Ruddlesden–Popper Oxyfluorides La2Ni1–xCuxO3F2 (0 ≤ x ≤ 1): Impact of the Ni/Cu Ratio on the Structure

Ruddlesden–Popper oxyfluorides La2Ni1–xCuxO3F2 (0 ≤ x ≤ 1) were obtained by topochemical reaction of oxide precursors La2Ni1–xCuxO4, prepared by citrate-based soft chemistry synthesis, with polyvinylidene fluoride (PVDF) as the fluorine source. Systematic changes of the crystal structure in the oxide as well as the oxyfluoride substitution series were investigated. For 0.2 ≤ x ≤ 0.9, the oxyfluorides adopt the monoclinic (C2/c) structural distortion previously solved for the x = 0.8 compound based on neutron powder diffraction data, whereas the sample with a lower Cu content of x = 0.1 crystallizes in the orthorhombic (Cccm) structure variant of La2NiO3F2. The orthorhombic-to-monoclinic structural transition was found to be the result of an additional tilt component of the Jahn–Teller elongated CuO4F2 octahedra. The structural transitions were additionally studied by DFT calculations, confirming the monoclinic space group symmetry. The “channel-like” anionic ordering of the endmembers La2NiO3F2 and La2CuO3F2 was checked by 19F MAS NMR experiments and was found to persist throughout the entire substitution series.


■ INTRODUCTION
Ruddlesden−Popper (RP) oxides have emerged as a focal point of research in the field of advanced materials due to their intriguing structural properties and potential applications in areas such as energy storage, 1,2 catalysis, 3,4 and electronics. 5,6uddlesden−Popper compounds are characterized by a layered crystal structure (often written as (AX)(ABX 3 ) n ), which distinguishes them from traditional perovskites (ABX 3 ).The arrangement of perovskite layers with thickness n alternating with one halite-type (AX) layer in the RP structures results in distinctive electronic, magnetic, and optical properties, making them interesting for both fundamental research and industrial applications.Ruddlesden−Popper cuprates A 2 CuO 4 are highly prominent representatives for the n = 1 member (K 2 NiF 4 structure type) of the series due to the first discovery of nonmetal superconductivity in La 2−x Ba x CuO 4−y by Bednorz et al. 7 Anionic substitution is widely used to obtain mixed ionic RP compounds with strongly altered structural distortions, which inflict the physical properties.One particular class of materials are the oxyfluorides, of which an increasing number of compounds with different O/F compositions are known to date.These oxyfluorides range from compounds with small fluoride excess like superconducting La 2 CuO 4 F δ (0 < δ < 0.25) 8−10 over stoichiometric compounds with O + F = 4 like Sr 2 FeO 3 F 11 to highly fluorinated compounds with O + F > 4 (e.g., LaSrMnO 4 F 1 , 12 La 2 CoO 4 F 1.2 , 2 Ba 2 ZrO 3 F 2 , 13 and Ba 2 SnO 2.5 F 3 14 ).Different low-temperature fluorination meth-ods like the solvothermal reaction with XeF 2 10 as well as reactions with the fluorine-containing polymers polyvinylidene fluoride (PVDF) 15−17 and polytetrafluoroethylene (PTFE) 18 have emerged in recent years, yielding less oxidative synthesis conditions compared to the reaction with F 2 gas, enabling the synthesis of less redox stable oxyfluorides.
Lately, oxyfluorides La 2 NiO 3 F 2 , 19 La 2 Ni 0.2 Cu 0.8 O 3 F 2 , and La 2 CuO 3 F 2 20 were successfully synthesized by the topochemical fluorination of corresponding oxides with PVDF.These compounds share an unusual structural distortion scheme with of strong octahedral tiltings, which result in the formation of a channel-like arrangement of the interstitial anionic positions within the AX layer.
For oxides La 2 NiO 4 −La 2 CuO 4 , the whole solid solution series has been described. 21,22In this study, we report on the successful synthesis of the corresponding oxyfluorides La 2 Ni 1−x Cu x O 3 F 2 (0.0 ≤ x ≤ 1.0 with Δx = 0.1).The samples were obtained through topochemical low-temperature fluorination of reactive oxide precursors prepared by the citrate route with PVDF as a fluorination agent.All oxyfluorides were found to adopt the distorted K 2 NiF 4 structure variant found for La 2 NiO 3 F 2 with a transition from orthorhombic (Cccm) to monoclinic (C2/c) symmetry for 0.2 ≤ x ≤ 0.9 due to the Jahn−Teller active Cu 2+ .The stability of both space groups was investigated by DFT calculations, verifying the structural transition.Additional 19 F MAS NMR experiments were applied to check the proposed anion ordering of both endmembers (i.e.x = 0 and 1) as well as two of the Ni/Cu-substituted compounds (i.e.x = 0.3 and 0.7).
