Growth and microhardness studies on NH4Sb3F10 single crystals


Journal of Crystal Growth 137 (1994) 295—298

Growth and microhardness studies on NH4Sb3F1Q single crystals Rani Christhu Dhas

1, j•

Benet Charles, F.D. Gnanam


Alagappa College of Technology, Anna Unit’ersity, Madras 600 025, India

Abstract Ammonium fluoride NH4F and antimony trifluoride SbF3 form many stoichiometric compounds, such as NH4SbF4, (NH4),SbF3, NH4Sb2F7, (NH4)3Sb2F9, NH4Sb3F10, etc. Since (NH4)2SbF~exhibits superionic conductivity and undergoes phase transitions, an attempt has been made to grow other stoichiometric compounds of this group. In this report,3we dealgrown with in thea growth andtwo microhardness studies of NH4Sb3F)() Crystals were period of months. Microhardness studies havesingle been crystals. carried out usingofa size 60 xdiamond Vickers 25 x 35 pyramid mm indenter. To study the anisotropy of this material, specimens cut in different orientations such as (001), (010), (100), (101), (101) have been subjected to microhardness studies. The results show strong anisotropy in the work-hardening coefficient for different orientations. This anisotropy is explained on the basis of a plastic deformation model.

1. Introduction Interest in the stoichiometric compounds of ammonium fluoride, NH 4F, and antimony trifluoride, SbF3, with chemical formulae NH4SbF4, (NH4)2SbF5, NH4Sb2F7, (NH4)3Sb2F9, NH4Sb3 Fit), etc., has increased since Avkhutskii et a!. [1] reported a superionic phase transition in (NH4)2 SbF5.Bandyopadhyay Moskvich et a!.et[2], and Czapla [3], al. Waskowska [4] and Trofimov et a!. [5] also reported studies on the various aspects of the superionic phase transitions of (NH 4)2SbF5. Microhardness studies of any of these compounds are not found in the literature. For hard and


Corresponding author. Ethiraj College for Women, Ethiraj Salai, Madras 600 105.


brittle materials, the hardness test has proved to be a valuable technique in the general study of plastic deformation [6]. Anisotropy in hardness can be very easily shown by microindentation experiments. In 1983, Ducourant et al. [7] reported the crystal data of this compound as monoclinic with space group ~2l/c’ a = 7.952 b= 13.830 ~ = 38.789 and After z = 4 that for and Pe =/3 ~= 95.9° g/cm3. pdate, = 3.89 g/cm no literature references have been found on this compound up to the period 1991. In the




present investigation, we report the microhardness anisotropy of NH4Sb3F10 single crystals grown by evaporation from solution and cut in five orientations, (001), (010), (100), (101), (101). A Vickers microindenter with optical microscope was used to measure the hardness numbers for various loads on the different planes.

0022-0248/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-0248(93)E0529-G


R.C. Dhas

Ct al. /Journal




137 (1994)


2. Experimental procedure 2.1. Crystal growth

60 -~(100) 171)

Appropriate amounts of NH4F and SbF3 were used to prepare the solution of NH4Sb3F10. By continuous stirring, the condition of supersaturation was reached. This supersaturated solution was kept in a constant temperature bath at 305 K in polyethylene containers with a stirrer of the same material. The crystals prepared by spontaneous nucleation were used as seed crystals. The crystals were grown by slow and controlled evaporation of the solvent in the constant temperature bath. Clear and highly transparent single crystals 3 have been of dimensions u~to 60 x 25 >< 35 mm grown in a period of two months. Fig. 1 shows a photograph of NH 4Sb3F10 single crystals grown in a period of one month. {0l0} and (l01} are the observed faces of the grown crystals. The grown crystals were identified using an X-ray powder method and chemical analysis. The crystal morphology was studied and thus verified that the system is monoclinic. 2.2. Indentation tests Specimens cut in different orientations were ground using silicon carbide powder and acetone, then polished using a disc polisher and cleaned using acetone. A Leitz microhardness tester with

(001 1 01





________________________________________________________________________ 25





Fig. 2. Variation of Vickers hardness with load on (100), (010). (001). (101) and (101) orientations.

a diamond pyramidal indenter was used for microhardness measurements. The mass was varied from 25 to 200 g. The indentation period was approximately 10 s. For each load, an average of at least six impressions was recorded for measuring the diagonal length (d) of impression. The Vickers hardness was measured using the formula 2 (1) H5—l.8544P/d where P is in kg and H,~,in kg/mm2.

3. Results The principal observation was that there were pronounced dislocation patterns for a load of 100

0 (0 (101( 1101 .~(010

____________________________ ____________________________ ___________

__________ __________________


___________ ____________________














1 6989 105

Fig. I. Single crysial of NI I~ Sh~F’,, eros’. n in a period ol days.


20 P

Fig. 3. Log P versus log d for orientations (100). (1)10). (001). (101) and (101).

