preparation and crystal structures of some a iv b...

Research ArticlePreparation and Crystal Structures of Some AIVB2

IIO4Compounds: Powder X-Ray Diffraction and Rietveld Analysis

K. Jeyadheepan1,2 and C. Sanjeeviraja1

1 Department of Physics, Alagappa Chettiar College of Engineering and Technology, Karaikudi, Tamilnadu 630 004, India2 School of Electrical and Electronics Engineering, SASTRA University, Tirumalaisamudram, Thanjavur 613401, India

Correspondence should be addressed to C. Sanjeeviraja; [email protected]

Received 10 May 2013; Revised 13 December 2013; Accepted 16 December 2013; Published 2 February 2014

Academic Editor: Ali Nokhodchi

Copyright © 2014 K. Jeyadheepan and C. Sanjeeviraja. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

TheAIVB2

IIO4compounds such as cadmium tin oxide (Cd

2SnO4or CTO) and zinc tin oxide (Zn

2SnO4or ZTO) are synthesized by

solid state reaction of the subsequent binary oxides. The synthesized powders were analyzed through the powder X-ray diffraction(PXRD). Cell search done on the PXRD patterns shows that the Cd

2SnO4crystallizes in orthorhombic structure with space group

Pbam and the cell parameters as 𝑎 = 5.568(2) A, 𝑏 = 9.894(3) A, and 𝑐 = 3.193(1) A and the Zn2SnO4crystallizes as cubic with

the space group 𝐹𝑑3𝑚 and with the cell parameter 𝑎 = 8.660(2) A. Rietveld refinement was done on the PXRD patterns to get thecrystal structure of the Cd

2SnO4and Zn

2SnO4and to define the site deficiency of atoms which causes the electrical properties of

the materials.

1. Introduction

Conducting oxide materials are interested for several energyconversion applications, including solid state electrolytes fornew types of storage and fuel cells, transparent electrodematerials for some types of solar cells, and photo-electrolysiselectrodes for the direct conversion of solar energy to hydro-gen [1]. The AIVB

2

IIO4compounds such as Cd

2SnO4and

Zn2SnO4are important prototypes for modern high efficient

transparent conducting oxides. The structure solution frompowder X-ray diffraction and Rietveld refinement of someAIVB2

IIO4compounds are reportedwhere the group-II atoms

are Mg, Zn, and Cd and group-IV atoms are Si, Ge, and Sn.But the structural studies of cadmium tin oxide (Cd

2SnO4or

CTO) and zinc tin oxide (Zn2SnO4or ZTO) lack a step in the

literature.Cadmium tin oxide was first synthesized by Smith [2] in

1960. Later, Nozik [3] studied its semiconducting propertiesand suggested it as a promising material for transparentelectrodes. It has been reported that the oxygen vacanciesin Cd

2SnO4provided the donor state for the conduction.

Few coworkers [4, 5] suggested that the cadmium interstitialsmay also contribute to the conductivity. Zn

2SnO4was first

prepared by Coffeen [6] through the wet chemical method,from its chemical precursors to precipitate the hydrated zincstannates and followed by a sintering process at elevatedtemperatures, found to be an oxide semiconductor, and itscrystal structure was determined to be cubic with inversespinel (MgAl

2O4) structure with the space group 𝐹𝑑3𝑚.

Chang and Kaldon [7] have studied the solid state reaction ofSnO2-ZnO and observed that the solid state reaction between

SnO2and ZnO is starting at above 1100∘C, and Zn

2SnO4

is the only resultant. Cun et al. [8] have studied the defectstructure of Zn

2SnO4and suggested that the defect structure

of Zn2SnO4is mainly due to the oxygen deficiency with n-

type conductivity.Over the past decade, structure determination from

powder diffraction (SDPD) hasmatured into a technique thatis widely and successfully used in the context of organic,inorganic, and organometallic compounds [9]. By defini-tion, the crystal belonging to the same structure type withsame space group should have the similar cell parameters

Hindawi Publishing CorporationJournal of ChemistryVolume 2014, Article ID 245918, 6 pageshttp://dx.doi.org/10.1155/2014/245918

2 Journal of Chemistry

and similar representative atom coordinates. But in thecase of deficit materials like ternary compounds (AIVB

2

IIO4

compounds) which have different atomic defects, have somemajor variations in the crystallographic parameters. Hence,the study of crystal structure of these compounds is necessaryto understand its physical properties, especially the electronicproperties. Hence, the present study aims at the preparationof Cd

2SnO4and Zn

2SnO4through the solid state reaction

method and the determination of the structure of Cd2SnO4

and Zn2SnO4powders through powder X-ray diffraction

technique.

