Home  About us  Contact
 

High-temperature Modification (high-temperature + modification)

Distribution by Scientific Domains
Chemistry100%

Selected Abstracts

ChemInform Abstract: High-Temperature Modification of Y5Sb3 and Its Ternary Analogue Y5NixSb3-x.

CHEMINFORM, Issue 32 2001
Yurij Mozharivskyj
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

Polymorphism of CaTeO3 and solid solutions CaxSr1,,,xTeO3

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2009
Berthold Stöger
Single crystals and microcrystalline samples of the calcium tellurate(IV) phases ,-, ,-, ,,- and ,-CaTeO3 as well as of two solid solutions CaxSr1,,,xTeO3 (x = 0.55 and 0.77) have been synthesized and characterized by X-ray diffraction and thermal analysis. A comparative description of the structures and the relations between the polymorphs is given. The main building units of the hitherto unknown structures are isolated [TeIVO3]2, units and [(Ca,Sr)Ox] (x = 6,8) polyhedra. All structures exhibit channels in which the TeIV electron lone pairs protrude. The low-temperature phase ,-CaTeO3 is stable up to 1168,K. It exhibits nearly cylindrical channels (diameter ,,4,Å) and differs structurally from the other phases, whereas the metastable high-temperature phases are closely related to each other. They feature oval channels (shortest and longest diameter ,,2 and 8,Å). ,-CaTeO3 can be described as an order,disorder (OD) structure of two non-polar layers with layer groups p121/m1 and p1211. The ,-CaTeO3 crystal under investigation consists of two polytypes with a maximum degree of order. The two phases CaxSr1,,,xTeO3 (x = 0.55 and 0.77) are isostructural to the MDO1 polytype of ,-CaTeO3. ,-CaTeO3 shows a distinct reversible phase transition at ,,293,K. The low-temperature modification ,,-CaTeO3 as well as its high-temperature modification ,-CaTeO3 can be considered as threefold superstructures along [100] based on the MDO1 polytype of ,-CaTeO3. [source]

Incommensurate structure of Ca2Al2O5 at high temperatures , structure investigation and Raman spectroscopy

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2008
Biljana Lazic
A high-temperature X-ray diffraction study revealed that brownmillerite-type Ca2Al2O5 transforms to an incommensurately modulated structure at elevated temperatures. Single crystals of Ca2Al2O5 were synthesized in an end-loaded piston cylinder press at 2.5,GPa and 1273,K. The diffraction pattern observed at 1090,(10),K by in situ single-crystal diffraction experiments can be indexed by an I -centred orthorhombic cell and a modulation wavevector of q = 0.595,(1)c*. A (3,+,1)-dimensional model in superspace group Imma(00,)s00 was used to refine the modulated structure. The structure is assembled from two building units: (i) layers of corner-sharing [AlO6] octahedra, stacked along b alternate with (ii) layers of zweier single chains of [AlO4] tetrahedra running along a. The modulated structure arises from an aperiodic sequence of two different configurations of the chains within the tetrahedral layers. The modulated high-temperature phase of Ca2Al2O5 is isotypic to the modulated high-temperature modification of Ca2Fe2O5. A large hysteresis was found in the phase-transition temperature. On heating, the transition occurs at ca 1075,(10),K; on cooling, satellite reflections can be observed down to 975,(10),K. The characterization of Ca2Al2O5 is completed by Raman spectroscopy, including a partial interpretation of the spectra. [source]

Hexagonal high-temperature form of aluminium phosphate tridymite from X-ray powder data

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2001
Heribert A. Graetsch
Similar to silica tridymite, AlPO4 tridymite shows a sequence of displacive phase transitions resulting in a dynamically disordered hexagonal high-temperature modification. Rietveld refinement reveals that the thermal motions of the tetrahedra can be described either by strongly anisotropic displacement parameters for oxy­gen or by split O atoms. Due to the ordered distribution of aluminium and phospho­rus over alternating tetrahedra, the space group symmetry of high-temperature AlPO4 tridymite is reduced with respect to SiO2 tridymite from P63/mmc to P63mc. [source]