H. J. Guggenheim

The Effects of Exchange Interactions in the Spectra of Octahedral Manganese. II. Compounds

Earlier experimental work (J. Ferguson, H.J. Guggenheim and Y. Tanabe, J. appl. Phys. 36 (1965) 1046, Phys. Rev. Letters 14 (1965) 737) on the Mn pair spectrum of KZnF 3 :Mn and the spectrum of KMnF 3 has been extended. The narrow bands of KMnF 3 and RbMnF 3 have been assigned and their abnormally high intensities established. The transition is an electric dipole one and oscillator strength (∼9.5×10 -7 for the 4000 A absorption) is temperature independent, corresponding to an allowed transition. The experimentally observed features of the pair absorption suggest a possible form of interaction between radiation and paramagnetic ion pair. The origin of this interaction is discussed and an order of magnitude argument is given to support the proposed mechanism. Related problems such as exchange interaction in excited states and the spectra of the concentrated materials are also considered.

Ferroelectricity in BaM2+F4

Abstract The piezoelectric compounds BaM2+F4 (M = Mg, Co, NiandZn) are shown to be ferroelectric at room temperature. The spontaneous polarizations are: PS(Mg) = 7.7 μC/cm2, PS(Co) = 8 μC/cm2, Ps(Ni) = 6.7 μC/cm2andPS(Zn = 9.7 μC/cm2.

Electronic Absorption Spectrum of Ni II in Cubic Perovskite Fluorides. I

The fine structure of the spectrum of Ni2+ in KNiF3 and KMgF3:Ni has been analyzed in terms of the spin—orbit splitting and vibrational intervals using a four‐parameter crystal‐field approach. The parameters obtained from the analysis differ from the free‐ion values by relatively small amounts, but the differences are not interpretable physically because configuration interaction has been neglected. We find that Dq=698 cm—1, λ=—(305–320) cm—1, F2=1520 cm—1, F4=114 cm—1 (F4/F2=0.675). For the free ion we find that λ=—334 cm—1, F2=1622 cm—1, and F4=116 cm—1 by fitting the published values for the 3d8 energy levels in our analysis with Dq=0.We find that the effects of exchange interaction on the excited state energy levels is small, and that no Jahn—Teller splitting is observed or expected for this crystal. An explanation is offered for temperature changes in the 3A2g→1Eg transition involving a breakdown of Born—Oppenheimer separability.

Crystal‐Field Spectra of d3,7 Ions. VII. Cr3+ in K2NaGaF6

The 4T2g level of Cr3+ in K2NaCrF6 and in K2NaGaF6:Cr3+ is strongly mixed with the 2Eg and 2T2g levels by spin–orbit coupling in the six‐coordinated cubic (Oh) site. The observed band profile for absorption and fluorescence in the transitions 4A2g ↔ 4T2g in the dilute system shows remarkably well‐resolved vibrational band progressions which have been analyzed in terms of a totally symmetric vibration at 568 cm−1 (556 cm−1 for the excited state) and a large number of nontotally symmetric vibrations ranging from 25–961 cm−1.

High resolution MCD spectroscopy of transition metal ions in fluoride crystals

A brief description of a high-resolution circular dichrograph is given. For MCD measurements a 0·52 T permanent magnet and silica flow tubes are used to obtain data with variable sample temperatures as low as 4·2 K. The MCD results provide an unambiguous assignment of the 4 A 1g and 4 Eg a states of Mn2+ in KMgF3 and KZnF3. The 4 Eg a state lies 90–100 cm-1 below 4 A 1g . A theoretical analysis of the MCD results confirms the assignment of the 4 A 1g state, while an analysis of the spin-orbit splitting of the 4 Eg a state has been carried out using all 252 |αLSJMJ > functions of the d5 configuration. The observed positions of the three components of 4 Eg a are accurately accounted for by Dq values appropriate to KMgF3 and KZnF3 and a spin-orbit coupling constant of approximately 300 cm-1.

