Calculation of the lattice deformation at the phase transitions of [Fe(ptz)₆](BF₄)₂ from powder diffraction patterns

The thermal and light-induced high-spin (HS: ⁵T_2g) low-spin (LS: ¹A_1g) transitions of [Fe(ptz)₆](BF₄)₂ (ptz = 1-propyltetrazole) are investigated by X-ray powder diffractometry in the range between ambient temperature and ~15 K. The anisotropic lattice deformation caused by variation of the HS fraction and by temperature change was calculated directly from the differences in peak positions of the powder diagrams measured at different temperatures. An analytical expression for the dependence of peak shifts and splittings on the deformation tensor elements was derived. The differences of the lattice deformations accompanying the thermal and light-induced spin transitions are discussed.