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Journal of Alloys and Compounds 678 (2016) 57e64

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Journal of Alloys and Compounds

journal homepage: http: / /www.elsevier .com/locate/ ja lcom

Structural and vibrational properties of phase-pure monoclinicNdLuO3 interlanthanides synthesized from nanostructured precursors

Júlia C. Soares a, Kisla P.F. Siqueira a, Roberto L. Moreira b, Anderson Dias a, *

a Departamento de Química, Universidade Federal de Ouro Preto, Campus Morro do Cruzeiro, ICEB II, Ouro Preto, MG, 35400-000, Brazilb Departamento de Física, ICEx, Universidade Federal de Minas Gerais, C.P. 702, Belo Horizonte, MG, 30123-970, Brazil

a r t i c l e i n f o

Article history:Received 8 December 2015Received in revised form17 February 2016Accepted 30 March 2016Available online 1 April 2016

Keywords:CeramicsElectron microscopyNanomaterialsRaman spectroscopyRare-earth

* Corresponding author.E-mail address: anderson_dias@iceb.ufop.br (A. Di

http://dx.doi.org/10.1016/j.jallcom.2016.03.2910925-8388/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

Phase-pure monoclinic NdLuO3 interlanthanides were successfully synthesized from hydrothermal-derived precursors. LuO(OH) and Nd(OH)3 materials were firstly obtained by hydrothermal synthesisat 250 �C, for 24 h. X-ray diffraction, micro-Raman spectroscopy, scanning and transmission electronmicroscopies were employed to investigate the thermal behavior of these precursors and NdLuO3interlanthanide up to 1600 �C. It was observed that LuO(OH) and Nd(OH)3 were converted to cubiclanthanide oxides below 1000 �C. Cubic Nd2O3 has changed to the monoclinic structure at 1000 �C,followed by reaction with cubic Lu2O3 at 1400 �C to produce monoclinic NdLuO3. Phase-pure monoclinic(C2/m space group, #12) interlanthanide was then produced at 1600 �C for the first time, as confirmed byall the employed techniques. Phonon modes were additionally determined and assigned for thismonoclinic structure.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

LnLn0O3 mixed oxides, known as interlanthanide materials, haverecently received an enormous scientific and technological interestbecause of their outstandingly complex structures toward prom-ising microwave-related, magnetic, and optical applications [1e5].These interlanthanides are frequently interpreted as mixtures ofsimple binary sesquioxides of trivalent lanthanides, which alsoexhibit intricate crystal chemistry and can be found in at least threedifferent modifications ranging from cubic fluorite-type (C-type),hexagonal (A-type), to monoclinic (B-type) polymorphs, with veryremarkable properties [6e11]. In the last decades, manyworks havebeen devoted to investigate a large number of trivalent lanthanidecombinations in order to obtain potentially ordered ternary mixed-lanthanide phases by systematically substituting smaller ions intolower coordination sites and larger ions into higher coordinationsites [1e4,12,13]. These attempts revealed a systematic occurrenceof order-disorder phenomena in many ternary interlanthanidecombinations, which can be advantageous from the perspective ofdoping (in disordered materials) or from the point of view of the

as).

polymorphism (in ordered materials). However, all compositionsdeserve investigation towards a complete assessment of structure-property relationships, where different lanthanide ions lead todiverse crystal structures.

The present work focuses the ternary lanthanide oxideNdLuO3, firstly studied by Schneider and Roth [14] and again re-ported by Berndt et al. [15] as a new material fifteen years later.Both papers did not presented any x-ray diffraction (XRD) datanor further evidence for their findings, only reporting either amixture of monoclinic and cubic phases after heat treatments at1650 �C for 6 h [14] or a mixture of 50% of the perovskite phase(orthorhombic) in combination with monoclinic and cubic phasesafter solid-state reactions of coprecipitated hydroxides at 1250 �C[15]. Su et al. [16] proposed the synthesis of many ternarylanthanide oxides, including NdLuO3, by using for the first timehigh pressure and high temperature methods. Although no XRDwas presented, the authors claimed a phase-pure NdLuO3 in aperovskite form at 1200 �C and 40 kbar. More recently, Duboket al. [13] reported the electrophysical properties of inter-lanthanide oxides, and NdLuO3 appeared as belonging to a C-typestructure after coprecipitation of hydroxides followed by thermaldecomposition and sintering at high temperatures. Bharathy et al.[12] presented a nice structure map for all known ternarylanthanide oxides in the literature, which shows that NdLuO3 is