Exchange Effects in the Electronic Absorption Spectrum of Mn(II) in Perovskite Fluorides

The electronic absorption spectrum of Mn in cubic perovskite fluorides is strongly modified by exchange interaction. This has been shown from the temperature dependence and concentration dependence of the spectrum of Mn in KZnF3 and the temperature dependence of the spectrum of KMnF3. The electronic transitions of particular interest are the ones near 4000 and 3300 A which have narrow line structure. This structure has been analyzed and the contribution to the intensity from single ions and pairs of ions (Mn–F–Mn) evaluated. Studies of the temperature dependence of the pair lines give a ground state J (JSa·Sb) interaction) of between +3.0° and +3.5°K and it is about twice as large in the excited state. The absorption lines correspond to pure electronic transitions and the selection rule is ΔS=0. The anomalously intense bands in the spectrum of KMnF3 are interpreted on the basis of pure electronic transitions involving more than one ion and the temperature shifts of the bands have been correlated with know...

Electronic Structure of Ni2+ in MgF2 and ZnF2

The absorption spectra of nickel (II) in MgF2 and ZnF2 are reported. The important feature of the spectra is that the first spin‐allowed absorption band shows strongly polarized line structure. From its polarization, the mechanism of the absorption for this line structure has been determined to be magnetic dipole. Furthermore, together with its temperature dependence, symmetry assignments have been made for the fine structure of 3T2 and the ground state. The energy separations and assignments of the ground state thus determined are compatible with the results of electron spin resonance. Choosing two parameters of the orthorhombic field so as to fit the energy separations of the ground state, each energy level of 3T2 under the combined action of the spin orbit force and orthorhombic fields has been calculated as a function of two remaining parameters and compared with experiment. The values of the four parameters and the discrepancy with experiment are discussed. It is shown that the effective Hamiltonian ...

Electronic Absorption Spectrum of Ni(II) in Cubic Perovskite Fluorides. II. Concentration and Exchange Effects

Spectral absorption data from crystals of composition KMg1—xNixF3 and KZnF3 (1% Ni2+) are reported. It is deduced, from a comparison of the spin—orbit energies at 20°K, that the excited states are only weakly perturbed by exchange interactions. The effect on the ground state is relatively large and it is stabilized by about 170 cm−1. Absorption by ion pairs and larger clusters has been observed in the near‐infrared magneticdipole transition. The mechanisms of the various electronic transitions have been studied and the vibrational structure of the electric‐dipole bands interpreted. The four perturbing k = 0 phonon modes observed in the KNiF3 spectrum are split in the dilute crystal spectra and correspond to localized vibrations. The mechanism of the transition to the 1Eg state has been discussed and it is concluded that it can be described as a single‐ion excitation which involves strong vibronic spin—orbit coupling with the nearby 3T1ga state.

Temperature dependence of the EPR linewidth in K2MnF4 and Rb2MnF4

Abstract EPR linewidths of Mn2+ in the quadratic layer antiferromagnets K2MnF4 and Rb2MnF4 have been measured at 24 GHz from 80 K downwards and found to increase steadily as TN is approached. The region of enhancement extends further above TN than is the case in MnF2, a three-dimensional system.

Absorption of Light by Pairs of Exchange‐Coupled Manganese and Nickel Ions in Cubic Perovskite Fluorides

The absorption of light by exchange‐coupled pairs of manganese and nickel ions in KZnF3 is reported. Three absorption regions are discussed, of which two (4000 and 3300 A) are associated with electronic excitation of the manganese and the third (6600 A) corresponds to electronic excitation of the nickel ion. The lines can be analyzed by means of an exchange interaction of the type JS·S and a value of J = 18 cm−1 for the manganese—nickel pair in its electronic ground state has been obtained. The mechanism of the electronic transitions has been discussed using the theory developed for like pairs in an earlier paper and reasons for the greater intensity of absorption by unlike pairs are considered. Corresponding effects of exchange interaction have also been observed in the spectra of KMnxNi1−xF3 crystals. The energies and intensities of the absorption bands have been measured as a function of x and a theoretical analysis of the main results has been given. The main features of these spectra can be interpret...