共查询到20条相似文献,搜索用时 31 毫秒
1.
S.N. Achary 《Journal of solid state chemistry》2007,180(1):84-91
The results of variable temperature powder X-ray diffraction and differential thermal analysis (DTA) studies on the orthorhombic (α) low-cristobalite to cubic (β) high-cristobalite phase transition for Al1−xGaxPO4, (0.00?x?1.00) are presented. These studies reveal that all these compositions undergo reversible phase transitions from orthorhombic to cubic form at higher temperature. The high-temperature behavior of GaPO4 is observed to have a different behavior compared to all other compositions in this series. Orthorhombic low-cristobalite-type GaPO4 transforms to cubic high-cristobalite form at ∼605 °C. Above ∼700 °C, the cubic high-cristobalite-type GaPO4 slowly transforms to trigonal quartz type structure. At about 960 °C, the quartz type GaPO4 transforms back to the cubic high-cristobalite form. During cooling cycles the cubic phase of GaPO4 reverts to trigonal quartz type phase. However, annealing of GaPO4 at higher temperatures for longer duration can stabilize the orthorhombic low cristobalite phase. The phase transition temperatures and associated enthalpies are related to the change in unit cell volume and the orthorhombicity of the respective low cristobalite lattice. 相似文献
2.
Stefan Saxin 《Journal of solid state chemistry》2011,184(6):1576-1579
Samples of Bi1−xTbxFeO3, with x=0.05, 0.10, 0.15, 0.20 and 0.25, have been synthesised by solid state reaction. The crystal structures of the perovskite phases, characterised via Rietveld analysis of high resolution powder neutron diffraction data, reveal a structural transition from the R3c symmetry of the parent phase BiFeO3 to orthorhombic Pnma symmetry, which is complete for x=0.20. The x=0.10 and 0.15 samples are bi-phasic. The transition from a rhombohedral to orthorhombic unit cell is suggested to be driven by the dilution of the stereochemistry of the Bi3+ lone pair at the A-site. The G-type antiferromagnetic spin structure, the size of the ordered magnetic moment (∼3.8 μB) and the TN (∼375 °C) are relatively insensitive to increasing Tb concentrations at the A-site. 相似文献
3.
Choong-Young Lee Rene Macquart Brendan J Kennedy 《Journal of solid state chemistry》2003,174(2):310-318
Polycrystalline samples of type BiTa1−xNbxO4 (0?x?1) in both the orthorhombic and triclinic phases have been characterized by a combination of powder X-ray diffraction, UV-Vis and Raman spectroscopy. The addition of Nb to BiTaO4 subtly alters the structure and spectroscopic properties of both the orthorhombic and triclinic oxides. The difference in bonding from the Nb 4d and Ta 5d electrons results in an unusual variation in the cell parameters in the orthorhombic form. In both structural types the addition of Nb results in a shift of the strong UV-Vis absorption feature towards the visible region. This feature noticeably broadens and shifts towards lower energy in the triclinic structures. 相似文献
4.
Thin (<15 μm) samples of lead tin telluride, Pb1?xSnxTe (x = 0.21, 0.25, 0.55, and 0.75) have been studied by temperature dependent Mössbauer spectroscopy using the 23.8 keV gamma radiation of 119mSn. The tin atom occupies a lattice site having cubic symmetry (QS = 0 ± 0.020 mm sec?1) over the temperature range 78 ≤ T ≤ 240 K, and there is no evidence for a rhombic (low temperature) to cubic (high temperature) phase transition such as that reported for SnTe in this temperature interval. The lattice temperature as probed by the Sn atom is independent of the compositional parameter x and is similar to that reported for SnTe from Mössbauer studies and for Pb0.63Sn0.37Te from X-ray powder diffraction data. Radiation damage produced by 2-MeV proton irradiation to a total fluence of ~1017 cm?2 at liquid nitrogen temperature does not have any effect on the Mössbauer parameters, possibly because the major damage is annealed at temperatures below 150 K. 相似文献
5.