2. Experimental Details

The Cd2SnO4powder was prepared from the solid state

reaction of the well-blendedmixture of required quantities ofCdO (99.99% pure-Merck) and SnO

2(99.99% pure-Merck)

in the atomic weight ratio of 2 : 1. The mixed binary oxides,after making them as a 5mm thick 50mm diameter pellet,were kept at 1050∘C for 6 hours in an autotuned, PIDcontrolled SiC furnace at air atmosphere.The optimized timeduration was selected based on the monophase compoundformation which was confirmed through XRD results. TheZn2SnO4powder was also prepared as mentioned above,

from the well-blended mixture of the ZnO (99.99% pure-Merck chemicals) and SnO

2(99.99% pure-Merck chemicals)

in the atomic weight ratio of 2 : 1. The pellet was kept at900∘C in air for 4 hours. The temperature conditions wereselected on the basis of thermal analysis results in bothcases.

The TG/DTA analysis of the CdO-SnO2and the ZnO-

SnO2mixed powders was made using TG/DTA analyzer

(Perkin Elmer Diamond TGA/DTA). The instrument wascalibrated using aluminum reference material which ishaving the transition point at 660.10∘C. TG/DTA curveswere measured in the constant flow of air at the rate of10∘C/min. X-ray diffraction analysis wasmade on the samplesat room temperature using the PANalytical X’Pert PROMPD diffractometer of the new generation equipped withtwo-dimensional solid state X’celerator detector. The datacollection was done with 2𝜃 ranging from 10 to 140∘ and thestep width of 0.02∘ with 212 seconds per step counting time.Powder pattern indexing and the manual space group testwere done in the softwareCMPR [10]; cell searchingwas doneusing TREOR 90 [11] algorithm. The Rietveld refinementwas done using the GSAS [12] with EXPGUI [13] interface.Structural illustrations were obtained from DIAMOND [14]using the crystallographic information file (cif file). Thematerials for the publication were done in the EXPGUIitself. Raman spectrum of the Cd

2SnO4

and Zn2SnO4

powders was recorded with the exciting laser wavelengthof 480 nm.

The electrical conductivity, bulk carrier concentration,and the mobility of the samples were measured using theEcopia Hall effect system model number HMS-3000 bymaking the powder as the hot pressed pellets.

Wei

ght (

mg)

11.4

11.3

11.2

11.1

11.0

10.9

10.8

Temperature (∘C)0 200 400 600 800 1000

Mic

rovo

lt (𝜇

V)/

K (e

ndo

dow

n)

0

−5

−10

−15

−20

−25

−30

Figure 1: TG/TDA analysis of mixed precursor for Cd2SnO4.

3. Results

TG/DTA studies were made to study the temperature effecton themixed sample of CdO and SnO

2(sample 1) and also on

themixed sample of ZnO and SnO2(sample 2).The TG/DTA

curves for the sample 1 are shown in Figure 1. In the TG curve,themultistage decomposition is observed at 210∘Cand 300∘C.After the decomposition, a stable intermediate stage has beenobserved up to 900∘C above which a slight increase in weightis observed up to 1050∘C.Then, above 1050∘C, another weightloss is observed. Figure 2 shows the TG/DTA analysis curveof the mixed sample of ZnO and SnO

2(sample 2). In the TG

curve of sample 2, rapid decomposition is observed sharplyat 210∘C. After the decomposition, a stable stage is observedapproximately up to 725∘C. After 725∘C, a gradual increase inweight is observed up to 1000∘C.

The prepared samples of Cd2SnO4and Zn

2SnO4were

subjected to the X-ray powder diffraction analysis with thestep value of 0.02∘ to avoid the crystallographic analysiserrors. Manual cell search done on the Cd

2SnO4sample

in CMPR using the TREOR90 algorithm revealed that theCd2SnO4powder crystallized in orthorhombic structurewith

cell parameters 𝑎 = 5.568 A, 𝑏 = 9.894 A, and 𝑐 = 3.193 Aand cell volume as 175.94 A3 with the figure of merit (FOM)of 62.0. Space group test, done with the peak positions,reveals that the space group is Pbam (55). Cell search doneon the Zn

2SnO4sample deduced that the Zn

2SnO4powder

crystallized in cubic structurewith cell parameter 𝑎 = 8.660 Aand the cell volume as 649.27 A3 with the FOM of 61. Spacegroup test done, with the peak positions only, reveals that thespace group is 𝐹𝑑3𝑚 (227).