Delta:1_given nameDelta:1_surnameDelta:1_given nameDelta:1_surnamemailto:anderson_dias@iceb.ufop.brhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.jallcom.2016.03.291&domain=pdfwww.sciencedirect.com/science/journal/09258388http://www.elsevier.com/locate/jalcomhttp://dx.doi.org/10.1016/j.jallcom.2016.03.291http://dx.doi.org/10.1016/j.jallcom.2016.03.291http://dx.doi.org/10.1016/j.jallcom.2016.03.291

J.C. Soares et al. / Journal of Alloys and Compounds 678 (2016) 57e6458

known, but its structure has not been reported by X-ray orneutron diffraction techniques. In fact, in their introduction,Barathy et al. [12] mentioned NdLuO3 as studied by Berndt et al.[15], Ito et al. [3] and Su et al. [16]. Unfortunately, Ito et al. [3] didnot presented any result for this interlanthanide.

The synthesis procedures employed to produce binary andeven ternary lanthanide oxides vary enormously, either as singlecrystals, ceramic powders or films. The standard solid-state re-action remains the major processing route for these materials[2,4], although other non-conventional methods are currentlyapplied, such as reverse-strike coprecipitation [17], mechano-chemical [18], thermal decomposition [5,6,13,19e21], sol-gelprocessing [22e24], combustion synthesis [25], molecular beamepitaxy technique [26,27], and hydroxide fluxes [12]. The hydro-thermal technology is currently applied by our research group tosynthesize nanostructured materials with technological interest[28e34]. In our previous investigations, crystal chemistry andmorphology are monitored and controlled by choosing theappropriate feed chemistry and processing conditions duringhydrothermal syntheses of nanostructured precursors [28e34]. Byusing experimental setups with fully-controlled temperature-pressure-time-pH environments, potentially reactive materialscan be produced in mild, reliable and reproducible conditions[28e34]. These processing parameters have decisive influence onthe crystal structure, purity, ordering degree, particle size andmorphology of the resulting materials [35,36]. It is well knownthat these features determine the performance of the materials,which lead us to conclude that the investigation of their pro-cessing conditions becomes very important in this context.

In view of that, this work introduces the hydrothermal pro-cessing route for the synthesis of phase-pure monoclinic NdLuO3.Based on the available literature discussed above, it seems that allattempts to produce phase-pure NdLuO3 failed. Thus, this paperreports the thermal behavior of nanostructured hydrothermally-derived precursors (hydroxides and oxy-hydroxides), which wasmonitored by XRD, Raman spectroscopy, scanning and trans-mission electron microscopies. Our procedures allowed us tosynthesize, for the first time, phase-pure NdLuO3 interlanthanidebelonging to the monoclinic C2/m (#12) space-group. Moreover,in order to expand the knowledge of this material, the completeset of non-polar phonon modes for the monoclinic NdLuO3 wasdetermined and assigned for the first time, with appropriate useof polarized micro-Raman scattering tools on sintered samples.

