Abstract
Neutron powder di raction has been used to investigate the structural and magnetic behavior of
the isoelectronically doped Fe pnictide material PrFe1-xRuxAsO. Substitution of Ru for Fe sup-
presses the structural and magnetic phase transitions that occur in the undoped compound PrFeAsO.
Contrary to the behavior usually observed in 1111 pnictide materials, the suppression of both the
structural and magnetic transitions does not result in the emergence of superconductivity or any
other new ground state. Interestingly, PrFeAsO itself shows an unusual negative thermal expansion
(NTE) along the c-axis, from 60K down to at least 4K; this does not occur in superconducting
samples such as those formed by doping with fluorine on the oxygen site. We nd that NTE is
present for all concentrations of PrFe1-xRuxAsO with x ranging from 0.05 to 0.75. These results
suggest that the absence of superconductivity in these materials could be related to the presence of
NTE.
