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1.
Enthalpies of drop solution (Δ H
drop-sol) of CaGeO 3, Ca(Si 0.1Ge 0.9)O 3, Ca(Si 0.2Ge 0.8)O 3, Ca(Si 0.3Ge 0.7)O 3 perovskite solid solutions and CaSiO 3 wollastonite were measured by high-temperature calorimetry using molten 2PbO · B 2O 3 solvent at 974 K. The obtained values were extrapolated linearly to the CaSiO 3 end member to give Δ H
drop-sol of CaSiO 3 perovskite of 0.2 ± 4.4 kJ mol −1. The difference in Δ H
drop-sol between CaSiO 3, wollastonite, and perovskite gives a transformation enthalpy (wo → pv) of 104.4 ± 4.4 kJ mol −1. The formation enthalpy of CaSiO 3 perovskite was determined as 14.8 ± 4.4 kJ mol −1 from lime + quartz or −22.2 ± 4.5 kJ mol −1 from lime + stishovite. A comparison of lattice energies among A 2+B 4+O 3 perovskites suggests that amorphization during decompression may be due to the destabilizing effect on CaSiO 3 perovskite from a large nonelectrostatic energy (repulsion energy) at atmospheric pressure. By using the formation enthalpy
for CaSiO 3 perovskite, phase boundaries between β-Ca 2SiO 4 + CaSi 2O 5 and CaSiO 3 perovskite were calculated thermodynamically utilizing two different reference points [where Δ G( P, T )=0] as the measured phase boundary. The calculations suggest that the phase equilibrium boundary occurs between 11.5 and
12.5 GPa around 1500 K. Its slope is still not well constrained.
Received: 20 September 2000 / Accepted: 17 January 2001 相似文献
2.
In-situ X-ray powder diffraction measurements conducted under high pressure confirmed the existence of an unquenchable orthorhombic perovskite in ZnGeO 3. ZnGeO 3 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 GeO 6 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 GeO 6 octahedra in the ilmenite–perovskite transition. The correlation of quenchability with rotation angle of GeO 6 octahedra for other germanate perovskites is also discussed. 相似文献
3.
Low-temperature isobaric heat capacities ( C
p
) of MgSiO 3 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 MgSiO 3 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 MgSiO 3 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. 相似文献
4.
The phase relations and compression behavior of MnTiO 3 perovskite were examined using a laser-heated diamond-anvil cell, X-ray diffraction, and analytical transmission electron
microscopy. The results show that MnTiO 3 perovskite becomes unstable and decomposes into MnO and orthorhombic MnTi 2O 5 phases at above 38 GPa and high temperature. This is the first example of ABO 3 perovskite decomposing into AO + AB 2O 5 phases at high pressure. The compression behavior of volume, axes, and the tilting angle of TiO 6 octahedron of MnTiO 3 perovskite are consistent with those of other A 2+B 4+O 3 perovskites, although no such decomposition was observed in other perovskites. FeTiO 3 is also known to decompose into two phases, instead of transforming into the CaIrO 3-type post-perovskite phase and we argue that one of the reasons for the peculiar behavior of titanate is the weak covalency
of the Ti–O chemical bonds. 相似文献
5.
The effect of crystal structure relaxation in oxygen-based Cr3+-containing minerals on the crystal field stabilization energy (CFSE) is considered. It is shown that the dependence of
\textCFSE\textCr 3+ {\text{CFSE}}_{{{\text{Cr}}^{ 3+ } }} , which is found from optical absorption spectra, on the average interatomic distances is described by the power function
with a negative exponent
c \mathord | / |
\vphantom c [`(R)]n [`(R)]n {c \mathord{\left/ {\vphantom {c {\bar{R}^{n} }}} \right. \kern-\nulldelimiterspace} {\bar{R}^{n} }} , where n approaches 5, as predicted theoretically, for pure Cr3+ compounds, but decreases to 1.0–1.5 for Cr3+-containing oxide and silicate solid solutions. The deviation of the experimental dependence for solid solutions from the
theoretical curve is due to structure relaxation, which tends to bring the local structure of Cr3+ ions closer to the structure in the pure Cr compound, thus producing changes in interatomic distances between the nearest
neighbors with respect to those in the average structure determined by X-ray diffraction. As a consequence, the mixing enthalpy
of Cr3+-bearing solid solutions can be represented by the sum of contributions from lattice strain and CFSE. The latter contribution
is most often negative in sign and, therefore, brings the Al–Cr solid solutions close to an ideal solid solution. It is supposed
that the increased Cr content in minerals from deep-seated mantle xenoliths and mineral inclusions in diamonds results from
the effect of
\textCFSE\textCr 3+ {\text{CFSE}}_{{{\text{Cr}}^{ 3+ } }} enhanced by high pressure. 相似文献
6.