2. Experimental section

Stoichiometric amounts of Nd(NO3)3$6H2O and Lu2O3 (Sigma-Aldrich, purity >99.9%) were used as starting materials. Lutetiumoxide was firstly treated with HNO3 65% to produce a mother so-lution of lutetium nitrate, according to the chemical reaction:

Lu2O3þ6HNO3/2LuðNO3Þ3þ3H2O: (1)Neodymium nitrate hydrate was then introduced and mixed to

this mother solution followed by adding of NaOH (10 mol/L) untilthe pH reach values above 13 under vigorous stirring. The resultingsolutions were loaded into stainless steel Parr® autoclaves equip-ped with pressure and temperature controllers. The reaction ves-sels were heated at 10 �C min�1 up to the processing temperatureand its corresponding saturated vapor pressure (250±1 �C and580 ± 2 psi) for 24 h. After synthesis, the obtained precursors werewashed with distilled water in order to remove any remaining Naþ

ion and dried at 70 �C. The general chemical reaction for the syn-thesis of the precursors can be written as:

LuðNO3Þ3þNdðNO3Þ3þ6NaOH/LuOðOHÞ þ NdðOHÞ3þ6NaNO3þH2O:

(2)

Following, the materials were calcined (air atmosphere) in thetemperature range 800e1600 �C for fixed times of 2 h in order toinvestigate their thermal behavior. For sintering, cylindrical pucksof about 5 mm height and 12.5 mm diameter were produced byapplying a pressure of 150 MPa in samples previously treated at1200 �C. The obtained pucks were then sintered in a conventionaloven (air atmosphere) at 1600 �C, for 8 h.

In the sequence, the structural properties were investigated byx-ray diffraction (XRD) using a Shimadzu D-6000 diffractometerwith graphite monochromator and a nickel filter in the range of10e60�2q (15 s/step of 0.02�2q), operating with FeKa radiation(l ¼ 0.1936 nm), 40 kV and 20 mA (the results were automaticallyconverted to CuKa radiation for data treatment and manipulation).The lattice parameters were determined by the software MDI Jade9.0. The vibrational properties were studied by micro-Ramanspectroscopy in back-scattering configuration by using anOlympus confocal microscope (100 � objective) attached to aHoriba/Jobin-Yvon LABRAM-HR spectrometer (equipped with 600and 1800 grooves/mm diffraction gratings). The 632.8 nm line of aHeeNe laser (maximum effective power of 6 mW at the surface ofeach sample) was used as exciting line and a Peltier-cooled chargecoupled device (CCD) detected the scattered light. An edge filterwas employed to stray light rejection (Rayleigh line). In order toavoid any kind of crystallization due to laser exposition, very lowintensities were employed, besides lens with low magnificationand defocused laser, when possible. Accumulation times of typi-cally 20 collections of 10 s were employed with spectral resolutionbetter than 1 cm�1. The resulting spectra were corrected by theBose-Einstein thermal factor besides individual baselinesubtractions.

Morphological and structural features of the samples were alsoinvestigated by electron microscopy. A Quanta FEG 3D (FEI) field-emission scanning electron microscope (FESEM) equipped withenergy-dispersive spectrometer (EDS) was employed to analyze themorphology and chemical features of the NdLuO3-derived mate-rials (applied voltages varied from5 to 30 kV). Images were taken inboth detection modes, i.e., secondary (SE) and backscattered (BSE)electrons, in order to differentiate the precursors and final mate-rials. High-resolution transmission electron microscopy (HRTEM)was carried out to investigate the crystalline aspects of the nano-powders in a Tecnai G2-20 (FEI) transmission microscope (appliedvoltage of 200 kV). Selected area electron diffraction (SAED) pat-terns were obtained for all samples in order to characterize theirpolycrystalline nature.

3. Results and discussion

Fig. 1 presents XRD pattern and Raman spectrum obtained for astoichiometric mixture of the precursors hydrothermally synthe-sized at 250 �C. The results showed that Nd(OH)3 and LuO(OH)were produced and may be indexed according to the ICDD (Inter-national Committee for Diffraction Data) cards #00-006-0601 and#01-072-0928, respectively (Fig. 1a). Fig. 1b presents the Ramanspectrum for this mixture of precursors at room temperature. As itcan be seen, a set of relatively broad Raman modes in the region100e950 cm�1 corresponds to the LuO(OH) and Nd(OH)3 vibra-tions. The bands related to LuO(OH) occur below 200 cm�1 and ataround 410, 660 and 830 cm�1, while the modes related to neo-dymium hydroxide occur at 297 and 371 cm�1 [20]. In thesequence, the thermal evolution aiming to the production of phase-

