Structure and optical non-linearity of PbO.2B(2)O(3)

The crystal structure of lead tetraborate, PbO.2B2O3, has been refined using single-crystal X-ray diffraction data (Mo Kα radiation, λ = 0.71069 Å). Crystal data at room temperature: Mr = 362.43, orthorhombic, P21 nm (C 7 2v ), a = 4.251 (2), b = 4.463 (3), c = 10.860 (3) Å, V = 206.04 Å3 with Z = 2, μ = 402.6 cm−1, Dx = 5.88 Mg m−3, F(000) = 316, final R = 0.022, wR = 0.025 over 655 reflections with I &gt; 2.5σ(I). Atomic coordinates are in general agreement with those previously reported for the isostructural compound, SrO.2B2O3, by Perloff &amp; Block [Acta Cryst. (1966), 20, 274–279]. All the borons are tetrahedrally coordinated with a three-dimensional network formed from O atoms that are common to either two or three tetrahedra. The tetrahedra show deformation because the B—O bonds involving the two-coordinated O atoms are much shorter than those involved with three-coordinated O atoms. The Pb atoms are situated in empty tunnels running along [010] left by the network of tetrahedra. The Pb atoms display a highly asymmetric distribution of Pb—O bonding, with the five shortest bonds covering the range 2.483 (5)–2.664 (5) Å, being all situated to one side of the Pb atom. Preliminary investigations of the non-linear optical behaviour of lead tetraborate are also discussed. The results indicate that doping with barium should lead to a new non-linear optical material that is both phase-matchable and has a high optical non-linearity.</jats:p

A re-investigation of the crystal structure of the perovskite PbZrO3 by X-ray and neutron diffraction

The crystal structure of the perovskite lead zirconate PbZrO3 has been redetermined using single-crystal X-ray diffraction (Mo Kα radiation, λ = 0.71069 Å). Single-crystal data at 100 K: space group. Pbam, a = 5.884 (1), b = 11.787 (3), c = 8.231 (2) Å, V = 570.85 Å3 with Z = 8, μ = 612.6 cm−1, D x = 8.06 Mg m−3, F(000) = 1168, final R = 0.033, wR = 0.061 over 555 reflections with I &gt; 2σ(I). An investigation is made into previous contradicting reports of a possible disorder in the O atoms and their origin by examining the crystal pseudo-symmetry. Information distinguishing an ordered and disordered oxygen substructure is shown to reside in weak l odd reflections. Because of their extremely low intensities these reflections have not contributed sufficiently in previous X-ray structure investigations and hence, to date, conclusive evidence differentiating between ordered and disordered models has not been possible. By collecting single-crystal X-ray data at low temperature and by using exceptionally long scans on selected hkl, l odd, reflections, a new accurate structure determination is presented and discussed, showing the true ordered oxygen positions. Because of the large difference in scattering factors between lead and oxygen when using X-rays, a neutron diffraction Rietveld refinement using polycrystalline samples (D1A instrument, ILL, λ = 1.90788 Å) is also reported as further evidence to support the true ordered oxygen structure revealed by the low-temperature X-ray analysis.</jats:p

Investigation into the crystal structure of the perovskite lead hafnate, PbHfO3

The room-temperature crystal structure of the perovskite lead hafnate PbHfO3 is investigated using both low-temperature single crystal X-ray diffraction (Mo Kα radiation, λ = 0.71069 Å) and polycrystalline neutron diffraction (D1A instrument, ILL, λ = 1.90788 Å). Single crystal X-ray data at 100 K: space group Pbam, a = 5.856 (1), b = 11.729 (3), c = 8.212 (2) Å, V = 564.04 Å3 with Z = 8, μ = 97.2 mm−1, F(000) = 1424, final R = 0.038, wR = 0.045 over 439 reflections with F &gt;1.4σ(F). Polycrystalline neutron data at 383 K: a = 5.8582 (3), b = 11.7224 (5), c = 8.2246 (3) Å, V = 564.80 Å3 with χ2 = 1.62. Although lead hafnate has been thought to be isostructural with lead zirconate, no complete structure determination has been reported, as crystal structure analysis in both these materials is not straightforward. One of the main difficulties encountered is the determination of the oxygen positions, as necessary information lies in extremely weak l = 2n + 1 X-ray reflections. To maximize the intensity of these reflections the X-ray data are collected at 100 K with unusually long scans, a procedure which had previously been found successful with lead zirconate. In order to establish that no phase transitions exist between room temperature and 100 K, and hence that the collected X-ray data are relevant to the room-temperature structure, birefringence measurements for both PbZrO3 and PbHfO3 are also reported.</jats:p