Eiken Haussühl

Single-crystal structure refinement of diaspore at 50 GPa

Abstract The crystal structure of diaspore, AlO(OH), has been investigated by in situ single-crystal synchrotron X-ray diffraction at ~50 GPa using the diamond-anvil cell technique. Diaspore is found to retain its structure up to 51.5 GPa at room temperature, which is more than 30 GPa above the transition pressure to δ-AlO(OH) found in quenched high-temperature experiments and derived from density functional theory calculations. The compression is anisotropic and largest for the a axis. This can be explained by the fact that the structural response to pressure is mainly due to the shortening of the hydrogen bond, which is oriented nearly parallel to the a axis. The hydrogen bond becomes significantly more symmetric with pressure up to 50 GPa.

Structural compression and vibrational properties of Bi12SiO20 sillenite from experiment and theory

The crystal structure of the bismuth silicon oxide Bi(12)SiO(20) was determined by single-crystal x-ray diffraction at ambient conditions and at high pressure. Single-crystal intensity data between 0.0001 and 16.8(3) GPa were collected in house with Mo Kα radiation and with synchrotron radiation (λ = 0.45 Å) at HASYLAB (D3), while lattice parameters were measured up to 23.0(3) GPa. The large cavities which exist in the crystal structure and host the lone electron pairs of the Bi(3 + ) ions are considerably compressed at high pressure. The crystal structure, however, remains stable and the lone electron pair is stereochemically active up to at least 16.8 GPa. A larger compression in the direction of the lone electron pairs by shear deformation was not observed. Raman spectra of Bi(12)SiO(20) were measured on powder samples during pressure decrease from 39.1(1) GPa down to ambient pressure and on single crystals during pressure increase up to 12.50(3) GPa. Density functional perturbation theory was used to compute Raman frequencies and intensities at ambient pressure and to investigate pressure-induced changes up to 50 GPa.

Persistence of the stereochemical activity of the Bi3+ lone electron pair in Bi2Ga4O9 up to 50 GPa and crystal structure of the high-pressure phase

The crystal structure of the high-pressure phase of bismuth gallium oxide, Bi(2)Ga(4)O(9), was determined up to 30.5 (5) GPa from in situ single-crystal in-house and synchrotron X-ray diffraction. Structures were refined at ambient conditions and at pressures of 3.3 (2), 6.2 (3), 8.9 (1) and 14.9 (3) GPa for the low-pressure phase, and at 21.4 (5) and 30.5 (5) GPa for the high-pressure phase. The mode-Grüneisen parameters for the Raman modes of the low-pressure structure and the changes of the modes induced by the phase transition were obtained from Raman spectroscopic measurements. Complementary quantum-mechanical calculations based on density-functional theory were performed between 0 and 50 GPa. The phase transition is driven by a large spontaneous displacement of one O atom from a fully constrained position. The density-functional theory (DFT) model confirmed the persistence of the stereochemical activity of the lone electron pair up to at least 50 GPa in accordance with the crystal structure of the high-pressure phase. While the stereochemical activity of the lone electron pair of Bi(3+) is reduced at increasing pressure, a symmetrization of the bismuth coordination was not observed in this pressure range. This shows an unexpected stability of the localization of the lone electron pair and of its stereochemical activity at high pressure.

High temperature elastic properties of Mg-cordierite: experimental studies and atomistic simulations

Abstract The temperature dependence of the elastic stiffness coefficients of natural orthorhombic Mg-cordierite was studied between 295 K and 1573 K using resonant ultrasound spectroscopy. The measurements revealed a continuous decrease of all the elastic constants with increasing temperature. The bulk modulus softens from about 129(2) GPa at 295 K to 110(2) GPa at 1473 K. Irreversible anomalies in the temperature evolution of the resonance frequencies of certain eigenmodes were observed above 920 K due to the escape of volatiles and the occurrence of microcracks. However, the dehydrated samples still showed integrity on the macroscopic scale. Therefore, despite the occurence of the micro-cracks, a reasonable quantitative analysis of the high-temperature RUS data of cordierite samples was still feasible. The thermal expansion was studied between 100 K and 1570 K using dilatometry. The new data are consistent with earlier experimental results and confirm the expansion of the a and b unit cell parameters and the contraction of the c parameter with increasing temperature. Posäsible contributions of the Al/Si disorder to the elastic properties of Mg-cordierite were estimated on the basis of force-field and quantum mechanical calculations. The behaviour of individual elastic stiffness coefficients was followed across the order/disorder transition by Monte Carlo simulations. The simulations predicted a decrease in the bulk modulus with increasing Al/Si disorder. However, this effect is much smaller than that observed experimentally. The measured decrease in the elastic stiffness coefficients is mainly due to phonon softening effects.

