Neutron scattering data are reported for II•VI zincblende crystals, which are believed to be of suf• ficient precision to refine earlier ZnS ambiguities and to provide a basis for model fitting compar• able to existing III• V results. Valence shell models, including 9-12 parameters (VSM) and a vari• able shell charge extension (VCM), were fit to the data and used to generate phonon density of states and Debye temperatures. Very good fits to the neutron data were obtained, but no model was found that also predicts an accurate set of electric and mechanical constants. It is concluded that an unambiguous ionic charge Z cannot be assigned from the neutron results in either case.

The lattice dynamics of KCN and NaCN in their disordered cubic phase has been studied by coherent inelastic neutron scattering and infrared reflection measurements. Acoustic phonon branches for cyanide single crystals were measured in the [001] direction, with more limited measurements taken in the [110] direction. The onephonon scattering generally appeared as weak peaks superimposed on a large background of non-one phonon scattering, particularly as measurements were extended to higher energy. No peaks clearly assignable to optical phonons were observed in the neutron measurements but the infrared reflection spectra for polycrystalline pellets were analyzed to derive TO and LO phonon energies (at q =0) of 17.0±0.6 and 28.5±2.5 meV. These measurements (and a comparison with the rather easily measured acoustic and optical phonons in a KBr crystal of similar volume) indicate that phonons do not exist as well-characterized excitations in the cyanides at higher frequencies and finite wave vectors. The unusual character of these results is attributed to the dynamical disorder of the cyanide ions in the fcc crystal and is compared with recent phonon measurements in the analogous fcc phase of NO.1. The [00 IJ transverse acoustic branches of both KCN and NaCN show unusually low frequencies below q = 0.5, which are consistent with recent elastic constant results and with the very large Oebye-Waller factors derived from previous neutron diffraction results, and suggest further that the phase transitions in the cyanides are "driven" by a soft shear mode.

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The acoustic modes of the phonon dispersion relation in Nb, NbDp l5 and NbD045 have been measured at 473 K for phonons with wave vectors a1ong the [100], [110],and [111]axes by coherent neutron scattering. The observed neutron groups for both alloys were well defined, with little or no apparent broadening. The results are compared to similar data for Nb-Mo alloys and with previous lattice-dynamics results for PdDO". This comparison shows that despite differences in detail, the general features of the dispersion relations of NbD"and Nb-Mo are similar after allowing for the differences in lattice parameters for the two alloys. The measured dispersion curves and derived phonon frequency distributions for the Nb-D alloys are quite difFerent from the analogous results for PdD063 in that the average acoustic phonon frequencies increase with increasing deuterium concentration and lattice parameter.

Molecular orbital calculations based on the Extended Huckel Theory have been performed forIt is noted that the residual resistivities for clusters of two and eight Si0 2 molecules. We Ag alloyed with rare earth impurities from the report here the results for an ordered structure~n latter half of the rare earth series are approximately which periodic boundary conditions are imposed three times the residual resistivity for any other to saturate the peripheral bonds which otherwise trivalent impurity. Moreover, these residual cause extraneous localized states to appear. We resistivities are essentially independent of the find an energy gap of 13 eV as compared to the magnetic character of the impurities. New thermoexperimental value of -s1ẽV. The removal of an electric power data indicate the impurity oxygen results in several levels in the energy gap contributions to the diffusion thermoelectric power which move upon displacement of the two are positive. These anomalies are shown to be neighboring silicons. The levels appear to account consistent with a very simple model which includes for the observed ultraviolet optical absorption core contributions to the scattering. band and are also a possible origin for the Received 20 July 1970 positively charged slow surface states present near a Si02-Si interface. The addition of either Revised 5 October 1970 atomic or molecular hydrogen to a perfect crystal is predicted to yield preferentially a double hydrogen-single oxygen center. If ambient oxygen atoms are present, a two hydroxl group center is 7. THE OPTICAL SPECTRUM AND then most favorable. MAGNETIC PROPETIES OF TmFeO3 IN THE SINGLE-ION MODEL