High-pressure transformations of ilmenite to perovskite, and lithium niobate to perovskite in zinc germanate

In-situ X-ray powder diffraction measurements conducted under high pressure confirmed the existence of an unquenchable orthorhombic perovskite in ZnGeO₃. ZnGeO₃ ilmenite transformed into perovskite at 30.0 GPa and 1300±150 K in a laser-heated diamond anvil cell. After releasing the pressure, the lithium niobate phase was recovered as a quenched product. The perovskite was also obtained by recompression of the lithium niobate phase at room temperature under a lower pressure than the equilibrium phase boundary of the ilmenite–perovskite transition. Bulk moduli of ilmenite, lithium niobate, and perovskite phases were calculated on the basis of the refined X-ray diffraction data. The structural relations among these phases are considered in terms of the rotation of GeO₆ octahedra. A slight rotation of the octahedra plays an important role for the transition from lithium niobate to perovskite at ambient temperature. On the other hand, high temperature is needed to rearrange GeO₆ octahedra in the ilmenite–perovskite transition. The correlation of quenchability with rotation angle of GeO₆ octahedra for other germanate perovskites is also discussed.     

Phase equilibrium and calorimetric study of the transition of MnTiO3 from the ilmenite to the lithium niobate structure and implications for the stability field of perovskite

The phase boundary between MnTiO₃ I (ilmenite structure) and MnTiO₃ II (lithium niobate structure) has been determined by analysis of quench products from reversal experiments in a cubic anvil apparatus at 1073–1673 K and 43–75 kbar using mixtures of MnTiO₃ I and II as starting materials. Tight brackets of the boundary give P(kbar)=121.2−0.045 T(K). Thermodynamic analysis of this boundary gives ΔH^o=5300±1000 J·mol⁻¹, ΔS^o = 1.98 ±1J·K⁻¹· mol⁻¹. The enthalpy of transformation obtained directly by transposed-temperature-drop calorimetry is 8359 ±2575 J·mol⁻¹. Possible topologies of the phase relations among the ilmenite, lithium niobate, and perovskite polymorphs are constrained using the above data and the observed (reversible with hysteresis) transformation of II to III at 298 K and 20–30 kbar (Ross et al. 1989). The observed II–III transition is likely to lie on a metastable extension of the II–III boundary into the ilmenite field. However the reversed I–II boundary, with its negative dP/ dT does represent stable equilibrium between ilmenite and lithium niobate, as opposed to the lithium niobate being a quench product of perovskite. We suggest a topology in which the perovskite occurs stably at low T and high P with a triple point (I, II, III) at or below 1073 K near 70 kbar. The I–II boundary would have a negative P-T slope while the II–III and I–III boundaries would be positive, implying that entropy decreases in the order lithium niobate, ilmenite, perovskite. The inferred positive slope of the ilmenite-perovskite transition in MnTiO₃ is different from the negative slopes in silicates and germanates. These thermochemical parameters are discussed in terms of crystal structure and lattice vibrations.     

Unquenchable high-pressure perovskite polymorphs of MnSnO3 and FeTiO3

New high-pressure orthorhombic (GdFeO₃-type) perovskite polymorphs of MnSnO₃ and FeTiO₃ have been observed using in situ powder X-ray diffraction in a diamond-anvil cell with synchrotron radiation. The materials are produced by the compression of the lithium niobate polymorphs of MnSnO₃ and FeTiO₃ at room temperature. The lithium niobate to perovskite transition occurs reversibly at 7 GPa in MnSnO₃, with a volume change of -1.5%, and at 16 GPa in FeTiO₃, with a volume change of -2.8%. Both transitions show hysteresis at room temperature. For MnSnO₃ perovskite at 7.35 (8) GPa, the orthorhombic cell parameters are a=5.301 (2) A, b=5.445 (2) Å, c=7.690 (8) Å and V= 221.99 (15) ų. Volume compression data were collected between 7 and 20 GPa. The bulk modulus calculated from the compression data is 257 (18) GPa in this pressure region. For FeTiO₃ perovskite at 18.0 (5) GPa, cell parameters are a=5.022 (6) Å, b=5.169 (5) Å, c=7.239 (9) Å and V= 187.94 (36) ų. Based on published data on the quench phases, the FeTiO₃ perovskite breaks down to a rocksalt + baddelyite mixture of FeO and TiO₂ at 23 GPa. This is the first experimental verification of the pressure-induced breakdown of a perovskite to simple oxides.     

