[en] Crystal structures and thermal expansion properties of solid solutions Ln_2-xDy_xW₃O₁₂ (Ln = Er and Y) were studied by room and high temperature X-ray diffraction. Rietveld analysis results show that compounds Er_2-xDy_xW₃O₁₂ with 0 ≤ x ≤ 0.7 and Y_2-xDy_xW₃O₁₂ with 0 ≤ x ≤ 1.0 have orthorhombic structures and show negative thermal expansion in temperature range of 200-800 deg. C while compounds Er_2-xDy_xW₃O₁₂ with 1.2 ≤ x ≤ 2.0 and Y_2-xDy_xW₃O₁₂ with 1.5 ≤ x ≤ 2.0 have monoclinic structures and exhibit positive thermal expansion in temperature range of 25-800 deg. C. Thermogravimetric (TG) and differential thermal analysis (DTA) were used to study the hygroscopicity and the phase transition temperature