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CaAl12O19, which can incorporate Ti as both Ti3+ and Ti4+ (charge coupled substitution with Mg2+), is one of the first minerals to condense from a gas of solar composition and is used as a ceramic. It is variously known as hibonite, calcium hexaluminate (CaO.6Al2O3), and CA6. The lattice parameters and unit cell volumes of Ti-substituted hibonite (P63/mmc) with the formulae CaAl11.8Ti3+0.2O19 and CaAl9.8Ti3+0.54Mg0.83Ti4+0.83O19 were determined as a function of temperature from ~ 10 to 275 K by neutron powder diffraction. The thermal expansion is highly anisotropic with the expansion in c a factor of ~ 5 greater than that in a. The change in a is approximately equal for the two compounds whereas the change in c is almost 50% larger for CaAl11.8Ti3+0.2O19. CaAl11.8Ti3+0.2O19 also exhibits negative thermal expansion between 10 and 70 K. The change in unit cell volume with temperature of both compositions is well described by a two term Einstein expression. The large change in c is consistent with substitution of Ti onto the M2 and M4 sites of the R-block structural unit. [ABSTRACT FROM AUTHOR]

The response of the trimethylammonium–iodinechloride and diiodide (TMA–ICl/I2) crystal structures have been examined under high pressure using neutron powder diffraction. TMA–ICl exhibits impressive pressure-driven electronic flexibility, where the N⋯I–Cl interactions progressively encompass all the distances represented in analogous structures recorded in the Cambridge Structural Database. Comparison with the TMA–I2 complex reveals that this flexibility is owed to the electronegativity of the chlorine atom which induces increased distortion of the iodine electron cloud. This structural flexibility may be influential in the future design of functional molecular materials. [ABSTRACT FROM AUTHOR]

The structure of the complex formed between 1,4-dioxane and iodine monochloride has been studied as a function of pressure using neutron powder diffraction. Initial compression was accompanied by a decrease in the O⋯I halogen bond length together with an increase in the intramolecular I–Cl bond length. Two phase transitions were observed at ∼2.8 and ∼4.5 GPa. The transient intermediate phase coexists with the ambient pressure phase during the initial phase transition and with the final high-pressure phase at the second phase transition, before its disappearance. The driving force for the first phase transition is a shearing motion of the complex causing a reduction in the dipolar interaction of two I–Cl moieties. The formation of the highest pressure phase is accompanied by a net reduction of 2 C–H⋯Cl hydrogen bonds per formula unit. From these changes we conclude that Cl⋯Cl halogen bonds are favoured over C–H⋯Cl hydrogen bonds at high pressures. [ABSTRACT FROM AUTHOR]

Experimental and computational studies of ammonium carbamate have been carried out, with the objective of studying the elastic anisotropy of the framework manifested in (i) the thermal expansion and (ii) the compressibility; furthermore, the relative thermodynamic stability of the two known polymorphs has been evaluated computationally. Using high-resolution neutron powder diffraction data, the crystal structure of α-ammonium carbamate (ND4αND2CO2) has been refined [space group Pbca, Z = 8, with a = 17.05189 (15), b = 6.43531 (7), c = 6.68093 (7) A ° and V = 733.126 (9) A ° 3 at 4.2 K] and the thermal expansivity of α-ammonium carbamate has been measured over the temperature range 4.2-180 K. The expansivity shows a high degree of anisotropy, with the b axis most expandable. The ab initio computational studies were carried out on the α- and β-polymorphs of ammonium carbamate using density functional theory. Fitting equations of state to the P(V) points of the simulations (run athermally) gave the following values: V0 = 744 (2) A³ and bulk modulus K0 = 16.5 (4) GPa for the α-polymorph, and V0 = 713.6 (5) A³ and K0 = 24.4 (4) GPa for the β-polymorph. The simulations show good agreement with the thermoelastic behaviour of α-ammonium carbamate. Both phases show a high-degree of anisotropy; in particular, α-ammonium carbamate shows unusual compressive behaviour, being determined to have negative linear compressibility (NLC) along its a axis above 5 GPa. The thermodynamically stable phase at ambient pressure is the α-polymorph, with a calculated enthalpy difference with respect to the β-polymorph of 0.399 kJ molβ1; a transition to the β-polymorph could occur at ~0.4 GPa. [ABSTRACT FROM AUTHOR]

Mg[Ti.sub.2][O.sub.5] (karrooite) exhibits cation exchange between the two non-equivalent octahedral M1 and M2 sites. The temperature dependence of Ti-Mg disorder has been determined using in situ time-of-flight powder neutron diffraction to establish the cation population of each site. The equilibrium Ti-Mg exchange commences above 700-800[degrees]C, and continues up to 1300[degrees]C. At ~1350[degrees]C Mg[Ti.sub.2][O.sub.5] appears to undergo a reversible, displacive phase transition, although the structure can still be refined in space group 63. This transition shows discontinuities in the degree of order, the c cell parameter, and in the M-O bond lengths, quadratic elongation, and bond angle variance for the M1 octahedron, which becomes pseudo-tetrahedral. By contrast, the M2 octahedron shows no significant change. An Arrhenius plot exhibits linear behavior from 750-1300[degrees]C, gives an exchange enthalpy of ordering of 33.6 kJ/mol, and a value of 10.7 kJ/mol for the entropy of disordering. A value of 5.92 [cm.sup.3]/mol is deduced for the [DELTA] V of the high-temperature reaction of geikielite (MgTi[O.sub.3]) + ruffle (Ti[O.sub.2]) to form fully disordered Mg[Ti.sub.2][O.sub.5] at an extrapolated temperature of 1860[degrees]C. Keywords: Crystal structure, high-temperature, neutron diffraction, order-disorder, oxides, karrooite, ruffle, geikielite

