Materials Research Bulletin 48 (2013) 813–818

Preparation and neutron diffraction study of Dion-Jacobson type oxynitridesLiLaTa2O7�3xN2x (x = 0.09, 0.29)

Eunhye Lee a, Seung-Joo Kim b, Younkee Paik c, Young-Il Kim a,*a Department of Chemistry, Yeungnam University, Gyeongsan 712-749, Republic of Koreab Department of Chemistry, Division of Energy Systems Research, Ajou University, Suwon 443-749, Republic of Koreac Korea Basic Science Institute, Daegu 701-702, Republic of Korea

A R T I C L E I N F O

Article history:

Received 24 September 2012

Received in revised form 26 October 2012

Accepted 12 November 2012

Available online 22 November 2012

Keywords:

A. Nitrides

B. Neutron scattering

B. Nuclear magnetic resonance

A B S T R A C T

Ammonolytic heating of Dion-Jacobson type layered perovskite LiLaTa2O7 produced oxynitride

derivatives LiLaTa2O7�3xN2x&x via the substitution, 3O2� ! 2N3� + &. The nitridation level increased

with raising the temperature and/or increasing the time of reaction, reaching the limit at x � 0.3 by the

ammonolysis at 650 8C for 48 h. Upon the nitridation, tetragonal pristine lattice underwent a axis

expansion together with c axis contraction. Neutron Rietveld refinement of LiLaTa2O7 and

LiLaTa2O7�3xN2x (x = 0.09, 0.29) revealed that the Li-centered tetrahedra are gradually compressed

along the 2-fold rotation axis, with the increase of x. Solid state 7Li nuclear magnetic resonance

spectroscopy suggested that the anion vacancy resides mostly on the interlayer surface of perovskite

block. As measured from the diffuse-reflectance spectroscopy, the nitridation induced marked

reduction in the band gap magnitude, from 4.11 eV (x = 0) to 3.35 eV (x = 0.09) to 2.76 eV (x = 0.29).

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Materials Research Bulletin

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1. Introduction

Recently, there have been notable research activities in the areaof oxynitride type mixed anion systems, where it was demon-strated that anion control of existing solids can allow for rationaland facile design of material functions [1–9]. The hybridization ofO2�/N3� in the anion sublattice often favorably combines theattributes of oxides and nitrides. For instance, the oxynitrides tendto possess narrower band gaps and greater lattice polarizationsthan oxides and also enhanced chemical stabilities than nitrides.With such a prospect, the oxynitride families emerged aspromising photocatalysts [4,5], pigments [6,7], and dielectrics[8,9].

Exploration of novel oxynitrides has focused mainly on thethree-dimensional perovskites [1–3], while there were also studiesusing other structure types including layered perovskite [10–16],pyrochlore [17–20], and spinel [21–23]. Among them, the layeredperovskite oxynitrides are expected to be a suitable system fordeveloping ionic conductors and photocatalysts, as are the oxideanalogs [24–26]. The synthesis and crystal structure of layeredperovskite oxynitrides have been reported for Ruddlesden–Popperphases Li2LaTa2O6N [10,11], Rb1+xCa2Nb3O10�xNx�yH2O [12],Rb1+xLaNb2O7�xNx�yH2O [12], A2TaO3N (A = Ca, Sr, Ba) [13,14],Nd2AlO3N [15], and SrO(SrNbO2�xN)n (n = 1, 2) [16]. However, the

* Corresponding author. Fax: +82 53 810 4613.

E-mail address: yikim@ynu.ac.kr (Y.-I. Kim).

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.11.069

experimental investigation of their photocatalytic or ionic con-ducting behaviors was reported only in few cases: The Li+ ionicconductivity of Li2LaTa2O6N was measured to be 3.5 � 10�6 S/cmwith activation energy of 0.66 eV at 300 8C [10], and thephotocatalytic oxygen evolution by Rb1.8LaNb2O6.3N0.7�yH2O wasobserved with a quantum efficiency of 0.025% [12].

Here we explored the oxynitride modification of Dion-Jacobsonlayered perovskite LiLaTa2O7 by the direct ammonolytic heating,from which solid solutions could be derived with the generalformula LiLaTa2O7�3xN2x&x. It was found that the ammonolysistemperature and time influence the anion composition, band gapmagnitude, and lattice constants of the product LiLa-Ta2O7�3xN2x&x. In order to understand the local structuralevolution around the interlayer lithium, neutron diffraction andsolid state 7Li nuclear magnetic resonance spectroscopy wereemployed.

