LaTe_1.82(1): modulated crystal structure and chemical bonding of a chalcogen-deficient rare earth metal polytelluride

Abstract

Crystals of the rare earth metal polytelluride LaTe1.82(1), namely, lanthanum telluride (1/1.8), have been grown by molten alkali halide flux reactions and vapour‐assisted crystallization with iodine. The two‐dimensionally incommensurately modulated crystal structure has been investigated by X‐ray diffraction experiments. In contrast to the tetragonal average structure with unit‐cell dimensions of a = 4.4996 (5) and c = 9.179 (1) Å at 296 (1) K, which was solved and refined in the space group P4/nmm (No. 129), the satellite reflections are not compatible with a tetragonal symmetry but enforce a symmetry reduction. Possible space groups have been derived by group–subgroup relationships and by consideration of previous reports on similar rare earth metal polychalcogenide structures. Two structural models in the orthorhombic superspace group, i.e.Pmmn(α,β,)000(−α,β,)000 (No. 59.2.51.39) and Pm21n(α,β,)000(−α,β,)000 (No. 31.2.51.35), with modulation wave vectors q1 = αa* + βb* + c* and q2 = −αa* + βb* + c* [α = 0.272 (1) and β = 0.314 (1)], have been established and evaluated against each other. The modulation describes the distribution of defects in the planar [Te] layer, coupled to a displacive modulation due to the formation of different Te anions. The bonding situation in the planar [Te] layer and the different Te anion species have been investigated by density functional theory (DFT) methods and an electron localizability indicator (ELI‐D)‐based bonding analysis on three different approximants. The temperature‐dependent electrical resistance revealed a semiconducting behaviour with an estimated band gap of 0.17 eV.

Distorted Te nets in the modulated crystal structures ofRETe1.94(1)(RE= La, Pr, Nd)

“…The band gap of LaTe 1.94 (1), according to this fit, is 0.14 eV (Fig. 7), which is similar to the reported band gaps of NdTe 1.89 (1) (0.14 eV) (Stö we, 2001), LaTe 1.82 (1) (0.17 eV) (Poddig et al, 2020), GdTe 1.8 (0.19 eV) (Poddig et al, 2018) and SmTe 1.84 (0.04 eV) (Park et al, 1998). As expected, the band gap of LaTe 1.94 (1) is slightly smaller than the one reported for LaTe 1.82 (1) , but matches that of NdTe 1.89 (1) very well, hinting towards a more metallic behaviour with an increasing Te content in the RETe 2-series.…”

“…Special care has to be taken when performing the mineralization experiments, as the Te-richer RETe 3 compounds can be formed if the initial ratio of the rare earth metal and tellurium is not corrected for the amount of REI 3 formed due to the addition of I 2 (Poddig et al, 2018). Powder and single-crystal diffraction clearly indicate a tetragonal basic unit cell for all compounds with a ' 4.4 Å and c ' 9.1 Å , as also seen for LaTe 1.82 (1) (Poddig et al, 2020). The EDS analysis of different single crystals, give a ratio RE:Te of about 1:1.9 (Table 1), which was later on confirmed by the structure refinements.…”

“…CeTe 1.9 (Ijjaali & Ibers, 2006) and incommensurately modulated structures such as e.g. LaTe 1.82 (1) (Poddig et al, 2020) a reduced site occupancy should only be present in the planar Te layer. Hence, a freely refined site occupancy factor of Te2 converged with about 0.93 (1) as expected.…”

“…The development of a structural model was performed analogous to the procedure for LaTe 1.82 (1) reported recently (Poddig et al, 2020) by considering first group-subgroup relations starting from the highest possible three-dimensional space group P4/nmm (No. 129).…”

“…The planar [Te] layer features mainly Te 2 2À dumbbells and isolated Te 2À anions along with some vacancies and bent Te 3 units. Allowing higher occupancy cut-off values than 0.825 additionally result in apparent Te 4 squares with Te-Te distances of about 2.93 1 for different RETe 2-compounds, like CeTe 2 , PrTe 2 and LaTe 1.82 (1) (Stö we, 2000b,c;Poddig et al, 2020). However, these Te 4 fragments are presumably the result of an unresolved disorder of two differently orientated Te 2 2À anions and two adjacent vacancies in the [Te] layer.…”