Fluorination of the Ruddlesden–Popper type cuprates, Ln2-xA1+xCu2O6-y (Ln=La, Nd; A=Ca, Sr) Peter R. Slater,*a Jason P. Hodges,b M. Grazia. Francesconi,b Colin Greavesb and Marcin Slaskic aSchool of Chemistry,University of St. Andrews, St. Andrews, Fife, UK KY16 9ST bSchool of Chemistry, University of Birmingham, Edgbaston, Birmingham, UK B15 2T T cSchool of Physics and Space Research, University of Birmingham, Edgbaston, Birmingham, UK B15 2T T The low-temperature (200–350 °C) fluorination of the Ruddlesden–Popper type cuprates, Ln2-xA1+xCu2O6-y (Ln=La, Nd; A= Ca, Sr) using F2 gas, CuF2, and NH4F is reported.The incorporation of large levels of fluoride ions is observed for each of these fluorinating agents. The general characteristics of each method are discussed, and it is shown that for this system, fluorination mainly occurs by insertion of fluorine for reaction with F2, substitution of fluorine for oxygen for NH4F, and a mixture of the two processes for CuF2.For A=Sr, it is assumed that fluorine inserts mainly between the two CuO2 layers, since large expansions of the unit cell along the c direction are observed. No evidence for bulk superconductivity has so far been observed after fluorination.Recently we have reported that the reaction of the alkaline- La1.9Sr1.1Cu2O6-y and La1.9Ca1.1Cu2O6-y. The intimately earth cuprates A2CuO3 (A=Ca, Sr) with F2 gas at low ground powders were heated in air at a temperature between temperatures (ca. 200 °C) yields the oxide fluorides 1050 and 1075 °C for 18 h, reground, and then reheated at the A2CuO2F2+d , with superconductivity (Tc ca. 46 K) being same temperature for a further 18 h, followed by furnace observed for A=Sr.1,2 We have subsequently demonstrated cooling. two other simple low-temperature solid-state fluorination routes to these oxide fluorides, involving the facile reaction (at Fluorination by solid/F2 gas reaction ca. 230 °C) of A2CuO3 with NH4F3 or transition-metal difluor- The samples were heated in 10%F2–90%N2 (passed over NaF ides (e.g.CuF2, ZnF2).4 Moreover partial substitution of Ba to remove HF) at a temperature between 200 and 250 °C for Sr has been shown to raise the Tc as high as 66 K, for a time ranging from 15 to 150 min. The temperature significantly the highest Tc for any phase with the confirmed used depended on the system [Nd1.3Sr1.7Cu2O6-y (205 °C), La2CuO4 structure.Since for other cuprates, it has been shown La1.9Sr1.1Cu2O6-y (240 °C), La1.9Ca1.1Cu2O6-y (250 °C)] and that Tc increases with increasing number of Cu layers (generally reaction times, which ranged from 15 min to 5 h, determined up to three layers), it was of extreme interest to look at rethe extent of fluorination.lated materials with multiple Cu layers. Sr2CuO3 may be considered as the first member of the homologous series, Fluorination by NH4F (solid-state reaction) Srn+1CunO2n+1+d. The n=2 member has been prepared by high-pressure synthesis,5 but for ambient pressure synthesis, For fluorination using NH4F a reaction temperature of rare-earth elements, Ln, are required to partially substituted 300–350 °C was required and up to 20 mol of NH4F were used for the alkaline-earth metals, and this represents the widely known Ruddlesden–Popper type double-layer cuprates, Ln2-xA1+xCu2O6-y (Ln=rare-earth, A=Ca, Sr).6–9 As in the case of Sr2CuO3, significant oxygen vacancies are located in these phases, mainly between the double CuO2 layers such that the coordination of Cu is square pyramidal (Fig. 1). These phases are non-superconducting as synthesised, but a number of groups have induced superconductivity (Tc up to 60 K) using a variety of routes including high-oxygen-pressure annealing, control of synthesis conditions, or use of an oxidizing agent such as KClO3.10–12 Our aim was to attempt to induce superconductivity through fluorination. In phases with high Sr content, e.g.Nd1.3Sr1.7Cu2O6-y, significant vacancies are also located in the CuO2 planes, and an ordering of these vacancies occurs leading to a tripled cell along one direction.9 This gives rise to a unique one-dimensional tunnelled copper–oxygen sublattice built from vertex linked CuO5 square pyramids (Fig. 2), where it should be possible to insert fluorine similar to the case of A2CuO3 (A= Ca, Sr, Ba).In this paper we report detailed studies on the fluorination of these double-layer cuprates, showing the extent and characteristics of fluorination by each method. In addition we highlight the diVerences in the characteristics of the three methods. Experimental High-purity Nd2O3, La2O3, SrCO3, CaCO3, and