Observation of longitudinal acoustic phonons in layer-silicates by neutron inelastic scattering

Abstract

A B S T R A C T:The technique of neutron inelastic scattering has demonstrated the presence of longitudinal acoustic phonons in two layer-silicate minerals. Values of the elastic constants determined from the dispersion curves have established the validity of the technique for this class of materials.

Determination of acoustic phonon dispersion curves in layer silicates by inelastic neutron scattering and computer simulation techniques

“…The three calculated acoustic branches along the [0, 0, ~l wavevector for both energy minimized (0 K) and free energy minimized (300 K) muscovite structures, together with our experimental data and that determined previously (Cebula et al 1982), are shown in Fig. 5.…”

“…where V is the velocity of sound though the crystal along [0, 0, 4] (as determined from the initial slope), and p is the density; for muscovite we use a value of 2.79 x 103 Kgm -3 (Cebula et al 1982). From our phonon data we calculate an initial velocity of 4604 ms-1, which gives a value of 5.92 x 101~ Nm -2 for the C33 constant of muscovite.…”

“…By determining the frequencies for phonons which propagate perpendicular to the layers, appropriate ionformation can be obtained regarding forces acting between the layers at small separations. The possibility of measuring longitudinal acoustic phonons in layer silicates by inelastic neutron scattering has previously been demonstrated using muscovite and a vermiculite termed "hydrobiotite" (Cebula et al 1982). Recently computer simulation techniques have been used in several studies to calculate phonon dispersion curves and lattice dynamical properties of minerals (Sanders et al 1984;Price et al 1987;Wall and Price 1988).…”