The bond lengths and bond angles of orthorhombic black phosphorus have been determined as a function of hydrostatic pressures to 26.6(5) kbar using time-of-flight neutron powder diffraction. We show that the markedly anisotropic compression reported previously results from a large pressure-induced shortening of the van der Waals bonds separating layers of atoms combined with a shear motion within the layers. Covalently bonded chains of atoms along the a direction remain very rigid. The average effective linear compressibility for van der Waals bonds is 1.48(9) ×10−3 kbar−1 while the average effective linear compressibility for covalent bonds is an order of magnitude smaller, 2.6(8) ×10−4 kbar−1.

Two time-of-flight powder diffractometers have operated at the Intense Pulsed Neutron Source (IPNS) since August 1981. These instruments use dedicated microcomputers to focus time-of-flight events so that data from different detectors can be summed into a single histogram. Thus, large multidetector arrays can be employed at any scattering angle from 12 to 157 °. This design permits data to be collected over a uniquely wide range of d spacings while maintaining high resolution and count rates. The performance of the two instruments is evaluated by analyzing data from a standard A1203 sample by the Rietveld method. These instruments provide the capability for moderate-to high-resolution measurements with the duration of a typical run being a few hours.

Structural parameters for D2O ice VIII have been determined by Rietveld refinement of time-of-flight powder neutron diffraction data collected in situ at 28 kbar and −4 °C. The D2O molecule has a bond length of 0.973 (11) Å and a bond angle of 104(2)°. O–D⋅⋅⋅O bonds are slightly bent with an angle of 177(1)°. Thus, the D2O molecules in ice VIII are essentially identical to those observed in other high pressure ice structures for which accurate structural data are available.

Neutron powder diffraction data have been collected for the ice VII phase ofD 2 0 at 26 kbar and 22 •C by the time-of-flight technique. The data confirm the cubic Pn3m space group with completely disordered hydrogen bonding. Rietveld analysis of the data to obtain a bond length and bond angle for the DzO molecule is hampered by combined static and thermal disorder of both the deuterium and oxygen atom positions. However, simple models give results which are consistent with the D 2 0 molecule parameters for other ice structures.