CuO were Fig. 1 Structure of La2ACu2O6 (A=Ca, Sr) showing square-pyramidal Cu (spheres: A, Sr) used to prepare the following samples, Nd1.3Sr1.7Cu2O6-y, J.Mater. Chem., 1997, 7(10), 2077–2083 2077for the sample. The fluorine contents of samples containing significant amounts of this impurity should therefore be treated as only approximate, and this is referred to further in the results section. The TISAB solution was prepared as follows; 57 cm3 glacial acetic acid, 58 g of NaCl and 4 g of trans-1,2-diaminocyclohexane- N,N,N,N-tetraacetic acid were dissolved in 500 cm3 of distilled water, and suYcient 5 M NaOH was added to adjust the pH to ca. 5.3. The buVer solution was then made up to 1000 cm3 with distilled water. Copper oxidation states were determined by iodometric titration. Two complementary titrations were performed for each sample.The first titration involved dissolving ca. 0.05 g of the sample in 50 cm3 of distilled water by the addition of dilute (2 M) HCl. The solution was then boiled to ensure complete conversion of any Cu3+/Cu+ to Cu2+, before excess KI was added. The I2 generated was then titrated under N2 against standard (0.02 M) Na2S2O3 solution. This titration determines the total amount of Cu in the sample.The second titration involved dissolving under N2 the same mass of sample Fig. 2 Structure of Nd1.3Sr1.7Cu2O6-y showing square-pyramidal Cu in excess KI solution by addition of dilute HCl. The liberated (spheres: Nd, Sr) I2 was then titrated as before. From the diVerence in the titration values obtained, the copper oxidation state can be to prepare samples with the highest F content.In order to determined. In samples fluorinated by direct solid-state reacminimize SrF2 impurities at higher F levels, the NH4F was tion with CuF2, CuO is present as an impurity, and so this added in stages, e.g. for 10 mol of NH4F, 5 mol was added will influence the titration. The quantity of CuO present is first and reacted at 300–350 °C; the sample was then reground however known (equal to the amount of CuF2 added) and so with a further 5 mol of NH4F and heated as before. this can be corrected for in the determination of the Cu oxidation state of the sample: good agreement between the Cu Fluorination by CuF2 (solid-state reaction) oxidation state calculations was obtained for samples fluori- Anhydrous CuF2 (0–2 mol) was added to the precursor oxides; nated to a similar extent by the direct solid-state reaction the mixture was ground and heated in the range 245–350 °C method and the autoclave method (where the CuF2 is kept in air for 12 h.separate and so no correction is required). Autoclave method (solid–gas reaction with CuF2) Results The drawback of the CuF2 route is the presence in the samples Fluorination of Ln2-xA1+xCu2O6-y (L=La, Nd; A=Ca, Sr) of CuO impurity deriving from the decomposition of CuF2.In order to avoid this problem, a number of experiments have Powder X-ray diVraction suggested significant fluorine incorbeen performed in enclosed vessels with the starting oxides poration for all fluorination methods, with large peak shifts in and CuF2 separated. In this route, the oxides were weighed most cases.The magnitude of the shifts increased with increased out into a Teflon vessel and the required amount of CuF2 was fluorine content. Fig. 3–5 show the X-ray diVraction patterns then placed in a small nickel pot, which was put into the for the starting materials, and the ‘fully’ fluorinated products Teflon vessel, and the lid fitted.The Teflon vessel was then for each method. placed in an autoclave and heated to 245 °C. Similar results were obtained to the solid-state reaction with CuF2, with the (a) Reaction with NH4F. In order to successfully fluorinate exception that no CuO impurities were observed in this case, the double-layer cuprates by this method, a higher temperature as the CuF2 was kept separate.