~ Preparation and electrical conductivity of neodymium-europium oxide fluorides Masayuki Takashima, Susumu Yonezawa, Kiyoshi Horita, Kouichi Ohwaki and Hiroshi Takahashi Department of Material Science and Engineering, Faculty of Engineering, Fukui University, 3-9-1 Bunkyo, Fukui-shi 910, Japan Neodymium-europium oxide fluoride, Nd-EuO,F,, was prepared by heating appropriate mixtures of Nd203 and EuF, at temperatures ranging from 1273 to 1673 K for 2 h in argon. Two monophases, rhombohedral and tetragonal were identified. The rhombohedral phase was obtained at an EuF, composition of 50-55 mol%, and the tetragonal one was obtained in the composition range between 65 and 75 mol%. Nd2Eu2O3F6 gave a high electrical conductivity of 5 x lov2S cm-' at 923 K under an oxygen partial pressure of 0.4 Pa.The charge carrier was determined to be mainly the oxide ion. The transport numbers of the oxide ion and the electron were measured to be ca. 0.9 and (0.05, respectively, at temperatures between 723 and 923 K. Resulting from XPS measurements, it was found that: (1) the valence of Nd in Nd2Eu20,F6 was higher than in Nd203 and that of Eu was lower than in EuF,, i.e. the variance of the valency state of Nd (+ 3-+ 4) and Eu (+ 2-+ 3) affects the oxide ion conducting structure; (2) the covalency in the bonds between metal ions and oxide ions in Nd2Eu2O3F6 was weaker than that in Eu203 or Nd203; (3) the bond between the fluoride ion and the rare-earth-metal ion in Nd2Ln203F, was stronger than in the individual rare-earth-metal fluorides.These might result in increased oxide ion mobility. The complex oxides of zirconia or ceria have been widely investigated as oxide-ion-conducting solid electrolytes used for solid-state fuel cells and/or an oxygen sensors.1-6 These solid electrolytes must be heated to ca. 1273 K to reach the required conductivity, > 1 S cm-' for a fuel cell.7 In order to utilize the oxide-ion-conducting solid electrolyte for practical devices, it is desirable to lower the operating temperature to at least ca. 800 K. Many attempts to develop a new compound having a higher oxide-ion conductivity at even lower temperatures have been made.'-'' Neodymium-containing binary rare-earth-metal oxide fluorides (Nd&n,O,F,), which can be obtained as a solid solution of Nd203 and LnF,, have an oxide-ion conductivity more than ten times higher than that found for a stabilized zirconia, (Zr02)o~89(Y203)o~11, provided commer-cially as YSZ-1 1.I2-l6 Among these compounds, Nd2Eu2O3F6 was found to have the highest conductivity.In this paper we report the preparation and electrical conducting properties of NdzEu203F, and the results of our investigation of the valency state of each element in Nd2Eu203F6 in order to clarify the ionic conduction mechanism. Experimenta1 Nd203 and EuF, were commercially obtained from Shin-Etsu Chemical Ltd (purity 99.99%). Nd203 and EuF,, which were powdered (passed 400 mesh sieve), were mixed thoroughly together in appropriate molar ratios in a glove box filled with dry argon.The mixtures were compacted into an alumina boat made of 99.5% pure A120,, and fired for 2-3 h at temperatures ranging from 473 to 1673 K in argon. The products were analysed by X-ray diffractometry (XRD; Shimadzu XD-3As; Cu-Ka, 30 kV, 20 mA), X-ray photoelectron spectroscopy (XPS; Shimadzu ESCA-750; Mg target, 8 kV, 30 mA) and electrical conductivity measurements. The disk samples (diam- eter 20 mm, thickness 2 mm) for the electrical conductivity measurements were prepared by hot-pressing at 1473 K under 25 MPa cm-2 for 3 h in argon. Fig. 1 shows the cell assembly designed specially to measure the electrical conductivity and the transport number of a sintered disk sample. The impedance was measured by an ac method scanning from 10MHz to 20kHz at temperatures