  In this work we develop new finite element discretisations of the
shear-deformable Reissner--Mindlin plate problem based on the
Hellinger-Reissner principle of symmetric stresses. Specifically, we use
conforming Hu-Zhang elements to discretise the bending moments in the space of
symmetric square integrable fields with a square integrable divergence
$\boldsymbol{M} \in \mathcal{HZ} \subset H^{\mathrm{sym}}(\mathrm{Div})$. The
latter results in highly accurate approximations of the bending moments
$\boldsymbol{M}$ and in the rotation field being in the discontinuous Lebesgue
space $\boldsymbol{\phi} \in [L]^2$, such that the Kirchhoff-Love constraint
can be satisfied for $t \to 0$. In order to preserve optimal convergence rates
across all variables for the case $t \to 0$, we present an extension of the
formulation using Raviart-Thomas elements for the shear stress $\mathbf{q} \in
\mathcal{RT} \subset H(\mathrm{div})$.
  We prove existence and uniqueness in the continuous setting and rely on exact
complexes for inheritance of well-posedness in the discrete setting.
  This work introduces an efficient construction of the Hu-Zhang base functions
on the reference element via the polytopal template methodology and Legendre
polynomials, making it applicable to hp-FEM. The base functions on the
reference element are then mapped to the physical element using novel polytopal
transformations, which are suitable also for curved geometries.
  The robustness of the formulations and the construction of the Hu-Zhang
element are tested for shear-locking, curved geometries and an L-shaped domain
with a singularity in the bending moments $\boldsymbol{M}$. Further, we compare
the performance of the novel formulations with the primal-, MITC- and recently
introduced TDNNS methods.