Wadeite K 2ZrSi 3O 9 and its analogues K 2TiSi 3O 9 and Cs 2ZrSi 3O 9, synthesized by high-temperature solid-state sintering, have been investigated using powder X-ray diffraction coupled with
Rietveld analysis and high-temperature oxide melt solution calorimetry. The crystal chemistry and energetics of these phases,
together with K 2Si VISi 3
IVO 9, a high-pressure wadeite analogue containing both tetrahedral and octahedral Si, are discussed in term of ionic substitutions.
As the size of the octahedral framework cation increases, Si 4+ → Ti 4+ → Zr 4+, the cell parameter c increases at a much higher rate than a. In contrast, increasing the interstitial alkali cation size (K + → Cs +) results in a higher rate of increase in a compared with c. This behavior can be attributed to framework distortion around the interstitial cation. The enthalpies of formation from
the constituent oxides (ΔH f,ox0) and from the elements (ΔH f,el0) have been determined from drop-solution calorimetry into 2PbO·B 2O 3 solvent at 975 K. The obtained values (in kJ/mol) are as follows: ΔH f,ox0 (K 2TiSi 3O 9) = −355.8 ± 3.0, ΔH f,el0 (K 2TiSi 3O 9) = −4395.1 ± 4.8, ΔH f,ox0 (K 2ZrSi 3O 9) = −374.3 ± 3.3, ΔH f,el0 (K 2ZrSi 3O 9) = −4569.9 ± 5.0, ΔH f,ox0 (Cs 2ZrSi 3O 9) = −396.6 ± 4.4, and ΔH f,el0 (Cs 2ZrSi 3O 9) = −4575.0 ± 5.5. The enthalpies of formation for K 2Si VISi 3
IVO 9 were calculated from its drop-solution enthalpy of an earlier study (Akaogi et al. 2004), and the obtained ΔH f,ox0 (K 2SiSi 3O 9) = −319.7 ± 3.4 and ΔH f,el0 (K 2SiSi 3O 9) = −4288.7 ± 5.1 kJ/mol. With increasing the size of the octahedral framework cation or of the interstitial alkali cation,
the formation enthalpies become more exothermic. This trend is consistent with the general behavior of increasing energetic
stability with decreasing ionic potential ( z/ r) seen in many oxide and silicate systems. Further, increasing the size of the octahedral framework cation appears to induce
more rapid increase in stability than increasing the interstitial alkali cation size, suggesting that framework cations play
a more dominant role in wadeite stability. 相似文献
7.
A wide set of aqueous chemistry data (574 water analyses) from natural environments has been used to testify and validate
of the solubility of synthetic hydroxyaluminosilicate (HAS B) , Al 2Si 2O 5(OH) 4. The ground and surface waters represent regolith and/or fissure aquifers in various (magmatic, sedimentary and metamorphic)
bedrocks in the Sudetes Mts. (SW Poland). The solubility of HAS B in natural waters was calculated using the method proposed by Schneider et al. (Polyhedron 23:3185–3191, 2004). Results confirm
usefulness and validity of this method. The HAS B solubility obtained from the field data (logK sp = −44.7 ± 0.58) is lower than it was estimated (logK sp = −40.6 ± 0.15) experimentally (Schneider et al. Polyhedron 23:3185–3191, 2004). In the waters studied the equilibrium with
HAS B is maintained at pH above 6.7 and at [Al 3+] ≤ 10 −10. Silicon activity (log[H 4SiO 4]) ranges between −4.2 and −3.4. Due to the calculation method used, the K sp mentioned above cannot be considered as a classical solubility constant. However, it can be used in the interpretation of
aluminium solubility in natural waters. The HAS B has solubility lower than amorphous Al(OH) 3, and higher than proto-imogolite. From water samples that are in equilibrium with respect to HAS B, the solubility product described by the reaction,
is calculated to be logK sp = 14.0 (±0.7) at 7°C. 相似文献
8.
