Pound's theory of the dependence of electric quadrupole splitting of nuclear magnetic resonance absorption lines in a single crystal on the orientation of the crystal in an external magnetic field is extended to cover the case of a crystal with nonaxially symmetric electric field gradient at the site of the nuclei being investigated. It is shown that an experimental study of the angular dependence of this splitting for three independent rotations of the crystal about any three mutually perpendicular axes will yield complete information about the orientation of the principal axes and the degree of axial asymmetry of the electric field gradient tensor at the site of the nuclei, and also will give the absolute value of the quadrupole coupling constant for those nuclei.The authors' experiments on the splitting of the Li7 absorption lines in a single crystal of LiAl(SiO3)2 (spodumene) are described and are used to illustrate the theory. The absolute value of the quadrupole coupling constant for the Li7 nuclei in spodumene is found to be [Formula: see text]. per sec. The axial asymmetry parameter of the field gradient tensor at the site of the Li nuclei is found to be η≡(ϕxx−ϕvv)ϕzz=0.79 ± 0.01. One of the principal axes of this tensor (the y axis corresponding to the eigenvalue of intermediate magnitude) is experimentally found to coincide with the b crystallographic axis of monoclinic spodumene as required by the known symmetry of the crystal. The other two principal axes are in the ac plane, the z axis (corresponding to the eigenvalue ϕzz of greatest magnitude) lying between the a and c axes at an angle of 48° ± 2° with the c axis.

X-ray diffraction and Fourier synthesis techniques have been used to determine the electron distribution in a crystal of portlandite with sufficient accuracy to establish the hydrogen positions. The atoms have been found to occupy the following positions:with u=0-2330 +__0-0004 and v =0"395 +0"008. Independent anisotropic temperature factors were introduced for the calcium and oxygen ions. Also, in the final stages of refinement, systematic differences between the observed and calculated structure factors became apparent and their causes were traced to the Hartree scattering curves ~or Ca ++ and0--which were ~irst used in the determination of the calculated structure factors. Semi-empirical scattering curves were derived and their use leads to an excellent agreement between the observed and calculated structure factors. After this work was completed new theoretical values for the scattering factors Ca ++ and O-became available and these have been compared with the derived semi-empirical values.

The structure of the paraelectric phase of RbHSO4 has been determined by X-ray and neutron diffraction at T = 23 °C. The structure consists of two crystallographically independent molecules in the asymmetric unit. Each of the independent sulphate tetrahedra is linked to a translationally equivalent sulphate by acentrically ordered hydrogen bonds. Some evidence is found for the disordering of one of the sulphate groups in the asymmetric unit. The bond lengths and angles are similar to those found in isomorphous NH4HSO4.

The complete structure of colemanite, CaB3O4(OH)3∙H2O, in both its nonpolar and ferroelectric phases, has been determined by neutron diffraction. This work shows that one of the hydrogen atoms of the water molecule and the hydrogen of an adjacent hydroxyl group, which are in a state of dynamic disorder in the nonpolar phase, settle into ordered noncentric positions in the ferroelectric phase. The ordering of the hydrogen atoms is accompanied by small displacements of the other atoms from the positions they occupy in the nonpolar phase. Assuming reasonable charge assignments, the magnitude of the spontaneous polarization calculated from the observed positional changes is of the same order of magnitude as the experimental value. On the basis of these results, a model for the atomistic mechanism of the transition to the ferroelectric state is advanced.

A coincidence circuit for use with anthracene or stilbene scintillation counters is described. Its resolving time lies in the 10−9 sec. range, in agreement with theoretical estimates of the minimum possible resolving time with these scintillation counters. Examples are given of the use of the circuit in the measurement of short gamma-ray half-lives and, in conjunction with beta-ray spectrometers, in the determination of the details of radioactive disintegration schemes. The new results given in these examples are as follows:[Formula: see text]