Structural and magnetic characterization of the elusive Jahn-Teller active NaCrF3
Fabian L. M. Bernal, Jonas Sottmann, Oystein S. Fjellvaag, Christina Drathen, Wojciech A. Slawinski, Ole Martin Loevvik, David S. Wragg, Helmer Fjellvaag
SStructural and magnetic characterization of the elusive Jahn-Teller active
NaCrF Fabian L. M. Bernal, Jonas Sottmann, David S. Wragg, and Helmer Fjellv˚ag
Chemistry Department and Center for Material Science and Nanotechnology, University of Oslo, NO-0315, Norway
Øystein S. Fjellv˚ag
Department for Neutron Materials Characterization,Institute for Energy Technology, PO Box 40, NO-2027, Kjeller, Norway
Christina Drathen
ESRF- The European Synchrotron, 71, Avenue des Martyrs, Grenoble 38043, France andCurrent Address: Bruker AXS, ¨Oestliche Rheinbrueckenstr. 49, 76187 Karlsruhe, Germany.
Wojciech A. S(cid:32)lawi´nski
Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland andISIS Facility, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire OX11 0QX, U.K.
Ole Martin Løvvik
Department of Physics, University of Oslo, NO-0315, Norway
We report on the structural and magnetic properties of the elusive Jahn-Teller active compoundNaCrF , for first time synthesized in large quantities allowing detailed characterization. The crystalstructure of NaCrF is initially described from a DFT model which helped serve as basis for indexingand structure determination confirmed by high-resolution synchrotron X-ray diffraction experiments.NaCrF adopts the triclinic space group P ¯1 (isostructural with NaCuF ). Magnetometry studiesat low temperature show that NaCrF is a weak antiferromagnet, Curie temperature θ = − T N = 21 . µ = 4 . µ B in accordancewith the theoretical S = 2. Field-dependent measurements between 2 and 12 K unveil the onsetof metamagnetic behavior. Our experiments revealed a weakly canted A -type magnetic structureobserved by neutron powder diffraction, with a magnetic propagation vector (1 / , / ,
0) and amagnetic moment of 3.51 µ B at 1.5 K. Our results shed further light on the Jahn-Teller effects andstrong correlations as a function of A -ion size in the family A CrF . I. INTRODUCTION
The cooperative Jahn-Teller [1] (JT) effect is com-monly ascribed to structural distortions caused by thecoupling between electronically degenerate orbital statesof transition metal ions and their normal modes of vibra-tion. This coupling results in reduction of the symmetryof the bonding environment around the JT ion to lowerthe total energy. JT- active perovskite-type materialsare at the center of intensive research within the mate-rial science community for their wide range of physicalproperties and structural diversity. Superconductivity,colossal magnetoresistance (CMR) and polaron confine-ment are known for these compounds, giving applicationsin information storage and spintronics [2–4]. Perovskiteshave the chemical formula
ABX . JT-active ions suchas Mn , Cr and Cu (with electron configurations3 d , 3 d and 3 d , respectively) can occupy the octahe-dral B -site (e.g., [MnO ]). The octahedra are linked bytheir vertices forming sets of B - X - B bond angles ξ ◦ (de-fined here as the perovskite angle). The electron-phononcoupling (ie., E ⊗ e ) causes octahedral distortion whichfavor the occupation of one of the originally degenerateorbital states. At the same time, the choice of orbitalstate induces an orbital ordering (OO).The best known JT-active oxide perovskite is lan- thanum manganite LaMnO , a parent compound for sev-eral derivative crystalline compounds exhibiting CMR.An essential feature of the manganites is the role playedby the atom occupying the A − site in influencing defor-mations of the perovskite type structure, and therebyalso the JT-structural distortions, leading to a rich di-versity of spin, orbital and charge orderings. In fluoridesJT-ions are well known for showing interesting phenom-ena under external stimuli. Alkali ternary manganese(III) fluorides with formula A x MnF x (with A =Na, K,Cs) show significant structural diversity, adopting 0-,1-and 2- dimensional vertex-sharing arrangements of theoctahedral units depending on the value of x . [5, 6].3-dimensional vertex sharing high spin 3 d elec-tronic configuration can form perovskite-type fluoridestructures (fluoroperovskites). These include ternarychromium (II) fluoroperovskites with formula A CrF (where A = alkali metals). KCrF has two structural-phase transitions at elevated temperatures: I /m → I /mcm at 250 K and I /mcm → P m m at 973 K[7, 8], and theoretical studies have associated the metalto insulator