Raman spectroscopy and heat capacity measurements have been used to study the post-perovskite phase of CaIr 0.5Pt 0.5O 3, recovered from synthesis at a pressure of 15 GPa. Laser heating CaIr 0.5Pt 0.5O 3 to 1,900 K at 60 GPa produces a new perovskite phase which is not recoverable and reverts to the post-perovskite polymorph
between 20 and 9 GPa on decompression. This implies that Pt-rich CaIr 1−xPt xO 3 perovskites including the end member CaPtO 3 cannot easily be recovered to ambient pressure from high P–T synthesis. We estimate an increase in the thermodynamic Grüneisen
parameter across the post-perovskite to perovskite transition of 34%, of similar magnitude to those for (Mg,Fe)SiO 3 and MgGeO 3, suggesting that CaIr 0.5Pt 0.5O 3 is a promising analogue for experimental studies of the competition in energetics between perovskite and post-perovskite
phases of magnesium silicates in Earth’s lowermost mantle. Low-temperature heat capacity measurements show that CaIrO 3 has a significant Sommerfeld coefficient of 11.7 mJ/mol K 2 and an entropy change of only 1.1% of Rln2 at the 108 K Curie transition, consistent with the near-itinerant electron magnetism. Heat capacity results for post-perovskite
CaIr 0.5Rh 0.5O 3 are also reported. 相似文献
9.
Comparison of polarized optical absorption spectra of natural Ca-rich diopsides and synthetic NaCrSi 2O 6 and LiCrSi 2O 6 clinopyroxenes, evidences as vivid similarities, as noticeable differences. The similarities reflect the fact that in all
cases Cr 3+ enters the small octahedral M1-site of the clinopyroxene structure. The differences are due to some iron content in the natural
samples causing broad intense near infrared bands of electronic spin-allowed dd transitions of Fe 2+(M1, M2) and intervalence Fe 2+/Fe 3+ charge-transfer transition, and by different symmetry and different local crystal fields strength of Cr 3+ in the crystal structures. The positions of the spin-allowed bands of Cr 3+, especially of the low energy one caused by the electronic 4
A
2g → 2
T
1g transition, are found to be in accordance with mean M1–O distances. The local relaxation parameter ε calculated for lim Cr
3+ → 0 from the spectra and interatomic
á Cr - O
ñ \left\langle {Cr - O} \right\rangle and
á Mg - O
ñ \left\langle {Mg - O} \right\rangle distances yields a very high value, 0.96, indicating that in the clinopyroxene structure the local lattice relaxation around
the “guest” ion, Cr 3+, deviates greatly from the “diffraction” value, ε = 0, than in any other known Cr 3+-bearing systems studied so far. Under pressure the spin-allowed bands of Cr 3+ shift to higher energies and decrease in intensity quite in accordance with the crystal field theoretical expectations, while
the spin-forbidden absorption lines remain practically unshifted, but also undergo a strong weakening. There is no evident
dependence of the Racah parameter B of Cr 3+ reflecting the covalence of the oxygen-chromium bond under pressure: within the uncertainty of determination it may be regarded
as practically constant. The values of CrO 6 octahedral modulus,
k\textpoly\textloc k_{\text{poly}}^{\text{loc}} , derived from high-pressure spectra of natural chromium diopside and synthetic NaCrSi 2O 6 kosmochlor are very close, ~203 and ~196 GPa, respectively, being, however, nearly twice higher than that of MgO 6 octahedron in diopside, 105(4) GPa, obtained by Thompson and Downs ( 2008). Such a strong stiffening of the structural octahedron, i.e. twice higher value of
k\textCr3 + \textloc k_{{{\text{Cr}}^{3 + } }}^{\text{loc}} comparing with that of
k\textMg2 + \textloc k_{{{\text{Mg}}^{2 + } }}^{\text{loc}} , may be caused by simultaneous substitution of Ca 2+ by larger Na + in the neighboring M2 sites at so-called jadeite-coupled substitution Mg 2+ + Ca 2+ → Cr 3+ + Na +. It is also remarkable that the values of CrO 6 octahedral modulus of NaCrSi 2O 6 kosmochlor obtained here are nearly twice larger than that of 90(16) GPa, evaluated by high-pressure X-ray structural refinement
by Origlieri et al. ( 2003). Taking into account that the overall compressibility of the clinopyroxene structure should mainly be due to the compressibility
of M1- and M2-sites, our
k\textCr3 + \textloc k_{{{\text{Cr}}^{3 + } }}^{\text{loc}} -value, ~196 GPa, looks much more consistent with the bulk modulus value, 134(1) GPa. 相似文献
10.