Lithium silicate, LiAlSi4O10 (petalite)—a novel monoclinic SRS-active crystal

The discovery of χ (3)–nonlinear lasing by stimulated Raman scattering (SRS) in natural monoclinic crystal of petalite, LiAlSi4O10, is reported. All recorded Stokes and anti-Stokes lasing sidebands under picosecond laser pumping in the visible are identified and attributed to the two occurring SRS-promoting phonon modes with ω SRS1 ≈ 490 cm−1 and ω SRS2 ≈ 357 cm−1. A short review of known SRS-active crystalline minerals and observed manifestations of their χ (2)– and χ (3)–nonlinear interactions is given as well.

Structural and magnetic properties of betaine adducts with transition metals: I. ((CH3)3NCH2COO)3MnMCl4 with M = Mn2+, Co2+, Zn2+

Large single crystals with diameters up to 20 mm of betaine adducts with 3d metals, namely 3bMnCl2MCl2 with M = Mn2+ (BMM), Co2+ (BMC) and Zn2+ (BMZ), were grown from aqueous solution by slow evaporation of the solvent. The isomorphous crystal structures, space group , are built up from carboxylate-bridged octahedral chains and isolated MCl4 tetrahedra. The magnetic susceptibilities of these low-dimensional spin-systems were determined by superconducting quantum interference device (SQUID) measurements and are interpreted with an antiferromagnetic Heisenberg-chain model. We studied by experiment and density functional theory (DFT) calculations how the replacement of half of the Mn content (S = 5/2) in 3b2MnCl2 by metals with different spin (Co, S = 3/2; Zn, S = 0) modifies the structural and magnetic properties. The results show that, predominantly, Mn occupies the octahedral sites in the chains and is responsible for the magnetic interaction, whereas the other metal (Co or Zn) is found on the isolated tetrahedral sites.

Elastic stiffness coefficients of thenardite and their pressure and temperature dependence

Abstract The elastic stiffness coefficients, cij, of orthorhombic Na2SO4 thenardite (space group Fddd) were measäured with an ultrasonic plane wave technique at ambient temperature as a function of hydrostatic pressure in the range of 0.1–70 MPa. The variation of the cij in the range of 1–5000 MPa was studied with density functional theory (DFT) based calculations. The experimental results and the DFT calculations were used to derive a force-field model, which was then employed to compute lattice parameters and elastic stiffness tensors of thenardite and of two other Na2SO4 polymorphs as functions of the temperature based on quasi-harmonic lattice dynamics. The structural parameters of the three polymorphs measured at high temperatures are reproduced to within 1.7% by the present calculations. Phases II (space group Pbnm) and III (Cmcm) appear to have significantly higher entropies than thenardite in agreement with their metastable formation at higher temperatures.

Structural and physical properties of bis(guanidinium) X bis(nitrilotriacetate) hydrate, (C[NH2]3)2X(N[CH2COO]3)2·H2O (X = Zr, Sn, Hf) and bis(methylammonium) X bis(nitrilotriacetate dihydrate, (CH3NH3)2X(N[CH2COO]3)2·2 H2O (X = Sn, Zr)

Abstract Single crystals of bis(guanidinium) tin bis(nitrilotriacetate) hydrate (GuSN) and bis(methylammonium) tin bis(nitrilotriacetate) dihydrate (MetamSN) having optical quality and dimensions up to 15 × 15 × 20 mm were grown from aqueous solutions. The isotypy with the corresponding zirconium and hafnium compounds (GuZN, GuHN, and MetamZN, respectively)) was proved by X-ray structure analyses (GuSN: space group Ccc21, a1 = 11.338 Å, a2 = 20.285 Å, a3 = 10.178 Å, Z = 4, density 1.796 g cm-3; MetamSN: space group P212121, a1 = 9.851 Å, a2 = 14.164 Å, a3 = 15.548 Å, Z = 4, density 1.821 g cm-3). Distinct differences in the dimensions of the tin bis(nitrilotriacetate) and zirconium bis(nitrilotriacetate) anions could be derived from structural features. The molecular volume of the latter is about 6 Å3 larger than that of the tin complex. Along one direction perpendicular to the twofold or quasi-twofold axis the diameter of the tin complex is about 0.18 Å shorter. Thermal expansion, dielectric, piezoelectric, elastic, thermoelastic and piezo elastic properties of corresponding tin, zirconium and hafnium compounds are quite similar. The small differences in the dimensions of the anions, however, are clearly reflected in the elastic longitudinal stiffnesses. The earlier described phase transitions in GuZN exist also in GuSN as confirmed by X-ray analysis at 100 K. The temperature of the transition region II is shifted to lower temperatures by about 4 and 6 K, if Zr is substituted by Sn and Hf, respectively. All anomalous phenomena observed during the transition in GuZN and GuHN are also found in GuSN.

