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1.
An order parameter treatment of the phase transitions in leucite, KAlSi 2O 6, at approximately 950 and 920 K:
(cubic) I4 1
acd(tetragonal) I4 1
a(tetragonal) is presented in terms of Landau theory and induced representation theory. The Al-Si order with decreasing temperature is taken as the primary order parameter to which other distortions (K + ion displacements, strain components, etc.) couple linearly. The expected Al-Si ordering behavior and the associated K + ion displacements for both transitions are derived and the resulting twin domain orientations are listed. The sequence of phase transitions results from a coupling of
3
+
and
4
+
representations. The Landau free energy for the five-dimensional reducible representation has been simplified to two components resulting in a linearquadratic coupling of the components. Possible phase diagrams are derived by free energy minimization. The cubic tetragonal transition is first-order, whereas the tetragonal-tetragonal transition may be second order. A tricritical point exists at which the first-order transition changes to second-order. 相似文献
2.
Relaxation times (T 1) and lineshapes were examined as a function of temperature through the - transition for 29Si in a single crystal of amethyst, and for 29Si and 17O in cristobalite powders. For single crystal quartz, the three 29Si peaks observed at room temperature, representing each of the three differently oriented SiO 4 tetrahedra in the unit cell, coalesce with increasing temperature such that at the - transition only one peak is observed. 29Si T 1's decrease with increasing temperature up to the transition, above which they remain constant. Although these results are not uniquely interpretable, hopping between the Dauphiné twin related configurations, 1 and 2, may be the fluctuations responsible for both effects. This exchange becomes observable up to 150° C below the transition, and persists above the transition, resulting in -quartz being a time and space average of 1 and 2. 29Si T 1's for isotopically enriched powdered cristobalite show much the same behavior as observed for quartz. In addition, 17O T 1's decrease slowly up to the - transition at which point there is an abrupt 1.5 order of magnitude drop. Fitting of static powder 17O spectra for cristobalite gives an asymmetry parameter () of 0.125 at room T, which decreases to <0.040 at=" the=" transition=" temperature.=" the=" electric=" field=" gradient=" (efg)=" and=" chemical=" shift=" anisotropy=" (csa),=" however,=" remain=" the=" same,=" suggesting=" that=" the=" decrease=" in="> is caused by a dynamical rotation of the tetrahedra below the transition. Thus, the mechanisms of the - phase transitions in quartz and cristobalite are similar: there appears to be some fluctuation of the tetrahedra between twin-related orientations below the transition temperature, and the -phase is characterized by a dynamical average of the twin domains on a unit cell scale. 相似文献
3.
Zusammenfussung Ein einfaches Verfahren zur genauen Bestimmung kleiner optischer Gangunterschiede zwischen 0 und /2 bzw. 1 Na-Licht (oder auch underer Wellenlängen zwischen C- und F-Linie) durch Kompensation auf einen Restgangunterschied
r
= 1 und 2 anstatt 0 wird beschrieben und an einigen Beispielen geprüft. 相似文献
4.
Zusammenfassung Eine Differenzmethode zur Erhöhung den Genauigkeit der Messung von optischen Gangunterschieden durch Drehkompensatoren und Erweiterung von deren Mcßbereich wind beschrieben und durch Beispiele geprüft und erläutert.Der Vorteil dieses Verfahrens liegt in der erreichbaren Genauigkeit der Bestimmung kleiner und großer
, die meist haher liegt als die der direkten Ermittelung durch Kompensation auf
. = 0.Bei kleinen und großen
, kann durch Messung einer Summe von Gangunterschieden oder eines Restgangunterschiedes anstatt des im Meßpräparat vorliegenden
die Messung stets in den besten Meßbereich des jeweils vorliegenden Kompensators gerückt werden. Darüberhinaus sind höhere , durch Kompensatoren mit kleinerem Meßbereich ebenso genau bzw. nosh genauer bestimmbar. Es muß nun durch einen Quarzkeil oder einen underen Kompensator die ungefähre Lage der Kompensationsstelle auf
= 0 ermittelt werden.Unter Verwendung monoehromatischen Lichtes können
auch in der Additionsstellung von Meßpraparat und Kompensator ebensogut und genau bestimmt werden wie in der bisher traditionell bevorzugten Subtraktionsstellung. 相似文献
5.
