Compositional analysis of single crystals of flux-grown magnetic garnets by atomic absorption spectrophotometry

Compositional analysis of single crystals of flux-grown magnetic garnets by atomic absorption spectrophotometry

cn39-9140/78/0601-0317 T~mta, Vol. 25, pp. 317-323. 0 Rrgamon Press Ltd., 1978 Rinted I” Great Britm. so2.tm/o COMPOSITIONAL ANALYSiS OF SINGLE CRY...

798KB Sizes 1 Downloads 22 Views

cn39-9140/78/0601-0317

T~mta, Vol. 25, pp. 317-323. 0 Rrgamon Press Ltd., 1978 Rinted I” Great Britm.

so2.tm/o

COMPOSITIONAL ANALYSiS OF SINGLE CRYSTALS OF FLUX-GROWN MAGNETIC GARNETS BY ATOMIC ABSORPTION SPECTROPHOTOMETRY” RCA Research Laboratories, Inc., Zushi-machi, Machida-shy, Tokyo 194-02, Japan (Received 31 August 1977. Accepted 28 October 1977) Summary-A

method of determining the chemical composition of single crystals of flux-grown magnetic garnets (R,, R,)J (Fe, Ga),G,, (R, and R, = Y, Eu, Gd, or Er) is described. Elements to be determined were rare earths, iron and gallium as main constituents, and lead as an impurity introduced from the flux. Lanthanum chloride was added to the samples and the standard solutions to remove the interference effects ofconcomitantforeign elements on the atomic-absorption measurement of the rare earths and to improve the measurement precision for iron, gallium and lead. The atomic-absorption measurements in the lanthanum chloride matrix are discussed. Finally, results for some garnets are presented and problems regarding the composition of the materials are described.

Rare-earth iron garnets have been investigated for their applications in magnetic bubble domain devices by a number of workers. The single crystals are usually grown by the flux method and the crystal composition is usually expressed as a “nominal” formula determined empirically by use ofdata on phase equilibria and distribution coefficients of components between the liquid (flux) and solid (garnet) phases ; the nominal composition differs from the crystal component ratio in the melt.’ Chemical analyses for all the constituents have not been reported and only a partial analysis has been described.2*3 In garnets with the general formula R3FesO12, where R is a rare-earth metal, the R sites can be occupied by combinations of several kinds of rareearth and the Fe sites can be partially substituted with tervalent ions such as Al3 + and Ga3+. The variation of rare-earth atom ratios causes a change in the lattice parameter3 and the,Fe substitutions change the saturation magnetic moment4 It is also known that lead from the flux agent is incorporated as an impurity in the crystal and that the lattice parameter increases with. increasing lead content3 A complete chemical analysis is desirable in interpretation of the physical properties of materials ; a partial analysis fails to detect compositional variation of components other than the elements of interest. In this paper a method for determining the chemical composition of the fluxgrown single crystals is described. The materials analysed were single crystals of magnetic garnets with the composition (R,, R,),(Fe, Ga),O,, (R, and R, = Y, Eu, Gd, or Er). The garnets were grown from a flux consisting of lead oxide, lead fluoride and boron oxide,? so that the elements to be determined were rare earths, iron, and gallium as main constituents, and lead as an impurity in the crystals.

Detonation of the rare earths has been reported previously.5 It was found that the addition of lanthanum chloride to the sample solution increased the analytical sensitivity for the rare earths and removed the interference effects of accompanying foreign ebments. A discussion on the effects of lanthanum chloride is given here because the major factors contributing to the role of lanthanum chloride were -not clearly underst,ood earlier. The effect of lanthanum chloride on the measurement of iron, gallium and lead was also examined in the present work. The add&n of lanthanum chloride improved the me~~ement precision for these elements as well. A~or~n~y, all the elements were determined in the lan~anum chloride matrix, on the same starting sample. It is recommended that a wide variety of metals in the same sample be determined by a one-solution procedure. To demonstrate the applicability of this method for the analysis of individual single crystals, a maximum sample weight of 20 mg was used. Finally, the results of analysis for various samples are presented.

