Ronald I. Smith

The Polaris powder diffractometer at ISIS

Abstract The Polaris instrument at the ISIS spallation neutron source operates as a medium resolution powder diffractometer. The high incident neutron flux enables datasets to be collected with comparatively short counting times or from extremely small sample volumes. Examples of recent experiments performed on Polaris, which exploit the high count rate and the particular advantages offered by fixed geometry diffraction measurements performed on a pulsed neutron source, are presented.

Structure and electrical properties of oxygen-deficient hexagonal BaTiO3

Rietveld refinements using neutron diffraction data have been used to determine the crystal structure of a series of oxygen-deficient barium titanate powders, BaTi IV 1–x Ti III x O 3–x/2 (0<x<0.30). The powders were prepared by reduction of stoichiometric, tetragonal BaTiO 3 in a vacuum furnace at temperatures above 1300°C and under an oxygen partial pressure of 0.1 mbar. The 6H-BaTiO 3 hexagonal perovskite structure is retained throughout and partial reduction of Ti IV to Ti III is accompanied by the formation of O(1) oxygen vacancies in the h-BaO 3 layers which separate pairs of occupied face-sharing octahedra, Ti 2 O 9 . There is no evidence of O(2) vacancies associated with corner sharing TiO 6 octahedra. The Ti(2)-Ti(2) separation within face sharing dimers increases from 2.690(4) A for x=0 to 2.7469(30) A for x=0.30. BaTiO 2.85 is a band-gap semiconductor at 300 K with a resistivity of ca. 1 Ω cm and activation energy 0.16 eV. A switch in conduction mechanism to variable range hopping (VRH) of electrons between Ti III and Ti IV ions occurs on cooling below 240 K.

In-situ study of the R3̅ to R3̅c phase transition in the ilmenite-hematite solid solution using time-of-flight neutron powder diffraction

Abstract The R3̅ - to R3̅c cation ordering phase transition in the ilmenite (FeTiO3) - hematite (Fe2O3) solid solution has been investigated using in-situ time-of-flight neutron powder diffraction. Four synthetic samples of the solid solution containing 70, 80, 90, and 100% FeTiO3 (ilm70, ilm80, ilm90, and ilm100, respectively) were heated under vacuum to a maximum of 1350 °C. Powder diffraction patterns were collected at several temperatures on heating and cooling, with a Rietveld refinement performed in each case. Samples ilm80, ilm90, and ilm100 were fully ordered after quenching from the synthesis temperature to room temperature. Sample ilm70 had a higher degree of quenched in disorder, which is the result of chemical heterogeneities produced during quenching and subsequent heating in the neutron experiments. The degree of order in all samples decreased smoothly at high temperatures, with second-order transitions to the R3̅c phase being observed at 1000, 1175, and 1325 °C in ilm70, ilm80, and ilm90, respectively. The transition temperature in ilm100 was higher than the maximum temperature reached in the neutron experiments, and is estimated as ~1400 °C. The character of the transition is typical of that predicted by three-dimensional Ising models and appears to become more first-order in character with increasing Ti-content. The temperature-dependence of the cell parameters reveals that components of the spontaneous strain tensor, e11 and e33, are negative and positive, respectively. Little volume strain is associated with long-range ordering. A small negative volume strain due to short-range ordering within the R3̅c phase is identified. The variations in cell parameters and cation-cation distances can be understood in terms of the competing effects of long- and short-range ordering as a function of temperature and composition.

Putting pressure on elusive polymorphs and solvates

The reproducible crystallisation of elusive polymorphs and solvates of molecular compounds at high pressure has been demonstrated through studies on maleic acid, malonamide, and paracetamol. These high-pressure methods can be scaled-up to produce ‘bulk’ quantities of metastable forms that can be recovered to ambient pressure for subsequent seeding experiments. This has been demonstrated for paracetamol form II and paracetamol monohydrate. The studies also show that the particular solid form can be tuned by both pressure and concentration.

