Effect of lanthanides on phase transformation and structural properties of LnNbO4 and LnTaO4 thin films

Accepted Manuscript Effect of lanthanides on phase transformation and structural properties of LnNbO4 and LnTaO4 thin films

Helena Brunckova, Erika Mudra, Lubomir Medvecky, Alexandra Kovalcikova, Juraj Durisin, Martin Sebek, Vladimir Girman PII: DOI: Reference:

S0264-1275(17)30810-9 doi: 10.1016/j.matdes.2017.08.068 JMADE 3327

To appear in:

Materials & Design

Received date: Revised date: Accepted date:

5 June 2017 31 July 2017 25 August 2017

Please cite this article as: Helena Brunckova, Erika Mudra, Lubomir Medvecky, Alexandra Kovalcikova, Juraj Durisin, Martin Sebek, Vladimir Girman , Effect of lanthanides on phase transformation and structural properties of LnNbO4 and LnTaO4 thin films, Materials & Design (2017), doi: 10.1016/j.matdes.2017.08.068

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ACCEPTED MANUSCRIPT Effect of lanthanides on phase transformation and structural properties of LnNbO4 and LnTaO4 thin films Helena Brunckovaa,*, Erika Mudraa, Lubomir Medveckya, Alexandra Kovalcikovaa, Juraj Durisina, Martin Sebeka, Vladimir Girmanb * Corresponding author : Helena Brunckova aInstitute of Materials Research, Slovak Academy of Sciences,

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Watsonova 47, 040 01 Kosice, Slovakia, tel:+421 55 7922 445, fax: +421 55 7922 408, e-mail: [email protected]

Faculty of Science, Pavol Jozef Safarik University, Jesenna 566/5, 040 01 Kosice

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b

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Abstract

Lanthanide niobate LnNbO4 (LnNO) and tantalate LnTaO4 (LnTO) thin films (~100 nm) were prepared by sol-

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gel/spin-coating process on PZT/Al2O3 substrates and annealing at 1000°C (Ln = Nd, Sm, Eu and Gd). The LnNO (NNO, SNO, ENO and GNO) and LnTO (NTO, STO, ETO and GTO) precursors of films were synthesized using Nb or Ta tartrate complexes. The different phase transformations were identified in LnNO or

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LnTO precursors from fluorite T´-structure via tetragonal T-LnNbO4 (NNO, SNO and GNO) or T-EuNb5O14 and

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T-NdTaO4, T-SmTa7O19, hexagonal H-EuTa7O19 and cubic C-GdTa7O19 to monoclinic M-LnNbO4 or M´LnTaO4 during annealing at 700-1200°C. The XRD results of LnNO or LnTO films confirmed the coexistence of monoclinic LnNbO4 or LnTaO4 phase in addition to the tetragonal as main phase at 1000°C. In STO film, the

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single monoclinic M´-SmTaO4 phase was revealed. The surface morphology and topography of thin films were investigated by the SEM and AFM analysis. STO film was smoother with roughness of 3.23 nm in comparison

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with SNO (6.35 nm). Effect of lanthanides can contribute to the formation different polymorphs of these films for the application to environmental electrolytic thin film devices.

Keywords: Lanthanides; Sol-gel; Thin films; Phase transformation; Microstructures.

1. Introduction Recently, the lanthanide (Ln = Nd, Sm, Eu and Gd) orthoniobates LnNbO4 (LnNO) and orthotantalates LnTaO4 (LnTO) materials have been developed for wide applications in 1

ACCEPTED MANUSCRIPT satelite communication systems [1,2,3]. LnNO and LnTO with fergusonite structure have attracted a great deal of attention due to their interesting physical properties, such as high dielectric constants, electro-optical, photoelastic, photocatalytic and luminescent properties as well as good mechanical and chemical stability. The Ca-doped LnNO and LnTO represent the high levels of proton conductivity, and thus provide interesting candidate electrolytes for thin-

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film solid oxide fuel cells (TF-SOFCs) that operate in CO2-containing atmospheres [4,5]. The

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proton conductivity of Ln1-xAxMO4 (Ln = La, Nd, Gd, Tb, Er, Y; A = Ca, Sr, Ba; x = 0.01-

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0.05; M = Nb,Ta) materials reaches values of the order of 10-1 S cm-1 and have interesting for hydrogen and humidity sensors at temperatures below around 700C.

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For LnNO, there are two crystalline forms, the low temperature M-phase isostructural with monoclinic form of the fergusonite and the high temperature T-phase corresponding to

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tetragonal scheelite. The transition between two phases occurs reversibly in the range 500800°C [6]. The LnTO crystallize in monoclinic form, besides their ability to possess two

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fergusonite-type structures, first known as M-type (I2/a) and second called M´-type (P2/a)

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[7,8,9,10,11]. Formation of these structure types depends on the atomic radius of Ln and synthesis conditions. The family of LnNO and LnTO ceramics and thin films with fergusonite

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structure that changes to scheelite structure. LnNbO4 and LnTaO4 precursors of thin films can be prepared by different methods:

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conventional solid state reaction (SSR) [12,13,14,15], hydrothermal [16] and sol-gel [1,2]. NdNbO4 (NNO) [12,13,17], SmNbO4 (SNO) [18,19], EuNbO4 (ENO) [16] and GdNbO4 (GNO) [20] ceramics were prepared by SSR and sintered with monoclinic M-type structure and small amounts of NdNb3O9 as second phase [17] at 1225-1300°C. Single monoclinic M´type structure of NdTaO4 (NTO) [21], SmTaO4 (STO) [22], EuTaO4 (ETO) [15] and GdTaO4 (GTO) [14] were obtained by heating at 1400°C. The sol-gel method has been used to prepare LnNbO4 and LnTaO4 at low temperature using Ln(NO3)3, ethanol, NbCl5 or TaCl5, ethylene

