Growth and characterization of single crystals of sodium metaperiodate (NaIO4)

Growth and characterization of single crystals of sodium metaperiodate (NaIO4)

Journal of Crystal Growth 57(1982)113—117 North-Holland Publishing Company 113 GROWTH AND CHARACTERIZATION OF SINGLE CRYSTALS OF SODIUM METAPERIODAT...

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Journal of Crystal Growth 57(1982)113—117 North-Holland Publishing Company

113

GROWTH AND CHARACTERIZATION OF SINGLE CRYSTALS OF SODIUM METAPERIODATE (Na104) Vilas DESHPANDE, S.V. SURYANARAYANA and V.T. DESPHANDE Department of Physics, University College of Science, Osmania University, Hyderabad 500007, India Received 4 March 1981; manuscript received in final form 19 October 1981

The solubility of anhydrous sodium metaperiodate (NalO 4) in water, from 29 to 70.2°C,is given by S = 16.31 + 0.526(T — 25) — 0.0038(T — 25)2, where S is the weight percent solubility and T is the temperature in °C.Well-developed, cm size, single crystals of2ethis andsalt ~.iowere at 4358, grown 5461 by and the slow 5893 cooling A are respectively: method. Indices 1.7909 of refraction ±0.0003,were 1.7395 determined ±0.0003,using 1.7513 the ± prism 0.0004, method. 1.7085 The ± 0.0003 of and values ‘ 1.7420 ±0.0011, 1.7014 ±0.0012. The Na10 4 crystal exhibits positive birefringence.

1. Introduction For a long time this laboratory has been interested in the study of the physical properties of sodium metaperiodate (Na104) and potassium metaperiodate (Kb4). Precision determinations of the lattice parameters and the principal coefficients of thermal expansion have been reported earlier [1,2]. Limited structural studies of potassium metaperiodate have also been reported [3]. Because large single crystals were not available, many physical property investigations could not be undertaken. Earlier attempts to grow these crystals produced crystals too small for the- planned measurements. A perusal of literature and the information received from Connolly [41mdicated that there were no earlier reports on the growth and characterization of single crystals of these substances. The growth of well-developed, centimeter size, single crystals of Na104 is reported here, Na104 belongs to the space group I4~/a and is isostructural with the well-known mineral scheelite (CaWO4). The crystal cannot be grown from the melt since the substance decomposes before melting [5]. Evaporation of an aqueous solution at room temperature yields crystals of the hydrated form Na104 3H20, which has a trigonal structure. Single crystals of the anhydrous form can be obtained only above 34.5°C [6]. Hence in the present study single crystals of the anhydrous salt were grown by slowly cooling a saturated aqueous solution from 72 to 39°C.In a pre.

0022-0248/82/0000—0000/$02.75 © 1982 North-Holland

liminary study the solubility of Na104 in water was determined. An earlier report by Hill [6] provides solubiity data over the range from 5.8 to 5 1.5°C.We have determined the solubility from 29 to 70.2°C, thereby covering the range over which crystal growth by slow cooling was planned. As part of our plan to investigate the physical properties of this crystal, we have measured, in first instance, the indices of refraction and the dispersion of natural birefringence over the entire visible region.

2. Experimental 2.1. Solubility Solubiity was determined by finding the weight of the dissolved salt in a measured weight of the aqueous solution, saturated at any temperature. To start with, 300 ml of the saturated solution was prepared at about 70°Cin a stoppered conical flask using double distilled water and recrystallized Na104. The flask was immersed up to its neck in a water bath. The temperature of the bath could be controlled with an accuracy of ±0.1°C. Hill [6] pointed out that Na104 takes about a day to attain equilibrium in an aqueous solution. In view of this, the saturated solution was kept at chosen temperatures for at least 36 h before withdrawing a small portion to estimate the weight percent solubiity. Precautions, such as pre-heating

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32

/ Growth

~

~ 24 Equatiurs t Preseut data

//

20H

Hills data

// / 0

30

40

50 Temperature

60

70

C)

