A. Manoogian

Electron spin resonance measurements of the spin Hamiltonian zero‐field splitting parameter D as a function of temperature for trigonally distorted Cr3+⋅6H2O magnetic complexes in families of hydrated crystals. I

The spin Hamiltonian zero‐field splitting parameter D is measured as a function of temperature for trigonally distorted Cr3+⋅6H2O magnetic complexes in families of hydrated crystals. The purpose of the study is to classify the isomorphs of the crystals via their magnetic behavior so that use can be made of similarities and differences between the members when analyzing data obtained from electron–nuclear double resonance (ENDOR) measurements. The D vs T curves are found to form well ordered patterns within a family of isomorphs, and the patterns are significantly different for each type of crystal. When the straight line part of the curves between 195 and 297 °K are extrapolated to high temperature, the existence of common crossing points is found. The orderly nature of the patterns is expected to provide an additional aid in describing the physical behavior of the crystals. The crystals studied are alums, guanidinium aluminum sulfate hexahydrate (GAlSH) and its isomorphs, and aluminum chloride hexahydrat...

Interpretation of 53Cr3+ electron–nuclear double resonance data obtained in hydrated crystals in terms of vibrational and static crystal distortion contributions to the spin Hamiltonian parameters. II

Electron nuclear double resonance (ENDOR) measurements of 53Cr3+ are extended to more isomorphs of guanidinium aluminum sulfate hexahydrate (GAlSH) and the alums. This is done because a satisfactory analysis of the varied ENDOR data obtained in the crystals can be obtained only by studying families of crystal isomorphs. An important relationship obtained in the combined double resonance studies at 4.2 °K is a plot of the quadrupole hyperfine parameter Q′ vs the zero field splitting parameter D. It is found that the Q′ points fall on a series of straight horizontal lines, indicating that Q′ is independent of D. The simple theory shows that if the trigonal distortion in the Cr3+⋅6H20 magnetic complexes is due to static effects alone then a direct proportionality between Q′ and D is expected. It is possible to give a satisfactory explanation to the experimental Q′ vs D diagram if the vibrational effect of the crystals is assumed to play an essential role. An analysis of the vibrational and static crystal dis...

Interpretation of the low temperature phase transitions in ammonium and potassium chromic alums in terms of vibrational and static crystal distortion contributions to the spin Hamiltonian zero‐field splitting parameter D. III

The spin Hamiltonian zero‐field splitting parameter D obtained in ammonium and potassium chromic alums and their isomorphs is analyzed in terms of the temperature dependence of vibrational and static crystal distortion contributions. The analysis shows that the low temperature phase transitions existing in the chromic alums are due to a change in sign of the static trigonal distortion at the chromium sites. For NH4Al and KAl sulfate alums dilutely doped with Cr3+ it is found that the distortions at the Cr3+ sites change sign at temperatures higher than the known transition points in the undoped crystals. Dilutely doped NH4In sulfate alum represents a special case in which the crystal changes phase but the static distortion at the Cr3+ site does not change sign.

Magnetic susceptibility data and exchange interaction parameters for some pseudo-ternary semimagnetic semiconductor alloys

Abstract Previously reported values of the spin-glass transition temperature T g from magnetic susceptibility measurements on the alloys Cd x Zn y Mn z Te, Cd x Zn y Mn z Se and Cd 1- z Mn z Te 1- y Se y have been analyzed in terms of an indirect antiferro magnetic exchange interaction having the form J = I 0 r −2 exp (− αr ). Values of I 0 and α so obtained were found to be characteristic of the two structures, zinc blende and wurtzite. Values of nearest and next nearest neighbour exchange obtained from I 0 and α are shown to be in very good agreement with values determined previously from other types of measurement. The values of I 0 and α have also been used to predict values of the Curie-Weiss θ for the alloys and these show very good agreement with the values determined from magnetic susceptibility measurements. The exchange is attributed to virtual transitions between the valence band and a non-localized 3d band, and the values of α are discussed in these terms.

Magnetic properties of Cd1-zMnzTe1-ySey alloys

Abstract Measurements of electron spin resonance (ESR) and low-field magnetic susceptibility have been made as a function of temperature in the range 4.2-500 K on polycrystalline samples of both the zinc blende and wurtzite phase fields of the alloy system Cd1-zMnzTe1-ySey. From the susceptibility measurements, values have been obtained for the spin-glass transition temperature Tg, the Curie-Weiss temperature θ and the Curie constant C. The curves of ESR linewidth ΔH versus temperature have been fitted to the relation ΔH = T exp(-T/T0)+B(1-θ/T){1-exp(-T/T0}, and hence values obtained for the parameters T, T0 and B. The variations of the six parameters Tg, θ, C, T, T0 and B with both composition and crystal structure are considered in some detail.

