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Volume is V = 3^.5/2a^2c =425*10^-24 cm^3

Have you thought about the exact meaning of Z and M? This seems to be your problem. Bi2Se3 contains 6 Bi and 9 Se atoms in the R centred unit cell. As it concerns the formula you state by yourself it means that you have 3 formula units per unit cell. The mass per unit cell is (6 M(Bi) + 9M(Se)/(6.022e23 mol^-1). Your formula with (Z x Atomic mass) works only for elemental crystals without further thinking. Always check whether your formulas are adequate for the situation and whether you understand the meaning of the quantities you use.

Dear Muneeb Ahme ,

your 6*Z is wrong.

You have 9*Z , i.e. in total 9 * Bi2Se3 molecules in the hexagon.

See for example Fig. 1 (a) in:

Article Model Hamiltonian for Topological Insulators

and count all the atoms in the (net) hexagon...( Please pay attention: the atoms at the rims of the hexagon have to be shared with the neighbourhood when adding them.)

With this number you end up at about 7,66 g/cm3; the same number, which you will get from Andreas Leineweber 's calculation.

This number, which is called the 'x-ray density', is a bit larger than the experimental density value. Experimental values suffer from defects in the crystal lattice and thus are a bit smaller...

Best regards

G.M.

The dimensions of the unit cell and the number of atoms in it are obtained from the XRD of the sample and apply the equation Dth(g/cm3) =1.66N M/ V(Ao).

There is a clear answer given by Prof. Andreas Leineweber.

A = Mass of the Unit Cell and Z = nor of formula Units in the Unit cell

so the mass M = A *Z grams Volume of the hexagonal unit cell is

V = 3^(0.5)/2 * a^2 * c cm^(3)

where a and c are the lattice parameters. So Density is M/V gm /cm^(3)