X-ray and electrical characterization of NaSb2F7 single crystals

Materials

ELSEVIER

Chemistry and Physics 38 (1994) 337-341

X-ray and electrical characterization J. Benet Alagappa

College

Charles

of Technology,

of NaSb2F7 single crystals

and F.D. Anna

Gnanam

University.

Madras-600

025 (India)

K. Sivakumar Depurtment

of Physics,

Collegr

of Engineering,

Annu

University.

Madras-600

025 (India)

(Received August 9, 1993:accepted April 13. 1994)

Abstract Single crystal X-ray analysis of NaSbzFT have been carried out and cell parameters are determined. From thermal analysis the weight loss, thermal stability and decomposition temperatures have been determined. The d.c. electrical conductivity of NaSbzF, at (loo), (010) and (001) orientations have been carried out in the temperature range from 30°C to 175°C. Dielectric constant (E ‘) and dielectric loss (tan4 have been measured in the range from 0.1 KHz to 100 KHz and at temperaturestietween 30°C to 175°C.

Introduction The study of physicochemical properties of inorganic fluorides is gaining interest because of the extensive possibilities for their applications in optical instruments and laser technology. Phase diagram [l] studies reveal that sodium fluoride and antimony trifluoride form many complexes such as NaSbF,, NaZSbFS and NaSb3F,” etc. The lattice parameters and their space group are reported in the literature. Thermal and X-ray analyses of NaSbF4 have been reported by Ptaszynski [2]. Single crystals of NaSbzF7 are grown by low temperature solution growth method [3]. The perfection of the single crystals of NaSbF, and Na,Sb4FlS have been carried out and the results are reported [4]. Extensive studies of crack patterns in NaSbF4 and NaSb2F, crystals have been reported in our earlier paper [5]. In the present communication, we report the results of X-ray, thermal and electrical studies of NaSb2F, crystals.

Experimental The single crystal X-ray analysis of this compound have been carried out using X-ray Oscillation, Weissenberg and Precision techniques to determine the lattice

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parameters and space group. X-ray powder diffraction studies of this crystal is also carried out using Cu-K, radiation using, ‘Reich-seifert’ x-ray powder difractometer. A nickel-foil filter is used to minimize Cu-K, The sample is scanned at a radiation (A = 1.5418 A). speed of 1” in 20 per minute. Thermogravimetric analyses of sodium fluoroantimonate compound NaSb2F, have been carried out using METTLER TA 3000 system. Both TGA and DSC, the measurements are made in air in the temperature range from 30 to 500°C at a heating rate of 20°C per minute. Single crystals of NaSbzF-, have been cut in the form of square and dimensions of nearly 10 mm2 cross section and 1 mm thickness are used for the measurements of electrical conductivity. The thickness variation is less than 10 micron. The opposite faces of the specimen are coated with a thin layer of silver paint. This ensures good contact with the electrodes. Conductivity measurements are made by measuring the resistance of the crystal using 610 C Solid State Keithley Electrometer. The measurements are made both during heating as well as cooling. The experiments have been repeated on different samples and the results are found to be reproducible within the experimental error. The resistance values are measured in the temperature range from 30 to 175°C.

338

J. Benet Charles et al. I Materials Chemtby

The sample preparation for dielectric measurement is as in the case of electrical conductivity studies explained above. Prior to resistance and dielectric measurements the crystals are heated and then cooled to permit reproducible data and avoid surface moisture too. The accuracy of measurement in dielectric constant is 2% and in dielectric loss is about 6%. Two samples in each orientation of NaSb2F, are studied and the agreement is within the limits mentioned above. The capacitance and dissipation factor of the specimen are measured as a function of frequency in the range from 100 Hz to 100 KHz using AND0 model AG-4311 LCR meter.

Fig. 1. a shows NaSbzFT single crystal grown by low temperature solution growth method and the growth details are explained in our previous paper [3]. From the results of X-ray analysis it is found that the compound NaSbrF, crystallises in monoclinic system with the following lattice parameters: p= 94.15 ,

V = 379.29 A3, d,,,. = 3.49 glcm3, d,,,,.=3.48 glcm3, z=2. The mirror symmetry in the Oscillation photograph indicated a monoclinic crystal system and from the systematic absences (OkO, k odd absent; h01, h odd absent) in the Weissenberg photograph the space group was determined to be P2Ja. The X-ray powder diffration data for NaSbzF, is given in Table 1. Typical thermoanalytical curves (TG & DSC) of sodium heptafluoroantimonate are shown in Fig. 1.b.

