CROATICA CHEMICA ACTA CCACAA 79 (2) 299¿309 (2006) ISSN-0011-1643 CCA-3094 Conference paper Crystal Structure Dynamics: Evidence by Diffraction and Spectroscopy Eugen Libowitzky Institut für Mineralogie und Kristallographie, Universität Wien – Geozentrum, Althanstr. 14, A-1090 Wien, Austria (E-mail: [email protected]) RECEIVED DECEMBER 12, 2005; REVISED MARCH 7, 2006; ACCEPTED MARCH 10, 2006 Bragg diffraction is a major tool to solve and refine crystal structures, though it is limited as results obtained from the bulk sample are averaged in time and space. In contrast, spectroscopy is site sensitive, and thus a probe for local structure with high time and space resolution. Com- Keywords bination of both methods may reveal important additional information on crystal structures hydrogen bonding such as disorder and dynamics, and may even help avoid pitfalls in structure solution. Among disorder the given examples are three minerals, i.e., lawsonite, hemimorphite, leonite, which show phase transition phase transitions from dynamically disordered to ordered structures. Continuous evolution infrared from order to dynamic disorder, however without a phase transition, is found in washing soda. water Finally, examples of proton dynamics in a tetragonal garnet and in minerals with very strong hydroxyl hydrogen bonds are presented. INTRODUCTION man spectroscopy are employed, will never result in full description of a crystal structure. However, since it does Since the first days of X-ray diffraction, crystal structu- not rely on crystalline periodicity, non-crystalline mate- res have been solved and refined using the positions and rials such as gases, liquids, melts, glasses and metamict intensities of Bragg reflections from single-crystal and samples can be investigated. Moreover, because it is si- powder diffraction experiments. Later on, the use of te-specific (a number of spectroscopic methods are even »light« sources different from X-rays, i.e., neutrons, element-sensitive), even defects at low concentrations in electrons, extended the use of diffraction experiments to a host structure and short range order phenomena may crystals with very light elements, e.g., hydrogen-bearing be studied. Finally, the excellent time resolution facili- samples, and micro-samples in an electron microscope. tates resolving dynamic processes much better than dif- However, though the method can give the full image of a fraction.3 crystal structure, information is always based on the more or less strict periodicity (long range order) of the crystal lattice.1 Thus, the data are averaged over the STRUCTURAL INFORMATION FROM SPECTRO- whole sample, both in space and time. Therefore, infor- mation on local disorder (space) and dynamics (time), SCOPY and on defects at only trace concentrations may be limi- Using vibrational spectroscopy, quantitative data can be ted. (The recent method of measuring diffuse X-ray scat- obtained that provide useful structural information in tering2 in the »background« region between the Bragg three ways. Since the band energy (frequency, wave- peaks to address these problems is not considered here.) number) of a vibrational band is dependent on the mass On the other hand, spectroscopy, and in this paper and bond strength of the vibrating unit, e.g., an O–H vibrational spectroscopy such as infrared (IR) and Ra- group, the peak positions provide important data on 300 E. LIBOWITZKY bond lengths, like in the case of hydrogen bonds (see be- 4000 low). Absorption (IR) and scattering experiments (Ra- 3500 man) with polarized radiation on oriented, optically ani- –1 sotropic samples result in data on the spatial orientation of a vibrating unit in a crystal structure (see below). Fi- 3000 nally, by calibration of band intensities in a number of different ways, even information on the amount of a cer- 2500 tain molecular species (down to trace concentration lev- els) in a sample can be obtained (see below). Because IR 2000 spectroscopy is frequently used in the examples given below, these quantitative IR techniques are described briefly (however, they work in a similar way also for Ra- 1500 man spectroscopy). O–H stretching wavenumber / cm 1000 In a basic IR absorption experiment, a beam of IR light (modulated in frequency by a monochromator or in 2.4 2.6 2.8 3.0 3.2 3.4 modern spectrometers by an interferometer) with inten- d(O···O) /Å sity I0 is absorbed by a molecular vibration in the sample and the remaining transmitted intensity I is recorded by 4000 the detector. The basic quantity I/I0 is the transmittance T (usually expressed in [100 %]). However, since it is in 3500 logarithmic relation with thickness d and absorber con- –1 centration c, a more convenient unit is absorbance A =– log T. According to the Beer-Lambert law, A = e ·c·d,it 3000 is in linear relation to sample thickness and absorber 4 concentration; e is the molar absorption coefficient. 