George A. Lager

NEUTRON POWDER DIFFRACTION STUDY OF HYDROGARNET TO 9.0 GPA

Abstract The crystal structure of synthetic, deuterated katoite [Ca3Al2(O4D4)3] has been refined at 0.0001,0.78,3.6,6.1,7.9, and 9.0 GPa using the opposed-anvil Paris-Edinburgh cell and the POLARIS powder diffractometer at the U.K. pulsed spallation source, ISIS. A constrained Birch-Murnaghan fit to the unit-cell volume yields K0 = 52(1) GPa for K0= 4.0. The lower bulk modulus of katoite relative to anhydrous Ca-bearing garnets (K0 = 159-179 GPa) is due to the greater compressibility of the Ca dodecahedron and O4D4 tetrahedron. At high pressure, comer-sharing tetrahedra and octahedra undergo a rigidbody rotation, which causes a shift in the a position. This rotation, coupled with the large compression of the two tetrahedral edges shared with the dodecahedron, changes the relative lengths of the Ca-O bonds at high pressure, such that Ca2-O < Ca1-O for P > 6 GPa. With increasing pressure, the O-D···O angles widen slightly, as the O-D vector rotates toward the tetrahedral face. Both O-D and O···D bond lengths decrease as a function of pressure, which was unexpected in view of the results of recent high-pressure studies. Comparison of crystallographic and spectroscopic data collected at high pressure for hydrogarnets suggests that empirical relationships derived for hydrogen-bond systems at ambient conditions, where a relaxed, equilibrium state is attained, may not always apply to O-D···O hydrogen bonds under compression.

A combined temperature dependent57Fe Mssbauer and single crystal X-ray diffraction study of synthetic almandine: evidence for the Gol'danskii-Karyagin effect

Synthetic almandine garnet, Fe3Al2Si3O12, has been studied by temperature — dependent single crystal X-ray diffraction and57Fe Mössbauer spectroscopy. The Fe2+ doublet in almandine is characterized by a small asymmetry between the high and low-velocity peaks that decreases in magnitude with decreasing temperature from 420 to 15 K. The X-ray results show that the Fe2+ cation is dynamically disordered with an anisotropic motion within the eight-coordinated site in garnet. The magnitudes of the X-ray determined mean-square-vibrational amplitudes of this motion parallel,x∥, and perpendicular,x⊥, to the principle axes of the electric field gradient give a calculated angular dependence of the electric quadrupole interaction of theI1/2 toI3/2 transitions that agree with the experimentally measured peak ratios. This is the first recognition of anisotropic recoil free fraction of57Fe in silicates.

Temperature dependence of IR absorption of hydrous/hydroxyl species in minerals and synthetic materials

Abstract We report on temperature dependencies of infrared (IR) fundamental, combination, and overtone vibrations of hydroxyl species (OH) in nominally anhydrous minerals (e.g., titanite), ferroelectric crystals (KTa1-xNbxO3, KTN), layer silicates (talc, mica, and pyrophyllite), and hydrous minerals such as apatite and synthetic hydrous/deuterated garnets [Ca3Al2(O4H4)3 and Ca3Al2(O4D4)3] for the temperature range of 20-300 K. Data obtained by high-resolution FTIR spectroscopy show that increasing temperature generally leads to a decrease in the band height and band area of fundamental vibrations of hydroxyl species, whereas the combination and first-overtone bands commonly show different temperature dependencies. The results show that in the investigated temperature range, the variations of the band height and area for different OH bands (especially for combinations and overtones) on cooling or heating do not reflect changes in OH concentrations in the materials, but relate to temperature-dependent absorption coefficients. The observations imply that absorption coefficients calibrated at room temperature cannot necessarily be used for the determination of hydroxyl contents at other temperatures

Re-examination of the hydrogarnet structure at high pressure using neutron powder diffraction and infrared spectroscopy

