METAL OXYHALIDE MATERIALS, THEIR USE AND METHODS FOR THEIR PRODUCTION

Abstract: A halogen-containing copper oxide of the formula An+1CunO2nX2n+ alpha where A represents one or more alkaline earth metals, X represents halogen, preferably fluorine, n is 1 or 2 and 0 < alpha </= 0.6 is useful as a superconductive material. The oxide may further contain carbonate. The oxide is produced by halogenating a copper oxide of the formula An+1CunO2n+1, where A and n are as defined above. The halogen plays a dominant structural role in the material.

Description: METAL OXYHALIDE MATERIALS, THEIR USE AND METHODS FOR THEIR PRODUCTION This invention relates to metal oxyhalides, their use as superconductors and methods for their production. High pressure synthesis has proved a useful technique for obtaining new, metastable copper oxide superconductors; for example, oxygen insertion into Sr 2 Cu0 3 at 6 GPa (Hirol, Z. et al, Nature 364, 315-317 (1993)) yields superconducting Sr 2 Cu0 3 ,, with a superconducting transition temperature (T c ) of 70 K, in which the superconducting Cu0 2 layers are generated by pressure-induced oxygen migration from apical to equatorial sites. Although the simple structure and high transition temperatures make this family (general formula Sr n+ 1 Cu n 0 2n+ 1 +δ ) of interest, the stringent synthesis conditions limit its value for applications. Metal oxide superconductors containing fluorine as an electronic dopant are per se known, for example La 2 Cu0 4 F x is disclosed by Tissue, B. et al in Solid St. Commun. 65, 51-54 (1988), and Nd 2 Cuθ 4 _ x F y is disclosed by James, A. et al in Nature 338, 240-241 (1989). We have discovered a novel class of metal oxyhalide materials in which the halide plays a dominant structural role, rather than merely being an electronic dopant, and which are superconductors with a maximum T c of 46 K. Thus, according to a first aspect of the present invention, there is provided a halogen-containing copper oxide of the formula A n+ 1 Cu n 0 2n X 2n+α where A represents one or more alkaline earth metals, X represents halogen, n is 1 or 2 and 0 < α < 0.6 . According to a second aspect of the present invention, there is provided a superconductive material comprising a halogen-containing copper oxide of the formula A n+ 1 Cu n 0 2n X 2n+a where A, X, n and α are as defined above. Preferably, α is 0.2 to 0.5. High pressure methods have allowed the synthesis of metastable phases (such as YBa 2 Cu 4 0 8 and YSr 2 Cu 3 0 7 ) which are difficult, or impossible, to obtain under ambient conditions. A recent observation relates to the oxidation of Sr 2 Cu0 3 , the structure of which can simply be derived from that oi La 2 Cu0 4 by removing chains of oxygen atoms from the Cu0 2 layers responsible for superconductivity in doped variants such as (La,. x Sr x ) 2 Cu0 4/ and La 2 Cu0 4+x . At 6 G Pa and 800-900°C, Sr 2 Cu0 3 absorbs small amounts of oxygen and transforms structurally (by oxygen transfer from apical to equatorial positions around Cu) to a metastable superconducting phase Sr 2 Cu0 3 ,, T c = 70 K (see Hirol et al, supra), Sr 3 Cu 2 0 5+0 , the next member of this superconductor family, was reported to have T c = 100 K (see Hirol et al, supra). Given that in both superconducting alloys and oxides the highest transition temperatures appear to occur in metastable phases, the development of alternative chemical routes to simulate the thermodynamic effects of high pressures is of fundamental importance. We have found that it is possible to insert elemental fluorine at relatively low temperatures and ambient pressure. Thus, according to a third aspect of the present invention, there is provided method of producing a halogen-containing copper oxide of the formula A n+ 1 Cu π θ 2n X 2rι+α where A, X, n and α are as defined above, comprising halogenating a copper oxide of the formula A n+ 1 Cu n 0 2n+ 1 , where A and n are as defined above. Halogenation (most preferably fluorination) may be effected using a gaseous halogenating agent (eg the halogen itself) at a temperature in the range of 100 to 400°C, more preferably 150 to 