Temperature and composition dependence of crystal structures and magnetic and electronic properties of the double perovskitesLa2−xSrxCoIrO

X-ray, synchrotron, and neutron powder diffraction techniques were combined to investigate the evolution of crystal structure and physical properties of ${\text{La}}_{2\ensuremath{-}x}{\text{Sr}}_{x}{\text{CoIrO}}_{6}$ with temperature and composition $x$. The following sequence of first- and second-order phase transitions is observed in this system, induced by increasing Sr content and temperature: $P{2}_{1}/n\ensuremath{\leftrightarrow}P{2}_{1}/n+I2/m\ensuremath{\leftrightarrow}I2/m\ensuremath{\leftrightarrow}I4/m\ensuremath{\leftrightarrow}Fm\overline{3}m$. The low-temperature magnetic structures are characterized by the propagation vector $k=(0,0,0)$ for $x=0$, $k=(1/2,0,1/2)$ for $x=1$, and $k=(0,1/2,1/2)$ or $k=(1/2,0,1/2)$ for 1.5 and 2. Different noncollinear magnetic structures are concluded from the combination of magnetization measurements and neutron powder diffraction. Resistivity measurements reveal that the whole series behaves like nonmetals with electronic transport described by a combination of thermal activation and variable range hopping. Band gaps determined by electronic structure calculations agree very well with the experimental data for $x=0$ and 1, and the calculated occupation of the $d$ bands of Co and Ir are in good agreement with a transition $\text{IS}/{\text{HS-Co}}^{2+}/{\text{LS-Ir}}^{4+}\ensuremath{\rightarrow}{\text{HS-Co}}^{3+}/{\text{LS-Ir}}^{5+}$ with increasing Sr content.