■ EXPERIMENTAL SECTION Synthesis.Precursor oxides La 2 Ni 1−x Cu x O 4 were synthesized in steps of x = 0.1 by a citric acid-assisted combustion method. 23toichiometric amounts of La 2 O 3 (Merck) (dried at 900 °C for 10 h), Ni powder (Sigma-Aldrich), and copper(II) acetate (98%; Sigma-Aldrich) were dissolved in approximately 25 mL of distilled water with the addition of a few drops of concentrated HNO 3 .Citric acid (molar ratio metal ions:citric acid = 1:3) was added while stirring.The obtained clear solutions were subsequently dried on a hot plate at 100 °C until gel formation.The gels were further heated at 350 °C until ignition.The resulting powders were calcinated at 950 °C for 6 h.Oxyfluorides La 2 Ni 1−x Cu x O 3 F 2 were synthesized from the precursor oxides by mixing with polyvinylidene fluoride powder (PVDF/(CH 2 CF 2 ) n ) (Alfa Aesar) in a molar ratio of 1:1 (oxide:CH 2 CF 2 ) with a small excess (5%) of the polymer.The mixtures were slowly heated 2 K/min to 350 °C, kept at this temperature for 48 h, and afterward allowed to cool down to room temperature in the box furnace.
Characterization.X-ray diffraction (XRD) patterns were recorded with Cu-K α1,2 radiation on a Bragg−Brentano Bruker D8-Advance diffractometer equipped with a 1D silicon strip detector (LynXeye).Patterns were recorded in the angular range of 2θ = 10− 140°with a step size of 0.01°and 3 s per step.Additionally, a STOE STADI-MP transmission geometry diffractometer operating with monochromatic Mo-K α1 radiation and equipped with a DECTRIS MYTHEN 1K detector was used to record patterns in the angular range of 2θ = 5−75°.
The samples were checked for residual polymer by ATR-FT infrared spectroscopy in the range of 4000−250 cm −1 with a Bruker Tensor 27 spectrometer.
The La, Ni, and Cu contents of the samples were quantified by Xray fluorescence spectroscopy (XRF) with a Panalytical Epsilon 4 spectrometer.Data analysis was carried out based on the fundamental parameter approach for pelletized powder samples.
Iodometric titration was used for the determination of the average Ni/Cu oxidation states.About 35 mg of the samples was dissolved in concentrated HCl containing KI in excess.Afterward, 1 g of NaHCO 3 was added to create a CO 2 -saturated atmosphere, preventing the solutions from autoxidation.For titration, a 0.005 M Na 2 S 2 O 3 solution was used.The obtained oxidation state was calculated from three averaged titrations per sample.
DFT calculations were performed using the projector augmented wave (PAW) setups 25,26 as implemented in the Vienna ab initio simulation (VASP V5.4.) package.For structural optimization, uniform Γ-centered k-point grids with a density of 2000 k-points per reciprocal atom were used to sample the Brillouin zones.For calculations of energy differences between magnetic orderings, denser k-grids with a density of 5 k-points/Å −1 were applied.A 520 eV cutoff for the plane-wave basis was used, and the total energies were converged to less than 0.01 meV/cell.The Perdew−Burke− Ernzerhof 27 exchange-correlation functional was used with an onsite 6.2 eV repulsive U correction for Ni 3d states.For the compositions containing both Ni and Cu, different magnetic and structural configurations were generated using the cluster expansion method as implemented in the ATAT package. 28 19 Fmagic angle spinning (MAS) NMR experiments were performed on a 7.04 T magnet and a 1.4 T magnet operated with an Avance II Bruker NMR console with Topspin V2.1pl8, operating at frequencies of 286 and 56.4 MHz, respectively.Magic angle sample spinning on the 7.04 T magnet was carried out with a homemade MAS probe head for cylindrical rotors with 4 mm outer diameter spinning at around 8.5 to 10 kHz.The samples were packed in 3Dprinted inserts made of polypropylene and using a 3D-printed stator. 29Magic angle sample spinning on the 1.4 T magnet was carried out with a homemade conical stator with the sample packed in 3D-printed conical rotors, spinning at approximately 20 kHz. 30The chemical shift values refer to neat CFCl 3 . 31The spectrum was analyzed with deconv2Dxy. 32RESULTS AND DISCUSSION Crystal Structure Evaluation of Oxides La 2 Ni 1−x Cu x O 4 .The oxyfluoride La 2 NiO 3 F 2 was previously prepared by topochemical fluorination of La 2 NiO 4 obtained from solidstate synthesis with La 2 O 3 and NiO as starting materials. 19Our own first experiments showed that a successful formation of the desired Cu-containing oxyfluorides was not possible using this approach.Instead, mixtures of the targeted oxyfluorides, LaOF, and unreacted oxides were usually obtained as products.