(300–350 °C) was required compared with similar reactions with Sr2-xAxCuO3 (230 °C). Moreover, since the double-layer Characterization methods materials appear to be less moisture sensitive, Sr/CaF2 impurities were only observed for samples with the highest fluorine The resulting products were characterised by powder X-ray diVraction (Cu-Ka1 radiation, Siemens D-5000 diVractometer).levels. Tables 1 and 2 list cell parameters and fluorine contents for a range of samples prepared by this method. The presence Potential superconducting properties were examined using a DC SQUID magnetometer (Cryogenics Model S100). of Sr/CaF2 impurities in the high fluorine content samples means that the calculated fluorine contents for these samples The fluorine contents were determined using a fluoride ion selective electrode.Prior to measurements being made, the should be viewed as only approximate. For La1.9Sr1.1Cu2O6-y and Nd1.3Sr1.7Cu2O6-y there was no electrode was calibrated using freshly prepared solutions containing known concentrations of NaF. The sample solution change in cell symmetry but the cell parameters increased by up to ca. 5% along a and b, and ca. 1% along c after was then prepared as follows: ca. 0.02–0.05 g of sample was dissolved in 5 cm3 of 0.5 M HCl, to which was added 45 cm3 fluorination. Fluorination of La1.9Sr1.1Cu2O6-y proved more interesting. In this system at low fluorine contents the cell was of distilled water followed by 50 cm3 of a pH ca. 5.3 total ionic strength adjustment buVer (TISAB) solution (preparation tetragonal as for the parent compound, while for high fluorine levels the cell became orthorhombic. At intermediate fluorine described below).The fluorine content of the sample was then determined from the electrode reading of the solution using contents both phases were observed (Fig. 6), with the ratio of the phases with low fluorine content (tetragonal, a=b#3.92, the NaF calibration graph.No noticeable residual fluorinating agent (NH4F or CuF2) was present in any of the samples c#20.14 A° ) and high fluorine content (orthorhombic, a#3.86, b#3.90 A ° , c#21.68 A ° ) varying with overall fluorine content; analysed. The major errors in the analysis resulted from any presence of Sr/CaF2 impurities which tended to be observed at high fluorine levels, only the latter phase was observed.In the low fluorine content phase, the largest changes in cell mainly for samples with high fluorine contents. The presence of this impurity will result in a higher apparent fluorine content parameters were along a and b (similar to La1.9Ca1.1Cu2O6-y 2078 J. Mater. Chem., 1997, 7(10), 2077–2083Fig. 4 Powder X-ray diVraction patterns for (a) La1.9Sr1.1Cu2O6.05, (b) Fig. 3 Powder X-ray diVraction patterns for (a) La1.9Ca1.1Cu2O5.95, La1.9Sr1.1Cu2O6.05F1.5 (F2 gas), (c) La1.9Sr1.1Cu2O4.9F2.3 (NH4F) and (b) La1.9Ca1.1Cu2O5.95F0.4 (F2 gas), (c) La1.9Ca1.1Cu2O5.3F1.3 (NH4F) (d) La1.9Sr1.1Cu2O5.45F2 (CuF2, 245 °C autoclave method) and (d) La1.9Ca1.1Cu2O5.5F0.9 (CuF2, 350 °C) and Nd1.3Sr1.7Cu2O6-y) whereas an expansion along c by 8% was observed for the high fluorine content phase, with much lower expansions along a and b (Table 1).If we consider the change in cell parameters from the low to the high fluorine content phase, then we can see that a and b actually contract slightly, with a large expansion along c. Moreover a change from tetragonal to orthorhombic symmetry is observed which may be related to an ordering of O/F and vacancies.It is possible that the cell may be tripled along b similar to that observed for Nd1.3Sr1.7Cu2O6-y, although there is no conclusive evidence for this. Iodometric titrations indicated a negligible increase in the copper oxidation state following fluorination, with the copper oxidation state for all three systems remaining close to 2.0+.Thus fluorination via NH4F appears to be essentially a nonoxidative process, and so probably involves a substitution reaction in which one oxygen is replaced by two fluorine atoms. The reaction with NH4F appears to proceed via the addition of HF, derived from the decomposition reaction NH4F�NH3+HF. Evidence for this is provided by an attempt to fluorinate La1.9Sr1.1Cu2O6-y via a quantitative reaction with a solution of NH4F in H2O in a hydrothermal bomb.Reaction at 100 °C suggested only a minor change in cell parameters, and subsequent heating of the filtered and washed solid at 350 °C resulted in a large change in cell parameters indicating successful fluorine incorporation. Moreover, the filtrate was alkaline, suggesting that the cuprate had incorporated HF, leaving NH3 in solution.Monitoring the reaction more carefully with a pH meter showed that at 60 °C, a fast reaction (total reaction time ca. 10 min) occurred with the pH changing from acidic to alkaline as the cuprate was added to Fig. 5 Powder X-ray diVraction patterns for (a) Nd1.3Sr1.7Cu2O5.65, the aqueous solution of NH4F. A similar experiment performed (b) Nd1.3Sr1.7Cu2O5.65F1.8 (F2 gas), (c) Nd1.3Sr1.7Cu2O4.65F2 (NH4F) and (d) Nd1.3Sr1.7Cu2O4.8F2.8 (CuF2, 245 °C autoclave method) at room temperature, however, showed a pH change to neutral J.Mater. Chem., 1997, 7(10), 2077–2083 2079Table 1 Cell parameters for La1.9Sr1.1Cu2O6.05 (LSC), of