Al-containing MgSiO 3 perovskites of four different compositions were synthesized at 27 GPa and 1,873 K using a Kawai-type high-pressure apparatus:
stoichiometric compositions of Mg 0.975Si 0.975Al 0.05O 3 and Mg 0.95Si 0.95Al 0.10O 3 considering only coupled substitution Mg 2+ + Si 4+ = 2Al 3+, and nonstoichiometric compositions of Mg 0.99Si 0.96Al 0.05O 2.985 and Mg 0.97Si 0.93Al 0.10O 2.98 taking account of not only the coupled substitution but also oxygen vacancy substitution 2Si 4+ = 2Al 3+ + V O¨. Using the X-ray diffraction profiles, Rietveld analyses were performed, and the results were compared between the stoichiometric
and nonstoichiometric perovskites. Lattice parameter–composition relations, in space group Pbnm, were obtained as follows. The a parameters of both of the stoichiometric and nonstoichiometric perovskites are almost constant in the X
Al range of 0–0.05, where X
Al is Al number on the basis of total cation of two ( X
Al = 2Al/(Mg + Si + Al)), and decrease with further increasing X
Al. The b and c parameters of the stoichiometric perovskites increase linearly with increasing Al content. The change in the b parameter of the nonstoichiometric perovskites with Al content is the same as that of the stoichiometric perovskites within
the uncertainties. The c parameter of the nonstoichiometric perovskites is slightly smaller than that of the stoichiometric perovskites at X
Al of 0.10, though they are the same as each other at X
Al of 0.05. The Si(Al)–O1 distance, Si(Al)–O1–Si(Al) angle and minimum Mg(Al)–O distance of the nonstoichiometric perovskites
keep almost constant up to X
Al of 0.05, and then the Si(Al)–O1 increases and both of the Si(Al)–O1–Si(Al) angle and minimum Mg(Al)–O decrease with further
Al substitution. These results suggest that the oxygen vacancy substitution may be superior to the coupled substitution up
to X
Al of about 0.05 and that more Al could be substituted only by the coupled substitution at 27 GPa. The Si(Al)–O1 distance and
one of two independent Si(Al)–O2 distances in Si(Al)O 6 octahedra in the nonstoichiometric perovskites are always shorter than those in the stoichiometric perovskite at the same
Al content. These results imply that oxygen defects may exist in the nonstoichiometric perovskites and distribute randomly. 相似文献
11.
Magnesium silicate perovskite is the predominant phase in the Earth’s lower mantle, and it is well known that incorporation of iron has a strong effect on its crystal structure and physical properties. To constrain the crystal chemistry of (Mg, Fe)SiO 3 perovskite more accurately, we synthesized single crystals of Mg 0.946(17)Fe 0.056(12)Si 0.997(16)O 3 perovskite at 26 GPa and 2,073 K using a multianvil press and investigated its crystal structure, oxidation state and iron-site occupancy using single-crystal X-ray diffraction and energy-domain Synchrotron Mössbauer Source spectroscopy. Single-crystal refinements indicate that all iron (Fe 2+ and Fe 3+) substitutes on the A-site only, where \( {\text{Fe}}^{ 3+ } /\Upsigma {\text{Fe}}\sim 20\,\% \) based on Mössbauer spectroscopy. Charge balance likely occurs through a small number of cation vacancies on either the A- or the B-site. The octahedral tilt angle ( Φ) calculated for our sample from the refined atomic coordinates is 20.3°, which is 2° higher than the value calculated from the unit-cell parameters ( a = 4.7877 Å, b = 4.9480 Å, c = 6.915 Å) which assumes undistorted octahedra. A compilation of all available single-crystal data (atomic coordinates) for (Mg, Fe)(Si, Al)O 3 perovskite from the literature shows a smooth increase of Φ with composition that is independent of the nature of cation substitution (e.g., \( {\text{Mg}}^{ 2+ } - {\text{Fe}}^{ 2+ } \) or \( {\text{Mg}}^{ 2+ } {\text{Si}}^{ 4+ } - {\text{Fe}}^{ 3+ } {\text{Al}}^{ 3+ } \) substitution mechanism), contrary to previous observations based on unit-cell parameter calculations. 相似文献
12.
