I believe you got it wrong. A ray hitting the sample with an angle theta is reflected also with an angle theta. Therefore, the XR source and the XR sensor are separated by an angle 2theta. In your experimental setup you change the angle between source and sensor, that is why your x-axis should be 2theta. However, for measuring the crystallographic distance (Bragg´s law) the important magnitude is the angle of the incident beam.

hi sir but in Bragg's law we take it as sintheta only .....not two theta y....

In Bragg's law angle between incident ray and normal to the surface is represented as theta. in the xrd spectrometer 2theta is the angle between incident and reflected rays, which the instrument measures. both are same.

@Javier Fernandez......that's a nice comment. We know that angle of incidence = angle of reflection. Could you please elaborate your comment by drawing ray diagram?

Its little bit confusing which angle you are taking into account.

Many thanks!

@Suresh.....I do agree with you. Could you please draw a ray diagram for better understanding of your comment?

Early X-ray sources were large and difficult to move. For this reason, early powder diffractometers were constructed such that the source was fixed, the sample moved through an angle theta and the detector moved through angle 2theta. This is what Javier was talking about, and the relevant ray diagram is here:

http://content.answcdn.com/main/content/img/McGrawHill/Encyclopedia/images/CE750600FG0010.gif

It is for this historical reason that powder patterns are usually plotted against 2theta. The angle relevant to Bragg's analysis, however, is theta. The reasons behind this and the derivation of Bragg's equation can be found in any elementary text on diffraction. In order to obtain correct values of the lattice parameters, you must divide 2theta by 2 and use theta in Bragg's equation.