Figure 3 shows the Rietveld refinement plot of Cd2SnO4

powder diffraction data. The difference plot and the plot ofcumulative 𝜒2 value show the convergence of the Rietveldrefinement.The crystal and experimental data of the Rietveldrefinement is given in Table 1. Figure 4 shows the Rietveldrefinement plot of Zn

2SnO4powder diffraction data. The

crystal and the Rietveld refinement data of Zn2SnO4is given

in Table 2.Figures 5 and 6(a) show the schematic representations

of the crystal structures of the orthorhombic Cd2SnO4and

Journal of Chemistry 3W

eigh

t (m

g)

7.7

7.6

7.5

7.4

7.3

7.2

Temperature (∘C)0 200 400 600 800 1000

Mic

rovo

lt (𝜇

V)/

K (e

ndo

dow

n)

0

−5

−10

−15

−20

−25

Figure 2: TG/DTA analysis curve ofmixed precursors for Zn2SnO4.

Chi2bckgrCalc

ObsdiffPhase1

CTO cycle 124 hist 1

50 100

Inte

nsity

0

1000

Cum

ulat

ive c

hi sq

uare

d

0

0.2

0.4

0.6

0.8

2𝜃

Figure 3: Rietveld refinement plot of Cd2SnO4.

cubic spinel Zn2SnO4, respectively. Figures 6(b) and 6(c)

show the octahedrally and tetrahedrally coordinated cations,respectively.

Raman spectrum of the Cd2SnO4and Zn

2SnO4powders

is shown in Figures 7 and 8, respectively. In Figure 7,the strong Raman shift peaks at 605.9 cm−1, 548 cm−1,and 474.6 cm−1 correspond to the Cd

2SnO4peaks [14].

In Figure 8, strong Raman shift peaks at 670.3 cm−1 and527.8 cm−1 correspond to the well-known ZTO peaks [15].

The conductivity of the prepared Cd2SnO4is 2.095 ×

103 S/cm with the bulk concentration 4.439 × 1020 cm−3and the mobility 29.46 cm2/Vs and the conductivity of theZn2SnO4is 65.38 S/cm with bulk concentration 5.375 ×

1018 cm3 and the mobility 7.592 cm2/Vs.

Table 1: Crystal and refinement data of Cd2SnO4.

Empirical formula: Cd2SnO4

Chemical formula: Cd3.58Sn1.78O8.05

Formula weight: 741.99Wavelength: 𝐾

𝛼1= 1.54060 A; 𝐾

𝛼2= 1.5444 A

Temperature: 300KCrystal system: orthorhombicSpace group: Pbam (55) 𝑍 = 1

𝑎 = 5.568(2) A 𝛼 = 𝛽 = 𝛾 = 90∘

𝑏 = 9.894(4) A𝑐 = 3.193(12) A𝑉 = 175.96(10) A3

𝐷𝑥= 7.002 g/cm3

No. of reflections used = 3942𝜃min = 10.00

∘, 2𝜃max = 140.00∘

𝑅𝑝= 0.105

𝑅𝑤𝑝= 0.145

𝑅exp = 0.155

𝑅 = 0.0407

Reduced 𝜒2 = 0.867𝑆 = 0.93

(Δ/𝜎)max = 0.02No. of parameters = 82Least squares matrix: fullNo. of cycles = 124Crystallite size = 156.73 nmStrain = 0.094 × 10−3

Table 2: Fractional atomic coordinates and isotropic displacementparameters of Cd2SnO4 (in A).

Atom Site 𝑋 𝑦 𝑧 Uiso∗/Ueq Occ. (<1)Sn1 2a 0 0 0 0.0032(27) 0.890(4)Cd2 4h 0.0585(10) 0.3224(7) 0.5 0.0057(22) 0.893(33)O3 4h 0.2369(9) 0.0418(5) 0.5 0.0100(21) 0.994(20)O4 4g 0.3620(10) 0.3052(6) 0 0.0230(25) 1.019(18)

4. Discussion

In the TG curve of Figure 1, the initial mass loss observedup to 100∘C is characteristics of desorption or drying of themixed CdO and SnO

2powders. The multistage decomposi-

tion at 207∘C and 300∘C may be attributed to the ionizationof excess of Cd and Sn present in the CdO and SnO