Time-resolved luminescence spectra of natural and synthetic hydrous volcanic glasses with different colors and different
Fe, Mn, and H 2O content were measured, and the implications for the glass structure are discussed. Three luminescence ranges are observed
at about 380–460, 500–560, and 700–760 nm. The very short-living (lifetimes less than 40 ns) blue band (380–460 nm) is most
probably due to the 4T 2( 4D) → 6A 1( 6S) and 4A 1( 4G) → 6A 1( 6S) ligand field transitions of Fe 3+. The green luminescence (500–560 nm) arises from the Mn 2+ transition 4T 1( 4G) → 6A 1( 6S). It shows weak vibronic structure, short lifetimes less than 250 μs, and indicates that Mn 2+ is tetrahedrally coordinated, occupying sites with similar distortions and ion–oxygen interactions in all samples studied.
The red luminescence (700–760 nm) arising from the 4T 1( 4G) → 6A 1( 6S) transition of Fe 3+ has much longer lifetimes of the order of several ms, and indicates that ferric iron is also mainly tetrahedrally coordinated.
Increasing the total water content of the glasses leads to quenching of the red luminescence and decrease of the distortions
of the Fe 3+ polyhedra.
Received: 30 July 2001 / Accepted: 15 November 2001 相似文献
13.
Room-temperature-polarized single-crystal Raman spectra have been measured for both GdAlO 3 and YAlO 3. Both aluminates crystallize in the orthorhombic (Pbnm) perovskite structure. Of the 24 possible Raman modes in 4 symmetries,
20 and 17 modes were observed for gadolinium and yttrium aluminates, respectively. Comparisons of the Raman spectra of these
two aluminates to those of 28 other orthorhombic ABO 3 perovskites revealed remarkably similar spectral patterns, regardless of chemistry or valency of the cations. Closer examination
of the effect of mass, valencies, and size of the cations on the Raman spectra versus composition revealed that for the observed
modes, the A cation plays the dominant role in determining the Raman shift. In particular, the one to two lowest energy modes
in each symmetry are determined by cation mass and valency no matter what the chemistry. For some perovskites with common
A cations, higher energy modes were also strikingly similar. In particular, the calcium perovskites had almost all A g modes at the same energies despite the greatly varying B cations. The second to the lowest mode in A g and B 1g depended only on A cation mass for all perovskites. The volume plays a minor role throughout but is hard to separate from
mass effects because the most massive cations are also the largest. However, if the B-cation is common, for example, aluminates
or ferrites, the volume has a minor effect on the higher energy modes. These trends were not observed for all perovskites.
Notable exceptions were found if a perovskite is near a phase transition or metastable, as found for three manganites. The
effect of increased valency of the A cation from 2–4 to 3–3 perovskites expresses itself as relatively larger Raman shifts
for the lowest energy modes. Analog studies of MgSiO 3 perovskites should be undertaken with only 2–4 perovskites. The increased understanding for the mode distributions of perovskites
allows for better estimates of their thermodynamic properties through vibrational modeling. 相似文献
14.