The use of more reactive precursor oxides obtained by the citrate-based route at lower calcination temperatures has been successful in the synthesis of La 2 CuO 3 F 2 20 as well as La 2 NiO 2.5 F 3 . 33Therefore, this approach was used for the precursor oxides in this work.Phase-pure samples of the whole oxide substitution series La 2 Ni 1−x Cu x O 4 (Δx = 0.1) were successfully obtained, and their XRD patterns are shown in Figure 1.For substitution levels of x ≤ 0.4, an orthorhombic unit cell (Fmmm) is indicated by broadening of some peaks with mainly (hk0) contribution like the (110) reflection (with respect to the tetragonal (I4/mmm) K 2 NiF 4 unit cell).For the range of 0.5 ≤ x ≤ 0.7, no such broadening is observed and the compounds possess the tetragonal I4/mmm unit cell symmetry.For copper contents of x ≥ 0.7, again, a clear splitting of the (110) reflection into two separated reflections ((200)/(020)) indicates a transition from the tetragonal (I4/ mmm) to orthorhombic distorted B-centered (Bmab) unit cell of La 2 CuO 4 .Such phase transitions were previously found by Aguadero et al. 22 for oxides which were pepared via classical solid-state synthesis with copper contents of x = 0.0, 0.2, 0.4, 0.6, and 1. Boehm et al. 21reported slightly different structural behavior with an orthorhombic (Fmmm)-to-orthorhombic (Bmab) transition at x > 0.75 but without the presence of a tetragonal structure throughout the substitution series.The structural transitions observed in this work are therefore in good agreement with the already published data yet provide more details because smaller increments of x have been used.
Rietveld refinements were applied to study the structural evolution and to obtain unit cell parameters for all samples.The Rietveld plots and all crystallographic parameters are shown in the Supporting Information (Table S1 and Figures S1 and S2).The obtained unit cell parameters are plotted in Figure 2a,b, depending on the Cu content x.The increase in unit cell volume is not linear due to an opposed change in parameters a and c.While c increases almost linearly with x throughout the whole substitution series, the parameters a and b decrease with a slightly different slope up to the orthorhombic-to-tetragonal transition at x ≥ 0.4.Parameters a and b are the same for the compounds with 0.5 ≤ x ≤ 0.7 due to the tetragonal unit cell symmetry, and they decrease further with a similar slope as before.Above x ≥ 0.8, the cell parameter a further decreases, while b even slightly increases, resulting in a stronger increase in the cell volume.
The overall increase in the unit cell volume is interpreted as the result of an increasing Jahn−Teller distortion of the Ni/ CuO 6 octahedra with x.For Ni 2+ (3d 8 configuration), no Jahn−Teller distortion is to be expected, while for La 2 CuO 4 , a strong elongation of the octahedra occurs 34,35 due to the 3d 9 electron configuration of Cu 2+ .This increasing octahedral distortion is reflected by a strong and almost linear increase in the apical atomic distance d(Ni/Cu−O ap ) from 2.2 to 2.4 Å (x = 0.0 to x = 1.0) accompanied by a less pronounced decrease in the equatorial atomic distance d(Ni/Cu−O eq ) from 1.93 to 1.90 Å as plotted in Figure 2c,d, respectively.
