Crystal structure and phase transition studies in perovskite-type oxides using powder-diffraction techniques and symmetry-mode analysis. Na0.5K0.5NbO3,La2MMnO6, Sr2LnSbO6 (Ln=Nd, Eu, Gd, Dy, Ho, Er, Tm and Yb), Sr2MSbO6 (M=In, Y and Sc), Ca2MSbO6 (M=Ln,