J.A. ColladoM.A.G. Aranda A. CabezaP. Olivera-Pastor S. Bruque 《Journal of solid state chemistry》2002,167(1):80-85
The La2W2−xMoxO9 series has been synthesized by the ceramic method. An alternative synthesis using microwave radiation is also reported. La2W2O9 has two polymorphs and the low-temperature phase (α) transforms to the high-temperature form (β) at 1077°C. The influence of the W/Mo substitution in this phase transition has been investigated by DTA. The β structure for x≥0.7 compositions can be prepared as single phase at any cooling rate. The β phase for 0.3≤x≤0.7 compounds can be prepared as single phase by quenching, whereas a mixture of α and β phases is obtained by slow cooling. The W/Mo ratio in both coexisting phases is different with the β-phase having a higher Mo content. The x=0.1 and 0.2 compounds have been prepared as mixtures of phases. The room temperature structure of β-La2W1.7Mo0.3O9 has been analyzed by the Rietveld method in P213 space group. The final R-factors were RWP=9.0% and RF=5.6% with a structure similar to that of β-La2Mo2O9. Finally, the thermal expansion of both types of structures has been determined from a thermodiffractometric study. The thermal expansion coefficients were 2.9×10−6 and 9.7×10−6°C−1 for α-La2W2O9 and β-La2W1.2Mo0.8O9, respectively. 相似文献
6.
L. Vasylechko V. Vashook A. Senyshyn M. Knapp M. Berkowski U. Bismayer 《Journal of solid state chemistry》2003,172(2):396-411
Crystal structure and anisotropy of the thermal expansion of single crystals of La1−xSrxGa1−2xMg2xO3−y (x=0.05 and 0.1) were measured in the temperature range 300-1270 K. High-resolution X-ray powder diffraction data obtained by synchrotron experiments have been used to determine the crystal structure and thermal expansion. The room temperature structure of the crystal with x=0.05 was found to be orthorhombic (Imma, Z=4, a=7.79423(3) Å, b=5.49896(2) Å, c=5.53806(2) Å), whereas the symmetry of the x=0.1 crystal is monoclinic (I2/a, Z=4, a=7.82129(5) Å, b=5.54361(3) Å, c=5.51654(4) Å, β=90.040(1)°). The conductivity in two orthogonal directions of the crystals has been studied. Both, the conductivity and the structural data indicate three phase transitions in La0.95Sr0.05Ga0.9Mg0.1O2.92 at 520-570 K (Imma-I2/a), 770 K (I2/a-R3c) and at 870 K (R3c-R-3c), respectively. Two transitions at 770 K (I2/a-R3c) and in the range 870-970 K (R3c-R-3c) occur in La0.9Sr0.1Ga0.8Mg0.2O2.85. 相似文献
7.
The compounds Ti3Se4?xTex are prepared. Their structure closely depends upon thermal conditions of synthesis. Treatments at 600°C and quenching give pure monoclinic compounds isotypic with Ti3Se4 and Ti3Te4 in narrow ranges of composition (x ? 0.5 and x ? 3.5). At 800°C, polycrystalline pure products are obtained with an hexagonal B8 unit cell (a′, ?') when x ? 3. At 1000°C, for x ? 3.5, almost pure compounds crystallize with that same hexagonal unit cell; an orthorhombic lattice is observed for x = 4.Moreover, twinning is observed on single crystals obtained after quenching from 800 or 1000°C, with x = 1, 2, 3 and 4. These crystals seem to exhibit an hexagonal unit cell (2a′, 2?'), but in fact their lattice is orthorhombic or monoclinic with a real I unit cell: a ≈ a′√3, b ≈ a′, c ≈ 2c′. The explanation of the phenomenon allows us to correct some misinterpretations found in the literature on “TiTe” single crystals. 相似文献
8.
L.J. GillieA.J. Wright J. HadermannG. Van Tendeloo C. Greaves 《Journal of solid state chemistry》2002,167(1):145-151
The nuclear and magnetic structures of polycrystalline Sr2MnO3.5 have been determined by the Rietveld analysis of neutron powder diffraction data and electron diffraction techniques. The pure Mn3+ single-layered phase crystallizes in the primitive monoclinic space-group P21/c with lattice constants a=6.8524(3) Å b=10.8131(4) Å c=10.8068(4) Å β=113.247(4)°. The oxygen defects form an ordered superstructure within the perovskite block layers consisting of interconnected MnO5 square pyramids, slightly different from those observed for the defect perovskites SrMnO2.5 and Ca2MnO3.5. Magnetic susceptibility studies show a broad transition at ∼280 K, which is attributed to an overall antiferromagnetic ordering of spins, which leads to doubling of the unit cell along [100]. The magnetic unit cell comprises ferromagnetic clusters of four corner-sharing MnO5 pyramids, which are antiferromagnetically aligned to other similar clusters within the perovskite block layers. 相似文献
9.