EXPERIMENTAL

Reagents

St~dard so~~tjo~s.The rare-earth solutions were prepared by dissolving rare-earth chloride hexahydrates in demineralized water and were standardized by titration with EDTA. The iron solution was prepared by-dissolving iron metal (99.9X nure)in hydrochloric acid. The aallium solution was prepared by dissolving gallium metal (99.99% pure) in nitric acid, evaporating with hydrochloric acid, and then dissolving the residue in demineralized water. The lead solution was prepared by dissolving lead nitrate in demineralized water. The standard solutions, except for lead, were prepared in 0.1M hydrochloric acid medium. The lead solution was prepared in &Oliw nitric acid medium. All stock solutions had an analyte concentration of 1 mg/ml. The * Part of the thesis submitted to the University of Tokyo to reagents were of analytical grade. fulfil the requirements for the degree of R~~~~-~~~~j ~~~~m chloride solution, Lanthanum chloride hepta(Ph.D.). hydrate (reagent grade) was dissolved in demineralized water t Unpublished work by Y. Wada and S. Harada, RCA and the stock solution bad an LaCl, concentration of Research Laboratories, Inc., Tokyo, Japan. 5Omg/ml. 317

318

KOHEI AMETA~I

Table 1. Operating conditions for measurements of rare earths, iron, gallium and lead

Anal+ Y Eu Cd Er Fe Ga Pb

Wa~eIength,~ am

tamp current, F&4

Scale expansion

Flow-rate,? t/&n G%.&O

h&&t;

41ff.2 459.4 368.4 4GO.8 312.0 281.4 283.3

10 10 10 IO 15 10 IO

O-2.5 0 o-2.5 O-2 O-f O-2 O-2.5

S&5.0 5.5-5.8 5.~6.0 5.8-6.0 5.8 5.8 5.8

1-2 3 1-2 i-2 3 3 3

BUi-ilfX

*Spectral width 0.32 nm. t Pressures were 1.8 kg/cm2 for N,O and 0.8 kp/cma far C,H, ; nitrous oxide flowrate 8.0I./min throughout. $The instrument has a 5-cm single-slot burner hl;ad and the beam is passed twice through the flame. The burner height is denoted by the numerals 0,1,X and 3. A measurement made with a screen placed on the burner head sbowed the following relation between these numerals and the height (mm) above the burner head at which the beam-centre enters or leaves ths R&me: beam position 1st entry

1st exit

I4 19 24 29

9 14 19 24

Working conditionsfor atomic-absorption measurements

2nd entry

2nd exit -

: 11

2 7 12

effects were suppressed by the addition of lanthanum

chloride. For example, the error in the europium A nitrous oxide-acetylene flame was u&, in a Hitachi measurement was about 50-80~ in the presence of mod& 208 atomic-absorption s~ctrophotomet~~. Westinghouse bo~ow-cath~e lamps were used for the rare earths and gadol~n~~rn or gal.Eum for solutions without langal&m, and Hitachi HLA-3 lamps were used for iron and thanum chloride, but was only l--2% when it was lead, Table 1 shows the o~ra~ngcon~tjons for the measurepresent, Similar effects with ~~than~rn chloride were ments; they were optimized with tl~e of the standard soaha observed for other rare earths_ The error in #he hrtions. Conditions were chosen to prevent a noisy absorption and to avoid blocking of the burner slot by reduced me~urement ofthe rare earths, arising from the eifects carbon particles. of all the foreign elements examined for the garnet As indicated m Tabb 1, m~surement at a Iower position in anafysis, was reduced to within ~-2% in the presence of the flame was suitable for yttrium, gado~~~~~rnand erbium, 5000 ppm lanthanum chloride. whereas the use of a higher position was bt?tter for other Figure 1 shows the effects of lanthanum chloride on &ments. The measurement positions may be related to the dissociation energies of the monoxides of tha elements: YO the atomic absorption of gallium and lead. The 7,SOeV, EuO 5.85 eV, GdO 7.55 eV, ErO 663 eV, Fe0 4.29 absorption of these elements was slightly increased by eV, OaQ 3,04 eV, and PbO 4.29 eV. Effects of hydrochloric the presence of lanth&num chloride whereas the iron acid on atomic-absorption measurements ofrare earths have absorption was not generally affected by the addition b+en studied by severai workers,6-9 but the eBcts reported differ. In this work a constant concentration of 0.I.M far the of: the reagent. I0 Table 2 presents the comparison of hydrochloric acid was used; no significant effects of the ~as~ements for 50 ppm lead with and without the hydrochloric acid were found for any of the elements.