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ACCEPTED MANUSCRIPT glycol and citric acid as chelating agent and annealing at 1000-1200°C [1]. Because both LnNO and LnTO crystallize at 600 and 800°C, respectively in the same metastable fluorite tetragonal T´ structure (similar to that of tetragonal zirconia, t-ZrO2) also initially might transform to stable M-LnNbO4 at 800°C and M´-LnTaO4 at 900°C [1]. T´ structure is the first crystalline product. NNO [2], SNO [23] and GNO [24] precursor powders were obtained with

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single M phase at 1000-1200°C. The phase transition of ENO was revealed that an

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irreversible phase transition from M-EuNbO4 to tetagonal T and the two phases coexisted

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[25]. SNO thin films, prepared on Si substrates during crystallization at 900-1200°C exhibited polycrystalline microstructure [26]. High quality GTO luminescence film on silica glass

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substrate were obtained heating at 1200°C with single monoclinic M´-GdTaO4 phase [27] and K+ doped GTO film composed of M´-GdTaO4 and tetragonal T-GdTa7O19 [28].

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Recently, several groups published studies on the relationship between electrical behavior and phase and crystallographic orientation by the introduction of buffer layers such

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as SrRuO3, LaNiO3, SrTiO3, (La,Sr)MnO3, (La,Sr)CoO3 is effective method to improve

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properties of thin films [29,30]. The significant impact of SrTiO3 [29] or (La,Sr)CoO3 [30] acting as buffer layers on structural, microstructural and ferroelectric properties for

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(Bi,Nd)4Ti3O12 or (Pb,Ca)TiO3 thin films was clearly revealed. By using PbTiO3 and (Pb0.72La0.28)TiO3 or Pb(Zr0.52Ti0.48)O3 (PZT) interlayer it was possible to control the PZT film

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orientation [31,32].

In this work, we demonstrate a method to prepare novel LnNbO4 and LnTaO4 thin films from polymeric Nb and Ta-tartrate solutions by sol-gel/spin-coating process (Ln = Nd, Sm, Eu and Gd). The precursors of films were analysed by XRD, Raman, FTIR spectra, SEM and TEM. We also report for the first time the structural properties and morphology of LnNO and LnTO thin films deposited on the alumina substrates with PZT interlayer and annealed at

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ACCEPTED MANUSCRIPT 1000°C. The results of this work can contribute to the fabrication of these films for the application to environmental electrolytic thin film devices. 2. Experimental 2.1. Synthesis of LnNO and LnTO sol precursors

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LnNbO4 (LnNO = NNO, SNO, ENO and GNO) and LnTaO4 (LnTO = NTO, STO, ETO and GTO), (Ln = Nd, Sm, Eu and Gd) precursors were prepared by modified polymeric

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complex sol-gel method [33]. The novel sol precursors were synthesized from

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Nd(NO3)3.xH2O, Sm(NO3)3.xH2O, Eu(NO3)3.xH2O or Gd(NO3)3.xH2O and Nb- or Ta-tartrate complex in solvent (ethylene glycol EG) with molar ratio of Ln/Nb or Ln/Ta = 1/1. The

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Ln(NO3)3.xH2O solutions were prepared by the dissolution of Ln2O3 in HNO3 at 50°C. All

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chemicals were of analytical grade and were purchased from Merck (Darmstadt, Germany). After homogenization at 80°C, the solutions were stirred, heated at 130°C for 5 hours with the formation of transparent viscous sols. Basic (0.5 M) sols were diluted in stabilization solution

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(n-propanol). The sols remained stable at room temperature for two months. Pb(Zr0.52Ti0.48)O3 (PZT) sol was prepared by sol-gel method [34]. The raw materials, lead acetate trihydrate and zirconium acetylacetonate were dissolved separately at a temperature of 80°C in glacial acetic

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acid in two closed flasks. The solutions were dehydrated at 105°C for 2 hours and after

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cooling to 80°C, they were mixed with titanium orthotetrabutylate Ti(OR)4 in the required ratio of Pb:Zr:Ti = 1:0.52:0.48. After adding ethylene glycol at the temperature of 80°C, a orange sol was formed. The PZT sol concentration after dilution with n-propanol was 1.0 M. 2.2. Preparation of LnNO and LnTO powder precursors The LnNO and LnTO orange, pink and yellow gels were obtained after drying at 135°C for 12 hours to achieve crystallization of the compounds. The final powder precursors were annealed at 600-1000 °C for 1 hour in air. 2.3. Preparation of LnNO and LnTO thin films

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ACCEPTED MANUSCRIPT LnNbO4 (LnNO = NNO, SNO, ENO and GNO) and LnTaO4 (LnTO = NTO, STO, ETO and GTO), (Ln = Nd, Sm, Eu and Gd) thin films were prepared from the adequate sols synthesized in the stoichiometric ratio and deposited on alumina substrates. The polycrystalline alumina wafer (50.8 mm x 50.8 mm x 0.63 mm and 630 m thickness) was used to deposit the films. The alumina substrates were spin-coated with PZT sol precursor at

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2000 rpm for 30 s (PZT interlayer) followed by drying at 110°C for 3 min and calcining at

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400°C for 3 min. Single LnNO or LnTO film layer was deposited on PZT/Al2O3 substrate

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with a drying step at 110°C for 3 min and calcined at 400°C for 3 min. The coating cycling was repeated three times to obtain three layers of thin films. Final 3-layered LnNO and LnTO

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films were annealed at 1000°C for 1 hour in air. 2. 4. Materials characterization

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The thermal decomposition of gels were analysed by differential scanning calorimetry (DSC), thermogravimetric (TG) analysis (JUPITER STA 449-F1 NETZSCH) in

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platinum crucibles, in the temperature range 50 to 1300°C at a heating rate of 10°C min-1 in