250°C, and most preferably 180 to 230°C. This fluorination step has the advantage that it can be conducted at ambient pressure. As an attractive alternative to the use of a halogen, halogenation may be effected in the solid phase using an ammonium halide (preferably ammonium fluoride or ammonium chloride) or in the gaseous or liquid phase using an organoammonium halide (eg an alkylammonium halide such as a tetraalkylammonium halide, for example tetramethyl- or tetraethyl- ammonium fluoride). Such halogenation processes may be used to advantage for the production of halogen-containing metal oxide or metal oxide carbonate superconductors generally. Thus, according to a fourth aspect of the present invention, there is provided a method of producing a halogen-containing, metal oxide or metal oxide carbonate superconductor comprising introducing a halogen into a metal oxide or metal oxide carbonate using an ammonium halide or an organoammonium halide, eg ammonium fluoride or a tetraalkylammonium fluoride. In one convenient embodiment, 1 mole of Sr 2 Cu0 3 is reacted with 2 to 20 moles of NH 4 F at 225°C for 10 hours to produce a mixture of SrF 2 and Sr 2 Cu0 2 F 3+α with a T c of 46K. In another convenient embodiment, 1 mole of (S s Ba 05 )CuO 3 is reacted with 2 to 20 moles of NH 4 F at 225°C for 10 hours to produce a mixture of (Sr,Ba)F 2 and Sr, 5 Ba 05 Cu 2 F 2+α with a Tc of 60K. In a further convenient embodiment, the cuprate carbonate (Ba x Sr,. x ) 2 Cu0 2 (C0 3 ) is fluorinated using NH 4 F to produce (Ba x Sr 1 . x ) 2 Cu0 2 (C0 3 )F α , where 0 < x < 1 . Thus, in a modification the halogen-containing copper oxide according to said first and second aspects of the present invention may also contain carbonate. Whilst the method according to the fourth aspect of the present invention may be applied to the production of per se known halogen-containing metal oxide superconductors such as La 2 Cu0 4 F x and Nd 2 Cu0 4.x F y noted above, it is preferably applied to the production of the novel superconductors according to said second aspect of the present invention, possibly modified by the additional presence of carbonate. The fluorination step in the method according to said third and fourth aspects of the present invention favours production of A n+ 1 Cu n 0 2n X 2n+ o. 6 - Most preferably, the material which has been subjected to halogenation is heat-treated in an inert gas atmosphere, eg nitrogen or a mixture of hydrogen and nitrogen, (without the need to use an elevated pressure) to promote loss of halide whereby to decrease the value of α until it is in the range of 0.2 to less than 0.6, preferably in the range of 0.2 to 0.5 in the case where a superconductive, halogen-containing copper oxide according to said second aspect of the present invention is required. In one embodiment, heat treatment of Sr 2 Cu0 2 F 2 . 6 in nitrogen for up to 3 hours at temperatures up to 330°C reduced the α value from 0.6 down to as low as 0.4. In another embodiment, heat treatment of Sr 2 Cu0 2 F 2 6 in a 10% hydrogen/nitrogen mixture for 15 minutes at temperatures up to 220°C reduced the α value from 0.6 to between 0.2 and 0.4 inclusive. The alkaline earth metal represented by A is preferably at least one of magnesium, calcium strontium and barium, more preferably at least one of strontium and barium. The halide represented by X is preferably fluorine or chlorine, with fluorine being more preferred. In the accompanying drawings:- FIG. 1 shows powder X-ray diffraction patterns (Cu Kα1) of Sr 2 Cu0 3 before (top) and after (bottom) fluorination. The SrF 2 impurity is marked* FIG. 2 shows the variation of T c (top) and superconducting fraction (bottom) with copper oxidation state for Sr 2 CuO 2 F 2+0 . FIG. 3 shows the variation of molar susceptibility of Sr 2 Cu0 2 F 233 with temperature for both field-cooled (FC) and 'zero field-cooled (ZFC) experiments, FIG. 4 shows a simplified structure of Sr 2 Cu0 2 F 2+δ showing idealized apical (F) and interstitial (F 1 ) sites, FIG. 5 shows the powder X-ray diffraction pattern of fluorinated Sr, s Ba 0 S CuO 2 (CO 3 ), and FIG. 6 shows the