2,

respectively. After the decomposition, a stable intermediatestage has been observed up to 900∘C above which a slightincrease in weight indicates the oxidation of the sampleat higher temperature [16]. At about 1050∘C, the observedweight indicates an oxidative decomposition reaction again athigher temperature. The sharp peaks observed in DTA curveat 210∘C and 300∘C reveal the purity of the precursors taken.From TG/DTA analysis, it is clear that, above 1000∘C, theoxidation takes place. So the mixed sample, after making it


ZTO cycle 197 Hist 1

50 100

Inte

nsity

0

2000

4000

6000

Cum

ulat

ive c

hi sq

uare

d

0

0.2

0.4

0.6

0.8

Chi2bckgrCalc

ObsdiffPhase1

2𝜃

Figure 4: Rietveld refinement plot of Zn2SnO4.

a

b

c

SnCdO

Figure 5: Schematic representation of the crystal structure oforthorhombic Cd

2SnO4.

as a 5mm thick 50mm diameter target, was kept at 1050∘Cfor 6 hours in air.

In the TG/DTA curve of Zn2SnO4precursor, there are

only two weight loses present in the TG curve. Accordingto Cun et al. [17], a weight loss observed at about 120∘Cmay be attributed to the liberation of the surface-adsorbedwater and another weight loss at about 210∘C might beattributed to the liberation of the crystal water. There is noany recrystallization taking place at about 724∘C. Only aconstant intermediate phase is observed.Thegradual increasein weight observed above the 725∘C may be attributed tothe oxidation of the Zn

2SnO4precursor. This oxidation is

observed up to 1000∘C also. So the precursor, after makingit as a 5mm thick 50mm diameter target, was kept at 950∘Cfor 6 hours in air atmosphere.

Table 3: Crystal and refinement data of Zn2SnO4.

Empirical formula: Zn2SnO4

Chemical formula: Zn60.53Sn36.12O138.74

Formula weight: 10463.77Wavelength: 𝐾

𝛼1= 1.54060 A; 𝐾

𝛼2= 1.5444 A

Temperature: 300KCrystal system: cubicSpace group: Fd 3m (227) 𝑍 = 1

𝑎 = 8.6604(2) A 𝛼 = 𝛽 = 𝛾 = 90∘

𝑉 = 649.56(5) A3

𝐷𝑥= 26.75 g/cm3

No. of reflections used = 902𝜃min = 10.00

∘, 2𝜃max = 140.00∘

𝑅𝑝= 0.03

𝑅𝑤𝑝

= 0.051𝑅exp = 0.052𝑅 = 0.034

Reduced 𝜒2 = 0.959𝑆 = 0.98

(Δ/𝜎)max = 0.01No. of parameters = 21Least squares matrix: fullNo. of cycles = 197Crystallite size = 253.39 nmStrain = 0.626 × 10−3

The cadmium stannate may be crystallized as monocad-mium stannate (CdSnO

3) or dicadmium stannate which are

of the same orthorhombic structure. To avoid the wrongindexing problem due to the same crystal structure, highquality powder diffraction data is needed. Hence, the powderX-ray diffraction analysis was done on the samples with the2𝜃 step value of 0.02∘. Since the powder diffraction data is ofhigh quality, the highest FOM values of 62 for Cd

2SnO4and

61 for Zn2SnO4were achieved.

In the crystallographic analysis, the possibility of themonocadmium stannate (CdSnO

3) was also checked and

the results showed that the space group pnma, which is forCdSnO

3, has the figure ofmerit of 5.6637with 10 unexplained

lines. Hence, it is confirmed that the orthorhombic Cd2SnO4

is the only product in the solid state reaction of CdO, andSnO2in the ratio of 2 : 1 at the temperature of 1050∘C, as the

orthorhombic structure is mostly favored for bulk Cd2SnO4

[18].There are no any secondary phases like CdSnO3, CdO or

SnO2. Characteristic peaks obtained in the Raman spectrum

also confirm the Cd2SnO4formation. The broadening and

asymmetry of the peaks indicate the typical features of thenanoscale materials which supports the crystallite size resultsof XRD.

The result of the Rietveld refinement of Cd2SnO4is

given in Table 1. Fractional atomic coordinates and isotropicdisplacement parameters of Cd

2SnO4are given in Table 2.

The Rietveld refinement was done with manual backgroundfitting and with the pseudo-Voigt profile function. The leastsquare refinement was converged with the 𝑅 value of 0.0407

Journal of Chemistry 5

a

b

c

ZnSnZn

O

(a) (b)

a

b c

ZnSnZn

O

(c)

Figure 6: (a): Schematic representation of the crystal structure of “inverse spinel” Zn2SnO4. (b) Octahedrally coordinated “SnZn” cation. (c)

Tetrahedrally coordinated Zn atom.