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air on the bulk sample 17 mg. The phase composition of samples were analysed FTIR spectroscopy Shimadzu IRAffinity1 (KBr pellets) and Raman spectra were collected by a

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Raman spectroscopy (HORIBA BX 41TF). The phase composition of precursors and films was determined by X-ray diffraction analysis (XRD), (a model X’ Pert Pro, Philips, The

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Netherlands) using CuKα radiation. The surface of LnNO and LnTO powder microstructures were characterized using scanning electron microscope (SEM), (a model JSM-7000F, Jeol, Japan) equipped with an energy dispersive X-ray (EDS) analyser link ISIS (Oxford, UK) and transmission electron microscopy (TEM), JEOL (JEM 2100F) with the addition of EDS analysis. The surface and cross-section of film microstructures were characterized using SEM equipped with focused ion beam (FIB-SEM), (Auriga Compact, Carl Zeiss Germany). Cross sections of films were opened to observation after cutting with the focused Ga+ ion beam and

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ACCEPTED MANUSCRIPT removing of a small piece of film layer from substrate. Surface topography and root mean square roughness of films were characterized by atomic force microscopy (AFM, Dimension ICON, by Veeco Instruments).

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3. Results and discussion

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3.1. Structural characterization of precursors

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TG and DSC curves of prepared xerogels heated to 1300°C in air are shown in Fig. 1. The small weight losses of 6% at temperatures up to 200°C are due to evaporation of remains of

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water residuals and EG. It was shown, that all chemical reactions involving weight loss, such as decomposition of organic polymeric network, finished below 500°C. In Fig. 1a, the

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exothermic peaks at 323 and 344°C (NNO) or 276, 315 and 360°C (SNO) could be attributed to thermal decomposition of tartrate complexes with corresponding about 47.3% (NNO) or

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43.0% (SNO) weight losses. The formation of amorphous oxides started above 400°C with a

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small exo-effects. The presence of small exothermic peaks between 450 and 600°C is the result of the combustion of residual carbon and following crystallization of LnNbO4 phase at

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640°C (fluorite tetragonal phase unknown structure denoted as T´ when it is prepared from amorphous) [1]. The exothermic effect above 890 (NNO), 889 (SNO), 815 (ENO) and 859°C

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(GNO) corresponds to the transformation from T´ to the stable T or M phase. In DSC curves of NNO and SNO, broad and distinct endothermic peaks above 1090 (NNO), 1100 (SNO, GNO) or 1150°C (ENO) and characterize the T-M transformation of LnNbO4 (see Table 1). The last approximately 5-9 % weight loss on TG curves between 900 to 1300°C relates to above transformations. On basis of TG analysis, the total weight losses were about 53% (NNO, ENO, GNO) or 55% (SNO). In DSC curves (Fig. 2b), two distinct exothermic peaks at 200 and 330°C (ETO and GTO) were observed. DSC analysis showed the broad exothermic

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ACCEPTED MANUSCRIPT peaks at 328 and 625°C (NTO) or 324 and 600°C (STO) correspond to organic removal by oxidation with weight losses of about 40.0% (NTO) or 61.0% (STO). The two small exothermic peaks at 900 and 994°C (NTO) or 890 and 964°C (STO) were observed. The first exo-peak is caused by crystallization of T´ phase and the second correspond the residual carbon oxidation. The next exothermic peaks at 1070 (NTO) and 1162°C (STO) related to

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transformation from metastable T´ structure to T or M´ phase. The distinct endothermic peaks

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above 1200°C characterize the T-M transformation of LnTaO4 (see Table 2). The three stage

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of weight losses above 1200°C are visible on TG curves of NTO or STO xerogels over the temperature range 200°C, 700°C and 1200°C. From TG analysis resulted the total weight

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losses were 51% (NTO), 70% (STO) and 45% (ETO and GTO). Table 1 and 2 are summarized all the phases corresponding to the XRD spectra and phases corresponding to

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DSC peaks of LaNbO4 and LaTaO4. Being a typical member of the lanthanide series, europium usually assumes the oxidation state +3, but the oxidation state +2 is also common

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and leading to chemical reaction differences [15].

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The XRD diffractograms of LnNO and LnTO precursors after annealing between 600-1200°C are shown in Fig. 2. The XRD patterns verified the formation of LnNbO4

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amorphous a (600-700°C), fluorite T´(800°C), tetragonal T, orthorhombic O (900°C) and monoclinic M (1000°C) phases, which correspond to O-NdNbO4 (JCPDS no. 81-1974), M-

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NdNbO4 (22-0680) in NNO, O-SmNbO4 (72-2143), M-SmNbO4 (22-1303) in SNO, TEuNb5O14 (26-0632), M-EuNbO4 (22-1099) in ENO, O-GdNbO4 (72-2144) and M-GdNbO4 (22-1104) in GNO powders (seeTable 1). In LnTO powders were identified amorphous a (700-800°C), fluorite T´(900°C), T-NdTaO4 (16-0745), M´-NdTaO4 (83-0408) in NTO, TSmTa7O19 (23-0626), M´-SmTaO4 (24-1010) in STO, hexagonal H-EuTa7O19 (45-1119) and monoclinic M´-EuTaO4 (24-0412) in ETO, cubic C-GdTa7O19 (39-0728) and M´-GdTaO4 (24-0441) phases in GTO powders (see Table 2). The T´ formation of metastable phase

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ACCEPTED MANUSCRIPT reported Mather and Davies [1,35,36] (synthetized by sol-gel) with fluorite structure similar to the tetragonal zirconia (t-ZrO2). As resulted from the XRD analysis, the annealing temperature for preparation of LnNbO4 and LnTaO4 phases was remarkably decreased to 1000-1200°C and it was lower than the in conventional mixed route (1400-1500°C for 10 hours) [12,20].

