The ternary rare-earth polychalcogenides LaSeTe2, CeSeTe2, PrSeTe2, NdSeTe2, and SmSeTe2:: Syntheses, crystal structures, electronic properties, and charge-density-wave-transitions

Phase transitions have been observed for LaSeTe2, CeSeTe2, PrSeTe2, and NdSeTe2 during temperature dependent single-crystal structure determinations and measurements of the electrical transport properties. The high temperature structure of all compounds is an ordered ternary variant of the NdTe3 type and can be described in the space group Cmcm (no. 63). The low temperature structures are incommensurately modulated due to the occurrence of a charge density wave (CDW). The LT structures of LaSeTe2, PrSeTe2, and NdSeTe2 can be described in the (3 + 1)-dimensional superspace group Cmcm(00y)s00 with the following lattice parameters and translational parts gamma of the modulation vectors:.a = 4.295(1) angstrom, b = 25.371(4) angstrom, c = 4.306(1) angstrom, gamma = 0.29(1) for LaScTe2 (at T = 173 K); a = 4.262(1) angstrom, b = 25.034(4) angstrom, c = 4.272(1) angstrom, gamma = 0.29(1) for PrSeTe2 (at 120 K); a = 4.256(1) b = 24.993(3) angstrom, c = 4.258(1) angstrom, gamma = 0.30(1) for NdSeTe2 (at 173 K). The square tellurium nets of the basic structure are distorted at lower temperatures to give zigzag chains of tellurium atoms. SNED patterns show that CeSeTe2 exhibits a two-dimensional modulation with modulation vectors of q(1) = (-0.12(1), 0, 0.12(1)) and q(2) = (0. 12(1), 0, 0.12(1)), and can be described in the superspace group C2/c(s0q., r0p)00 with lattice parameters of a = 4.257(1) angstrom, b = 25.176(5) angstrom, c = 4.259(1) angstrom (at T = 120 K). Resistance measurements show that the non-modulated HT phases exhibit a metal-like temperature dependence of the resistance whereas the modulated LT phases behave like semiconductors. The transition temperatures T-CDW have been found to be 491(5) K for LaSeTe2, 226(3) K for PrSeTe2, and 207(3) K for NdSeTe2 by X-ray diffraction and 275(3) K for CeSeTe2 by measurements of the electrical resistance. (c) 2005 Elsevier SAS. All rights reserved.