Table 4: Fractional atomic coordinates and isotropic displacement parameters of Zn2SnO4 (in A).

Atom Site 𝑋 𝑦 𝑧 Uiso∗/Ueq Occ. (<1)Zn1 16c 0.0 0.0 0.0 0.0151∗ 1.714 (21)Zn2 8b 0.375 0.375 0.375 0.0085∗ 4.138 (20)Sn3 16c 0.0 0.0 0.0 0.0246∗ 2.26O4 32e 0.2416 (13) 0.2416 (13) 0.2416 (13) 0.0130∗ 4.336 (26)

Inte

nsity

(a.u

.)

474.8

605.04

Wavenumber (cm−1)200 400 600 800 1000

Figure 7: Raman spectrum of Cd2SnO4powder.

with 𝑅𝑝value of 0.105, 𝜒2 of 0.8675, and the goodness of fit

(GOF) of 0.93. The values of 𝜒2 and GOF confirm the best fitof the calculated profile function to the observed diffractiondata. The weight ratio of Cd, Sn, and O (given in Table 1)reveals that there is no any oxygen vacancy in the compoundto contribute to the conductivity.The excess proportion of Cdatom shows that the Cd interstitials only contribute to thehigh conductivity which pronounced the high conductivityvalue of 2.095 × 103 S/cm. Since there is no any disorder inthe site of cations (Cd and Sn), the mobility will be increased

Inte

nsity

(a.u

.)

400

350

300

250

200

150

100

50

223.4

527.8

670.3

Wavenumber (cm−1)200 400 600 800 1000

Figure 8: Raman spectrum of Zn2SnO4powder.

to the value of 29.46 cm2/Vs, even though there is the highbulk carrier concentration of 4.439 × 1020 cm−3. It makes thenormal orthorhombic Cd

2SnO4more conductive in nature

than the cubic spinel structure.Approximate fractional atomic coordinates found from

the structure refinement show that the tetrahedral voids (8b)are occupied by Zn atoms and the octahedral voids (16c)are occupied randomly by an equal number of Zn and Snatoms [19] which confirms the inverse spinel structure ofthe Zn

2SnO4in which the Zn

2SnO4is more stable [18]. The


Rietveld refinement was converged with the 𝑅 value of 0.034,𝑅𝑝value of 0.030,𝑅

𝑤𝑝of 0.051, and reduced 𝜒2 value of 0.959.

The crystallographic and refinement data of Zn2SnO4were

given in Table 3. The atomic coordinates and the isothermaldisplacement parameters were given in Table 4. The weightratio of the Zn

2SnO4given in Table 3 reveals that the oxygen

vacancy (𝑉𝑜) is the main reason for the conductivity of

Zn2SnO4to the value of 65.38 S/cm. Normally, Zn

2SnO4is

composed of closely packed oxygen ions and rutile chainswhich are connected by cations in the tetrahedral sitesand which run through the lattice. The rutile chains areconsidered as conduction paths for electrons as unoccupiedorbital of cations which significantly overlap in the chainsbecause of short cation-cation separation distances due tothe edge sharing of cation octahedra. But the spinel latticeof Zn2SnO4is locally distorted enough to form two distinct

octahedrally coordinated Sn and Zn sites.The disorder in theSn and Zn sites in the lattice significantly limits the mobilityof the carriers (7.592 cm2/Vs), possibly by disrupting theedge-sharing nature of the octahedrally coordinated cations[20]. The characteristic Raman peaks confirm the Zn

2SnO4

formation.

5. Conclusion

The accurate X-ray structural investigations of orthorhombicCd2SnO4and cubic inverse spinel Zn

2SnO4and the use of

the experimental data with high resolution of Δ2𝜃 = 0.02∘allowed us to obtain the results with high relative precision(reduced 𝜒2 ∼ 1). As the inclusion of a large number ofhigh angle weak reflections in the analysis leads to a decreasein accuracy of the results of the refinement, the design andthe performance of the experiment and the methods used toprocess the experimental data made it possible to overcomethe influence of this factor. The electrical conductivity andthe mobility values were correlated with the site occupancy,atomic site defects, and also the weight percentage of theelement present in the compounds.

Conflict of Interests

The author(s) declare(s) that there is no conflict of interestsregarding the publication of this paper.

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