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It is well known that the radii of Ln3+ ions decrease regularly from Nd3+ to Gd3+

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("lanthanide contraction"). The Nd3+ ion has a radius of 99.5 (pm), Sm3+ 96.4 (pm), Eu3+ 95.0

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(pm), whereas the heavier Gd3+ ion has a radius of 93.8 (pm). We are interested in an effect of the ionic radii on the phase transformation. From XRD results of crystallization of a large

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series of different lanthanide structures, we have demonstrated that crystallization processes by Ln3+ ions are very sensitive to ionic radii in LnNO and LnTO precursors. The structures of

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the LnNbO4 (Ln = Nd, Sm, Eu, Gd) compounds have monoclinic structure (according JCPDS database) with lattice parameters a=5.467, 5.421, 5.393, 5.369 Å; and β=94.50, 94.70, 94.67,

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94.55, respectively. The reduction in the lattice parameters of monoclinic LnNbO4 in going

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from Nd to Gd correlates with the ionic radii of Ln: r(Nd3+) >r(Sm3+) >r(Eu3+) >r(Gd3+). The similar trend was observed in LnNb7O12 compounds [37]. The lattice parameters of LnTaO4

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(a=5.516, 5.456, 5.426, 5.405 Å; and β=95.74, 95.73, 95.67, 95.62, respectively) decrease as the ionic radius of Ln3+ decreases: r(Nd3+) >r(Sm3+)>r(Eu3+) >r(Gd3+) and atomic number

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increases Nd (60) to Gd (64). The phase transition temperature of LnNbO4 and LnTaO4 was sensitive to the ionic radius of both the Ln and Nb- or Ta-site cation. As well known, niobium oxide compounds can exist in two kinds of coordinated structures: octahedrally coordinated NbO6 with different extents of distortion and tetrahedrally coordinated NbO4, but the tetrahedrally coordinated NbO4 structure is not a typical structure for niobium oxide because an Nb atom is too large to fit into an oxygen-anion tetrahedron. Only a few Ln orthoniobates, LnNbO4 (Ln =Y, Yb, Sm

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ACCEPTED MANUSCRIPT and La) possess tetrahedron coordination [25]. LnTaO4 (Ln = Nd, Sm, Eu and Gd) with the monoclinic cell dimensions structures have a coordination sphere of eight oxygen atoms for the lanthanide trications shaped as distorted as trigonal dodecahedron for the structure with the smaller lanthanides. The coordination environment about the Ta5+ cations can be described as a heavily distorted octahedron (coordination number CN = 6) for the structure of LnTaO4

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[38,39]. In general, the average oxygen coordination number for Ln3+ is higher than that for

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Nb5+ or Ta5+ with ionic radius 78 (pm) with values of CN 8 and 6, respectively.

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Raman spectra of precursors after annealing at 700-1200°C are shown in Fig. 3. The 18 optical Raman active phonon modes are expected [9,40,41]. The results evidenced all the

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bands whose wavenumbers are summarized in Table 3. Fig. 3 presents the wavenumber of the Raman active bands as a function of the ionic radii for M-type and M´-type structures. The

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wavenumbers increased from Nd to Gd for decreasing ionic radii from 99.5 to 93.8 (pm). This behavior is because of the phenomenon of lanthanide contraction (crystal radii of the

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Ln3+ ions decrease for increasing atomic number) [40,41]. The phonons between ~ 400-500

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cm-1 and ~ 600-700 cm-1 are assigned to anti-symmetric Nb-O vibrations, while the main modes (Ag and Bg) at approximately 330 and 810 cm-1 are related to the symmetric Nb-O

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vibrations of the NbO4 tetrahedron [42] in LnNbO4 precursors annealed at 1000°C (Fig. 3a). The peaks around 375 and 805 cm-1 (NTO and STO) or 460 cm-1 (ETO) and 510 and 740 cm-1

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(GTO) represent the mixture of T-NdTaO4 and M´-NdTaO4 and T-SmTa7O19 or H-EuTa7O19 and C-GdTa7O19 derived from fluorite T´-type structure (Fig. 3c) [12]. The Raman spectrum of NdNbO4 precursor (Fig. 3b) was strongly changed after heat treatment at 700°C with the bands at 310 and 800 cm-1 (amorphous structure), at 800°C bands at 178, 223, 330, 400 and 800 cm-1 (fluorite T´-type structure) and at 1000 and 1100°C the bands at 178, 223, 330, 400, 480, 675 and 803 cm-1 (tetragonal T-type and M-type structure) (Fig. 3b). The most intense Raman active vibrational modes of the monoclinic phase may be superimposed with the

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ACCEPTED MANUSCRIPT modes of the tetragonal phase at 800°C (6 modes), with similar vibrational frequencies [23,25]. In Fig. 3d, the peaks around 330 and 740 cm-1 can be assigned to the amorphous and coexistence of amorphous and T´-type structure in SmTaO4 precursor (at 700 and 800°C) and to tetragonal T-SmTa7O19 structure at 1000°C. The FTIR spectra of NdNbO4 and NdTaO4 precursors are shown in Fig. 4. Comparing

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with NNO and NTO powders prepared at 800°C, different phase transition behaviour was

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found during thermal annealing at 1100°C. Fig. 4a reports FTIR spectra in the range 4000-400 cm-1. Absorption peak located at 3433, 1630 and 1460 cm-1 correspond to physically adsorbed

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water and CO3-2 respectively, which are air pollutant. The summary 15 IR active phonon

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modes are expected. The region below 850 cm-1 is typical for the vibrational frequencies of metal-oxygen bonds formed at relatively low temperature. The clear differences in FTIR

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spectra of NNO products at 800°C (fluorite T´-NbNbO4), 900°C (orthorhombic O-NbNbO4) and 1100°C for 1h (O and monoclinic M-NbNbO4) are visible in Fig. 4b. In the fluorite

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structure, bands at ~ 790, 665, 598, 531 and 412 cm-1 were observed in T´-NdTaO4 precursor

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(900°C). In the case of the mixture of tetragonal T and monoclinic M´-NdTaO4 phase, the bands at ~ 834, 749, 701, 643, 591, 533 and 459 cm-1 were found in NTO spectra [1]. The

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presence of broad and intense absorption band with maximum at ~ 643 cm-1, which characterizes the Ta-O stretching vibration of O-Ta-O bridge bonds of the polymeric type.