transition with the onset of the tetragonal-to-cubic phase transition [9]. In addition, KCrF dis-plays a rich magnetic phase diagram at low tempera-tures: an incommensurate antiferromagnetic ordering at79.5 K, an incommensurate-to-commensurate antifferro- a r X i v : . [ c ond - m a t . m t r l - s c i ] J a n INTRODUCTION I n t e n s i t y ( a . u . ) Q ( ˚A − ) . . . . I n t e n s i t y ( a . u . ) Q (˚A − ) ( a ) ( b ) FIG. 1. ( a ) Final observed (black dots), calculated (red line) and difference (blue line) synchrotron X-ray powder diffractionprofiles ( λ = 0 . at 298 K ( a = 5 . b = 5 . c = 8 . α = 90 . ◦ , β = 92 . ◦ , γ = 86 . ◦ ). R wp = 11 . R exp = 5 . viewed along the [110]-direction.FIG. 2. ( a ) ls - bond length motif of the four crystallographic chromium sites of NaCrF . Cr1, Cr2, Cr3 and Cr4 are representedhere with blue, cyan, purple and green spheres, respectively. The Cr-F-Cr bond angles are labelled: ξ ◦ : ξ ◦ = Cr1 − F1 − Cr2, ξ ◦ = Cr2 − F6 − Cr4, ξ ◦ = Cr4 − F3 − Cr3, ξ ◦ = Cr3 − F5 − Cr1. ( b ) Packed crystal structure of NaCrF with red and blueplanes marking layers of Cu in which the ls-bond length motif (a) is rotated by 90 ◦ relative to the adjacent layers. The midplane (gray) cuts through the connecting m -bond distances, and represents the stacking directions of the canted AOO. Theunit cell is shown in pale grey. magnetic transition at 45.8 K, and below 9.5 K a cantedantiferromagnetic ordering with weak ferromagnetic in-teractions [10]. Further studies of the role played by the A − site in A CrF are currently lacking despite the inter-esting phase diagram of KCrF . The main reason forthis is the lack of proper synthetic protocols for the re- actions of Cr compounds with fluorides. The synthesisof NaCrF has until now proved extremely problematicdue to the sensitivity of Cr to oxidation. None of thesynthesis routes described by Deyrup and Earnshaw etal. resulted in NaCrF [11, 12]. To the best of our knowl-edge, the only evidence of the preparation of NaCrF was2 I EXPERIMENTAL AND COMPUTATION SECTION given by the work of Vollmer and UV-vis spectroscopystudies performed by Kruger [13, 14]. Our new reliablesynthetic protocol for NaCrF opens up further possi-bilities for synthesizing analogous materials of interestfor information storage technologies, with rich states ofmatter and novel physical phenomena to appear in sto-ichiometric and non-stoichiometric modifications of the A - and B sites in the A CrF family. We report for thefirst time the crystal and magnetic structure of the elu-sive JT- active compound, NaCrF prepared by a novelwet-chemistry method. These results are complementedby magnetometry studies. II. EXPERIMENTAL AND COMPUTATIONSECTIONA. Synthesis of
NaCrF Chromium (II) acetate dihydrate(Cr (CH CO ) (H O) ) (0.5g 1.33 mmol) and 2mL of degassed water is loaded into a 85 mL polycar-bonate (PC) vial closed with a septum under a constantflow of Ar. NaHF (0.45 g 5.45 mmol) is dissolved in 10mL deoxygenated water in a second PC vial under Arby heating to above 50 ◦ C. The hot-solution of NaHF is carefully and quickly injected into the vial containingCr (CH CO ) (H O) under vigorous stirring. NaCrF precipitates after few seconds. The supernatant isdecanted off and the solid product is washed once with2 mL 50:50 deoxygenated water and methanol solution,and subsequently with 5 mL deoxygenated methanol.Finally, the product is vacuum-dried overnight to yieldair-stable NaCrF . B. Computational simulations
For the structural phase model of NaCrF , densityfunctional theory (DFT) was applied using the Vienna Ab initio
Simulation Package ([15, 16]), with the PBEgeneral gradient approximation (GGA) [17]. The cutoffenergy of the plane wave basis set expansion was set to atleast 450 eV. The density of k points was determined bya maximum of 0.25 ˚A − . The structure was relaxed withremaining forces below 0.05 eV/˚A using a quasi Newtonmethod. C. Synchrotron X-ray diffraction
High- resolution synchrotron powder X-ray diffrac-tion (HR-SPXRD) experiments were conducted at ID22beamline of the European Synchrotron (ESRF), Greno-ble, France where the diffraction patterns were recordedusing a wavelength of λ = 0 . has been refinedusing TOPAS v5 (Bruker AXS) [18]. The initial model was obtained by DFT minimisation of a symmetry free(space group P