fluorine (Table 3), whereas La1.9Sr1.1Cu2O6-y and La1.9Ca1.1Cu2O5.95 (LCC) and Nd1.3Sr1.7Cu2O5.65 (NSC) fluorinated Nd1.3Sr1.7Cu2O6-y showed larger expansions, consistent with using NH4F high fluorine levels.For these phases the largest increase in cell parameters was along the c direction, suggesting the compound mol. NH4F a/A° b/A° c/A° probable incorporation of F between Cu ions in adjacent LCC — 3.828(1) =a 19.410(3) CuO2 layers. Unlike the case of the fluorination of A2CuO3 LCC 1 3.831(1) =a 19.442(6) (A=Ca, Sr, Ba), where fluorination led immediately to a large LCC 2 3.834(1) =a 19.475(5) structural change to the oxide fluoride Sr2CuO2F2+d, with LCC 10 3.895(2) =a 19.56(1) little non-stoichiometry range (d) for F, the fluorination of the LCC 20 3.899(1) =a 19.57(1) Nd1.3Sr1.7Cu2O6-y (L=Eu, Nd) showed a wide fluorine solid LSC — 3.851(1) =a 20.048(6) solution from the oxide endmember to the highest fluorine LSC 0.5 3.896(3) =a 20.12(2) LSC 1 3.922(1) =a 20.141(9) content, the XRD peaks shifting to lower 2h values as the LSC 2 3.929(1) =a 20.177(7) fluorine content was increased.Indeed, broad peaks, or should- LSC 5 3.937(1) =a 20.199(5) ers on peaks were commonly observed, indicating a mixture LSC 20 3.859(2) 3.903(2) 21.68(1) of compositions with slightly diVerent cell parameters in the NSC — 3.767(3) 11.366(9) 20.10(1) sample, due to the inhomogeneity of fluorination.NSC 0.5 3.798(5) 11.49(1) 20.29(4) As in the case of fluorination using NH4F, a distinct two- NSC 1 3.818(4) 11.54(1) 20.35(3) NSC 2 3.847(1) 11.555(6) 20.38(2) phase region (high and low F content phases) was observed NSC 5 3.851(2) 11.56(1) 20.37(2) for La1.9Sr1.1Cu2O6-y at intermediate F levels, the cell param- NSC 10 3.863(3) 11.62(1) 20.31(2) eters for these two phases being vastly diVerent due to a large NSC 20 3.944(3) 11.92(1) 20.10(3) expansion along c for the high F content phase (a=b#3.90, c#20.32 A ° compared to a=b#3.90, c#22.11 A ° ).In this case, however, no change to orthorhombic symmetry was observed Table 2 F contents, Cu oxidation states and suggested final for the high fluorine content phase.The fluorine contents for compositions for selected samples of La1.9Sr1.1Cu2O6.05 (LSC), a range of samples prepared using F2 gas are given in Table 3. La1.9Ca1.1Cu2O5.95 (LCC) and Nd1.3Sr1.7Cu2O5.65 (NSC) fluorinated using NH4F These results show that for the phases with A=Sr, very high flrine contents are achieved, such as 1.8 fluorine atoms per F Cu suggested formula unit after fluorination of Nd1.3Sr1.7Cu2O5.65. mol.content oxidation composition Iodometric titrations showed that the reaction with F2 gas compound NH4F mol-1 state of sample was highly oxidative, as might be expected, with the copper oxidation state increasing with increasing fluorination up to LCC — — 2.0 La1.9Ca1.1Cu2O5.95 LCC 1 0.5 2.0 La1.9Ca1.1Cu2O5.7F0.5 values close to 3.0+ (Table 3).These results, and the fluorine LCC 2 0.7 2.0 La1.9Ca1.1Cu2O5.6F0.7 contents determined, suggest that the fluorination of the LCC 10 1.3 2.0 La1.9Ca1.1Cu2O5.3F1.3 double-layer cuprates by F2 gas proceeds via a simple insertion LSC — — 2.1 La1.9Sr1.1Cu2O6.05 process, in which the anion site vacancies are gradually filled LSC 1 0.6 2.1 La1.9Sr1.1Cu2O5.75F0.6 by fluorine.The fluorine contents and copper oxidation states LSC 20 2.3 2.1 La1.9Sr1.1Cu2O4.9F2.3 determined, however, indicate that fluorine must also occupy NSC — — 2.0 Nd1.3Sr1.7Cu2O5.65 NSC 5 1.2 2.0 Nd1.3Sr1.7Cu2O5.05F1.2 some interstitial sites for high fluorine contents, similar to that NSC 10 1.8 2.0 Nd1.3Sr1.7Cu2O4.75F1.8 observed in Sr2CuO2F2+d, since, for Nd1.3Sr1.7Cu2O5.65F1.8 NSC 20 2.0 2.0 Nd1.3Sr1.7Cu2O4.65F2.0 and La1.9Sr1.1Cu2O6F1.5 there are seven ‘ideal’ anion sites, leaving the remaining 0.45 or 0.5 fluorine atoms to occupy interstitial sites presumably in the Ln/Sr (L=Nd, La) bilayers. The alternative possibility of partial substitution of fluorine for oxygen was not supported by the copper oxidation state determinations.Slightly higher fluorine contents were also observed on prolonged fluorination, demonstrated by larger peak shifts. The Nd1.3Sr1.7Cu2O5.65 sample heated at 205 °C for >2.5 h showed a shoulder at higher d-spacing, which increased with increasing fluorination time, thus indicating the presence of two phases, Nd1.3Sr1.7Cu2O5.65F1.8 and a phase with larger peak shift and therefore presumably higher F content.Unfortunately, at these high fluorination levels, it is extremely diYcult to avoid decomposition to give SrF2, and Fig. 6 Powder X-ray diVraction patterns for La1.9Sr1.1Cu2O6 fluori- it was not possible to isolate the pure phase with the higher F nated with 7.5 moles of NH4F, showing two fluorinated phases: main content.Fluorine analysis of the mixed sample gave an anomalpeaks from the high fluorine content phase are marked* ously high fluorine content, 3.5, presumably due to the presence of the SrF2 impurity, and so the exact fluorine content of this phase is not known. only, suggesting that NH4F itself was adsorbed by the sample, with no decomposition to HF and NH3 occurring at this temperature.