High-pressure single-crystal neutron diffraction (to 20 kbar) using a pulsed source: preliminary investigation of Tl₃PSe₄

A new technique is described for performing high-pressure single-crystal neutron diffraction [up to 20 kbar (2 GPa) at room temperature], using a BeCu pressure cell, an area detector and the Los Alamos National Laboratory pulsed neutron source. Success of this method depends on the increase in information available with a multi-wavelength pulse neutron source, a novel orientation of a cylindrically symmetric pressure cell with its axis coincident with the neutron beam and a specific crystal orientation within the pressure cell. Bragg scattering from the pressure cell is avoided and background for a given 2θ is constant. For a crystal of orthorhombic or higher symmetry oriented with the incident beam passing midway between the major lattice vectors, it will be possible to refine a complete three-dimensional structure with data collected from only one pressure loading. Preliminary investigations of Tl₃PSe₄ lattice parameters (space group Pcmn) at 15(1)kbar yielded linear compressibilities (× 1000 in kbar⁻¹) of K_a = 1.05(8), K_b = 1.50(10), K_c = 1.20(8). The anisotropic compressibility is explained by examination of the ambient-pressure room-temperature structure.