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Thanks! It works with \pgfmatrixnextcell

Thanks kaiserkarl. But unfortunately this does not change the outcome. I get always the same error messages

I need to use the matrix command to create tikz diagrams like
\begin{tikzpicture}[-stealth',looseness=.5,auto]
\matrix [matrix of math nodes, column sep={1.5cm,between origins}, row sep={1.5cm,between origins}]
{|(a)| A& |(b)| B& |(c)| C \\[12pt]
&|(x)| \alpha & \\
};
\end{tikzpicture}
But I'd ...