No abstract

The structure of KMgF 3 has been determined by high-resolution neutron powder diffraction at 4.2 K, room temperature and at 10 K intervals from 373 K to 1223 K. The material remains cubic at all temperatures. The average volumetric coef®cient of thermal expansion in the range 373±1223 K was found to be 7.11 (3) Â 10 À5 K À1. For temperatures between 4.2 and 1223 K, a secondorder Gru È neisen approximation to the zero-pressure equation of state, with the internal energy calculated via a Debye model, was found to ®t well, with the following parameters: D = 536 (9) K, V o = 62.876 (6) A Ê 3 , K H is a Gru È neisen parameter. The atomic displacement parameters were found to increase smoothly with T and could be ®tted using Debye models with D in the range 305±581 K. At 1223 K, the displacement of the F ions was found to be much less anisotropic than that in NaMgF 3 at this temperature.

No abstract

In situ FTIR spectroscopy is applied to characterise structural transitions occurring in zeolites NaP and NaA upon hydrationÈ dehydration in conjunction with adsorption measurements. Little spectral variation is observed in the region of structural vibrations of zeolite NaA. By contrast, NaP shows prominent reversible changes in the region of asymmetric stretching TÈO vibrations, 900È1150 cm~1, including both " internal Ï and " external Ï linkages, although the former are usually considered as structure insensitive. Based on regression analysis of our IR data and on XRD results reported previously, phase composition of NaP is determined and four stages in the hydration of P zeolite are outlined.

14The nature of the apparently continuous structural phase transition at 1049 K in the 15 perovskite-structured, MgSiO3 isomorph, neighborite (NaMgF3) ,