Normally we also use to estimate the pore width from XRD first peak position. If it is so, in some layered materials and thermally unstable materials are having peak at lower two theta. How do judge the samples. Beside some of the Inorganic or Organo Inorganic salts are also having peak at lower two theta values.
X-ray diffraction primarily measures regular arrangements of atoms. If the regular arrangement of atoms lead to a pore, then the pore size can be estimated from the crystal structure. In layered materials, and in particular if the 'top' and 'bottom' layers have the same structural arrangement, the pore size can then be deduced from the XRD-pattern.
Yes, My assumption is If the starting peak is higher than 8 degree that is small pore, in between 6 to 7 medium pore, 5 to 6 large pore 5 to 5.5 very large pore, lesser than 5 ie around 2.5 or still lesser will be mesopore. I hope these are reflected in most of the samples. However, this is not shown by all the samples. Why such deviation occurs. It is a matter of discussion.
N. Venkatathri is right, however, it very often disobeys the relationship. Specially when your first peak of XRD is small peak. It reflect the only dimension of unit cell of structure of zeolite. When the first peak is very strong, the most of case are follow the rules mentioned by N. Venkatathri. For example,ITQ-22 sample has its peak below 5, however, it is only 12*10*8 ring structure. ITQ-43 has very strong peak at around 4 degree. It is an extra-large pore zeolite with 28*12*12 channel system.
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If your sample having some porosity then you have to measure XRD at lower angel. After measuring the XRD, analyzed this data by using JCPDS data. From this softwere you known the unit cell structure of sample, also this data give (h,k,l) value and d spacing value. Use dx = 8M/ Na^3(means 'a' cube) , where 8 is number of molecule in unit cell of spinel lattice, M for M.W of sample , N is avogardo's number and 'a' is lattice constant, dx is X-ray density. Then for porosity (P) calculation use P= (1- ds/dx) where, ds is bulk density of sample this can be calculated from geometry of mass sample.
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