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Main bands at 500-800 cm-1, are attributed to the lattice vibrations from Ta-O stretching and Ta-O-Ta bridging modes. The band at 420 cm-1 involves the asymmetric bending mode while the broad band at 591 cm-1 can be assigned to the asymmetric stretching mode of the TaO6 octahedron. 3.2. Morphological characterization of precursors The particle morphology of LnNO precursors prepared at different temperatures were investigated by SEM and TEM (Fig. 5). The micrographs of the NNO and SNO precursors at

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ACCEPTED MANUSCRIPT 800°C (Fig. 5a,b) demonstrate formation of the extremely fine particles (~ 20 nm) connected into large agglomerates (EDS spectrum of NdNbO4 and HRTEM image of fluorite T´SmNbO4). After annealing at 1000°C, the spherical particle coarsening to size around 50-80 nm and cuboidal 100-150 nm in NNO and SNO (Fig. 5c,d) in ENO and GNO (Fig. 5e,f) was observed. Similar morphology and size of the sol-gel prepared GdNbO4 powders were

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observed by Liu et al. [43,44]. By means of the SEM/EDS, local point by point probing made

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it possible to define position and qualitative composition of such NdNbO4 and EuNbO4

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analyses in NNO and ENO powders at 800 and 1000°C, respectively are shown in Fig. 5a,e (The Nd/Nb and Eu/Nb atomic ratio was approximately 1.0). In Fig. 6, the morphologies of

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particle agglomerates after annealing of LnTO samples at 800 and 1000°C are shown. In amorphous NTO and STO powders (Fig. 6a,b), particle agglomerates (up to 1µm size) with a

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more compact irregular morphology (T´- NdTaO4) and plate-like shape (T´-SmTa7O19) were identified [45]. They were consisted of small approximately 100 nm globular clusters with

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very fine structure (The Nd/Ta atomic ratio was approximately 1.0 in NdTaO4 spectrum). On

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other side, in NTO after annealing at 1000°C, the large agglomerates were characterized by a higher degree of microporosity with the separation of individual spherical clusters of NdTaO4

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particles, T and M´-structure (Fig. 6c). In LnTO (STO, ETO and GTO) powders, the large particles (T-SmTa7O19, H-EuTa7O19 and C-GdTa7O19) with the shape of leaves or flakes

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mixed with smaller particles can be visible (Fig. 6d,e,f). The particles were formed by highly coalesced spherical particles with diameter ~ 50-100 nm [23,46,47]. TEM and HRTEM observation correspond probably to the formation of fluorite at 800°C and orthorhombic or monoclinic SmNbO4 phase (small portion) at 1000°C (Fig. 7a,b). The polycrystalline compact agglomerates composed of 100-150 nm sized and irregularly shaped nanoparticles of orthorhombic and monoclinic NdNbO4 or T-EuNb5O14 and MEuNbO4 phases were found in NNO or ENO powders (Fig. 7c,d). In Figs. 7e,f, GdNbO4

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ACCEPTED MANUSCRIPT particles with well-ordered lattice can be seen. The lattice fringes show the imaging characteristics of the GdNbO4 crystal with size of domains of 5-10 nm. EDS spectra of ENO and GNO (Fig. 7d,e) confirmed the EuNbO4 and GdNbO4 (The Eu/Nb and Gd/Nb ratio was approximately 1.0). TEM observation and selected area electron diffraction (SAED) patterns clearly

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verified the formation of amorphous and fluorite T´-SmTa7O19 at 800°C and tetragonal T-

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SmTa7O19 phase at 1000°C (Fig. 8a,b). The spherical about 10 nm amorphous clusters are

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connected to the form of coarse irregular agglomerates with small amount of fluorite structure (Fig.8a). In Fig. 8b, particle agglomerates of 100 nm size with well-ordered crystalline lattice

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of T-SmTa7O19 are visible (in inset HRTEM and SAED patterns). In NTO at 1000°C, the formation of crystalline nanosized (about 50 nm) particles of the tetragonal and monoclinic

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NdTaO4 phase (Fig. 8c), were observed. The irregular agglomerates up to 150 nm size were observed in ETO, which were composed of fine rounded and ordered prolonged (probably

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hexagonal H-EuTa7O19) and spherical (orthorhombic O-Ta2O5) particles of 10-20 nm in

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diameter (Fig.8d). The GTO powders contained larger agglomerates up to 100 nm size with fine spherical ordered clusters about 10 nm size (Fig. 8e,f). These clusters contained cuboidal

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C-GdTa7O19 phase (see diffractogram). The Nd/Ta and Gd/Ta atomic ratio was approximately 1.0 and 1/7, respectively in EDS spectra in Fig. 8c,f.

