I was reading about this compound and found that it has been reported that on increasingly substituting the Cu with Ag, the lattice constant a (and b) increase while c remain unchanged. What bothers me is that since Ag is bigger than Cu in size the lattice constant a, b and c al should increase. Why c is not changing ?.
This compound crystallice in a tetragonal structure: a=b; if a increases then b also increases. The c parameter value in this structure is affected by the anion, as anion remains the same, c don't change.
Best regards,
Pedro
Dear Pedro thanks for your input. This is what my doubt actually is.... on substituting cation, why c does not change.
See the picture for the structure of CuZnSnS4. Cu has occupied 2 sites 2a and 4d (share 50:50 with Zn). At low substitution it may go to the largest posible site, in this case 2a (Cu) and 2b (Sn). The lattices parameter of a=b=5.435, c=10.843, slightly increase in a could be difficult to detect in c. By the atomic radius, it's favourable for Au (1.79) to replace Sn (1.72) rather than Cu (1.57) which may be explained by a thermodynamic analysis. G = U + PV +1/2hv − TS. At low temperatures and pressures, G ≈ U, i.e. the lattice energy determines which structure is stable.
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