2. Experimental

2.1. Sample preparation

Dion-Jacobson type layered perovskite LiLaTa2O7 was synthe-sized as reported previously [27]. First, RbLaTa2O7 was prepared bysolid state reaction, where a mixture of Rb2CO3 (Alfa, 99.9%), La2O3

(Aldrich, 99.99%, calcined at 1000 8C for 12 h) and Ta2O5 (Alfa,99.85%), in a mole ratio of 1.5:1:2, was heated to 1100 8C for 30 hwith an intermittent grinding. Finely ground RbLaTa2O7 wasdispersed in an equimolar amount of molten LiNO3 (Aldrich,

Fig. 1. XRPD patterns of (a) LLTO and its ammonolysis products from explorative

conditions: (b) 550 8C, 10 h; (c) 650 8C, 10 h; (d) 700 8C, 10 h; and (e) 750 8C, 10 h.

E. Lee et al. / Materials Research Bulletin 48 (2013) 813–818814

99.99%), which was kept at 250 8C for 24 h with magnetic stirring.After the above Rb/Li ion exchange procedure, white polycrystalswere collected by filtration, rinsing with deionized water, anddrying at 55 8C for 12 h. Thus obtained powder had a compositionof LiLaTa2O7(CO2)x(H2O)y with x = 0.15 and y = 0.65, and will bedenoted as LLTO hereafter. For the ammonolysis, LLTO was heatedin a stream of dry NH3 (Wonik Materials, 99.999%) to a constanttemperature point chosen from the range 550 to 750 8C, and for thetotal dwell time of 10–48 h. As a cautionary note, the ammonolysisprocedure had to be conducted inside a fume hood to avoid anybiohazard.

2.2. Characterizations

Inductively coupled plasma atomic emission spectroscopy (ICP-AES; Jobin Yvon, 138 Ultrace) and combustion analysis (CEInstruments, Flash EA 1112) were conducted for the quantitativedeterminations of lithium and nitrogen, respectively. For ICP-AES,sample powder was dissolved in concentrated HNO3 using amicrowave digestion system (Milestone, ETHOS). In both cases, theelemental composition was determined by averaging the results ofthree measurements. Thermal behaviors of LLTO and its nitrida-tion products were examined by thermogravimetry coupled withmass spectrometry (TG–MS), using NETZSCH STA 409 simulta-neous thermal analysis system. Diffuse-reflectance absorbancespectroscopy was performed in the wavelength range 200–800 nmusing a double-beam spectrometer (Scinco, Neosys-2000)equipped with 35 mm integrating sphere. The measured reflec-tance was converted to absorbance by Kubelka–Munk equation[28], from which the band gap energy was estimated by Shapiro’smethod [29].

For examining the lattice evolution upon the ammonolysis, X-ray powder diffraction (XRPD) was conducted in a laboratorydiffractometer (PANalytical X’Pert Pro MPD) with Cu Ka radiation(40 kV, 30 mA). However for the detailed structural study,constant-wavelength neutron powder diffraction (NPD) measure-ments were carried out using the powder diffractometer HRPD ofthe HANARO reactor at Korea Atomic Energy Research Institute(KAERI). The NPD data were recorded using powder samplesloaded in a vanadium can, with a wavelength of 1.8367 A over the2u range 10–1608 and at a step size 0.058. In order to minimize thecontribution from adsorbed water, sample powders were dried at100 8C for 2 h prior to the measurement. The GSAS-EXPGUIsoftware suite [30,31] was used for the Rietveld refinement. Theneutron scattering coherence lengths b (in fm) of constituentatoms are �1.90 (Li), 8.24 (La), 6.91 (Ta), 5.803 (O), and 9.36 (N)[32].

Solid state 7Li nuclear magnetic resonance (NMR) spectroscopywas carried out under magic angle spinning (MAS) condition at10 kHz, using a Varian 200 spectrometer. The 7Li 908 pulse wasapplied for 2.5 ms with the pulse repetition delay of 60 s. Thechemical shift was referenced to 1.0 M aqueous LiCl (d = 0 ppm).