Thermal effect on the electron donating and accepting properties of Na-Y zeolite, and the reduction of SO2 and CO

Abstract A study was made of the formation of phenothiazine cation radical (TPQD+) and tetracyanoethylene anion radical (TCNE−) on the surface of Na-Y zeolite. It was observed using electron spin resonance (e.s.r.) that the number of cation formation sites decreased with higher activation temperature while the number of anion sites increased, the respective changes not being in proportion. The formation of the SO2− anion radical on Na-Y zeolite was studied at 450°C. It was observed that the e.s.r. line intensity of SO2− increased with the reaction time, and also with the length of degassing time during thermal activation. CO+ cation radicals were formed when the SO2-treated zeolite was heated at 450°C in contact with CO. The CO+ radicals disappeared either on prolonged heating or heating in the presence of air. The SO2 ions do not take part in the reduction of SO2− with the CO over zeolite. Mechanisms for the formation of CO+ and the other observed effects is discussed.

Electron spin resonance of vanadium pentoxide molybdenum trioxide catalysts

X‐ray, infrared, and electron spin resonance (ESR) studies were made on molybdenum trioxide catalysts doped with 0.25, 0.5, 2.0, 5.0, 10.0, and 20% by weight vanadium pentoxide. X‐ray diagrams indicate the existence of two kinds of patterns for catalysts containing more than 2% V2O5. All the samples produced ESR signals, including pure V2O5. For low dopant concentrations the signals were well resolved, and the paramagnetic center was identified as the VO2+ ion in the unit structure VO2+5O2−. The resonance lines increased in width as the dopant concentration was increased to 10%, and only a single broad line existed for 20% V2O5 or pure V2O5.

Electron spin resonance studies of some Canadian coals

Abstract Nine Canadian coals of different rank and composition were studied by electron spin resonance. For percentage of fixed carbon in the range of 43 to 78 wt%, the free radical g values were found to increase with decreasing carbon content, and did not level off for the low rank coals. The free radical linewidths are attributed to atomic species such as oxygen and not to protons of hydrogen.

Interpretation of ESR results for some semimagnetic semiconductor alloys

Abstract Previously reported values of the ESR linewidth variation with temperature for the alloys (i) Cd x Zn y Mn z Se (ii) Cd x Zn y Mn z Te and (iii) Cd 1− z Mn z Te 1− y Se y have been analyzed in terms of the relation, δ H = Γ exp (- T / T 0 ) + B {1 - exp(- T / T 0 )}(1−θ/ T ), where the first term is attributed to spatial inhomogeneity effects and the second to paramagnetic behaviour. Values of the parameters, Γ, T 0 and B have been determined as a function of composition in each alloy system. The parameter T 0 , which measures the potential barrier separating two neighbouring ground states of the disordered spin system, is found to satisfy the relation T 0 = (α + β f ) z , where f = y /( x + y ) for (i) and (ii) and f = y for (iii). β is positive for (i) and (ii) and negative for (iii), consistent with previous suggestions of Kremer and Furdyna, and may be correlated with the exchange constants J sp-d discussed by Hass et al. The parameter Γ, which measures the width of the distribution of the local field seen by manganese ions in the lattice, is found to satisfy the relation Γ = Dz 2 (1 − z ), where D appears to be characteristic of the crystal structure. The parameter B which measures the linewidth δ H at high temperatures due to paramagnetic effects is found to vary as B = B 0 + γ z and the values of the coefficient γ appear to be related to the exchange parameters of the materials.

Reaction of CO and SO2 over Cu-mordenite

Abstract A study is made of the chemisorption of CO and SO2 over Cu-mordenite by means of electron spin resonance (ESR). Two Cu2+ magnetic complexes are observed in Cu-mordenite, and the reaction with CO causes a differential reduction in their respective line intensities. For reaction at room temperature, the effect is reversible upon removal of the CO, but when the system is heated the catalyst changes irreversibly. Introduction of SO2 to the Cu-mordenite-CO system causes the reappearance of the Cu2+ spectrum in a single, different, magnetic complex. Models are proposed to explain the chemical processes operative in the reactions.