(c) CuF2 method (solid-state and autoclave routes). Both the solid-state and the autoclave methods gave similar results, although the latter had the advantage that because the sample (b) F2 gas. In order to achieve successful fluorination, the samples La1.9Ca1.1Cu2O6-y and La1.9Sr1.1Cu2O6-y required a and CuF2 were kept separate, no CuO impurity was observed.Cell parameters, copper oxidation states and F contents are higher temperature (240–250 °C) than Nd1.3Sr1.7Cu2O6-y (205 °C), which may relate to the slightly diVerent structures. given in Tables 4 and 5. At high temperatures (>300 °C) the XRD patterns of the Moreover, at temperatures >205 °C the Nd containing phase decomposed to give SrF2.The fluorine content was observed products were similar to those from reaction with NH4F, whereas at lower temperatures (ca. 245 °C) the reaction to increase with increasing reaction time at these temperatures. Fluorination of La1.9Ca1.1Cu2O6-y resulted in a small appeared to be similar to reaction with F2 gas. Iodometric titrations supported this view, with the copper oxidation state increase in the cell parameters, indicating a low incorporation 2080 J.Mater. Chem., 1997, 7(10), 2077–2083Table 3 Cell parameters, Cu oxidation states and F contents (x) for La1.9Sr1.1Cu2O6.05Fx (LSC), La1.9Ca1.1Cu2O5.95Fx (LCC) and Nd1.3Sr1.7Cu2O5.65Fx (NSC) prepared using F2 Cu reaction reaction oxidation compound temp./°C time/min a/A ° b/A ° c/A ° state x LCC — — 3.828(1) =a 19.410(3) 2.0 — LCC 250 15 3.834(2) =a 19.475(5) 2.1 0.2 LCC 250 50 3.839(1) =a 19.506(4) 2.15 0.4 LSC — — 3.851(1) =a 20.048(6) 2.1 — LSC 220 30 3.853(1) =a 20.05(1) 2.1 0.1 LSC 220 60 3.876(3) =a 20.18(4) 2.25 0.4 LSC 240 15 3.898(1) =a 20.32(2) 2.3 0.6 LSC 240 60 3.893(1) =a 21.95(1) 2.9 1.5 NSC — — 3.767(3) 11.366(9) 20.10(1) 2.0 — NSC 205 20 3.787(4) 11.399(6) 20.18(1) 2.15 0.2 NSC 205 60 3.842(3) 11.544(7) 20.54(2) 2.3 0.8 NSC 205 120 3.837(1) 11.600(4) 21.04(2) 2.6 1.3 NSC 205 150 3.851(2) 11.679(5) 21.37(2) 2.9 1.8 Table 4 Cell parameters for La1.9Sr1.1Cu2O6.05 (LSC), achieved by this method was, however, lower than that La1.9Ca1.1Cu2O5.95 (LCC) and Nd1.3Sr1.7Cu2O5.65 (NSC) fluorinated obtained using F2 gas (2.5+ compared to 3.0+), suggesting using CuF2 that although highly oxidative, the oxidising power of the reaction with MF2 is not as great as for the reaction with F2 mol.reaction gas. Copper oxidation states and fluorine contents for selected compound CuF2 temp./°C a/A ° b/A ° c/A ° samples are listed in Table 5. Taken together, the copper LCC — — 3.828(1) =a 19.410(3) oxidation states and fluorine contents indicate that at high LCC 0.5 245 3.827(1) =a 19.427(5) temperatures the reaction proceeds via substitution (2FO10), LCC 1 245 3.834(1) =a 19.51(1) whereas at low temperatures the reaction is probably a mixture LCC 2 245 3.843(3) =a 19.58(2) of insertion (as in the case of F2) and substitution.Suggested LCC 1 350 3.860(2) =a 19.50(1) final compositions of the samples after fluorination were LSC — — 3.851(1) =a 20.048(6) determined from the fluorine contents and copper oxidation LSC 0.25 245 3.898(3) =a 20.18(2) LSC 0.5a 245 3.902(2) =a 20.14(2) states and are reported in Table 5.The slight diVerence in the LSC 1b 245 3.887(7) =a 21.99(6) MF2 (low temperature) and F2 reaction routes is demonstrated LSC 1.5 245 3.896(5) =a 22.09(2) by the fact that for Nd1.3Sr1.7Cu2O5.65, the orthorhombic LSC 2 245 3.93(1) =a 22.2(1) splitting at the highest fluorine content, designated by the ratio LSC 1.5 300 3.864(2) 3.929(2) 21.53(2) a : b/3, was significantly larger for samples derived from the LSC 1.5 350 3.856(1) 3.911(1) 21.45(1) MF2 route (ca. 1.025 compared to ca. 1.01). NSC — — 3.767(3) 11.366(9) 20.10(1) In all three methods, the fluorine content could be varied NSC 0.25 245 3.817(1) 11.455(1) 20.33(1) NSC 0.5 245 3.832(3) 11.55(1) 20.50(2) by suitable control, e.g.amount of CuF2/NH4F or temperature NSC 1.25b 245 3.804(1) 11.690(4) 21.70(2) and time for F2 gas. For the CuF2 and F2 methods, the