Fig. I. Solubility of Na104 in water,

the pipette to the temperature of the bath, avoiding

any evaporation losses from solution withdrawn for weighing and complete drying of the salt, as ascertamed by repeated weighings, were taken to remove all possible uncertainties. Measurements were made at five or six different temperatures while the bath was progressively cooled. Two independent determinations were performed. The combined results are given in fig. 1. Least squares fitting of the experimental data gave the following equation S = 16.31 + 0.526(T 25) 0.0038(T 25)2 —

where S is the weight percent solubility and T is the temperature in °C.An error analysis showed that this expression gives the solubility with an error of less than 4%. The data from the literature [6] are also shown in fig. 1 and it is seen that there is good agreement between the present results and those in Huh’s paper. It is interesting to note that the solubiity values at 29 o

.

and 33 C fall on the smooth curve passing through the data for higher temperatures. Both these temperatures are below the transition point 34.5°C.This mdicates that a transition had not taken place during the measurement. This confirms a similar observation by Hill [6]. 2.2.

4

tilled twice in an all glass distillation unit and recrystallized Na104 were used. The solution was then heated to and maintained at 81°Cin a thermostatic oven. This solution was filtered hot, inside the oven, into a conical flask through a sintered glass filter. The resulting solution was clear and “shining”. The flask

~_—‘~

28~

and characterization of Na10

Crystal growth

Using the solubihity data, a saturated aqueous solution of Na104 was prepared at about 72°C.Water dis-

was closed and transferred to the water bath which had been brought to 81°C.The bath has a large capacity (32 litres), a mercury regulator with a coarse and a fine control, eight different heaters and glass panels which allowed continuous observation of the flask and its contents. A layer of paraffin oil was spread on the surface of the water to prevent evaporation. After allowing sufficient time for the flask to reach thermal equilibrium with the bath at 81°Cthe cooling process was begun. In the absence of a programmable cooling device the following procedure was adopted for slowly reducing the temperature. The controls of the mercury regulator were adjusted to new positions, corresponding to a temperature a fraction of a degree lower than the previous value, and the power was reduced so that it took a few hours for the temperature to reach the new value. Initial trial runs were conducted to ascertain suitable cooling rates. During these trials the growth of crystals was visually monitored after spontaneous nudeation had occurred. In the final run three seeds were introduced in the flask when the temperature was 72 C. Initially the seeds appeared to dissolve. However when the temperature reached 7 1°C,the crystals

I

80

~

601

o

~ 40

0

40

80

Timeb)

tOO

200

240

Fig. 2. Variation of temperature of the solution with time during the slow cooling experiment.

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and characterization of Na10

4

115

112 /

I I

112

/ 112

I

--

/

/

101,,

,c Fig. 3. Na104 crystals grown by the slow cooling method. The heaviest lines demark 1 cm by 1 cm squares.

a

/

/

/ /

112



101

\\

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Fig. 4. Morphology of a typical Na104 crystal.

appeared to be growing and cooling was continued until the temperature reached to 39°C.It took more than 245 Ii for the temperature to fall from 72 to 39°C.The observed fall of temperature with time is shown in fig. 2. The three seeds grew into clear and well faceted crystals. Two of these crystals are shown in fig. 3. In addition to these, a few smaller good quality crystals grew on spontaneously nucleated seeds. Before removing the crystals, carbon tetrachloride at 39°Cwas poured into the flask to isolate the crystals from the mother liquor. 2.3. Characterization The crystals were identified as anhydrous Na104 by taking X-ray powder photographs and comparing the d values with those given in the ASTM files and obtained by earlier measurements. The following impurities, in less than 100 ppm quantities, were detected by DC arc emission spectroscopy: Si, Mg, Al, Cu, Ag, Fe, Ca and Cr. The morphology of these crystals was studied by goniometry. Fig. 4 shows the faces exhibited by a typical crystal. The only forms present were found to be {112} and {lOl} the former being the more prominent of the two, The “as grown” crystals have clean, smooth and clear surfaces which are preserved if the crystals are

kept in a desiccator. If exposed to moist air for a long period all the faces get tarnished. All our efforts to cleave the Na104 crystals were unsuccessful although CaWO4 crystals, which are isostructural with Na104, are known to show distinct cleavage on (111) [7].

