Ad Fortes

Negative linear compressibility and massive anisotropic thermal expansion in methanol monohydrate

At low temperatures, a simple molecular crystal can shrink along one axis when heated and expand along it when compressed. The vast majority of materials shrink in all directions when hydrostatically compressed; exceptions include certain metallic or polymer foam structures, which may exhibit negative linear compressibility (NLC) (that is, they expand in one or more directions under hydrostatic compression). Materials that exhibit this property at the molecular level—crystalline solids with intrinsic NLC—are extremely uncommon. With the use of neutron powder diffraction, we have discovered and characterized both NLC and extremely anisotropic thermal expansion, including negative thermal expansion (NTE) along the NLC axis, in a simple molecular crystal (the deuterated 1:1 compound of methanol and water). Apically linked rhombuses, which are formed by the bridging of hydroxyl-water chains with methyl groups, extend along the axis of NLC/NTE and lead to the observed behavior.

The high-pressure phase diagram of ammonia dihydrate

We have investigated the P–T phase diagram of ammonia dihydrate (ADH), ND3·2D2O, using powder neutron diffraction methods over the range 0–9 GPa, 170–300 K. In addition to the ambient pressure phase, ADH I, we have identified three high-pressure phases, ADH II, III, and IV, each of which has been reproduced in at least three separate experiments. Another, apparently body-centred-cubic, phase of ADH has been observed on a single occasion above 6 GPa at 170 K. The existence of a dehydration boundary has been confirmed where, upon compression or warming, ADH IV decomposes to a high-pressure ice phase (ice VII or VIII) and a high-pressure phase of ammonia monohydrate (AMH V or VI).

Ab initio simulation of ammonia monohydrate (NH3⋅H2O) and ammonium hydroxide (NH4OH)

Ab initio pseudopotential plane-wave calculations on the ambient pressure monohydrate of ammonia were carried out. Significant differences in the pressure dependence of covalent O-H and N-H bond lengths from ice VIII and solid ammonia were observed. Simulated structure spontaneously transforms to an ionic solid (NH4OH) at ∼5 GPa. It was found that the enthalpy difference between AMH I, and ammonium hydroxide at zero Kelvin is ∼15 KJ mol-1.

The thermoelastic properties of MgSO4·7D2O (epsomite) from powder neutron diffraction and ab initio calculation

time-of-flight powder neutron diffraction has been used to measure the molar volume of MgSO4.7D(2)O (i) from 1.8 - 300 K at ambient pressure, (ii) from 50 - 290 K at 1.4, 3.0, and 4.5 kbar, (iii) from 0 - 5.5 kbar at 290 K, and (iv) from 0 - 4.5 kbar at 50 K. The data have allowed us to determine the temperature dependence of the incompressibility, (partial derivative K/partial derivative T)(p), (thermodynamically equivalent to the pressure dependence of the thermal expansion, (partial derivative alpha/partial derivative P)(T)) of epsomite throughout its stability field. We observed that the a-axis exhibits negative thermal expansion, alpha(a), from 30 - 250 K at room pressure, turning positive above 250 K and being zero below 30 K. However, each of the crystallographic axes exhibits a sharp change in (partial derivative alpha/partial derivative T) at similar to 125 K, and this appears to correspond to significant changes in the axial incompressibilities with the a- and c-axes softening, and the b-axis stiffening considerably below similar to 125 K. Our thermoelastic results are in agreement with ab initio calculations at zero Kelvin; however the calculations offer no obvious insight into the mechanism responsible for the change in behaviour at low temperature.

Crystal structures and thermal expansion of α-MgSO4 and β-MgSO4 from 4.2 to 300 K by neutron powder diffraction

Detailed neutron powder diffraction measurements have been carried out on two polymorphs of anhydrous magnesium sulfate, alpha-MgSO4 and beta-MgSO4. alpha-MgSO4 is orthorhombic, space group Cmcm (Z = 4); at 4.2 K the unit-cell dimensions are a = 5.16863 (3), b = 7.86781 (5), c = 6.46674 (5) angstrom, V = 262.975 (2) angstrom(3) [rho(calc) = 3040.16 (2) kg m(-3)], and at 300 K, a = 5.17471 (3), b = 7.87563 (5), c = 6.49517 (5) angstrom, V = 264.705 (2) angstrom(3) [rho(calc) = 3020.29 (2) kg m(-3)]. The axial and volumetric thermal expansion coefficients are positive at all temperatures and exhibit no unusual behaviour. Structures were refined at 4.2 and 300 K to R-P < 3%; less precise structural parameters were determined during warming from 4.2 to 300 K. beta-MgSO4 has a more complex structure, crystallizing in space group Pbnm (Z = 4); the unit-cell dimensions at 4.2 K are a = 4.73431 (8), b = 8.58170 (12), c = 6.67266 (11) angstrom, V = 271.100 (5) angstrom(3) [rho(calc) = 2949.04 (5) kg m(-3)], and at 300 K, a = 4.74598 (7), b = 8.58310 (10), c = 6.70933 (10) angstrom, V = 273.306 (4) angstrom(3) [rho(calc) = 2925.42 (4) kg m(-3)]. The thermal expansivities of the a and c axes, and the volumetric thermal expansion coefficient, are positive at all temperatures and normally behaved. However, the thermal expansion of the b axis is both very small and negative below similar to 125 K. Structural and thermal motion parameters for beta-MgSO4 as a function of temperature are also reported.

