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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]

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

High resolution neutron powder diffraction has been used to determine lattice parameter variations of deuterated lawsonite across the Cmcm [left and right arrow] Pmcn and Pmcn [left and right arrow] [P2.sub.1]cn phase transitions, in the temperature interval 1.6-505 K. The variations are reversible through heating and cooling cycles. Strain analysis, based on a displacive model of the transitions with saturation temperatures to describe the temperature-independent behavior as T [right arrow] 0 K, shows that the data are consistent with a tricritical transition at 271 [+ or -] 2 K and a second-order transition at 155 [+ or -] 1 K. Comparisons with strains from published dilatation data for a natural (hydrogenated) sample highlight aspects of the transitions that are most dependent on the behavior of protons in the structure. Replacing H by D causes the low temperature transition point to be increased by ~27 K and an anomaly in the strain evolution of the Pmcn transition to increase from ~225 to ~250 K. The transition point of the high-temperature transition remains the same within [+ or -] ~2 K. We conclude that proton ordering and displacive contributions are both important in the 271 K transition, though with the displacive component providing the initial symmetry-breaking mechanism. Structural changes that are more dependent on the behavior of H or D become important ~20-50 K below this. Strains for the Pmcn [left and right arrow] [P2.sub.1]cn transition are consistent with a transition driven by a single order parameter. The data are used to determine values for strain/order parameter coupling coefficients for calculation of elastic anomalies due to the phase transitions.

The proton positions in leucophoenicite, ideally [Mn.sub.7][Si.sub.3][O.sub.12][(OH).sub.2], have been determined by neutron powder diffraction under ambient conditions on a natural sample from Franklin, New Jersey. Refinement in the P[2.sub.1]/a space group gave [R.sub.p] = 2.0% (w[R.sub.p] = 2.1%), [chi square] = 4.04 for 110 refined parameters. The two non-equivalent protons form a pair of hydroxyl groups and make hydrogen bonds to the same O7 atom of the Si1 tetrahedron at distances of 1.99(1) and 2.09(1) [Angstrom]. The H.... H distance in leucophoenicite is 2.16(1) [Angstrom], which is more than twice the Van der Waal's radius of H (>2 [Angstrom]) and so no proton positional disorder is expected in leucophoenicite. A comparison of the H environments is made between leucophoenicite and Phase B, Superhydrous B, and Phase A.

Non-convergent cation ordering in the [([Fe.sub.3][O.sub.4]).sub.1-x] [(Mg[Al.sub.2][O.sub.4]).sub.x] solid solution was investigated using in-situ time-of-flight neutron powder diffraction. The approach to equilibrium in a sample with x = 0.75 was observed at 923 K by performing in-situ structure refinements at intervals of 5 min, and the ordering behavior was traced through the time-dependence of the lattice parameter, the cation-oxygen bond lengths, and the cation-site scattering lengths. The data are consistent with a two-stage kinetic process in which relatively rapid exchange of [Fe.sup.3+] with Mg and [Fe.sup.2+] between tetrahedral and octahedral sites was followed by slower exchange of Mg with Al. The [Fe.sup.3+] cations are shown to order onto tetrahedral sites, contrary to the predictions of thermodynamic models for the solid solution. Equilibrium cation distributions in samples with x = 0.4, 0.5, and 0.75 were determined between 1073 and 1273 K by combining the structure refinements with measurements of saturation magnetization in quenched material. The adopted cation distribution was a compromise between the normal and inverse distributions observed in the end-members. The conflict of site preference between these two ordering schemes resulted in a simple behavior in the middle of the solid solution in which Al occurred predominantly on octahedral sites and the Mg, [Fe.sup.2+], and [Fe.sup.3+] cations were distributed randomly over the remaining sites. The ordering scheme adopted away from the middle of the solid solution was obtained by combining this pseudo-random scheme with a tetrahedral site preference of [Fe.sup.3+] relative to Mg and [Fe.sup.2+]. Comparison of the structure refinements with published thermodynamic models demonstrates that quantitative agreement was poor between calculated and observed behavior in this system. Qualitative agreement with the O'Neill-Navrotsky thermodynamic model was found near the middle of the solid solution.