Thermal expansion of lemmleinite-Ba had been studied by high-temperature X-ray powder diffraction method. Parameters at room temperature are: a= 14.183(9), b= 13.744(3), c= 7.754(5) Å, β = 116.66(5)°, V= 1351(2) Å3. Unit-cell parameters change linearly with the increase of temperature and may be described by the following polynomials: a = 14.184 + 0.1661 10-3 t; b = 13.758 + 0.06508·10-3 ·t; с = 7.747 + 0.1379·10-3 ·t [Å]; β = 116.64 + 0.9522 ·10-3 ·t [°]; V= 1349.872 + 42 ·10-3 ·t [Å3]. Temperature of the beginning of dehydration can be estimated as 275° ± 25° (with heating rate of 50 °C/h)/ Thermal expansion coefficients within range of temperature 25-275 °C are: αa = 12, αb = 5, αc = 18, α11 = 20, α22 = 5,033 = 1 ( 10-6°C-1), µ = 69°. Peculiarities of thermal expansion can be explained as a result of hinge deformations which is especially clear from the v