10 15 20 25 30 35 40 45 50 55 60

Inte

nsity

(arb

. uni

ts)

2 Theta (°)

Nd(OH)3LuO(OH)

(a)

150 300 450 600 750 900

LuO(OH)

Nd(OH)3

(b)

Ram

an in

tens

ity (a

rb. u

nits

)

Wavenumber (cm-1)

Fig. 1. (a) XRD pattern and (b) Raman spectrum for an equimolar mixture of thehydrothermally-synthesized Nd(OH)3 and LuO(OH) precursors. In (b), the main bandsfor each phase are indicated in different colors.(For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. (a) FESEM image for the precursors obtained by hydrothermal synthesis at250 �C, for 24 h. LuO(OH) tabular particles are identified by a circle, while Nd(OH)3particles are indicated by an arrow; (b) HRTEM images for the Nd(OH)3. Tabular,faceted particles are shown (left image) with interplanar spacing at 3.19 Å, corre-sponding to the (110) plane of the P63/m hexagonal space group (right image).

J.C. Soares et al. / Journal of Alloys and Compounds 678 (2016) 57e64 59

pure monoclinic NdLuO3 from hydrothermal-derived precursorswas studied in the temperature range 800e1600 �C. Morphologicaland structural aspects of the precursors were investigated byFESEM and HRTEM. Fig. 2a shows a FESEM image where nanosized,agglomerated particles can be observed for the precursors syn-thesized by hydrothermal processing at 250 �C, for 24 h. As it can beseen, two different morphologies are present and could be associ-ated after chemical analyses (EDS) with the precursors obtainedafter synthesis. LuO(OH) exhibits tabular, faceted particles (easilyidentified by a circle in Fig. 2a), while Nd(OH)3 shows nano-structured, heavily agglomerated particles (identified by an arrowin Fig. 2a). Fig. 2b shows representative HRTEM images obtained inNd(OH)3 precursor hydrothermally synthesized at 250 �C, in whichnanosized, well-dispersed tabular particles can be visualized. Forthis sample, the interplanar spacing was computed to be about3.19 Å, which correspond to the (110) plane of the P63/m hexagonalspace group.

Fig. 3 shows the XRD results for the samples thermally treatedfor 2 h at selected temperatures (800, 1000, 1200, 1400 and1600 �C). At temperatures below 1000 �C, the precursors haveconverted to lanthanide oxides according to the ICDD cards #00-

021-0579 (Nd2O3) and #01-072-6366 (Lu2O3). These oxides belongto the Ia3 (#206) space group (cubic fluorite-type phase) and can bevisualized as phase-pure materials at 800 �C in Fig. 3b with latticeparameters a ¼ 10.96 Å for Nd2O3 and a ¼ 10.39 Å for Lu2O3. At1000 �C, cubic Nd2O3 changed (see Fig. 3b) to monoclinic neo-dymium oxide, C2/m space-group (#12), according to the ICDD card#01-076-7416, with lattice parameters a ¼ 14.12 Å, b ¼ 3.63 Å,c ¼ 8.80 Å and b ¼ 100.2�. This result was unexpected, since ahexagonal structure (space group P3m1) is frequently found forpure Nd2O3 [20,37,38]. At 1200 �C, a mixture of pure monoclinicNd2O3 and cubic Lu2O3 phases can be observed. At this temperatureand above, Fig. 3b shows clearly the chemical reaction and crystalphase evolutions, since it focuses on the range 25e35�2Q, wherethe most important changes in the diffraction patterns of Fig. 3aoccur. In Fig. 3b, each peak could be identified in agreement withICDD cards of previously published works [5,17,39,40]. At 1400 �C,the NdLuO3 interlanthanides become visible as a monoclinic phase(C2/m space group, #12) together with residual cubic lutetiumoxide. Finally, heat treatment at 1600 �C produced phase-puremonoclinic NdLuO3, whose identification and assignment couldonly be accomplished by comparing its XRD pattern with thatobserved for the LaYO3 material (ICDD card#70-3117) [39].

