Thermal Expansion, Compressibility, and Polymorphism in Hafnium and Zirconium Titanates

Abstract

Using X-ray diffraction techniques, thermal expansion and compressibility were measured on the orthorhombic compounds HfTiO,, Hf,.,,Tk,.,40,, and ZrTiO, (both quenched and cooled slowly from 1300°C). The thermal expansion of HfTiO, is highly anisotropic; the thermal expansion coefficients along the crystallographic axes are an= +(8.7+0.5) XlO-, "C-', ab= -(5.2+0.5)Xlo-B "C", and a c = +(5.3+0.5)X104 "CY. The thermal expansion of Hf,.,,Tio.,,04 was similar to that of HfTiO, but that of ZrTiO, was markedly less anisotropic. The compressibilities of HfTiO, and ZrTiO4 also differed markedly. All compounds investigated, however, behaved similarly in exhibiting a polymorphic transition to a high-pressure phase having the monoclinic baddeleyite (ZrO,) structure. The polymorphism can be explained qualitatively on the basis of crystal structure.

The thermodynamics of formation, molar heat capacity, and thermodynamic functions ofZrTiO4(cr)

“…(9,10) It contains highly distorted edge-sharing [MO 6 ] octahedra. (11) When cooled slowly at T < 1470 K, it undergoes a transformation into an incommensurate phase characterized as being "interface modulated". (12) This phase, in turn, transforms on slow cooling to a commensurate phase at T = 1110 K. (13) The commensurate phase has the same space group as the high-temperature structure, but with a doubling of the a-lattice dimension from its value in the high temperature phase.…”

Ti-catalyzed HfSiO4 formation in HfTiO4 films on SiO2 studied by Z-contrast scanning electron microscopy

“…We previously showed 10 that demixing in o-HfTiO 4 initiates by a polymorphic second order phase transition to monoclinic (m) HfTiO 4 (or m-Hf 0.5 Ti 0.5 O 2 ) after 1 h at 1193 K. m-HfTiO 4 is an elevated pressure phase unstable at atmospheric pressure at any temperature. 21 In the films, m-HfTiO 4 ultimately evolves into one of the demixed phases, m-HfO 2 , by Ti exsolution. 10 We suggest here that Ti exsoluted from m-HfTiO 4 catalyzes HfSiO 4 formation by the following two-step reaction.…”

Dense mullite–zirconia–zirconium titanate ceramic composites by reaction sintering

“…1 also shows the complete dilatometric curve of the unfired sample from room temperature to 1500 1C and the reverse cooling cycle down to 300 1C. The sintering starts at Table 1 Formula and properties of the studied ceramics phases [38][39][40][41][42][43][44]. Fig.…”