between 500 and 1000K under a partial pressure of oxygen ranging from 1 x to 2 x 104 Pa.The ionic and electronic transport numbers were examined by the emf method using an oxygen gas concentration cell17 and the dc polarization method," respectively. The charge-carrying species in NdzEu20,F, was identified by an electrolysis method using a cell as shown in Fig. 2. After electrolysis by a constant current of 1 mA for 12 h at 923 K in argon, the anode mixture (equimolar mixture of NiO and Ni) was analysed by means of X-ray diffractometry and a fluoride-ion-selective electrode method. Details of the electrical conductivity measurements have been reported previo~s1y.l~ Fig. 1 Cell assembly used for the electrical conductivity measurements.(1)Sintered sample; (2) Pt electrode; (3) Pt lead wire; (4)thermocouple (CA); (5) quartz mantle; (6) electrode holder; (7) quartz spacer; (8) guard electrode; (9) brass rod with spring to set electrode; (10) gas inlet/outlet; ( 11)furnace; ( 12) ground. Fig. 2 Cell used to examine the charge carrier via electrolysis. (1) Sintered sample; (2) Pt electrode; (3) compacted powder layer of Ni +NiO mixture; (4) quartz tubing; (5) thermocouple (CA). J. Muter. Chem., 1996, 6(5), 795-799 795 Results and Discussion Fig 3 shows the XRD profiles of powder samples, prepared from the Nd20,-EuF, system in a molar ratio of 1 2 by firing for 2 h at various temperatures In profile (2) of the product at 473 K, a small new diffraction line appeared around 28= 32" as shown in the inset This diffraction line became distinct as the reaction temperature was increased Profile (3) at 873 K could be separated into two patterns assigned to rhombohedral and tetragonal phases Compared with the XRD profiles of Nd and Eu oxide fluorides prepared separately, the rhombo- hedral product (lattice parameters a, =0 71 nm, a=34 2") was identified as NdOF and the tetragonal phase (ao=O 557 nm, c, =0 562 nm) corresponded to EU403F6 Diffraction profile (5) showed that the product at 1273K was a monophase which was identified as the tetragonal structure, as indicated by the Miller indices shown During the sample firing at 1273 K, the mass of the sample was found to decrease by <10 mass% Therefore, Nd,Eu,O,F, was produced stoichio-metrically from the mixture of 1mol Nd203 and 2 mol EuF, at 1273K Fig 4 shows the phases of the products from Nd20,-EuF, mixtures at various molar ratios The boundary lines shown are drawn on the basis of the intensity of the main peak of the XRD profile for Nd203, rhombohedral product, tetragonal product and EuF, The x axis shows the nominal compositions of EuF, The rhombohedral monophase was obtained at 50-55 mol% EuF,, and Nd,EuO,F, was formed from the equimolar mixture of Nd203 and EuF, The tetra- gonal monophase showed a wide homogeneous composition range from 65 to 75molYo EuF, The stoichiometric com- pound, Nd2Eu2O3F6, was obtained from a mixture of 1 mol Nd203 and 2 mol EuF, Fig 5 shows the relationship between the electrical conductivity and the nominal composition of EuF, in the products from the Nd203-EuF3 system The 1 I 1 I 1 20 30 40 50 60 2eldegrees (CU-KU) Fig.3 XRD profiles of the products prepared from the mixture of 1 mol Nd20, and 2 mol EuF, by finng at various temperatures for 2 h in argon Reaction temperature (1) starting mixture, (2) 473 K, (3) 873 K, (4) 1173 K, (5) 1273K 0,Tetragonal, 0,rhombohedral 796 J Muter Chem , 1996, 6(5), 795-799 1 v) a,.!