A STUDY OF TK3 MGN3TIC PROPERTIES OF FERROCYAKTDij^S AND FERRICYANIDES by JACK ED7ARD RANDORFF, B. S. A THESIS IN PHYSICS Submitted to the Graduate Faculty of Texas Technological College in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCISIICE Approved Accented June, 1967 f^(A^fjin Cop. a ACKNO/JLEDGS-IENTS I am deeply appreciative of Dr. Billy J. I4arshall for his direction of this thesis; to the Robert A. Welch Foundation of Texas for financial support given to this project; to Dr. V/, 0, Milligan, research director of the forementioned Foundatior^ for the helpful and stimulating discussion with regard to this work and to his research group at Baylor University for the preparation of samples; and to the U. S. Department of Health, Education and V'elfrro for tho presentation of an N,D,E,A. Title IV Fellowship as financial support to pursue this study. ii TABLE OF COirrSxNTS ACKNO'WLEDG&IEOTS ii TABLE OF CONTENTS iii LIST OF TABLES iv LIST OF FIGURES v I, INTRODUCnON 1 II. EXPERIMEOTAL TECHMIQUE 3 Sample Preparation 3 Apparatus 3 Temperature Control and Detection 5 Calibrations and Corrections 6 Set-up Procedure 8 III, RESULTS Aim Discussion l6 IV. FUTURE WORK 33 LIST OF REFERENCES 34 APPENDIX I 35 APPENDIX II 42 APPENDIX III 43 iii LIST OF TABLES Table 1, The values of susceptibility and temperature.,, 19 Table 2, Effective number of Bohr Diagnetons per atom or per molecule, 20 iv LIST OF FIGURES Figure 1, Apparatus Assembly. 9 Figure lA. Internal view of Cahn RM Automatic Electrobalance, 10 Figure 2. Schematic of Germanium Probe Thermometer Current Standard, 11 Figure 3, Detail of Dewar Assembly, 12 Figure 4. Sample Arrangement and Associated Apparatus, 13 Figure 5» Plot of Am/m versus Fieldial Regulator Readings for diamagnetic results, 14 Figure 6, Plot of A^/m versus Fieldial Regulator Readings for paramagnetic results, 15 Figure ?. Plot of x x 10^ vs. T for Fe3(Fe(CN)5)2. 21 Figure 8. Plot of x x 10^ vs. T for ?Qi^{FeiCi:i)^)y 22 Figure 9. Plot of x x 10^ vs, T for Cu3(Fe(CIl)^)2^ 23 Figure 10, Pl^ot of x x 10^ vs. T for Cu2Fe(CN)£. 24 Figure 11. Plot of x x 10^ vs. T for Cu2Fe(CN)5*, 25 Figure 12. Plot of x x 10^ vs. T for Cd3(Fe(CN)5)2. 26 Figure 13. Plot of l/x^ vs. T for Fe3(Fe(CN)^)2. 2? Figure 14, Plot of l/x^ vs, T for Fei^(Fe(CN)5)3. 28 Figure 15. Plot of l/x^.j vs, T for Cu3(Fe(CN)^)2. 29 Figure 16. Plot of l/x- vs, T for Cu2Fe(CN)5. 30 Figure 1?. Plot of l/x^ vs. T for Cu2Fe(C.l)5*. 31 Figure 18. Plot of l/x^., vs. T for Cd3(Fe(CN)5)2. 32 Fitnire 19. Energy levels splitting schame... 3^ •^s CHAPTER I lOTRODUCTION According to the existing theoiy, all substances in nature are diariiagnetic and a relatively few exhibit a paramagnetism which overshadows the diamagnetic properties. Those substances which exhibit a negative magnetic susceptibility are termed diamagnetic while those displaying a positive susceptibility are classified as paramagnetic. Appendix I relates the theory associated vdth each effect. In the late 1920's, questions began to arise over the fori^iation of certain iron-cyanides. The argument centered around two substances in particular, Prussian blue, Fe4(Fe(CN)^)o, and Turnbull's blue, Fe3(Fe(CN)^)2. The original thought was that ferric salts interacting with alkali ferrocyanides gave Prussian blue and the interaction of ferrous salts and alkali ferricyanides produced Turnbull's blue. The problem is actually complicated because of the oxidation-reduction reaction which occurs upon mixing iron salts and alkali iron-cyanides. Some authorities have supported the production of Prussian blue in both cases wliile others have supported Turnbull's blue. X-ray diffraction and density measurements have failed to resolve this disagreement. In an attempt to answer this question, a systematic study of the magnetic properties of ferrocyanide and ferricyanide coiTipounds has boen undertaken, Ferrocyanide is characterised by Fe(CM)^ while ferricyanide is denoted by Fe(CN)^ • As a further study and to attempt a clearer understanding of the nature of these •t^ compounds, the magnetic properties of Cu2Fe(CN)^, Cuo(Fe(CK)^)2 and Cd3(Fe(CN)^)2 will be discussed. The study was conducted by observing the behavior of the magnetic susceptibilities as a function of the temperature over a range of approximately 2 to 300° Kelvin, CHAPTER II EXPERIMENTAL TECHNIQUE SAMPLE PREPARATION The sample crystals were obtained from Dr. W. 0, Milligan's research group at Baylor University, (The methods for crystal 2 "^ growth are given by I'^-lligan and Davidson-^, For instance, Fe^(Fe(CN)^)^ is formed by combination of ferric and ferrocyanide