page_title: Molybdenum ditelluride
section_title: Crystal form
hierarchical_section_title: Molybdenum ditelluride / Physical properties / Crystal form
caption_attribution_description: Crystal structures of 1T′ (shadow) and Td (solid) phases
context_page_description: Molybdenum(IV) telluride, molybdenum ditelluride or just molybdenum telluride is a compound of molybdenum and tellurium with formula MoTe₂, corresponding to a mass percentage of 27.32% molybdenum and 72.68% tellurium. It can crystallise in two dimensional sheets which can be thinned down to monolayers that are flexible and almost transparent. It is a semiconductor, and can fluoresce. It is part of a class of materials called transition metal dichalcogenides. As a semiconductor the band gap lies in the infrared region. This raises the potential use as a semiconductor in electronics or an infrared detector.
context_section_description: MoTe₂ commonly exists in three crystalline forms with rather similar layered structures: hexagonal α, monoclinic β and orthorhombic β'. At room temperature it crystallises in the hexagonal system similar to molybdenum disulfide. Crystals are platy or flat. MoTe₂ has unit cell sizes of a=3.519 Å c=13.964 Å and a specific gravity of 7.78 g·cm⁻³. Each molybdenum atom is surrounded by six tellurium atoms in a trigonal prism with the separation of these Mo and Te atoms being 2.73 Å. This results in sublayers of molybdenum sandwiched between two sublayers of tellurium atoms, and then this three layer structure is stacked. Each layer is 6.97 Å thick. Within this layer two tellurium atoms in the same sublayer subtend an angle of 80.7°. The tellurium atoms on one sublayer are directly above those in the lower sublayer, and they subtend an angle of 83.1° at the molybdenum atom. The other Te-Mo-Te angle across sublayers is 136.0°. The distance between molybdenum atoms within a sublayer is 3.518 Å. This is the same as the distance between tellurium atoms in a sublayer. The distance between a tellurium atom in one sublayer and the atom in the other sublayer is 3.60 Å.
  The layers are only bonded together with van der Waals force. The distance between tellurium atoms across the layers is 3.95 Å. The tellurium atom at the bottom of one layer is aligned with the centre of a triangle of tellurium atoms on the top of the layer below. The layers are thus in two different positions. The crystal is very easily cleaved on the plane between the three layer sheets. The sizes change with temperature, at 100 K a=3.492 Å and at 400 K is 3.53 Å. In the same range c changes from 13.67 Å to 14.32 Å due to thermal expansion. The hexagonal form is also called 2H-MoTe₂, where "H" stands for hexagonal, and "2" means that the layers are in two different positions. Every second layer is positioned the same.
  At temperatures above 900 °C MoTe₂ crystallises in the monoclinic 1T form, with space group P2₁/m with unit cell sizes of a=6.33 Å b=3.469 Å and c=13.86 Å with the angle β=93°55′. The high-temperature form has rod shaped crystals. The measured density of this polymorph is 7.5 g·cm⁻³, but in theory it should be 7.67 g·cm⁻³. Tellurium atoms form a distorted octahedron around the molybdenum atoms. This high-temperature form, termed β–MoTe₂ can be quenched to room temperature by rapid cooling. In this metastable state β-MoTe₂ can survive below 500 °C. When metastable β–MoTe₂ is cooled below −20 °C, its crystal form changes to orthorhombic. This is because the monoclinic angle c changes to 90°. This form is called β' or, misleadingly, Td.
  The transition from α- to β-MoTe₂ happens at 820 °C, but if Te is reduced by 5% the required transition temperature increases to 880 °C. K. Ueno and K. Fukushima claim that when the α form is heated in a low or high vacuum that it oxidises to form MoO₂ and that reversible phase transitions do not take place.
  In bulk, MoTe₂ can be produced as a single crystal with difficulty, but can also be made as a powder, as a polycrystalline form, as a thin film, as a nanolayer consisting of a few TeMoTe sheets, a bilayer consisting of two sheets or as a monolayer with one sheet.
  Thin nanolayer forms of α-MoTe₂ have different symmetry depending on how many layers there are. With an odd number of layers the symmetry group is D¹₃ₕ without inversion, but for an even number of layers, the lattice is the same if inverted and the symmetry group is D³3d.Nanotubes with a 20–60 nm diameter can be made by heat treating amorphous MoTe₂.
caption: Crystal structure of orthorhombic (β', 1T' or Td) and monoclinic (β or 1T, shadow) MoTe2
exif: {}