On the sharpness of Tians criterion for K-stability

Tians criterion for K-stability states that a Fano variety of dimension $n$ whose alpha invariant is greater than $frac{n}{n+1}$ is K-stable. We show that this criterion is sharp by constructing singular Fano varieties with alpha invariants $frac{n}{n+1}$ that are not K-polystable for sufficiently large $n$. We also construct K-unstable Fano varieties with alpha invariants $frac{n-1}{n}$.