LaSe1.85, CeSe1.83, NdSe1.83 and SmSe1.84 – four new rare earth metal polyselenides with incommensurate site occupancy and displacive modulation

Abstract

Abstract Crystals of the four new binary rare earth metal polyselenides LaSe1.85, CeSe1.83, NdSe1.83 and SmSe1.84 were obtained by flux reactions. Their two-dimensionally incommensurately modulated structures were solved from X-ray diffraction data. Structure models were established in super space group X4/n(–αβ0)(βα0)00 with modulation wave vectors q 1 = –α a * + β b * and q 2 = β a * + α b * and the centering vector (0, 0, 1/2, 1/2, 1/2). The unit cell parameters at T = 293 K are a = 4.216(1) Å and c = 17.048(2) Å for LaSe1.85, a = 4.178(1) Å and c = 16.942(3) Å for CeSe1.83, a = 4.123(1) Å and c = 16.792(2) Å for NdSe1.83 and a = 4.080(1) Å and c = 16.672(2) Å for SmSe1.84; the components of the modulation wave vectors are α = β = 0.292(1) for LaSe1.85 and CeSe1.83, and α = β = 0.293(1) for NdSe1.83 and SmSe1.84, respectively. The modulation originates from a site occupancy wave caused by defects in planar selenium layers coupled to a displacive modulation.