Samples weighing IO-2Omg were dissolved in concentrated hydrochforic acid and the sam& salu6ons were [email protected] to give appropriate con~ntrations of the analytes. Ths; final sampIe solutions were adjusted to be O.lM in hydrochloric acid and to contain 5000ppm af lanthanum chloride. The same adjustment was carried out for the standards. The nitrous oxide-acetylene flame WM used for all the determinations.

As reported previo~dy,~ the absorption of the rare earths was enhanced by increasing the fanthanum chloride concentration, and the mutud interference

addition of ~~than~rn chloride. The observed values nre the average of three measurements for the same sample solution. As seen from Table 2, the addition of

Fig. 1. EfXmts of tanthmum chloride on absorption gallium and lead in 0.1M hydrochloric acid solution.

319

Analysis of magnetic garnets Table 2. Comparison of measurements for 50 ppm lead with and without addition of lanthanum chloride

Y IOOppm

Sample solution (in O.lM HCl)

No LaCI,

5000 ppm LaCl,

Foreign element,* ppm

obsd., ppm

error, %

obsd., PPm

error, %

Y Eu Gd Er Y Fe Ga Eu Fe Ga Gd Fe Ga Er Fe Ga

1000 2000 500 2000 1000 2000 200 2000 2000 200 500 2000 200 2000 2000 200

52.3 52.6 52.0 51.3

+4.6 + 5.2 + 4.0 + 2.6

+ 0.4 +3.0 + 1.0 + 0.2

51.1

+2.2

-0.4

51.2

+2.4

+0.8

51.4

+2.8

+l.O

51.3

+ 2.6

+ 1.2

51.6,

+3.3,

+ 0.9,

av. std. devn.

3.7,

Y law line (371 .O nm) Sol’n.OlMHCI

1.3,

*Added as chlorides. lanthanum chloride improves the precision. Similar effects were also observed for iron and gallium. In the case of the alkaline earths, it is known that lanthanum chloride acts as a releasing reagent.“**’ The effect is generally attributed to formation of lanthanum compounds of the interfering elements that are more stable than the corresponding alkaline earth compounds and so prevent formation of refractory compounds of these analytes. However, the releasing agent may possibly act by a simple sputtering mechanism that leads to production of smaller residual solid particles which can be more easily volatilized. In the case of the rare earths, however, the major factors contributing to the effect of lanthanum chloride have not been reported. The effects of iron, gallium and gadolinium on yttrium atomic and ionic absorption was therefore studied with and without the addition of 5000 ppm of lanthanum chloride. The ionic line used was 371.0 nm. Results are shown in Fig. 2, where the variation of lanthanum ionic absorption at 407.7 or 408.7 nm is also given. As can be seen from Fig. 2, in solutions without lanthanum chloride, the presence of iron increased the yttrium atomic absorption but had little effect on the ionic absorption. With the addition of lanthanum chloride, the atomic absorption of yttrium was constant over the range O-2000 ppm of iron and the yttrium ionization was almost suppressed ; the lanthanum ionic absorption was unchanged. The presence of gadolinium decreased or increased the yttrium atomic absorption, depending on the gadolinium concentration, but suppressed the yttrium ionic absorption under all conditions in the absence of lan-

