Thermal Properties and Phase Transition of 2ZrO₂●P₂O₅ Studied by In Situ Synchrotron X‐ray Diffraction

Abstract

High‐temperature in situ synchrotron X‐ray powder diffraction experiments were carried out to investigate the phase transition mechanism of Zr2P2O9 (2ZrO2·P2O5 or “Z2P”). Linear thermal expansion coefficients were calculated for the low‐temperature phase (α‐Z2P) and the high‐temperature phase (β‐Z2P) from temperature‐dependent changes in lattice parameters. The crystal structures of α‐ and β‐Z2P were determined as a function of temperature by performing Rietveld crystal structure refinements. The structural changes at the phase transition are accompanied by an increase in the average atomic distance between neighboring Zr atoms. The occurrence of shorter metal–metal distances in α‐Z2P is interpreted to result from stronger interactions between partly occupied valence orbitals of the d0 metal atoms. The bond valence method was used to calculate the valence sums of the atoms of α‐ and β‐Z2P, respectively, considering also contributions resulting from covalently bonded atoms. As the bond strength between the metal atoms in Z2P decreases with the transition into the high‐temperature phase, notably, the metal–metal interactions are regarded to constitute a prerequisite for the stabilization of the α‐phase over the energetically favored β‐phase.

In Situ Synchrotron X-ray Diffraction Analysis of the Setting Process of Brushite Cement: Reaction and Crystal Growth

“…Synchrotron X-ray diffraction (SXRD) is a powerful tool for revealing detailed structural and morphological information by real-time monitoring of crystal evolution and phase transition. − The short exposure time makes it possible to provide fast data acquisition with high angular resolution and signal-to-noise ratio, thereby following rapid reactions. It has already been used to study the setting reaction of α-TCP-based cement, which however focused on the setting reaction after 24 h and even longer, as the setting of α-TCP-based cement is much slower than brushite cements.…”

Effect of the sintering method on microstructure and thermal and mechanical properties of zirconium oxophosphate ceramics Zr2O(PO4)2

“…In 2003 and 2008, Wallez et al cleared up the atomic mechanism responsible for the low mean linear thermal expansion of the two forms  and -Zr 2 O(PO 4 ) 2 (2.6 10 -6 K -1 and 1.6 10 -6 K -1 , respectively) [7,8]. The thermal expansion of the  form can be explained by a mechanism which involves a classical ring deformation and the rocking of bridging oxygen atoms, but this phase, which turns irreversibly into the -form between 1100 and 1250 °C, is of secondary interest [9,10]. The low thermal expansion of the form is more atypical and can be explained by a dual thermal contraction mechanism based on cation-cation Coulombic repulsion that leads to the resorption of cavities of the structure and a rocking effect of the Zr-O-Zr bonds.…”