${\mathrm{Cr}}_{1+x}{\mathrm{Te}}_2$ is a self-intercalated van der Waals system that is of current interest for its room-temperature room-temperature ferromagnetic (FM) phases and tunable topological properties. In bulk samples, the strain from the interstitial Cr ions leads to distinct structural phases for different ranges of $x$. Early neutron powder diffraction (NPD) measurements on the monoclinic phase ${\mathrm{Cr}}_3{\mathrm{Te}}_4$ ($x=0.5$) presented evidence for competing FM and antiferromagnetic (AFM) phases. Here we apply neutron diffraction to a single crystal of ${\mathrm{Cr}}_{3+\delta }{\mathrm{Te}}_4$ with $\delta =-0.10$ and discover that it consists of two distinct monoclinic phases, one with FM order below $T_{\mathrm{C}}\approx 321$ K and another that develops AFM order below $T_{\mathrm{N}}\approx 86$ K. In contrast, we find that a crystal with $\delta =-0.26$ exhibits only FM order below $T_{\mathrm{C}}\approx 285$ K. The single-crystal analysis is complemented by results obtained with NPD, x-ray powder diffraction, and transmission electron microscopy (TEM) measurements on the $\delta =-0.10$ composition. From observations of spontaneous magnetostriction of opposite sign at $T_{\mathrm{C}}$ and $T_{\mathrm{N}}$, along with the TEM evidence for both monoclinic phases in a single thin ($\approx 100$ nm) grain, we conclude that the two phases must have a fine-grained ($\lesssim 100$ nm) intergrowth character, as might occur from high-temperature spinodal decomposition during the growth process. Calculations of the relaxed lattice structures for the FM and AFM phases with density functional theory provide a rationalization of the observed spontaneous magnetostrictions. Correlations between the magnitude and orientation of the magnetic moments with lattice parameter variation demonstrate that the magnetic orders are sensitive to strain, thus explaining why magnetic ordering temperatures and anisotropies can be different between bulk and thin-film samples, when the latter are subject to epitaxial strain. Our results point to the need to investigate the supposed coexistence FM and AFM phases reported elsewhere in the ${\mathrm{Cr}}_{1+x}{\mathrm{Te}}_2$ system, such as in the ${\mathrm{Cr}}_5{\mathrm{Te}}_8$ phase ($x=0.25$).

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• Magnetic order
• Magnetism
• Magnetostriction
• Order parameters
• Single crystal materials
• van der Waals systems
• Crystal growth
• Neutron scattering
• X-ray diffraction