Fig. 1. a. Solution grown single crystal of NaSbZF7.

The first endothermic peak at 264.3”C is due to the melting of the sample. The second exothermic reaction begins at 350°C and ends at 460°C. The third exothermic peak has its maximum at 484.2”C. From the TG curve it is observed that there is no change in weight up to 298°C. In the last stage weight loss corresponds to the removal of SbF,. This is confirmed by X-ray analysis. Moreover, the ratio of the number of moles of SbF3 liberated to those bonded in the compound decreases, suggesting an increase of resistance against thermal decomposition. At higher temTABLE 1. X-ray powder diffraction data for NaSb2F-I crystals Peak No.

Results

a = 8.564 A, b = 5.513 A, c = 8.054 A,

and Physics 38 (1994) 337-341

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 3.5 36 37 38 39 40 41 42 43 44 45

28

hkl

20.88

200 lli 111 201

21.52 22.42 22.92 23.55 24.56 24.87 26.43 27.56 29.42 32.65 33.04 33.49 34.35 35.18 35.72 37.77 38.51 40.02 40.93 41.75 42.33 43.81 44.28 45.07 45.57 45.88 47.22 48.02 49.03 49.65 50.22 51.51 51.88 52.42 54.19 54.80 55.63 59.64 60.34 66.03 66.98 68.31 69.10 73.39

io2 201 102 210 012 211 020 3oi 301 021 310 212 311 113 302 221 122 400 312 213 401 320 222 104 014 123 030 114 031 204 322 230 032 501 015 502 404 503 324 034 206

d (A) measured 4.261 4.125 3.862 3.878 3.774 3.622 3.577 3.369 3.234 3.026 2.741 2.709 2.674 2.601 2.549 2.512 2.380 2.279 2.251 2.203 2.157 2.133 2.065 2.044 2.010 1.989 1.976 1.924 1.893 1.857 1.835 1.815 1.773 1.761 1.744 1.691 1.674 1.651 1.549 1.533 1.414 1.396 1.372 1.358 1.289

d (A) calculated 4.271 4.083 3.946 3.889 3.740 3.663 3.538 3.376 3.246 3.051 2.757 2.747 2.625 2.607 2.530 2.525 2.370 2.280 2.248 2.203 2.174 2.136 2.081 2.019 2.028 1.980 1.973 1.924 1.887 1.853 1.838 1.817 1.791 1.769 1.742 1.688 1.671 1.647 1.542 1.533 1.413 1.395 1.376 1.356 1.252

I/IO

12 19 5 34 71 59 26 38 100 6 2 11 4 8 7 6 6 11 7 15 14 8 24 21 7 18 10 21 8 4 10 11 6 16 6 6 8 7 10 9 5 6 10 6 4

J. Benet Charles et ul. I Material

339

Chemistry and Physics 38 (1994) 337-341

peratures the decrease in mass may be connected with further slow decomposition and evaporation of sodium fluorides. The conductivity versus temperature of NaSbTF, single crystals along (100) (010) and (001) directions are shown in Fig. 2. The measurements have been taken in the temperature range from 30 to 175°C. There are differences in the absolute magnitudes of the conductivity and transition temperatures in the three directions. It is very clear from the figure that the

-8

2.2

2.6

3.0 1000 T

Fig 2.

3.4

(K-l)

A plot of log oT versus 1000/T for NaSb2F-i along (100) and (001) directions.

TABLE 2. Activation energy of NaSbzF, Region

Intrinsic Extrinsic

TGA and DSC curve of NaSb*F,.