ions and Fe3(Fe(CN)^)2 is formed id.th ferrous and ferricyanide ions.) The sample material was in the form of a fine poxder, hence it had to be placed in a container before measurements could be made. The container consisted of a spiall glass bubble irdth the approximate dimensions: 1,5 millimeters in diameter; stem, 1,5 centimeters in length; mass, 50 milligrams. The container was blo;>ni from a one millimeter diameter, low melting point, capillary tube. After being cleaned, the container's mass was measured and recorded. The sample material was placed in the bubble through the open stem and the total mass was recorded. The end of the stem was sealed and a glass hook attached. Mass measurements were made on a Mettler model B-6 Semi-Micro balance which gave an accuracy of 1 0.01 milligrarns. The masses of the samples studied varied from 0.31 to 2.7? milligrams, APPARATUS The susceptibilities were measured using the Faraday method , which consists of measuring the force exerted on a hypothetical point source of the magnetic material placed in a gradient magnetic field, H(3H/9Z). The equipment used in this research is shown in Figure 1. The gradient field, H(9H/9Z), was produced by a Varian model 3600-12 inch electromagnet, a Varian model V-FR2501 Fieldial Regulated 7 Kilowatt Power Supply and set of constant H(9H/9Z) pole faces. The sample \jas held, via the container, in the gradient field ty a thin glass fiber vrhich, as shoi-m in Figure 1 and Figure lA, was suspended from the lever arm of a Cahn RM Automatic Electrobalance. When the sample experienced a vertical force, it was transmitted immediately through the fiber to the lever arm which was momentarily displaced from equilibri\im. A servo signal was then relayed to an electromagnet in the balance which created a restoring torque equal to tho torque exerted by the sample. Thus the sample essentially remained in the same position. The servo signal vras directly proportional to the force change on the sample, hence this voltage was displayed on a Hewlett-Packard model 3440A digital voltmeter. We were thus able to obtain a continuous, visual readout. For our studies a signal of 1.0 millivolts corresponded to a mass change of 0.1 milligrams. The digital voltmeter was adjusted to read to the nearest 0.01 millivolt. As shown in Figure 1 and Figure 3> the sample was suspended inside a 3/4 inch pyrox glass tube which was connected to the Cahn balance ty a ball joint vacuum seal. The tube and balance were then evacuated to a pressure of 5 x 10"^ torr. The low temperature cryostat, as pictured in Figure 3» >^s designed such that the region above the cryogenic liquid in the inner De;«rar could be evacuated by a moans of a 120 cfm. Stokes pump. This was necessary to produce two more temperatures of investigation and allow cryogenic transfer in a closed atmosphere. The liquid helium and liquid nitrogen Devrars ijere purchased from the Scientific Glass Blowing Co, TEi^SRATU'RE COIITROL PJJD DETECTION Temperatures were measured using a copper-eonstantan thermocouple at room temperatures and a gennaniuin resistance thermometer at cryogenic temperatures. The thermocouple was obtained from the Leeds and Northi'up Co, while the germanium probe was purchased from Cryocal Inc, Calibrations for both sensors were obtained from the manufacturers. The potentials across the tvjo sensors were determined x-dth the aid of a Leeds and Northrup Guarded Potentiometer model 755^ and a D,C, Null Indicator model 983^1 • The germanium probe was adjusted to optimum current conditions as suggested by the manufacturer "vdth the use of a current calibration circuit as pictured in Figure 2, The ranges were 1°-2°K, 1,0 microamperes; 2°-15°K, 10 microamperes; 15°-40°K, 0,1 milliamperes. These ranges were the approximate temperatures of investigation. Determination of room temperature (300°K) gave no particular problem. The temperature was monitored by a copper-constantan thermocouple referenced to a second thermocouple in an ice bath and assumed to be at 273*^K. The approximate temperatures of 77°, ^^^^ 4,2° and 2°K were obtained vdth liquid nitrogen, pumped nitrogen, liqiiid helium and pumped helium, respectively, {By the term "pumped" it is meant that the equilibriuii vapor pressure of the liquid is reduced, hence lowering the temperature. This is maintained using a vacuum pur^p.) The position of the geiTJianium probe relative to the sample as indicated in Figure 4 i.^s about one centimeter. The temperature of the sample was assumed to be that of the germanium probe ty using a few microns pressure of helium exchange gas.