Cristobalite, a high temperature phase of silica, SiO 2, undergoes a (metastable) first-order phase transition from a cubic,
, to a tetragonal, P4 32 12 (or P4 12 12), structure at around 220° C. The cubic C9-type structure for -cristobalite (Wyckoff 1925) is improbable because of two stereochemically unfavorable features: a 180° Si-O-Si angle and an Si-O bond length of 1.54 Å, whereas the corresponding values in tetragonal -cristobalite are 146° and 1.609 Å respectively. The structure of the -phase is still controversial. To resolve this problem, a symmetry analysis of the
(or P4 12 12) transition in cristobalite has been carried out based on the Landau formalism and projection operator methods. The starting point is the ideal cubic (
) C9-type structure with the unit cell dimension a (7.432 Å) slightly larger than the known a dimension (7.195 Å at 205° C) of -cristobalite, such that the Si-O-Si angle is still 180°, but the Si-O bond length is 1.609 Å. The six-component order parameter driving the phase transition transforms according to the X 4 representation. The transition mechanism essentially involves a simultaneous translation and rotation of the silicate tetrahedra coupled along 110. A Landau free-energy expression is given as well as a listing of the three types of domains expected in -cristobalite from the transition. These domains are: (i) transformation twins from a loss of 3-fold axes, (ii) enantiomorphous twins from a loss of the inversion center, and (iii) antiphase domains from a loss of translation vectors 1/2 110 (FP). These domains are macroscopic and static in -cristobalite, and microscopic and dynamic in -cristobalite. The order parameter , couples with the strain components as 2, which initiates the structural fluctuations, thereby causing the domain configurations to dynamically interchange in the -phase. Hence, the - cristobalite transition is a fluctuation-induced first-order transition and the -phase is a dynamic average of -type domains. 相似文献
6.
The timescale of structural relaxation in a silicate melt defines the transition from liquid (relaxed) to glassy (unrelaxed) behavior. Structural relaxation in silicate melts can be described by a relaxation time, , consistent with the observation that the timescales of both volume and shear relaxation are of the same order of magnitude. The onset of significantly unrelaxed behavior occurs 2 log 10 units of time above . In the case of shear relaxation, the relaxation time can be quantified using the Maxwell relationship for a viscoelastic material; S = S/ G
(where S is the shear relaxation time, G
is the shear modulus at infinite frequency and S is the zero frequency shear viscosity). The value of G
known for SiO 2 and several other silicate glasses. The shear modulus, G
, and the bulk modulus, K
, are similar in magnitude for every glass, with both moduli being relatively insensitive to changes in temperature and composition. In contrast, the shear viscosity of silicate melts ranges over at least ten orders of magnitude, with composition at fixed temperature, and with temperature at fixed composition. Therefore, relative to S, G
may be considered a constant (independent of composition and temperature) and the value of S, the relaxation time, may be estimated directly for the large number of silicate melts for which the shear viscosity is known.For silicate melts, the relaxation times calculated from the Maxwell relationship agree well with available data for the onset of the frequency-dependence (dispersion) of acoustic velocities, the onset of non-Newtonian viscosities, the scan-rate dependence of the calorimetric glass transition, with the timescale of an oxygen diffusive jump and with the Si-O bond exchange frequency obtained from 29Si NMR studies. 相似文献
7.
Thorium(IV) sorption onto hematite (-Fe 2O 3) was examined as a function of pH and ionic strength. Sorption behaved Langmuirian over an eleven order of magnitude range in adsorption densities, : 10 –12 to 10 –1 moles Th sorbed per mole hematite sites, indicating that the overall free energy of Th adsorption is independent of adsorption density. Modeling of Th sorption was conducted with the Triple Layer Model of Davis and Leckie; reactions considered included solution-phase hydroxy and carbonato complexes of thorium, and carbonate/hematite surface complexes. The entire Th sorption isotherm can be modeled with a single surface complex formation reaction |
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8.