magnetic susceptibility temperature dependence of fluorinated Sr, s Ba 05 CuO 2 (CO 3 ). The present invention will now be described in further detail in the following Examples. Example 1 Sr 2 Cu0 3 (prepared from SrC0 3 and CuO, using two heat treatments at 950°C for 16 h in air) was subjected to thermal treatments in a nickel furnace tube in a mixture of 10% F 2 /90% N 2 from which traces of HF have been removed by passing over NaF. Although moderate temperatures (400°C) were found to yield predominantly SrF 2 owing to its high thermodynamic stability, at 210°C this yield of this reaction product was minimised (estimated at ~ 5 - 10% by X-ray diffraction). Upon treatment for 15 min at 210°C, a new orthorhombic (Fmmm) phase was formed with unit-cell dimensions implying a close structural relationship to La 4 Cu0 4 : a = 5.394(1 ), b = 5.513(1 ), c= 13.468(3) A (see attached Fig 1 ). In one particular treatment, a 200-mg sample of Sr 2 Cu0 3 in a nickel boat was heated to 210°C in dry N 2 at normal ambient pressure, subjected to a flowing F 2 /N 2 atmosphere for 15 min at this temperature, and allowed to cool in N 2 or air after flushing all F 2 from the system. The F content was estimated by precipitation titrations using thorium nitrate solution with methyl thymol blue indicator and comparison with standard solutions containing similar cation concentrations. Thermogravi metric analysis (H 2 /N 2 mixture at 930°C to form SrF 2 , SrO, Cu) followed by quantitative X-ray diffraction determination of the SrF 2 :SrO ratio, provided an independent measure of the F:0 ratio in the material; the Cu oxidation state was evaluated by iodometric titration. All results were consistent with the composition Sr 2 Cu0 2 F 2 6 (formal Cu oxidation state + 2.6), which implies significant fluorine substitution for oxygen, in addition to fluorine insertion. In view of the instability of Sr 2 Cu0 2 F 2 6 at sintering temperatures, resistance measurements were not attempted and its superconducting properties were examined magnetically with a d.c. SQUID magnetometer, using a field of 10 "3 T. Some samples were superconducting but with small ( < 1 %) superconducting volume fractions; low temperature reductions resulted in fluorine loss to give Sr 2 Cu0 2 F 2+α with improved superconducting properties. Samples where therefore annealed for up to 3h in N 2 (temperatures up to 330°C, 0.4 < α < 0.6) or for 15 min in a 10%> H 2 /N 2 mixture ( temperatures up to 220°C, 0.2 < α< 0.4). The superconducting properties (T c and percentage flux expulsion) are shown in attached Fig 2 in relation to the Cu oxidation state, and the susceptibility for the sample with T c = 46 K is shown in attached Fig 3. The calculated superconducting volume fractions are small for all samples; this is attributed to particle size effects as it is well established that the magnetization is reduced for powders comprising crystallites with dimensions comparable to the London penetration depth. This interpretation is supported by the broad X-ray diffraction peaks for Sr 2 Cu0 2 F 2 6 (Fig 1), which indicate a mean crystallite size of only - 700A, and the small divergence between field-cooled and zero field-cooled susceptibilities (Fig 3). The data in Fig 2 suggests that T c and the superconducting volume fraction may show a broad maximum at a formal Cu oxidation state of +2.2 to +2.3, which is similar to other copper oxide superconductors. The wide range of oxidation states for which superconductivity is observed is thought to reflect composition variations, which are to be expected due to the low temperatures of the synthesis conditions. Although neither X-ray nor neutron diffraction can differentiate O and F ions, bulk structural characteristics were investigated using powder neutron diffraction data (collected on a 4-g sample using the POLARIS diffractometer, Isis Rutherford Appleton Laboratory) to aid accurate location of the light atoms. Although structure refinement based on a simple La 2 Cu0 4 - type model produced an unacceptably high and anisotropic temperature factor for the apical anions (B u = 6A 2 , B 22 = 