The lattice parameters extracted from Lebail analysis of neutron powder diffraction data collected between 2 and 300 K have been used to calculate the temperature evolution of the thermal expansion tensor for hopeite, [Zn.sub.3][(P[O.sub.4]).sub.2] x 2[H.sub.2]O, Pnma, Z = 4 with a 10.6065(4) [Angstrom] , b = 18.2977(4) [Angstrom] , c = 5.0257(2) [Angstrom] , at 275 K. The a lattice parameter shows a negative thermal expansion, the b lattice parameter appears to saturate at 275 K while the c lattice parameter has a more typical positive thermal expansion. At 275 K, the magnitudes of the thermal expansion coefficients are [[alpha].sub.a] = -1.1 (4) x [10.sup.-5] [K.sup.-1], [[alpha].sub.b] = 2.4(9) x [10.sup.-6] [K.sup.-1] and [[alpha].sub.c] = 3.6(2) x [10.sup.-5] [K.sup.-1]. Under the conditions of these experiments, hopeite begins to dehydrate to the dihydrate between 300 and 325 K, and between 480 and 500 K the monohydrate is formed. The thermal expansion of the dihydrate has been calculated between 335 and 480 and at 480 K the magnitudes of the thermal expansion coefficients are [[alpha].sub.a] = 1(2) x [10.sup.-5] [K.sup.-1], [[alpha].sub.b] = 4(1) x [10.sup.-6] [K.sup.-1], [[alpha].sub.c] = 4(2) x [10.sup.-5] [K.sup.-1], [[alpha].sub.[beta]]= 1(1) x [10.sup.-5] [K.sup.-1], and [[alpha].sub.V] = 2(2) x [10.sup.-5] [K.sup.-1]. The thermal expansion of hopeite is described in terms of its crystal structure and possible dehydration mechanisms for the a and modifications of hopeite are discussed. Keywords: Hopeite, thermal expansion, dehydration, neutron diffraction

Crystallographic distortions in the alternating aluminium and silicon tetrahedral framework of sodalite (Na8Al6Si6O24Cl2), and beryllium and silicon in helvine (Mn8Be6Si6O24S2), (framework designated SOD) are described in terms of a set of condensed normal mode amplitudes and phases derived from an ideal tetrahedron of a theoretical aristotype phase. For a sodalite-structured hettotype phase in space group $P{\overline 4} 3n$ , these normal modes transform as the irreducible representations A1, E(α) and T1(z) of point-group ${\overline 4} 3m$ , where to a good approximation A1 acts as a pure breathing mode, E(α) as a polyhedral distortive mode and T1(z) as a rigid unit rotation about the unique ${\overline 4}$ axis of the T-site under consideration. Parameterisation of the mode amplitudes in terms of low-order polynomials as a function of thermodynamic variable permits the crystal structure of sodalite-structured phases to be accurately interpolated at intermediate values of the thermodynamic variable. Published data for the high-temperature behaviour of sodalite have been re-analysed in terms of mode amplitudes which accurately reproduce the temperature dependence of the bond lengths, bond angles and the Al–O–Si inter-polyhedral angle. Full expressions for these derived structural parameters in terms of mode amplitudes and the lattice parameter are tabulated and agree with experimental results to within one estimated standard deviation of the experimental parameter. The potential for mode decomposition in lower symmetry SOD framework crystal structures is illustrated by deriving an aristotype structure for tugtupite (Na8Al2Be2Si8O24Cl2) at room temperature in space group $I{\overline 4}$. [ABSTRACT FROM AUTHOR]

Sol-gel methods are used for preparing a polar phase of NaNb[O.sub.3] and their properties are characterized by using high-resolution powder diffraction and [super 23]Na multiple-quantum (MQ) MAS NMR methods. The studies have shown that by monitoring the molten salt methods, the polar polymorph is eradicated and the pure Pbcm phase is synthesized.