Abstract Time-of-flight neutron powder data and synchrotron infrared absorption spectra were collected for katoite hydrogarnet [Ca3Al2(O4D4)3] at pressures to 9.4 and 9.8 GPa, respectively. The phase transition from space group Ia3d to I4̄3d was observed in the neutron spectrum at ~7.5 GPa, as indicated by the presence of two weak reflections (730 and 530) that violate the hkl conditions (hk0, h ≠ 2n) imposed by the a-glide operation. However, attempts to refine the high-pressure structure in space group I4̄3d did not significantly improve the fit and produced a chemically unreasonable O-D bond length at the second D position. Structure refinements in Ia3d indicate that (1) the O-D bond length, corrected for the effects of thermal motion, remains essentially constant (~0.95 Å) with increasing pressure; (2) hydrogen bond lengths shorten with increasing pressure; however, the variation in O-D···O angles indicates a preferential strengthening of H bonds; and (3) the compression mechanism is characterized by bond shortening rather than bond bending. The new results are in excellent agreement with both high-pressure X-ray diffraction experiments and ab initio calculations, and illustrate the need to eliminate peak broadening in high-pressure neutron powder experiments. IR spectra collected for the same sample showed discontinuities in both O-H and O-D vibrational frequencies at ~5 GPa, suggesting that deuteration does not significantly affect the pressure of the transition. The higher pressure observed for the transition in the neutron data is probably due to lower signal-to-noise levels, which mask the weaker, symmetry-forbidden reflections at lower pressure.

O-D…O bond geometry in OD-chondrodite

Abstract The crystal structure of OD-chondrodite [Mg5Si2O8(OD)2, P21/b (a unique), a = 4.74711(5), b = 10.34888(16), c = 7.90228(13) Å, α = 108.678(1)°] was refined to wRp = 0.0218, χ2 = 3.545 at ambient conditions using time-of-flight neutron powder data. The disordered H model proposed for OH-chondrodite on the basis of single-crystal X-ray data is confirmed. The occupations of the D1 and D2 sites are, respectively, 0.52(1) and 0.48(1). The long O5-D1 [1.076(4) Å] and O5-D2 [1.111(4) Å] bond lengths, which are two of the longest O-H(D) bonds observed in mineral structures, reflect the positional disorder of the O5 atom in the unshared OH-OH edge. Both D1 [1.968(4), 2.489(4) Å] and D2 [2.149(4), 2.251(4) Å] atoms are involved in two hydrogen bonds. A re-examination of the origin of positive OH frequency shifts in both F-bearing and OH-chondrodite at high pressure is warranted in view of the crystallographic data.

Temperature-dependent single-crystal neutron diffraction study of natural chondrodite and clinohumites

ABSTRACT The crystal structures of natural F-bearing chondrodite [Mg4.64Fe0.28Mn0.014Ti0.023(Si1.01O4)2 F1.02OH0.97] from the Tilley Foster mine (Brewster, New York), F-bearing titanian clinohumite [Mg8.805Fe0.006Ti0.214(Si0.993O4)4F0.484OH0.516] from Kukh-i-Lal (Tadjikistan) and F-free titanian hydroxylclinohumite [Mg7.378Fe1.12Mn0.052Ni0.014Ti0.453(Si0.996O4)4OH1.0] from Val Malenco (Italy) were refined in space group P21/b (unique axis a) from single-crystal neutron diffraction data, collected on a four-circle diffractometer at the High Flux Isotope Reactor at Oak Ridge National Laboratory. Accurate H atom positions were determined at 295 K, 100 K, and 20 (10) K. Only one H position of approximately 50% occupancy was observed for each structure, which confirms a disordered H model. Time-of-flight single-crystal neutron data were also collected at 295 K and 20 K for the Val Malenco clinohumite as an additional check on space group symmetry. The crystal structure of the Kukh-i-Lal clinohumite was further investigated by X-ray single-crystal refinement at 295 K and by piezoelectric measurements. A few, very weak, symmetry-forbidden reflections were observed for each crystal at both ambient and lower temperatures. The same reflections were observed by all methods used. No temperature dependence is indicated, as no additional peaks appear at low temperature, and the intensity of the reflections are sample dependent. It appears that the real structure is made up of P21 and Pb domains so that violations are due to ordering of both H and Ti. No distinct piezoelectric effect was observed that would indicate the absence of a center of symmetry. This points to the simultaneous presence of various enantiomorphic domains, which cancels the piezoelectric effect of individual domains. The decrease in unit-cell volume with F substitution in clinohumites can be explained by the higher concentration of H-site vacancies and the coupled cationic and anionic substitution on the M3 and O/F site.