Fe,Ga

or Gd added.

ppm

Fig. 2. Variation of absorption Intensities of the yttrium atomic and ionic lines and the lanthanum ionic line with and without lanthanum chloride present in the solution.

thanum chloride. In the presence of 5000 ppm If lanthanum chloride, the atomic absorption of yttrium was constant for O-10OOppm gadolinium, but slightly increased for the higher concentrations of gadolinium up to 2000 ppm. The 2000 ppm of gadolinium also increased the lanthanum ionic absorp tion. The presence of gallium increased the yttrium atomic absorption but suppressed the ionic absorp tion for the solutions without lanthanum chloride. In the presence of SOOOppm of lanthanum chloride, the atomic absorption of yttrium was constant over the range O-2000ppm of gallium, but the yttrium ionization was almost suppressed and the lanthanum ionic absorption did not vary. The formation of free.atoms in the flame is governed by the processes of solvent evaporation, vaporization of solid particles, and the dissociation of gaseous compounds into atoms. The rare earths have a relatively low ionization energy (5.5-6.8 eV;,Y 6.38 eV) so that easily ionized elements are usually added to the sample solutions to suppress the ionization and thus increase the measurement sensitivity for the rare earths.6~7~13-i6 However, it seems that not all the interference effects can be explained in terms of only ionization suppression, because the dissociation energies of the rare-earth monoxides are relatively high (6-8.5eV; YO 7SOeV). Metals which form monoxides with dissociation energies less than about

6.5 eV are completely atomized if solute vaporization is complete,” but most concomitant foreign metal species strongly influence the atomization process of the elements of interest. t B As shown in Fig, 2, the increase in the yttrium atomic absorption in the presence of iron cannot be attributed to suppression of the ionization of yttrium. The presence of gadolinium or gallium truly contributes to suppression of yttrium ionization, however, because elements with ionization energies b&w &5eV are amicably ionized and suppress io~~~o~ of other elements in the game.lg The ionization energies are 6.16 and 6 eV for gadolinium and gallium, respectively. However, other factors such as the vaporization of samples and dissociation rate of yttrium compounds in the flame have to be taken into account; inhering the vaporization or dissociation rate would apparently produce the same e&W as ionization suppression. If the addition of gadolinium or gallium did nat affect the vaporization and dissociation rates for the yttrium measurement, a constant number ratio of yttrium atoms to ions should. be obtained Fclr various ~n~n~a~o~s of gadolinium or gallium This relation can be expressed as follows:2*

y/k

= (1. -nyg3-1)s

chmnical properties, compared to those of yttrium, rather than that in the ionization energies. A change in dissociation or vaporization rate seems to occur also in the presence of iron because the yttrium ionization is not suppressed but the yttrium atomic absorption increases as shown in Fig. 2. This. suggests that the interference effects of gadolinium and iron cannot be eliminated by addition of easily ianizable elements, e,& sodium and potassium.s Another reagent, which will control the d~sso~a~on or vapo~za~on rate_ can be more e&ctive for suppr~in~ the interference eflects. IQ

r

@I

whm x and g are @te fractions of yearnpresent in the flameas atoms and as ions, respectively, without

addition of foreign elements. The terms a and b are the ratios of the atomic and ionic absorptioas with and without addition of the foreign elements, respectively, Table 3 shows the ratios ofy to x ~~c~la~ from the data in Fig. 2 for ~d~~~~urn and gallium. The ratio For gallium is fairly constant but that for gadolinium varies widely. This imlicates that the effect of gallium is mainly the suppression of ionization. Therefore, an easily ionizable element, for example sodium, can eliminate the gallium interference as reported previ5udps In the ase of gado~~u~ the variation in values of the ratio suggests that there are contributing faStam other than the suppression of ionization. There [email protected] to be a change in dissociation or vaporization rate. The difference between the gallium and gadolinium e&e& may be due to the di~i~la~ty in Table 3. Ratios of fractions of yttrium present as ions and atoms in the flame (y/x),* as a function of concentrations of gadolinium or gallium added Concentration, ppm