Y. Hotta Y. Ueda N. Nakayama K. Kosuge S. Kachi M. Shimada M. Koizumi 《Journal of solid state chemistry》1984,55(3):314-319
The pressure-products diagram of the FexV1?xO2 system (0 ≤ x ≤ 0.5) was investigated under the conditions of 5 ~ 55 kbar and 800°C. In high-pressure synthesis, FeV3O8 phase (monoclinic), which is stable under ambient pressure, was converted to the O phase (orthorhombic) and the region of O phase was extended from x = 0.1 up to ~0.3. The O phase so obtained, showed the characteristic reversible temperature-induced phase transition to the high-temperature rutile phase under ambient pressure, prior to the decomposition to an ambient pressure form. At x = 0.5, successive transformations, FeVO4-I (monoclinic) → -II (orthorhombic) → -III (α-PbO2 type) → -IV (wolframite type), were observed with increasing pressure. 相似文献
10.
The full LaCo1−xRhxO3 solid solution was investigated utilizing structural, electrical transport, magnetic, and thermal conductivity characterization. Strong evidence for at least some conversion of Rh3+/Co3+ to Rh4+/Co2+ is found in both structural and electrical transport data. The crystal structure is that of a rhombohedrally distorted perovskite over the range 0.0≤x≤0.1. The common orthorhombic distortion of the perovskite structure is found over the range 0.2≤x≤1.0. A crossover of all three orthorhombic cell edges occurs at x=0.5 giving the appearance of a cubic structure, which actually remains orthorhombic. The octahedra in the orthorhombic structure must be distorted for x values less than 0.5, and the observed distortion suggests orbital ordering for Co2+. Electrical resistivity measurements as a function of temperature show semiconducting-like regions for all compositions. There is a steady increase in electrical resistivity as the Rh content increases. Large positive thermopower values are generally obtained above 475 K. With increasing Rh substitution there is a decrease in thermal conductivity, which slowly rises with increasing temperature due to increased electrical conductivity. The electronic part of the thermal conductivity is suppressed significantly upon Rh substitution. A thermoelectric figure-of-merit (ZT) of about 0.075 has been achieved for LaCo0.5Rh0.5O3 at 775 K, and is expected to reach 0.15 at 1000 K. 相似文献
11.
Yoshio Ohashi Yuji Masubuchi Keiichiro Murai 《Journal of solid state chemistry》2011,184(8):2061-2065
New niobium oxynitrides containing either magnesium or silicon were prepared at 1000 °C by ammonia nitridation of oxide precursors obtained via the citrate route. The products had rock-salt type crystal structures. Crystallinity was improved by annealing in 0.5 MPa N2 and the final compositions were (Nb0.95Mg0.05)(N0.92O0.08) at 1500 °C and (Nb0.87Si0.09□0.04)(N0.87O0.13) at 1200 °C. The magnesium and oxide ions partially co-substitute the niobium and nitride ions in the octahedral sites of the δ-NbN lattice, respectively. Silicon ions were also successfully doped together with oxide ions into the rock-salt type NbN lattice. The Si doped product exhibited relatively large displacement at the octahedral sites and was accompanied by a small amount of cation vacancies. Superconductivity was improved by annealing to obtain critical temperatures/volume fractions of Tc=17.6 K/100% for Mg- and Tc=16.2 K/95% for the Si-doped niobium oxynitrides. 相似文献
12.
D. KoK.R. Poeppelmeier D.R. KammlerG.B. Gonzalez T.O. MasonD.L. Williamson D.L. YoungT.J. Coutts 《Journal of solid state chemistry》2002,163(1):259-266
The cation distribution in the transparent conducting oxide Cd1+xIn2−xSnxO4 was investigated to determine if there is a correlation between structure and electronic properties. Combined Rietveld refinements of neutron and X-ray diffraction data and 119Sn Mössbauer spectroscopic analysis were used to show that the cation distribution changed with x(0≤x≤0.7) from a primarily normal spinel (x=0) to an increasingly random spinel. CdIn2O4 quenched from 1175°C has an inversion parameter of 0.31 (i.e., (Cd0.69In0.31)tet(In1.69Cd0.31)octO4). The inversion parameter decreases to 0.27 as the quench temperature is lowered from 1175°C to 1000°C. The decrease in inversion parameter with temperature correlates with an increase in optical gap from 3.0 eV to 3.3 eV for specimens prepared at 1175°C and 800°C, respectively. We show that this is a consequence of an increase in the fundamental band gap. 相似文献
13.