1) triclinic model based on the crystalstructure of NaCuF [13, 19] with Cr replacing Cu. Thismodel was refined against the HR-PXRD data to obtainthe correct lattice parameters and crystallite size peakbroadening. The model was then processed using theADDSYMM routine in PLATON [20] to determine thecrystallographic symmetry. The new model, now in spacegroup P ¯1, was refined against the HR-SPXRD data.Scale, lattice parameters, 13 term Chebyshev polyno-mial background function, Gaussian crystallite size andstrain and Lorentzian strain broadening terms (funda-mental parameters peak shape) and all Na and F atomiccoordinates and isotropic displacement parameters wererefined. Atoms of the same type (Na, Cr and F) wereconstrained to have identical isotropic thermal parame-ters. D. Magnetic characterization
Magnetometry experiments were performed on aQuantum Design 14 T Physical Property MeasurementSystem (PPMS). Temperature dependent DC magneticsusceptibility χ ( T ) measurements were conducted duringheating from 4 to 300 K in zero-field-cooled field-cooledmode (ZFC-FC). The magnetic susceptibility is calcu-lated by χ = M/ H where M is the magnetization givenin emu mol − and the magnetic field H = 1 T. Isothermalfield dependent measurements ( M ( H )) were collected at2 K, and half-loop isothermal measurements at 4, 12 and23 K up to 14 T. E. Neutron Powder Diffraction
Neutron powder diffraction (NPD) patterns were col-lected at ISIS Neutron and Muon Source (UK) by usingthe WISH long-wavelength diffractometer [21]. The sam-ple was placed in thin wall vanadium container (7 mmin diameter) and cooled down to 1.5 K. The measure-ments were performed while heating from 1.5 K up to127 K at several temperature steps. The raw data wasintegrated by using the Mantid suite [22] and analysedusing the Jana2006 software [23]. The structure refine-ment was performed using data from the four detectorbanks with highest resolution. The lowest resolutionbank was discarded as the it contained no informationnot present in the other detector banks. The background(10 term Chebyshev polynomial), peak-shape, isotropicthermal displacement parameters for each element type,lattice parameters and angles, and scale parameters wererefined. The superspace formalism for commensuratemagnetic moment modulation was used for the magneticstructure description. The magnetic form factor of Cr was employed in the refinements. Spherical coordinateswere used to refine the magnetic moments. The fourCr sites were constrained to have a single magnetic3 Neutron Powder Diffraction III RESULTS
TABLE I. Structural parameters from Rietveld refinement of HR-SPXRD dataset of NaCrF at ambient conditions. l, m and s are long, medium and short bond distances, respectively. Space Group : P ¯1 a d Cr = 78 . × − ˚A b Octahedral distortions : ∆ d Cr = 59 . × − ˚A c d Cr = 72 . × − ˚A α ◦ ∆ d Cr = 76 . × − ˚A β ◦ γ ◦ V R wp , R wp − bkg R p , R p − bkg R exp , R exp − bkg χ ◦ of independent parameters 53Restrains, constrains 0, 3Rigid bodies 0 Z Selected Bond Distances
Cr1-F Cr2-F Cr3-F Cr4-F l × m × s × moment magnitude. Polar angles ( ϕ and ϕ ) were re-fined for Cr1 and Cr3, with the polar angles of Cr2 andCr4 constrained to values of 180+ ϕ and 180+ ϕ respec-tively. Independent azimuthal angles were refined for allCr sites. These constraints are summarised in Table II.At 17 and 19 K, the azimuthal angle of Cr1 and Cr3,and Cr2 and Cr4 were constrained to be equal. Also at19 K the polar angle for Cr1 and Cr3 was fixed at valuesobtained at 17 K. This is due to the low intensity of mag-netic Bragg reflections near the N´eel temperature and fitinstability. III. RESULTSA. Crystal structure determination
To the best of our knowledge, no reliable synthesis pro-tocol for NaCrF has previously been described, and thecrystal structure of the compound has not been describedin detail. The air-sensitivity of Cr is intrinsically dif-ficult to combine with fluorine chemistry. Conventionalsolid-state methods are therefore unsuitable, so we de-veloped a novel own wet-chemistry protocol. Using thiswe can work under conditions where Cr is stable andobtain pure, single phase NaCrF in large quantities.We expect that other fluorides can be prepared usingthe same approach. Results of the Rietveld refinementagainst HR-PXRD data are shown in Figure 1 and TableI. The plot, fitting statistics and bond lengths and angles obtained indicate that the model is an excellent repre-sentation of the real structure. Table I and Table III SIshow the structural parameters and atomic coordinates,as obtained from Rietveld refinements.The Cr cations occupy four non-equivalent crystallo-graphic sites. Although the structure is triclinic, the celledges and angles are close to those of a tetragonal unitcell. The corresponding approximated tetragonal distor-tion, c/a = 1 .
