Powder samples of pure BaAlO and doped with 4.9 atom % Eu in relation to Ba were prepared by a hydrothermal route. The samples were characterized by X-ray diffraction, Eu Mössbauer spectroscopy, synchrotron-based X-ray absorption spectroscopy at the Ba L- and Eu L-edges, and photoluminescence measurements. Diffraction lines were broadened, indicating that the samples were nanocrystallline. The samples possessed a hexagonal crystal structure, space group P6. Eu Mössbauer spectroscopy revealed the presence of Eu in the 3+ oxidation state. The same information on the Eu oxidation state was also obtained by the Eu L-edge X-ray absorption near-edge structure of the doped sample. Extended X-ray absorption fine structure showed an Eu ion substituted for Ba on the Ba2 site in the BaAlO host structure, with charge compensation by an interstitial O in the vicinity of the Ba2 site. That was confirmed by a Rietveld structure refinement for the Eu-doped BaAlO sample. Analysis of the diffraction line broadening for the prepared samples was performed simultaneously with the structure refinement. Both the dopant Eu and the interstitial O acted as defects in the host BaAlO lattice, which increased the lattice strain from 0.02% for pure BaAlO to 0.17% for the Eu-doped sample. Crystallite sizes in the samples increased with Eu doping from 32 nm for pure BaAlO to 36 nm for Eu-doped BaAlO. This could likely be related to the increase in the diffusion rate of the cations in the sample when a part of the Ba cation content was exchanged with smaller Eu cations. The Eu-doped BaAlO sample exhibited red photoluminescence under excitation with λ = 308 nm. The observed emission spectrum indicated that Eu ions occupied the Ba site with lower symmetry in the doped sample.

We have studied the magnetic and magnetocaloric (MC) properties of lithium lanthanide tetraphosphates LiLnP4O12 (Ln = Gd, Tb, Dy) single crystals (SCs) upon the magnetization (M) and heat-capacity (Cp) measurements. Detailed investigations in the temperature range T = 2–300 K indicate that all SCs exhibit paramagnetic (PM) character with effective PM moments close to those expected for free Ln 3+ ions. The magnetic contribution to Cp takes place strongly at temperatures below 20 K that enhances the values of the parameters characteristic for the MC effect. Under an applied field H = 50 kOe, the values of the maximum magnetic entropy change (| Δ S m max |) at 2 K are about 27.6, 15.9 and 11.7 J kg−1 K−1 for Ln = Gd, Tb and Dy, respectively, corresponding to refrigerant capacity (RC) of 133 ∼ 254 J kg−1. Among the studied SCs, the maximum adiabatic temperature change ( Δ T a d max ) obtained for the system Ln = Gd at a field change H = 20 kOe is about 13 K, which can be achieved by using a permanent magnet. With the large values of | Δ S m max |, RC and Δ T a d max , and the absence of magnetic hysteresis, LiLnP4O12 are considered as alternatively potential candidates for adiabatic magnetic refrigeration applications at sub-helium temperatures. It is believed that the different magnetic and MC properties among the LiLnP4O12 SCs are related to the ground-state nature and spin-orbit interaction of Ln 3+ ions.

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