23A 2 , B 33 = OA 2 ) when considered as a single site, splitting this site into two positions was more satisfactory and resulted in sensible isotropic thermal parameters. This feature is similar to that observed in La 2 Cu0 4+δ ( see Chaillot, C. et al, Physics C159, 183 (1989) and Radaell, et al Phys. Rv. B48,499 (1993) ) and suggests the presence of interstitial F ions, consistent with the formula Sr 2 Cu0 2 F 2+α . Inclusion of such ions near the ideal/(!4, IVA , IVA) position produced a stable refinement, and the results are shown in Table 1 below. The interstitial F (F(3)), which would have four unacceptably short contacts ( -2.15A) to neighbouring F ions in an undistorted structure, seems to be displaced away from three neighbours towards the fourth, which subsequently shifts by -0.6Λ to the F(2) site. In this way, four satisfactory F-F distances are achieved (Table 1). Although the defect model is slightly different from those proposed for La 2 CuO,^s , one of which involves a very short O-O distance, it is chemically plausible and avoids all close F-F contacts. In accordance with this model, the F(3) and F(2) occupancies were constrained to be equal in the final refinement, which suggested the composition Sr 2 Cu0 2 F 2 S7 , in excellent agreement with analysis. Bond valence sum calculations suggested a Cu oxidation state of 2.23 (using Cu 2+ values for r 0 -see Brown, I et al Acta Crystallogr. B41, 244-247 (1985) ), which is lower than the expected value of +2.6. Madelung energies were calculated for different distributions of O and F atoms in the parent Sr 2 Cu0 2 F 2 , and indicated a substantial preference for apical F (1 1 ,072 kjmol- 1 ) rather than equatorial F (10,138 kjmol '1 ) and provided strong support for the presence of only apical F. The structure of Sr 2 Cu0 2 F 2 6 is shown in simplified form in Fig 4. The evidence that F avoids the equatorial sites clearly demonstrates the potential role of oxy-fluorides for forming new high temperature copper oxide superconductors; superconductivity in Sr 2 Cu0 2 F 2 has been induced by incorporating interstitial F. The partial replacement of the apical F ions by O 2" could provide an alternative means of an ionic chemical control of the electron carrier density, in analogous fashion to Sr and Ba substitutions for La in La 3 Cu0 4 . Of greater significance is the demonstration that the F site preference can promote interchange of O and F ions to create the structural features necessary to support superconductivity in phases which are otherwise unsuited, for example, Sr 2 Cu0 3 . TABLE 1 Refined structural data for 5r 7 CuO,F, Atom Position X y z B(A 2 ) ι Cell I occupancy Sr δi 0 0 0.3680(1) 0.67(2) 8 Cu 4a 0 0 0 0.66(3) 4 0 8e 0.25 0.25 0 0.76(3) 8 F(1) 32p 0.0358(8) 0.0483(7) 0.1802(4) 0.67(7) 5.72(5) F(2) 32p 0.059 0.151 (2) 0.1802(4) 0.67(7) 2.28(5) F(3) 32p 0.196(2) 0.239(2) 0.2212(8) 0.67(7) 2.28(5) Unit cell dimensions a = 5.394(1)A, b = 5.513(1)A, c = 13.468(3)A; space group Fmmm; R factors; R wp = 3.7%, R exp = 1.2 o, R,= 7.2%. Bond distances (A): Cu-O, 1.928(1) [x4]; Cu-F(1 ), 2.45(1); Cu-0/F(2), 2.59(1 ); F(3)-F(1); 2..50(3). 2.65(3), 2.56(3); F(3)-F(2), 2.55(3). Example 2 1 mmol of Sr, s Ba 05 CuO 2 (CO 3 ) (having a (Ba x Sr 1 . x ) 2 Cu0 2 (C0 3 ) structure) was intimately ground with 0.5 mmol of NH 4 F and the resultant brown coloured mixture was then heated in air for 12 hours at 200°C to produce a black powder. Such black powder was characterised by both X-ray diffraction (PXD) and magnetic susceptibility measurements. PXD confirmed that this new material retained the (Ba x Sr 1 . x ) 2 Cu0 2 (C0 3 ) structure, but gave no information as to the location of the fluorine (see Fig. 5). The magnetic susceptibility measurements showed the presence of a superconducting transition at a temperature of about 55 K (see Fig. 6). Although the location of the fluorine has not yet been established, it is possible to envisage that fluorine insertion occurs in the carbonate layer, thus doping the Cu0 2 planes. The resulting structure could therefore be described as (Ba x Sr 1 . x ) 2 Cu0 2 (C0 3 )Fα.

