantum chemically calculated atom positions for MnF 3 .Thus, the crystal structure of MnF 3 is herewith corrected and its symmetry relation related by a direct group-subgroup transition to the VF 3 structure-type is shown.We performed quantum chemical calculations on the crystal structure to allow for the assignment of bands in the recorded IR and Raman spectra.

Table 7 .
Selected crystallographic data and details of the Rietveld refinement of MnF 3 .

Table 8 .
Atomic coordinates and isotropic displacement parameters U iso for MnF 3 from Rietveld refinement at 293 K.

Table 9 .
Selected interatomic distances d and their multiplicities m for MnF 3 from Rietveld refinement at 293 K.