Belomarinaite, ideally KNaSO₄, is a new sulfate mineral discovered in the Toludskoe lava field, formed during the 2012–2013 Tolbachik Fissure eruption. The mineral occurs as arborescent aggregates of tabular crystals (1 mm × 0.3 mm × 0.1 mm) comprising hematite impurities. The average size of the aggregates is 0.5–0.7 mm. The empirical formula is (K_0.95Na_0.92Cu_0.04)_Σ1.91S_1.01O₄. The crystal structure of belomarinaite was determined using single-crystal X-ray diffraction data; the space group isP3m1,a= 5.6072(3),c= 7.1781(4) Å,V= 195.45(2) ų,Z= 2 andR₁= 2.6%. In the crystal structure of belomarinaite, there are six cation sites: the^[4]S1 and^[4]S2 sites are occupied by S, the^[6]Na and^[12]K sites are occupied by Na and K, respectively, giving Na_0.5K_0.5apfu and the^[10]M1 and^[10]M2 sites are occupied by Na_0.78K_0.22and K_0.78Na_0.22apfu, respectively. The crystal structure is a framework of SO₄tetrahedra, Na octahedra and K,M1 andM2 polyhedra. Belomarinaite is isostructural with the synthetic compound KNaSO₄. In belomarinaite, Na and K are disordered overM1 andM2 sites; in its synthetic analogue, Na and K are ordered overM1 andM2 sites, respectively. The Mohs’ hardness is 2–3. The mineral is uniaxial (+), with ω = 1.485(3) and ε = 1.488(3) (λ = 589 nm). The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 4.022(31)(101); 3.591(26)(002); 2.884(74)(102); 2.800(100)(110); 2.391(16)(003); 2.296(8)201; 2.008(38)(022); and 1.634(10)(212). The mineral was named in honour of Russian volcanologist Marina Gennadievna Belousova (b. 1960) for her significant contributions to the monitoring of the Tolbachik Fissure eruption.

One of the most critical problems for phosphors used in light-emitting diodes (LEDs) is the thermal quenching of luminescence. Herein, novel (Y_1−xEu_x)₂(SO₄)₃ (x = 0, 0.17, 0.33, 0.5, 0.66) phosphors with anti-thermal quenching of luminescence due to giant negative thermal expansion were obtained by crystallization from aqueous solutions. The Y₂(SO₄)₃ crystal structure is described as a framework, which consists of corner-sharing SO₄ tetrahedra and YO₆ octahedra. Thermal expansion of x = 0, 0.17, 0.5 solid solutions was investigated by high-temperature powder X-ray diffraction. The crystal structure of the x = 0.5 solid solution was refined by the Rietveld refinement using X-ray powder diffraction data at 400 and 500 °C to R_B = 1.7 and 1.9%, respectively. The rotation of the SO₄ tetrahedra results in giant negative volumetric thermal expansion: the structure contracts in all directions (α_a = −7(3), α_b = −15(3), α_c = −22(2), α_V = −44(2) · 10⁻⁶ °C⁻¹ at 500 °C for YEu(SO₄)₃). The photoluminescence intensity is enhanced 1.8 times with the temperature increasing up to 500 °C. The anti-thermal quenching of Eu³⁺ luminescence can be described in terms of thermal-enhanced energy transfer between Eu³⁺ ions due to the negative thermal expansion of the Y₂(SO₄)₃ matrix in all non-equivalent directions.

Belomarinaite KNaSO₄ (space group P3m1, a = 5.6072(3), c = 7.1781(4) Å and Z = 2) has been studied by high-temperature single crystal X-ray diffraction. The K and Na atoms are disordered, and M1(1c) and M2(1b) sites merge into an M1(2d)(K50%/Na50%) site with increasing temperature, and novel translationsgleiche phase transition of belomarinaite KNaSO₄ (P3m1 → P-3m1) was revealed at 123 °C. Then, the temperature increase leads to phase transition of $P\overline{3}m1$-polymorph of KNaSO₄ to α-KNaSO₄ (P6₃/mmc) at 446 °C, and sodium and potassium atoms in K1(1a) and Na1(1b) sites merged into an M4(2a)(K50%/Na50%) site. Crystal structures of KNaSO₄ were refined at 300, 500 and 750 °C to R₁ = 0.057, 0.056 and 0.048, respectively. The BVS maps and bond-valence energy landscape (BVEL) have been calculated, and the probability of the Na⁺ migration has been predicted using structural data for belomarinaite at 25 °C. The Na⁺ ion migration can occur at 2.14 eV in the ab plane and 2.80 eV along the c axis at 25 °C.

Novel Ba₃Sr₃B₄O₁₂: Eu³⁺ phosphors were synthesized by crystallization from a melt. The crystal structures of Ba₃(Sr_3−1.5xEu_x)B₄O₁₂ (x = 0.03, 0.06, 0.15, 0.20, 0.25) solid solutions were refined from SCXRD data. The crystal structures of Ba₃(Sr_3−1.5xEu_x)B₄O₁₂ phosphors can be described in terms of the cationic sublattice and belong to the “anti-zeolite” family of borates. Its cationic framework is constructed of Ba and Sr atoms. The Eu³⁺ ions occupy the Sr(1) extraframework cationic site in the Ba₃(Sr_3−1.5xEu_x)B₄O₁₂ (x = 0.01–0.20) phosphors. The Ba₃Sr_2.625Eu_0.25B₄O₁₂ borate crystallizes in a new structure type (I4/mcm, a = 13.132(3), c = 14.633(4) Å, V = 2523.5(11) ų, Z = 8, R₁ = 0.067). In the Ba₃Sr_2.625Eu_0.25B₄O₁₂ crystal structure, the Eu³⁺ ions occupy Sr(1) and Ba/Sr(1) sites, which leads to changes in the crystal structure. The Wyckoff letter and occupancy of the O(5) site are changed; B–O anion groups contain two BO₃ triangles (B(3) and B(4)), orientationally disordered over the four orientations, and two ordered BO₃ triangles (B(1) and B(2)) in contrast to Ba₃Sr₃B₄O₁₂, in which these groups are disordered over the 4 and 8 orientations. The emission spectra of Ba₃Sr₃B₄O₁₂: Eu³⁺ show characteristic lines corresponding to the intraconfigurational 4f-4f transitions of Eu³⁺ ions. Ba₃Sr_2.7Eu_0.20B₄O₁₂ demonstrates the strongest luminescent intensity among Ba₃(Sr_3−1.5xEu_x)B₄O₁₂ solid solutions. The increase in the Eu³⁺ content results in a gradual change in chromaticity from light red to orange-red/red. It can be concluded that Ba₃Sr₃B₄O₁₂: Eu³⁺ is a promising red phosphor.