The structure of dehydrated zeolite 3A (Si/Al = 1.01) by neutron profile refinement

A neutron powder diffraction study of a dehydrated commercially available potassium exchanged zeolite A (Linde 3A) has shown that the diffraction pattern can be indexed in cubic space group $\text{Fm}\text{3}\text{c}$. For this sample there is 63% exchange of potassium for sodium (K⁺/Na⁺ = 1.69). Data collected at a neutron wavelength of 2.98 Å shows no evidence of rhombohedral distortion and suggests that the assignment of space group $\text{Fm}\text{3}\text{c}$ is correct. The final structural model is closely analogous to that found for dehydrated sodium zeolite A (J. M. Adams, D. A. Haselden, and A. W. Hewat, J. Solid State Chem.44, 245 (1982); J. J. Pluth and J. V. Smith, J. Amer. Chem. Soc.102, 4074 (1980). Unusual features of previous refinements of potassium containing zeolite A samples, i.e., “zero coordinate” cations (P. C. W. Leung, M. B. Kunz, K. Seff, and I. E. Maxwell, J. Phys. Chem.83, 741 (1979)) or potassium inside the β-cage (J. J. Pluth and J. V. Smith, J. Phys. Chem.83, 741 (1979)) have not been found. Refinements using the same 1.9 Å neutron powder diffraction data were also obtained with the models of Leung et al. and Pluth and Smith (1979) as starting points (denoted LKSM and PS, respectively) and comparison is made with these. The final R factors for the three refinements were R_pw (A. K. Cheetham and J. C. Taylor, J. Solid State Chem.21, 253 (1977)) = 10.24% for the model presented here, R_pw = 10.38% (PS model), and R_pw = 10.61% (LKSM model).