Synchrotron infrared spectroscopy of OH-chondrodite and OH-clinohumite at high pressure

Abstract High-pressure synchrotron infrared (IR) absorption spectra were collected at ambient temperature for OH-chondrodite and OH-clinohumite up to 38 and 29 GPa, respectively, using argon as the pressure- transmitting medium. The crystal structures of both clinohumite and chondrodite are preserved up to the maximum pressure. However, disordering of the silicate framework appears to become more pronounced at high pressure based on significant broadening of the IR bands with increasing pressure. All three OH bands in both structures shift linearly to higher frequency with pressure up to 18 GPa. Above 18 GPa, the variation of OH frequency with pressure remains linear; however, the slopes for the three OH bands are significantly different as a result of different degrees of hydrogen bonding. The IR results are compared to those from recent Raman studies in which water was used as the pressuretransmitting medium.

Neutron powder diffraction study of defect spinel structures: Tetrahedrally coordinated Ti4+ in Ni2.62Ti0.69O4 and Ni2.42Ti0.74Si0.05O4

Cation-excess spinels with the general formula Ni2(1 + x)2 +(Si4+, Ti4+)(1 − x)O4 (0.16 ≤ x ≤ 1) can be considered as defect NiO rocksalt structures in which two octahedral metal atoms are replaced by one tetrahedrally coordinated atom. Neutron time-of-flight powder data have been collected for two members of the solid-solution series with the ZING-P′ high-resolution diffractometer at Argonne National Laboratory. Least-squares refinement, using a modified Rietveld profile analysis, confirms that all Ti4+ in Ni2.62Ti0.69O4 [R(profile) = 3.27%; R(Rietveld) = 9.06%] and Ni2.42Ti0.74Si0.05O4 [R(profile) = 3.07%; R(Rietveld) = 8.54%] partially occupies the tetrahedral site 8(a) (Fd3m). Ni2+ totally occupies the octahedral position 16(d) and partially occupies position 16(c) which is vacant in stoichiometric spinels. The presence of small amounts of Si4+ (~ 3 mol%) in the tetrahedral site 8(a) stabilizes the structure and results in a large decrease [0.0194 (1) A] in the a cell parameter.

Neutron powder diffraction of forsterite, Mg2SiO4: a comparison with single-crystal investigations

Neutron time-of-flight powder data have been collected for forsterite [Mg2SiO4; Pbnm (D162h, No. 62); Z = 4, a = 4.7534 (1), b = 10.1989 (2), c = 5.9813 (1) A] with the ZING-P′ high-resolution diffractometer at Argonne National Laboratory. Forty-seven variables, including anisotropic temperature-factor coefficients, were refined with a profile-fitting procedure to R(profile) = 2.10% and R(Rietveld) = 3.53%. Positional and thermal parameters are in good agreement with those determined from recent X-ray and neutron single-crystal diffraction experiments. Results suggest that powder data can be used to obtain accurate positional parameters and reasonable temperature factors for moderately complex structures.

Fe, Ti ordering and octahedral distortions in acentric neptunite: Temperature dependent X-ray and neutron structure refinements and Mssbauer spectroscopy

Single-crystal X-ray and neutron structure refinements carried out on neptunite (KNa2Li(Fe, Mg, Mn)2Ti2Si8O24) from San Benito, California at various temperatures (neutrons: 15 K and 293 K; X-rays: 110 K, 293 K and 493 K) indicate that this mineral crystallizes in the acentric space group Cc (T=293K: a=16.427 Å, b=12.478 Å, c=9.975 Å, β= 115.56°, Z=4, V=1844.53 Å3) due to ordering of octahedrally coordinated metals (Ti, Fe, Mn, Mg). In the neptunite structure, Ti and (Fe, Mn, Mg) octahedra share edges to form chains that run along [110] and [110]. These chains are, in turn, linked through shared corners along [001]. The resulting octahedral framework is interwoven by a similar [Si8O22] tetrahedral framework. Li, Na and K occupy 6-, 8- and 10- coordinated sites within the framework. The metal-containing polyhedra show strong distortions at all temperatures. In particular, Ti exhibits a strong off-center displacement (≈0.25 Å) within its octahedron, leading to four Ti-O distances of 2.0 Å, one of 2.2 Å and one of 1.7 Å. The displaced Ti position is in good agreement with a position that minimizes differences between ionic bond strengths and is interpreted as an energy minimum in an ionic potential model. Mössbauer spectra collected at 77 K, 293 K and 400 K indicate all Fe to be present as octahedral Fe2+. Although two distinct Fe positions were found in the structure, 77 K and 293 K spectra display only one quadrupole doublet. Two Fe sites can only be resolved in the 400 K spectrum. It is suggested that the temperature dependence of octahedral edge distortions is responsible for the separation of the Mössbauer doublets.