A selected set of five different kyanite samples was analysed by electron microprobe and found to contain chromium between <0.001 and 0.055 per formula unit (pfu). Polarized electronic absorption spectroscopy on oriented single crystals,
R 1, R 2-sharp line luminescence and spectra of excitation of λ 3- and λ 4-components of R 1-line of Cr 3+-emission had the following results: (1) The Fe 2+–Ti 4+ charge transfer in c-parallel chains of edge connected M(1) and M(2) octahedra shows up in the electronic absorption spectra
as an almost exclusively c(||Z′)-polarized, very strong and broad band at 16000 cm −1 if <, in this case the only band in the spectrum, and at an invariably lower energy of 15400 cm −1 in crystals with ≥ . The energy difference is explained by an expansion of the O f–O k, and O b–O m edges, by which the M(1) and M(2) octahedra are interconnected (Burnham 1963), when Cr 3+ substitutes for Al compared to the chromium-free case. (2) The Cr 3+ is proven in two greatly differing crystal fields a and b, giving rise to two sets of bands, derived from the well known
dd transitions of Cr 3+
4A 2g→ 4T 2g(F)(I), → 4T 1g(F)(II), and → 4T 1g(P)(III). Band energies in the two sets a and b, as obtained by absorption, A, and excitation, E, agree well: I: 17300(a, A),
17200(a, E), 16000(b, A), 16200(b, E); II: 24800(a, A), 24400(a, E); 22300(b, A), 22200(b, E); III: 28800(b,A) cm −1. Evaluation of crystal field parameters from the bands in the electronic spectra yield Dq( a)=1730 cm −1, Dq( b)=1600 cm −1, B( a)=790 cm −1, B( b)=620 cm −1 (errors ca. ±10 cm −1), again in agreement with values extracted from the λ 3, λ 4 excitation spectra. The CF-values of set a are close to those typical of Cr 3+ substituting for Al in octahedra of other silicate minerals without constitutional OH − as for sapphirine, mantle garnets or beryl, and are, therefore, interpreted as caused by Cr 3+ substituting for Al in some or all of the M(1) to M(4) octaheda of the kyanite structure, which are crystallographically
different but close in their mean Al–O distances, ranging from 1.896 to 1.919 A (Burnham 1963), and slight degrees of distortion.
Hence, band set a originates from substitutive Cr 3+ in the kyanite structural matrix. The CF-data of Cr 3+ type b, expecially B, resemble those of Cr 3+ in oxides, especially of corundum type solid solutions or eskolaite. This may be interpreted by the assumption that a fraction
of the total chromium contents might be allocated in a precursor of a corundum type exsolution.
Received: 3 January 1997 / Revised, accepted: 2 May 1997 相似文献
15.
The spin Hamiltonian (SH) parameters ( g factors g
x
, g
y
and g
z
and the hyperfine structure constants A
x
, A
y
and A
z
) and local structure for the rhombic Rh 4+ and Ir 4+ centers in TiO 2 (rutile) are theoretically studied from the perturbation formulas of these parameters for a low spin ( S = 1/2) d
5 ion under rhombically distorted octahedra. In the calculations, the ligand orbital and spin–orbit coupling contributions
as well as the influence of the local lattice distortions are taken into account using the cluster approach. The local axial
elongation ratios are found to be about 1.7 and 3 times, respectively, larger for the Rh 4+ and Ir 4+ centers than that (≈0.0075) for the host Ti 4+ site in rutile, while the perpendicular distortion angles (≈−0.28° and −0.42°, respectively) are more than one order in magnitude
smaller than the host value (≈−9.12°). This means that the impurity centers exhibit further elongations of the oxygen octahedra
and much smaller perpendicular rhombic distortions as compared with those of the host Ti 4+ site in TiO 2. The above local lattice distortions can be mainly ascribed to the substitution of the host Ti 4+ by the nd
5 impurities, which may induce different physical and chemical properties for the metal–ligand clusters. In addition, the influence
of the Jahn–Teller effect on the local structure may not be completely excluded. The calculated SH parameters show reasonable
agreement with the observed values. 相似文献
16.
An exploratory high-pressure study of the join CaTiO 3-FeTiO 3 has uncovered two intermediate perovskites with the compositions CaFe 3Ti 4O 12 and CaFeTi 2O 6. These perovskites have ordering of Ca 2+ and Fe 2+ on the A sites. Both of these perovskites are unusual in that the A sites containing Fe 2+ are either square planar or tetrahedral, due to the particular tilt geometries of the octahedral frameworks. For CaFe 3Ti 4O 12, the structure has been refined from a powder using the Rietveld technique. This compound is a cubic double perovskite (SG Im $\bar 3$ , a = 7.4672 Å), isostructural with NaMn 7O 12. Fe 2+ is in a square-planar A site (similar to Mn 3+ in NaMn 7O 12) with Fe-O = 2.042(3) Å, with distant second neighbors in a rectangle at Fe-O = 2.780(6) Å. Calcium is in a distorted icosahedron with Ca-O =2.635(5) Å. CaFeTi 2O 6 crystallizes in a unique tetragonal double perovskite structure (SG P4 2/nmc, a = 7.5157(2), c = 7.5548(2)), with A-site iron in square-planar (Fe-O = 2.097(2) Å) and tetrahedral (Fe-O = 2.084(2) Å) coordination, again with distant second neighbor oxygens near 2.8 Å. Rietveld refinement was also performed for the previously known perovskite-related form of FeTiO 3 recovered from high pressure (lithium niobate type). This compound is trigonal R3c, with a = 5.1233(1) and c = 13.7602(2). The ordered perovskites appear to be stable at 1215 GPa and CaFe 3Ti 4O 12 is found as low as 5 GPa. Thus these perovskites may be important to upper mantle mineralogy, particularly in kimberlites. These compounds are the first known quenchable perovskites with large amounts of A-site ferrous iron, and add greatly to the known occurrences of ferrous iron in perovskites. 相似文献
17.
