The pressure-induced crystal structure of lead sulfide (PbS) above 2.2 GPa has been studied with single-crystal x-ray diffraction in a diamond anvil cell at room temperature. It has been found to be twinned and of the TlI type (Cmcm, Z=4), in which the Pb atoms are surrounded by seven S atoms in a capped trigonal prism coordination. The twin laws in relation to the parent B1 (NaCl) type structure (Fm ̄3m, Z=4) at atmospheric pressure have been discussed.

The high-pressure behaviour of (NH₄)₂V₃O₈ with the fresnoite structure (P4bm, Z = 2) has been studied at room temperature with single-crystal X-ray diffraction in diamond anvil cells using laboratory and synchrotron facilities. At ambient conditions, the crystal structure is composed of layers of corner-sharing V⁵⁺O₄ tetrahedra and V⁴⁺O₅ square pyramids separated by layers of the NH₄ ⁺ cations. At about 3 GPa, there occurs a reversible first-order phase transition to a three-dimensional structure (P4/mbm, Z = 2) built of corner-sharing V⁵⁺O₅ trigonal bipyramids and V⁴⁺O₆ octahedra. The NH₄ ⁺ cations fill up the interstitial sites in the tunnels formed by the vanadate framework. Up to the phase transition, the a lattice parameter of the low-pressure polymorph does not change while the contraction perpendicular to the stacking of the V₃O₈ slabs accounts entirely for the bulk compressibility. Above the phase transition, the a lattice parameter slightly expands. The structural features of the high-pressure phase of (NH₄)₂V₃O₈ are compared to those of other vanadium oxides.

It is shown that it is possible to perform combined X-ray and neutron single-crystal studies in the same diamond anvil cell (DAC). A modified Merrill–Bassett DAC equipped with an inflatable membrane filled with He gas has been developed. It can be used on laboratory X-ray and synchrotron diffractometers as well as on neutron instruments. The data processing procedures and a joint structural refinement of the high-pressure synchrotron and neutron single-crystal data are presented and discussed for the first time.

High-pressure behaviours of isomorphous HoMn(2)O(5) and BiMn(2)O(5) (Pbam, Z = 4) were studied, using Raman spectroscopy and single-crystal x-ray diffraction, with the samples in diamond anvil cells pressurized to 10.09 GPa and 8.54 GPa, respectively, at room temperature. No phase transition was detected in either material. The two manganates are most compressible along the a axis with the lattice contractions along the b and c directions nearly the same. Their P-V data fitted with the second-order Birch-Murnaghan equation of state (B' = 4.0) give the bulk moduli B(0) = 138(2) GPa for BiMn(2)O(5) and B(0) = 173(3) GPa for HoMn(2)O(5). The difference in bulk moduli results from BiMn(2)O(5) being more compressible than HoMn(2)O(5) along the a axis. Our study shows that the Mn-O frameworks in isomorphous RMn(2)O(5) compounds (R = Bi and rare earth) are not rigid at high pressures, but rather adapt to the behaviour of the coordination spheres around the R atoms. Bond valence calculations indicate that the pressure-induced charge transfers between the two Mn sites have different characters for the two compounds.