48, corresponds to the (stretching) normalmode Q of the octahedral units CrF − . Figure 1 ( b )shows the crystal structure of NaCrF with vertex sharedoctahedral units (blue) with Na + ions (red) in interstices.We calculate the octahedral distortion according to theequation ∆ d = 1 / (cid:80) n =1 | l i − l av | /l av where l i are theindividual bond distances of the octahedral unit, and l av is the average bond distance. Figure 2 ( a ) shows the l and s bonds building a tilted ls -motif connected throughthe Cr-F-Cr angles ξ ◦ i . Figure 2 ( b ) shows the ls - mo-tif stacking along [110], with the bonding-motif rotated90 ◦ (represented here as blue and red planes to indicatethe 90 ◦ rotation), whereas the m bonds propagate aboveand below the (001)-plane in the [1¯10]-direction. Thefour CrF − distortions can be found in Table I. The non-equivalent octahedra are sharply tilted, corresponding tothe Glazer notation a − b − c − [24].4 Magnetic characterization and Neutron diffraction studies III RESULTS
TABLE II. Parameters used to describe the magnetic structure of NaCrF from Rietveld Refinements at 1.5 K in sphericalcoordinates with a modulation vector of k = (1 / , / ,
0) in superspace group P ¯1( αβ ϕ and azimuthal ϑ angles were given degrees of freedom.Atom Label Atom position Magnetic moment Polar angle Azimuthal angleCr1 (1/2 0 0) M = 3.520(6) ϕ = 223 . ϑ = 38 . . ϕ + 180 = 43 . ϑ = 128 . . ϕ = 223 . ϑ = 55 . . ϕ + 180 = 43 . ϑ = 135 . . . . . . . . . .
08 0 50 100 150 200 250 300020406080100120140 χ ( e m u · m o l − O e − ) χ − ( e m u − m o l · O e ) T (K) ZFCFC 02000400060008000100001200014000 0 2 4 6 8 10 12 14 M ( e m u · m o l − ) H (T) − − − − H (T) ( a ) ( b ) FIG. 3. ( a ) ZFC-FC temperature dependency of the magnetic susceptibility measured χ ( T ) at H = 1 T (left axis), and theirinverse χ − (right) with the linear regression at θ = − b ) Isothermal half-loop magnetization curves magnetic field ( M ( H ))applied from 0 to 14 T and then back to 0 T at 2, 4, 12 and 23 K. The inset is the full M ( H ) hysteresis loop at 2 K to showthe symmetry at the negative quadrant. B. Magnetic characterization and Neutrondiffraction studies
DC temperature dependent magnetic susceptibility ex-periments on a polycrystalline sample of NaCrF between4 and 300 K show a kink corresponding to the onsetof long-range antiferromagnetic ordering on reaching theN´eel temperature at T N = 21 . a ). Further-more, an upswing at around 9 K reveals the emergenceof a weak ferromagnetic component at lower tempera-ture. The Curie - Weiss (CW) law is applicable for thetemperature range 300 - 24 K. The fit to the inverse sus-ceptibility curves 1 /χ show a linear behaviour where thecalculated paramagnetic moment of µ eff = 4 . µ B is ingood agreement with the theoretical value of the spin-only configuration S = 2 for Cr . The Curie temper-ature is θ = − which displays weak ferromagnetic interactions θ = 2 . A -site is of paramount importance infinetuning the magnetic exchange interactions.Magnetic field dependent isothermal M ( H ) half-loops(forward and reverse field application) for NaCrF arepresented in Figure 3 ( b ). These loops were measured at2, 4, 12 and 23 K in applied magnetic fields up to 14 T. At23 K the half-loop shows almost linear behaviour, never-theless with a small hysteresis indicating the presence offerromagnetic interactions. The half-loop at 12 K retainsthe hysteresis with additional signatures of metamagnetictransitions identified by a clear S-shape occuring between6 and 8 T.The metamagnetic transition becomes more pro-nounced with decreasing temperature as observed at 4and 2 K. At 4 K the hysteresis is at its widest. How-ever, as shown by complete isothermal loop in the insetto Figure 3 ( b ), there is no longer any hysteresis at 2 K.This means that the ferromagnetic components are sup-pressed by lowering the temperature. In order to identifythe point of metamagnetic transition we calculated the5 Magnetic characterization and Neutron diffraction studies III RESULTS . . .
53 0 2 4 6 8 10 12 14 − d M / d H H (T) FIG. 4. First derivative dM/dH of the isothermal half-loops at 2, 4, 12, and 23 K. The upswing is represented withconnected line-guides. A metamagnetic transition occurs at8 T in the half-loops at 2 and 4 K. Up and downswing dataare emphasized by arrows for the 4 T data. first derivative d M /d H of the magnetization M with re-spect to applied field H as shown in 4. An emergent peakat 8 T is observed below T N with well-defined singulari-ties at 4 and 2 K.The derived synthesis protocol made it possible toprepare large scale samples with high purity and crys-tallinity, well suited for detailed neutron diffraction stud-ies. We conducted powder neutron diffraction experi-ments between 1.5 and 127 K to study the structural andmagnetic changes in NaCrF above and below the Neltemperature. Visual inspection of the neutron diffractionpatterns reveals a transition originating from the order-ing of magnetic moments in the proximity of the Neltemperature, e.g. a strong reflection due to magnetic or-dering occurs at d = 7 .
