2210
A. REISXlBN, A[. BERKESBLIT, .4ND l f . WITZEX
Yol. 66
a (since r < 0). Consequently, Ifpasses through a maximum at a value of t which is readily derived. The overvoltage V , which is measured, was computed from eq. 2-4 for several values of re, and results are listed in Table TI. It is seen that the in-
and the measured overvoltage is too high. This effect can be quite pronounced and may lead to serious error. For instance, the experimental / q / values a t 10 psec. for Zn(I1) discharge under the same conditions as above, except for insertion of a resistance in series with the cell, mere 2.75, 4.5, T 4 B L E 11 CALCULATED OVERVOLTAGE (TX MV.) FOR ])IFRERENT TIVES 6.6, and 20 mv. (cf. Table 11),respectively, for T , = 13, 560, 1210, and 2710 ohms. This type of error A I D c(F,LL RESISTANCES FOR THE D I S C H S R G E O F 10 MAT Zrl" could be minimized by insertion of a filter network 7, between the cell and the cathode-ray oscilloscope. ohms psec. t psec ;\lore simply, a three-electrode cell could be used so 1 2 5 5 10 20 40 0 0 4 28 3 96 that only a fraction of the ohmic drop is applied to 3 4s z e6 1 50 o 54 13 o 0037 4 28 3 43 2 66 3 9fi I 5s o 54 the oscilloscope. Further, one should select the 560 16 22 2 3 96 3 45 2 66 1 56 0 54 smallest possible VO and the largest capacity c 1210 33 43 6 7 5 3 31 2 54 1 49 0 52 which are compatible with the desired time con2710 71 227 4 28 3 11 4 2 52 1 48 0 51 701
a
I.
Other conditions, except for re, are the same as for Table
fluence of the resistance is negligible when t >> 7. I n practice, however, the large ohmic drop across the cell for short times may saturate the amplifiers of the cathode-ray oscilloscope. Recovery of the oscilloscope amplifiers may be relatively sluggish,
stant
T.
Acknowledgment.-This work T?sas supported by the Office of Naval Research. The authors are indebted to Professor L. W. Morris of the Physics Department, Louisiana State University, for suggesting capacity discharge as a practical means of devising a coulostat.
SOX-STOICHIOMETRY IS CADAIIUPIP SELESIDE ASD EQUILIBRLI BYA. REISMAN, 11.BERKEXBLIT, AND h4. WITZEK Thomas J . Watson Research Center of International Business Machines, Yorktown Heights, New Yo& Received M a g 6,1982
Studies of the condensed system Cd-Se have indicated a solubility limit of Cd in CdSe corresponding to a Cd/Se molar ratio of from 1.010-1.015. The system shows the formation of a two-liquid region in the selenium-rich portions, and a tendency ton-ard such formation in the cadmium-rich portions, The melting point of the selenide was determined as 1239 i 3'. Using precision X-ray fluorescence techniques to study products of vacuum heat treatments of initially stoichiometric CdSe, it has been concluded that this compound vaporizes incongruently and achieves a steady-state vaporization along the three phase line Cdl+sSe solid solution-liquid-vapor.
Introduction The first paper in this series dealing with some aspects of the detailed chemistry of compound semiconductors discussed direct synthesis of 11-VI compounds from the elements a t temperatures not much in excess of the melting point of the higher melting constituent.2 The present report is concerned specifically with deviations from stoichiometry in CdSe as a function of temperature, steady state vaporization of this material a t a given temperature, phase equilibria in the system Cd-Se under equilibrium pressures, and the solid solubility limits of Cd and Se for a system under its equilibrium pressure. CdSe always has been observed as an "n" type semiconductor, indicatiing that it tends to be selenium deficient. Hines and Banksj3in studies of CdSe, observed rather large conductivity changes along a gi\Ten crystal but could not relate these conductivity differences numerically to varying selenium vacancy counts. The only published (1) This paper was presented in part ilt the Symposium on Nonstoichiometric Comuounds, 141st National MeetinK of the American Chemical Society, Washington, D. C., March 20-29, 1962. (2) A. Reisman and M. Berkenblit, J Phys, Chem., in press. (3) D. Hanea and E. Banks, J . Cham. i'hvs., 24, 301 (1956).
work on the system Cd-Se dates back to work by
Chikashige and Hitosaka* in 1917. The proposed diagram appears to be a non-equilibrium one, since the data for all compositions simultaneously show thermal effects attributable to the meltings of Cd and Se. Similar behavior has been described in connection with the studies of t&e synthesis of CdSe from equimolar mixt'ures of Cd and Se, and an explanation, believed valid for all compositions, has been presented.2 Experimental Procedure A. Thermal Studies.-Differential thermal analysis (d.t,.a.) was used to define the phase diagram for the system Cd-Se. Discussions of equipment and other relevant information are given elsewhere.2,6 B. Fluorescence Analysis.-The a.nalysis of CdSe by wet chemical techniques lacks sufficient precision for purposes of studying non-stoichiometry . Of the available instrumental methods, X-ray fluorescence analysis appeared to be the only one with potential for charact.erizing major (4) b1. Chikashige and R. Hitosalca, M'em. CoZZ. Sci., Univ. Kuolo, 2, 239 (1917). (6) See for example: S.Reisman, Ph.D. Thesis, Univ. Nic. No. 882876, Chem. Phys.; A. Reisman, Anal. Chem., 32, 1566 (1960); A. Reisman and J. Mineo, J. Phgs. Chem., 64, 748 (1960): A. Reisman and J. Karlak, J. Am. Chem. Sac., 80, 6500 (1958); F. Holtzberp, A. Reisman, M. Berry, arid R.1, Rorkenblit, ibid.. 79, 2039 (1957).
Nov., 1962
NOS-STOICHIONETRY IN CADMICM SE,LESIDE
element conbent, and this method was explored as a means for studying non-stoichiometry. A Philips vacuum spectrograph was employed, and with proper sample preparation and equipment modification, the technique was found suitable for the present studies. It was found that simple manual powder packing on the sample holder resulted in appreciable scatter of data, and that solution techniques presented serious problems because the CdSe required strong acids to achieve solution. A successful method of sample preparation involves pressing of