3. Results and discussion

3.1. Ammonolysis of LLTO

For preliminarily studying the nitridation behavior of LLTO,ammonolysis was attempted at different temperature points (550,650, 700, and 750 8C), with the dwell time of 10 h. In all those cases,the ammonolytic treatment gave rise to discernible changes in thesample color. The pristine LLTO, which was white, turned intoivory (550 8C), light yellow (650 8C), yellow (700 8C), and palebrick-red products (750 8C). Considering the general trend of theband gap reduction upon the oxide-to-oxynitride conversion of asemiconducting solid [3,6], the above color change was indicative

of the gradual progress of the nitridation along with the increase ofammonolysis temperature.

Examination of the XRPD patterns revealed an additional aspectof the temperature effect on the ammonolysis of LLTO. Fig. 1 showsthe XRPD data measured for LLTO and its ammonolysis products.The pattern for LLTO (Fig. 1a) corresponds to a tetragonal lattice ofa = 3.88 A and c = 20.42 A, which agree well with the previouslyreported cell parameters of Dion-Jacobson type LiLaTa2O7 [27].There were also detected, in Fig. 1a, weak peaks assignable toLaTa3O9 (space group Pnma, a = 6.543 A, b = 7.649 A, c = 12.583 A)[33]. We speculate that local compositional fluctuation could occurin the molten salt medium for Rb/Li exchange, leading to theformation of thermodynamically stable La–Ta–O phases. A similarobservation of La3TaO7 phase during the synthesis of LiLaTa2O7 hadbeen reported previously [27]. The ammonolysis productsobtained at 550–700 8C exhibited similar patterns (Fig. 1b–d) tothat of LLTO, although the peak positions were more or less shifted.On the other hand, the ammonolysis at higher temperatures (e.g.

750 8C) resulted in decomposition of the Dion-Jacobson lattice intoLaTaON2 and Ta3N5 (Fig. 1e). Even for the ammonolysis tempera-ture of 700 8C, a similar decomposition was observed when alonger heating time was applied. Therefore, it is judged that theammonolysis of LLTO can be achieved in the temperature range of550–700 8C, but suffers a phase instability at higher temperatureregions.

In order to investigate the lattice evolution depending on theammonolysis temperature and time, we determined the cellparameters of differently prepared oxynitride products by X-rayLe Bail refinement [34]. As presented in Fig. 2, the partialnitridation of LLTO caused c axial compression of the tetragonallattice, to the greater extent by the higher temperature and thelonger time of ammonolysis. For detailed compositional andstructural analyses of LLTO-derived oxynitrides, we selected twodifferent ammonolysis conditions, 550 8C for 48 h and 650 8C for48 h. The resulting phases are denoted below as LLTN1 and LLTN2,respectively.

Fig. 2. Lattice constants for various ammonolysis products of LLTO. Ammonolysis

temperature (8C) and time (h) are shown near the corresponding data point.

Bidirectional error bars indicate 10 times of estimated standard deviation.

Fig. 4. (a) TG and (b) TG–MS profiles of LLTN2.

E. Lee et al. / Materials Research Bulletin 48 (2013) 813–818 815

3.2. Composition and band gap

Ammonolytic heating of LLTO should affect the compositions ofO, N, and possibly Li as well. We examined the chemicalcompositions of LLTN1, LLTN2, and LLTO, by TG–MS, ICP-AES,and combustion analysis. Fig. 3 shows the TG and TG–MS profilesof LLTO recorded in Ar atmosphere. As observed from the TG–MSexamination up to 800 8C, thermal decomposition of LLTOliberated gaseous species with m/e = 18, 44, and 46, whichsupposedly corresponded to H2O, CO2, and Li2O2, respectively.From the temperature range of each evaporation step, we infer that

Fig. 3. (a) TG and (b) TG–MS profiles of LLTO.

LLTO contained weakly adsorbed H2O together with stronglybound CO3

2�, and that the de-intercalation of lithium commencedat �450 8C. The amounts of H2O and CO2 could be estimated fromthe weight decreases in the temperature regions 20–60 8C and 60–460 8C, respectively, from which the composition of LLTO wasdetermined as LiLaTa2O7(CO2)0.15(H2O)0.65.

Compared with the pristine oxide, LLTN1 and LLTN2 showedsignificantly different TG profiles, while their behaviors were verysimilar to each other’s. The TG and TG–MS profiles of LLTN2,obtained in Ar, are displayed in Fig. 4. From 20 8C to 600 8C, theweight loss from LLTN2 was only �0.8% in contrast to �4.2% fromLLTO. The TG–MS examinations of LLTN1 and LLTN2 detected onlyCO2, hence their compositions were assumed as LizLaTa2O7�3xN2x-(CO2)y. By combining the results of the TG, ICP-AES, andcombustion analysis (Table 1), the compositions were obtainedas LiLaTa2O6.75N0.17(CO2)0.11 for LLTN1 and LiLaTa2O6.15-N0.57(CO2)0.11 for LLTN2.

