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