copper NSC 1.5 245 3.798(1) 11.699(2) 21.91(1) oxidation state can also be controlled between 2+ and 3.0+, NSC 1.5 270 3.823(1) 11.746(4) 21.75(2) such that it is quite readily possible to prepare samples with NSC 1.5 300 3.937(1) 11.800(2) 19.96(1) copper oxidation states in the region 2.2–2.3+, which should NSC 1.5 350 3.884(7) 11.81(1) 19.89(2) be optimum for superconductivity.aSample consisted of two phases: the cell parameters for the lower F All samples were examined for possible superconductivity.content (most abundant) phase are given. bSample consisted of two For the sample La1.9Sr1.1Cu2O6-y, a very weak superconphases: the cell parameters for the higher F content (most abundant) ducting signal (ca. 0.1% volume fraction), Tc ca. 20 K, was phase are given. observed after light fluorination using CuF2 (ca. 0.1–0.2 moles). Under such conditions, samples showed broad X-ray peaks increasing at low temperatures, while remaining close to 2.0+ consistent with a mixture of fluorine contents.Further work has failed to increase the superconducting fraction, or increase at higher temperatures. The maximum copper oxidation state Table 5 F contents, Cu oxidation states and suggested final compositions for selected samples of La1.9Sr1.1Cu2O6.05 (LSC), La1.9Ca1.1Cu2O5.95 (LCC) and Nd1.3Sr1.7Cu2O5.65 (NSC) fluorinated using CuF2 mol.reaction F content Cu oxidation suggested composition compound CuF2 temp./°C mol-1 state of sample LCC — — — 2.0 La1.9Ca1.1Cu2O5.95 LCC 2 245 0.5 2.1 La1.9Ca1.1Cu2O5.8F0.5 LCC 1 350 0.9 2.0 La1.9Ca1.1Cu2O5.5F0.9 LSC — — — 2.1 La1.9Sr1.1Cu2O6.05 LSC 0.25 245 0.4 2.25 La1.9Sr1.1Cu2O6.0F0.4 LSC 1.5 245 2.0 2.5 La1.9Sr1.1Cu2O5.45F2.0 LSC 1.5 300 1.8 2.15 La1.9Sr1.1Cu2O5.2F1.8 LSC 1.5 350 1.7 2.1 La1.9Sr1.1Cu2O5.2F1.7 NSC — — — 2.0 Nd1.3Sr1.7Cu2O5.65 NSC 0.5 245 0.7 2.2 Nd1.3Sr1.7Cu2O5.5F0.7 NSC 1.5 245 2.8 2.55 Nd1.3Sr1.7Cu2O4.8F2.8 NSC 1.5 270 2.5 2.35 Nd1.3Sr1.7Cu2O4.75F2.5 NSC 1.5 300 2.2 2.1 Nd1.3Sr1.7Cu2O4.65F2.2 NSC 1.5 350 1.8 2.1 Nd1.3Sr1.7Cu2O4.85F1.8 J.Mater. Chem., 1997, 7(10), 2077–2083 2081Tc.No evidence for superconductivity was found for any atom by two fluorine atoms in La1.9Sr1.1Cu2O6), with octahedral copper similar to the situation in Sr2CuO2F2+d . This is other sample. supported by fluorine analysis (Table 2) although the presence of SrF2 impurities at the higher fluorine levels does cast some Discussion doubts over the absolute reliability of the values.Such a compound, with all the anion sites filled, might be expected to The results clearly demonstrate that the Ruddlesden–Popper type cuprates, Ln2-xA1+xCu2O6-y (L=Nd, La, Eu; A=Sr, be very stable, and this perhaps explains the two-phase region. The reason why fluorination of Nd1.3Sr1.7Cu2O6-y with NH4F Ca) can incorporate significant fluorine levels.This system is interesting because it demonstrates some diVerent character- does not form a similar phase may be related in some way to the slightly diVerent structure of the parent oxide or to the istics of each of the three fluorination routes leading to large diVerences in the final fluorinated products. In particular, fact that the starting oxygen content is lower ( y#0.35) so that an anion content (O+F) of 7.0 is not achieved after fluorination reaction with F2 gas appears to proceed via fluorine insertion, while reaction with NH4F appears to involve fluorine substi- of this phase.One problem with this postulation is that the fully fluorinated La1.9Sr1.1Cu2O6-y is orthorhombic, whereas tution for oxygen. The reaction with CuF2 at low temperatures (ca. 245 °C) probably involves both insertion and substitution the proposed compound La1.9Sr1.1Cu2O5F2 might be expected to be tetragonal. This indicates some sort of ordering must be mechanisms, while at higher temperatures (>300 °C) the substitution process dominates, and the copper oxidation state present, which may be related to O/F ordering, perhaps in the apical sites (if we assume that fluorine occupies only the apical remains at 2.0+.In addition, it has been found that heating samples prepared by the F2 gas route to temperatures >300 °C sites, then the composition of the apical positions consists of one oxygen plus two fluorine atoms). Alternatively it may be also results in a similar XRD pattern to reaction with NH4F. Thus the high copper oxidation states are only stable at low- related to a small excess of interstitial fluorine, which is suggested by the fluorine analysis.temperatures (ca. 200–250 °C), and so this fact can be used as a post-synthesis means of control of the fluorine content and The