Tetragonal-orthorhombic transformation

The transformations above room temperature (monoclinic-orthorhombic, orthorhombic-tetragonal I) are more easily understood. One notes the expected decrease in transition temperature with increasing sodium content, as is found in many alloy phase diagrams. Attempts to observe transitions to a cubic phase at higher temperatures failed, owing to sublimation of samples of very low sodium content (x < 0.02) or decomposition at higher sodium concentrations. 

Fig. fc.2-27 Orthorhombic to tetragonal transformation temperature Ttrans at an oxygen partial pressure of p= atm as a function of the ionic radii of the ions CN = 8)... 

Y. Nakabayashi, Y. Kubo, T. Manato, J. Tabuchi, A. Ochi, K. Utsumi, H. Igarashi, M. Yonezawa The orthorhombic-tetragonal phase transformation an doxygen deficiency in LnBa2Cu3P7 8, Jpn. J. Appl. Phys. 27, L6A-L66 (1988)... 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Figure 8.11 Transformation of a tetragonal crystal, with lattice parameters aT = bT, into a multiply twinned orthorhombic crystal with lattice parameters a0, bQ. The twinned regions are often called domains and the boundaries may occur on a variety of crystallographic planes.

Silvery-white metal exhibits three crystalline modifications an orthorhombic alpha form, stable at ordinary temperatures and density 20.45 g/cm the alpha-form transforms to a tetragonal beta allotrope of density 19.36 g/cm when heated at 280°C the beta form converts to a body-centered cubic crystaUine gamma modification at 577°C, having a density 18.0 g/cm . 

According to the results of Hebecker, v. Schnering and Hoppe (133) Na2SnFe is monoclinic, but because of the size of its unit cell there are pseudosymmetries that may lead to its tetragonal or orthorhombic description. Presumably the mentioned compounds Na2MeFe (Me = Mo, Re, Os) are also monoclinic but show pseudo-cells that pretend an orthorhombic symmetry. Under this assumption (133) the monocUnic lattice constants have been calculated on the base of the transformation given by these authors for the compound Na2SnFe. 

PbO PbO exists in two polymorphic forms - tetragonal a-PbO and orthorhombic yS-PbO they are photosensitive substances. Basic electrochemical properties of PbO have been studied in acidic [105, 111], neutral [165,] and alkaline [153] media. In acidic solutions, the /I-PbO formation is followed by transformation to a- PbO [166, 167]. In sulfuric acidic... 

The y phase of bismuth molybdate underwent a reversible transformation to the metastable tetragonal y" modification. This metastable modification was observed in the temperature range of 520° to 550°C and underwent an irreversible transformation to the y modification which readily formed at 700°C. The results indicated that the y modification corresponds to that reported by Blasse (83). However, refinement of the crystal data utilizing a single crystal revealed that this y modification was orthorhombic with lattice parameters a = 15.99 A, b = 15.92 A, and c = 17.43 A. An additional observation was the reversible transformation of the y modification to y at 900°C. 

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