Low-temperature tetragonal phase

Fig. 89. Magnetic data for the mixed system (Zni Ax)CMns, where A Al, Ga, Mn. (a) Variation of Bohr magnetons per Mn atom with composition. (Ordinary values obtained b r extrapolation from cubic phase extraordinary value corresponds to low-temperature tetragonal phase.) (b) (facing page) Variation of Curie and phase-transition temperatures with composition. (After Howe and Myers (282).)...

The transition temperature from the low-temperature tetragonal phase to the monoclinic phase occurs at a lower temperature (6(X)-700 C) for the more acidic gels (pH =1.5 and 6.0) than the more basic gels (pH = 12) which transforms at 800 C [Colomban and Bruneton, 1992]. Like in other ceramic systems, a small addition of other oxide can change the nature of the phase(s) formed at a given temperature. For example, with 10% molar Ce02 present, the zirconia cubic phase can exist with tetragonal which is stable up to 13(X) C [Colomban and Bruneton, 1992]. 

The high catalytic activity and surface area observed for the catalyst CG can be explained by the formation of Pb involved active sites and stabilization of low temperature tetragonal phase for the carrier material ZrOj (XRD results [11]). High oxidation state of Zr(IV) and low Pb Zr ratio will favour the variation of oxidation state of counter cation Pb to higher ones, creating certain active sites M = Pb or M = Pb, M =... 

C specific heat LTT phase low-temperature tetragonal phase... 

Whereas the first microscopic theory of BaTiOs [1,2] was based on order-disorder behavior, later on BaTiOs was considered as a classical example of displacive soft-mode transitions [3,4] which can be described by anharmonic lattice dynamics [5] (Fig. 1). BaTiOs shows three transitions at around 408 K it undergoes a paraelectric to ferroelectric transition from the cubic Pm3m to the tetragonal P4mm structure at 278 K it becomes orthorhombic, C2mm and at 183 K a transition into the rhombohedral low-temperature Rm3 phase occurs. 

The presence of the higher order harmonics of the magnetic helix in the incommensurate phase is characteristic for the temperature interval where the Lifshits invariant is comparable with an anisotropy invariant [11], For magnetic systems with a one-parametric thermodynamic potential the propagation vector q is not equal to zero already at the temperature where the system orders, T1 =TP. As an anisotropy invariant is proportional to rj for a crystal with tetragonal symmetry, then it becomes comparable with Lifshits invariant proportional to q t] 2 much below Ti near the transition into a low-temperature commensurate phase. However, in copper metaborate q grows sharply from approximately zero at temperature 7) < Tp (Fig. 7) [5],... 

Finally, interest in zirconia membranes has increased in recent years. Zirconia exhibits three well-defined phases in the order of increasing temperature the monoclinic, tetragonal and cubic phases. However, it has been suggested that a low-temperature metastable tetragonal phase exists in contrast to the high-temperature tetragonal phase [Cot, 1991 Colomban and Bruneton, 1992]. For pure zirconia membranes, it appears that the following phase uansitions occur [Stevens, 1986 Colomban and Bruneton, 1992] ... 

Two phases, a high-temperature (fergusonite-(Y)) T-phase with tetragonal symmetry and a low temperature M-phase (fergusonite-)S-(Y)) with monoclinic symmetry, are known for fergusonite. Komkov (1959) determined the crystal structures of both phases. Later, the crystal structure of the M-phase was refined independently by Weitzel and Schrocke (1980) and by Trunov et al. (1981). Their unit cells were different from those of Kinzhibalo et al. (1982) a = 7.037, b = 10.945, c = 5.298 A, i = 134.07° (Trunov et al. 1981). 

Neutron scattering experiments also detected static charge stripes in the low-temperature tetragonal (LTT) phase of lanthanum cuprates Lai 6-. rNdo 4Ba tCu04 and La2- BajrCu04 [16-19]. One of the manifestations of the stripe formation is the anomalous suppression of superconductivity near the hole concentration x = in the latter crystal. A weaker... 

In all appHcations involving zirconia, the thermal instabiHty of the tetragonal phase presents limitations especially for prolonged use at temperatures greater than - 1000° C or uses involving thermal cycling. Additionally, the sensitivity of Y—TZP ceramics to aqueous environments at low temperatures has to be taken into account. High raw material costs have precluded some appHcations particularly in the automotive industry. 

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