No evidence for large-scale proton ordering in Antarctic ice from powder neutron diffraction

We have examined a sample of 3000 year old Antarctic ice, collected at the Kohnen Station, by time-of-flight powder neutron diffraction to test the hypothesis of Fukazawa et al. [e.g., Ann. Glaciol. 31, 247 (2000)] that such ice may be partially proton ordered. Great care was taken to keep our sample below the proposed ordering temperature (237 K) at all times, but we did not observe any evidence of proton ordering.

Hydrogen bonding in solid ammonia from ab initio calculations

We have carried out ab initio simulations on the ambient pressure phase I of solid ammonia, and on the high-pressure phase IV. Our plane-wave pseudopotential calculations yield very good agreement with existing structural data, lattice energies, and equations of state. We have also studied the tendency toward symmetrization of the hydrogen bonds at high pressures and find that, unlike pure ice, this process should not occur at experimentally achievable pressures, i.e., <300 GPa. Moreover, our results show that ammonia IV does not contain a bifurcated hydrogen bond, as has previously been suggested.

The structure, ordering and equation of state of ammonia dihydrate (nh3 · 2h2o)

Abstract We present the first ab initio simulations of the low-pressure phase of ammonia dihydrate (NH3 · 2H2O), ADH I, a likely constituent of many volatile-rich solid bodies in the outer Solar System (e.g., Saturn’s moons). Ordered monoclinic (space group P21) and orthorhombic (space group P212121) variants of the experimentally observed cubic cell (space group P213) may be constructed, with fully ordered water molecule orientations that obey the ice rules. Our calculations show that the most stable structure at 0 K is orthorhombic (P212121), the monoclinic variants (P21) being energetically disfavored. We provisionally call this ordered orthorhombic phase ADH III. The, as-yet-unmeasured, bulk modulus, K0, is predicted to be 10.67−0.44+0.56 GPa at 0 K. Our results are also combined with literature data to arrive at a revised coefficient of volume thermal expansion, αv = 2.81 × 10−7 T1.39 (from 0–176 K), with the density at 0 K, ρ0 = 991.7(39) kg m−3. We also present a case, based on literature data, that argues for a gradual transformation from a paraelectrically disordered cubic structure (P213) to the proposed antiferroelectrically ordered orthorhombic structure (P212121) around 130–150 K (cf. ice III → IX), a temperature regime that applies to the surfaces and interiors of many medium-sized (radii ∼500–700 km) icy bodies.

Phase behaviour and thermoelastic properties of perdeuterated ammonia hydrate and ice polymorphs from 0 to 2 GPa

The results are described of a series of neutron powder diffraction experiments over the pressure and temperature ranges 0 < P < 2 GPa, 150 < T < 240 K, which were carried out with the objective of determining the phase behaviour and thermoelastic properties of perdeuterated ammonia dihydrate (ND3·2D2O). In addition to the low-pressure cubic crystalline phase, ADH I, two closely related monoclinic polymorphs of ammonia dihydrate have been identified, which commonly occur as a composite in the range 450–550 MPa at 175 K; these are labelled ADH IIa and IIb, and each has unit-cell volume V ≃ 310 A3 and number of formula units per unit cell Z = 4. It has been determined that this composite dissociates to a mixture of ammonia monohydrate (ND3·D2O) phase II (AMH II) and ice II when warmed to ∼190 K at 550 MPa, which in turn partially melts to ice II + liquid at T = 196 K; AMH II has a large orthorhombic unit cell (V ≃ 890 A3, Z = 16). Above 600 MPa, an orthorhombic polymorph of ammonia dihydrate (with V ≃ 530 A3, Z = 8), which has been referred to previously as ADH IV, persists to pressures greater than 2 GPa and appears to be the liquidus phase over this whole pressure range. This phase has been observed co-existing with ice II, ice VI and AMH II. The most plausible synthesis of the high-pressure phase behaviour is described here. This model explains the reported observations, and provides measurements of the densities, thermal expansion, bulk moduli and crystal growth kinetics of the high-pressure ammonia dihydrate, ammonia monohydrate and ice polymorphs.

Ab initio simulation of the ice II structure

We have carried out ab initio simulations on the high-pressure polymorph of solid water, ice II, a phase for which there is a surprising lack of experimental data. We report our calculated third-order Birch–Murnaghan equation of state for ice II: the zero pressure and temperature density, ρ0=1240.27±0.62 kg m−3, bulk modulus, K0=16.18±0.12 GPa, with the first pressure derivative of the bulk modulus, K0′, fixed equal to 6.0. These parameters, the unit cell dimensions, and the atomic positions are in good agreement with experimental values. We also describe the way in which the change in unit cell volume is accommodated within the structure, primarily by contraction of the distance between neighboring hexagonal tubes—the principal structural element of ice II. This is in agreement with existing experimental data.