Novel apparatus for the in situ study of hydrothermal crystallizations using time-resolved neutron diffraction

The design and use of a pressure cell for the in situ study of hydrothermal crystallizations using time-resolved neutron diffraction are described. This novel apparatus allows the kinetics and mechanism of the crystallization of complex inorganic solids, such as zeolites and other microporous materials, to be investigated under laboratory conditions. Reactions can be studied over a wide range of temperatures (25–250 °C) under conditions of autogenous pressure (1–22 bar). The use of the cell is demonstrated by a study of the hydrothermal crystallization of the zeolite sodium hydroxosodalite with time-of-flight neutron diffraction data collected using the Polaris diffractometer of the United Kingdom’s pulsed spallation neutron source, ISIS.

Crystal structure of the Li+ ion-conducting phases, Li0.5 – 3x Re0.5 +x TiO3: RE = Pr, Nd; x≈ 0.05

The crystal structure of the C polymorph of the solid solutions, Li0.5 – 3x RE0.5 +x TiO3(RE = Pr, Nd) determined from powder neutron diffraction using Rietveld refinement, is an ordered perovskite of the GdFeO3 type. The structures of both phases are similar: the A sites contain (0.5 +x) RE cations and (0.5 – 3x) off-centre lithium ions, with 2x sites vacant. Orthorhombic unit cell, RE = Nd: a= 5.43661(7)A, b= 7.6647(1)A, c= 5.39752(7)A; RE = Pr: a= 5.43793(5)A, b= 7.66828(8)A, c= 5.40769(5)A; space group Pnma(no. 62). Conduction occurs by a Li+ vacancy mechanism through interconnecting A sites, avoiding those sites that are occupied by the RE cations.

Composition and temperature dependence of cation ordering in Ni-Mg olivine solid solutions: a time-of-flight neutron powder diffraction and EXAFS study

Abstract The non-convergent ordering of Mg and Ni over the M1 and M2 sites of synthetic olivines has been studied using “time of night” neutron powder diffraction and X-ray absorption spectroscopy (EXAFS). The compositional dependence of order/disorder at room temperature was established for solid solutions of general formula (Mg1-xNix)2SiO4. where X= 0.15, 0.2, 0.25,0.3, 0.5, and 0.8 atoms Ni (XNi; i.e.. mole fraction of Ni-olivine end-member). Ni orders into M1 with KD = (Ni/Mg in M1)/ (Ni/Mg in M2) reaching a maximum of 9.5 at a composition of Mg1.0Ni0.4SiO4. The temperature dependence of order/disorder at up to 1100 °C was determined for two samples (XNi = 0.2 and 0.5). Between about 600 and 750 °C the samples show an increase in order due to kinetic effects, while above 750 °C the samples show a progressive decrease in order and describe an equilibrium disordering path. Equilibrium data define a Ni-Mg, M1-M2 intersite exchange energy of 21.5 ± 1.9 kJ/ mol. On cooling, the blocking temperature for cation exchange is about 800 °C. The kinetics of disordering behavior were analyzed using a Ginzburg-Landau model giving activation energies for Mg-Ni exchange between M1 and M2 for samples of composition Mg1.6Ni0.4SiO4 and Mg1.0Ni1.0SiO4 of 145 ± 5 and 160 ± 5 kJ/mol. respectively. The model also shows that the characteristic time scale for re-equilibration of M1-M2 order decreases from around 2.5 s at 1000° to 0.03 s at 1300 °C. This points to the inapplicability of intracrystalline Ni-Mg partitioning for obtaining geothermometry and geospeedometry information for magmatic conditions. Ni X-edge EXAFS data show that samples with XNi = 0.15.0.2,0.25 and 0.3 all show Ni clustering on adjacent M1 sites, indicating the presence of domains of Ni-rich and Mg-rich regions on a nanolength scale of < 10 A. These “precipitates” are at least an order of magnitude too small to be detectable by neutron powder diffraction. We suggest that the elastic strain at the interfaces between the Ni-rich precipitates and the Mg-rich matrix is responsible for the plateau or possible maximum in the b unit-cell parameter as a function of composition across the solid solution, which is observed at a composition of Mg1.6Ni0.4SiO4 at room temperature. Comparison of our data with earlier studies at high P and T on Mg1.0Ni1.0SiO4 olivine suggests that the effect of P is to increase the degree of order of Ni into M1 and to slow down the kinetics of intersite exchange with a ΔVdisorder of 0.039 J/bar.

