In this study, the effect of chromium impurities on the crystal structure and lattice microstrains of tin oxide nanoparticles was investigated. Pure SnO2 nanoparticles were synthesized and sub-jected to calcination at different temperatures. Additionally, various concentrations (5%, 8%, 10% and 15%) of Cr-doped SnO2 nanoparticles were prepared using the sol-gel technique and subse-quently calcined at 550 °C. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques were utilized to examine the structure and morphology of the doped nanoparticles. The XRD patterns of tin oxide nanoparticles with different percentages of chromium impurities showed a tetragonal structure without any additional phase. The TEM images of pure SnO2 na-noparticles showed a uniform distribution of size and shape, with relatively smaller sizes com-pared to Cr-doped nanoparticles. To investigate the peak broadening of Cr-doped SnO2 nano-particles, the Halder-Wagner method and Williamson-Hall models were employed to examine the effects of crystallite sizes and lattice strain. The results showed that increasing the impurity has a dual effect on nanoparticle sizes. Increasing the chromium impurity up to 8% led to an in-crease in compressive stress caused by the substitution of Sn ions with Cr ions on the crystal structure of rutile, resulting in an increase in the magnitude of lattice strain. However, when the chromium impurity was increased more, up to 15%, interstitial doping was preferred over sub-stitutional doping. The compressive stress was subsequently converted to tensile stress, requir-ing the system to spend some of its energy to overcome the compressive stress, with the re-maining energy reflected in the form of tensile stress. Furthermore, Fourier transform infrared (FTIR) spectra were obtained for all of the samples, confirming the XRD analyses.

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