origin of the weak superconducting signal in La1.9Sr1.1Cu2O6-y at 20 K for low fluorine levels is unclear. copper oxidation state.If we consider the reactions with F2 gas and MF2 (low This signal has been seen reproducibly in a number of lightly fluorinated samples, but without any significant increase in temperature), the highest fluorine levels were achieved for A= Sr leading to a large expansion of the lattice parameters along signal strength, and is not present in the parent undoped material.It is possible that this signal may be due to the c (up to 2 A ° ) for such samples, suggesting that fluorine is being inserted into the vacant sites between the CuO2 layers, so that presence of a very small amount of the single copper layer phase La2-xSrxCuO4 impurity, which becomes supercon- the copper coordination changes from square pyramidal to octahedral. The magnitudes of the changes in c are also ducting after fluorination. The lack of bulk superconductivity in these systems may be possibly explained by the partial consistent with the expansion that would be needed for insertion of fluorine between the CuO2 layers to give two CuMF occupation of the anion sites between the CuO2 layers, since the location of isolated fluorine or oxygen atoms in these sites bonds of reasonable length (ca. 1.8 A ° ). There is also evidence that some excess fluorine is introduced in these systems, which would be expected to result in hole trapping. Recently superconducting related phases, Sr2Can-1 must be located in interstitial sites, presumably between the Ln/Sr bilayers, similar to the interstitial fluorine sites between CunO2n+xF2-y (n=2, 3), have been synthesised by a highpressure route with high Tcs (n=2, 99 K; n=3, 111 K).15 These the Sr bilayers in Sr2CuO2F2+d .4 A further interesting point is the distinct two-phase character observed for results support the view that it should be possible to induce superconductivity in the systems studied by suitable synthesis La1.9Sr1.1Cu2O6-y on fluorination, suggesting the stability of the high fluorine content phase.In the case of A=Ca, lower control. In this respect, if we assume that the occurrence of superconductivity is quenched by hole trapping by localised fluorine levels are observed, and the expansion along c is small (ca. 0.17 A ° ), suggesting that very little, if any, fluorine is now oxygen or fluorine atoms linking the CuO2 layers, then in principle, complete occupation of these anion sites should located between the CuO2 layers, with the fluorine possibly being located in the interstitial sites instead.eliminate such hole trapping. However, such a situation ultimately results in copper oxidation states which are too high These results can be explained by considering the size of the cation separating the two CuO2 layers. For phases with A= for superconductivity, e.g.for La1.9Sr1.1Cu2O6F the copper oxidation state would be 2.55+. It may therefore be possible Sr, there is a mixture of rare earth, Ln, and Sr separating the CuO2 layers, whereas for phases containing Ca, it is the Ca to use a combination of fluorination methods, involving the non-oxidative fluorination of the sample first with NH4F, that occupies these sites.13 Thus for the former systems the larger size of Sr compared to Ca (ionic radii for 8-coordination, which we believe leads to the replacement of one oxygen by two fluorine atoms and a consequent increase in the occupation Sr2+ 1.26 A ° , Ca 1.12 A ° )14 means that the separation of the layers is larger (e.g.La2SrCu2O6 ca. 3.7 A° , La2CaCu2O6 of the anion sites between the CuO2 layers.Reaction with F2 or MF2 could then be used to fill the remaining sites and ca. 3.3 A ° )13 and so fluorine can be inserted much more readily into the vacant sites between the layers. provide the necessary Cu oxidation. Alternatively, we have suggested that the high fluorine content phase produced by An interesting point is the fact that for the reactions with NH4F and CuF2 (high temperature) the samples the reaction of La1.9Sr1.1Cu2O6 with NH4F consists of replacement of one oxygen atom by two fluorine atoms to give a La1.9Sr1.1Cu2O6-y and Nd1.3Sr1.7Cu2O6-y showed diVerent behaviour, whereas for the other methods they behaved simi- stable system La1.9Sr1.1Cu2O5F2, where all the anion sites are filled, i.e.larily. For the former, a large increase in c was observed along with a distinct two-phase region similar to that observed for La1.9Sr1.1Cu2O6+2 F�La1.9Sr1.1Cu2O5F2+1 O reaction with F2 and MF2 (low temperature). In contrast, for both La1.9Ca1.1Cu2O6-y