63 ˚A, Figure 5 ( a ). The additionalmagnetic reflections were indexed in a supercell with dou-bled a - and b -unit cell parameters (2 a × b × c ), corre-sponding to a propagation vector of k = (1 / , / ,
0) formodulation of the magnetic structure. To describe themagnetic structure in detail, we use magnetic superspacegroup formalism. The magnetic structure is describedin the superspace group P ¯1( αβ
0) with a commensuratemodulation vector (1 / , / , adopts a canted A -type antiferromagneticstructure where chromium has an ordered magnetic mo-ment of µ = 3.520(6) µ B at 1.5 K, Figure 5 ( b ). Themagnetic moments of chromium atoms are ferromagnet-ically ordered in the (1¯10)-planes, i.e. along [110] and[001]. We observe canting in the (1¯10)-plane. This can-cels out within the magnetic unit cell due to AFM stack-ing along [1¯10], which are shown by red and blue coloredplanes in Figure 5. In the triclinic structure, the mag-netic moments of chromium atoms point almost directly through the middle of the edge between the equatorialand axial fluorine atoms of the JT distorted CrF oc-tahedra. Consequently, the magnetic moments formingchains in the [11¯1]-direction. The canted A-type antifer-romagnetic structure is in agreement with the structural ls -motif corresponding to ferromagnetic interactions inthe (1¯10)-plane and antiferromagnetic interactions per-pendicular to it. The a − b − c − tilts reduces the 3 d p over-lap and weaken thereby the superexchange interactionsin the presence of Na + ions.When the direction of the magnetic moments of thefour chromium sites were constrained to be either par-allel or anti-parallel, several weaker reflections originat-ing from magnetic ordering at i.e. d = 5 .
64 and 5 . d = 5 .
64 and 5 .
86) clearly showsthat the four chromium sites have slightly different cant-ing of their magnetic moments. These subtle aspects ofthe magnetic structure could only be described due thehigh resolution and excellent signal to noise ratio of theneutron diffraction data obtained from the WISH instru-ment at ISIS (UK). The magnetic structure of NaCrF is accurately described at 1.5 K, and details are given inTable V.The evolution of the magnetic structure was furtherstudied below the N´eel temperature ( T N = 21.3 K). Inaccordance with the spin only approximation ( µ eff =4.47 µ B in the paramagnetic regime, see above), the or-dered magnetic moment of chromium is 3.520(6) µ B at1.5 K. The slightly lower experimental value comparedto the theoretical value (of 4 µ B ) is attributed to hy-bridization in the chemical bonding which effectively re-duces the number of electrons contributing to the mag-netic moment. The ordered magnetic moment steadilydecreases from µ = 3.520(6) µ B at 1.5 K with increasingtemperature up to the Nel temperature at 21 K wherethe magnetic ordering disappears (see Fig. 6( a )).The polar angle difference between the magnetic mo-ments of Cr1 and Cr3 is fairly constant (see Fig. 6( b, c )).The two pairs of azimuthal angles (Cr1, Cr3) and (Cr2,Cr4) show similar values, but cannot be constrained tobecome equal without worsening the fit. However, at 17and 19 K the azimuthal angle ϑ of Cr1 and Cr3, and Cr2and Cr4 could successfully be constrained. For the at19 K data all angle values were frozen at values obtainedat 17 K due to fit instability. The antiferromagnetic or-dering at the Nel temperature is associated with a signifi-cant thermal contraction of the lattice upon cooling, Fig-ure 7. At the ordering temperature, changes in the tiltingof the octahedra is revealed by the analyzed changes inthe perovskite bond angles. These observations indicatea clear magnetostructural coupling in NaCrF .6 V DISCUSSION
FIG. 5. ( a ) Rietveld refinements of NPD dataset of NaCrF at 1.5 K from detector bank 2 (lowest resolution bank used inrefinements) showing the peak at 7.63 ˚A with the inlet showing small peaks of the magnetic phase. The purple and green ticscorrespond to the crystal and magnetic phase, respectively. ( b ) Magnetic structure of NaCrF in the [1,-1,0]-direction. Theanti-parallel alignment of the spins is represented by the blue-red sequence. Blue, cyan, purple and green atoms correspond toCr1, Cr2, Cr3 and Cr4 respectively. − − − − − − − . . . . . . . . ( ϑ ◦ ) T (K) Cr Cr Cr Cr ( ϕ ◦ ) Cr Cr ( µ B )( µ B ) ( a )( b )( c ) FIG. 6. ( a ) Magnetic moment of the chromium cations inNaCrF determined by neutron diffraction as a function oftemperature. Temperature evolution of the ( b ) polar and ( c )azimuthal angles. Constraints are described in Table II. IV. DISCUSSION
The reliable new synthesis route for NaCrF allowedus to undertake a detailed study of its structure and magnetic properties for the first time. The JT-activeions Cr of NaCrF occupy four non-identical crystal-lographic sites with different octahedral distortions. Ourresults demonstrate the importance of the ion size atthe A -site in tuning the properties of the JT-active B -site ions. A -site dependent physical phenomena havepreviously been observed in the d isoelectronic low-dimensional manganese (III) fluoroperovskites, wherevariarions in the A -site ion size give rise to rich and in-teresting phase diagrams under external stimuli.A significant feature of NaCrF is its metamagneticsignatures below T N under field dependent measurementsin addition to weak residual ferromagnetic interactions at23 K. The presence of metamagnetism in NaCrF resem-bles in some aspects other known systems with exoticproperties (See Ref. [25], [26], [27]). We believe that thisbehavior is related to correlations between the orbitalstructure and magnetic ordering as discussed by Kugeland Khomskii [28]. The temperature dependent NPDdata reveals a smooth decrease in the unit cell volumeand γ angle above T N , with a rapid collapse at lowertemperatures Figure 7 ( a ). One would expect that the ξ ◦ angle would reduce for all four Cr sites, however,they follow independent patterns as shown in Figure 7( b ). ξ ◦ displays a slight decrease upon cooling while ξ ◦ increases. Perovskite angle reduction further decreasesthe orbital overlap, weakening the magnetic interactionswhile reinforcing Cr-to-Cr interactions. The refined mag-netic moments of Cr ions in NaCrF are in agreementwith NPD studies on KCrF by Xiao et al. [10]. Com-pared to other sodium transition metal fluoroperovskites,NaCrF deviates from the family trend by displaying acanted A − type magnetic ordering compared to the G -types found in NaNiF and NaCoF [29, 30].To further investigate the role of the A -site in A CrF I AKNOWLEDGEMENTS . . . ξ ◦ ξ ◦ ξ ◦ ξ ◦ . . . . . . . . . .