Yuichi Michiue Mitsuko OnodaAkiteru Watanabe Mamoru WatanabeFrancisco Brown Noboru Kimizuka 《Journal of solid state chemistry》2002,163(2):455-458
The structure of pseudorhombohedral-type InFe1−xTixO3−x/2 (x=2/3) was refined by Rietveld profile fitting. The crystal is a commensurate member of a series in a solution range on InFeO3-In2Ti2O7 including incommensurate structures. The structure with the unit cell of a=5.9188(1), b=10.1112(2), and c=6.3896(1) Å, β=108.018(2)°, and a space group P21/a is the alternate stacking of an edge-shared InO6 octahedral layer and an Fe/Ti-O plane along c*. Metal sites on the Fe/Ti-O plane are surrounded by four oxygen atoms on the Fe/Ti-O plane and two axial ones. Electric conductivities of the order 10−4 S/cm were observed for the samples at 1000 K, while the oxide ion transport number is almost zero as no electromotive force was detected by an oxygen concentration cell. 相似文献
14.
Klaus W. Richter Hans FlandorferHugo F. Franzen 《Journal of solid state chemistry》2002,167(2):517-524
The title compounds MxTa11−xGe8 (M=Ti, Zr, Hf) were prepared from the pure elements by arc-melting and subsequent induction heating at temperatures between 1200°C and 1400°C. X-ray powder diffraction studies of the samples were performed using the Guinier technique and the respective powder patterns were refined with a structure model based on the orthorhombic Cr11Ge8-structure type (oP76, Pnma). The homogeneity ranges of the compounds were determined to be 0.9<x<1.3 (M=Ti), 0.7<x<1.3 (M=Zr) and 0.7<x<2.4> (M=Hf) by means of electron probe microanalysis. Chemical bonding, electronic structure and site preferences are discussed based on extended Hückel calculations performed on hypothetical binary Ta11Ge8. 相似文献
15.
CdRE2S4 (RE = Gd, Tb, Dy, Ho, Er, Tm, and Yb) and Mg(GdxYb1?x)2S4 were prepared by solid-state reactions. All the cadmium-containing compounds are cubic, i.e., the Th3P4 structure for Gd, Tb, and Dy and the spinel type for all the others. The first three compounds were deficient in CdS. In the case of the Mg system, for x = 1 the system is cubic Th3P4, for x = 0 cubic spinel, and for 0 < x < 1 orthorhombic MnY2S4 (Cmc21). All the materials studied are paramagnetic above 77 K. Below 77 K in the magnesium family both cubic materials are paramagnetic down to 4.2 K and the orthorhombic materials show magnetic ordering. In the cadmium family all but CdTm2S4 show exchange coupling. 相似文献
16.
Navida Nasir Nataliya Melnychenko-Koblyuk Ingeborg Bednar 《Journal of solid state chemistry》2010,183(10):2329-2342
Three series of vacancy-free quaternary clathrates of type I, Ba8ZnxGe46−x−ySiy, Ba8(Zn,Cu)xGe46−x, and Ba8(Zn,Pd)xGe46−x, have been prepared by reactions of elemental ingots in vacuum sealed quartz at 800 °C. In all cases cubic primitive symmetry (space group Pm3?n, a∼1.1 nm) was confirmed for the clathrate phase by X-ray powder diffraction and X-ray single crystal analyses. The lattice parameters show a linear increase with increase in Ge for Ba8ZnxGe46−x−ySiy. M atoms (Zn, Pd, Cu) preferably occupy the 6d site in random mixtures. No defects were observed for the 6d site. Site preference of Ge and Si in Ba8ZnxGe46−x−ySiy has been elucidated from X-ray refinement: Ge atoms linearly substitute Si in the 24k site whilst a significant deviation from linearity is observed for occupation of the 16i site. A connectivity scheme for the phase equilibria in the “Ba8Ge46” corner at 800 °C has been derived and a three-dimensional isothermal section at 800 °C is presented for the Ba-Pd-Zn-Ge system. Studies of transport properties carried out for Ba8{Cu,Pd,Zn}xGe46−x and Ba8ZnxSiyGe46−x−y evidenced predominantly electrons as charge carriers and the closeness of the systems to a metal-to-insulator transition, fine-tuned by substitution and mechanical processing of starting material Ba8Ge43. A promising figure of merit, ZT ∼0.45 at 750 K, has been derived for Ba8Zn7.4Ge19.8Si18.8, where pricey germanium is exchanged by reasonably cheap silicon. 相似文献
17.