2500 Correlation diagrams to derive hydrogen bond lengths from O–H stretching vibrations have been devel- 2000 oped since the 1950s both from theoretical calculations5 6–9 and from empirical studies. In general, hydrogen 1500 bonds can be classified according to their bond lengths, i.e., d(O···O), d(H···O), d(O–H), into very strong (very O–H stretching wavenumber / cm short): d(O···O) < 2.50 Å, strong (short): 2.50 < d < 2.70 1000 Å, and weak (long): d > 2.70 Å with a barely defined 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 10 upper limit beyond 3 Å. Figure 1 shows the positive, d(H···O) /Å curved correlation between O–H stretching wavenumber and O···O / H···O hydrogen bond lengths. The principle Figure 1. Correlation diagram between O···O / H···O hydrogen bond lengths and O–H stretching wavenumbers.9 of this correlation is that with increasing hydrogen bond strength (decreasing H bond length), the H bond accep- tor attracts the proton and thus elongates and weakens lipsoid) in optically anisotropic crystals16 (it is therefore the O–H bond (~ 0.98 Å without H bonding), which in impossible in cubic crystals). According to Figure 2, the turn results in decreased stretching frequency (wavenumber). Further effects that correlate with increa- absorbance components along the principal axes Ax, Ay, sing H bond strength are increasing peak width, and in- Az are in a simple cosine squared relation to the total creasing anharmonicity.4,7,11–14 Besides Figure 1, it is magnitude of the absorbance (as if measured parallel to important for the examples below to realize that stretch- the absorber). Thus, by measurements with IR radiation ing vibrations are observed at 3200–3700 cm–1 for weak polarized parallel to all three main directions, the angles H bonds, at 1600–3200 cm–1 for strong H bonds, and be- and consequently the orientation of the absorber are ob- low 1600 cm–1 for very strong bonds.10 Finally, it must tained. Moreover, the sum of the three components re- be stressed that in addition to H bonding, the cationic sults in total absorbance, which is the correct quantity to environment has also a (minor) influence on the wave- estimate the concentration of the absorber. number of O–H stretching vibrations.15 Information about the concentration of a certain ab- The spatial orientation of an O–H vector (or any sorbing species is obtained by calibration of IR band in- other vibrational direction) can be derived from mea- tensities (preferably band areas are used instead of peak surements with polarized IR radiation along the princi- heights) with different analytical methods. In the case of pal optical directions (the main axes of the indicatrix el- hydrous species and defects in minerals, water contents Croat. Chem. Acta 79 (2) 299¿309 (2006) CRYSTAL STRUCTURE DYNAMICS 301 A simple, illustrating example of the above mentio- ned structural constraints from IR data is given in Figure 3, showing spectra20 acquired with polarized IR radiati- on of the tetragonal hydroxyl-bearing silicate mineral vesuvianite. The spectrum with the electric vector E of the polarized light parallel to c shows a band at ~ 3150 cm–1, which is absent in the spectrum perpendicular to c. Thus, an OH– group with strong hydrogen bonding is expected (using Figure 1, an O···O distance of ~ 2.7Åis derived) with orientation parallel to the c axis. This hy- droxyl group (refined O···O H bond length ~ 2.70 Å) is actually observed by diffraction methods (inset in Figure 3).21 In addition, weak or non-H bonded hydroxyl groups with various cationic environments and different orientations (~ 30° from the c axis) are confirmed by the bands at higher wavenumbers in both spectra. Using the A Figure 2. Sketch of an absorber with total absorbance and band areas (integrated absorbances) and the general cali- components Ax, Ay, Az that can be obtained by polarized IR ab- 19 sorption measurements parallel to main optical directions x, y, z.16 bration trend of Libowitzky and Rossman, a concentra- tion ratio of 1 : 4 between the former and the latter hy- droxyl groups is obtained (please note that the areas are may be calibrated by heating the crushed (and milled) not directly comparable!), which is in excellent agree- sample, and detection of the expelled water vapor by va- ment with the crystal structure refinement.20,21 rious methods.17 Because of water adsorbed at grain sur- faces, results are sometimes inaccurate. Bulk methods, e.g., proton magic angle spinning nuclear magnetic reso- LAWSONITE nance (1H MAS NMR), nuclear reaction analysis (NRA), and secondary ion mass spectrometry (SIMS), Lawsonite, CaAl2Si2O7(OH)2 ·H2O, space group Cmcm, avoid this disadvantage but are available only in rare, is a hydrous high-pressure mineral.