A few points are worthy to note from the compositionalanalyses. First, it is interesting that the Li compositions of LLTN1,LLTN2, and LLTO were practically same. That is, the ammonolyticheating of LLTO did not cause the loss of Li, in contrast to thethermal decomposition under the TG condition. The TG examina-tion also revealed that Li evaporation is minimal in LLTN1 andLLTN2, as evidenced by the weight retentions up to 700 8C and theabsence of MS peak with m/e = 46. Finally, those two oxynitridesshowed negligible water-adsorption. Above observations alto-gether suggest that the incorporation of N3� into LLTO substan-tially modified the chemical environment of interlayer Li, in a waythe covalent interaction between Li and the perovskite layer wasenhanced via the formation of Li–N bonds.

The oxynitride samples LLTN1 and LLTN2 exhibited chromaticcolors of light yellow and yellow, respectively. The diffuse-reflectance spectroscopy revealed that such optical behaviorswere associated with the band gap transition of the primary phase

Table 1Compositions of LLTO, LLTN1, and LLTN2, as determined from TG, ICP-AES, and

combustion analyses.

Sample wt% Composition

Li N H2O CO2

LLTO 1.12(5) – 1.84(2) 1.06(2) LiLaTa2O7(H2O)0.65(CO2)0.15

LLTN1 1.13(5) 0.38(1) – 0.81(2) LiLaTa2O6.75N0.17(CO2)0.11

LLTN2 1.15(5) 1.29(3) – 0.80(2) LiLaTa2O6.15N0.57(CO2)0.11

Fig. 5. Ultraviolet–visible absorbance spectra for (a) LLTO, (b) LLTN1, and (c) LLTN2.

E. Lee et al. / Materials Research Bulletin 48 (2013) 813–818816

rather than any impurity-related absorption. As shown in Fig. 5,the band gap energy (Eg) decreased with the increase of nitrogencontent, from 4.1 eV (LLTO) to 3.3 eV (LLTN1) to 2.7 eV (LLTN2).This is well consistent with the previous observations that the(oxy)nitride derivatives have higher valence band edges andnarrower band gaps than the related oxides [35]. The Eg of LLTN2was smaller than that of LLTO by �1.4 eV, and fell in the visiblelight range, giving promises for photocatalytic applications.

3.3. Solid state 7Li NMR

As a local probe for the chemical environment of Li, weemployed solid state MAS 7Li NMR spectroscopy. The 10 kHz MAS7Li NMR spectra for LLTO, LLTN1, and LLTN2 are presented in Fig. 6.The spectra of LLTO consisted of a prominent centerband and well-recognizable sidebands, indicative of the highly symmetricenvironment of Li. Compared with LLTO, the oxynitrides LLTN1and LLTN2 displayed broader and weaker sideband peaks. Weascribe such a depression of sideband signal to the symmetry-lowering around the Li site, as the nitridation of LLTO cantransform the coordination shell of Li from LiO4 to Li(O,N,&)4,accompanying the geometric distortion of tetrahedral shell and theconfigurational mixing of the ligand components as well.

Fig. 6. Solid state Li-7 NMR of (a) LLTO, (b) LLTN1, and (c) LLTN2, acquired at 10 kHz

MAS condition. Inset shows the zoom-in view of the centerbands.

From the Lorentzian peak fitting, the centerband positions (d)were determined as 0.2 ppm, 0.9 ppm, and 1.2 ppm for LLTO,LLTN1, and LLTN2, respectively. Therefore, the transformation ofLiLaTa2O7! LiLaTa2O7�3xN2x&x induced the downfield shift of 7Liresonance peak, or in other words, deshielding at the Li nucleus.Indeed, the changes in the anion composition can directlyinfluence the Li chemical shift. The replacement of O by N canenhance the bond covalency around Li, and will contribute toincrease the shielding at the nucleus. On the other hand, the anionvacancy can potentially reduce the coordination number of Li, todecrease the local shielding. Here the above two factors haveopposite effects on the displacements of 7Li shift, upward for theformer and downward for the latter. The observed result,downfield 7Li shift upon the nitridation, points that the lattervacancy effect dominated, and also strongly suggests that LLTN1and LLTN2 contained the anion vacancy in the vicinity of Li.