Sequential high-pressure transformations of FeGeO3 high-P clinopyroxene (C 2 /c) at temperatures up to 365 ¯C

 In order to elucidate high-pressure transformations of high-P clinopyroxene (C2/c) at kinetically low temperature where atoms are not thermally activated, the transformation processes of FeGeO₃ clinopyroxene (C2/c) have been investigated at pressures up to 20 GPa and 365 °C by powder X-ray diffraction using a synchrotron radiation source and TEM observation. With increasing pressure up to 20 GPa at room temperature, FeGeO₃ high-P clinopyroxene (C2/c) reversibly transforms into a new high-pressure phase, FeGeO₃(II). On increasing the temperature up to 365 °C, this phase rapidly transforms into FeGeO₃ ilmenite within about 2 h. Intensity analysis of the X-ray diffraction pattern reveals that the high-pressure phase of FeGeO₃(II) has an intermediate structure between clinopyroxene and ilmenite: the cation arrangement is similar to that of clinopyroxene and the oxygen arrangement is similar to that of ilmenite. The comparison of the crystal structures of these polymorphs suggests that clinopyroxene to FeGeO₃(II) and FeGeO₃(II) to ilmenite transformations are performed by the slight deformation of the oxygen packing and the short-range movement of cations, respectively. It is shown that this high-P clinopyroxene transforms into ilmenite through a low-activation energy path under the low-temperature condition. Received: 30 August 2000 / Accepted: 10 February 2001     

Electron density distribution of FeTiO<Subscript>3</Subscript> ilmenite under high pressure analyzed by MEM using single crystal diffraction intensities

Many of ilmenites ABO₃ compounds bearing transition elements have semiconductive, ferroelectric and antiferromagnetic properties. The high-pressure diffraction studies of FeTiO₃ have been conducted up to 8.2 GPa using synchrotron radiation in KEK at Tsukuba with diamond anvil cell. The compression mechanism of FeTiO₃ ilmenite has been investigated by the structure refinements converged to the reliable factors R = 0.05. The deformations of the FeO₆ and TiO₆ octahedra were reduced with increasing pressure. In order to elucidate the electric conductivity change with pressure, electron density distribution of ilmenite have been executed by maximum entropy method (MEM) using single-crystal diffraction intensity data. MEM based on F _obs(hkl) of FeTiO₃ clearly shows electron density in comparison with the difference Fourier synthesis based on F _obs(hkl) − F _calc(hkl). The radial distribution of the electron density indicates electron localization around the cation positions. The bonding electron density found in bond Fe–O and Ti–O is lowered with pressure. The isotropic temperature factors B _iso become smaller with increasing pressure. Nevertheless the thermal vibration is considerably restrained by the compression, the electric conductivity is enhanced with pressure. Neither charge transfer nor electron hopping between Fe and Ti along the c axis in FeTiO₃ is plausible under high pressure. But the electric conductivity due to electron super-exchange in Fe–Fe and Ti–Ti has been clarified by the MEM electron density distribution. The anisotropy in the electric conductivity has been clarified.     

MgSiO3 ilmenite: Heat capacity,thermal expansivity,and enthalpy of transformation

A new determination, using high temperature drop-solution calorimetry, of the enthalpy of transformation of MgSiO₃ pyroxene to ilmenite gives H ₂₉₈ = 59.03 ±4.26 kJ/mol. The heat capacity of the ilmenite and orthopyroxene phases has been measured by differential scanning calorimetry at 170–700 K; C_p of MgSiO₃ ilmenite is 4–10 percent less than that of MgSiO₃ pyroxene throughout the range studied. The heat capacity differences are consistent with lattice vibrational models proposed by McMillan and Ross (1987) and suggest an entropy change of -18 ± 3 J-K^-1 ·mol^-1, approximately independent of temperature, for the pyroxene-ilmenite transition. The unit cell parameters of MgSiO₃ ilmenite were measured at 298–876 K and yield an average volume thermal expansion coefficient of 2.44 × 10^-5 K^-1. The thermochemical data are used to calculate phase relations involving pyroxene, -Mg₂SiO₄ plus stishovite, Mg₂SiO₄ spinel plus stishovite, and ilmenite in good agreement with the results of high pressure studies.     

