Thermal, Structural, and Phase Evolution of the Y₂(SO₄)₃*8H₂O–Eu₂(SO₄)₃*8H₂O System via Dehydration and Volatilization to Y₂(SO₄)₃–Eu₂(SO₄)₃ and Y₂O₂(SO₄)–Eu₂O₂(SO₄) and Its Thermal Expansion

The synthesis, crystal structure, phase transformations, and thermal expansion of (Y_1–xEu_x)₂(SO₄)₃*8H₂O (where x = 0, 0.17, 0.33, 0.50, 0.66, 0.83, and 1) are presented. (Y_1–xEu_x)₂(SO₄)₃*8H₂O solid solutions were synthesized via crystallization from an aqueous solution. (Y_1–xEu_x)₂(SO₄)₃*8H₂O (C2/c) ↔ (Y_1–xEu_x)₂(SO₄)₃ (Pbcn) → (Y_1–xEu_x)₂O₂SO₄ (C2/c) and Eu₂(SO₄)₃*8H₂O (C2/c) ↔ Eu₂(SO₄)₃ (C2/c) → Eu₂O₂SO₄ (C2/c) phase transformations for all samples were investigated by high-temperature powder X-ray diffraction, differential scanning calorimetry and thermogravimetry in the temperature ranges of 25–750 and 25–1350 °C, respectively. The aim of this work is to identify the structural heredity of the phases formed during thermal transformations of (Y_1–xEu_x)₂(SO₄)₃*8H₂O solid solutions, and to study the mechanisms of the thermal deformations of the crystal structure. Structural relations between these phases were found. The crystal structures of YEu(SO₄)₃*8H₂O and (Y_0.83Eu_0.17)₂(SO₄)₃*8H₂O were refined at −173, −123, −73, −23, 27, and 77 °С. Thermal expansion coefficients for (Y_1–xEu_x)₂(SO₄)₃*8H₂O, Eu₂(SO₄)₃, (Y_1–xEu_x)₂O₂SO₄ (where x = 0, 0.17, 0.33, 0.50, 0.66, 0.83, and 1) compounds and solid solutions were calculated for the first time. The thermal expansion of Eu₂(SO₄)₃ was highest in the direction approximately coinciding with the c-axis, because the Eu–O chains extended in this direction. As temperature increased, the crystal structure of (Y_1–xEu_x)₂(SO₄)₃*8H₂O expanded significantly in the ac plane along directions close to the a and c axes, while thermal expansion along the b axis was relatively low. The distance between layers in the (Y_1–xEu_x)₂(SO₄)₃*8H₂O crystal structure increased with increasing temperature, and corrugated layers (parallel to (101) direction) straightened out due to the rotation of the S2O₄ tetrahedra. At high temperature, thermal expansion of Y₂O₂SO₄ was highest along the longer diagonal of the ac parallelogram perpendicular to the plane of the oxo-centered ${\displaystyle \frac{2}{\infty }}$[YO] layers.