According to the previous literature discussed above,perovskite-like NdLuO3 interlanthanides can be formed as a stable

10 20 30 40 50 60

2 Theta (°)

800°C

1000°C

Inte

nsity

(arb

. uni

ts)

1200°C

1400°C

1600°C

(a)

26 28 30 32 34

cubic Lu Ocubic Nd O

NdLuO

monoclinic Nd O

2 Theta (°)

800°C

1000°C

Inte

nsity

(arb

. uni

ts)

1200°C

1400°C

1600°C

(b)

Fig. 3. XRD patterns exhibiting the thermal behavior for samples produced in the temperature range 800e1600 �C (from bottom to top). (a) General XRD patterns in the 10e60�2Qrange, and (b) particular view in the region 25e35�2Q with identification of all observed phases for better visualization.

J.C. Soares et al. / Journal of Alloys and Compounds 678 (2016) 57e6460

phase [15,16]. In fact, the structural stability and formability ofperovskite materials are based on the so-called tolerance factor (t),a criterion proposed by Goldschmidt through the relationship [41]:

t ¼ ðRA þ ROÞffiffiffi2

pðRB þ ROÞ

; (3)

where RA, RB and RO are the ionic radii of A, B and oxygen in thegeneral formula ABO3. In an ideal cubic perovskite structure, thetolerance factor is equal to the unity. Lower t values could occur,which means that distorted structures would be present. It is

150 300 450 600 750 900

1600°C

1400°C

1000°C

800°C

cubic Lu2O

3

Wavenumber (cm-1)

cubic Nd2O

3

Ram

an In

tens

ity (a

rb. u

nits

)

monoclinic Nd2O

3

NdLuO3

Fig. 4. Raman spectra (100e1000 cm�1 range) showing the thermal evolution(800e1600 �C) of the samples produced from hydrothermally-processed precursors at250 �C. The main bands for each phase are indicated in different colors.(For inter-pretation of the references to colour in this figure legend, the reader is referred to theweb version of this article.)

currently accepted that almost all perovskites have t values rangingfrom 0.75 to 1.00. However, the tolerance factor is a necessary butnot a sufficient condition for the formation of perovskite structures.The Goldschmidt's criterion neglects the influence of the coordi-nation number on the effective ionic radii, and was debated by theliterature by years without a definitive solution. For NdLuO3, thetolerance factor could be calculated as 0.7847, which indicates thatthis material could be produced in a (highly distorted) perovskitephase, even if one considers this value in the border for the tradi-tional perovskite atomic arrangement. In this work, the phasebehavior followed by our NdLuO3 was quite different: the hydro-thermal precursors were converted to monoclinic Nd2O3 and cubicLu2O3 at 1200 �C, followed by a chemical reaction at higher tem-peratures allowing the production of phase-puremonoclinic (C2/m,#12) materials at 1600 �C. Fig. S1 (Supplementary Material) pre-sents XRD data for the phase-pure monoclinic NdLuO3 togetherwith its Miller indexes for the monoclinic C2/m space group. In a

200 400 600 800 1000 1200

Ram

an in

tens

ity (a

rb. u

nits

)

Wavenumber (cm-1)

Fig. 5. Micro-Raman spectra for the NdLuO3 sample in the spectral region 50-1200 cm�1. Experimental data are in open circles, whereas the fitting curve is repre-sented by a red line. Green lines represent the phonon modes adjusted by Lorentziancurves. (For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

Table 1Phonon frequencies, FWHM and assignment of the gerade modes determined fromthe adjustment of the Raman experimental data by Lorentzian curves, for the phase-pure monoclinic NdLuO3.

