Stefan T. Norberg

Neutron total scattering study of the delta and beta phases of Bi2O3

The highly disordered structure of the delta phase of Bi2O3, which possesses the highest known oxide-ion conductivity, has been studied using neutron powder diffraction. A detailed analysis of data collected at 1033(3) K using Rietveld refinement indicates that the time-averaged structure of delta-Bi2O3 can be described using the accepted model of a disordered, anion-deficient fluorite structure in space group Fm3m. However, reverse Monte Carlo modelling of the total (Bragg plus diffuse) scattering demonstrates that the local anion environment around the Bi3+ resembles the distorted square pyramidal arrangement found within the stable alpha and metastable beta phases at ambient temperature, which is characteristic of the cations 6s2 lone-pair configuration. Similarities between the structures of the highly disordered delta phase and the ambient temperature metastable beta phase are used to support this assignment and assess the validity of previous structural models based on short-range ordering of vacancies within the cubic lattice of delta-Bi2O3.

New phosphate langbeinites, K2MTi(PO4)3 (M = Er, Yb or Y), and an alternative description of the langbeinite framework.

Three new potassium rare-earth/titanium phosphate structures, K(2)ErTi(PO(4))(3) (KErTP), K(2)YbTi(PO(4))(3) (KYbTP) and K(2)YTi(PO(4))(3) (KYTP), are presented, all of which are characterized by single-crystal X-ray diffraction studies. In addition, a fourth structure, K(2)CrTi(PO(4))(3) (KCrTP), has been reinvestigated. All structures are isostructural to the langbeinite-type structure and result from changes made to the growth constituents in high-temperature flux-growth experiments intended to give structurally modified potassium titanyl phosphate (KTP). The two crystallographically independent octahedra sites (site symmetry 3) have a mixed Ti/M (M = Er, Yb, Y or Cr) population, although the rare-earth metals favour one site while chromium favours the other. An alternative approach for the description of the channels and cation cages in langbeinite and related structures is given using [M(5)X(6)O(39)] units. The framework of langbeinite is compared with that of nasicon using these alternative building units. All of the investigated structures crystallize in space group P2(1)3 with Z = 4; a = 10.1053 (2) A, R = 0.023 (KErTP); a = 10.0939 (8) A, R = 0.022 (KYbTP); a = 10.1318 (6) A, R = 0.047 (KYTP); a = 9.8001 (2) A, R = 0.016 (KCrTP).

K-site splitting in KTiOPO4 at room temperature

The room-temperature structure of potassium titanyl phosphate (KTiOPO4, KTP) with Pna2(1) symmetry has been studied by means of synchrotron radiation. Each of the two crystallographically unique K1 and K2 cations is split over two sites that are shifted along the c direction by 0.287 (13) and 0.255 (13) A for the K1a/b and K2a/b pairs, respectively. The refined populations of the minor K1b and K2b sites are 0.102 (12) and 0.132 (17), respectively. It is shown that accurate high-resolution synchrotron data (Rmerged = 0.015 for 25 010 reflections, 9456 unique, sintheta/lambda limit > 1.0) are required for the determination of a reliable structure model.

The proton conducting electrolyte BaTi0.5In0.5O2.75: determination of the deuteron site and its local environment

Deuterated BaTi0.5In0.5O2.75 has been studied with neutron total (Bragg plus diffuse) scattering data, using both the Rietveld refinement method and the reverse Monte Carlo (RMC) modelling technique, to investigate the preferred proton site and its local structural environment. The Rietveld analysis shows an excellent fit between experimental data and a long-range cubic description of the BaTi0.5In0.5O2.53(OD)0.44 perovskite structure containing a statistical distribution of Ti and In ions at the centre of regular (Ti/In)O6 octahedra. However, an RMC analysis of the data reveals substantial local structural features that reflect limitations of the Rietveld method for studies of this type. The Ti-O and In-O pair distribution functions given by the RMC analysis are markedly different from each other, with average Ti-O and In-O bond distances of 2.035 Å and 2.159 Å, respectively. The InO6 octahedra are regular in shape whereas the TiO6 octahedra are distorted. The average O-D bond distance is roughly 0.96 Å, and the preferred deuteron sites have a second nearest oxygen distance of 2.13 Å, which confirms localized tilting of the deuteron and indicates a substantial degree of hydrogen bonding. The impact of octahedral distortion and hydrogen bonding on the proton conduction mechanism is discussed.

Two new phosphate langbeinites, Rb2YbTi(PO4)3 and Rb2Yb0.32Ti1.68(PO4)3, investigated at 293 and 150 K

The rubidium ytterbium titanium phosphates Rb2YbTi(PO4)3, (I), and Rb2Yb(0.32)Ti(1.68)(PO4)3, (II), have been structurally characterized from X-ray data collected at both 293 and 150 K. Compound (II) is blue owing to the presence of mixed-valence titanium (41% Ti3+ + 59% Ti4+). Both (I) and (II) belong to the langbeinite structure type, with mixed Yb/Ti populations in the two crystallographically independent octahedral sites (of symmetry 3). Ytterbium favours one of these sites, where about two-thirds of the Yb atoms are found. The O-atom displacement parameters are large in both compounds at both temperatures.