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3.3. Structural characterization of thin films The Fig. 9a,b shows XRD patterns of LnNO films annealed at 1000°C. XRD analyses verified formation of the monoclinic ZrO2 phase (JCPDS 83-0944) as interlayer on substrate Al2O3 and phases: O-NdNbO4 (81-1974), M-NdNbO4 (32-0680) in NNO, O-SmNbO4 (722143), M-SmNbO4 (22-1303) in SNO, M-EuNbO4 (22-1099), T-EuNb5O14 (26-0632) in ENO, O-GdNbO4 (72-2144) and M-GdNbO4 (22-1104) in GNO films. In Fig. 9c,d are shown the XRD patterns of NTO, STO, ETO and GTO films. Lanthanide tantalates usually crystallize in

12

ACCEPTED MANUSCRIPT three different structures at room temperature [46,48]. One belongs to I2/a symmetry or Mtype (fergusonite structure), the other is P2/a symmetry or M´-type, which is true equilibrium phase at room temperature with the M-type modification and another is the high temperature tetragonal phase of the scheelite structure or T-type. As shown in Fig. 9c,d the NTO film coincides with T-NdTaO4 (16-0745) and M´-NdTaO4 (83-0408) phases. As illustrated in Fig.

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9c,d the STO film is composed of single M´-SmTaO4 (24-1010), ETO film contains

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hexagonal H-EuTa7O19 (45-1191) and M´-EuTaO4 (24-0412) phases. In GTO film, the cubic

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C-GdTa7O19 (39-0728) and M´-GdTaO4 (24-0441) phases are shown. From XRD patterns resulted that the tetragonal phase transforms to monoclinic M or M´phase in LnNbO4 or

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LnTaO4 films.

Raman spectra of films after annealing at 1000°C are shown in Fig. 10 and Table 3.

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Two dominant bands at 331 cm-1 and 800-805 cm-1 were clearly identified in all films. These bands are assigned to the bending and stretching modes of MO6 (M=Nb5+,Ta5+) octahedron,

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respectively. The wavenumbers were increased for decreasing ionic radii of Ln (from Nd to

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Gd). Intense peaks at 333, 378, 420 cm-1 and peak between 600 and 850 cm-1 are visible in GNO, contrary to broad bands at 331, 407 and 471 cm-1 in NNO, respectively (Fig. 10a).

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Different peaks assigned to tetragonal NdTaO4, hexagonal EuTa7O19 and cubic GdTa7O19) in coexistence with monoclinic LnTaO4 structure were found in spectra of LnTO thin films (Fig.

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10b). Frequencies in range 230-450 cm-1 are influenced by Ln3+ cation displacements. 550850 cm-1 range could be assigned to the Nb-O or Ta-O stretching modes essentially involving oxygen atom shifts [42]. 3.4. Morphological characterization of thin films The cross-section microstructures of ENO (Fig. 11a,b) and GTO (Fig. 11c,d) thin films deposited on PZT/Al2O3 substrates with EDS lines analysis were characterized using SEM equipped with focused ion beam (FIB-SEM). From image analysis of films (Fig. 11a

13

ACCEPTED MANUSCRIPT and 11c) resulted that the ENO, GTO and PZT film thickness was ~100, 150 and 50 nm, respectively. EDS analysis (Fig. 11b,d) demonstrate the distribution of elements across ENO and GTO films. SEM micrographs (Fig. 12a,b) with EDS spectra (1 and 2) demonstrate the distribution of elements in ENO and GTO films and results confirm the presence of Eu, Nb and Gd, Ta, in the film and Pb, Zr and Ti from PZT interlayer and Al and O from Al2O3

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substrate.

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Fig. 13 demonstrates the SEM and 3D AFM micrographs of PZT interlayer on Al2O3

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substrate. The film microstructure contained two phases (monoclinic ZrO2 and pyrochore PZT). Pyrochlore Pb2(Zr, Ti)2O7 phase occurs at 29° (222), 33° (400) and 45°(200) and was

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not confirmed by XRD in Fig. 9 (one PZT layer - only 50 nm thickness). The SEM and 3D

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AFM micrographs of the LnNO thin films are shown in Fig. 14. It was observed that the particles in the ceramic films were close packed and uniformly distributed at 1000°C. The monoclinic

NdNbO4

phase

shows

typical

ferroelasticity

rubber-like

behaviour

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(pseudoelasticity) in NNO film (Fig. 14b) and domain structure was observed in NNO precursor [49,50]. The surface morphology of the film was affected by introduction of the lanthanide elements (Eu) resulting in strong effect on surface roughness (EuNb5O14 particles).

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The microstructures of NNO, SNO and GNO films composed of NdNbO4, SmNbO4 and

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GdNbO4, respectively were smoother than ENO film. As shown in Fig. 14f, the AFM images illustrate that average particle size (~ 50 and ~ 100 nm) corresponding to the size of EuNbO4 and EuNb5O14 particles in ENO film and it is evidently larger than that in NNO, SNO and GNO films (~ 20-50 nm). The root mean square (Rq) surface roughness values of the films were calculated as 6.95 nm for NNO, 6.35, 8.98 and 7.31 nm for SNO, ENO and GNO films and 24 nm for PZT layer on Al2O3 substrate. The effect of surface roughness is significant on physical properties of these extremely thin films. Rough surfaces/interfaces of thin films contribute to electron scattering and thus to the film conductivity in electrolytic devices.

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ACCEPTED MANUSCRIPT SEM and 3D AFM topography images in Fig. 15 reveal surface morphologies of NTO (Fig. 15a), STO (Fig. 15b), ETO (Fig. 15c) and GTO (Fig. 15d) thin films after annealing at 1000°C. The surface of NTO, ETO and GTO films were rougher and more heterogeneous as compared with STO. The Rq values of the films were 5.24 nm for NTO, 3.52 and 3.53 nm for ETO and GTO films and 3.23 nm for STO film. The heterogeneous microstructure of the

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ETO or GTO and NTO is characterized by the bimodal particle size distribution and contains

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smaller spherical monoclinic EuTaO4 or GdTaO4 and NdTaO4 (~ 30-50 nm) and bigger

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needle-like EuTa7O19 or GdTa7O19 and NdTaO4 particles of about ~100 nm. The hexagonal EuTa7O19 particles were observed in Ta2O5:Eu film annealed at 1000 and 1100°C [51]. The

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homogeneous surface of STO film composed of SmTaO4 spherical particles (~50 nm). The structural properties of LnNO and LnTO films depend on their phase composition.