Band Frequency (cm�1) FWHM (cm�1) Assignment

1 65.7 5 Ag2 96.5 19 Ag3 110.5 6 Bg4 123.6 14 Ag5 168.4 11 Ag6 181.1 16 Ag7 200.2 19 Ag8 218.4 18 Bg9 265.9 43 Bg10 306.0 52 Ag11 338.5 47 Bg12 381.7 43 Bg13 408.7 42 Ag14 439.9 27 Ag15 486.9 9 Bg16 572.2 36 Ag17 595.2 29 Bg18 702.3 56 Ag19 815.2 48 Ag20 840.0 40 Ag21 953.4 17 Ag

200 400 600 800 1000 1200

B

B

B

B

B

B

Ram

an in

tens

ity (a

rb. u

nits

)

Wavenumber (cm-1)

B

//

T

Fig. 6. Polarized micro-Raman scattering for the sintered NdLuO3. Parallel (//) andcross-polarized (t) configurations are indicated in red and blue lines, respectively. Therelative strengthening of the Ag modes in the parallel configuration are accompaniedby the relative weakening of the Bg modes (indicated in blue). Conversely, Bg modesare favored by crossed light configuration, for the same sample position (particulargrain size orientation). (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article.)

J.C. Soares et al. / Journal of Alloys and Compounds 678 (2016) 57e64 61

previous publication [39], Yamamura et al. studied the synthesis ofmonoclinic LaYO3 and their results were employed in the presentwork to indexing our NdLuO3 samples, since no XRD pattern for thismaterial in the monoclinic phase could be found in the literature.The lattice parameters were determined as a ¼ 14.08 Å, b ¼ 3.54 Å,c ¼ 8.67 Å and b ¼ 101.3� for the monoclinic C2/m space group,according to the equation [42]:

1d2

¼ 1sen2b

�h2

a2þ k

2sen2bb2

þ l2

c2� 2hl cos b

ac

�: (4)

It is important to highlight that conventional solid-state reac-tion using neodymium oxidewith lutetium oxide at 1600 �C did notresult in a phase-pure monoclinic NdLuO3. Fig. S1 also presentsXRD results for conventionally-synthesized materials by solid-statereaction. We can note the presence of both unreacted Lu2O3 andNd2O3 in the final product obtained throughout solid-statereaction.

The thermal behavior of hydrothermally treated precursors forthe synthesis of NdLuO3 was also monitored by micro-Ramanscattering. Fig. 4 presents selected spectra for the samplessequentially treated at 800 �C, 1000 �C, 1400 �C and 1600 �C. Thespectrum for the samples heat treated at 800 �C is quite similar tothose obtained by Ubaldini and Carnasciali [20] for lutetium andneodymium cubic oxides. The strongest bands for these materialsare located, respectively, at 338 and 390 cm�1. For the samplesprepared at 1000 �C, we can also observe the strongest band relatedto the Lu2O3 (390 cm�1) together with vibrational modes associatedwithmonoclinic Nd2O3.20 These additional modes are also reportedfor the same crystal structure by Mele et al. [5], for Nd2O3eGd2O3mixed system, Martel et al. [43], for Sm2O3, and Heiba et al. [22], forDy2-xHoxO3 (x > 0.4). At 1400 �C, monoclinic NdLuO3 can now beobserved, in accordancewith Tompsett et al. [17], particularly in thefrequency range 350e500 cm�1. Although some secondary phases(lanthanide oxides) are still present, as verified by XRD and dis-cussed previously, in the range 350e500 cm�1 a broad group ofRaman modes could be assumed as a fingerprint of the monoclinicC2/m (#12) space group, as previously proposed for the LaGdO3material [17] and for the (Gd0.5Nd0.5)2O3 system [3].

