Large effects of A -site average cation size on the properties of the double perovskites Ba 2 − x Sr x MnReO 6 : A d 5 − d 1 system

${\mathrm{Ba}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnReO}}_{6}$ $(x=0,$ 0.5, 1, 2) phases with a double-perovskite structure were prepared by solid-state techniques in evacuated sealed silica tubes. ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Re}}^{6+}$ are virtually completely ordered on the B sites. The compounds are ferrimagnetic below 120 K. The maximum saturation moment was obtained for a compound with $x=0.5$ whose tolerance factor is closest to 1. The whole series of compounds, $0.0l~xl~2.0,$ exhibits semiconducting behavior with variable-range hopping type of conduction. ${\mathrm{Sr}}_{2}{\mathrm{MnReO}}_{6}$ has an unusually high coercive field (2.6 T at 5 K) and two transitions in the $M\ensuremath{-}H$ loop. ${\mathrm{Ba}}_{2}{\mathrm{MnReO}}_{6}$ shows large positive magnetoresistance (14% at 80 K, 5 T) below 140 K, while the other compositions studied exhibit negative magnetoresistance in the temperature range measured.