[Tex/LaTex] Horizontal and vertical lines in pmatrix

Question

I am trying to replicate the following matrix in TeX:

So far, I have this (the elements in my TeX are supposed to stay in that way):

\documentclass[11pt,a4paper]{article}
\usepackage{amssymb, amsmath, bm}
\usepackage{float, booktabs, makecell, caption, tabularx}
\usepackage{siunitx}

\begin{document}

\begin{gather}
\centering
\boldsymbol{F} = 
\begin{pmatrix}
I_3 & \mathrm{Unrestricted} \\
0_{k \times k} & I_k \\
\end{pmatrix}
.
\end{gather}

\end{document} 

How can I add horizontal and vertical lines in my matrix such that it matches the one in the picture?

Thank you in advance.

Answer 1

Using array instead of pmatrix you can easily add vertical and horizontal lines:

\documentclass[11pt,a4paper]{article}
\usepackage{amssymb, amsmath, bm}

\begin{document}

\begin{gather}
\centering
\boldsymbol{F} = 
\left(
\begin{array}{c|c}
I_3 & \mathrm{Unrestricted} \\ \hline
0_{k \times k} & I_k \\
\end{array}
\right)
.
\end{gather}

\end{document} 

For a bit more vertical white space around the matrix entries, you can change the value of \arraystretch:

\documentclass[11pt,a4paper]{article}
\usepackage{amssymb, amsmath, bm}

\begin{document}
\renewcommand{\arraystretch}{1.5}
\begin{gather}
\centering
\boldsymbol{F} = 
\left(
\begin{array}{c|c}
I_3 & \mathrm{Unrestricted} \\ \hline
0_{k \times k} & I_k \\
\end{array}
\right)
.
\end{gather}

\end{document}