(100) orientation eV

(010) orientation eV

(001) orientation eV

2.9 2.0

4.7 1.1

0.71 0.35

activation energies are different from each other and are given in Table 2. The variation of dielectric constant against frquency for NaSb,F7 crystals along (100) (010) (001) and (101) orientations are shown in Fig. 3. The dielectric constant decreases with increasing frequency and attaining a constant value beyond 10 KHz for all orientatons studied. The figure show that dielectric constant is nearly independent of the frequency at above 10 KHz. The variation of 6 ’ with temperature at different orientations are shown in Fig. 4. The dielectric loss as a function of frequency at 100°C for (001) and (101) orientations are shown in Fig. 5. Table 3 show that the dielectric constant of NaSbzF, along different orienta-

.I. Benet Charles et al. / Materials

340

Chemistry and Physics 38 (1994) 337-341

12c

I-

8C )-

c’ II

25

(001)

I (100) A (101)

4c I-

E’

I Frequency Fig 3.

0 I_

IO

I

I

( kHz 1

I

I

I

I

I

I

I

a (001)

Variation of dielectric constant with frequency along different orientations.

A

4c I-

(101)

Jf

c’ tions at different frequencies. From the results it is clear that the dielectric constant along (100) (001) and (101) orientations are close with each other. For frequency at 1 KHz the dielectric constant is large along (010) orientation compared with other orientations. The large variation indicates the anisotrophy of the crystal. The magnitude of E’ is a measure of the electrostatic binding strength between ions. The large measurement value of E‘along (010) direction indicates the low electrostatic binding strength [6]. Fig. 6 shows that the dielectric loss (tan 8) as a function of frequency for NaSbzFT crystal along (100) and (101) orientations. Initially there is an increase in dielectric loss reaching maximum at the frequency of 0.4 KHz and then decreases. Similarly in (010) and (001) orientations, it reaches maximum at 0.6 KHz and decreases with the increase in frequency. The dielectric loss as a function of frequency at 100°C for (001) and (101) orientations are shown in Fig. 6. The dielectric loss decreases with the increase of frquency. For (010) and (100) orientations also the dielectric loss decreases with the increase of frequency. The larger values of E’ and tan8 at lower frequencies may be attributed to space charge polarisation due to lattice defects [7].

2c

)-

-_-___ IO kHz ---

0

I

0

I

I

I

,I

60

100 kHz

I

120

Temperature Fig 4.

I

-

I

I

180

( “C )

Variation of dielectric constant as a function of temperature along different orientations at different frequencies

TABLE 3. Dielectric constant of NaSbZF7 along different orientations Frequency HKZ 1

5 100

(100)

(010)

Wl)

(101)

14.82 12.34 10.81

56.25 42.93 34.78

13.85 11.36 9.61

13.09 10.65 9.22

341

J. Benet Churles et al. / Materials Chemistry and Physics 38 (1994) 337-341

1

6-

1

I

. IO

I Frequency

(

loo

ktiz 1

Fig. 6. Variation of dielectric loss as a function of frequency at room temperature.

0

I Frequency

Fig 5.

IO

( kHz 1

Variation of dielectric loss as a function of frequency

at 100°C.

One of the authors (J.B.C.) is grateful to Council of Scientific and Industrial Research (CSIR), New Delhi for the award of Senior Research Fellowship.

Conclusion Single crystal of NaSb,F; crystallises in monoclinic system with space group P2i/a. From the TGA curve, we observed that there is no weight loss is observed up to 298°C. It indicates that the compound is highly stable. The dieIectric studies of NaSb2F7 reveal that at room temperature the dielectric constant decrease with frequency up to 10 KHz beyond which it attains almost a constant value. Tan6 also behaves in a similar way. From dielectric and electrical conductivity studies it is observed that NaSbzF, exhibit anisotropy.

References 1. F.B. Kalinchenko, M.P. Borzenkova and A.V. Novoselova. Russ. J. Inorg. Chem., 27 (1982) 1653. 2. B. Ptaszynski, Recz. Gem., 51 (1977) 1597. 3. J. B. Charles and F.D. Gnanam, J. Muter. Sci. Letis., 9 (1990) 165. 4. J. B. Charles and F.D. Gnanam, Crysr. Res. Tech., 9 (1990) 1063. 5. J. B. Charles and F.D. Gnanam, Cryst. Res. Tech., 12 (1990) 1451. 6.

S. Hirano. P. C. Kim, H. Orihara, H. Umeda and Y. Ishibashi, J. Mater. Scl, 25 (1990) 2800. 7. B. Narasimha, R.N.P. Choudhary and K.V. Rao. J. Mater. Sci., 23 (1988) 1416.