Gd

Ga

50 loo 250 500 1000 2000

-0.162 -0.204 -0.078 - 0.004 0.225 0.461

0.127 0.165 0.187 0.185 0.174 0,176

* See eqtration (2) in the text

The effect of lanthanum chloride on the atomic and iouic absorption of yttrium is shown in Fig. 3, together with the variation in the ionic abso~tion of lantaco, From these data, similar ca~~a~o~s were made for, various concentrations of lanthanum chloride. Non-constant values were obtained, indic&ting that lanthanum chloride affects the vaporization and dissociation as well as the ionization process. However, it seems certain that lanthanum chloride acts as an ionization suppressor by pr~u~~~el~t~on~ in-the flame, because the lanthanum ionic absorption was not altered by the other species presen.nt,in most cases examined, as shown in Fig. 2. Lanthanum itself ionizes in the ffame, as shown in Fig. 3. s The vaporizatian or dissociation rate is a complex f~~t~o~ of many factors such as variation of droplet size, and diffusion of vaporized and flame species. Although we do not understand all the phenomena in the present case, lanthanum chloride would appear to affect the volatility of the analytes (rare earths). The most probable explanation for the interference effects is a change in the volatility of the analytes when O&KS metal species are present in the sample soiutionf~~~ This effect occurs because a high concentration of salts in sample solutiona can change the droplet size but does not affect the aspiration rateA2i In the present work, the sample solutions were aspirated at a flowrate of 2.00 +0.025 ml)min.

321

Analysis of magnetic garnets Table 4. Results of mixed-oxidesanalysis (means of three experiments) Analyte, % w/w Sample*

Pb

Y

1

calcd. { obsd. range

0.27 0.26 0.02

2

calcd. i obsd. range

3

4

Eu

Gd

Er

35.24 35.5 0.4

-

-

0.21 0.25 0.02

30.89 31.5 0.2

5.87 5.77 0.06

-

calcd. 1 obsd. range

3.76 3.70 0.05

25.82 25.5 0.5

-

calcd. ( obsd. range

0.53 0.50 0

-

11.42 10.9 0.7

15.62 15.3 0.4

-

Fe

Ga

-

28.04 27.8 0.2

11.05 10.9 0.1

-

26.59 26.2 0.1

11.66 11.5 0

-

27.04 26.6 0.2

8.44 8.37 0.06

24.68 24.4 0.2

5.02 5.02 0.04

34.38 34.2 0.2

* Oxides mixed in the following mole ratios:

1: 2: 3: 4:

PbO

Y,O,

Eu,O,

Gd,O,

Er,O,

0.02 0.02 0.30 0.05

3.0 2.7 2.4 -

0; 1.0

r 0.6 -

I

For measurements of iron, gallium and lead, the addition of lanthanum chloride is effective for matching the concentration of salts in the sample solutions nearly to that in the standard solutions. Thus lanthanum chloride can eliminate some of the matrix effects and thereby improve the measurement precision as already described Interferences due to the formation of lead chloridezZ were not observed. Analysisof samples Test analyses were first performed on a mixture of oxides of known composition. Results are given in Table 4 for rare earths (Y, Eu, Gd, and Er), iron, gallium and lead. All the oxides had a purity of at least 99.9%. The atomic absorption was measured with 5000 ppm of lanthanum chloride present, as described in the experimental section. The values shown in Table 4 are the averages from three experiments. The range is also shown. In all cases the results agree with the calculated values, indicating that the procedure is suitable for analysis of single-crystal samples. The results for single crystals of several magnetic garnets are shown in Table 5, calculated as mole ratios. The average and range of results from three experiments are shown. In all cases both the ratio of rare earth to iron plus gallium, [R/(Fe+Ga) where R = Y, Eu, Gd, and/or Er] and the rare-earth ratio, RJR,, were close to the ratios expected from the nominal compositions. For different crystals grown in the same batch, the observed rare-earth ratios also agreed with each other as shown by the results for samples 2 and 3 and for samples 4 and 5. A similar result was reported by Barns3 for X-ray tluorescence milliprobe analysis of the Gd,.J,Tb 0.66Fe,012 crystal. He reported that application of Vega&s law2’ gave a quantitative