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The structural robustness of thin films has significant importance for the reliable operation of smart flexible electronics including electrolytic devices. It is important to identify

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surface states and their topological properties. Compared with powder, thin film is more

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outstanding because of high contrast and resolution superior thermal conductivity as well as high degree of uniformity and better adhesion. Sol-gel coating technique is widely used in the

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fabrication of functional films. However, the critical thickness, i.e. the maximum thickness achievable without crack formation via non-repetitive deposition, is often less than 100 nm

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for films (our case). The thermal evolution for LnNO and LnTO films showed that crystal phases are formed up to 1000ºC, while single well-crystallized M-LnNbO4 or M´-LnTaO4 materials are obtained in temperatures higher than 1000ºC. Also, the influence of the annealing time will be investigated for the LnNO and LnTO films: it is necessary longer time to obtain samples in the single monoclinic structure (in future work).

15

ACCEPTED MANUSCRIPT 4. Conclusions In conclusion, we have developed novel lanthanide (Ln = Nd, Eu, Sm and Gd) niobate LnNbO4 (LnNO) and tantalate LnTaO4 (LnTO) thin films using tartrate solutions. The different mechanism of the phase transformation in heating LnNO and LnTO precursors from fluorite (at 800 and 900°C, respectively) via orthorhombic LnNbO4 (Nd, Sm and Gd) or

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tetragonal EuNb5O14 (at 900°C) and tetragonal NdTaO4, SmTa7O19, hexagonal EuTa7O19 or

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cubic GdTa7O19 phase (at 1000°C) to monoclinic M-LnNbO4 (at 1000°C) or M´-LnTaO4 (at

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1100°C) was determined.

The ceramic thin films (~ 100 nm) were fabricated by sol-gel/spin-coating method on

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Al2O3 substrates with ZrO2 interlayer and annealed at 1000°C. The structural characterization of EuNbO4 film demonstrate strong different phase formation (tetragonal EuNb5O14) in

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comparison with orthorhombic and monoclinic M-LnNbO4 (Nd, Sm and Gd). While all lanthanides form relatively large trivalent Ln3+ ions, europium has additional valence and

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forms Eu2+ ions, leading to chemical reaction differences in how this ion can partition versus

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the Eu3+ (europium anomaly). The coordination number of Ln3+ ions depends on their ionic raddii. XRD results confirmed the presence of tetragonal NdTaO4, hexagonal EuTa7O19 or

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cubic GdTa7O19 and monoclinic M´-LnTaO4 phases in LnTO thin films. In heterogeneous microstructure of Eu or Gd tantalate films were identified spherical EuTaO4 or GdTaO4 and

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needle-like EuTa7O19 or GdTa7O19 particles, which is difference as compared with the SmTaO4 film composed of spherical particles (single monoclinic SmTaO4 phase). This study indicated that the temperature, time and lanthanide elements are directly correlated with the crystalline arrangement of the niobate and tantalate electrolytic thin films.

Acknowledgment This work was supported by the Grant Agency of the Slovak Academy of Sciences through project VEGA No. 2/0036/17.

16

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ACCEPTED MANUSCRIPT Table 1 The phases corresponding to the XRD spectra and DSC peaks of LnNbO4 (aamorphous, T´-fluorite, O-orthorhombic, T-tetragonal, M-monoclinic phase) in the temperature (T) range 600-1200°C .

XRD - 1100°C DSC XRD - 1200°C

ENO

GNO

a 450-690°C / a a 640°Cexo / a T´-NdNbO4 760°Cendo / T´ O-NdNbO4 890°Cexo / O O-NdNbO4 M- NdNbO4 960-1090°C / O+M O-NdNbO4 M- NdNbO4 1090°Cendo / M O-NdNbO4 M- NdNbO4

a 520-720°C / a a 660°Cexo / a T´-SmNbO4 750°Cendo / T´ O- SmNbO4 889°Cexo / O O-SmNbO4 M-SmNbO4 970-1100°C / O+M O-SmNbO4 M-SmNbO4 1100°Cendo / M O-SmNbO4 M-SmNbO4

a 530-710°C / a a 650°Cexo / a T´-EuNb5O14 750°Cendo / T´ T- EuNb5O14 815°Cexo / T T-EuNb5O14 M-EuNbO4 950-1220°C / T+M T-EuNb5O14 M-EuNbO4 1120-1220°C / M T-EuNb5O14 M-EuNbO4 1220°Cendo / M

a 450-720°C / a a 690°Cexo / a T´-GdNbO4 750°Cendo / T´ O-GdNbO4 859°Cexo / O O-GdNbO4 M-GdNbO4 960-1098°C / O+M O-GdNbO4 M-GdNbO4 1098°Cendo / M O-GdNbO4 M-GdNbO4

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MA

DSC

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SNO

SC

DSC

NNO

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Precursor XRD - T [°C] / phase DSC peak - T / phase XRD - 600°C DSC XRD - 700°C DSC XRD - 800°C DSC XRD - 900°C DSC XRD -1000°C

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Table 2 The phases corresponding to the XRD spectra and DSC peaks of LaTaO4 (aamorphous, T´-fluorite, O-orthorhombic, T-tetragonal, H-hexagonal, C-cubic, M´-monoclinic phase) in the temperature (T) range 700-1200°C . NTO