493 5 . . . . . . . . a (˚A) b (˚A)86 . . . . . . . . . .
37 252 . . . . . . γ ( ◦ )Vol (˚A ) ξ ◦ , ξ ◦ ξ ◦ , ξ ◦ T (K) a ( ˚A ) b ( ˚A ) γ ( ◦ ) V o l u m e u . c . ( ˚A ) . . . . . . . . . c ( ˚A ) T (K) ( a )( b )( c ) FIG. 7. ( a ) Temperature dependence of unit cells dimen-sions ( a, b, γ, V) and ξ ◦ angles. ( b ) Temperature depen-dence of the unit cell lattice parameters ( c ) Perovskite an-gles composing the canted ls -motif NaCrF as function oftemperature: ξ ◦ = Cr1 − F1 − Cr2, ξ ◦ = Cr2 − F6 − Cr4, ξ ◦ = Cr4 − F3 − Cr3, ξ ◦ = Cr3 − F5 − Cr1. Vertical dashedline at T N to emphasize the place where the magnetic longrange order sets in. we report elsewhere the use of UV-vis spectroscopy alongwith magnetic characterization studies to compare thelocal electronic structure of the JT-systems KCrF andNaCrF as a function of temperature and magnetic field [31]. Such experiments could provide more detailed in-formation on the strength of the JT-distortions and beused to assess OO-melting points in JT-active fluorides. V. CONCLUSIONS
This work provides compelling evidence of the exis-tence of the JT-active compound NaCrF , and describesits structural and magnetic properties. The successful de-velopment of a reliable and reproducible synthesis route,provided the required materials basis for shedding morelight on the properties of the A CrF family, which previ-ously proved elusive owing to the air-sensitive chemistryof Cr . The structural and magnetic phase diagram ofNaCrF is much simpler than the diverse situation ob-served for KCrF at low temperature. This is due to thesmaller A -ion size causing the NaCrF structure to adoptthe low symmetry space group P ¯1 at relatively high tem-perature. The low symmetry structure is responsible forthe unusual metamagnetic behavior of NaCrF , whichcan be clearly linked to variations in both the crystalstructure (perovskite angles and lattice parameters) andthe magnetic structure (polar and azimuthal angles ofthe magnetic moments), observed in the variable temper-ature NPD data. The new synthesis protocol opens upthe possibility of preparing numerous novel stoichiomet-ric compounds by tuning the A and B sites in fluoroper-ovskites, which in turn may reveal new and interestingphysical properties. VI. AKNOWLEDGEMENTS
We thank Serena Margadonna (Swansea University,Swansea, UK) for granted financing support by the Nor-wegian Research Council (Norges Forskningsr˚ad NFR)project 214260. The U.K. Science and Technology Facil-ities Council (STFC) is thanked for allocating beamtimeat the ISIS Facility. We also thank Pascal Manuel forhelp during the experiment. We aknowldge CRISMATlaboratory (Caen France) for the magnetization measur-aments up to 14 T and Fabien Veillon and Bruno Gonanofor technical and analysis help. We thank Susmit Kumarfor discussions. [1] H.A. Jahn and E. Teller.
