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Several high-speed, high-temperature in situ diffraction studies are conducted to explain the mechanisms involved in the direct synthesis of cubic Zr[Mo.sub.2][O.sub.8]. The results prove that the material is highly metastable at all the temperatures.

A combination of high-resolution solid-state NMR spectroscopy and first-principles calculations is utilized to determine the [super 17]O and [super 29]Si NMR parameters of six different phases of MgSi[O.sub.3].

The high-pressure structural chemistry of α-zirconium phosphate, α-Zr(HPO4)2·H2O, was studied using in-situ high-pressure diffraction and synchrotron radiation. The layered phosphate was studied under both hydrostatic and non-hydrostatic conditions and Rietveld refinement carried out on the resulting diffraction patterns. It was found that under hydrostatic conditions no uptake of additional water molecules from the pressure-transmitting medium occurred, contrary to what had previously been observed with some zeolite materials and a layered titanium phosphate. Under hydrostatic conditions the sample remained crystalline up to 10 GPa, but under non-hydrostatic conditions the sample amorphized between 7.3 and 9.5 GPa. The calculated bulk modulus, K0 = 15.2 GPa, showed the material to be very compressible with the weak linkages in the structure of the type Zr-O-P. [ABSTRACT FROM AUTHOR]

The acidity and unique porous structures of zeolites play an important role in controlling the activity and selectivity of many zeolite-based catalysts. Although 27Al, 29Si and 1H NMR spectroscopy represent standard analytical tools with which to study these materials, 17O-NMR investigations are much less routine, owing to the very low natural abundance of 17O (0.037%), its relatively low resonant frequency and its large quadrupole moment. 17O-NMR resonances from framework oxygen sites in a variety of zeolites have been detected, but the 17O-NMR resonance from oxygen directly bound to the Brønsted acid site (Si–O(H)–Al) has remained elusive. Here we report the direct observation of this resonance in dehydrated zeolite HY, by using high magnetic-field strengths. 17O–1H double-resonance NMR experiments are used to prove unambiguously that the 17O signal arises from O nearby H atoms. A large quadrupolar coupling constant, the measure of the local distortion of this site, of 6.6 MHz is determined, which is similar to that obtained in ab initio calculations of zeolite HY-like clusters; this value drops to 5 MHz on acetone binding. The results presented in this paper open up methods for characterizing zeolite acidity and investigating H+-sorbent interactions. [ABSTRACT FROM AUTHOR]

17O NMR studies of various cation-exchanged LTA and LSX zeolites have shown similarities between the two systems. LSX samples containing divalent cations contain resonances with similar chemical shifts to those previously assigned to ‘bare’ framework oxygen atoms in Ca-LTA and Sr-LTA. The assignments are consistent with the trends seen in the spectra of monovalent cation-containing LSX and LTA zeolites, which show an increase in the average chemical shift with increasing cationic radius. The spectrum of Li-LSX, like Na-LSX, can be assigned based on the T–O–T bond angles. Gas sorption studies on Li-LSX are used to help identify the framework oxygen atoms that form the β-cages and demonstrate the sensitivity of the 17O shifts to gas loading. [Copyright &y& Elsevier]

The article discusses a study which examines the direct synthesis of cubic gamma-ZrMo2O8 from the constituent oxides at different temperatures with rapid in-situ powder X-ray diffraction. It focuses on the thermodynamics of cubic gamma-ZrMo2O8 which is essential in the production of zero expansion composite materials. It also compares the thermodynamic properties of ZrMo2O8 and ZrW2O8.

The 29Si and 17O NMR parameters of six polymorphs of MgSiO3 were determined through a combination of high-resolution solid-state NMR and first-principles gauge including projector augmented wave (GIPAW) formalism calculations using periodic boundary conditions. MgSiO3 is an important component of the Earth's mantle that undergoes structural changes as a function of pressure and temperature. For the lower pressure polymorphs (ortho-, clino-, and protoenstatite), all oxygen species in the 17O high- resolution triple-quantum magic angle spinning (MAS) NMR spectra were resolved and assigned. These assignments differ from those tentatively suggested in previous work on the basis of empirical experimental correlations. The higher pressure polymorphs of MgSiO3 (majorite, akimotoite, and perovskite) are stabilized at pressures corresponding to the Earth's transition zone and lower mantle, with perovskite being the major constituent at depths >660 km. We present the first 17O NMR data for these materials and confirm previous 29Si work in the literature. The use of high-resolution multiple-quantum MAS (MQMAS) and satellite-transition MAS (STMAS) experiments allows us to resolve distinct oxygen species, and full assignments are suggested. The six polymorphs exhibit a wide variety of structure types, providing an ideal opportunity to consider the variation of NMR parameters (both shielding and quadrupolar) with local structure, including changes in coordination number, local geometry (bond distances and angles), and bonding. For example, we find that, although there is a general correlation of increasing 17O chemical shift with increasing Si-O bond length, the shift observed also depends upon the exact coordination environment. [ABSTRACT FROM AUTHOR]

The understanding of the structural framework and metal locations within zeolites are two critical aspects to tune and further exploit their properties as heterogeneous catalysts, advanced optical devices, or as water remediation materials. The development of electron microscopy has made it possible to observe directly the zeolitic framework and the distribution of extra framework cations within the pores at the atomic level. Here, we have studied the EMT framework providing data with an unprecedented spatial resolution, which have allowed the analysis of the FAU domains present in the framework. Additionally, potassium and lead (introduced by aqueous ion exchange), which were nonperiodically distributed in the pores have been located. An alternative image mode, annular bright field, has been used, proving its usefulness to extend the spatial resolution and increase the sensitivity toward light elements such as bridging oxygen. Finally, the combination of atomic imaging (local information) with the three-dimensional electron diffraction tomography analysis (averaged information) determined the lead- EMT crystal symmetry to be P 6 3 mc ., Competing Interests: The authors declare no competing financial interest., (© 2021 American Chemical Society.)