R. C. Dhas et a!. /Journal of Crystal Growth 137 (1994) 295 —298


Table 1

metallic compounds. She attributed the humps

Work-hardening coefficient and correction factor for different orientations 2k Orientation n k (k 2/k1)~’ 2 (x 10 m) 010 2.50 8.51 2.86 6.95x 101 4 —2339 8.05 101 3.00 102 —16180 001 388 12.59 1862 8.33 667x10’ 8.74x 0828x10

seen for boron carbide and tungsten carbide to impure phases present. When the load is in-

100 101

ers hardness with load. This relationship is not always satisfied, as reported by authors like Saraswati [10], Kotru et a!. [9], Pratap and Han Babu [11] among others. The present observa-

1.82 4.57 6.06x10’ 1.68 + 1.65 4.40 19.95 2.50>1 102 2.47x io~ —30.00

creased beyond a certain value, the effect of defects in the crystal may vanish and the hardness will depend only on the material properties. Eq. (3) demands a linear relationship of Vick-

g. The values of H~calculated using Eq. (1) for various loads for each orientation were plotted for H~versus load. These plots are given in Fig. 2. When Meyer’s relation is used in Eq. (1) we get

tions also support this. When a correction, x, is applied to the observed d value, Kick’s law may be satisfied as given below


P= k



The above relation indicates that H~should increase with P if n> 2 and decrease with P when n<2. We can determine n from the slope of a plot of log P versus log d. Fig. 3 shows such plots of log P versus log d using the least-squares fit method for all orientations. As there is a fall of H~with load after 100 g for (001), (010) and (101) and a rise for (100), only the values up to 100 g were used to draw Fig. 3. The values of n, calculated from the slopes of the straight lines of Fig. 3 for all the orientations are given in Table 1. In accordance with Fig. 2, the work-hardening coefficient is greater than 2 for (001), (010), (101) and (101) and less than 2 for (100). The values in Table 1 show that the crystal is highly anisotropic in microhardness.

2. (3) 2(d +x) Substituting for P from Meyer’s relation we get k d’0 k d 2 4 2( +x) Therefore, d’0 /2 (k /k ~i/2d + (k /k ) ~2x (5 —









The values of x were determined from the intercepts of straight lines obtained by plotting dno,002 versus d. These values were substituted in Eq. (3) and a plot of log P versus log(d +x) is shown in Fig. 4; a set of parallel straight lines with a slope 20

/ I

/ //

4. Discussion




When brittle materials or oriented structures are involved, microhardness measurements become dependent on load [8]. Kotru et a!. [9] observed that the dependance of microhardness on load is an important property which needs to be thoroughly investigated in order to gain information concerning the laws governing the mechanical properties of materials. The striking factor of Fig. 2 is the rise and fall of microhardness with load. Saraswati [10] has done extensive research on the microhardness of various non-






(1011 1101)












// ///




1 8


log P

Fig. 4. Log P versus log(d+ x) for orientations (100), (010), (001), (101) and (101).


R.C. Dhas eta!. /Journal of Ctyootal Growth 137 (/994) 295—298





I -12r








__________________ 0182



-1 875




Fig. 5. (2— n)vcrsus


nearly equal to 2 satisfying Kick’s law is obtained. Table 1 gives the various values obtained for x, n and k 1. The striking factor is that x is positive only when n <2 and negative for n > 2. Since the sign of x depends on whether n is less than or greater than 2, an attempt was made to find the relationship of x with n. Hence, we propose the following relationship between n and x. xcx(2—n),

creases with load up to 100 g and then increases. Orientation (101) does not show any such maximum or minimum in microhardness measurements at 100 g and the microhardness number increases with load from 25 to 200 g. Meyer’s index, n, is less than 2 for orientation (100) and more_than 2 for orientations (001), (010), (10!) and (101). Verification of Kick’s law, after applying a correction to the indentation diagonal length, yielded negative corrections for orientations with n > 2 and positive correction when n <2. It is found that the corrected diagonal lengths satisfied Kick’s law.

6. Acknowledgement This research was supported by the University Grants Commission of India. This support is gratefully acknowledged.

7. References

which gives X =

R( 2



where R is a constant for that material, which depends on the plastic and elastic properties of the material. To test the validity of Eq. (6), x was plotted against (2 n) as per Table I to give Fig. 5. This plot is essentially a straight line, as expected from Eq. (6). The slope of this line gives R and is found to be 7 and positive. We expect that x and hence R should give an idea about the nature of the dislocations in the particular orientation of the crystal. —

5. Conclusion For single crystals of NH4Sb3F10, the microhardness number increases with load up to 100 g and then decreases, for orientations (101), (001) and (010); whereas for orientation (100), it de-

[I] L.M. Avkhutskii, R.L. Davidovich, L.A. Zemnukhova, P.S. Gordienko, V. Urhonavicius and J. Grigas, Phys. Status Solidi (h) 116 (1983) 483. Yu.N. Moskvich. B.J. Cherkasov, AM. Polyakov, A.A. Sukhovskii and R.L. Davidovich, Phys. Status Solidi (h) 156 (1989) 615. [3] A. Waskowska and Z. Czapla, Cryst. Rcs. Technol. 24 (1989) 1259. [4] B. Bandyopadhyay, A. Ghoshray. R. Mukhopadhyay and R.M. Kadam. Pramana 33 (1989) 713. 151 G.L. Trofimov, V.1. Sergienko and G.M. Larin, Ivy Akad. Nauk. SSS. Ncorg. Mater. 26(199))) 1943. [6] J.H. Westhrook and H. Conrad. The Science of hardness Testing and Its Research Applications (American Society for Metals. Metals Park, OH. 1971) p. 23. [7] B. Ducourant. R. Fourcade and (,. Mascherpa, Rev. Chim. Mineral. 2)) (1983) 314. [81 A. Szymanski and J.M. Szymanski, Hardness Estimation of Minerals, Rocks~~ Ceramic Material (PWN-Polish Scientific, Warsaw, 1989) p. 65. [9] P.N. Kotru, AK. Razdan and B.M. Wank)yn, J. Mater. Sc. 24 (1989) 793. 110] V. Saraswati. J. Mater. Sci 23(1988) 3161. 1111 K.J. Pratap and V. Ilari Bahu. Bull. Mater. Sci. 2 (198))) 43.