Identification of a new structural polymorph of quenched high-pressure CrTe₃ (P2/m)

Abstract text (incl. figure legends and references)

CrTe (P2₁/c) is of potential interest for spintronic devices because of its layered structural motif and antiferromagnetic and semiconducting properties (Eg = 0.3 eV)[1]. Modifying these properties towards a ferromagnetic state and metallic-like conduction upon structure deformation is suggested by theoretical studies which describe a high strain sensitivity of the material when the dimensions are reduced to a monolayer of CrTe₃ [2]. Such studies are motivation to investigate the CrTe3 system further.

In the past, structural polymorphs of binary TE have been identified and described using high-p and high-T experiments as a vital approach to explore structural modifications with unprecedented properties [3,4]. In this work, the layered phase of CrTe3 was used as a starting material and subjected to pressures as high as 6GPa at an applied temperature of 250°C to investigate new structural polymorphs of CrTe₃ and their magnetic properties.

The structural polymorph of the quenched CrTe₃ high-p phase was investigated by the combined methods of synchrotron radiation scattering experiments (PDF and XRD) and PED as well as atomic resolution STEM. Model-based simulations of STEM and ED data were conducted and compared with structural refinements by Rietveld analysis for structure determination.

With our experiments, we were able to identify a phase transition from the monoclinic CrTe₃ phase (P2₁/c) into a structural polymorph with space group P2/m. At first, the superposition of two potential structure candidates of CrTe₂ (Pnn2) and CrTe₄ (P2₁/m) were refined. These initial models were tested to PED data which excluded the CrTe₂ model and suggested the introduction of a 1/3 occupied Cr position at the corners of the unit cell into the supposed CrTe₄ structure to explain the kinematic intensity patterns. Further, large area EDX analysis confirmed the chemical homogeneity within large grains of the CrTe₃ polymorph with stoichiometry of Cr:Te 1:3. The here proposed structure of the new polymorph after refinement to the synchrotron diffraction data is depicted in Figure 1. Atomic scale imaging by aberration corrected STEM supported the proposed model by imaging the Te sub-structure.

In conclusion, a new structural polymorph of the CrTe₃ system was identified after high-pressure synthesis and subsequent quenching by the combined approach of TEM and synchrotron diffraction. In ongoing research, first SQUID measurements indicate interesting changes of the magnetic properties which could provide similarities to the theoretical calculations made by Z.-W. Lu et al[2].

[1] M. A. McGuire et al., Phys. Rev. B, vol. 95, no. 14, p. 144421, Apr. 2017, doi: 10.1103/PhysRevB.95.144421.

[2] Z.-W. Lu, S.-B. Qiu, W.-Q. Xie, X.-B. Yang, and Y.-J. Zhao, J. Appl. Phys., vol. 127, no. 3, p. 033903, Jan. 2020, doi: 10.1063/1.5126246.

[3] A. Pawbake et al., Phys. Rev. Lett., vol. 122, no. 14, p. 145701, Apr. 2019, doi: 10.1103/PhysRevLett.122.145701.

[4] W. C. Yu and P. J. Gielisse, Mater. Res. Bull., vol. 6, no. 7, pp. 621–638, Jul. 1971, doi: 10.1016/0025-5408(71)90011-0.

Figure 1: Crystal structure of the quenched high-pressure phase of CrTe₃ with space group P2/m shown for a unit cell of 2x2x2 size. The corner-sharing motif can be best seen by a view along the crystallographic b-axis(a). Golden atoms are Te, blue are Cr. The edge-sharing motif and columnar ordering of the CrTe₆ octahedra can be best seen by a view along the crystallographic c-axis (b).