Crystal structures of the NaLnF₄ materials (Ln = La, Ce, Pr, Nd, Sm or Gd) were studied with synchrotron single-crystal and powder diffraction. The materials with Ln = La, Ce, Nd, Sm and Gd have the average β structure (P, Z = 1) with partial ordering of the cations. A new type of a superstructure due to ordering of the cations and vacancies was found in NaPrF₄ (P3, Z = 6). It could be described using the group–subgroup relationships P ↔ P3. Our observations suggest that the β structure is unstable and that the ordering is a slow process at ambient conditions. Upon compression, β-NaNdF₄, β-NaGdF₄ and the superstructure NaPrF₄ are stable to at least 8 GPa with no evidence for any pressure-induced disorder–order phenomena.

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.

Mn₂Sb is ferrimagnetic below its Curie temperature (T_C) and passes through a spin flip transition with decreasing temperature. The Co substitution induces an additional first-order phase transition from the ferrimagnetic (FRI) to an antiferromagnetic (AFM) state. This phase transition is connected to a sizable magnetocaloric effect (MCE). To understand the underlying mechanisms, the temperature dependence of structural and magnetic changes was analyzed. At the same time, the influence of the Co substitution was explored. Three Mn_2−xCo_xSb (x = 0.1, 0.15, 0.2) compounds were synthesized by cold crucible induction melting. Neutron powder diffraction was performed to determine the magnetic structures and to obtain the individual magnetic moments on both symmetrically independent Mn sites. In combination with the temperature-dependent magnetization measurements, the magnetic phase transition temperatures were identified. In the low-temperature range, additional antiferromagnetic peaks were detected, which could be indexed with a propagation vector of (0 0 ½). In Mn_1.9Co_0.1Sb at 50 K and in Mn_1.8Co_0.2Sb at 200 K, a co-existence of the FRI and the AFM state was observed. The pure AFM state only occurs in Mn_1.8Co_0.2Sb at 50 K.