and Nd1.3Sr1.7Cu2O6-y the major (Cu oxidation state=2.05) expansion was along a and b, with little change along c.The Fluorine could then be inserted into the ierstitial sites of this origin of this diVerence is unclear.Further work, includphase to raise the Cu oxidation state to the optimum level by ing powder neutron diVraction studies and Eu Mo�ssbauer the oxidative fluorination with F2 gas or CuF2, i.e. experiments on Eu1.3Sr1.7Cu2O6-y (isostructural with Nd1.3Sr1.7Cu2O6-y), is planned to help rationalise this result. La1.9Sr1.1Cu2O5F2+0.5 F�La1.9Sr1.1Cu2O5F2.5 We suggest that the high fluorine content phase has a composition La1.9Sr1.1Cu2O5F2 (i.e.the substitution of one oxygen (Cu oxidation state=2.3) 2082 J. Mater. Chem., 1997, 7(10), 2077–2083Such two-stage fluorination processes could be used for a of superconductivity, these results further demonstrate that low-temperature fluorination is a powerful tool for the incor- range of cuprate systems to vary the O/F ratio, while achieving a copper oxidation state (ca. 2.3+) suitable for super- poration of fluorine and control of copper oxidation states in cuprates. Recent preliminary results have shown that these conductivity. Neutron diVraction studies are planned to try to confirm fluorination routes can also be applied to non-cuprate systems.these conclusions, particularly relating to the location of the We thank the EPSRC for financial support. fluorine atoms. In addition Eu Mo�ssbauer studies of the Eu1.3Sr1.7Cu2O5.65 system are planned to examine the variation in the Eu environment with increasing F content. With respect References to this system, and the related Nd-containing system, 1 M. Al-Mamouri, P.P. Edwards, C. Greaves and M. Slaski, Nature Nd1.3Sr1.7Cu2O5.65, one might expect that fluorine would (L ondon), 1994, 369, 382. initially insert along the one-dimensional channels in 2 M. Al-Mamouri, P. P. Edwards, C. Greaves, P. R. Slater and the structure (Fig. 2). Then after these sites have been filled, M. Slaski, J.Mater. Chem., 1995, 5, 913. the fluorine would presumably occupy the sites between the 3 P.R. Slater, P. P. Edwards, C. Greaves, I. Gameson, J. P. Hodges, CuO2 sheets. M. G. Francesconi, M. Al-Mamouri and M. Slaski, Physica C, 1995, 241, 151. Despite the lack of superconductivity, these results taken 4 P. R. Slater, J. P. Hodges, M. G. Francesconi, P. P. Edwards, together with previous studies on A2CuO3 (A=Ca, Sr, Ba), C. Greaves, I.Gameson and M. Slaski, Physica C, 1995, 253, 16. show that low-temperature fluorination is a powerful tool for 5 Z. Hiroi, M. Takano, M. Azuma and Y. Takeda, Nature (L ondon), the incorporation of fluorine in samples of this type. Moreover, 1993, 364, 315. these results on the double-layer cuprates show a clear diVer- 6 N. Nguyen, L. Er-rakho, C. Michel, J. Choisnet and B. Raveau, ence between the nature of the fluorination processes. Namely Mater. Res. Bull., 1980, 15, 891. 7 C. Michel and B. Raveau, Rev. Chim.Miner., 1984, 21, 407. the fluorination with NH4F appears to be essentially a 8 N. Nguyen, C. Michel, F. Studer and B. Raveau, Mater. Chem., non-oxidative substitution reaction, although the fact that 1982, 7, 413. superconducting Sr2CuO2F2+d can be prepared from Sr2CuO3 9 N. Nguyen, J. Choisnet and B. Raveau, Mater. Res. Bull., 1982, suggests that some oxidation, probably aerial oxidation, can 17, 567. occur.3 In contrast, the fluorination with F2 gas appears to 10 R. J. Cava, B. Batlogg, R. B. van Dover, J. J. Krajewski, proceed via an oxidative insertion reaction in this system, while J. V. Waszczak, R. M. Fleming, W. F. Peck Jr., L. W. Rupp Jr., P. Marsh, A. C. W. P. James and L. F. Schneemeyer, Nature reaction with MF2 (low temperature) involves both substi- (L ondon), 1990, 345, 602. tution and insertion. The reaction with Sr2CuO3 to give 11 K. Kinoshita, H. Shibata and T. Yamada, Physica C, 1991, 176, Sr2CuO2F2+d shows that the fluorination by F2 gas can also 433. involve partial substitition in addition to oxidative insertion. 12 R. Mahesh, R. Vijayaraghavan and C. N. R. Rao,Mater. Res. Bull., 1994, 29, 303. 13 R. C. Lobo Ph.D. Thesis, University of Birmingham, 1990. Summary 14 R. D. Shannon, Acta. Crystallogr., Sect. A, 1976, 32, 751. 15 T. Kawashima, Y. Matsui and E. Takayama-Muromachi, Physica The results clearly demonstrate that large amounts of fluorine C, 1996, 256, 313. can be incorporated into the Ruddlesden–Popper type cuprates Ln2-xA1+xCu2O6-y (L=La, Nd; A=Ca, Sr). Despite the lack Paper 7/03735J; Received 29th May, 1997 J. Mater. Chem., 1997, 7(10), 2077–208