P. Roy. Soc A-Math Phy , ,220, 1937.[2] H. M. Rønnow, Ch. Renner, G. Aeppli, T. Kimura, andY. Tokura. Nature , , 1025, 2006.[3] J. M. De Teresa, M. R. Ibarra, P. Algarabel, L. Morellon,B. Garc´ıa-Landa, C. Marquina, C. Ritter, A. Maignan,C. Martin, B. Raveau, A. Kurbakov, and V. Trounov. Phys. Rev. B , , 100403, 2002. [4] G. Alvarez, M. Mayr, A. Moreo, D. Dagotto. Phys. Rev.B , , 014514, 2005.[5] F. Rodriguez and F. Aguado J. Chem. Phys , , 10867,2003.[6] F. Aguado, F. Rodriguez, and P. N´u˜nez. Phys. Rev. B , , 094417, 2007.[7] S. Margadonna and G. Karotsis. J. Amer. Chem. Soc. , , 16436, 2006. I AKNOWLEDGEMENTS [8] S. Margadonna and G. Karotsis.
J. Mater Chem , ,2013, 2007.[9] G. Wang, Z. Li, L. Zheng, and Z. Yang Phys. Rev. B , , 045111, 2011.[10] Y. Xiao, Y. Su, H.F. Li, C.M.N. Kumar, R. Mittal,J. Persson, A. Senyshyn, K. Gross, and Th. Brueckel. Phys. Rev. B , , 094437, 2010.[11] A.J. Deyrup Inorg. Chem , , 1645, 1964.[12] A. Earnshaw, L.F. Larkworthy, and K.S. Patel. J. Chem.Soc. A , 363, 1966.[13] G Vollmer. PhD thesis, Tubingen, 1966.[14] V.D. Oelkrug.
Berichte der Bunsengesellschaft f¨urphysikalische Chemie , , 736, 1966.[15] G. Kresse,J. Furthmuller. Phys. Rev. B. , , 11169, 1996.[16] G. Kresse,G. Hafner. Phys. Rev. B. , , 558, 1997.[17] Perdew, John P. and Burke, Kieron and Ernzerhof,Matthias Phys. Rev. B , , 3865, 1996.[18] A. Coelho J. Appl. Cryst. , , 210, 2018.[19] V. Keiser, M. Otto, F. Binder, and D. Babel. Z. Anorg.Allg. Chem. , , 93, 1990.[20] A.L. Spek. Acta Crystallographica Section D , D65 , 148,2009.[21] L.C. Chapon, P. Manuel.
Neutron News , , 22, 2011[22] O. Arnold and J.C. Bilheux and J.M. Borreguero and A.Buts and S.I. Campbell and L. Chapon and M. Doucetand N. Draper and R. Ferraz Leal and M.A. Gigg and V.E. Lynch and A. Markvardsen and D.J. Mikkelsonand R.L. Mikkelson and R. Miller and K. Palmen andP. Parker and G. Passos and T.G. Perring and P.F. Pe-terson and S. Ren and M.A. Reuter and A.T. Savici andJ.W. Taylor and R.J. Taylor and R. Tolchenov and W.Zhou and J. Zikovsky Nucl. Instrum. Methods Phys. Res.A , , 156, 2014.[23] V. Petricek, M. Dusek, L. Palatinus Z.Kristallogr. Cryst.Mater , , 345, 2014[24] A.M. Glazer. Acta Cryst. , B28 , 3384, 1972.[25] M. Matsubara, C. Martin, B. Vertruyen, A. Maignan,F. Fauth, P. Manuel, V. Hardy, D. Khalyavin, E. Elkaim,and F. Damay.
Phys. Rev. B , , 014409, 2019.[26] Y. Zhou, X. Zhu, S. Huang, X. Chen, Y. Zhou, C. An,B. Zhang, Y. Yuan, Z. Xia, C. Gu, and Z. Yang. Phys.Rev. B , , 205122, 2017.[27] J.P. Bolleta, F. Pomiro, R.D. Sanchez, V. Pomjakushin,G. Aurelio, A. Maignan, C. Martin, and R.E. Carbonio. Phys. Rev. B , , 134417, 2018.[28] K.I. Kugel and D.I. Khomskii Sov. Phys. Usp , , 231,1982.[29] A. Epstein, J. Makovsky, M. Melamud, H. Shaked. Phys.Rev. , , 560, 1968.[30] Z. Friedman, M. Melamud, J. Makovsky, H. Shaked. Phys. Rev. B , , 179, 1970.[31] F.L.M Bernal, F. Lundvall, S. Kumar, P.A. Hansen,D.S. Wragg, O.M. Løvvik and H. Fjellv˚ag. I AKNOWLEDGEMENTS