To investigate high-temperature creep and kinetic decomposition of nickel orthosilicate (Ni 2SiO 4), aggregates containing 3 vol% amorphous SiO 2 have been deformed in uniaxial compression at a total pressure of one atomsphere. Twenty-three samples with grain sizes (d) from 9 to 30 m were deformed at temperatures (T) from 1573 to 1813 K, differential stresses () from 3 to 20 MPa, and oxygen fugacities ( f
o
2) from 10 -1 to 10 5 Pa. At temperatures up to 1773 K, the steady-state creep rate () can be described by the flow law |
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9.
Al-Si ordering in Sr-feldspar has been followed by isothermal annealing, starting from a disordered metastable configuration. Ordering could not be followed by changes in the spontaneous strain as cell parameters did not show significant changes with thermal treatment from 0.016 h to 452 h at T=1350° C, while, on the contrary, significant changes in IR spectra are observed. A single crystal obtained from melt ( Q
od 0) has been progressively heated up to 678 h at T=1350° C and the relevant structural refinements enabled to monitor changes in degree of Al-Si order up to Q od = 0.86. In isothermal treatment for Sr-feldspar it is observed a significantly lower Q
od than in anorthite after the same annealing time. TEM observation has shown in Sr-feldspar, also for shortest annealing, b type reflections, while in anorthite, in the same conditions, e type reflections have been observed (Carpenter 1991a). In the first stages of ordering b APDs sized 100 Å (at T=1350° C, 0.33 h) have been observed in Sr-feldspar; APD coarsening occurs with an activation energy of 120±7 kcal mol -1, not significantly different from anorthite. The ordering process seems to be a slower process in Sr-feldspar than in anorthite, even though data from longer annealing suggest that the Q
od close to the equilibrium is the same in Sr and Ca-feldspar ( Q
od = 0.86 at T=1350° C). 相似文献
10.
Neutron powder diffraction measurements of the temperature dependence of superlattice reflections in calcite have shown that there is a continuous phase transition at 1260 K. The change in space group symmetry
and the halving of the unit cell size on heating indicate that this transition is an orientational order/disorder transition. The intensities of the superlattice reflections show that the temperature dependence of the order parameter, Q, is of the form ( T
c
– T) , where is 0.25, indicating that the transition is tricritical. The transition is accompanied by a large contraction along the c axis on cooling, defining a spontaneous strain e 3 which is related to the order parameter (and hence temperature) via e 3 Q
2. No evidence for critical lowering of the value of was found. These measurements confirm that, apart from the detailed critical behaviour, the phase transition in calcite is similar to that observed in NaNO 3. 相似文献
11.
Electron microprobe analysis of Pb-Cu(Fe)-Sb-Bi sulfosalts from Bazoges and Les Chalanches (France), and Pedra Luz (Portugal), give new data about (Bi, Sb) solid-solution and incorporation of the minor elements Cu, Fe or Ag in jaskolskiite, and in izoklakeite-giessenite and kobellite-tintinaite series. Jaskolskiite from Pedra Luz has high Sb contents (from 17.9 to 20.7 wt.%), leading to the extended general formula: Cu
x
Pb 2+x
(Sb 1–y
Bi
y
) 2–x
S 5, with 0.10 x 0.22 and 0.19 y 0.41. Fe-free, Bi-rich izoklakeite from Bazoges has high Ag contents (up to 2.2 wt. %), leading to the simplified formula Cu 2Pb 22Ag 2(Bi, Sb) 22S 57; in Les Chalanches it contains less Ag content (1.2 wt.%), but has an excess of Cu that gives the formula: Cu 2.00 (Cu 0.49Ag 1.18) =1.67Pb 22.70(Bi 12.63Sb 8.99) =21.62S 57.27.In tintinaite from Pedra Luz, the variation of the Fe/Cu ratio can be explained by the substitution: Cu + (Bi, Sb) Fe + Pb; Fe-free kobellite from Les Chalanches has a Cu-excess, corresponding to the formula Cu 2.81Ag 0.54Pb 9.88(Bi 10.37Sb 5.21) =15.38S 35.09. Eclarite from the type locality, structurally related to kobellite, shows a Cu excess too. In natural samples of the kobellite homologous series, Fe is positively correlated with Pb, and its contents never exceed that of Cu. Ag substitutes for Pb, together with (Bi, Sb). Taking into account the possibility of Cu excess, but excluding formal Cu 2+ and Fe 3+, general formulae can be written: 相似文献
12.