Low-temperature heat capacities,entropies and high-pressure phase relations of MgSiO<Subscript>3</Subscript> ilmenite and perovskite

Low-temperature isobaric heat capacities (C _p ) of MgSiO₃ ilmenite and perovskite were measured in the temperature range of 1.9–302.4 K with a thermal relaxation method using the Physical Properties Measurement System. The measured C _p of perovskite was higher than that of ilmenite in the whole temperature range studied. From the measured C _p , standard entropies at 298.15 K of MgSiO₃ ilmenite and perovskite were determined to be 53.7 ± 0.4 and 57.9 ± 0.3 J/mol K, respectively. The positive entropy change (4.2 ± 0.5 J/mol K) of the ilmenite–perovskite transition in MgSiO₃ is compatible with structural change across the transition in which coordination of Mg atoms is changed from sixfold to eightfold. Calculation of the ilmenite–perovskite transition boundary using the measured entropies and published enthalpy data gives an equilibrium transition boundary at about 20–23 GPa at 1,000–2,000 K with a Clapeyron slope of −2.4 ± 0.4 MPa/K at 1,600 K. The calculated boundary is almost consistent within the errors with those determined by high-pressure high-temperature in situ X-ray diffraction experiments.     

Vibrational and thermodynamic properties of Ni<Subscript>3</Subscript>S<Subscript>2</Subscript> polymorphs from first-principles calculations

We have calculated the compressional, vibrational, and thermodynamic properties of Ni₃S₂ heazlewoodite and the high-pressure orthorhombic phase (with Cmcm symmetry) using the generalized gradient approximation to the density functional theory in conjunction with the quasi-harmonic approximation. The predicted Raman frequencies of heazlewoodite are in good agreement with room-temperature measurements. The calculated thermodynamic properties of heazlewoodite at room conditions agree very well with experiments, but at high temperatures (especially above 500 K) the heat capacity data from experiments are significantly larger than the quasi-harmonic results, indicating that heazlewoodite is anharmonic. On the other hand, the obtained vibrational density of states of the orthorhombic phase at 20 GPa reveals a group of low-frequency vibrational modes which are absent in heazlewoodite. These low-frequency modes contribute substantially to thermal expansivity, heat capacity, entropy, and Grüneisen parameter of the orthorhombic phase. The calculated phase boundary between heazlewoodite and the orthorhombic phase is consistent with high-pressure experiments; the predicted transition pressure is 17.9 GPa at 300 K with a negative Clapeyron slope of −8.5 MPa/K.     

Phase boundary between ilmenite and perovskite structures in MnGeO3 determined by in situ X-ray diffraction measurements

In situ X-ray observations of the phase transition from ilmenite to perovskite structure in MnGeO₃ were carried out in a Kawai-type high-pressure apparatus interfaced with synchrotron radiation. The phase boundary between the ilmenite and perovskite structures in the temperature range of 700–1,400°C was determined to be P (GPa) = 16.5(±0.6) − 0.0034(±0.0006)T (°C) based on Anderson’s gold pressure scale. The Clapeyron slope, dP/dT, determined in this study is consistent with that for the transition boundary between the ilmenite and the perovskite structure in MgSiO₃.     

