MWE and test file that shows the problem, if compiled without LuaTeX/XeTeX:
\documentclass{article}
\usepackage{ifluatex,ifxetex}
\ifluatex
\usepackage{fontspec}
\else
\ifxetex
\usepackage{fontspec}
\else
\usepackage[utf8]{inputenc}
\fi
\fi
\usepackage[
backend=biber,
]{biblatex}
\begin{filecontents}{\jobname.bib}
@book{foobar,
author = {Kru\u{z}kov and Kru\v{z}kov},
title = {About foobar},
year = {1970},
}
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
\cite{foobar}
\printbibliography
\end{document}
The program biber
normalizes the data input to NFD UTF-8, where all accented characters are decomposed. From its documentation:
3.2 Unicode
Biber uses NFD UTF-8 internally. All data is converted to NFD UTF-8 when read.
If UTF-8 output is requested (to .bbl for example), the UTF-8 will always be > NFC.
In the final file \jobname.bib
, the decompositions are replaced by equivalent precomposed characters, if these exist.
LaTeX → internal biber (NFD UTF-8) → output of biber (NFC UTF-8)
\v{z}
→ U+007A
(z) U+030C
(combining caron) → U+017E
(latin small letter z with caron)
\u{z}
→ U+007A
(z) U+0306
(combining breve) → U+007A
U+0306
Thus \u{z}
remains decomposed and this is a serious problem, because
TeX cannot handle combining accents easyly, if they are following the symbol.
At this time the accent is seen, the symbol is usually already set and the accent cannot modify the base symbol any more. Even worse, it does not even know
the base symbol.
Package ucs
can handle combining accents to some degree by looking ahead for combining accents. But this package is not compatible with package biblatex
.
Also it could get \u{z}
working, probably because a precomposed character does not exist for it.
LaTeX's utf8.def
for package inputenc
cannot handle them.
The following options remain:
Using \v{z}
instead of \u{z}
, probably the correct spelling anyway according to the comments.
The example runs with LuaTeX and XeTeX that can handle the Unicode combining accents.
Option safeinputenc
for package biblatex
, see pst's answer.
Using bibtex
instead of biber
as backend.

bibtex
orbiber
to compile your bibliography.ğ
is used in Turkish, whileǧ
is used in the Latin Berber alphabet and in Lakota. The letteră
is used in Romanian, whileǎ
is used in romanization of Chinese.