Synchrotron X-ray Diffraction Study of Nanostructured Er 2 O 3 TiO 2 (50 60 mol % Er 2 O 3 ) Solid Solutions

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1 ISSN -8, Inorganic Materials,, Vol. 9, No., pp. 9. Pleiades Publishing, Ltd., Original Russian Text L.P. Lyashenko, L.G. Shcherbakova, E.S. Kulik, R.D. Svetogorov, Ya.V. Zubavichus,, published in Neorganicheskie Materialy,, Vol. 9, No., pp.. Synchrotron X-ray Diffraction Study of Nanostructured Er O TiO ( mol % Er O ) Solid Solutions L. P. Lyashenko a, L. G. Shcherbakova b, E. S. Kulik c, R. D. Svetogorov c, and Ya. V. Zubavichus c a Institute of Problems of Chemical Physics, Russian Academy of Sciences, pr. Akademika Semenova, Chernogolovka, Moscow oblast, Russia b Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina, Moscow, 999 Russia c National Research Centre Kurchatov Institute, pl. Kurchatova, Moscow, 8 Russia lyash@icp.ac.ru Received June, Abstract Monochromatic synchrotron X-ray diffraction data demonstrate that single-crystal and polycrystalline xer O ( x)tio (x =..) solid solutions consist of a fluorite-like disordered (Fmm) phase and a nanoscale ( nm) pyrochlore-like ordered phase (Fdm) of the same composition in the range. x.7, coherent with the disordered phase. Reducing the density of structural defects in the unit cell of Er TiO. (x =.) leads to a structural transformation of the pyrochlore-like phase into a Ta O -type ordered phase (Ia), derived from the fluorite phase. In the composition range of the solid solutions (. < x <.), the lattice parameter of the fluorite-like phase follows Vegard s law. The formation of nanodomains with different degrees of order is shown to be caused by the internal strain due to the high density of structural defects in their unit cells. DOI:./S88 INTRODUCTION The formation of nanostructured materials was first observed in microstructural studies of fluorite-like R MO (R = lanthanide, Y, Sc; M = Ti, Zr, Hf) phases with a high density of structural defects [ ]. Nanostructured materials were also obtained by doping a number of fluorite phases with rare-earth elements: M x R x F +x, where M = Ca, Sr, or Ba []. A characteristic feature of melted crystals of these compounds is that, having an inhomogeneous structure, they behave as single crystals when studied by X-ray diffraction [, ]. Structural features of nanostructured fluorite-like R TiO and solid solutions based on these compounds are essentially unexplored. As determined by the diffusion layer method, solid solutions in the TiO Er O system exist in the composition range mol % Er O, with Er TiO and Er TiO. as end-members []. According to scanning electron microscopic examination and X-ray diffraction characterization with monochromatized CuK α radiation, these solid solutions have an inhomogeneous structure, which consists of a fluorite-like disordered phase and a nanoscale ( nm) pyrochlore-like ordered phase coherent with the matrix [7]. In this paper, we report a detailed structural study of high-temperature nanostructured Er O TiO ( mol % Er O ) solid solutions by synchrotron X-ray diffraction with the aim of gaining additional information about these materials. EXPERIMENTAL хer O ( х)tio (x =.,.,.7,.) samples for this investigation were prepared by a ceramic processing technique, using appropriate mechanical mixtures of TiO (extrapure grade) and Er O (Er--). In the synthesis of the samples by the ceramic processing technique, appropriate ratios of the starting oxides were mixed by grinding in a jasper mortar with ethanol. The mixtures were pressed into disks.7 cm in diameter and. cm in thickness at a pressure of MPa, which were then fired in an electric furnace with a platinum rhodium heater at a temperature of C for h in air. In our studies, we also used single-crystal samples prepared through induction skull melting. Monochromatic synchrotron X-ray diffraction measurements (λ =.888 Å, Si monochromator) were performed at the Station for X-ray Crystallography and Materials Science (Kurchatov Center for Synchrotron Radiation and Nanotechnology) in transmission geometry using a Fujifilm image plate detector. X-ray diffraction intensity data were collected at C in integral mode. The use of a highintensity monochromatic synchrotron radiation

