D1B

D20

D2B

High-resolutiontwo-axis

diffractometer D2B

Applications

NEUTRONSFOR SCIENCE

Selected examples

• The structural chemistry ofnon-rigid molecules

• Ab-initio structure solutionfrom powders

The diamminehydrogen ion N2D7+

Proton transfer along hydrogen bonds is animportant process in biological and chemicalsystems.There has been considerable theoreticalinterest in model systems of the type:H3O+-H2O, NH4+-H2O, NH4+-NH3 and others.The diaquahydrogen ion O2H5+ is known to existin the crystalline state and has been characterizedstructurally. The isoelectronic N2H7+ ion wasknown to exist in the gas phase revealing a[NH3__H.....NH3]+ topology.

High-resolution neutron powder diffraction usingD2B was able to show that this cation exists notonly in the solid state but also reveals the "freezing"of various internal motions when cooling, and itssubsequent distortion. Besides that, by comparingthe deuterated and hydrogenated compound aninteresting difference in the phase behaviour wasfound.

Both compounds N2H7I and N2D7I crystallize atroom temperature in a cubic phase.This cubic phaseis the consequence of an orientational disorder ofthe cation: its centre of gravity is located in themiddle of the cube of iodine atoms.The N-N axis isstatistically equally orientated along the three cubicaxis and the terminal hydrogen (deuterium) atomsare disordered around each N-N axis (Fig.1).

The molecule nowundergoes differenttransitions whencooling down depen-ding on whetherthe hydrogenated ordeuterated cation ispresent. In the hydro-genated case a tetra-gonal phase is obser-ved at 220K followedby an orthorhombicphase at 202K, whe-ras the deuteratedcompound alreadyundergoes a transi-tion to an orthor-hombic phase whencooling to 260K. Bothcompounds trans-form into a monocli-nic low-temperatureform at around 155K.

1 - Group-subgroup relations of the phasesN2H7I/N2D7I.

(1) N--axis disordered in three dimensions.H atoms disordered.

(2) a´= b´~acubic, c´~acubic, N--N axis disorde-red in two dimensions. H atoms disordered.

(3) a´´= b´´= Ï2a´, c´´= 2c´.N-N axis ordered. H atoms still disordered,

origin shifted to 01/20.

(4) a´´´~ a´´, b´´´~ b´´ (a´´´ not= b´´´),c´´´ ~ c´´.Terminal H atoms ordered,

bridging H atoms distorted, cation on 2/m.

(5) aIV~ a´´´, bIV~ b´´´, cIV~ c´´´.Terminal H atoms ordered,

bridging H atom disordered, cation on 1.

2 - Orientation of the N2D7I+ - cations in theiodine "cube" of the orthorhombic phase.

One D atom of each terminal ND3 group formsa strong linear hydrogen bond to an iodine atom,

the other two D atoms point approximately tothe middle of the edges of the cubes

(bifurcated hydrogen bond).

The structure of the orthorhombic phase wasdetermined from high resolution powder diffrac-tion data.

On cooling-down, the three-dimensional disorderof the N-N axis along the three cubic axes isreduced to a two-dimensional disorder leading tothe contraction of the axis along which the mole-cule is no longer disordered. The reduction oforientational disorder leads to a larger N-Ndistance (2.81(1)Å) than observed in the disorde-red cubic phase (2.52(4)Å).

The temperature dependent powder diffractionpatterns show that on transforming into theorthorhombic phase the diffuse scattering due tothe rotational disorder of the terminal deuteriumatoms spinning around the N-N axis vanishes.The"freezing-out" of this rotational disorder leads tothe appearance of hydrogen bonds betweenthese terminal hydrogens and the surroundingiodine atoms. A close examination of the structu-re revealed that one D-I distance (2.78(2)Å) wassignificantly shorter than the other two (3.03(3)Å)and (3.17(4)Å).This is the result of an almost linearhydrogen bond of the type N-D......I. The otherN-D bonds point between two iodines qualifyingthem as bifurcated hydrogen bonds (Fig.2).

1

2

The cell distortion when going from the cubic tothe orthorhombic cell shows that the cell cons-tant b along which the strong linear hydrogenbonds are orientated is shorter than a. The for-mation of one stronger and two weaker hydro-gen bonds explains this distortion. In the tetrago-nal hydrogenated phase the terminal hydrogenatoms are still spinning around the N-N axis as inthe cubic phase, hence no hydrogen bond net-work is formed which can induce an orthorhom-bic distortion. Figure 1 shows the group-subgrouprelations of the N2D7I/N2H7I phases.The tetrago-nal structure is only found in N2H7I, whereas thePm3m and Pcan structures are found for bothN2D7I and N2H7I.

D2B is a very high-resolution powder diffractometer designed to achieve the ultimateresolution, limited only by powder particle size (Dd/d ~ 5x10-4). It was built so that analternative high flux option, with resolution comparable to that of D1A, but muchhigher intensity, could be chosen at the touch of a button. D2B then has very highintensity at D1A resolution, or very high resolution at D1A intensity. Being on a beamtube in the reactor hall, it can use wavelengths as short as 1.05 Å, impossible on D1A,and can collect complete diffraction patterns in as little as 5-10 minutes.This instrument was upgraded as a part of the ILL’s Millennium Programme.

The diffractometer D2B is characterised by a very high take-off angle (135°) forthe monochromator, which has a relatively large mosaic spread of 20' to com-

pensate for the corresponding intensity (Dl/l) loss. It is 300 mm high, focusing ver-tically onto about 50 mm; this large incident vertical divergence is matched by200 mm high detectors and collimators. A complete diffraction pattern is obtai-

ned after about 25 steps of 0.05° in 2u, since the 128 detectors are spaced at1.25° intervals. Such scans take typically 30 minutes; they are repeated to impro-ve statistics.

Apart from the work on superconductors, D2B is particularly well suited for theRietveld refinement of relatively large structures, such as zeolites containing absor-bed molecules. It has also proved successful for the solution of some of the new'quasi-crystalline' materials.

With the new 2D-detector, the efficiency of D2B has increased by an order ofmagnitude, and it is now possible to measure very small samples of about 200 mgwith high resolution.

Instrument description

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Instrument layout

Instrument Data

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2B

web: www.ill.fr/YellowBook/D2B/

2425

monochromator

28 Ge[115] crystals of 1 x 5 x 1 cm 3

take-off-angle 135°germanium [hkl] wavelengths l/Å 557 1.051 337 1.277 551 1.464 335 1.594 (optimum l)331 2.398 113 3.152 flux at sample l= 1.594 Å 10 6 high resolution

10 7 high intensity

Reactor hall, thermal beam H11

sample

beam size at sample 2 x 5 cm 2

angular range 5° < 2u <165°0° < v <360°

detectors

128 3 He counting tubes background without sample 0.1 Hz

sample environment

cryostat 1.5 ... 300 K cryofurnace 1.5 ... 525 K furnace 200 ...1000 K cryomagnet dilution cryostat 50 ... 4000 mK pressure cell 2 GPa and 4 ... 300 Kcryocooler 3.5...700 K

D2B was also designed for work on magnetismand high resolution of very large d-spacings usingwavelengths of between 2.4 Å and 6 Å.Wavelengths can easily be changed under compu-ter control, since they are all obtained by a simplerotation within the Ge[hhl] plane. A large graphi-te filter can be switched in to provide a very cleanbeam at 2.4 Å, and a cold Be-filter can be used forlonger wavelengths.