3. Refractive indices The optical properties of Na104 do not appear to have been investigated. The only data which the authors could locate indicate that they show positive birefringence [9]. We determined the ordinary and extra-ordinary indices of refraction for three different wave lengths, employing the prism technique. The prism was fabricated from a basal crystal plate which was free from all visible flaws. The refracting faces of the prism were optically ground and p0lished. The edge of the prism was almost parallel to the optic axis. A discussion on the departure from this parallelism and some other features studied with a conoscope are given in section 4. The angle of the prism and the angles of minimum deviation for different wave lengths were measured with a spectrometer. Five independent measurements were made in each case. Errors in the mean values of the angles and of the refractive indices were evaluated

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characterization of Na10

4

Table I Refractive indices of Na104

.,

Wavelength

~e

___________

5893

1.7420±0.0011

1.7014±0.0012

~

~

______________________

_________

by the use of standard formulae [8]. The prism angle was found to be 24.300° ±0.008°.The values of the refractive indices are given in table 1 along with estimates of their probable errors. The uncertainty in the indices for sodium light was larger than for other wavelengths; probably because the D1—D2 doublet was not resolved by the spectrometer, and the angle of minimum deviation was measured with reduced accuracy. The two images obtained for any wavelength were examined with a Nicol prism and it was established that the extraordinary ray had suffered the greater deviation. This confirms the observation, given in Barker’s crystal index [9], that Na104 exhibits positive birefringence. 4. Conoscopic observations In order to test the parallelism of the prism edge with the optic axis the prism was subjected to a conoscopic observations. The two triangular basal faces of the prism were optically polished. The prism was then placed on the table of a polarizing microscope

Fig. 6. Separation of the uniaxial cross into brushes.

between crossed Nicols. On introducing the converging lens and the Bertrand lens a typical uniaxial interference figure was observed. Fig. 5 shows the pattern observed with sodium light. The centre of the cross of the uniaxial figure was found to be shifted slightly from the intersection of the dross wires. This indicates that the optic axis was at a small angle with the prism edge. The method suggested by Bloss [10] was used to estimate this angle, which was found to be 1°.The effect of this small departure from parallehism of 1e~ the the prism edge with the optic axis on was the extraordinary refractive index, value of l and found to be insignificant when comevaluated pared to the errors given in table 1. However during these observations an unexpected feature in the interference pattern was observed. Rotating the microscope table through 45° from the position at which fig. S was taken, caused the cross to separate into brushes which are characteristic of biaxial interference figures. However the separation was quite small. A photograph of this feature, obtained with another basal plate of smaller thickness than that of the prism, is reproduced in fig. 6. It was not clear, how this biaxial character is shown by a crystal which is known to be uniaxial. Further investigations on the causes and origin of this small biaxiality are in progress.

Acknowledgements The authors are grateful to Professor G. Sivarama Sastry, Head, Department of Physics, Osmania UniFig. 5. Uniaxial interference figure exhibited by Na10 4.

versity, for providing the facilities for this work. They

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also wish to express their sincere thanks to Sri Baliguddin Hussain and Dr. Shanker Narayan of the J~ partment of Geology, Osmania University, for their help in taking photographs of the interference figures.

References [1] VT. Deshpande, S.V. Suryanarayana and Rama Rao Pawar, Acta Cryst. A24 (1968) 398. [2] VT. Deshpande, Rama Rao Pawar and S.V. Suryanarayana, Current Sci. (India) 36 (1967) 513. [3] S.V. Suryanarayana, PhD Thesis, Osmania University (1971). [4] T.F. Connolly, Research Materials Information Centre, Oak Ridge National Laboratory, USA, private communication.

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T.P. Whaley, in: Comprehensive Inorganic Chemistry, Vol. I, Eds. J.C. Bailar, H.J. Emele’us, R. Nyhoim and A.F. Trotman-Dickenson (Pergamon, 1973) p. 519. [6] A.H. Hill, J. Am. Chem. Soc. 50 (1928) 2678. [7] E.S. Dana, A Text book of Mineralogy, 4th ed., Ed. W.E. Ford (1932) p. 772. [8] R. Livingston, Physico Chemical Experiments, 3rd ed. (Macmillan, New York, 1957) p. 29. [9] The Barker Index of Crystals, Vol. I, Eds. M.W. Porter and R.C. Spiller (Heffer, Cambridge, UK, 1951) table 120. [10] F.D. Bloss, An Introduction to the Methods of Optical Crystallography (Holt, Rinehart and Winston, New York, 1961) p. 135.