2 LYASHENKO et al. source in combination with the area detector and Si monochromator considerably increased the intensity of diffraction peaks and improved the resolving power of our measurements in comparison with conventional X-ray diffractometry. The nanostructure of the samples was examined by scanning electron microscopy (SEM) on a Zeiss LEO SUPRA, and their composition was determined by electron probe microanalysis using an Oxford Instruments Inca-Sight accessory. Molecular-scale structural characterization was carried out by infrared spectroscopy (Specord M7 spectrometer, KBr pellet, -mg samples). RESULTS AND DISCUSSION The synchrotron X-ray diffraction patterns of the single-crystal and polycrystalline solid solutions in the composition range mol % Er O showed reflections from a major fluorite-like disordered phase (F) (Fmm (), Z = ) with a unit-cell parameter a =..8 Å and a nano/microscale pyrochlorelike ordered phase (P) (Fdm (7), Z = 8) with a unitcell parameter a =.8.8 Å (..9 Å in a fluorite setting) in the range 7 mol % Er O, coherent with the matrix (Figs., ). Also presented for comparison is the X-ray diffraction pattern of polycrystalline pyrochlore-like Er Ti O 7. The lattice parameters of the pyrochlore-like nanophase in the solid solutions were determined using superlattice reflections. The fundamental reflections of the pyrochlore-like phase had a lower intensity in comparison with that in conventional (CuK α, λ =. Å) X-ray diffraction patterns. One possible reason for this is that the smallest crystallites of the pyrochlore-like phase were disordered under the effect of the energy of the harder synchrotron X-rays (λ =.888 Å). The large-angle reflections were weak, so only diffraction peaks of the fluorite phase were seen for θ >. The diffraction peaks in this angular range were broadened, indicating the presence of internal strain in the fluorite-like phase, which had a high density of structural defects in its unit cell (Fig., scans ). Increasing the Er O content of the solid solutions reduces the number of cation and oxygen vacancies per unit cell and the internal strain, and the diffraction peaks become sharper (Fig., scans, ). The superlattice reflections from the pyrochlorelike phase in the samples that were prepared at C, without subsequent annealing at lower temperatures ( C), were very broad (Fig., scan ). After annealing at C and exposure to an electromagnetic field of frequency Hz, both the fundamental and superlattice reflections from the pyrochlore-like phase were sharper because of the increase in nanoparticle size as a result of sintering under the effect of the energy of the electromagnetic field and temperature (Fig., scans, ). We measured the electrical conductivity of those samples in the range C at C intervals at frequencies of. to Hz at each temperature using impedance spectroscopy. A similar picture was observed in neutron diffraction studies of a fluorite-like ZrO -based solid solution stabilized with % CaO and annealed at C for a long time ( h) [8]. Reducing the defect density in the unit cell of the solid solutions reduces the intensity of the superlattice reflections from the pyrochlore-like phase (Fig., scans ). This is particularly well illustrated by the superlattice reflection, located at θ.7 (Fig., scans ). At х =. (Er TiO. ), the pyrochlore-like phase converts to a C-type ordered phase (Ta O structure, а =. Å, sp. gr. Ia (), Z = 8), derived from the fluorite-like phase (Fig., scan ). The lattice parameters of the F- and C-phases in the sample with the composition Er TiO. differ little in a fluorite setting:.8 and.7 Å, respectively. For this reason, these phases coexist in the nanostructured material and can be coherently joined through transition layers created by stacking faults and dislocations. The lattice parameters of the C-phase of Er O and those of the accompanying phase in Er TiO. also differ little:.8 and. Å, respectively [9]. Note however that, in the composition range of the solid solutions (. х.), Vegard s law is obeyed, according to which the lattice parameter of the solid solutions is a linear function of doping level (Er O content) (Fig., curve ). In the case of the formation of two phases of different compositions, that is, Er O and the F-phase, in the sample with the composition Er TiO., the lattice parameter of the F-phase should remain constant at the lattice parameter of the F-phase in the sample containing 7 mol % Er O. Moreover, no evidence for the presence of Er O was found in the IR spectrum of Er TiO. (Fig., spectrum ). It is, therefore, reasonable to believe that the accompanying phase in Er TiO. has a С-type ordered structure and is identical in composition to the major fluorite-like disordered phase. The IR spectra of the solid solutions show a broad absorption band in the range 9 cm (characteristic of the fluorite-like matrix) in combination with sharp, weak absorption bands at 9 and 7 cm, characteristic of an ordered pyrochlore structure (Fig., spectra 8). The pyrochlore-related absorption bands are no doubt due to nanodomains. Also presented in Fig. are the IR spectra of single-crystal and polycrystalline pyrochlore-like Er Ti О 7 phases (spectra, ). The use of synchrotron X-rays allowed us to observe additional diffraction peaks at small diffraction angles, which are indiscernible in conventional X-ray diffraction patterns. According to indexing INORGANIC MATERIALS Vol. 9 No.