Thanner H. Krempl P. W. Selic R. Wallnöfer W. Worsch P. M. 《Journal of Thermal Analysis and Calorimetry》2003,71(1):53-59
Quartz-homeotypic gallium (ortho-) phosphate, GaPO4, is of special interest for resonator applications asking for temperature compensated cuts with higher electro-mechanical coupling than quartz and operational temperatures up to 970°C. The crystal microbalance technique, well known for quartz (QCM) which can be used only at moderate temperatures, can now be extended to much higher temperatures using GaPO4 crystals, benefiting from all three advantages mentioned above. Two different experiments were done to demonstrate the advantages of a crystal microbalance based on GaPO4. First, the GaPO4 resonator was used for film thickness determination and compared with a commercial QCM. This experiment demonstrated that the measuring range can be extended by using GaPO4 resonators instead of quartz. The second experiment demonstrates the possibility for thermogravimetric analysis up to 720°C by using a new concept for resonator mounting.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
18.
Emily Asenath-Smith Scott T. Misture Doreen D. Edwards 《Journal of solid state chemistry》2011,184(8):2167-2177
The Ca2(ZnxFe2−x)O5 series was synthesized and characterized to determine the influence of zinc dopant on the brownmillerite structure for thermoelectric applications. All single-phase compounds exhibited Pnma symmetry at room temperature up to the solubility limit at x=0.10. High-temperature X-ray powder diffraction was used to show that the nature of the Pnma-Imma(0 0 γ)s00 transition in Ca2Fe2O5 is modified by the presence of zinc. While the Zn-free composition transitions to an incommensurate phase, the Zn-containing phases transition instead to a commensurate phase, Imma(0 0 γ)s00 with γ=1/2. Both the Néel temperature and the onset temperature of the Pnma-Imma(0 0 γ)s00 phase transition decreased with increasing zinc concentration. Rietveld analysis of the in situ diffraction pattern for the x=0 sample at 1300 °C demonstrates that the structure contains statistically disordered chain orientations as described by space group Imma. Thermoelectric properties were analyzed in air from 100 to 800 °C. The positive Seebeck coefficient revealed hole-type conduction for all compositions. Doped samples exhibited electrical conductivities up to 3.4 S/cm and thermal conductivity of 1.5 W/mK. Transport analysis revealed thermally activated mobility consistent with polaron conduction behavior for all compositions. 相似文献
19.
Fluorinated transition aluminas (Al2−x/3O3−xFx) with hexagonal platelet shape were synthesized via decomposition of α-AlF3 under air; they are thermally stable up to 1000 °C and exhibit at 1150 °C a weight loss with volume reduction caused by fluorine departure corresponding to a phase transition toward corundum alumina. The different characterizations performed in this study are structural (XRD), chemical (TGA-MS and microprobe analysis) and morphological (SEM, TEM and dilatometry). The evidence provided from this study is consistent with the formation of an Al-O-F phase as an intermediate compound in the pyrohydrolysis of an aluminium trifluoride phase to α-Al2O3. 相似文献
20.
C. Tenailleau A. Pring Y. Liu S. Tarantino M.A. Carpenter 《Journal of solid state chemistry》2005,178(3):882-891
Composition-induced structural phase changes across the high temperature, fast oxide ion conducting (Ba1−xLax)2In2O5+x, 0?x?0.6, system have been carefully analysed using hard mode infrared (IR) powder absorption spectroscopy, X-ray powder diffraction and electron diffraction. An orthorhombic brownmillerite to three-dimensionally disordered cubic perovskite phase transition in this system is signalled by a drastic change in slope of both wavenumber and average line widths of IR spectra as a function of composition. Some evidence is found for the existence of an intermediate tetragonal phase (previously reported to exist from electron diffraction data) around x∼0.2. The new spectroscopic data have been used to compare microscopic and macroscopic strain parameters arising from variation in composition. The strain and spectroscopic data are consistent with first-order character for the tetragonal→orthorhombic transition, while the cubic→tetragonal transition could be continuous. Differences between the variation with composition of spectral parameters and of macroscopic strain parameters are consistent with a substantial order/disorder component for the transitions. There is also evidence for precursor effects within the cubic structure before symmetry is broken. 相似文献