Typesetting chemical element names in bibliography

Question

This is a request of clarification for a quirk I've found with the accepted solution of Typesetting chemical element names

If I attempt to use such solution, and I define a couple of new macros as

\newcommand{\chem}[1]{\ensuremath{\mathrm{#1}}}
\newcommand{\AlO}{\chem{Al_{2}O_{3}}}

using the latter in the bibliography (managed through Biblatex + Biber backend) gives me lots of errors of the kind:

! Missing } inserted.
<inserted text>
}
l.835
?
! Extra }, or forgotten \endgroup.
\MakeLowercase  ... {\lowercase {#1}}\reserved@a }
l.835
?
! LaTeX Error: \mathrm allowed only in math mode.
See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...
l.835
?
! Missing $ inserted.
<inserted text>
$
l.835

In the rest of the text things seem working fine. I'm relatively new to Latex, is there something I'm doing wrong?

Answer 1

An error like this appears when you use \AlO in the title of an entry that your bibliography style converts into sentence case with \MakeSentenceCase*.

@book{hacker,
  author  = {James Hacker},
  title   = {On the Properties of \AlO and More},
  date    = {1980},
}

You need to protect \AlO from the workings of \MakeSentenceCase* and case conversion with double curly braces.

Note that since curly braces are used as argument delimiters for macros and as protection from sentence casing you may have to use more pairs of braces than one would initially think, the minimum number of braces can also depend on the definition of the involved macro.

For protection from case changing for certain words (like proper nouns), see BibTeX loses capitals when creating .bbl file. Also related What is the proper casing to use when storing titles in the bibliography database?, Biblatex changes capitalisation of titles and Biblatex-apa error message when compiling.

\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}

\usepackage[style=phys, backend=biber]{biblatex}

\newcommand{\chem}[1]{\ensuremath{\mathrm{#1}}}
\newcommand{\AlO}{\chem{Al_{2}O_{3}}}

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@book{hacker,
  author  = {James Hacker},
  title   = {On the Properties of {{\AlO}} and More},
  date    = {1980},
}
\end{filecontents}

\addbibresource{\jobname.bib}

\begin{document}
\cite{hacker}
\printbibliography
\end{document}