Crystal structures of sulfamates MeLi(NH2SO3)2 (Me : K, Rb and Cs) and physical properties of KLi(NH2SO3)2 (refractive indices, thermal expansion, elastic properties)

Abstract New sulfamates KLi(NH2SO3)2, RbLi ⋅ (NH2SO3)2 and CsLi(NH2SO3)2 were prepared from non-stoichiometric (ratio Li: Me = 3:1) aqueous solutions of LiNH2SO3 and MeNH2SO3 (Me = K, Rb, Cs) at room temperature. Large single crystals of KLi(NH2SO3)2 were grown from these solutions by controlled lowering of the temperature between 293 K and 288 K. The crystal structures were investigated using single-crystal X-ray diffraction methods. KLi(NH2SO3)2: space group P21, Z = 2, a = 5.196(1) Å, b = 8.475(1) Å, c = 8.868(1) Å, β = 105.05(1)°, Tmeas. = 293 K, R1 = 0.026; RbLi(NH2SO3)2: space group P1̅, Z = 4, a = 7.844(1) Å, b = 9.804(1) Å, c = 10.825(1) Å, α = 108.77(1)°, β = 93.29(1)°, γ = 98.64(1)°, Tmeas. = 293 K, R1 = 0.033; CsLi(NH2SO3)2 : space group P1̅, Z = 2, a = 5.535(1) Å, b = 7.877(1) Å, c = 9.857(1) Å, α = 74.07(1)°, β = 76.33(1)°, γ = 87.07(1)°, Tmeas. = 200 K; R1 = 0.023. The common feature of the three crystal structures are alternating layers of Me+ and lithium cations together with anionic [NH2SO3]– tetrahedra. Li+ coordination polyhedra and [NH2SO3]– anions, linked via common corners, form one type of these sheets. Additionally, these layers are stabilized by hydrogen bonds. Me+ coordination polyhedra are linked to each other and to the above mentioned layers by common corners and edges. RbLi(NH2SO3)2 and CsLi(NH2SO3)2 possess similar structures. Generally, the observed interatomic distances and angles agree with crystal chemical expectations. Differential scanning calorimetry measurements showed that KLi(NH2SO3)2 undergoes a first-order structural phase transition at 401 K and melts at 423 K. Between 153 K and 423 K no indications of further phase transitions were observed. In addition, the refractive indices and the tensor of thermal expansion were measured. Further, the elastic and thermoelastic constants of KLi(NH2SO3)2 were determined from ultrasonic resonant frequencies of thick plane-parallel plates and their shift upon variation of temperature, respectively. The elastic behaviour of KLi(NH2SO3)2 is similar to the average elastic properties of LiNH2SO3 and KNH2SO3.

Influence of deuteration on lithium acetate dihydrate studied by inelastic X-ray scattering, density functional theory, thermal expansion, elastic and thermodynamic measurements

The influence of deuteration on the properties of lithium acetate dihydrate has been investigated by thermal expansion measurements, ultrasound spectroscopy and calorimetry. Inelastic X-ray scattering has been employed to investigate if the low temperature structural phase transition can be detected by a change in the vibrational spectrum. Density functional theory, DFT, calculations have been employed to complement the experimental investigations. The thermal expansion coefficients and the specific heat of the deuterated compound differ significantly from the protonated form. The differences in the elastic stiffness coefficients are just above the detection limit of the technique employed here. Temperature dependent inelastic X-ray spectroscopic measurements show no significant change of the vibrational spectrum when crossing the transition temperature. The DFT calculations show that the methyl group dynamics are best described in the framework of coupled rotators of opposing methyl groups. One of the coupled rotational modes corresponds to a hindered rotator with a barrier of 15 meV, while the other is a free rotator.