The spectrum for the calcined sample at 1600 �C (Fig. 4, top)

shows the Raman-active modes for the phase-pure NdLuO3. In thismonoclinic C2/m space group, there are six formula units per uni-tary cell (Z ¼ 6) [5,44]. Four oxygen ions and three lanthanide ionsoccupy the 4i Wyckoff position with Cs symmetry (2Ag 4 Bg 4 Au4 2Bu), and one oxygen ion sites on an 2b position with C2h sym-metry (Au 4 2Bu). Based on these occupation sites, the site-groupmethod of Rousseau et al. [45] was applied to obtain thefollowing representation of the Raman-activemodes (gerade) at theBrillouin-zone center:

GRAMAN¼ 14Agðxx; yy; zz; xyÞ47Bgðxz; yzÞ: (5)According to this model, one would expect to observe up to 21

first-order Raman modes in the spectrum of NdLuO3 sampletreated at 1600 �C (beside 24 infrared bands, 8Au (y) 4 16Bu (x,z)).However, the spectrum of this sample showed in Fig. 4 presentsrelatively broad peaks and does not allow one to visualize morethan 10 bands. Therefore, in order to resolve the superimposedphonon modes, peak fitting procedures were required. The Ramanspectrum for this sample was then fitted by Lorentzian curves andthe results are presented in Fig. 5. The experimental data are inopen circles and the fitting curve is presented by a red line. Theresults showed that 21 Raman modes were required for the fitting(green curves), in perfect agreement with the theoretical pre-dictions. Based on this fitted spectrum, the frequencies and full-width at half maxima (FWHM) for all bands were determined forour phase-pure monoclinic NdLuO3 and are presented in Table 1.

The Raman spectrum depicted in Fig. 5 could represent afingerprint of the NdLuO3 interlanthanides with monoclinic C2/mstructure. In addition, as already mentioned, 21 Raman-activemodes were expected, with the following distribution in terms ofthe irreducible representations (i.r.) of the C32h point group: 14Ag 47Bg [2,22,44]. In order to corroborate our findings and assign thesenon-polar gerade modes to their respective i.r., additional experi-ments were carried out by using polarized micro-Raman spec-troscopy. For this study, a ceramic sample was sintered at 1600 �Cfor 8 h. These processing conditions were employed because thegoal was to obtain ceramics with large grains in order to get suit-able polarized micro-Raman spectra. It is well known that the in-elastic scattered light intensities due to the Raman effect are

Fig. 7. FESEM images for samples obtained at different conditions. (a) 800 �C, (b) 900 �C, (c) 1000 �C, (d) 1400 �C and (e) 1600 �C.

J.C. Soares et al. / Journal of Alloys and Compounds 678 (2016) 57e6462

proportional to the square of the elements of the polarizabilitytensor (second-order tensor). Then, the base functions of the i.r.that are Raman-active have quadratic forms, i.e., they transform likethe product of the Cartesian coordinates (e.g., xx, xy, xz, yy, etc). Forsingle crystals, we take benefit of the crystal symmetry to assignthe lattice vibrations to the different i.r. [45,46]. However, in thecase of the ceramics, although the group-theory predictions remainvalid, the symmetries of the modes are generally mixed up due tothe random orientation of the crystalline grains. In this work, wehave used a confocal microscope with an objective of magnificationof 100�, which allows the selection of observation regions smaller

than the grain sizes (scattering volumes of circa 1 mm3). However,we do not know anything about the crystallographic axes of thesegrains, which have also random orientation throughout the sample.By measuring the micro-Raman spectra of NdLuO3 sintered samplewith polarized light (Fig. 6), we observed that for some grains thespectra of parallel (red line) and crossed light (blue line) becomequite different. We can observed the relative strengthening of thetotally symmetric 14Ag modes in the parallel configuration(because these modes are particularly observed for xx, yy and zzconfigurations of polarizer and analyzer), accompanied by therelative weakening of the 7Bg modes, which are favored by crossed

Fig. 8. HRTEM images for the phase-pure monoclinic NdLuO3 produced at 1600 �C. Thick samples were the result of the solid-state reactions that occurred at this temperature (leftimage). Interplanar spacing of 3.13 Å corresponds to the (111) plane of the monoclinic C2/m space group (right image). The inset shows the SAED pattern for this sample, in whichthe aligned spots confirm its monoclinic structure.

