Haruo Ohashi

Spontaneous strain variations through the low temperature displacive phase transition of LiGaSi2O6 clinopyroxene

The evolution of the lattice parameters and crystal structure of LiGaSi 2 O 6 clinopyroxene has been studied by single-crystal X-ray diffraction from 100 to 1073 K. Complete single-crystal X-ray diffraction data have been collected at 22 temperatures. A displacive P 2 1 / c ↔ C 2/ c phase transition is observed at T c = 286 (10) K. The evolution of the spontaneous strain related to the phase transition with temperature has been studied in detail and compared to available data for pyroxenes with related compositions. Particular attention has been devoted to the ubiquitous presence of residual spontaneous strain after the phase transition by following the structure evolution in order to determine the related microscopic mechanism. The observed mean thermal expansion for the C 2/ c phase in the temperature interval 423–1073 K is α = 24.1(1) × 10 −6 (K −1 ), which is similar to that of spodumene, and it is highly anisotropic. The ratio of axial thermal expansion coefficients is α a :α b :α c = 2.8:3.6:1, while in spodumene it is 1:2.9:1.3. This represents a clear difference in the mechanism of thermal expansion between the two structures which only differ in the substitution of Al by Ga at the M (1) site. The particular composition studied has fully extended chains of tetrahedra at T > T c , although significant thermal expansion along c -axis is still observed in the C 2/ c phase. Extension along the c -axis is resolved therefore by dilatation of the (SiO 4 ) tetrahedron, consequently increasing the thermal expansion along d (100) by 44 % compared to spodumene.

High-pressure behavior of Ca/Na clinopyroxenes: The effect of divalent and trivalent 3d-transition elements

Abstract Two C2/c pyroxenes, CaMnSi2O6 and CaZnSi2O6, were investigated at high pressure by singlecrystal X-ray diffraction. Their unit-cell parameters were determined at 9 different pressures up to 7.55 and 8.23 GPa, respectively. No evidence of phase transformations was found for either sample throughout the investigated pressure range. The pressure-volume data were fit using a third-order Birch-Murnaghan equation of state and it was possible to refine simultaneously the unit-cell volume V0, the bulk modulus KT0 and its first pressure derivative K′ providing the following coefficients: V0 = 462.01(3) Å3, KT0 = 97.3(4) GPa, K′ = 7.5(2) for CaMnSi2O6 and V0 = 444.79(2) Å3, KT0 = 109.6(5) GPa, K′ = 7.0(2) for CaZnSi2O6. The substitution of Zn for Mn at the M1 crystallographic site causes a strong decrease of the bulk modulus (by about 11%). Intensity data were collected at several pressures for both samples. The main compression mechanism is related to the tetrahedral chain kinking and the consequent shortening of the Ca-O3long bond distances, which show a markedly larger decrease for the CaMnSi2O6 sample. Comparing the results obtained in this work with previous data for Ca/ Na-bearing C2/c pyroxenes containing trivalent and divalent 3d-transition elements at M1 site, we identified two distinct compressibility trends: for a similar unit-cell volume at room pressure the pyroxenes with Na on M2 and trivalent cations on M1 are more compressible than those having Ca on M2 and divalent cations at the M1 site. The analysis of the structure evolution for all samples indicates that this is strongly correlated with the evolution of kinking of the tetrahedral chains and the shortening of the M2-O3 bonds.

Bulk modulus variation along the diopside–kosmochlor solid solution

Three pyroxene single-crystals with compositions NaCrSi 2 O 6 (Ko 100 Di 0 ), (Na 0.75 Ca 0.25 )(Cr 0.75 Mg 0.25 )Si 2 O 6 (Ko 75 Di 25 ) and (Na 0.25 Ca 0.75 )(Cr 0.25 Mg 0.75 )Si 2 O 6 (Ko 25 Di 75 ) have been studied by means of in situ high-pressure X-ray single-crystal diffraction using a diamond anvil cell up to ~7.6 GPa. A third-order Birch-Murnaghan equation of state (BM3-EoS) was used to fit the P–V data measured for the three samples. The bulk modulus, K T0 , increases from 113.1(5) GPa for Ko 25 Di 75 to 130.2(7) GPa for Ko 100 Di 0 , whereas the first pressure derivative, K ′, decreases from 5.8 to 4.1 with increasing Ko content. The bulk modulus varies linearly with composition according to the expression: K T0 (GPa) = 107.4(1) + 0.228(1) × % molar Ko and the extrapolated value for pure diopside is in good agreement with the datum obtained from Brillouin scattering measurements and lies between data obtained from other X-ray diffraction studies. An inverse behaviour of volume compressibility and thermal expansion with composition is observed, i.e . Na-richer compositions are stiffer and less expandable. Softening of the clinopyroxene structure with increasing diopside component is mainly due to the softening of the b direction and can be ascribed to the narrowing of the M1 octahedral chains which is greater in diopside than in kosmochlor.

Lattice compression and structural behavior of NaVSi2O6 clinopyroxene to 11 GPa

Abstract High-pressure behavior of a synthetic single crystal of the C2/c clinopyroxene NaVSi2O6 was investigated using X-ray diffraction and Raman spectroscopy in combination with diamond-anvil cell techniques. Lattice-parameters, single-crystal intensity data, and Raman spectra were collected up to pressures of about 11 GPa, revealing no discontinuity in the evolution of the lattice with increasing pressure. The fit of a third-order Birch-Murnaghan equation of state to the pressure-volume data yields a bulk modulus of K0 = 114.7 (1.8) GPa with its pressure derivative K′ = 5.4 (4). Strain analysis reveals compressional anisotropy with unit strain axial ratios of 1:2.9:3.1. The direction of largest compression was found to be perpendicular to the (3̅02) plane, which matches closely the direction of largest interlayer spacing between oxygen monolayers forming the structure of clinopyroxenes. The relatively large pressure dependency of the bulk modulus, compared to that of isostructural transition-metal compounds, was related to an increase in stiffness of the M2 polyhedron occurring above 4.6 GPa.