The synthesis boundaries of the phase transformation; ++ in (Mg 0.9, Fe 0.1)SiO 4, have been clarified at temperatures to 2000° C and pressures up to 20 GPa in order to synthesize single crystals of high quality. A single crystal of (Mg 0.9, Fe 0.1) 2SiO 4 was grown successfully to a size of 500 m. The crystal structure has been refined from single-crystal X-ray intensities. The ferrous ions prefer M1 and M3 sites to over the larger M2 site. The volume change of all the occupied polyhedra does not contribute to the decrease of total volume in the transformation; rather it tends to increase the bulk volume through the expansion of occupied tetrahedra. The volume reduction in the phase transformations is accounted for by unoccupied polyhedra, with the octahedra contributory 60% and the tetrahedra 40% to the V of the transition. The volume change in the transformation is caused also partly by the volume decrease of MO
6 (25%), partly the unoccupied tetrahedra (45%) and octahedra (30%). 相似文献
13.
Zusammenfassung Emmonsit kristallisiert triklin, Raumgruppe P
, Gitterkonstanten: a
0=7,90 Å, b
0=8,00 Å, c
0=7,62 Å, =96 o44, =95 o 0, =84 o 28, Z=2. Der Strukturtyp wurde aus 3-dimensionalen photographischen Röntgendaten ermittelt. Die Eisenatome werden je von 6 Sauerstoffen verzerrt oktaedrisch koordiniert. Jedes Telluratom wird von 3 Sauerstoffen in einem Abstand <2,0 Å umgeben. Ein vierter Sauerstoff hat bezüglich dieser drei einen um etwa 25–35% größeren Abstand, so daß jedes Telluratom im weiteren Sinne eine (3+1)-Koordination aufweist.
The structure type of emmonsite, {Fe2[TeO3]3·H2O}·xxH2O (x=0–1) Summary Emmonsite is triclinic with space groupP
, and lattice constantsa
0=7.90 Å,b
0=8.00 Å,c
0=7.62 Å, =96o 44, =95o 0, =840 28,Z=2. The structure type is derived from 3-dimensional photographic X-ray data. The iron atoms are coordinated by six oxygens in the form of a distorted octahedron. Each tellurium atom is coordinated to 3 oxygens at a distance <2.0 Å. Compared with these 3 Te–O distance the distance of a fourth oxygen is only 25 to 35% greater; therefore each tellurium atom has a (3+1)-coordination of oxygens.
Mit 2 Abbildungen 相似文献
15.
Thirteen energy-dispersive x-ray diffraction spectra for -Fe 2SiO 4 (spinel) collected in situ at 400° C and pressures to 24 GPa constitute the basis for an elevated-temperature static compression isotherm for this important high-pressure phase. A Murnaghan regression of these molar volume measurements yields 177.3 (±17.4) GPa and 5.4(±2.5) for the 400° C, room pressure values of the isothermal bulk modulus ( K
P
0) and its first pressure derivative ( K
P
0), respectively. When compared to the room- Tdeterminations of K
P
0 available in the literature, our 400° C K
P
0 yields -4.1 (±6.2)×10 -2 GPa/degree for the average value of ( K/ T)
P
0 over the temperature interval 25° C<><400°>A five-parameter V(P, T) equation for -Fe 2SiO 4 based on simultaneous regression of our data combined with the elevated P-Tdata of Yagi et al. (1987) and the extrapolated thermal expansion values from Suzuki et al. (1979) yields isochores which have very little curvature [( 2
T/ P
2)
v
0], in marked contrast to the isochores for fayalite (Plymate and Stout 1990) which exhibit pronounced negative curvature [( T/ P
2)
v
<0]. along=" the="> -Fe 2SiO 4 reaction boundary V Rvaries from a minimum of approximately 8.3% at approximately 450° C to approximately 8.9% at 1200° C. Extrapolation of the fayalite and -Fe 2SiO 4
V(P, T) relationships to the temperature and pressure of the 400 km discontinuity suggests a V
R
of approximately 8.4% at that depth, approximately 10% less than the 9.3% V
R
at ambient conditions. 相似文献
16.