3 SYNCHROTRON X-RAY DIFFRACTION STUDY OF NANOSTRUCTURED Intensity, arb. units θ, deg Fig.. X-ray diffraction patterns of ( ) nanostructured хer O ( x)tio solid solutions and () pyrochlore-like Er Ti O 7 : () х =. (F + P), () х =. (F + P), () х =. (F + P), () х =.7 (F + P), () х =. (F + C) (synchrotron radiation, λ=.888 Å); (, ) polycrystals, () single crystal. INORGANIC MATERIALS Vol. 9 No.

4 LYASHENKO et al. Intensity θ, deg Fig.. X-ray diffraction patterns of ( ) nanostructured хer O ( x)tio solid solutions and () pyrochlore-like Er Ti O 7 : ( ) same as in Fig.. INORGANIC MATERIALS Vol. 9 No.

5 SYNCHROTRON X-RAY DIFFRACTION STUDY OF NANOSTRUCTURED Intensity, arb. units θ, deg Fig.. Shape of the reflections from ( ) fluorite-like хer O ( x)tio solid solutions with х = ()., ()., ()., ().7, and (). and () pyrochlore-like Er Ti O 7 in the angular range θ =. (synchrotron radiation, λ =.888 Å). INORGANIC MATERIALS Vol. 9 No.

6 8 LYASHENKO et al. a, nm mol % Er O Fig.. Composition dependences of the unit-cell parameter for (, ) the F-phase and () P-phase, obtained using super-lattice reflections; (, ) synchrotron radiation, λ =.888 Å; () СuK α radiation, λ =. Å. results, the lines at θ =. and 8.7 correspond to the ( h i =.89) and ( h i =.9) reflections and cannot be due to a face-centered cubic lattice (Fig., scans, ). Also, they cannot be attributed to an impurity phase because their intensity correlates with the percentage of the pyrochlore-like phase in the samples. It may be that these reflections are from 8 7 severely distorted planes in interfacial layers between the F- and P-phases. Our results demonstrate that the formation of nano- and microdomains with different degrees of structural order in the solid solutions is caused by the internal strain due to the high density of structural defects in their crystal lattice. According to SEM examination results, the fluorite-like solid solutions had an inhomogeneous microstructure, formed by grains that had their own substructure: nano- and microdomains nm to µm in size, coherently embedded in the matrix. Such nanoand microdomains were observed not only in the grain bulk but also on grain boundaries, which were a buffer layer upon monolithic intergranular intergrowth (Fig. a). The melted single crystals also had an inhomogeneous microstructure, which contained nanodomains ranging in size from to nm (Fig. b). Transmission Fig.. Infrared spectra of the F- and P-phases in the Er O TiO system: () TiO, () Er O, () P (Er Ti O 7 ceramic, C, h; ν abs =,, and 7 cm ), () P (Er Ti O 7 single crystal, induction skull melting; ν abs =,, and 7 cm ), () F + С (Er TiO. ceramic, C, h; ν abs =, 7,, and Wavenumber, cm 9 cm ), () F + P (Er TiO ceramic, C, h; ν abs = 9,, and 9 cm ), (7) F + P (Er TiO single crystal, induction skull melting, quenching from C; ν abs = 9, 8, and 8 cm ), (8) Er TiO single crystal, induction skull melting, annealing at C followed by furnace-cooling; ν abs = 9, 7, and 8 cm ). INORGANIC MATERIALS Vol. 9 No.

