pub fn equal_bond_wurtzite_u(a_m: f64, c_m: f64) -> f64Expand description
The wurtzite u at which the bond along [0001] and the three other
bonds of each atom have the same length: u = 1/4 + a^2 / (3 c^2).
Derived here from the wurtzite sites above: a cation on column B at
height 0 has its anion neighbour at u c straight above it, and three
anions of the C layer at height (u - 1/2) c, displaced in the plane by
the B-C column spacing a / sqrt(3). Equal lengths,
(u c)^2 = a^2/3 + (1/2 - u)^2 c^2, give the formula. At the ideal ratio
c/a = sqrt(8/3) it is 3/8.
This is a geometric model value, not a measurement. It is what
Lattice::gallium_nitride uses until a measured u is sourced.