High-temperature properties of geikielite (MgTiO3-ilmenite) from high-temperature high-pressure Raman spectroscopy — Some implications for MgSiO3-ilmenite

The Raman spectra of geikielite (MgTiO₃-ilmenite) have been recorded at high pressure (up to 27 GPa) and at high temperature (up to 1820 K). No phase transitions could be evidenced in both cases. In particular, no cation disordering can be evidenced from the high temperature spectra. The observed Raman wavenumber shifts with pressure and with temperature are used to calculate the intrinsic mode anharmonic parameters. The low absolute values of these parameters indicate that geikielite has a nearly quasi-harmonic behaviour, at least to moderate temperatures. However, systematics and the temperature evolution of Raman linewidths suggest that the absolute values of the anharmonic parameters increase at high temperatures. Anharmonic corrections are applied to Kieffer modelling of the constant volume heat capacity of geikielite. They amount to +4 J · mol^-1 · K^-1 at 1800 K, i.e. are much lower than those inferred for, for instance, olivine and garnet structures. These results are used to discuss some implications on the phase relations of the high-pressure MgSiO₃-ilmenite, and the factors controlling the occurence of order-disorder transitions in ilmenite structures.     

A new high-pressure phase of strontium carbonate

High-pressure and high-temperature experiments conducted in a laser-heated diamond-anvil cell with a synchrotron X-ray diffraction method have revealed a phase transformation in the aragonite-type SrCO₃ at pressures above 10 GPa. The new phase has an orthorhombic symmetry and was confirmed to remain stable to 32 GPa. The Birch-Murnaghan equation of state for new phase was determined from the experimental unit cell parameters, with K₀ = 101 (± 16) GPa, K₀ = 4 (constrained value), and V₀ = 111.9 (± 2.2). This transformation in SrCO₃ is different from that in BaCO₃ as reported in previous studies. After decompression at ambient pressure, the high-pressure phase transforms to a metastable structure, which has an orthorhombic symmetry. This result should also resolve a dispute regarding the stable high-pressure phases in BaCO₃, which is an analog material of CaCO₃ and SrCO₃.This revised version was published in February 2005 with corrections to the Introduction and to the References.     

Chemical Characterisation of Natural Ilmenite: A Possible New Reference Material

Seven ilmenite (FeTiO₃) megacrysts derived from alnöite pipes (Island of Malaita, Solomon Islands) were characterised for their major and trace element compositions in relation to their potential use as secondary reference materials for in situ microanalysis. Abundances of thirteen trace elements obtained by laser ablation ICP‐MS analyses (using the NIST SRM 610 glass reference material) were compared with those determined by solution‐mode ICP‐MS measurements, and these indicated good agreement for most elements. The accuracy of the LA‐ICP‐MS protocol employed here was also assessed by repeated analysis of MPI‐DING international glass reference materials ML3B‐G and KL2‐G. Several of the Malaitan ilmenite megacrysts exhibited discrepancies between laser ablation and solution‐mode ICP‐MS analyses, primarily attributed to the presence of a titano‐magnetite exsolution phase (at the grain boundaries), which were incorporated solely in the solution‐mode runs. Element abundances obtained by LA‐ICP‐MS for three of the ilmenite megacrysts (CRN63E, CRN63H and CRN63K) investigated here had RSD (2s) values of < 20% and therefore can be considered as working values for reference purposes during routine LA‐ICP‐MS analyses of ilmenite.     

High-pressure transformations in silicates,germanates, and titanates with ABO3 stoichiometry

High-pressure phase transformations were investigated for two silicates, MgSiO₃ and ZnSiO₃; six germanates, MGeO₃ and six titanates, MTiO₃ (M=Ni, Mg, Co, Zn, Fe, and Mn) at about 1,000°C and pressures up to ca. 30 GPa. CoGeO₃ was found to assume the ilmenite form. The ilmenite phases were confirmed to transform in the following schemes: to perovskite in MgSiO₃ and MnGeO₃, to corundum in MgGeO₃ and ZnGeO₃, to rocksalt plus rutile in ZnSiO₃ and CoGeO₃ and to rocksalt plus TiO₂ (possibly of some denser structure) in NiTiO₃, MgTiO₃, CoTiO₃, ZnTiO₃ and FeTiO₃. In the case of FeTiO₃, the corundum form appeared as an intermediate phase. The possibility that the corundum type MnTiO₃ might transform to some denser modification could not be excluded. The compound NiGeO₃ was nonexistent throughout the pressure range studied. High-pressure phases of ABO₃ (A=Ni, Mg, Co, Zn, Fe, and Mn; B=Si, Ge and Ti) are summarized, and those stabilized at pressures higher than 20 GPa are discussed.     

