Alicia Batana

Pressure dependence of the effective charge of ionic crystals

Abstract For ionic crystals of NaCl and CsCl structures, the volume dependence of the effective ionic charge ( d ln s d ln V ) defined by the second Szigeti relation is calculated using low temperature experimental data. The need for experimental data of γ T at T → 0°K is remarked upon. ( d ln s d ln V ) obtained independently with a deformation dipole model and a simple shell model. The results are compared. All models tend to give positive values of ( d ln s d ln V ), as expected.

On the pressure dependence of the effective ionic charge of cesium halides

Abstract The importance of temperature effects in the use of the second Szigeti relation to calculate the volume dependence of the effective ionic charge of cesium halide crystals is studied. The effect obtained is larger than the one for alkali halide crystals of the NaCl structure. Positive values of (∂ ln s /∂ In v ) are obtained using low temperature experimental data with the Gruineisen parameter γ t ( T → 0K) calculated using the generalized first Szigeti relation. Values of γ t are also obtained with a rigid ion model and they differ little from the previous ones.

Volume dependence of the effective charge of zinc-blende-type crystals

Abstract The volume dependence of the effective charge is calculated for crystals of the zinc-blende structure. The thermal effects and the experimental uncertainties in the use of the second Szigeti equation are studied. Both effects are evaluated for the pressure dependence of the dynamical effective charge. The volume dependence of the generalized first Szigeti equation is used to calculate the Gruneisen parameter γ t with low temperature data.

Volume dependence of the effective ionic charge of alkaline-earth fluorides

Abstract The importance of the use of low temperature data with the second Szigeti relation is shown for the fluorite structure; a comparison with the NaCl and CsCl structures is given. The volume dependence of the generalized first Szigeti relation is used to calculate the Gruneisen parameter γ 1 with low temperature data; γ 1 is calculated with different models. Values of the volume dependence of the effective ionic charge are obtained independently using Hardys model.

Ionic polarizabilities in crystals at high pressures

Abstract The pressure dependence of ionic polarizabilities in alkali halides and fluorite-structured crystals has been evaluated via different methods and their results compared. Some methods had to be extended for the fluorite structure. Recent experimental data for cesium halides, together with that for alkali halides, allowed for a more complete analysis of results. When high pressure data were available comparison allows for an evaluation of the validity of the methods themselves; these could then be applied to propose high pressure values for other crystals with measurements only at p =0.

Calculation of the pressure dependence of anion polarizabilities in crystals

Abstract A program has been developed in Maple V that allows for the calculation of anion polarizabilities of ionic crystals (based on the Clausius–Mossotti treatment) and its pressure dependence. It has been applied to alkali halides for which the results may be compared to those obtained from experimental data. It opens the possibility of its further use for the evaluation of the pressure dependence of the dielectric constant from the knowledge of experimental data at p=0; we here apply this to alkaline-earth fluorides obtaining very good results.

Thermal expansion and third order elastic constants of FCC metals

Abstract In this paper we develop the necessary expressions to evaluate third order elastic constants and temperature dependence of the thermal expansion of FCC metals. For this we use a previously proposed interatomic potential that can be expressed as a sum of two and three body force independent contributions. The very low temperature limit of the thermal expansion is also calculated using the continuum model. To do all the numerical calculations we write several FORTRAN programs. The necessary subroutines are grouped in several modules to allow maximum portability. We illustrate the usefulness of the programs by their application to gold.

Three body forces in the lattice dynamics of FCC metals

Abstract There are presently experimental data on FCC metals, concerning fundamentally harmonic properties (dispersion curves, second order elastic constants) and essentially anharmonic ones (third order elastic constants, thermal expansion). However, at present, there is not a simple interatomic potential that can reproduce adequately all this experimental information simultaneously. In this work we develop several programs to calculate the second order elastic constants and the dispersion curves using a general central two body interaction up to 19th neighbors and common nearest neighbor three body interactions. The difference from previous works is that here the three body potential is a purely angular one, uncoupled from the two body forces, allowing a quantitative evaluation of its relative importance.

Pressure dependence of the dielectric properties of alkali halide crystals within the framework of the exchange-charge model

Abstract For alkali halide crystals dϵ 0 dp is calculated with the exchange-charge model (ECM) and various evaluations of the exchange-charge parameter D and dD dV . The effect of using the effective ionic charge Zes and its volume derivative (d In s/d In V ) obtained from experimental data is studied. Comparison of these results with those of previous calculations within the ECM shows that the use of the experimental d In s/d In V gives better values of dϵ 0 dp for these salts.

Calculation of the effective charge of crystals and its volume dependence

Abstract A program for the calculation of the effective charge and its volume dependence of solids of NaCl, CsCl, CaF 2 and zincblende structures has been developed, which employs three different evaluations: (a) using the second Szigeti equation; (b) using the latter equation with γ t = -(∂ ln ω t /∂ ln V ) obtained from the generalized first Szigeti equation; and (c) using Hardys model for seven different short-range potential forms. It consists of the FORTRAN-77 program DLOGS.FOR, which reads the input data from up to three input files (INPUTx.INP, x = 1, 2, 3; each of them with up to four data sets), calls subroutines SZIG2.FOR, SZI G12.FOR and HARDY.FOR for the computations by the three evaluations mentioned, respectively, and produces the corresponding output files (OUTPUTx. OUT, x = 1, 2, 3). DLOGS.FOR uses subroutines DIFDAT.FOR and DIFMA.FOR for calculating percentual differences between data. HARDY.FOR uses subroutine TABDAT.FOR which performs calculations with the short-range potentials mentioned above.