Claims: CLAIMS 1. A halogen-containing copper oxide of the formula A n+ 1 Cu n 0 2n X 2n+α where A represents one or more alkaline earth metals, X represents halogen, n is 1 or 2 and 0 < α < 0.6 . 2. A halogen-containing copper oxide as claimed in claim 1 , wherein α is 0.2 to 0.5. 3. A superconductive material comprising a halogen-containing copper oxide of the formula A n+ 1 Cu n 0 2n X 2n+α where A, X, n and α are as defined in claim 1 or 2. 4. A method of producing a halogen-containing copper oxide of the formula A n+ 1 Cu n θ 2n X 2n+α where A, X, n and α are as in claim 1 or 2, comprising the step of halogenating a copper oxide of the formula A n+ 1 Cu n 0 2n+ 1 , where A and n are as defined in claim 1. 5. A method as claimed in claim 4, wherein the halogenating step is a fluorinating step. 6. A method as claimed in claim 4 or 5, wherein the halogenating step is effected using a gaseous halogenating agent. 7. A method as claimed in claim 6, wherein the halogenating agent is the halogen. 8. A method as claimed in claim 4, 5, 6 or 7, wherein the halogenating step is effected at a temperature in the range of 100 to 400°C. 9. A method as claimed in claim 8, wherein the halogenating step is effected at a temperature in the range of 150 to 250°C. 10. A method as claimed in claim 8, wherein the halogenating step is effected at a temperature in the range of 180 to 230°C. 1 1. A method as claimed in any one of claims 4 to 8, wherein the halogenating step is effected at ambient pressure. 12. A method as claimed in claim 4 or 5, wherein the halogenating step is effected in the solid phase using an ammonium halide or in the gaseous or liquid phase using an organoammonium halide. 13. A method of producing a halogen-containing metal oxide or metal oxide carbonate superconductor, comprising the step of introducing a halogen into a metal oxide or metal oxide carbonate using an ammonium halide or an organoammonium halide. 14. A method as claimed in claim 13, wherein the ammonium halide is ammonium fluoride. 15. A method as claimed in claim 13, wherein the organoammonium halide is a tetraalkylammonium fluoride. 16. A method as claimed in claim 4 or 13, wherein 1 mole of Sr 2 Cu0 3 ' is reacted with 2 to 20 moles of NH 4 F at 225°C for 10 hours to produce a mixture of SrF2 and Sr 2 Cu0 2 F 3+α with a T c of 46K. 17. A method as claimed in claim 4 or 13, wherein 1 mole of (Sr, s Ba 05 )CuO 3 is reacted with 2 to 20 moles of NH 4 F at 225°C for 10 hours to produce a mixture of (Sr,Ba)F 2 and Sr, s Ba 0 S Cu 2 F 2+α with a Tc of 60K. 18. A method as claimed in claim 4 or 13, wherein the cuprate carbonate (Ba x Sr 1 . x ) 2 Cu0 2 (C0 3 ) is fluorinated using NH 4 F to produce (Ba x Sr,. x ) 2 Cu0 2 (C0 3 )Fα, where 0 < x < 1 . 19. A modification of the halogen-containing copper oxide according to any one of claims 1 to 3, wherein the halogen-containing copper oxide also contains carbonate. 20. A method as claimed in claim 13, for the production of a halogen- containing copper oxide as claimed in any one of claims 1 to 3 or 19. 21. A method as claimed in claim 4 or 13, wherein the material which has been subjected to halogenation is heat-treated in an inert gas atmosphere to promote loss of halide whereby to decrease the value of α until it is in the range of 0.2 to less than 0.6. 22. A method as claimed in claim 4 or 13, wherein the material which has been subjected to halogenation is heat-treated in an inert gas atmosphere to promote loss of halide whereby to decrease the value of α until it is in the range of 0.2 to 0.5 in the case where a superconductive, halogen-containing copper oxide according to claim 3. 23. A method as claimed in claim 22, wherein heat treatment of Sr 2 Cu0 2 F 2 6 is effected in nitrogen for up to 3 hours at temperatures up to 330°C to reduce the α value from 0.6 down to 0.4. 24. A method as claimed in claim 22, wherein heat treatment of Sr 2 Cu0 2 F 2 6 is effected in a 10%. hydrogen/nitrogen mixture for 15 minutes at temperatures up to 220°C to reduce the α value from 0.6 to between 0.2 and 0.4 inclusive.

Inventors: AL-MAMOURI MALEK, EDWARDS PETER, FRANCESCONI MARIA GRAZIA, GAMESON IAN, GREAVES COLIN, HODGES JASON, SLATER PETER RAYMOND, AL-MAMOURI, MALEK, EDWARDS, PETER, FRANCESCONI, MARIA, GRAZIA, GAMESON, IAN, GREAVES, COLIN, HODGES, JASON, SLATER, PETER, RAYMOND
Applicants: AL MAMOURI MALEK, EDWARDS PETER, FRANCESCONI MARIA GRAZIA, GAMESON IAN, GREAVES COLIN, HODGES JASON, SLATER PETER RAYMOND, UNIV BIRMINGHAM, AL-MAMOURI, MALEK, EDWARDS, PETER, FRANCESCONI, MARIA, GRAZIA, GAMESON, IAN, GREAVES, COLIN, HODGES, JASON, SLATER, PETER, RAYMOND, THE UNIVERSITY OF BIRMINGHAM
Application year: 1995
Main classification: C01G3
IPC codes: C01G1, C01G3, H01L39