Lead(II) oxydiacetate hydrate

The coordination polyhedron around the Pb atom in the title compound, poly[aqualead(II)-mu(5)-oxydiacetato], [Pb(mu(5)-C4H4O5)(H2O)](n), is a distorted square antiprism formed by three O atoms from one oxydiacetate ligand, four O atoms from four other identical ligands and one water O atom, resulting in a three-dimensional network structure. The almost planar oxydiacetate ligand is disordered over two positions. The Pb lone-pair 6s(2) electrons show stereochemical activity in the coordination sphere.

Structural and vibrational properties of silyl (SiH3(-)) anions in KSiH3 and RbSiH3: new insight into Si-H interactions.

The alkali metal silyl hydrides ASiH3 (A = K, Rb) and their deuteride analogues were prepared from the Zintl phases ASi. The crystal structures of ASiH3 consist of metal cations and pyramidal SiH3(-) ions. At room temperature SiH3(-) moieties are randomly oriented (α modifications). At temperatures below 200 K ASiH3 exist as ordered low-temperature (β) modifications. Structural and vibrational properties of SiH3(-) in ASiH3 were characterized by a combination of neutron total scattering experiments, infrared and Raman spectroscopy, as well as density functional theory calculations. In disordered α-ASiH3 SiH3(-) ions relate closely to freely rotating moieties with C3v symmetry (Si-H bond length = 1.52 Å; H-Si-H angle 92.2 °). Observed stretches and bends are at 1909/1903 cm(-1) (ν1, A1), 1883/1872 cm(-1) (ν3, E), 988/986 cm(-1) (ν4, E), and 897/894 cm(-1) (ν2, A1) for A = K/Rb. In ordered β-ASiH3 silyl anions are slightly distorted with respect to their ideal C3v symmetry. Compared to α-ASiH3 the molar volume is by about 15% smaller and the Si-H stretching force constant is reduced by 4%. These peculiarities are attributed to reorientational dynamics of SiH3(-) anions in α-ASiH3. Si-H stretching force constants for SiH3(-) moieties in various environments fall in a range from 1.9 to 2.05 N cm(-1). These values are considerably smaller compared to silane, SiH4 (2.77 N cm(-1)). The reason for the drastic reduction of bond strength in SiH3(-) remains to be explored.

Phase transitions in KTP isostructures: correlation between structure and Tc in germanium-doped RbTiOPO4

Crystals of germanium-doped rubidium titanyl phosphate, Rb(2)(Ti)(Ge(0.121)Ti(0.879))O(2)(PO(4))(2) (GeRTP#1) and Rb(2)(Ge(0.125)Ti(0.875))(Ge(0.225)Ti(0.775))O(2)(PO(4))(2) (GeRTP#2), have been structurally characterized from X-ray diffraction data at room temperature. In addition, a third structure, Rb(2)(TiO)(2)(PO(4))(2) (RTP), has been reinvestigated. The exchange of titanium for germanium results in a less distorted octahedral coordination around the two crystallographically independent titanium sites. Additionally, rubidium split-cation positions have been found in these doped RTP crystals. Dielectric measurements show that the phase-transition temperature, T(c), decreases with increasing germanium concentration, and a direct correlation between the room-temperature split of the rubidium cations and T(c) has been discovered. General trends regarding the relationship between the room-temperature structures of KTP-like compounds and their T(c) values are discussed.

A neutron total scattering study of local coordination in KTiOPO4 from room temperature to 900??C

Neutron total (Bragg plus diffuse) scattering data have been analysed by means of bond valence constrained reverse Monte Carlo modelling, using RMCProfile software, in order to simultaneously probe the natures of the short- and long-range structural correlations in potassium titanyl phosphate, KTiOPO(4) (KTP). The diffraction data were collected at room temperature, 300, 500, 700 and 900 °C, and the resulting partial radial distribution functions, g(jk)(r), were thoroughly investigated in order to obtain information about the structural aspects that cause reduced second harmonic generation (SHG) with increasing temperature in KTP. The -TiO(6)-PO(4)- lattice shows very little rigid unit motion up to 900 °C and the PO(4) tetrahedra were likewise unchanged. However, subtle changes regarding oxygen disordering around the TiO(6) octahedra, i.e. the number of short titanyl bonds, and K(+) cation displacement along the c axis are carefully mapped out. The latter has a direct detrimental effect on the SHG response, whilst the anisotropic Ti-O bond distribution seems to be a prerequisite for the SHG effect.

Cation movement and phase transitions in KTP isostructures; X-ray study of sodium-doped KTP at 10.5 K

An accurate structure model of sodium-doped potassium titanyl phosphate, (Na(0.114)K(0.886))K(TiO)(2)(PO(4))(2), has been determined at 10.5 K by single-crystal X-ray diffraction. In addition to the low-temperature data, X-ray intensities have been collected at room temperature. When the temperature was decreased from room temperature to 10.5 K, both potassium cations moved 0.033 (2) A along the c-axis, i.e. in the polar direction within the rigid Ti-O-P network. This alkaline metal ion displacement can be related to the Abrahams-Jamieson-Kurtz T(C) criteria for oxygen framework ferroelectrics. Potassium titanyl phosphate (KTP) is a well known material for second harmonic generation (SHG), and the influence of sodium dopant on the TiO(6) octahedral geometry and SHG is discussed. The material studied crystallizes in the space group Pna2(1) with Z = 4, a = 12.7919 (5), b = 6.3798 (4), c = 10.5880 (7) A, V = 864.08 (9) A(3), T = 10.5 (3) K and R = 0.023.