Data on the mechanisms of mantle phase transformations have come primarily from studies of analogue systems reacted experimentally at low pressures. In order to study transformation mechanisms in Mg 2SiO 4 at mantle pressures, forsterite ( ) has been reacted in the stability field of -phase, at 15 GPa and temperatures up to 900° C, using a multianvil split-sphere apparatus. Transmission electron microscope studies of samples reacted for times ranging from 0.25–5.0 h show that forsterite transforms to -phase by an incoherent nucleation and growth mechanism involving nucleation on olivine grain boundaries. This mechanism and the resultant microstructures are very similar to those observed at much lower pressures in analogue systems (Mg 2GeO 4 and Ni 2SiO 4) as the result of the olivine to spinel ( ) transformation. Metastable spinel ( ) also forms from Mg 2SiO 4 olivine at 15 GPa, in addition to -phase, by the incoherent nucleation and growth mechanism. With time, the spinel progressively transforms to the stable -phase. After 1 h, spinels exhibit a highly striated microstructure along {110} and electron diffraction patterns show streaking parallel to [110] which indicates a high degree of structural disorder. High resolution imaging shows that the streaking results from thin lamellae of -phase intergrown with the spinel. The two phases have the orientation relationship [001] //[001] and [010] //[110] so that the quasi cubic-close-packed oxygen sublattices are continuous between both phases. These microstructures are similar to those observed in shocked meteorites and show that spinel transforms to -phase by a martensitic (shear) mechanism. There is also evidence that the mechanism changes to one involving diffusion-controlled growth at conditions close to equilibrium. 相似文献
17.
Zusammenfassung Das neue Mineral Koritnigit ist ein wasserhaltiges Zinkhydrogenarsenat der Formel Zn[H 2O|HOAsO 3]. Die chemische Analyse (Elektronenmikrosonde und T.G.A.) ergab: As 2O 5 51,75%, ZnO 35,97% und H 2O 12,3%, Summe 100,0%. Die HOAsO 3-Ionen wurden IR-spektroskopisch nachgewiesen. Koritnigit ist löslich in kalter, verdünnter HCl und HNO 3.Die Gitterkonstanten sind: a
0=7,948(2), b
0=15,829(5), c
0=6,668(2) Å, =90,86(2), =96,56(2), =90,05(2) o, V=833,2(4)Å 3, V=8. Die Raumgruppe ist
. Die stärksten Linien des Pulverdiagramms sind: 7,90(10) (020,100), 3,83(7) (
), 3,16(9) (
) 2,926(4) (150), 2,679(4) (
), 2,461(6) (
), 2,186(5) (
), 1,969(4) (400), 1,649(3) (004).Koritnigit ist wasserklar bis durchscheinend weiß. Idiomorphe Kristalle sind nicht bekannt. Die Spaltbarkeit nach {010} ist ausgezeichnet und auf {010} sind Spaltspuren nach [001] und nach [100] erkennbar. Härte 2. G=3,54 g·cm –3, D
x
=3,56 g·cm –3. Koritnigit ist optisch zweiachsig positiv, 2 V70(5) o. Die Werte der Lichtbrechung sind: n
=1,632(5), n
=1,652(3) und n
=1,693(3).Koritnigit wurde auf der 31. Sohle der Tsumeb-Mine, Südwestafrika gefunden. Er kommt als Sekundärmineral in Paragenese mit Cu-Adamin, Stranskiit und drei weiteren, vorerst nicht identifizierten mineralen in Zersetzungshohlräumen von Tennantit vor.