= v)c 0, r Y8L1 + 0 4-2 02 04 06 08 EuF, (mol%) Fig.4 Phases in the products from the Nd,O,-EuF, system (a) Rhombohedra1 phase, (b) tetragonal phase TetragonalP 0.5 0.6 0.7 0.8 LnF3(mol%) Fig.5 Relationship between the electrical conductivity and the nom- inal composition of LnF, in products prepared from Nd,O,-LnF, (Ln=Y, Eu) systems Electncal conductivities measured at 923 K under an oxygen partial pressure of 1 33 x 10 Pa (1) Nd203-YF3 system," (2) Nd,O,-EuF, system electrical conductivity of the tetragonal phase is ca 1OOx higher than that of the rhombohedral phase The charge carrier in Nd2Eu20,F6 was investigated by an electrolysis method If the fluoride ion is mobile within N~,Eu,~~F~,the anode mixture (Ni+NiO) should be con- verted by the electrolysis into fluorine-containing compounds such as nickel fluoride and/or nickel oxide fluorides However, no diffraction peaks due to fluorine-containing nickel com- pounds could be found in the X-ray diffraction profile of the anode mixture after electrolysis at 923 K The Ni peaks disap- peared and only those due to Ni0 remained There were no changes in the diffraction profiles of the anode mixtures after heat treatment at 923 K Fluoride ion could not be detected using a fluoride-ion selective electrode in the solution of the anode mixture after electrolysis at 923 K and dissolving in aqueous HC1 solution These facts indicate that the oxide ion is the charge carrier in Nd2Eu203F6 Fig 7(a) and (b) show the Arrhenius plots of the electrical conductivities measured under an oxygen partial pressure of 0 4 Pa and the oxide ion transport numbers for Nd2Eu2o3F6, Nd,Y,O,F, and YSZ-11 Nd2Eu20,F, and Nd2Y2O3F6 showed higher conductivities than YSZ-11 The conductivity at 873 K of Nd,Eu203F6 was ca 3 5 x lop2S cm-I which corresponds to the value for YSZ-11 at 1273 K The activation energy of Nd,Eu,O,F, was calculated to be 70 kJ mol-l, which is lower than those of Nd2Yzo3F6 and YSZ-11 (100 kJ mol-') This seems to reflect I NiO(200) 1058 K\ NiO( 1 1 1)L(1) 40 50 2eldegrees (Cu-Ka) Fig.6 XRD profiles of the Ni +NiO anode mixture. ( 1)After electroly- sis at 1 mA for 12 h at 923 K; (2) after heat treatment for 12 h at 923 K without electrolysis. TIK 973 873 773 723 1.1 1.2 1.3 1.4 l@ WT 0.9l'*/ g! 0.8 3 Uc& 0.7 5 A o-lLdJ0.0 673 773 873 973 TJK Fig. 7 (a) Arrhenius plots of the electrical conductivity of Nd2Eu203F6, Nd,Y,O,F, and (Zr02)o.89(Y203)o.11 under an oxygen partial pressure of 0.4Pa.(1) Nd,Eu,O,F,; (2) Nd,Y,O,F,; (3) YSZ-11. (b) Oxide ion transport numbers for Nd,Eu,O,F, (4) and Nd,Y,O3F, (5); electron transport number for Nd,Eu,O,F, (6). that Nd2Eu203F6 may have a crystal structure in which there is an easy path for the rapid migration of oxide ions. The oxide ion transport number for Nd2Y2O3F6 decreased with decreasing temperature [Fig. 7( b), curve (5)].I9 The oxide ion transport number of NdzEuz03F6 [Fig. 7( b), curve (4)] remains at ca. 0.9 between 723 and 923 K. Moreover, the electron transport number of this compound, measured by means of the dc polarization method, is <0.05, as shown in Fig. 7(b), curve (6).The TG curves of Nd2Eu20,F6 (Fig. 8), show that this compound is completely stable up to ca. 1000 K I I I I I I I I I 473 673 873 1073 1273 TIK Fig. 8 TG curves of Nd2Eu2O3F, in (1) air and (2) argon at 10°Cmin-' 0 Nd 0 Eu F @o t, Fig. 9 The double layered fluorite structure of Nd,Eu,O,F,. uo= 0.5641, co= 1.1227 nm. in argon and/or air. This result indicates that Nd,Eu2O3F6 is applicable as the solid electrolyte in a fuel cell and/or an oxygen sensor, which can be operated at most at 900K, a temperature which seems sufficiently low for practical uses. In order to obtain good ionic conduction, a special arrange- ment of the cations and anions is required, in order to produce an easy path for the rapid migration of ionic species.In oxide- ion-conducting ceramics such as stabilized zirconia, the oxide ion conduction has been explained by a mechanism based on the oxide ion defect ~tructure.