In order to find the lattice parameters you will need to do indexing. This is discovering the hkl values for each of the diffraction peaks. There are many tutorials to be found by searching the web that will show you how to do it with high symmetry structures. (You can also find the information in old fashioned things called books! These can can be found in establishments once frequented by students called libraries). Honestly - reading books is a great way to learn.

Anyway indexing gets more complicated the lower the symmetry. More commonly software is now employed to do 'autoindexing' This can often find the symmetry system and the approximate lattice parameters but it is more difficult to obtain a correct space group without going further into crystal structure determination.

Finding the lattice parameters requires that you have accurate peak positions to start with, so get high quality, high resolution data. It is better to work at lower angles since at higher angles the problem of overlapping peaks becomes a serious limitation to finding the peaks to index. If you need to do further crystal structure determination (finding atomic co-ordinates and accurate space group and lattice parameters) then you will need very high quality data that ranges up to very high 2-theta values. 

If you first understand the theory and try the manual method with sample cubic data, then you will understand what the software is doing with more complex situations.

I have added a link to a list of available software for indexing.

http://www.ccp14.ac.uk/solution/indexing/

The lattice parameter can be calculated using the Bragg's law:

2*d*sin(theta) = n*wavelenght

d = distance between interatomic plans

n = order of diffraction

theta = angle at the centre of the peak

Dear Khagesh,

For new compounds we need planes indexed powder XRD pattern. If you have identified the planes one can easily determine the lattice parameters.  If you want to find out the lattice parameters, do the single crystal XRD study, it will provide you the cell parameters.I think this is available at Delhi university and all IIT's in INDIA.

If it  is a new crystal better to collect entire structure data and it will be a nice piece of work.

Hi Kagesh,....in addition to previous answer, in my opinion, If  your sample is a new compound (new crystalline phase or polycrystalline phases). It is hard to find the lattice parameters (position = a, b,c ; angle = alpha, betha, and gamma; S.G and others). Since we have already known that Bragg's law (as mention in previous answer) actually is used related to  both hkl planes and diffracted beam's angle (in advance : to know d-spacing in cubic phase  is different to that of in monoclinic and others phase), so you can identify your knew compound (assumed) by : (1) identify your raw materials and predict  the possibility compound formed, (2). compare to the existed compounds, (3). match your closest predicted compound (cubic, orthorhombic, or monoclinic and the 2-theta/angle shift) and calculate the the cell parameters using Bragg's equation, (4). compare the pattern of your sample (cubic, monoclinic or others to the pattern of reference) that you have calculated in order to know which one is the closest structure. (5). Use the data of the closest one to run  Rietveld program.

Hope this information will be useful.

In order to find the lattice parameters you will need to do indexing. This is discovering the hkl values for each of the diffraction peaks. There are many tutorials to be found by searching the web that will show you how to do it with high symmetry structures. (You can also find the information in old fashioned things called books! These can can be found in establishments once frequented by students called libraries). Honestly - reading books is a great way to learn.

Anyway indexing gets more complicated the lower the symmetry. More commonly software is now employed to do 'autoindexing' This can often find the symmetry system and the approximate lattice parameters but it is more difficult to obtain a correct space group without going further into crystal structure determination.

Finding the lattice parameters requires that you have accurate peak positions to start with, so get high quality, high resolution data. It is better to work at lower angles since at higher angles the problem of overlapping peaks becomes a serious limitation to finding the peaks to index. If you need to do further crystal structure determination (finding atomic co-ordinates and accurate space group and lattice parameters) then you will need very high quality data that ranges up to very high 2-theta values. 

If you first understand the theory and try the manual method with sample cubic data, then you will understand what the software is doing with more complex situations.

I have added a link to a list of available software for indexing.

http://www.ccp14.ac.uk/solution/indexing/

the below post may give you more detailed view on how to go about with your data.

https://www.researchgate.net/post/How_can_I_analyse_a_new_powder_XRD_data_for_new_composition

If you know arc of symmetrical pairs

Bragg's law theta=S/4

Inter planar Spacing d = a/sqrt(h^2+k^2+l^2)

d= a/sqrt(N) N= h^2+k^2+l^2

therefore cell constant a= dsqrt(N) meter

if N = 1 == then hkl = 100

N=2 == = 110

N= 3========== = 111

---------------

--------------

---N=16 ========= = 400

where d= lamda/2sin(theta) lamda = 1.54 A0 if Cu k - alpha ray used

crystal size

Beta(2theta) = k lamda / L cos (theta)

K= Full width at half maximum value from the spectrum

L = volume average of the crystal thickness in the direction of normal to the reflecting plane