3.4. Neutron diffraction

The intracrystalline structures of LLTO, LLTN1, and LLTN2 werestudied by Rietveld analysis of the room temperature NPD pattern.Throughout the refinement, the composition of LLTO was fixed toLiLaTa2O7, while those of LLTN1 and LLTN2 were constrained asdetermined from the chemical analyses. In all three cases, thestructure model was taken from a previous report on the Dion-Jacobson LiLaTa2O7 phase [27], whose unit cell is represented inFig. 7. Briefly, LiLaTa2O7 has a body-centered tetragonal lattice,space group I4/mmm, in which Li, La, and Ta atoms occupy theWyckoff sites 4d (0, ½, ¼), 2a (0, 0, 0), and 4e (0, 0, �0.4),respectively. On the other hand, O atoms occupy three differentsites, X1 of 8g (0, ½, �0.1), X2 of 4e (0, 0, �0.3), and X3 of 2b (0, 0, ½)types. Each Ta atom is octahedrally surrounded by six O atoms,four of which sit on X1, one on X2, and one on X3 site.

Rietveld refinements of LLTO, LLTN1, and LLTN2 were carriedout as presented in Fig. 8 and Table 2, from which the atomicparameters and bond distances were obtained as listed in Tables 3and 4. In case of LLTO, the fraction of the secondary phase LaTa3O9

was found to be below 3 wt%. The refined structure of LLTO agreed

Fig. 7. Unit cell representation of LiLaTa2O7. Note that Li site is half-filled, whereas

the others are filled completely.

Fig. 8. Rietveld refinement of constant-wavelength NPD patterns for (a) LLTO, (b)

LLTN1, and (c) LLTN2. Calculated patterns (solid lines) are superimposed on

observed data (open circles), with Bragg positions and difference profiles at the

bottom. In (a), upper set of tick bars corresponds to Bragg positions for LiLaTa2O7,

and the lower set, LaTa3O9.

Table 3Atomic parametersa for LLTO, LLTN1, and LLTN2.

Atom Site LLTO LLTN1 LLTN2

Li 4d (0, ½, ¼) Uiso (A2) 0.056(3) 0.026(9) 0.019(4)

La 2a (0, 0, 0) Uiso (A2) 0.014(1) 0.025(2) 0.029(1)

Ta 4e (0, 0, z) z 0.3866(2) 0.3899(3) 0.3869(2)

Uiso (A2) 0.023(2) 0.016(1) 0.0056(8)

X1 8g (0, ½, z) z 0.0910(2) 0.0947(3) 0.0974(2)

Uiso (A2) 0.028(5) 0.021(1) 0.012(1)

Occupancy O 100% O 100% O 100%

X2 4e (0, 0, z) z 0.3038(4) 0.2975(7) 0.2860(4)

Uiso (A2) 0.046(3) 0.077(4) 0.054(3)

Occupancy O 100% O 87%

N 9%

O 57%

N 29%

X3 2b (0, 0, ½) Uiso (A2) 0.031(4) 0.054(3) 0.053(3)

Occupancy O 100% O 100% O 100%

a For all three compositions, occupancies of cation components were 50% for Li,

and 100% for La and Ta sites.

E. Lee et al. / Materials Research Bulletin 48 (2013) 813–818 817

with the literature data [27], except for small differences in thebond distances of Ta–O and Li–O pairs.

The oxynitrides LLTN1 and LLTN2 were isostructural to LLTO,and their lattice constants and atomic positions could be quicklyapproximated. However, the detailed structural description ofthese oxynitrides was complicated by the mixed occupation of theanion sublattice, where three different species (O, N, and vacancy)and three types of crystallographic sites (X1, X2, and X3, see Fig. 7)were involved. The Rietveld examinations for LLTN1 and LLTN2

Table 2Rietveld refinement of LLTO, LLTN1, and LLTN2, using room temperature NPD

(l = 1.8367 A) patterns.