A 29Si MAS NMR study of sub-Tg amorphization of stishovite at ambient pressure

Stishovite, a high-pressure SiO₂ polymorph in which each Si is coordinated by six O atoms, transforms to an amorphous phase when undergoing heat treatment below the glass transition temperature at ambient pressure. We have applied ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) to study this amorphization process. We found that the amorphous phase generated after heating stishovite for up to 3 days at around 600 ° C consisted exclusively of four-coordinate Si, similar to glasses quenched from melts at ambient pressure. Furthermore, our data suggest that there are subtle structural differences between the amorphous phase transformed from stishovite at 600 ° C and glasses quenched from melts at ambient pressure: the amorphous phase from stishovite had a smaller mean Si-O-Si angle initially, and it gradually relaxed toward the latter with increasing heating time. There was no detectable change in the stishovite structure even after about 80% of it had been converted to the amorphous phase. The mechanism of the amorphization of stishovite is discussed in light of these results.     

CdGeO3-phase transformations at high pressure and temperature and structural refinement of the perovskite polymorph

Four polymorphs of CdGeO₃ were synthesized at high temperatures (600 ～ 1200° C) and high pressures up to 12 GPa. The pyroxenoid phase synthesized under ambient pressure transforms to garnet, ilmenite and perovskite phases with increasing pressure. The phase boundary of ilmenite-perovskite had a slightly negative P-T slope in contrast to the positive P-T slopes of the pyroxenoid-garnet and garnet-ilmenite transition boundaries. CdGeO₃III has the ilmenite structure with hexagonal lattice parameters, a=5.098 Å and c =14.883 Å. The c/a ratio of 2.919 is greater than that of any other ilmenite. CdGeO₃IV has a distorted perovskite structure with orthorhombic lattice parameters a = 5.209 Å, b = 5.253 Å and c = 7.434 Å. Synthesis of a CdGeO₃IV single crystal was successful and structural refinement revealed that the structure is isomorphic to GdFeO₃ with the space group Pbnm. The increase of density with the CdGeO₃III→CdGeO₃IV transformation is the largest (9.8%) for any ilmenite-perovskite transition studied so far.     

Pressure-induced structural modifications in Mg2GeO4-olivine: A Raman spectroscopic study

Raman spectra of Mg₂GeO₄-olivine were obtained from ambient pressure up to 34 GPa at ambient temperature. Under quasi-hydrostatic pressure conditions, the following modifications in the Raman spectra occur as pressure increases: 1) near 11 GPa, two sharp extra bands appear in the 600–700 cm^?1 frequency range, and increase in intensity with respect to the olivine bands; 2) above 22 GPa, these two bands become very intense, and the number, position and relative intensity of the other vibrational bands drastically change; 3) the intensity of sharp bands progressively decreases above 25 GPa. The transformation occurs at lower pressures under non-hydrostatic conditions. During decompression to atmospheric pressure, the high-pressure phase partially reverts to olivine. These observations can be interpreted as the progressive metastable transformation from the olivine structure to a crystalline phase with four-fold coordinated Ge, in which the GeO₄ tetrahedra are polymerized. We propose that the metastable high-pressure phase is a structurally disordered spinelloid close to the hypothethical ω- or ?^*-phase, and forms by a shear mechanism assisted by the development of a dynamical instability in the olivine structure. Implications for the transformations undergone by olivines under far-from-equilibrium conditions (e.g. in subducting lithospheric slabs and in shocks) are discussed.     

Heat capacity calculations for Al2O3 corundum and MgSiO3 ilmenite

We have used Kieffer's vibrational model to calculate heat capacities and entropies for Al₂O₃ corundum and MgSiO₃ ilmenite, using available vibrational and elastic data for these phases. The calculated heat capacity for corundum is within 1–2 percent of the experimental values between 100 K and 1,800 K, while that for MgSiO₃ ilmenite is within 1–2 percent of the experimental data between 350 K and 500 K. We have calculated the heat capacity for MgSiO₃ ilmenite from 50 K to 1,800 K, which extends the range of available heat capacity data for this phase. The results of this calculation suggest that there may be differences in the vibrational properties of corundum and MgSiO₃ ilmenite. Finally, we have used the results of our calculation to obtain a transition entropy of near -18.8 J/mol.K for the MgSiO₃ pyroxene-ilmenite reaction.     