G

WA 3.8 3.7 4.0 4.3

Ga203 1.2 1.3 1.0 0.7

estimation of compositional gradients due to varying rare-earth atom ratios, but according to the X-ray fluorescence analysis there was no variation in the rare-earth ratios. He concluded that this discrepancy was due to a variation in the lead content incorporated from the flux, indicating that the application of Vega&s law is unsuitable for such analyses, just as was found for single crystals of chromium chalcogenide spinels.24 The ratio of iron to gallium always deviated from that calculated from the nominal composition. In general, the iron content was higher and the gallium content lower than the nominal content. Even in the same batch, the iron:gallium ratios were different for individual crystals, as shown by the results for samples 4 and 5. In this particular case, the lead content differed from crystal to crystal. This variation of lead content may cause the fluctuation in the iron and gallium content because the incorporation of gallium in the Y,Fe,_,O,, crystals is strongly dependent on the ratio of lead fluoride to lead oxide in the flux.’ Hence the lead fluoride to lead oxide ratio has to be well adjusted to optimize uniformity within crystals.2 It also became clear that a substantial amount of lead was incorporated into the crystals; the mole ratios for lead content given in Table 5 correspond to 0.4-2.1x w/w. The present results indicate that the crystalgrowth conditions must be carefully determined for growing garnets of the desired composition, particularly for iron and gallium, and that the analysis should be performed for all the co~tituents, including impu~ties. The results of the analysis are being utilized to investigate the associated physical properties of these materials. It has been found that the composition

0

0

0

0

0

0

0.01

0.03

0.02

0.08

0.02

0.03

2012.

tR designates

2.02

3.00

0.47

0.50

0.29

composition:

0.02

0.02

0.03

0.04

0.30

-

mean

EU

0.02

0

0

0.01

0.01

0.01

range

Gd

7Ga,[email protected]~

-

-

0.62

-

-

-

mean

0.04

range

Analyte,

0.97

-

-

-

mean

Er

garnets

0.01

range

of magnetic

mole ratio

analysis

value of the ratio is 0.6 for each sample.

5: Y,.,Eu,.,Fe,.,Ga,.,O,,. 8: Eu,ErFe,.,Ga,.,O,,.

2: Y,.,Eu,.,Fe,

rare earths (Y, Eu, Gd, and Er), and the expected

nominal

2.35

2.49

2.48

2.69

0.04

0.06

2.98

2.69

range

Y

mean

grown had the following

-

range

mean

4: Y&u, ,Fe, 7Gar @r2. 7: Eu,Fe,GaO,,.

1: Y,Fe,.,Ga,

*The crystals

Sample*

Pb

Table 5. Results of single-crystal

0.03

0

0.03

0.03

0.04

0.03

0.04

0.06

range

0.61

0.93

0.92

0.88

0.78

0.96

0.94

0.75

mean

6: Yr ,GdO.,Fe~.,GaI.&rz.

-. 3: Y2.,Eu,.3Fe3.,Ga,.3G,2.