LLTO LLTN1 LLTN2

Space group I4/mmm I4/mmm I4/mmm

a (A) 3.8892(3) 3.8971(3) 3.9132(3)

c (A) 20.457(2) 19.911(5) 18.887(5)

V (A3)/Z 309.44(7)/2 302.39(9)/2 289.21(9)/2

Rwp 0.0365 0.0734 0.0621

R(F2) 0.0739 0.1003 0.0417

x2 6.47 14.36 9.50

indicated that the three anion sites had fairly similar scatteringfactors. Although this might be ascribed simply to the randomdistribution of O/N/& over the entire anion sublattice, we regardedit implausible considering the markedly distinct chemical envir-onments of those anion sites. Meanwhile, it was noted that thereplacement of 3O ! 2N + & accompanies only a small change,�7% increase, in the overall scattering factor. That is, if both N andthe vacancy reside on the same type of site, all three anion siteswould have indistinguishable scattering factors. For deciding thesite for N and & we took into account the findings from TG and 7LiNMR investigations which were strongly indicative of the Li–Nbonding and the vacancy defects on the X2 site. Also it was recalledthat the X2 site is kinetically favored in the nitridation process.Above considerations led us to judge that the X2 site has the mixedO/N/& occupation, whereas X1 and X3 sites are filled completelyby O. For LLTN2, the occupants in X2 site consist of O 57%, N 29%,and & 14%, and for LLTN1, O 87%, N 9%, and & 4%.

Fig. 9 compares the local geometries of Ta and Li within LLTO,LLTN1, and LLTN2. With the progress of nitridation, theintraoctahedral distortion of TaX6 was gradually alleviated, wherethe difference between dTa�X3 and dTa�X2 markedly decreased from0.626 A (LLTO) to 0.352 A (LLTN1) to 0.231 A (LLTN2). Althoughthe lattice constant c decreased along the nitridation, the c axialheight of TaX6 octahedron slightly increased from 4.012 A (LLTO)to 4.032 A (LLTN1) to 4.043 A (LLTN2). We regard that thereplacement of O2� by the larger N3� ion led to the increase of Ta–X2 bond distances. The composition-weighted bond valence sum[36] (in valence unit) for Ta varied as +5.43 in LLTO, +5.19 in LLTN1,and +5.01 in LLTN2, owing to the displacement of Ta toward theoctahedral center. On the other hand, the above nitridation causedthe tetrahedral compression of LiX4 along the 2-fold rotation axis,together with the shortening of Li–X2 bond from 2.235 A (LLTO) to2.166 A (LLTN1) to 2.071 A (LLTN2). According to Shannon [37], forcomparison, the sums of the 4-coordinate ionic radii of Li+–O2� andLi+–N3� pairs are 1.97 A and 2.05 A. Thus, it is inferred that thenitridation of LLTO induced the structural relaxation around boththe overbonded Ta and the underbonded Li.

Table 4Selected interatomic distances (A) and bond angles (8) for LLTO, LLTN1, and LLTN2.

LLTO LLTN1 LLTN2

dTa�X1 (4�) 1.998(1) (4�) 1.972(1) (4�) 1.979(1)

dTa�X2 (1�) 1.693(9) (1�) 1.840(1) (1�) 1.906(8)

dTa�X3 (1�) 2.319(4) (1�) 2.192(7) (1�) 2.137(5)

dLi�X2 (4�) 2.235(4) (4�) 2.166(6) (4�) 2.071(2)

nX2�Li�X2 (4�) 104.0(2)

(2�) 121.0(2)

(4�) 101.0(3)

(2�) 128.2(3)

(4�) 96.2(1)

(2�) 141.7(4)

Fig. 9. Comparison of the bonding geometries in (a) LLTO, (b) LLTN1, and (c) LLTN2.

E. Lee et al. / Materials Research Bulletin 48 (2013) 813–818818

4. Conclusions

Oxynitride type Dion-Jacobson layered perovskite phases werederived from ammonolysis of LiLaTa2O7, where the resultingcomposition LiLaTa2O7�3xN2x (0 < x < 0.3) could be adjusted bythe reaction temperature and time. The solid state 7Li NMR andneutron Rietveld analyses of LiLaTa2O7�3xN2x provided convincingand complementary evidences that the O/N substitution andconcurrent vacancy creation occurred mostly on the layer surfaceof perovskite block. Compared with the oxide LiLaTa2O7, theoxynitride derivatives exhibited enhanced thermal stability andnarrower band gaps, which are stimulating in view of applicationsin photocatalysts or ionic conductors. As an interesting subject offurther study, exfoliation of LiLaTa2O7�3xN2x is proposed as a routeto realize nanosheets having oxynitride surface layers.

Acknowledgement

This work was supported by the Yeungnam University researchgrant in 2010.

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