Structural and Compositional Characterization of Oxyfluorides La 2 Ni 1−x Cu x O 3 F 2 .Fluorination of the precursor oxides was carried out with PVDF in the molar ratio of oxide/CH 2 CF 2 (PVDF) of 1:1.Optimized reaction conditions were established by in situ XRD experiments published before for the Cu-rich compounds with x = 0.8 and 1.0. 20As a result, long dwell times of ∼48 h at a comparatively low reaction temperature of 350 °C were found to give the best results.These optimizations were especially needed for the compounds with higher Cu content, as the thermal stability of the fluorinated phase was found to decrease with increasing Cu content.The corresponding oxyfluorides with the formula La 2 Ni 1−x Cu x O 3 F 2 were obtained phase-pure for the entire range of 0 ≤ x ≤ 1.The absence of any residual PVDF or other organic phases was confirmed by IR spectroscopy (shown in Figure S3) of the final products.The La:Ni:Cu ratio of both the starting oxides and oxyfluorides was analyzed by XRF measurements.The La:(Ni 1−x Cu x ) ratio was found to be in the range of (1.9−2.1):1,i.e., very close to the expected value, and no major deviations of the nominal nickel/copper ratio were observed.The anionic stoichiometry (3O + 2F) has been verified for both endmembers (x = 0.0 and 1.0) lately, 19,20 and it was also derived from the site occupation factors for the sample with x = 0.8 based on neutron powder diffraction data. 20As a further support, iodometric titration was applied to determine the bulk oxidation state of nickel/copper in the newly synthesized oxyfluorides.Titration was performed for selected samples (x = 0.2, 0.4, 0.6, and 0.8), and the average oxidation state of the B-type cation was found to be near +2 for both the oxides and oxyfluorides without any systematic dependence on the increasing Cu content.These results demonstrate the non-oxidative 36 nature of fluorination with PVDF and additionally confirm the O 3 F 2 anion stoichiometry under the assumption of 5 anions per formula.
The XRD patterns of the La 2 Ni 1−x Cu x O 3 F 2 oxyfluorides are shown in Figure 3.The overall diffraction patterns for x ≤ 0.3 resemble the one of La 2 NiO 3 F 2 , even though a significant decrease in the orthorhombic distortion of the unit cell is indicated by a decrease in the (020)(002) splitting.For x ≥ 0.4, an additional gradually increasing splitting of the (311) main reflection is observed with increasing x, indicating a transition of the orthorhombic structure of La 2 NiO 3 F 2 (Cccm) 19 to the monoclinic unit cell of La 2 Ni 0.2 CuO 3 F 2 with C2/c symmetry 20 (see Figure S4 for the symmetry relations of Cccm and C2/c).For La 2 CuO 3 F 2 (x = 1), we previously reported additional symmetry lowering to a triclinic structure (P-1), which is for example indicated by a shoulder found at the (31-1) reflection (marked with an arrow in Figure 3) arising from the triclinic cell metric.
Rietveld refinements were conducted as joint refinements based on two X-ray diffraction data sets recorded with monochromatic Mo-K α1 radiation and Cu-K α1,2 radiation.This approach was used to achieve higher redundancy and to combine the advantages of the absence of Ni fluorescence in the Mo-K α1 diffraction patterns and their higher accessible Qrange with the better angular resolution of the diffraction patterns taken with Cu radiation.In our previous refinements of the x = 0.8 compound, a split apical octahedral (X ap ) position was derived from the refinement of NPD data and this finding was interpreted as the result of a Jahn−Teller distortion of the CuO 4 F 2 octahedra in contrast to NiO 4 F 2 . 20This split position was not considered in the current refinements because the O/F positions cannot be determined with the required accuracy from XRD data, especially not in the presence of heavy scattering atoms like La/Ni/Cu.Rietveld plots of all refinements and the obtained crystallographic parameters can be found in the Supporting Information (Figures S5 and S6 and Table S2).For x = 0.0 and 0.1, good refinements were achieved with the structural model of La 2 NiO 3 F 2 in the space group Cccm.For x = 0.2 to 0.4, an increasing anisotropic broadening of the (311) reflection indicates the transitions from orthorhombic to monoclinic unit cell symmetry and the space group C2/c was therefore chosen for all compounds with a copper content of x ≥ 0.2.For x = 0.9, the monoclinic structural model of the x = 0.8 compound and the triclinic structural model of La 2 CuO 3 F 2 were used for the refinements.For both samples, no systematic broadening or splitting of selected reflections was observed even for the highest Q values.Therefore, the monoclinic structure model (C2/c) was chosen.Additional fits in P-1 did not yield significantly better residual values despite the larger number of refinable parameters.
