Kiyosi Motida

Effect of Covalency on Phonon Dispersion Relations in Nacl Type Alkali Halide Crystals

A lattice dynamical model is constructed for the NaCl type alkali halide crystals by incorporating the covalency effects in the framework of Hardys polarization dipole model. Three effects of covalency are taken into account: (1) the reduction of the effective charge of the long range Coulomb interaction between ions from the formal charge of ion, (2) the attractive force due to covalency, and (3) the field induced charge flow. The model is found to provide a closer description of the dispersion curves than the deformation dipole model that is obtained by incorporating the mechanical polarization effects in the framework of the polarization dipole model.

Szigeti Charge and Its Correlation with Hyperfine Coupling Constant of Doped Mn2+ Ion in Divalent Metal Compounds

Correlations are given between Szigeti charge and the hyperfine constants of the doped Mn 2+ ion in the divalent metal compounds and of Cr + ion in the alkali halides respectively. From the correlations it is concluded that the deviation of Szigeti charge from the formal charge Z e relates to covalency. On this conclusion a hypothesis is introduced that Szigeti charge represents substantially the static charge of the constituent ion.

Correlation between Szigeti Charge and Hyperfine Coupling Constants of Doped Ti2+(3d2), V2+(3d3) and Co2+(3d7) Ions in Divalent Metal Compounds

It is shown that the correlations between the Szigeti charge and the hyperfine constant of doped ions can be explained successfully on the assumptions adopted in a preceding paper (J. Phys. Soc. Jpn., 49 (1980) 213), of which the principal one is that Szigeti charge represents substantially the static charge of the constituent ion.

Relation between Elastic Modulus C44 and Covalency in NaCl Type Alkali Halides

It is shown that there exist empirical relations between the elastic modulus C 44 of the NaCl type alkali halides and the Szigeti charge. By reason of these empincal relations it is concluded that the elastic modulus C 44 relates to covalency on the hypothesis that the Szigeti charge represents substantially the static charge (J. Phys. Soc. Jpn. 49 (1980) 213, 218).

Origin of the Exchange Striction in NiO

Two kinds of exchange interactions which have possibilities to cause exchange striction in NiO are discussed. One is the direct exchange interaction between neighboring d γ orbits, which is shown to be too small to explain the observed exchange striction. The other is the interaction via both the anionic 2 p and 2 s orbits, which involves kinetic and potential exchanges. This interaction, to which the origin of the exchange striction is attributed, is shown to contribute by 222cos ϕ K (ϕ: Ni–O–Ni angle) to the Neel temperature. This interaction is shown to be necessary for understanding the Neel temperature of a perovskite-like substance Ni 2+ TeO 3 .

Correlation between Szigeti Charge and Hyperfine Constant of Doped 151 Eu 2+ Ion in Divalent Metal Compounds

It is shown that there exists a linear correlation between the Szigeti charge and the hyperfine constant of the doped 151 Eu 2+ ion in the divalent metal compounds. It is pointed out that this correlation is the same type as the case of the Mn 2+ ion given previously by the author (J. Phys. Soc. Jpn. 49 (1980) 213.), although in the Eu 2+ ion case the 4 f electrons possessing the spin have no appreciable covalency unlike the case of Mn 2+ ion.

Effects of Covalency on the Phonon Dispersion Relations in the NaCl-Type Alkali Halide Crystals.III.

A lattice dynamical model for the NaCl-type crystals are constructed which is a composite of the covalency model and the deformation dipole model. Using this model, the covalency parameter n c , the number of electrons occupying the outer most s orbitals of an alkali metal atom, is determined from the measured dispersion curves of 13 NaCl-type alkali halides by the least-squares analyses subject to the constraint that the model parameters reproduce the dielectric and elastic constants. The obtained n c values are contrary to the expectation of the traditional idea ( n c =0): about 0.08 for the fluorides, 0.10 the chlorides, and 0.15 the bromides and iodides.

Cation Dependence of Néel Point in Perovskite-Like Compounds RFe3+O3, Where R is a Rare-Earth

It is pointed out that the variation of Neel point in the series of orthoferrites RFe 3+ O 3 , (R; a rare-earth) deviates from the angular dependence of the type cos 2 ϕ (ϕ: bond angle), if the Fe 3+ –O 2- distance is assumed to be constant throughout the series. The change in the Fe–O distance, to which the deviation of Neel Set point is attributed, is shown to agree qualitatively with the one estimated from the change in the coordination number of the anion in the position 8(d). It is shown that the derived values agree with the measured Fe–O distances in the orthoferrites from Lu to Sm but do not from Nd to La.

Ratio of c/a of Wurtzite Type Crystals: Comparison of Keffer and Portis' Theory with Experiments

The ratio of c / a of five wurtzite type crystals are compared with the theory of Keffer and Portis under the hypothesis that the effective ionic charge equals the Szigeti effective charge. It is shown that the theory can explain reasonably the observation.

Correlations between Elastic Moduli and Covalency in the Alkaline Earth Oxides

For the alkaline earth oxides a correlation between C 44 and d C 44 /d P is shown to exist which is substantially the same as the corresponding one for the alkali halides in a previous paper (J. Phys. Soc. Jpn. 50 (1981) 102). On the other hand, a relation between d C 11 /d P and d C 44 /d P is shown to differ qualitatively from that for the alkali halides. Discussion about origins of these correlations is given.