High-pressure phase transformations were investigated for two silicates, MgSiO 3 and ZnSiO 3; six germanates, MGeO 3 and six titanates, MTiO 3 ( M=Ni, Mg, Co, Zn, Fe, and Mn) at about 1,000°C and pressures up to ca. 30 GPa. CoGeO 3 was found to assume the ilmenite form. The ilmenite phases were confirmed to transform in the following schemes: to perovskite in MgSiO 3 and MnGeO 3, to corundum in MgGeO 3 and ZnGeO 3, to rocksalt plus rutile in ZnSiO 3 and CoGeO 3 and to rocksalt plus TiO 2 (possibly of some denser structure) in NiTiO 3, MgTiO 3, CoTiO 3, ZnTiO 3 and FeTiO 3. In the case of FeTiO 3, the corundum form appeared as an intermediate phase. The possibility that the corundum type MnTiO 3 might transform to some denser modification could not be excluded. The compound NiGeO 3 was nonexistent throughout the pressure range studied. High-pressure phases of ABO 3 ( 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. 相似文献
18.
The pressure derivatives of elastic moduli (∂M/∂P; M=K S and G) for a suite of polycrystalline oxide perovskites (2 titanates, 1 stannate and 2 aluminates) have been measured up
to 3 GPa using the ultrasonic interferometry method combined with a buffer rod technique. Two empirical systematic relationships
(∂G/∂P vs K S/G and ∂K S/∂P vs K S (/ρ) 1/3) have been used to investigate the elasticity systematics of this suite of perovskites and to estimate ∂M/∂P of MgSiO 3 perovskite. The pressure derivatives ∂G/∂P and ∂K S/∂P for this suite of perovskites scatter between well-defined linear trends for the rutile, rocksalt and spinel structures.
The more diffuse trends observed for the perovskites might reflect greater flexibility in the response of its corner-connected
octahedral framework structure to changing pressure. The pressure derivatives of the elastic moduli for MgSiO 3 perovskite estimated by the “perovskite bands” are ∂G/∂P=1.6–2.2 and ∂K S/∂P=3.9–4.2.
Received: 13 November 1997 / Revised, accepted: 31 August 1998 相似文献
19.
Spatial factor analysis (SFA) is a multivariate method that determines linear combinations of variables with maximum autocorrelation at a given lag. This is achieved by deriving estimates of auto-/cross-correlations of the variables and calculating the corresponding eigenvectors of the covariance quotient matrix. A two-point spatial factor analysis model derives factors by the formation of transition matrix U comparing auto-/cross-correlations at lag 0, R
0, with those at a specified lag d, R
d, expressed as U
d= R
0
–1
R d. The matrix U
d can be decomposed into its spectral components which represent the spatial factors. The technique has been extended to include three points of reference. Spatial factors can be derived from the relationship: |
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20.
The luminescence spectra of Pr 3+ and Sm 3+ ions in apatite Ca 5[F∣(PO 4) 3] crystals from Spain and Russia have been compared with those for phosphate glasses doped with Pr 3+, Sm 3+ and Pr 3+, Sm 3+ ions. Time-resolved spectra measurements confirm that, in apatites, samarium ions occupy two non-equivalent crystal sites;
the same is assumed for praseodymium ions. For the first time in minerals, the Stark splitting energy levels Δ E for 3H 6 and 1D 2 of Pr 3+ ion and 6H 7/2 of Sm 3+ ion were determined. Some small differences in Δ E values for the Spanish and Russian apatite are discussed. The decay times of the excited levels of Pr 3+, Sm 3+ and Pr 3+, Sm 3+ doped in phosphate glass were measured at room temperature and at 77 K. The energy transfer process between samarium and
praseodymium ions was observed and the energy transfer rate was calculated. 相似文献
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