4.40

4.04

4.08

4.08

4.23

4.03

4.05

4.21

mean

Fe

Ga

0.01

0

0.02

0.02

0.02

0.01

0.01

0.02

range

0.595

0.604

0.594

0.596

0.594

0.598

0.597

0.592

mean

0.010

0.014 0.001

0.015 0.015 0.008

0.015

0.022

range

R/(Fe + Ga)t

F I% 3 6 2 5

Analysis of magnetic garnets determined by the present method is consistent with the physical measurements such as the saturation magnetization and the Curie temperature.* For example, in the case of the system EuB_xErxFeS_,GaYO,, (x=0,1,2,3, y=0.6-1.2) the saturation magne~ation at room temperature increases linearly with increasing iron to gallium ratio and does not depend primarily on the rare-earth ratio. The Curie temperature for the system also increases with increasing iron to gallium ratio. The correlation between the physical properties and the composition is useful for characterizing garnet films prepared by liquid-phase epitaxial growth. Acknowledgements-The author would like to express his sincere appreciation to Mr. Edward 0. Johnson and Dr. Fumio Okamoto for critical reading of the manuscript. He is also indebted to Dr. Yasuo Wada and Mr. Shigeo Harada for providing samples.

REFERENCES See for example, L. G. Van Uitert, W. A. Bormer, W. H. Grodkiewicz, L. Pictroski and G. J. Zydzik, Mat. Res. Bull., 1970,5,825. J. W. Neilsen, D. A. Lepore and D. C. Leo in H. S. Peiser, eds., Crystal Growth, p. 457. Pergamon Press, London, 1967. R. L. Barns, J. Appl. Phys., 1971,42,1623. A. H. Bobeck, E. G. Spencer, L. G. Van Uitert, S. C. *Unpublished work by Y. Wada and S. Harada, RCA Research Laboratories, Inc., Tokyo, Japan.

323

Abrahams, R. L. Barns, W. H. Grodkiewicz, R. C. Sherwood, P. H. Schmidt, D. H. Smith and E. M. Walters, Appl. Phys. Lett., 1970,17, 131. 5. K. Ametani, Bull. Chem. Sot. Japan, 1974,47,2238. 6. M. [email protected] H. Matsui and T. Matsubara, Bunseki Kagaku, 1971,[email protected],856. 7. A. M. Szaplonczay, Analyst, 1972,97,29. 8. T. Ishizuka and H. Sunahara, Bunseki K~~u, 1973,22, 59 : Anal. Chim. Acta, 1973.66.343. 9. T. .Ishizuka, Y. Uw&nd and H. Sunahara, Bunseki Kagaku, 1973,22,1450. 10. K. Ametani and T. Takahashi, Bull. Chem. Sot. Japan, 1975,48,721. 11. J. Yofk, R. Avni and M. Stiller, Anal. Chim Acta, 1963,28, 331. 12. I. Rubeeka and B. Moldan, ibid., 1967,37,421. 13. R. W. Cattrall and S. J. E. Slater, ibid., 1971,56,143. 14. D. R. Thomerson and W. J. Price, ibid., 1974,72, 188. 15. W. Ooghe and F. Verbeek. ibid.. 1974.73.87. 16. R. L. Scott, At. Abs. Newsjett., i970, d, 48. 17. J. 0. Rasmuson, V. A. Fassel and R. N. Kniseley, SDectrochim. Acta. 1973.28B. 365. 18. J.-Y. Marks and 6. G. ‘Welc‘her,AnaL Chem., 1970,42, 1033. 19. G. R. Kornbtun and L. deGalan,~~c~~h~rn. Acta, 1973, 28B. 139. 20. D. (?. Manning and L. Capacho-Delgado, Anaf. Chim. Acta, 1966,36,312.

21. J. Ramirez-Mufioz, Anal. Chem., 1970,42,517. 22. J. E. Baker, A. J. Kabat and V. G. Mossotti in J. A. Dean and T. C. Rains eds., Flame Emission and Atomic Absorption Spectrometry, Vol. 3, p. 191. Dekker, New York, 1975. 23. See for example, R. H. Bragg in E. F. Kaelble ed., Handbook of X-rays for Difjraction, Emission, Absorption __. __ _. _ __... __ _. and Microscopy, p. 12-11. M&raw-Hill, New York, 1967. 24. K. Ametani, Butt. Chem. Sot. Japan, 1976,49,450.