The obtained unit cell parameters are plotted in Figure 4a,b.For the longest axis (a in Cccm and C2/c), a strong increase is observed with increasing Cu content ranging from 12.84 Å (x = 0.0) to 13.24 Å (x = 1.0).This is similar to the observations made for the oxides and can be attributed to the higher space requirements of the Jahn−Teller elongated CuO 4 F 2 octahedra and the slightly larger ionic radius of Cu 2+ compared to Ni 2+ (0.73 Å vs 0.69 Å). 37 For x ≥ 0.7, a reduction in slope is observed, most probably because the elongation is compensated by the increasing octahedral tilting and, in turn, increased monoclinic distortion as reflected by the increase in the angle β.Additionally, the straining of the b/c plane becomes smaller as b decreases and c increases with x, again being the result of the altered octahedral tilting scheme with additional tilting component in the c direction as found for La 2 Ni 0.2 Cu 0.8 O 3 F 2 . 20he cell volume (Figure 4c) increases almost linearly with x from 410 to 427 Å 3 despite the observed structural transitions.The Ni/Cu−F ap and Ni/Cu−O eq distances are plotted in Figure 4d, highlighting the impact of substituting Ni 2+ with the Jahn−Teller active Cu 2+ .As can be seen, the increase in Ni/ Cu−F ap with x is less pronounced for the oxyfluorides (2.17 Å − 2.30 Å) compared to the one observed for the precursor oxides shown in Figure 2d.This overall reduction in the Ni/ Cu−F ap distance is most probably caused by the additional anion in the halite-type layers, which alters the amounts of La− O/F bonds while keeping the overall bond valence sum of +3, resulting in overall increased La−O/F distances, which in return yield a decreased Ni/Cu−F ap distance.The thermodynamic stability of the oxyfluorides crystallizing in space groups Cccm and C2/c depending on the Ni/Cu ratio was additionally studied by DFT calculations.Thirty-two atom unit cells, with increments of x = 0.25, were used to simulate all symmetry-inequivalent Ni/Cu occupational configurations as well as magnetic configurations.The relative energies (ΔE) were defined as the difference between the most stable configurations in each phase.The obtained energy difference between Cccm and C2/c can be found in Table 1.The results show that the monoclinic unit cell is lower in energy and, therefore, more stable for all calculated Ni/Cu ratios even for the pure Ni compound La 2 NiO 3 F 2 .This deviation from the experimental results is most probably due to the fact that the energies are calculated for 0 K. Similar results were found by Wissel et al.where the experimental structure of La 2 NiO 3 F 2 relaxes in a monoclinic unit cell after artificial straining. 38The difference in energy between the Cccm and C2/c structures increases with the copper content, which is also reflected by the increasing monoclinic unit cell distortion (i.e.increasing angle β).Based on these findings, the choice of a monoclinic unit cell for the compounds with x ≥ 0.2 is justified, although a clear splitting of the diffraction peaks is not in all cases observed due to the limitted resolution of the diffractometers used.To answer the question whether the compounds with x < 0.2 also crystallize in a monoclinic unit cell, high-resolution synchrotron XRD data would be needed.Additionally, the smaller value of ΔE for x = 1 compared to x = 0.75 might be seen as a hint to a structural transition in La 2 CuO 3 F 2 , which indeed crystallizes in the space group P-1 as reported earlier. 20his additional symmetry lowering was not considered by the DFT calculations. 19F MAS NMR experiments were conducted for both endmembers La 2 NiO 3 F 2 and La 2 CuO 3 F 2 as well as two members of the solid solution (x = 0.3 and 0.7) to check the previously reported anion ordering.For example, in the case of La 2 NiO 3 F 2 , a single signal at δ = −44 ppm was previously attributed to an apical ordered fluorine atom. 19One signal with chemical shifts of δ = −20 ppm (x = 0.3) and δ = −44 ppm (x = 0.0 and 0.7) is also found in our 19 F MAS NMR data (see Figure 5a).A possible explanation for the different chemical shifts may be varying La−F interactions due to changes in the chemical environment of fluorine indirectly caused by Ni/Cu substitution and the corresponding Jahn− Teller elongation.The proposed apical F-ordering is in agreement with the previously reported chemical shift values and our observed spectra.Additional NMR data of La 2 CuO 3 F 2 were recorded on two spectrometers with a 7.04 T magnet and a 1.4 T magnet (Figure 5b).The spectra obtained for the La 2 CuO 3 F 2 sample on the 7.04 T NMR spectrometer are dominated by spinning side bands of the most intense peak at −127 ppm.By using a 1.4 T magnet in combination with a homemade MAS sample holder, 30 the spinning side bands were successfully suppressed through high spinning frequencies of ∼20 kHz, yielding two overlapping and thus not wellresolved peaks for La 2 CuO 3 F 2 centered at 40 and −36 ppm.The peak at −127 ppm of the 7.04 T data is not observed.This signal is attributed to an unknown diamagnetic impurity, which due to the longer relaxation time T 1 of ∼3 s is not observed in the lower field data.This signal is not caused by LaOF for which signals are found at −24 ppm (β-LaOF) or −35 ppm (t-LaOF). 39The presence of two signals for La 2 CuO 3 F 2 is expected as two crystallographic sites (both with Wyckoff symbol 2e) exist for the apical position in the previously reported triclinic structure.As the sites share the same multiplicity, a peak area ratio of 50:50 is expected.This ratio is close to the experimental data, which show a ratio of ∼40:60, given that the broad peaks are not fully resolved.Also, the indication of a slight depopulation of one of the apical octahedral positions is possible, probably due to starting decomposition.This is in accordance with the small amounts of LaOF found in the XRD data of this sample.

