Nuclear magnetic resonance investigations of the structural phase transitions of AlPO 4 tridymite
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Al and 31 P NMR spectroscopic data are presented for the tridymite polymorph of A1PO4 (AlPO4-t) through its structural phase transition at about 80 °C. The RT 27A1 and 31 P spectra of AlPO4-t both contain doublets of broad peaks, indicating two well-separated groups of sites in the RT structure with mean A l - O - P bond angles per tetrahedron of ~ 147.8° and 153.1° (±1). With increasing temperature, the doublets remain the same up to about 74 °C, where the relative intensities of the two peaks start to change. The peak corresponding to smaller A l - O - P bond angles disappears, and above —88 °C the 27 Al and 31 P spectra contain single symmetrical peaks, corresponding to a mean A l - O - P bond angle of 153.4°. This bond angle increases gradually with increasing temperature to 153.7° at —150 °C and remains constant to about 500 °C. 27A1 quadrupole echo experiments suggests that the 27A1 nuclear quadrupole coupling constant (QCC) is small and decreases with increasing temperature. QCC remains nonzero in the high temperature phase of AlPO4-t, consistent with the previously proposed 3m local symmetry of Al in the high-temperature structure.
I. INTRODUCTION AIPO4 tridymite (AlPO4-t) is thought to be isostractural with SiO2 tridymite (SiO2-t) based on powder x-ray diffraction (XRD) results, 1 4 and has been studied extensively because of its potential application as a catalyst support3 and microwave substrate.5 There has been no single-crystal XRD study of this phase, partially due to the difficulties in making crystals of suitable quality, and its structure is not well understood. Powder XRD refinement at room temperature (RT) yields an orthorhombic unit cell with a = 9.638 ± 0.0019 A, b = 8.664 ± 0.0017 A and c = 18.280 ± 0.0036 A.4 Thermal analyses of AlPO4-t reveal a major phase transition near 93 °C (90 to 96 °C) and possibly a minor one near 130 °C.1>2 In terms of the number of transitions between RT and 500 °C, AlPO4-t is more similar to incommensurate MX-1 phase of SiO2-tridymite (SiO2-t)6>7 than to monoclinic MC SiO2-t.8'9 NMR spectroscopy has proved to be a powerful tool for studying structural phase transitions (SPT's) in solids because it probes short-range structure and relatively low-frequency dynamics (kHz to MHz). AlPO4-t has two 100% abundant NMR-sensitive nuclides, 31 P and 27 Al, and is ideally suited for NMR study. Because 27A1 is quadrapolar, its quadrupole coupling constant (QCC) and asymmetry parameter (17) relate directly to the electric field gradient at the 27A1 sites. Thus, 27A1 QCC and 17 values provide direct information concerning local site symmetry.10 2586
J. Mater. Res., Vol. 10, No. 10, Oct 1995
http://journals.cambridge.org
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To understand better the mechanisms of the structural phase transitions of AlPO4-t, we have carried out a variable temperature 31 P and 27A1 NMR spectroscopy and spin-lattice relaxation study of AlPO4-t. Based on these data we present here structural constraints for the various phases inferred from the NMR parameters (e.g., Z.Al-O-P, s
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