J.C. Soares et al. / Journal of Alloys and Compounds 678 (2016) 57e64 63

light configuration (selected only for xz and yz configurations).These crossed-polarized Bg modes are indicated in blue in Fig. 6.Therefore, we could assign all the 21 Raman-active gerademodes asbelonging to the Ag and Bg symmetries, as also presented in Table 1.These results are in complete agreement for the proposed mono-clinic C2/m (#12) space group for our phase-pure NdLuO3.

The thermal behavior of all samples was examined in theirmorphological, chemical and structural aspects by FESEM andHRTEM. Fig. 7 presents a sequence of FESEM images (backscatteredelectrons) for samples obtained under different conditions.Fig. 7aee shows representative FESEM images for samples obtainedat 800, 900, 1000, 1400 and 1600 �C. It was observed a generalincrease in their particle sizes; in particular, the morphologies dueto LuO(OH) and Nd(OH)3 precursors presented and discussed inFig. 2 were changed above 800 �C (Fig. 7aec) due to their conver-sion into lanthanide oxides. EDS analyses showed that Lu and Ndare still in opposite corners, which means that a chemical reactionbetween these precursors was not in progress. Above 900 �C(Fig. 7b and c), it is expected that some changes can occur since XRDdata verified a phase transformation for Nd2O3, from cubic tomonoclinic. However, the morphology had no significant changesabove 900 �C, except for a further increase in the particle size.Fig. 7d and e presents the morphology of the samples obtained at1400 �C and 1600 �C, which exhibit features of materials wheresome solid-state reaction cannot be neglected. Hard, compactedagglomerates can be seen, which indicate that a chemical reactionbetween the hydrothermally-derived precursors have nowoccurred. In fact, EDS analyses indicate that it is no longer possibleto discern between lutetium and neodymium along the samples.

Fig. 8 presents HRTEM images for the phase-pure monoclinicNdLuO3 produced at 1600 �C. It was observed that thick samplesdominate the scenario, which were the result of the solid-statereactions that occurred at this temperature (left image). Theinterplanar spacing was determined to be about 3.13 Å, corre-sponding to the (111) plane of the monoclinic C2/m space group(right image). The inset in Fig. 8 shows the SAED pattern for thesample obtained at 1600 �C, in which spots observed in well-aligned samples confirm their monoclinic structure.

4. Conclusions

In this work, phase-pure monoclinic NdLuO3 was synthesized

for the first time from nanostructured hydrothermally-derivedprecursors. Aqueous solutions of lutetium and neodymium ni-trates were hydrothermally treated at 250 �C for 24 h in order toproduce nanostructured LuO(OH) and Nd(OH)3 precursors. Thethermal behavior of this interlanthanide material was investigatedby XRD, micro-Raman spectroscopy and electron microscopy(FESEM and HRTEM). It was observed that the starting materialswere converted to cubic lanthanide oxides below 1000 �C. CubicNd2O3 has changed to themonoclinic structure at 1000 �C, followedby reaction with Lu2O3 at 1400 �C to produce monoclinic NdLuO3.Phase-pure monoclinic C2/m space group was obtained only at1600 �C, as verified by XRD, Raman spectroscopy and HRTEM. Inorder to validate our results, polarized micro-Raman scattering wasalso employed to probe the phonon modes in sintered NdLuO3. Allthe 21 non-polar vibrational modes predicted by group theorycalculations for the C2/m space group were identified and assigned,which also confirmed the production of phase-pure NdLuO3interlanthanides at 1600 �C.

Acknowledgements

The authors acknowledge the financial support from CAPES,CNPq, FINEP and FAPEMIG. The Center of Microscopy at the Uni-versidade Federal de Minas Gerais (http://www.microscopia.ufmg.br) is also acknowledged for providing the equipment and technicalsupport for experiments involving electron microscopy.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jallcom.2016.03.291.

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Structural and vibrational properties of phase-pure monoclinic NdLuO3 interlanthanides synthesized from nanostructured prec ...1. Introduction2. Experimental section3. Results and discussion4. ConclusionsAcknowledgementsAppendix A. Supplementary dataReferences