Temperature dependence of lattice parameters of langasite single crystals

To determine the coefficient of thermal expansion of trigonal langasite (La3Ga5SiO14) the two independent lattice parameters a and c are measured over a temperature range of 800 degrees C using X-ray diffraction on single crystal samples. From the given nonlinear temperature dependence the linear and quadratic thermal coefficients of expansion alpha(11), beta(11) and alpha(33), beta(33) for the two lattice parameters a and c could be deduced.