A high-pressure neutron diffraction study of the ferroelastic phase transition in RbCaF3

The fluoroperovskite phase RbCaF₃ has been investigated using high-pressure neutron powder diffraction in the pressure range ~0–7.9 GPa at room temperature. It has been found to undergo a first-order high-pressure structural phase transition at ~2.8 GPa from the cubic aristotype phase to a hettotype phase in the tetragonal space group I4/mcm. This transition, which also occurs at ~200 K at ambient pressure, is characterised by a linear phase boundary and a Clapeyron slope of 2.96 × 10^?5 GPa K^?1, which is in excellent agreement with earlier, low-pressure EPR investigations. The bulk modulus of the high-pressure phase (49.1 GPa) is very close to that determined for the low-pressure phase (50.0 GPa), and both are comparable with those determined for the aristotype phases of CsCdF₃, TlCdF₃, RbCdF₃, and KCaF₃. The evolution of the order parameter with pressure is consistent with recent modifications to Landau theory and, in conjunction with polynomial approximations to the pressure dependence of the lattice parameters, permits the pressure variation of the bond lengths and angles to be predicted. On entering the high-pressure phase, the Rb–F bond lengths decrease from their extrapolated values based on a third-order Birch–Murnaghan fit to the aristotype equation of state. By contrast, the Ca–F bond lengths behave atypically by exhibiting an increase from their extrapolated magnitudes, resulting in the volume and the effective bulk modulus of the CaF₆ octahedron being larger than the cubic phase. The bulk moduli for the two component polyhedra in the tetragonal phase are comparable with those determined for the constituent binary fluorides, RbF and CaF₂.     

The Thermodynamics of Ordered Perovskites on the CaTiO3-FeTiO3 Join

The enthalpies of formation from ilmenite, FeTiO₃, and perovskite, CaTiO₃, of two intermediate ordered perovskite phases, CaFeTi₂O₆ and CaFe₃Ti₄O₁₂, have been measured at 801°C using oxide melt solution calorimetry. These data, in combination with experiments at high pressure and temperature, indicate that below 1518±50°C CaFe₃Ti₄O₁₂ is stable at lower pressures (∼7 GPa at 1200°C) than CaFeTi₂O₆ (∼13 GPa at 1200°C). This relationship should be reversed, and CaFeTi₂O₆ should become stable at lower pressures than CaFe₃Ti₄O₁₂, at temperatures above 1518±50°C. These intermediate phases are of petrological interest because they form as a reaction between two minerals, ilmenite and perovskite, which are commonly associated in kimberlites, and because their pressure-temperature range of formation overlaps that of origin of kimberlites. Received: 10 November 1997 / Revised; accepted: 15 January 1998     

The ZnSiO3 clinopyroxene-ilmenite transition: Heat capacity,enthalpy of transition,and phase equilibria

ZnSiO₃ clinopyroxene stable above 3 GPa transforms to ilmenite at 10–12 GPa, which further decomposes into ZnO (rock salt) plus stishovite at 20–30 GPa. The enthalpy of the clinopyroxene-ilmenite transition was measured by high-temperature solution calorimetry, giving ΔH⁰=51.71 ±3.18 kJ/mol at 298 K. The heat capacities of clinopyroxene and ilmenite were measured by differential scanning calorimetry at 343–733 and 343–633 K, respectively. The C _p of ilmenite is 3–5% smaller than that of clinopyroxene. The entropy of transition was calculated using the measured enthalpy and the free energy calculated from the phase equilibrium data. The enthalpy, entropy and volume changes of the pyroxene-ilmenite transition in ZnSiO₃ are similar in magnitude to those in MgSiO₃. The present thermochemical data are used to calculate the phase boundary of the ZnSiO₃ clinopyroxene-ilmenite transition. The calculated boundary,