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Crystal structures of pure and doped KLnF4 (Ln = Y, Ho, Er, Tm, Yb) grown hydrothermally were studied with synchrotron single-crystal and powder diffraction as a function of temperature and pressure. At atmospheric conditions, KHoF4 and KErF4 crystallize in space group P31, while KTmF4, Er:KYbF4, and KYF4 crystallize in space group P32. In both enantiomorphic structures, the K+ and Ln3+ cations are completely ordered. The pseudo-symmetry of the structures with respect to the two minimal supergroups k = 3 (P31 and P32) and t = 2 (P3112 and P3212) increases with decreasing radius of the Ln3+ cation, respectively. No phase transition is detected in KYF4 at low temperatures down to 100 K at atmospheric pressure. Er:KYbF4 and KYF4 undergo irreversible pressure-induced phase transitions at about 4 GPa. In each case, the single crystals become fragmented into several crystallites as observed during single-crystal measurements in diamond anvil cells. Up to the phase transitions, both Er:KYbF4 and KYF4 are more compressible along the c axis and their bulk compressibility predominantly results from the contraction of the KF8 polyhedra. The application of pressure does not affect the distribution of the cations in the crystal structures of Er:KYbF4 and KYF4 up to the phase transitions at about 4 GPa.
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