TABLE III. Atomic positions of NaCrF from HR-SPXRD Rietveld refinement. See Table I for crystal structure details.Atom Multiplicity x y z Occ B iso Na1 2 0.5062(8) 0.5511(7) 0.2370(5) 1 1.53(7)Na2 2 0.9765(8) 0.0563(7) 0.2603(5) 1 1.53(7)Cr1 1 0.5 0 0 1 0.79(3)Cr2 1 0 0.5 0 1 0.79(3)Cr3 1 0.5 0 0.5 1 0.79(3)Cr4 1 0 0.5 0.5 1 0.79(3)F1 2 0.6788(9) 0.2832(9) 0.0558(6) 1 0.98(5)F2 2 0.2019(9) 0.2062(8) 0.9260(6) 1 0.98(5)F3 2 0.7153(9) 0.3238(8) 0.4292(6) 1 0.98(5)F4 2 0.1887(10) 0.1902(9) 0.5503(7) 1 0.98(5)F5 2 0.3801(9) 0.9405(8) 0.2724(7) 1 0.98(5)F6 2 0.1204(9) 0.4380(8) 0.2272(7) 1 0.98(5)TABLE IV. Selected bond angels in NaCrF from HR-SPXRD Rietveld refinementsF i -Cr-F j Cr1 ( i =2 , , j =1 , , — Cr2 ( i =2 , , j =6 , , — Cr3 ( i =4 , , j =5 , , — Cr4 ( i =4 , , j =3 , , i -Cr-F i at 1.5 K. l, m and s are long, mediumand short bond distances, respectively. Space Group : P ¯1 a d Cr = 79 . × − ˚A b Octahedral distortions : ∆ d Cr = 54 . × − ˚A c d Cr = 71 . × − ˚A α ◦ ∆ d Cr = 75 . × − ˚A β ◦ γ ◦ V R wp R p ◦ of independent parameters 111Restrains, constrains 0, 5Rigid bodies 0 Z Selected Bond Distances
Cr1-F Cr2-F Cr3-F Cr4-F l × m × s × I AKNOWLEDGEMENTS
TABLE VI. Atomic positions of NaCrF from NPD Rietveld refinements at 1.5 K. See Table V for crystal structure details.Atom Multiplicity x y z Occ U iso Na1 2 0.5084(5) 0.5551(4) 0.2358(3) 1 0.0181(4)Na2 2 0.9718(5) 0.0597(4) 0.2621(3) 1 0.0181(4)Cr1 1 0.5 0 0 1 0.0085(4)Cr2 1 0 0.5 0 1 0.0085(4)Cr3 1 0.5 0 0.5 1 0.0085(4)Cr4 1 0 0.5 0.5 1 0.0085(4)F1 2 0.6791(3) 0.2814(3) 0.0585(2) 1 0.0151(3)F2 2 0.2023(3) 0.2052(3) 0.9229(2) 1 0.0151(3)F3 2 0.7161(3) 0.3249(3) 0.4261(2) 1 0.0151(3)F4 2 0.1854(4) 0.1900(3) 0.5525(2) 1 0.0151(3)F5 2 0.3750(3) 0.9405(2) 0.2727(2) 1 0.0151(3)F6 2 0.1252(3) 0.4358(3) 0.2297(2) 1 0.0151(3)TABLE VII. Magnetic parameters of Cr ions in NaCrF from Rietveld refinements of PND as function of temperature.Temperature M ϕ (Cr1) ϑ (Cr1) ϕ (Cr2) ϑ (Cr2) ϕ (Cr3) ϑ (Cr3) ϕ (Cr4) ϑ (Cr4)1.5 3.51969 -136.9916 38.94935 43.00841 129.5479 -151.1129 56.12197 28.88715 136.50425 3.46608 -137.1359 39.5289 42.86414 129.5129 -149.6677 55.52545 30.33231 136.30857 3.40534 -136.6089 38.33702 43.39112 128.9114 -149.7235 54.45972 30.27652 134.95759 3.29101 -136.7846 39.77436 43.21539 133.1239 -149.2775 55.90062 30.72252 136.647111 3.12823 -135.8486 42.12699 44.15142 134.918 -150.2494 57.66518 29.75058 139.040713 2.9001 -133.409 36.74886 46.591 130.0525 -151.4175 52.1276 28.58247 134.712915 2.57635 -131.5435 40.35261 48.4565 133.1635 -154.1384 53.42544 25.86157 136.784517 2.02352 -129.6754 49.17003 50.32455 137.5972 -158.6962 49.17003 21.30376 137.597219 0.6124 -129.6754 49.17003 50.32462 137.5972 -158.6962 49.17001 21.30381 137.5972(Cr4)1.5 3.51969 -136.9916 38.94935 43.00841 129.5479 -151.1129 56.12197 28.88715 136.50425 3.46608 -137.1359 39.5289 42.86414 129.5129 -149.6677 55.52545 30.33231 136.30857 3.40534 -136.6089 38.33702 43.39112 128.9114 -149.7235 54.45972 30.27652 134.95759 3.29101 -136.7846 39.77436 43.21539 133.1239 -149.2775 55.90062 30.72252 136.647111 3.12823 -135.8486 42.12699 44.15142 134.918 -150.2494 57.66518 29.75058 139.040713 2.9001 -133.409 36.74886 46.591 130.0525 -151.4175 52.1276 28.58247 134.712915 2.57635 -131.5435 40.35261 48.4565 133.1635 -154.1384 53.42544 25.86157 136.784517 2.02352 -129.6754 49.17003 50.32455 137.5972 -158.6962 49.17003 21.30376 137.597219 0.6124 -129.6754 49.17003 50.32462 137.5972 -158.6962 49.17001 21.30381 137.5972