High-pressure optical spectroscopy and X-ray diffraction studies on synthetic cobalt aluminum silicate garnet

Abstract The pressure-induced behavior of spin-allowed dd-bands of VIIICo2+ in the absorption spectra of synthetic Co3Al2Si3O12 garnet was studied from 10-4 to 13 GPa. The plots of the peak energy vs. pressure for the three sharpest well resolved bands at ca. 5160, 17 680, and 18 740 cm-1 display small but discernible breaks in linear relations between 4 and 5 GPa. Data from single-crystal X-ray diffraction likewise show discontinuities in trends of CoO8 polyhedral volume and distortion, and Co-O and Si-O bond distances over this pressure range. These effects are related to a pressure-induced phase transition from the β- to α-isostructural polymorph of Co3Al2Si3O12.

Optical absorption spectroscopy study of three synthetic V 3+ -bearing clinopyroxenes

Optical absorption spectra of synthetic clinopyroxenes of NaVSi 2 O 6 , LiVSi 2 O 6 and CaVAlSiO 6 composition consist of a high-energy absorption edge and two broad absorption bands ν 1 and ν 2 at around 15,000 cm −1 and 22,000 cm −1 caused by spin-allowed electronic transitions 3 T 1 g ( 3 F ) → 3 T 2 g ( 3 F ) and 3 T 1 g ( 3 F ) → 3 T 1 g ( 3 P ) of V 3+ in octahedral M1-site of the clinopyroxene structure. In polarized spectra of NaVSi 2 O 6 and LiVSi 2 O 6 pyroxenes the relative intensities of the polarized components of ν 1 - and ν 2 -bands are rather different, revealing differences in the distortion of VO 6 -octahedra in the two structures. In spite of a formal similarity to the absorption spectra of Cr 3+ in Cr-bearing clinopyroxenes, the pressure-induced behaviour of the spin-allowed bands of V 3+ in NaVSi 2 O 6 -pyroxene is noticeably different: although they also shift to higher energies as do the bands of Cr 3+ , the integral intensity of the ν 1 -band remains nearly constant within uncertainty whereas that of the ν 2 -band somewhat decreases, in contrast to Cr 3+ for which the spin-allowed bands significantly decrease. Comparing with k oct loc -value of CrO 6 -octahedron in NaCrSi 2 O 6 kosmochlor, ~196 GPa, the VO 6 -octahedron in NaVSi 2 O 6 clinopyroxene structure is considerably softer, with k oct loc ≈ 163 GPa. The distances derived from the optical absorption spectra can fairly well be approximated by a linear dependence. The Racah parameter B of V 3+ evidently decreases with increasing pressure from ~554 cm −1 at 10 −4 GPa to 508 cm −1 at 10.7 GPa, thus indicating that V – O bonds in the VO 6 -octahedra become more covalent.

High temperature structure and thermal expansion of Co 3 Al 2 Si 3 O 12 garnet

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 135-144 - DOI: 10.2451/2011PM0011 Special Issue in memory of Sergio Lucchesi High temperature structure and thermal expansion of Co 3 Al 2 Si 3 O 12 garnet Mario Tribaudino 1,* and Haruo Ohashi 2 1 Dipartimento di Scienze della Terra, Universita di Parma, Italy 2 Hashi Institute for Silicate Science, Nishinakanobu 1-9-25, Shinagawa, Tokyo 142-0054, Japan *Corresponding author: mario.tribaudino@unipr.it Abstract Synchrotron radiation powder diffraction patterns were taken on synthetic Co 3 Al 2 Si 3 O 12 garnet at BM8-GILDA (ESRF), at T = 298, 423, 573, 723 and 873 K. The cell parameters and atomic positions were refined by Rietveld analysis; the volume thermal expansi on coefficient is α V = 28.5(9) x10 -6 K -1 . The results on synthetic Co 3 Al 2 Si 3 O 12 garnet were compared with the high temperature structure data of X 3 Al 2 Si 3 O 12 garnets with X = Mg, Ca, Mn and Fe taken from literature. The thermal expansion of the unit cell is very similar in X 3 Al 2 Si 3 O 12 garnets, whereas the thermal expansion of the longest bonds in the distorted cubic cage surrounding the X cation decreases with increasing cation size. Contrarily to the compression behaviour, the thermal expansion in X 3 Al 2 Si 3 O 12 garnets does not change significantly as a function of composition. Key words : garnet; thermal expansion; Cobalt; synchrotron radiation; powder diffraction; crystal structure.

Effect of the contraction of Zn-O bonds on X-ray emission spectra

The X-ray emission spectra were measured for tetrahedrally coordinated Zn ion in several synthetic compounds. Intensity ratios of Lβ and Lα spectral lines correlate strikingly with the Δ(Zn—O) values, which are the differences between theoretical and measured or calculated distances of Zn—O in the compounds and correspond to the contraction of the Zn—O bond. This difference increases with the Lβ/Lα intensity ratio. The increments of the Lβ/Lα intensity ratios are mainly attributed to the increments of Lβ intensity that accompany the contraction of Zn—O bonds. Coexistence of a transition metal ion increases the Lβ intensity. The Lβ transition probability and the Lβ/Lα intensity ratio are affected by bond contraction and chemical environment.