Cuprate Oxyfluorides La₂Cu_0.8Ni_0.2O₃F₂ and La₂CuO₃F₂ with “Channel-like” Anion Ordering

Abstract

Highly fluorinated cuprate Ruddlesden–Popper oxyfluorides La2Cu0.8Ni0.2O3F2 and La2CuO3F2 were obtained by topochemical reaction between poly(vinylidene fluoride) (PVDF) and the corresponding oxides La2Cu0.8Ni0.2O4 and La2CuO4 prepared by citrate-based soft chemistry synthesis. The crystal structures of both oxyfluorides were investigated by powder diffraction techniques. The structure of La2Cu0.8Ni0.2O3F2 was solved based on combined neutron and X-ray powder diffraction. It crystallizes in a new monoclinic distorted version [C2/c a = 13.1880(3) Å, b = 5.7244(1) Å, c = 5.6007(1) Å, and β = 90.85(1)°] of the anionic ordered structure lately reported for La2NiO3F2. For La2CuO3F2, an even less symmetrical triclinic structure was derived from X-ray powder diffraction data [P1̅ a = 5.6180(5) Å, b = 5.7316(6) Å, c = 7.1978(9) Å, α = 113.32(1)°, β = 90.89(9)°, and γ = 90.16(11)°]. For both compounds, an additional tilt component of the partially Jahn-Teller elongated (Cu,Ni)O4F2 octahedra was found as the origin for the lowered symmetry. The formation reaction of La2CuO3F2 was studied by in situ XRD measurements. In these investigations, two new reaction intermediates were identified. The magnetic properties of both oxyfluorides La2Cu0.8Ni0.2O3F2 and La2CuO3F2 were characterized by field- and temperature-dependent measurements. An antiferromagnetic ordering with T N = 240 K was found for La2Cu0.8Ni0.2O3F2. In La2CuO3F2, additional weak ferrimagnetism was observed, resulting in a pronounced hysteresis but a weak saturation moment, which was attributed to result from a canted antiferromagnetic spin arrangement.

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

“…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, , and it was also derived from the site occupation factors for the sample with x = 0.8 based on neutron powder diffraction data . As a further support, iodometric titration was applied to determine the bulk oxidation state of nickel/copper in the newly synthesized oxyfluorides.…”

“…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 . As a result, long dwell times of ∼48 h at a comparatively low reaction temperature of 350 °C were found to give the best results.…”

“…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 ) to the monoclinic unit cell of La 2 Ni 0.2 CuO 3 F 2 with C 2/ c symmetry (see Figure S4 for the symmetry relations of Cccm and C 2/ 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 ) arising from the triclinic cell metric.…”

“…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 . 20 The 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+ .…”

Topochemical Fluorination of LaBaInO4 to LaBaInO3F2, Their Optical Characterization, and Photocatalytic Activities for Hydrogen Evolution

“…Further analyses of the Ba and La spectra reveal the absence of additional peaks, meaning that there is no indication of the formation of side products at the surface. La, Ba, and In have precise oxidation states (no mixed valency as could be found in Ni/Co/Cu-RP-type compounds 45,50 ), and hence one can assign an anion content close to LaBaInO 3 F 2 to this compound.…”