■ CONCLUSIONS
The topochemical fluorination of the solid solution La 2 Ni 1−x Cu x O 4 was carried out with PVDF as a fluorination agent, yielding phase-pure oxyfluoride samples for the whole substitution series La 2 Ni 1−x Cu x O 3 F 2 .The structure of the obtained oxides was studied in increments of x = 0.1 by X-ray powder diffraction experiments, and two structural transitions were observed (orthorhombic (Fmmm; x ≤ 0.4) → tetragonal  Inorganic Chemistry (I4/mmm; 0.4 < x ≤ 0.7) → orthorhombic (Bmab; 0.7 < x ≤ 1.0)).For the oxyfluorides, we were able to show that the solid solution exhibits a structural transition at x = 0.2 from the orthorhombic structure of La 2 NiO 3 F 2 to the monoclinic distorted structure of La 2 Ni 0.2 Cu 0.8 O 3 F 2 , which we previously solved based on NPD data, 20 while for La 2 CuO 3 F 2 , even further symmetry lowering to P-1 was found.This transition is driven by an increased Jahn−Teller distortion.The structural transition was also studied by DFT calculations, and based on the difference in thermodynamic stability between both space groups, the presence of the monoclinic structure was further confirmed.The previously proposed apical fluoride anion ordering of La 2 NiO 3 F 2 and La 2 CuO 3 F 2 was further verified for x = 0.0, 0.3, 0.7, and 1.0 by additional 19 F MAS NMR experiments.In this paper, we were therefore able to show the existence of several new oxyfluorides all exhibiting a similar anion ordering scheme and to refine their strucutres.In a follow-up article, we will discuss the impact of Ni/Cu substitution on the thermal stability as well as the magnetic properties of La 2 Ni 1−x Cu x O 3 F 2 as these results would by far exceed the scope of this article.
■ ASSOCIATED CONTENT

Figure 2 .
Figure 2. Evolution of the unit cell parameters a, b, and c (a) and cell volume (b) as well as the apical (Ni/Cu−O ap ) and equatorial (Ni/ Cu−O eq ) distances (c, d) with the copper content (x) obtained by Rietveld refinements.The cell parameter a is given as √2a for I4/ mmm to achieve comparability between the different centered lattices (Fmmm, I4/mmm, and Bmab).

Figure 3 .
Figure 3. XRD patterns of the oxyfluoride solid solution La 2 Ni 1−x Cu x O 3 F 2 with a detailed view of the main reflections (311) and (020)(002), highlighting the position shifts and changing peak splits.

Figure 4 .
Figure 4. Evolution of the unit cell parameters a, b, and c (a), monoclinic angle β (b), and unit cell volume (c) of La 2 Ni 1−x Cu x O 3 F 2 obtained from Rietveld refinements.In panel (d), the evolution of the Ni/Cu−F ap and Ni/Cu−O eq bond distances is shown.

Figure 5 .
Figure 5. (a) 19 F MAS NMR spectra of La 2 NiO 3 F 2 , La 2 Ni 0.7 Cu 0.3 O 3 F 2 , La 2 Ni 0.3 Cu 0.7 O 3 F 2 , and La 2 CuO 3 F 2 measured in B = 1.4 T at the spinning frequency of ca.20 kHz.(b) 19 F MAS NMR spectra of La 2 CuO 3 F 2 measured in 7.04 T at spinning frequencies of 8.5 and 9.5 kHz and in 1.4 T at 20 kHz.The asterisks mark isotropic peaks.

Table 1 .
Energy Difference (ΔE) Obtained from DFT Calculations for Both Unit Cell Symmetries (Cccm and C2/ c) with Respect to the Copper Content