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

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(-alpha beta 0)(beta alpha 0)00 with modulation wave vectors q(1) = -alpha a* + beta b* and q(2) = beta a* + alpha 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) angstrom and c = 17.048(2) angstrom for LaSe1.85, a = 4.178(1) angstrom and c = 16.942(3) angstrom for CeSe1.83, a = 4.123(1) angstrom and c = 16.792(2) angstrom for NdSe1.83 and a = 4.080(1) angstrom and c = 16.672(2) angstrom for SmSe1.84; the components of the modulation wave vectors are alpha = beta = 0.292(1) for LaSe1.85 and CeSe1.83, and alpha = beta = 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.