Neutron diffraction studies as a function of temperature on solid solutions of MnSe and MnTe in the Se rich region are presented. Interestingly as Te is doped in MnSe, the structural transformation to NiAs phase diminishes, both in terms of % fractio
n of compound as well as in terms of transition temperature. In MnTe$_{0.3}$Se$_{0.7}$, the NaCl to NiAs phase transformation occurs at about 40K and although it is present at room temperature in MnTe$_{0.5}$Se$_{0.5}$, its volume fraction is only about 10% of the total volume of sample. The magnetic ordering temperature of the cubic phase decreases with increasing Te content while the hexagonal phase orders at the same temperature as in MnSe. Anomalies in thermal evolution of lattice parameters at magnetic ordering as well as structural transition temperatures indicate presence of magnetostructural coupling in these compounds.
Temperature (12K $le$ T $le$ 300K) dependent extended X-ray absorption fine structure (EXAFS) studies at the Fe K edge in FeSe$_{1-x}$Te$_x$ (x = 0, 0.5 and 1.0) compounds have been carried out to understand the reasons for increase in T$_C$ upon Te
doping in FeSe. While local distortions are present near superconducting onset in FeSe and FeSe$_{0.5}$Te$_{0.5}$, they seem to be absent in non superconducting FeTe. Of crucial importance is the variation of anion height. In FeSe$_{0.5}$Te$_{0.5}$, near superconducting onset, the two heights, $h_{Fe-Se}$ and $h_{Fe-Te}$ show a nearly opposite behaviour. These changes indicate a possible correlation between Fe-chalcogen hybridization and the superconducting transition temperature in these Fe-chalcogenides.
Effect of Co, Ru and Cu substitution at B and B sites on the magnetic and transport properties of LaSrCoRuO$_6$ have been investigated. All the doped compositions crystallize in the monoclinic structure in the space group $P2_1/n$ indicating a double
perovskite structure. While the magnetization and conductivity increase in Co and Ru doped compounds, antiferromagnetism is seen to strengthen in the Cu doped samples. These results are explained on the basis of a competition between linear Co-O-Ru-O-Co and perpendicular Co-O-O-Co antiferromagnetic interactions and due to formation of Ru-O-Ru ferromagnetic networks.