7 SYNCHROTRON X-RAY DIFFRACTION STUDY OF NANOSTRUCTURED 9 (а) µm (b) nm Fig.. SEM micrographs of the surface of nanostructured хer O ( x)tio solid solutions: (a) mol % Er O, polycrystal; (b). mol % Er O, melted single crystal (thermal etching at C for h). CONCLUSIONS Structural studies by monochromatic hard synchrotron X-ray diffraction confirmed that single-crystal and polycrystalline nanostructured хer O ( х)tio solid solutions with x =..7 consisted of coherent phases of the same composition: a fluoritelike disordered phase (Fmm) and a nanoscale ( nm) pyrochlore-like ordered phase (Fdm). Reducing the vacancy concentration in the unit cell of the solid solutions leads to a structural transformation of the pyrochlore-like phase. The Er TiO. (х =.) solid solution is composed of two phases identical in composition but differing in symmetry: a fluorite-like disordered phase (Fmm) and a nanoscale С-type (Ia) ordered phase derived from the fluorite phase. We have determined the composition dependences of the lattice parameter for the fluorite-like and pyrochlore-like phases. The lattice parameters of the disordered and ordered phases in a fluorite setting differ little. It is, therefore, reasonable to believe that the crystal lattices of the nano/microdomains and matrix are coherently joined through transition layers created by dislocations and other structural defects. The formation of nanophases with different degrees of order in fluorite-like solid solutions with a high density of structural defects is caused by internal strain in their unit cells. The solid solutions studied here are nanostructured materials. REFERENCES. Lyashenko, L.P., Nikonov, Yu.P., Raevskii, A.V., and Shcherbakova, L.G., Formation mechanisms of fluoritelike phases in the TiO R O (R = Y, Er, Sc) systems, Materialovedenie, 999, no., p. 9.. Lyashenko, L.P., Kolbanev, I.V., Shcherbakova, L.G., et al., Effect of a nonequilibrium state on phase relations in the system TiO Sc O ( mol % Sc O ), Inorg. Mater.,, vol., no. 8, p. 8.. Lyashenko, L.P., Shcherbakova, L.G., Belov, D.A., and Knotko, A.V., Electrical conductivity of nanostructured fluorite-like Sс Ti O, Inorg. Mater., 9, vol., no., p... Lyashenko, L.P., Shcherbakova, L.G., Belov, D.A., et al., Synthesis and electrical properties of Gd MO (M = Zr, Hf), Inorg. Mater.,, vol., no., p... Sobolev, B.P., Golubev, A.M., and Herrero, P., Fluorite M x R x F +x phases (M = Ca, Sr, Ba; R = rare earth elements) as nanostructured materials, Crystallogr. Rep.,, vol. 8, no., p... Shcherbakova, L.G., Glushkova, V.B., Lyashenko, L.P., et al., High-temperature phase relations and diffusion in the systems Er O TiO and Y O TiO, Dokl. Akad. Nauk SSSR, 977, vol., no., p Lyashenko, L.P., Shcherbakova, L.G., Belov, D.A., et al., Structural and electrical properties of nanostructured fluorite-like R +x TiO +.x (R = Y, Er; x ), Inorg. Mater.,, vol. 9, no., p Collongues, R., La nonstoechiometrie, Paris: Masson, Powder Diffraction File, Swarthmore: Joint Committee on Powder Diffraction Standards, card no. -7. Translated by O. Tsarev INORGANIC MATERIALS Vol. 9 No.