A universal aristotype crystal structure for leucite/pollucite-structured phases (A2B2IIIC4IVO12, A2BIIC5IVO12) with space group I a 3 ¯ d has been determined by a non-linear optimisation of the fractional coordinates. The resultant T-site tetrahedron exhibits close to perfect 4 ¯ 3 m point group symmetry (quadratic elongation 1.0000124, tetrahedral angle variance 1.53 × 10−5 degrees2). For any distorted leucite/pollucite crystal structure, an associated aristotype phase can be derived from the optimised fractional coordinates, with the aristotype phase lattice parameter calculated from the volume of the hettotype T-site tetrahedron. Assuming the aristotype T-site tetrahedron to be ideal, the distortion of the hettotype tetrahedron from that of the aristotype is described as the linear sum of the amplitudes of an appropriate set of symmetry-adapted basis-vectors of the effective regular TO4 molecule consistent with the observed point group symmetry within the crystal structure. The methodology is illustrated by reference to the disordered T-site, leucite-structured phase K2MgSi5O12. [ABSTRACT FROM AUTHOR]

The synthesis of polycrystalline samples of B-site doped SrRu1−xMxO3 with x≤ 0.2 by solid state methods is described for a number of dopants (M = Mg, Mn, Fe, Co, Ni, Cu, or Zn) and the structures of these established using Synchrotron X-ray powder diffraction, and for SrRu0.8Cu0.2O3 high resolution neutron diffraction. With the exception of M = Cu, samples with x = 0.2 form an orthorhombic Pbnm type perovskite structure at room temperature and these exhibit a sequence of phase transitions upon heating associated with the gradual reduction in the cooperative tilting of the corner sharing octahedra. SrRu0.8Cu0.2O3 forms a unique monoclinic structure at low temperatures and this transforms to the cubic Pm3̅m structure via an I4/mcm intermediate upon heating. The magnetic and electronic properties of the samples have been studied. Doping results in a decrease in the Curie temperature and at x = 0.2 all the samples are insulators. This is a consequence of the partial oxidation of the Ru cation that narrows the Ru 4d bands coupled with the suppression of the itinerant nature of the Ru 4d electrons due to the random distribution of the dopant cations. Ru L3-edge X-ray absorption spectroscopy of the Cu doped samples reveal a gradual increase in the average Ru oxidation sate upon doping. Electrical resistivity measurements show that doping increases the resistivity of the samples, and the temperature dependence of the resistivities are consistent with Arrhenius-type charge conduction. [ABSTRACT FROM AUTHOR]

Refinement of time-of-flight high-resolution neutron powder diffraction data for lueshite (Na, Ca)(Nb, Ta, Ti)O3, the natural analogue of synthetic NaNbO3, demonstrates that lueshite at room temperature (298 K) adopts an orthorhombic structure with a 2ap × 2ap × 4ap superlattice described by space group Pmmn [#59: a = 7.8032(4) Å; b = 7.8193(4) Å; c = 15.6156(9) Å]. This structure is analogous to that of phase S of synthetic NaNbO3 observed at 753–783 K (480–510 °C). In common with synthetic NaNbO3, lueshite exhibits a series of phase transitions with decreasing temperature from a cubic (Pm3¯m) aristotype through tetragonal (P4/mbm) and orthorhombic (Cmcm) structures. However, the further sequence of phase transitions differs in that for lueshite the series terminates with the room temperature S (Pmmn) phase, and the R (Pmmn or Pnma) and P (Pbcm) phases of NaNbO3 are not observed. The appearance of the S phase in lueshite at a lower temperature, relative to that of NaNbO3, is attributable to the effects of solid solution of Ti, Ta and Ca in lueshite. [ABSTRACT FROM AUTHOR]

The structure of the complex formed between trimethylamine and iodine monochloride had been redetermined by a combination of neutron powder diffraction and X-ray single crystal diffraction techniques. The structure determined using NPD shows that the geometry of the trimethylamine moiety resembles that in trimethylammonium cations rather than in the free trimethylamine molecule. The structure shows the presence of weak C–D…Cl hydrogen bonds, The I–Cl bond length is in agreement with that derived from an analysis of the pKb values of a series of nitrogen bases and previously determined structures containing N…ICl fragments. [ABSTRACT FROM AUTHOR]

The phase behavior of the layered perovskite CsBi0.6La0.4Nb2O7, of the Dion-Jacobson family, has been studied by high-resolution powder neutron diffraction between the temperatures of 25 < T < 850 °C. At ambient temperature, this material adopts the polar space group P21am; this represents an example of hybrid improper ferroelectricity caused by the interaction of two distinct octahedral tilt modes. Within the limits of our data resolution, the thermal evolution of the crystal structure is consistent with a first-order transition between 700 and 750 °C, with both tilt modes vanishing simultaneously, leading to the aristotype space group P4/mmm. This apparent "avalanche transition" behavior resembles that seen in the related Aurivillius phase SrBi2Nb2O9. [ABSTRACT FROM AUTHOR]

A combination of time-of-flight neutron diffraction and synchrotron X-ray powder diffraction has been used to investigate the thermal expansion of a synthetic deuterated natrojarosite from 80 to 440 K under ambient-pressure conditions. The variation in unit-cell volume for monoclinic jarosite over this temperature range can be well represented by an Einstein expression of the form V = 515.308 (5) + 8.5 (4)/{exp[319 (4)/ T] − 1}. Analysis of the behaviour of the polyhedra and hydrogen-bond network suggests that the strength of the hydrogen bonds connected to the sulfate tetrahedra is instrumental in determining the expansion of the structure, which manifests primarily in the c-axis direction. [ABSTRACT FROM AUTHOR]