STO

ETO

GTO

a 420-780°C/ a a 680°Cexo / a T´ - NdTaO4 890°Cexo / T´ T-NdTaO4

a 430-790°C/ a a 690°Cexo / a T´-SmTa7O19 900°Cexo / Tˇ T-SmTa7O19

a 420-790°C/ a a 680°Cexo / a T´-GdTa7O19 860°Cendo / T´ C- GdTa7O19

DSC

994°Cexo / T

964°Cexo / T

M´-NdTaO4 T-NdTaO4 1070-1230°C/ T + M´ M´-NdTaO4 T-NdTaO4 1230°Cendo / M´

T-SmTa7O19

a 520-750°C/ a a 650°Cexo / a T´-EuTa7O19 860°Cexo / T´ H-EuTa7O19 O-Ta2O5 960°Cendo / H 990°Cendo / O H-EuTa7O19 O-Ta2O5 1120-1190°C/ H + O + M´ H-EuTa7O19 O-Ta2O5 1190°Cendo / M´

AC

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Precursor XRD - T [°C] / phase DSC peak - T / phase XRD - 700°C DSC XRD - 800°C DSC XRD - 900°C DSC XRD - 1000°C

XRD - 1100°C DSC XRD - 1200°C DSC

1160-1220°C/ T + M´ T-SmTa7O19 M´-SmTaO4 1220°Cendo / M´

970°Cendo / C C-GdTa7O19 1100-1200°C/ C + M´ C-GdTa7O19 M´-GdTaO4 1250°Cendo / M´

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ACCEPTED MANUSCRIPT Table 3 List the observed frequencies of the Raman-active bands for LnNbO4 and LnTaO4 powders and fims annealed at 1000°C; (p - powders, f - films, wavenumbers [cm-1]). assign.

NNO

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181.7 180.1 229.6 229.4 330.2 331.0 373.1 377.2 407.1 407.2 474.5 471.2

178.9 180.6 233.8 232.1 330.5 331,9 377.2 414.3 416.2 -

177.8 180.5 221.6 230.2 329.2 332.4 413.3 413.5 -

178.2 181.0 233.5 235.1 331.4 333.1 375.3 378.2 419.2 420.5 471.1 472.2

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671.2 674.5 730.4 730.1 802.2 803.1

675.3 674.9 732.5 804.3 805.2

681.5 695.2 734.5 805.5 807.1

705.8 710.2 735.8 732.2 807.0 808.0

172.5 174.2 223.4 210.2 321.5 323.4 375.1 372.4 472.2 490.3 (537.2) 681.5 678.2 730.2 805.5 810.1

185.0 181.3 229.6 328.8 330.3 378.2 371.5 412.8 486.3 496.2 (546.3) 679.6 671.3 738.2 735.1 812.3

181.1 175.6 223.8 223.5 310.2 305.6 459.5 426.5 459.6 488.2 (548.2) 683.3 691.2 815.4

178.4 230.2 210.2 325.6 427.5 428.6 510.2 479.1 (553.1) 695.6 688.3 738.4 739.2 821.6 823.1

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1 DSC and TG curves of (a) LnNbO4 (LnNO) and (b) LnTaO4 (LnTO) gel precursors. Fig. 2 XRD patterns of (a) LnNO (Ln = Nd, Sm, Eu and Gd) and (b) LnTO precursors annealed at 600-1200°C. Fig. 3 Raman spectra of (a) LnNO and (c) LnTO precursors after annealing at 1000°C, (b) NdNbO4 at 700-1100°C and (d) SmTaO4 at 800-1100°C. 1100°C in the range (a) 4000-400 cm-1 and (b) 1000-400 cm-1.

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Fig. 4 FTIR spectra of NdNbO4 (NNO) and NdTaO4 (NTO) precursors annealed at 800-

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Fig. 5 SEM and TEM surface morphologies of (a) NNO and (b) SNO at 800°C and (c) NNO, (d) SNO, (e) ENO and (f) GNO powders after annealing at 1000°C and (a,e) in inset EDX

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spectra of NdNbO4 and EuNbO4.

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Fig. 7 TEM and HRTEM micrographs of LnNbO4 (a) SNO at 800°C and (b) SNO, (c) NNO, (d) ENO and (e,f) GNO at 1000°C and (d,e) in inset EDS analyses of EuNbO4 and GdNbO4. Fig. 8 TEM and HRTEM micrographs of LnTaO4 (a) STO at 800°C and (b) STO, (c) NTO, (d)

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ETO and (e,f) GTO at 1000°C, in inset SAED patterns of (a) amorphous STO, T-SmTa7O19

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and (e) C-GdTa7O19 and EDS analyses of (c) NdTaO4 and (f) GdTa7O19. Fig. 9 XRD patterns of (a,b) LnNbO4 and (c,d) LnTaO4 thin films on PZT/Al2O3 substrates after annealing at 1000°C, (b,d ) XRD details in the range 26-35°2θ.

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Fig. 10 Raman spectra of (a) LnNbO4 and (b) LnTaO4 thin films on PZT/Al2O3 substrates after annealing at 1000°C.

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Fig. 11 FIB-SEM cross-section microstructures and EDS lines of (a,b) ENO and (c,d) GTO

Fig. 12 FIB-SEM cross-section microstructures and EDS spectra of (a) ENO and (b) GTO thin films (spectrum1 and 2). Fig. 13 (a) SEM surface microstructures and (b) 3D AFM topography of PZT/Al 2O3 substrates. Fig. 14 SEM surface microstructures and 3D AFM topography of LnNbO4 (a,b) NNO, (c,d) SNO, (e,f) ENO and (g,h) GNO thin films. Fig. 15 SEM surface microstructures and 3D AFM topography of LnTaO 4 (a,b) NTO, (c,d) STO, (e,f) ETO and (g,h) GTO thin films.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights

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LnNbO4 and LnTaO4 thin films (~100 nm) were prepared using sol-gel method. Transformation from fluorite via tetragonal to monoclinic phase was determined. Different polymorphs in Eu niobate as compared to Nd, Sm and Gd was identified. Single monoclinic phase was confirmed in homogeneous SmTaO4 film at 1000°C.

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