The crystal structure of perdeuterated methanol hemiammoniate (CD3OD center dot 0.5ND(3)) determined from neutron powder diffraction data at 4.2 and 180 K

The crystal structure of perdeuterated methanol hemiammoniate, CD3OD center dot 0.5ND(3), has been solved from neutron powder diffraction data collected at 4.2 and 180 K. The structure is orthorhombic, space group Pn2(1)a (Z = 4), with unit-cell dimensions a = 12.70615 (16), b = 8.84589 (9), c = 4.73876 (4) angstrom, V = 532.623 (8) angstrom(3) [rho(calc) = 1149.57 (2) kg m(-3)] at 4.2 K, and a = 12.90413 (16), b = 8.96975 (8), c = 4.79198 (4) angstrom, V = 554.656 (7) angstrom(3) [rho(calc) = 1103.90 (1) kg m(-3)] at 180 K. The crystal structure was determined by ab initio methods from the powder data; atomic coordinates and isotropic displacement parameters were subsequently refined by the Rietveld method to R-p similar or equal to 2% at both temperatures. The crystal structure comprises a three-dimensionally hydrogen-bonded network in which the ND3 molecules are tetrahedrally coordinated by the hydroxy moieties of the methanol molecule. This connectivity leads to the formation of zigzag chains of ammonia-hydroxy groups extending along the c axis, formed via N-D center dot center dot center dot O hydrogen bonds; these chains are cross-linked along the a axis through the hydroxy moiety of the second methanol molecule via N-D center dot center dot center dot O and O-D center dot center dot center dot O hydrogen bonds. This 'bridging' hydroxy group in turn donates an O-D center dot center dot center dot N hydrogen bond to ammonia in adjacent chains stacked along the b axis. The methyl deuterons in methanol hemiammoniate, unlike those in methanol monoammoniate, do not participate in hydrogen bonding and reveal evidence of orientational disorder at 180 K. The relative volume change on warming from 4.2 to 180 K, Delta V/V, is + 4.14%, which is comparable to, but more nearly isotropic (as determined from the relative change in axial lengths, e. g. Delta a/a) than, that observed in deuterated methanol monohydrate, and very similar to what is observed in methanol monoammoniate

The microscopic origin of thermal cracking in rocks: An investigation by simultaneous time-of-flight neutron diffraction and acoustic emission monitoring

We demonstrate that neutron diffraction measurements make it possible to quantify elastic strains within the interior of solid samples, and thus have great potential for addressing a wide range of problems connected with the characterization of the mechanical properties of geological materials. We use the time-of-flight neutron diffraction technique, in combination with acoustic emission monitoring, to study the evolution of thermal strain within the interior of samples of a pure quartzite during slow heating, and the onset of the associated thermal cracking. Thermal cracking commences around 180 degreesC when the thermal strain deficit along the a-axes of quartz grains induces a thermal stress that is close to the bulk tensile strength of the rock

Crystal structures and thermal expansion of alpha-MgSO4 and beta-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

Structure, thermal expansion and incompressibility of MgSO4·9H2O, its relationship to meridianiite (MgSO4·11H2O) and possible natural occurrences

Since being discovered initially in mixed-cation systems, a method of forming end-member MgSO4·9H2O has been found. We have obtained powder diffraction data from protonated analogues (using X-rays) and deuterated analogues (using neutrons) of this compound over a range of temperatures and pressures. From these data we have determined the crystal structure, including all hydrogen positions, the thermal expansion over the range 9-260 K at ambient pressure, the incompressibility over the range 0-1.1 GPa at 240 K and studied the transitions to other stable and metastable phases. MgSO4·9D2O is monoclinic, space group P21/c, Z = 4, with unit-cell parameters at 9 K, a = 6.72764 (6), b = 11.91154 (9), c = 14.6424 (1) Å, β = 95.2046 (7)° and V = 1168.55 (1) Å3. The structure consists of two symmetry-inequivalent Mg(D2O)6 octahedra on sites of symmetry. These are directly joined by a water-water hydrogen bond to form chains of octahedra parallel with the b axis at a = 0. Three interstitial water molecules bridge the Mg(D2O)6 octahedra to the SO42- tetrahedral oxyanion. These tetrahedra sit at a ≃ 0.5 and are linked by two of the three interstitial water molecules in a pentagonal motif to form ribbons parallel with b. The temperature dependences of the lattice parameters from 9 to 260 K have been fitted with a modified Einstein oscillator model, which was used to obtain the coefficients of the thermal expansion tensor. The volume thermal expansion coefficient, αV, is substantially larger than that of either MgSO4·7D2O (epsomite) or MgSO4·11D2O (meridianiite), being ∼ 110 × 10-6 K-1 at 240 K. Fitting to a Murnaghan integrated linear equation of state gave a zero-pressure bulk modulus for MgSO4·9D2O at 240 K, K0 = 19.5 (3) GPa, with the first pressure derivative of the bulk modulus, K′ = 3.8 (4). The bulk modulus is virtually identical to meridianiite and only ∼ 14% smaller than that of epsomite. Above ∼ 1 GPa at 240 K the bulk modulus begins to decrease with pressure; this elastic softening may indicate a phase transition at a pressure above ∼ 2 GPa. Synthesis of MgSO4·9H2O from cation-pure aqueous solutions requires quench-freezing of small droplets, a situation that may be relevant to spraying of MgSO4-rich cryomagmas into the surface environments of icy satellites in the outer solar system. However, serendipitously, we obtained a mixture of MgSO4·9H2O, mirabilite (Na2SO4·10H2O) and ice by simply leaving a bottle of mid-winter brine from Spotted Lake (Mg/Na ratio = 3), British Columbia, in a domestic freezer for a few hours. This suggests that MgSO4·9H2O can occur naturally - albeit on a transient basis - in certain terrestrial and extraterrestrial environments