Koritnigite, Zn[H2O|HOAsO3], a new mineral from Tsumeb, South West Africa Summary The new mineral koritnigite is a hydrated zinc hydrogen arsenate with the formula Zn[H2O|HOAsO3]. Chemical analysis (electron microprobe and t.g.a.) gave: As2O5 51.75%, ZnO 35.97%, and H2O 12.3%, total 100.0%. The HOAsO3 ions were determined by IR spectroscopy. Koritnigite is soluble in cold diluted HCl and HNO3. The unit cell dimensions are:a
0=7.948(2),b
0=15.829(5),c
0=6.668(2)Å, =90.86(2), =96.56(2), =90.05(2)o,V=833.2(4) Å3,Z=8. The space group is
. The strongest lines of the powder pattern are: 7.90(10) (020, 100), 3.83(7) (
), 3.16(9) (
), 2.926(4) (150), 2.679(4) (
), 2.461(6) (
), 2.186(5) (
), 1.969(4)(400), 1.649(3) (004).
Mit 2 Abbildungen
Herrn Univ. Prof. Dr. H. Meixner zum 70. Geburtstag gewidmet. 相似文献
18.
The pressure dependence of the Raman spectrum of forsterite was measured over its entire frequency range to over 200 kbar. The shifts of the Raman modes were used to calculate the pressure dependence of the heat capacity, C
v, and entropy, S, by using statistical thermodynamics of the lattice vibrations. Using the pressure dependence of C
v and other previously measured thermodynamic parameters, the thermal expansion coefficient, , at room temperature was calculated from = K
S
(T/P)
S
C
V/ TVK
T, which yields a constant value of ( ln / ln V) T= 6.1(5) for forsterite to 10% compression. This value is in agreement with ( ln / ln V) T for a large variety of materials.At 91 kbar, the compression mechanism of the forsterite lattice abruptly changes causing a strong decrease of the pressure derivative of 6 Raman modes accompanied by large reductions in the intensities of all of the modes. This observation is in agreement with single crystal x-ray diffraction studies to 150 kbar and is interpreted as a second order phase transition. 相似文献
19.
A molecular dynamics simulation of quartz at different temperatures both in the a and in the phase has been conducted. The - phase transition could be observed. A phonon analysis of the -phase confirms and rounds out in a quantitative way the origin of the incommensurate (ic) modulated phase. In particular it traces the optic soft mode at becoming (to a good approximation) a so-called rigid unit mode (RUM) at q0, and elucidates its coupling to the transverse acoustic mode which precipitates the incommensurate transition. This success underpins and illuminates the concept of RUMs and their role in structural phase transitions. 相似文献
20.
Electron diffraction and electron microscopic evidence is presented for a dynamical and reversible
phase transition in anorthite at T
c=516 K. Antiphase boundaries with a displacement vector, R=1/2[111] become unstable at T
c, while other antiphase boundary loops with the same displacement vector are formed. These interfaces are very mobile and vibrate with a frequency which increases strongly with temperature. At temperatures considerably above T
c, a shimmering effect is observed on imaging in dark field using diffuse c reflections. These observations are in agreement with the interpretation of the high temperature body-centered phase as a statistical dynamical average of very small c type antiphase domains of primitive anorthite. We propose that the c type antiphase domains in primitive anorthite originate from ordered and anti-ordered configurations around Ca 2+ ions at (ooo) and (oio) [likewise (zoo) and (zio)] positions. The dynamical model for the transition involves a two-stage mechanism: a softmode mechanism causing the aluminosilicate framework to approach body-centered symmetry, followed by an orderdisorder of the Ca 2+ ion configurations. Close to T
c, statistical fluctuations set in and breathing motion type lattice vibrations of the aluminosilicate framework cause the configurations around Ca (ooo) and Ca(oio) [likewise Ca(zoo) and Ca(zio)] in the
configuration to dynamically interchange through an intermediate
configuration. The dynamical nature of the phase transition in anorthite is comparable to the — phase transition in quartz. 相似文献
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