~As a result of the crystal structure analyses by computer simulation,j- which shows a structure in which the oxide ion is mobile, we proposed a special model for the crystal structure, the 'double-layered fluorite structure', as shown in Fig. 9.20 It is supposed that the variable valences of Nd (+ 3-+4) and Eu (+2-+3) result in an anionic defect structure in which the migration of only oxide ions is possible. Fig. 10 shows the X-ray photoelectron t Details of the crystal structure analysis of Nd2Eu2O3F6 by Rietveld refinement are available as supplementary data (SUP No.57130)from the British Library. For details of the Supplementary Publications Scheme, see Information for Authors, Issue 1 of J. Muter. Chem. J. Muter. Chem., 1996, 6(5),795-799 797 990 9854 980 150 14&178 0 984 2 139 2 binding energy/eV Fig. 10 XP spectra for (a) Nd 3d,,, and (b) Eu 4d,,, electrons of NdzEu,03F, ( 1 ), Nd,O, (2)and EuF, (3) spectra (XPS) of the Nd 3d5,2 and Eu 4d5,2 electrons in Nd203, EuF, and Nd2Eu203F6 and the binding energy of the Nd 3d,,, electron in Nd2Eu20,F6 is higher than that in Nd203 In contrast, the binding energy of the Eu 4d5/2 electron (138 0 eV) showed a downward shift of 12 eV with respect to that in EuF, These facts indicate that the valence of Nd in Nd2Eu2O3F6 is higher than that in the pure oxide, and that of Eu is lower than that in its fluoride Fig 11 shows XP spectra of the 0 1s electrons in Nd2Eu203F6, Eu203 and Nd203 Each spectrum can be separated into two peaks as shown by the dashed lines The peak at higher binding energy is assumed to be due to a rare-earth-metal hydroxide formed on the sample surface We have confirmed that the rare-earth-metal oxide is easily hydrolysed on its surface by exposure to an ordinary atmosphere The peak at lower binding energy corre- sponds to the 0 1s electron involved in metal-oxygen bonding The peak for Nd2Eu203F6 is at a slightly lower binding energy than those of Eu203 and Nd203, ie the oxide ion in Nd2Eu20,F6 tends to be more negative than in the separate oxides This shows that the covalency in the bond between the 529.7 (1) 531 9A 535 530 525 535 530 525 binding energylev Fig.11 XP spectra for the 0 1s electrons of Nd,Eu,O,F, (l), Eu,03 (2) and Nd203 (3) 798 J Muter Chem, 1996, 6(5),795-799 6877 68157 binding energy/eV Fig. 12 XP spectra for the F 1s electrons of PTFE (l), EuF, (2) Nd,Eu,03F6 (3) and KF (4) P 4.0E 3.0 E;2.0 2 5 I I 0-(3) -686685 7 cV Fig. 13 Binding energy shifts of the F 1s electron from the standard (686 eV) for all compounds of Nd,Ln,03F, (Ln=Y-Lu) (1) PTFE, (2) BaF2, (3) KF, (4)LnF3, (5)Nd2Ln203Ffj metal ion and oxide ion in Nd2Eu203F6 is weaker than that in Eu20, or Nd203 Fig 12 shows the F 1s spectra of Nd2Eu2O3F6, PTFE (as a compound with covalent bonds), EuF, and KF (of which the chemical bond is almost completely ionic) The binding energy of the F 1s electron in Nd,Eu203F6 is lower than that in EuF, The binding energy shifts of the F 1s electron from the standard energy of 689eV for all Nd2Ln2O3F6compounds are summarized in Fig 13 All energy shifts for Nd2Ln2O3F6 compounds are smaller than those in the rare-earth-metal fluorides This indicates that the bond between the fluoride ion and the rare-earth-metal ion in Nd2Ln2O3F6is stronger than in the rare-earth-metal fluorides, ie the ionic interaction of the oxide ion with the rare-earth- metal ion becomes weaker, and the mobility of the oxide ion increases This work was partially supported by a Grant in Aid for Scientific Research on Priority Areas (grant no 07239218) from the Ministry of Education, Science and Culture of Japan References 1 T H Etsell and S N Flengas, Chem Rev, 1970,70,339 2 A Kvist, in Physics ofElectrolytes, ed J Hladik, Academic Press, London, 1972, vol 1, p 319 3 K S Goto, Solid State Electrochemistry and Its Applications to Sensors and Electronic Devices, Elsevier, Amsterdam, 1988 4 5 A.S. 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