Change the direction of arrows in the graph

Question

I am trying to draw a following figure and this is the code

 \[\begin{tikzpicture}
        ] 
        \draw[ arrow] (0,0)--(8,0);      
        \draw[arrow] (4,4)--(8,0);
        \draw[arrow] (0,0)--(4,4);
         \draw [arrow](2,2)--(4,0);
         \draw [arrow](6,2)--(4,0);
         \draw [arrow](6,2)--(2,2);
   \node[left] at (0,0) {$a_{i}$};
 \node[right] at (8,0) {$a_j$};
    \node[above] at (4,4) {$a_k$};
    \node[below] at (4,0) {$a_{ij}$};
    \node[above] at (2,2) {$a_{ik}$};
    \node[above] at (6,2) {$a_{jk}$};
    \end{tikzpicture}\]

I want to change the direction of the arrow as in the picture with the blue one.

Answer 1

Like this:

Code:

\documentclass[10pt,a4paper]{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\begin{document}
     \begin{tikzpicture}
        %\drawgray!25 grid(8,4);
        \begin{scope}[very thick,decoration={
                markings,
                mark=at position 0.5 with {\arrow{>}}}
            ] 
        \draw[postaction={decorate}] (0,0)--(4,0); 
           \draw[postaction={decorate}] (4,0)--(8,0); 
        \draw[postaction={decorate}] (4,0)--(2,2);
        \draw[postaction={decorate}] (0,0)--(2,2);
        \draw postaction={decorate}--(4,4);
        \draw postaction={decorate}--(4,0);
        \draw postaction={decorate}--(6,2);
        \draw[postaction={decorate}] (6,2)--(4,4);
        \draw[postaction={decorate}] (8,0)--(6,2);
    \end{scope}
        \node[left] at (0,0) {$a_{i}$};
        \node[right] at (8,0) {$a_j$};
        \node[above] at (4,4) {$a_k$};
        \node[below] at (4,0) {$a_{ij}$};
        \node[above] at (2,2) {$a_{ik}$};
        \node[above] at (6,2) {$a_{jk}$};
    \end{tikzpicture}
\end{document}

Answer 2

Two alternatives.

1. A normal TikZ picture with two triangles and a bunch of edge nodes.
2. A TikZ-CD where the nodes are in place of the intersections.

Code

\documentclass[tikz]{standalone}
\usetikzlibrary{cd, arrows.meta, quotes}
\tikzset{
  pics/arrow/.style={/tikz/sloped, /tikz/allow upside down,
    code=\pgfarrowdraw{#1}}, pics/arrow/.default=>}
\begin{document}
\tikz[
  >=Straight Barb, line join=round, thick,
  execute at begin node=$, execute at end node=$,
  ar2/.style={edge node={pic[near start]{arrow}pic[near end]{arrow}}},
  ar2'/.style={edge node={pic[near start]{arrow=<}pic[near end]{arrow=<}}},
  ar/.style={edge node={pic{arrow}}},
  auto=right, mc/.style={edge node={coordinate(#1)}},
]
\draw (0,0) node[left]{a_i} to["a_{ij}", ar2, mc=a] +(right:8) node[right]{a_j}
                            to["a_{jk}", ar2, mc=b] +(4,6)     node[above]{a_k}
                            to["a_{ik}", ar2',mc=c] cycle
                        (a) to[ar] (c) to[ar] (b) to[ar] cycle;
\begin{tikzcd}[
  row sep=huge,
  arrows={thick, dash,
    /tikz/every to/.append style={edge node={pic {arrow=Straight Barb}}}}]
                   &                     & a_k                                                 \
                   & a_{ik} \urar\ar[rr] &                     & a_{jk} \dlar\ular             \
  a_i \urar\ar[rr] &                     & a_{ij} \ular\ar[rr] &                   & a_j \ular
\end{tikzcd}
\end{document}

Output

Answer 3

Another possible solution:

\documentclass[10pt,a4paper]{article}
\usepackage{tikz}
\usetikzlibrary{arrows.meta,
                decorations.markings}
\begin{document}
     \begin{tikzpicture}[
                       > = Straight Barb, 
every edge/.append style = {very thick,
                            decoration={markings,
                                        mark=at position 0.5 with {\arrow{>}}},
                            postaction={decorate}
                            },
lbl/.style ={label=#1, coordinate, node contents={}}
                        ]
\draw   (0,0) node[lbl=left:$a_{i}$]    edge (4,0) 
        (4,0) node[lbl=below:$a_{ij}$]  edge (8,0)
        (8,0) node[lbl=right:$a_j$]     edge (6,2)
        (6,2) node[lbl=right:$a_{jk}$]  edge (4,4)

        (4,4) node[lbl=above:$a_{k}$] 
%
        (0,0) edge  (2,2)

        (2,2) node[lbl=left:$a_{ik}$]   edge (4,4)

%
        (6,2) edge (4,0)
        (4,0) edge (2,2)
        (2,2) edge (6,2)
        ;
    \end{tikzpicture}
\end{document}