Variable temperature study of the crystal and magnetic structures of the giant magnetoresistant materials LMnAsO (L =La, Nd)

A variable temperature neutron and synchrotron diffraction study have been performed on the giant magnetoresistant oxypnictides LnMnAsO (Ln = La, Nd). The low temperature magnetic structures have been studied and results show a spin reorientation of the Mn2+ spins below TN (Nd) for NdMnAsO. The Mn2+ spins rotate from alignment along c to alignment into the basal plane and the Mn2+ and Nd3+ moments refine to 3.54(4) \mu B and 1.93(4) \mu B respectively at 2 K. In contrast there is no change in magnetic structure with temperature for LaMnAsO. There is no evidence of a structural transition down to 2 K, however discontinuities in the cell volume, Ln-O and Mn-As bond lengths are detected at \sim 150 K for both materials. This temperature coincides with the electronic transition previously reported and suggests a coupling between electronic and lattice degrees of freedom.

The nuclear and magnetic structure of “white rust”—Fe(OH0.86D0.14)2

Abstract The structure of Fe(OH0.86,OD0.14)2 was refined by applying the Rietveld technique to neutron powder diffraction data collected at 300, 150, and 10 K. The nuclear structure, of the CdI2 type (P3̅ m1) and isostructural with Mg, Ca, Mn, Ni, and β-Co(OH)2, is maintained over the range of temperatures studied. At 10 K, the magnetic structure is ordered antiferromagnetically, with the magnetic moments aligned in the basal plane. The refined Fe2+ magnetic moment magnitude is 3.50(4) μB. This magnetic structure (space group P2c1̅) is unique amongst those encountered in the transition metal dihydroxides, for which the moment is either directed along the c axis [b-Co(OH)2 and Ni(OH)2], or at least has a considerable component along this direction [Mn(OH)2]. The dependence of the strength of potential hydrogen bonds with temperature is discussed.

Powder neutron diffraction studies of three low thermal expansion phases in the NZP family : K0.5Nb0.5Ti1.5(PO4)3, Ba0.5Ti2(PO4)3 and Ca0.25Sr0.25Zr2(PO4)3

We present the results of variable temperature neutron powder diffraction studies of three low thermal expansion materials belonging to the NZP family; K 0.5 Nb 0.5 Ti 1.5 (PO 4 ) 3 , Ba 0.5 Ti 2 (PO 4 ) 3 and Ca 0.25 Sr 0.25 Zr 2 (PO 4 ) 3 . Each of these structures has half occupancy of the crystallographic site for the large cation (MI = K, Ba or Ca/Sr), but differ in that the vacancies on this site are disordered in the former two (space group \[ R\overline 3 c \] ) but ordered in the latter (space group \[ R\overline 3 \] ). The thermal expansion behaviour is quantified from parameters obtained by Rietveld structure refinement, and is described in terms of rotations and distortions of linked MO 6 (M = Nb, Ti, Zr) and PO 4 polyhedra. The driving force for the anisotropic low thermal expansion behaviour is found to be in the thermal expansivity of the MI–O bonds. This behaviour is intricately linked to the order/disorder behaviour of the MI sites, as shown by a comparison of the three title structures, together with those of NaTi 2 (PO 4 ) 3 in which the MI sites are completely filled, and Sr 0.5 Ti 2 (PO 4 ) 3 , in which the MI sites are half-occupied in an ordered manner.