XRD Peaks in databases

I had a look at the JCPDS card of BiOCl and saw that there were very small peaks (such as 0.1 percent relative intensity) that I think wouldn't be observed in XRD pattern, they would just blend in with the noise, so I'd like to know how do they calculate the XRD peaks and know their relative intensities so well (this card referenced a 1935 paper).


Many thanks




***short answer:*** The record is based on the old record by Bannister 1934 *as first description of this phase* . The fine intensity scaling 1:1000 on the record card is based on a more recent re-investigation.


*recording the data:*


Beside the point detector / Geiger counter technique mentioned in Jon Custer's comment -- still *highly* valuable in powder diffraction analysis -- a common method was to record the diffraction pattern *on a photographic film*.


In the case of powder samples, you used for example Guinier cameras:


[![enter image description here](https://i.stack.imgur.com/HQlfl.png)](https://i.stack.imgur.com/HQlfl.png)


([source](https://www.researchgate.net/figure/a-Principle-of-Guiniers-camera_fig2_332306411))


with a stripe of a film placed in the inner of the housing. After the experiment, the film was developed; and the more diffraction intensity, the darker the band on the developed film. The following image depicts a simultaneous analysis of four samples:


[![enter image description here](https://i.stack.imgur.com/Y6vLr.jpg)](https://i.stack.imgur.com/Y6vLr.jpg)


([source](http://pd.chem.ucl.ac.uk/pdnn/inst1/optics2.htm)).


In case of single crystals, *one* method was to place the *oriented* sample in front of a film, for example with a Buerger precession camera


[![enter image description here](https://i.stack.imgur.com/QALoS.gif)](https://i.stack.imgur.com/QALoS.gif)


([source](http://oldwww.iucr.org/iucr-top/comm/cteach/pamphlets/2/node12.html))


to yield clichés like


[![enter image description here](https://i.stack.imgur.com/K1xwW.jpg)](https://i.stack.imgur.com/K1xwW.jpg)


([source](https://www.xtal.iqfr.csic.es/Cristalografia/parte_06_2-en.html))


where the *location* of the spots provides information about symmetry distance relationships of the lattice. (Relative intensity of the spots equally has a meaning, but beyond the scope of this question here.)


In both cases, the intensity of the signals were attributed by eye (e.g., strong, medium, weak), or later by [densitometry](https://en.wikipedia.org/wiki/Densitometry) and [densitometers](https://en.wikipedia.org/wiki/Densitometer) in numbers. While these film methods *today* are superseeded in convenience by positional detectors and CCD arrays (no wet chemistry to develop the films, quick numeric read-out), you still find these noted in volume C of the International Tables of Crystallography, section 7.1 «Detectors for X-rays».


[![enter image description here](https://i.stack.imgur.com/R8eeO.png)](https://i.stack.imgur.com/R8eeO.png)


([source](https://books.google.com/books?id=60FoFEGyShIC&pg=PA619&lpg=PA619&dq=densitometer%20crystallography&source=bl&ots=u29jIqE-fr&sig=ACfU3U0H6DbdkrCZx8Ra_aqbVrPv7GTsVQ&hl=de&sa=X&redir_esc=y#v=onepage&q&f=false))




---


*your phase:*


You find the first entry about $\ce{BiOCl}$ as entry [#1011175](http://www.crystallography.net/cod/1011175.html) published 1934 in *Nature*, and [#9011782](http://www.crystallography.net/cod/9011782.html) published 1935 in *Mineralogical Magazine* by the same author as indexed by the [COD](http://www.crystallography.net/cod/index.php) with their `.cif` files. You may use these to predict how a powder diffraction pattern of this phase should look like:


[![enter image description here](https://i.stack.imgur.com/9tFbB.png)](https://i.stack.imgur.com/9tFbB.png)


And indeed, there are some signals *expected* to be much weaker, than others, e.g., reflection $(103)$ (blue rectangle) would be weak and maybe not spot.


Interestingly, the [AMS database](http://rruff.geo.arizona.edu/AMS/minerals/Bismoclite) equally offers access to the Bismoclite data, including a copy of Bannister's publication in *Mineralogical Magazine*, too ([here](http://rruff.geo.arizona.edu/doclib/mm/vol24/MM24_49.pdf)). And table 2 (*loc. cit.*), based on the photographs and their visual readout lists only 16 entries. You find all of them in the subsequent re-investigation in 1983 ([here](http://rruff.geo.arizona.edu/AMS/xtal_data/DIFfiles/18036.txt), view text file *next to* download diffraction data), now with a total of observations equal to 37.


Thus, it is plausible the record card you accessed refers either to this set (1983) of data (or an even more recent study) to list $(h,k,l)$ and their diffraction intensities, but credits Bannister for the first description of the very same phase in the 1930s.


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