Published online by Cambridge University Press: 24 April 2020
We adapt a technique of Kisin to construct and study crystalline deformation rings of $G_{K}$ for a finite extension
$K/\mathbb{Q}_{p}$. This is done by considering a moduli space of Breuil–Kisin modules, satisfying an additional Galois condition, over the unrestricted deformation ring. For
$K$ unramified over
$\mathbb{Q}_{p}$ and Hodge–Tate weights in
$[0,p]$, we study the geometry of this space. As a consequence, we prove that, under a mild cyclotomic-freeness assumption, all crystalline representations of an unramified extension of
$\mathbb{Q}_{p}$, with Hodge–Tate weights in
$[0,p]$, are potentially diagonalizable.